ViDA 2021 Speakers

Bernardo Sabatini
Neurobiology, Harvard Medical School, HHMI
Dopaminergic control of cellular state and action selection
Dopamine is known to reinforce actions which, when performed in the right sensory context, led to its release. This is thought to occur via temporally restricted actions on direct and indirect pathway neurons of the striatum. Here we discuss how dopaminergic neuron firing affects the electrical and biochemical state of direct and indirect pathway striatum projection neurons. Furthermore, we demonstrate how the effects of dopamine on each neuron class depends on the receptors they express and how these vary during behavior. Lastly, we present a model for how these effects contribute to action reinforcement and devaluation.
Neurobiology, Harvard Medical School, HHMI
Dopaminergic control of cellular state and action selection
Dopamine is known to reinforce actions which, when performed in the right sensory context, led to its release. This is thought to occur via temporally restricted actions on direct and indirect pathway neurons of the striatum. Here we discuss how dopaminergic neuron firing affects the electrical and biochemical state of direct and indirect pathway striatum projection neurons. Furthermore, we demonstrate how the effects of dopamine on each neuron class depends on the receptors they express and how these vary during behavior. Lastly, we present a model for how these effects contribute to action reinforcement and devaluation.

Okihide Hikosaka
NIH/NEI
Dopamine neurons prepare actions based on objects and environments
Dopamine (DA) neurons are sensitive to the reward outcome. In addition, they predict reward (+) or (-) based on a particular object. This is important to control behavior: Saccade to the reward-predicting object (good object); Saccade away from the no-reward-predicting object (bad object). These selective eye movements are controlled by the basal ganglia projecting to the superior colliculus. These mechanisms start working after finding good or bad objects. However, these objects often appear in particular environments. Then, animals (& humans) try to find good objects in such environments. This raises a new question: Can DA neurons identify such environments? After the monkey learned objects and environments, we found that some DA neurons are tonically activated when a rich environment (i.e., contains good objects) appears. This is different from their phasic responses to objects. How do they get the tonic activity? It has been known that neurons in the amygdala project to the substantia nigra (SN), including pars reticulata neurons (SNr) and DA neurons. We found that neurons in the central amygdala (CeA) are tonically activated by rich environments, which disinhibited some DA neurons through two sequential inhibitions: 1) CeA-SNr, 2) SNr-DA. These data suggest that DA neurons contribute to goal-directed behavior based on two features: Object and Environment. It is important to start preparing actions in response to an environment, although its outcome may be uncertain.
NIH/NEI
Dopamine neurons prepare actions based on objects and environments
Dopamine (DA) neurons are sensitive to the reward outcome. In addition, they predict reward (+) or (-) based on a particular object. This is important to control behavior: Saccade to the reward-predicting object (good object); Saccade away from the no-reward-predicting object (bad object). These selective eye movements are controlled by the basal ganglia projecting to the superior colliculus. These mechanisms start working after finding good or bad objects. However, these objects often appear in particular environments. Then, animals (& humans) try to find good objects in such environments. This raises a new question: Can DA neurons identify such environments? After the monkey learned objects and environments, we found that some DA neurons are tonically activated when a rich environment (i.e., contains good objects) appears. This is different from their phasic responses to objects. How do they get the tonic activity? It has been known that neurons in the amygdala project to the substantia nigra (SN), including pars reticulata neurons (SNr) and DA neurons. We found that neurons in the central amygdala (CeA) are tonically activated by rich environments, which disinhibited some DA neurons through two sequential inhibitions: 1) CeA-SNr, 2) SNr-DA. These data suggest that DA neurons contribute to goal-directed behavior based on two features: Object and Environment. It is important to start preparing actions in response to an environment, although its outcome may be uncertain.

Roy Wise
NIH/NIDA
A Dopamine History
The history of dopamine is over 100 years. It began when Sir Henry Dale first tested a compound in 1910; he eventually named the compound dopamine in 1951. A handful of investigators took steps to develop and use the compound, and the major clinical ending was when dopamine depletion was iidentifed in Parkinsonian patients and when L-DOPA was found to alleviate its symptoms. The lack of movement in Parkinsonian patients—particularly those who had been infected in the 1919 flu epidemic—suggested that dopamine was involved in motor function; dopamine-depleted animals were akinetic and failed to seek food or water or other rewards and failed to avoid predictable punishment. These animals had reflex responses to rewards and punishers but could not learn to search for rewards—or to avoid predictable punishers. In normal animals, excitatory environmental stimuli cause burst-firing of dopaminergic neurons. This burst-firing is required for learning to respond to predictive stimuli; this learning involves two mechanisms. First, burst-responses must develop, through experience, to reward-predictors. This happens by a Hebbian process; rewards enable the development of burst-firing in response to stimuli that immediately precede them. Second, burst-firing in response to predictors enables the cellular basis for long-term potentiation (LTP) or long-term depression (LTD) in nearby sensory—glutamatergic—inputs to medium-spiny—GABAergic—output neurons in the striatum. By enabling LTP and LTD, burst-firing of dopamine neurons enables the stamping-in of learned responses to external stimuli; this is one of dopamine’s two major behavioral effects. A second behavioral function is the modulation of motivation. The level of pacemaker firing of the dopamine system drives or helps drive motivation; low levels of baseline dopamine are associated with low motivation, and moderate levels are associated with activation. High levels of dopamine—apparently caused only by addictive drugs—are associate with drug-satiety. The driving of dopaminergic burst-firing is induced by glutamate input (a correlate of, but separate from, glutamate input to the striatum). The driving of pacemaker firing is under the control of hormones and the modulation of inhibitory GABAergic input to the dopamine neurons. These two functions—reinforcement and motivation—are the major behavioral effects of dopamine in the brain, and they appear to account for its secondary effects.
NIH/NIDA
A Dopamine History
The history of dopamine is over 100 years. It began when Sir Henry Dale first tested a compound in 1910; he eventually named the compound dopamine in 1951. A handful of investigators took steps to develop and use the compound, and the major clinical ending was when dopamine depletion was iidentifed in Parkinsonian patients and when L-DOPA was found to alleviate its symptoms. The lack of movement in Parkinsonian patients—particularly those who had been infected in the 1919 flu epidemic—suggested that dopamine was involved in motor function; dopamine-depleted animals were akinetic and failed to seek food or water or other rewards and failed to avoid predictable punishment. These animals had reflex responses to rewards and punishers but could not learn to search for rewards—or to avoid predictable punishers. In normal animals, excitatory environmental stimuli cause burst-firing of dopaminergic neurons. This burst-firing is required for learning to respond to predictive stimuli; this learning involves two mechanisms. First, burst-responses must develop, through experience, to reward-predictors. This happens by a Hebbian process; rewards enable the development of burst-firing in response to stimuli that immediately precede them. Second, burst-firing in response to predictors enables the cellular basis for long-term potentiation (LTP) or long-term depression (LTD) in nearby sensory—glutamatergic—inputs to medium-spiny—GABAergic—output neurons in the striatum. By enabling LTP and LTD, burst-firing of dopamine neurons enables the stamping-in of learned responses to external stimuli; this is one of dopamine’s two major behavioral effects. A second behavioral function is the modulation of motivation. The level of pacemaker firing of the dopamine system drives or helps drive motivation; low levels of baseline dopamine are associated with low motivation, and moderate levels are associated with activation. High levels of dopamine—apparently caused only by addictive drugs—are associate with drug-satiety. The driving of dopaminergic burst-firing is induced by glutamate input (a correlate of, but separate from, glutamate input to the striatum). The driving of pacemaker firing is under the control of hormones and the modulation of inhibitory GABAergic input to the dopamine neurons. These two functions—reinforcement and motivation—are the major behavioral effects of dopamine in the brain, and they appear to account for its secondary effects.

Roshan Cools
Radboud university medical center & Donders Institute for Brain, Cognition and Behaviour
Chemistry of the adaptive mind: lessons from dopamine
The human brain faces a variety of computational dilemmas, including the flexibility/stability, the speed/accuracy and the labor/leisure tradeoff. I will argue that striatal dopamine is particularly well suited to dynamically regulate these computational tradeoffs depending on constantly changing task demands. This working hypothesis is grounded in evidence from recent studies on learning, motivation and cognitive control in human volunteers, using chemical PET, psychopharmacology, and/or fMRI. These studies also begin to elucidate the mechanisms underlying the huge variability in catecholaminergic drug effects across different individuals and across different task contexts. For example, I will demonstrate how effects of the most commonly used dopaminergic drug methylphenidate on learning, Pavlovian and effortful instrumental control depend on fluctuations in current environmental volatility, on individual differences in working memory capacity and on opportunity cost respectively.
Radboud university medical center & Donders Institute for Brain, Cognition and Behaviour
Chemistry of the adaptive mind: lessons from dopamine
The human brain faces a variety of computational dilemmas, including the flexibility/stability, the speed/accuracy and the labor/leisure tradeoff. I will argue that striatal dopamine is particularly well suited to dynamically regulate these computational tradeoffs depending on constantly changing task demands. This working hypothesis is grounded in evidence from recent studies on learning, motivation and cognitive control in human volunteers, using chemical PET, psychopharmacology, and/or fMRI. These studies also begin to elucidate the mechanisms underlying the huge variability in catecholaminergic drug effects across different individuals and across different task contexts. For example, I will demonstrate how effects of the most commonly used dopaminergic drug methylphenidate on learning, Pavlovian and effortful instrumental control depend on fluctuations in current environmental volatility, on individual differences in working memory capacity and on opportunity cost respectively.

Ann Graybiel
MIT
A Circuit For Controlling Dopamine
Dopamine holds great power through its actions on multiple sits in the brain. This power is displayed in everyday life and also in the problems that arise when circuits related to dopamine are compromised, as in a range of neurology and neuropsychiatric disorders extending to addictive states. Great care must therefor have been taken through evolution to have gates and other controls on dopamine, and to have homeostatic state control systems. Here I will talk about of these, which has a surprising influence on dopaminergic cells of the midbrain substantia nigra pars compacta (SNpc).This circuit forms part of the striato-nigral circuit, a crucial part of the nigro-striatal-nigral loop system renowned in the clinic. This striato-nigral circuit can act in part through striosome-dendron bouquets, elaborate input-output formations by which striosomal axons can engage directly dopamine-containing cells and dendrons of the SNpc. The existence of this circuit suggests that striosomes are in a position to switch the activity of dopamine neuron subgroups, as part of a larger network of cortico-striosome inputs directly and indirectly dopamine function.
MIT
A Circuit For Controlling Dopamine
Dopamine holds great power through its actions on multiple sits in the brain. This power is displayed in everyday life and also in the problems that arise when circuits related to dopamine are compromised, as in a range of neurology and neuropsychiatric disorders extending to addictive states. Great care must therefor have been taken through evolution to have gates and other controls on dopamine, and to have homeostatic state control systems. Here I will talk about of these, which has a surprising influence on dopaminergic cells of the midbrain substantia nigra pars compacta (SNpc).This circuit forms part of the striato-nigral circuit, a crucial part of the nigro-striatal-nigral loop system renowned in the clinic. This striato-nigral circuit can act in part through striosome-dendron bouquets, elaborate input-output formations by which striosomal axons can engage directly dopamine-containing cells and dendrons of the SNpc. The existence of this circuit suggests that striosomes are in a position to switch the activity of dopamine neuron subgroups, as part of a larger network of cortico-striosome inputs directly and indirectly dopamine function.

Ehud Isacoff
MCB, UC Berkeley
MP-D: cell-specific molecular gating of dopamine circuits
Dopamine circuits control diverse behaviors and their dysregulation contributes to many disorders. Our ability to understand and manipulate their function is hampered by the heterogenous nature of dopaminergic projections, the diversity of neurons that are regulated by dopamine, the varying distribution of dopamine receptors, and the complex dynamics of dopamine release. We developed a solution to these challenges in a generalizable photo-pharmacological approach called membrane-anchored photoswitchable orthogonal remotely tethered ligand (MP) that makes it possible to manipulate dopamine circuits with brain region, cell type, receptor subtype, and temporal specificity. Using a dopamine receptor MP (MP-D), we found that although dopaminergic input from the substantia nigra compacta has many targets in the dorsal striatum, its ability to promote movement in mice is accounted for by dopamine D1 receptor activation in direct pathway-medium spiny neurons. In contrast, conventional pharmacology does not mimic the actions of dopaminergic input, highlighting the need for the specificity of MP-D, which derives from the combination of cell specific expression of the M and ligand specificity of the P. Our results provide a template for analyzing dopamine circuits and suggests a novel way to treat dopamine-related disorders.
MCB, UC Berkeley
MP-D: cell-specific molecular gating of dopamine circuits
Dopamine circuits control diverse behaviors and their dysregulation contributes to many disorders. Our ability to understand and manipulate their function is hampered by the heterogenous nature of dopaminergic projections, the diversity of neurons that are regulated by dopamine, the varying distribution of dopamine receptors, and the complex dynamics of dopamine release. We developed a solution to these challenges in a generalizable photo-pharmacological approach called membrane-anchored photoswitchable orthogonal remotely tethered ligand (MP) that makes it possible to manipulate dopamine circuits with brain region, cell type, receptor subtype, and temporal specificity. Using a dopamine receptor MP (MP-D), we found that although dopaminergic input from the substantia nigra compacta has many targets in the dorsal striatum, its ability to promote movement in mice is accounted for by dopamine D1 receptor activation in direct pathway-medium spiny neurons. In contrast, conventional pharmacology does not mimic the actions of dopaminergic input, highlighting the need for the specificity of MP-D, which derives from the combination of cell specific expression of the M and ligand specificity of the P. Our results provide a template for analyzing dopamine circuits and suggests a novel way to treat dopamine-related disorders.

Melissa Warden
Neurobiology and Behavior, Cornell University
Ramping activity in midbrain dopamine neurons signifies the use of a cognitive map Journeys to novel and familiar destinations employ different navigational strategies. The first drive to a new restaurant relies on map-based planning, but after repeated trips the drive is automatic and guided by local environmental cues. Ventral striatal dopamine rises during navigation toward goals and reflects the spatial proximity and value of goals, but the impact of experience, the neural mechanisms, and the functional significance of dopamine ramps are unknown. Here, we used fiber photometry to record the evolution of activity in midbrain dopamine neurons as mice learned a variety of reward-seeking tasks, starting recordings before training had commenced and continuing daily for weeks. When mice navigated through space toward a goal, robust ramping activity in dopamine neurons appeared immediately – after the first rewarded trial on the first training day in completely naïve animals. In this task spatial cues were available to guide behavior, and although ramps were strong at first, they gradually faded away as training progressed. If instead mice learned to run a fixed distance on a stationary wheel for reward, a task that required an internal model of progress toward the goal, strong dopamine ramps persisted indefinitely. In a passive task in which a visible cue and reward moved together toward the mouse, ramps appeared and then faded over several days, but in an otherwise identical task with a stationary cue and reward ramps never appeared. Our findings provide strong evidence that ramping activity in midbrain dopamine neurons reflects the use of a cognitive map – an internal model of the distance already covered and the remaining distance until the goal is reached. We hypothesize that dopamine ramps may be used to reinforce locations on the way to newly-discovered rewards in order to build a graded ventral striatal value landscape for guiding routine spatial behavior.
Neurobiology and Behavior, Cornell University
Ramping activity in midbrain dopamine neurons signifies the use of a cognitive map Journeys to novel and familiar destinations employ different navigational strategies. The first drive to a new restaurant relies on map-based planning, but after repeated trips the drive is automatic and guided by local environmental cues. Ventral striatal dopamine rises during navigation toward goals and reflects the spatial proximity and value of goals, but the impact of experience, the neural mechanisms, and the functional significance of dopamine ramps are unknown. Here, we used fiber photometry to record the evolution of activity in midbrain dopamine neurons as mice learned a variety of reward-seeking tasks, starting recordings before training had commenced and continuing daily for weeks. When mice navigated through space toward a goal, robust ramping activity in dopamine neurons appeared immediately – after the first rewarded trial on the first training day in completely naïve animals. In this task spatial cues were available to guide behavior, and although ramps were strong at first, they gradually faded away as training progressed. If instead mice learned to run a fixed distance on a stationary wheel for reward, a task that required an internal model of progress toward the goal, strong dopamine ramps persisted indefinitely. In a passive task in which a visible cue and reward moved together toward the mouse, ramps appeared and then faded over several days, but in an otherwise identical task with a stationary cue and reward ramps never appeared. Our findings provide strong evidence that ramping activity in midbrain dopamine neurons reflects the use of a cognitive map – an internal model of the distance already covered and the remaining distance until the goal is reached. We hypothesize that dopamine ramps may be used to reinforce locations on the way to newly-discovered rewards in order to build a graded ventral striatal value landscape for guiding routine spatial behavior.

Stephan Lammel
MCB, UC Berkeley
A pain pathway that links the spinal cord to the midbrain dopamine system
Dopamine (DA) neurons in the ventral tegmental area (VTA) are critical in reward and motivation, but their crucial role in behavioral responses to pain is less clear. VTA DA neurons show heterogeneous responses to acute noxious stimuli. Chronic pain is common in hypodopaminergic conditions such as Parkinson’s disease, and DA dysfunction has been associated with neuropathic pain, which could be related to several of the comorbid symptoms such as reduced motivation and depression. Thus, while animal and human studies provide evidence that acute and chronic pain affect VTA DA neurons, which may modulate the affective motivational aspects of pain, the neural circuit that transmits nociceptive signals from the periphery to DA neurons is not well understood. In my presentation, I will discuss an unpublished study, in which we systematically analyzed each circuit node between the spinal cord and the VTA utilizing a combination of circuit-based optogenetics, genetic ablation, fiber photometry, in vivo and ex vivo electrophysiology, and anatomical tracing in order to reveal a previously unknown circuit, which can inhibit VTA DA neurons and their projections to the nucleus accumbens during pain. The identification of a neural circuit conveying nociceptive input to DA neurons is critical to our understanding of how pain influences learning and behavior.
MCB, UC Berkeley
A pain pathway that links the spinal cord to the midbrain dopamine system
Dopamine (DA) neurons in the ventral tegmental area (VTA) are critical in reward and motivation, but their crucial role in behavioral responses to pain is less clear. VTA DA neurons show heterogeneous responses to acute noxious stimuli. Chronic pain is common in hypodopaminergic conditions such as Parkinson’s disease, and DA dysfunction has been associated with neuropathic pain, which could be related to several of the comorbid symptoms such as reduced motivation and depression. Thus, while animal and human studies provide evidence that acute and chronic pain affect VTA DA neurons, which may modulate the affective motivational aspects of pain, the neural circuit that transmits nociceptive signals from the periphery to DA neurons is not well understood. In my presentation, I will discuss an unpublished study, in which we systematically analyzed each circuit node between the spinal cord and the VTA utilizing a combination of circuit-based optogenetics, genetic ablation, fiber photometry, in vivo and ex vivo electrophysiology, and anatomical tracing in order to reveal a previously unknown circuit, which can inhibit VTA DA neurons and their projections to the nucleus accumbens during pain. The identification of a neural circuit conveying nociceptive input to DA neurons is critical to our understanding of how pain influences learning and behavior.

David Lovinger
National Institute on Alcohol Abuse and Alcoholism
Dopamine, Dope and Sleep
Most psychoactive drugs alter sleep and long-term use and abuse of drugs can produce sleep disruption. Acute use of cannabis drugs generally promotes sleep, but long-term, heavy cannabis drug users experience sleep disruption. Indeed, sleep problems are often cited as a reason for relapse to cannabis use. The major psychoactive ingredient of cannabis drugs, delta-9-tetrahydrocannabinol (THC) is likely responsible for these effects. Given the role of dopamine (DA) in the neural effects of THC and other drugs of abuse, along with a growing literature implicating DA in the neural mechanisms of sleep, we have examined THC effects on sleep in mice using polysomnography and on striatal DA release using fast-scan cyclic voltammetry (FSCV). A chronic THC exposure regimen that produces tolerance to the drug alters sleep in a sex-dependent manner. Initial doses of THC enhanced non-rapid eye movement (NREM) sleep in both males and females. Following chronic exposure, both males and females showed tolerance to the sleep-inducing THC effects. Male mice exhibited reduced NREM sleep following this exposure, while female mice showed no such change but exhibited increased REM sleep. Examination of DA release in slices from dorsomedial and dorsolateral striatum (DMS and DLS) and nucleus accumbens (NAc), revealed region, sex and exposure-time-dependent THC effects. The largest chronic THC-induced changes were in DMS where males showed increased DA release after acute exposure on the 6th day after chronic exposure. In contrast, females showed decreased DA release in DMS after acute and on day 6 after chronic THC exposure, with similar effects in female DLS and NAc. We are currently exploring drug-induced DA changes in the different striatal subregions using dLight and in vivo fiber photometry. It will be interesting to examine how THC alters DA levels in vivo and how DA may contribute to the sex-dependent drug-induced sleep alterations.
National Institute on Alcohol Abuse and Alcoholism
Dopamine, Dope and Sleep
Most psychoactive drugs alter sleep and long-term use and abuse of drugs can produce sleep disruption. Acute use of cannabis drugs generally promotes sleep, but long-term, heavy cannabis drug users experience sleep disruption. Indeed, sleep problems are often cited as a reason for relapse to cannabis use. The major psychoactive ingredient of cannabis drugs, delta-9-tetrahydrocannabinol (THC) is likely responsible for these effects. Given the role of dopamine (DA) in the neural effects of THC and other drugs of abuse, along with a growing literature implicating DA in the neural mechanisms of sleep, we have examined THC effects on sleep in mice using polysomnography and on striatal DA release using fast-scan cyclic voltammetry (FSCV). A chronic THC exposure regimen that produces tolerance to the drug alters sleep in a sex-dependent manner. Initial doses of THC enhanced non-rapid eye movement (NREM) sleep in both males and females. Following chronic exposure, both males and females showed tolerance to the sleep-inducing THC effects. Male mice exhibited reduced NREM sleep following this exposure, while female mice showed no such change but exhibited increased REM sleep. Examination of DA release in slices from dorsomedial and dorsolateral striatum (DMS and DLS) and nucleus accumbens (NAc), revealed region, sex and exposure-time-dependent THC effects. The largest chronic THC-induced changes were in DMS where males showed increased DA release after acute exposure on the 6th day after chronic exposure. In contrast, females showed decreased DA release in DMS after acute and on day 6 after chronic THC exposure, with similar effects in female DLS and NAc. We are currently exploring drug-induced DA changes in the different striatal subregions using dLight and in vivo fiber photometry. It will be interesting to examine how THC alters DA levels in vivo and how DA may contribute to the sex-dependent drug-induced sleep alterations.

Anissa Abi-Dargham
Department of Psychiatry, Stony Brook University
Dopamine in schizophrenia: from molecules to behavior
This lecture will integrate findings from molecular and functional imaging studies in patients with schizophrenia aimed at understanding the topography of dopamine dysregulation in this disease as well as its functional impact on circuitry and behavior. We have used Positron Emission Tomography (PET) imaging of D2 radiotracers combined with pharmacological manipulations of the dopaminergic system, to examine indices of pre and post synaptic dopamine function and, in the same subjects, we used functional imaging to assess the significance of the molecular alterations. These studies have revealed topographical perturbations in presynaptic dopamine function where the associative striatum shows excess storage and release and extrastriatal regions, including cortex and midbrain, show a deficit in dopamine release. To understand the localized striatal enhancement in dopamine, in presence of low midbrain dopamine availability, we examined local striatal mechanisms that may dysregulate dopamine. One important such mechanism is the cholinergic modulation of dopamine release, via the striatal cholinergic interneurons. For this purpose, we measured the vesicular cholinergic transporter, using PET and a radiotracer specific for this target, which relates to the storage capacity for acetylcholine (ACH) in a small cohort of patients and controls. The molecular alterations described are linked to abnormal connectivity of the rostral caudate to the rest of the brain, particularly to cortical regions, and to measures of auditory perceptual bias and hallucinations. In addition, cortical dopamine deficit has implications for the role of cortical D1 receptors in modulating activity and cognitive processes. Understanding these molecular and functional abnormalities is a critical step in developing novel and targeted therapeutics for the various symptom domains of the disease. We will conclude by highlighting a new multisite trial of a D1 agonist in schizophrenia based in part on these findings and an overall model for the role of dopamine in the development and staging of the disease.
Department of Psychiatry, Stony Brook University
Dopamine in schizophrenia: from molecules to behavior
This lecture will integrate findings from molecular and functional imaging studies in patients with schizophrenia aimed at understanding the topography of dopamine dysregulation in this disease as well as its functional impact on circuitry and behavior. We have used Positron Emission Tomography (PET) imaging of D2 radiotracers combined with pharmacological manipulations of the dopaminergic system, to examine indices of pre and post synaptic dopamine function and, in the same subjects, we used functional imaging to assess the significance of the molecular alterations. These studies have revealed topographical perturbations in presynaptic dopamine function where the associative striatum shows excess storage and release and extrastriatal regions, including cortex and midbrain, show a deficit in dopamine release. To understand the localized striatal enhancement in dopamine, in presence of low midbrain dopamine availability, we examined local striatal mechanisms that may dysregulate dopamine. One important such mechanism is the cholinergic modulation of dopamine release, via the striatal cholinergic interneurons. For this purpose, we measured the vesicular cholinergic transporter, using PET and a radiotracer specific for this target, which relates to the storage capacity for acetylcholine (ACH) in a small cohort of patients and controls. The molecular alterations described are linked to abnormal connectivity of the rostral caudate to the rest of the brain, particularly to cortical regions, and to measures of auditory perceptual bias and hallucinations. In addition, cortical dopamine deficit has implications for the role of cortical D1 receptors in modulating activity and cognitive processes. Understanding these molecular and functional abnormalities is a critical step in developing novel and targeted therapeutics for the various symptom domains of the disease. We will conclude by highlighting a new multisite trial of a D1 agonist in schizophrenia based in part on these findings and an overall model for the role of dopamine in the development and staging of the disease.

Nicolas Tritsch
Neuroscience, NYU School of Medicine
State-dependent synchrony patterns striatal neuromodulation
It is widely believed that striatal circuits bathe in a steady tone of neuromodulators like acetylcholine (ACh) and dopamine (DA) and that phasic interruptions of this tone highlight stimuli that need to be attended to, acted on or learned from. However, the dynamics of the striatal neuromodulatory environment in vivo remains poorly understood, especially for ACh. I will present evidence that extracellular ACh levels in the dorsal striatum of mice fluctuate constantly, even in the absence of overt sensory stimuli. These fluctuations are driven by coordinated spiking of cholinergic interneurons and are modulated in amplitude by the degree of coherence across the population. Strikingly, ACh fluctuations maintain a specific temporal relationship to DA across behavioral states. These findings question the notion of a steady neuromodulatory tone in striatum, and point to the existence of intrinsically-structured ACh and DA oscillations that are neither rare nor uniquely associated with salient sensory stimuli.
Neuroscience, NYU School of Medicine
State-dependent synchrony patterns striatal neuromodulation
It is widely believed that striatal circuits bathe in a steady tone of neuromodulators like acetylcholine (ACh) and dopamine (DA) and that phasic interruptions of this tone highlight stimuli that need to be attended to, acted on or learned from. However, the dynamics of the striatal neuromodulatory environment in vivo remains poorly understood, especially for ACh. I will present evidence that extracellular ACh levels in the dorsal striatum of mice fluctuate constantly, even in the absence of overt sensory stimuli. These fluctuations are driven by coordinated spiking of cholinergic interneurons and are modulated in amplitude by the degree of coherence across the population. Strikingly, ACh fluctuations maintain a specific temporal relationship to DA across behavioral states. These findings question the notion of a steady neuromodulatory tone in striatum, and point to the existence of intrinsically-structured ACh and DA oscillations that are neither rare nor uniquely associated with salient sensory stimuli.

Stephanie Borgland
Department of Physiology & Pharmacology, University of Calgary
Optogenetic stimulation of lateral hypothalamic orexin inputs to the VTA active dopamine neurons in a circuit-specific manner to drive reward-seeking
Dopamine neurons in the ventral tegmental area (VTA) respond to motivationally relevant cues. Circuit-specific signaling of these neurons drives different aspects of motivated behavior. VTA dopamine neurons receive complex innervation and various neuromodulatory factors, including input from lateral hypothalamic orexin/hypocretin (LHox) neurons which also express and co-release the neuropeptide, dynorphin (LHox/dyn). While LHox promotes motivated behavior, dynorphin inhibits reward-seeking behavior and produces aversive conditioning through the Kappa opioid receptor (KOR) and decreases dopamine when administered intra-VTA. Exogenous application of orexin or orexin 1 receptor (OXR1) antagonists in the VTA bidirectionally modulates dopamine-driven motivation and reward-seeking behaviours. However, the effect of endogenous stimulation of LHox/dyn-containing projections to the VTA and the potential contribution of co-released dynorphin on mesolimbic dopamine neuronal activity, dopamine release, and reward-related processes remains uncharacterised. We combined optogenetic, electrophysiological, electrochemical, and behavioural approaches to examine this. We demonstrated a diverse response of LHox/dyn photostimulation on dopamine neuronal firing rate. Photostimulation of LHox/dyn inputs in the VTA inhibited firing of the majority of BLA projecting dopamine neurons. However, photostimulation of LHox/dyn inputs in the VTA both increased and reduced firing of dopamine neurons that project to either the lateral or medial nucleus accumbens (NAc) shell. SB334687, an OXR1 receptor inhibitor or NorBNI, a KOR inhibitor reversed the potentiation or inhibition of firing, respectively. Furthermore, we found that optical stimulation of LHox/dyn inputs in the VTA potentiates mesolimbic dopamine in the NAc, and produces real time and conditioned place preference. LHox/dyn potentiation of NAc dopamine release and real time place preference was completely blocked by an OXR1 antagonist, but not a KOR antagonist. Thus, rewarding effects associated with optical stimulation of LHox/dyn inputs in the VTA are predominantly driven by orexin rather than dynorphin.
Department of Physiology & Pharmacology, University of Calgary
Optogenetic stimulation of lateral hypothalamic orexin inputs to the VTA active dopamine neurons in a circuit-specific manner to drive reward-seeking
Dopamine neurons in the ventral tegmental area (VTA) respond to motivationally relevant cues. Circuit-specific signaling of these neurons drives different aspects of motivated behavior. VTA dopamine neurons receive complex innervation and various neuromodulatory factors, including input from lateral hypothalamic orexin/hypocretin (LHox) neurons which also express and co-release the neuropeptide, dynorphin (LHox/dyn). While LHox promotes motivated behavior, dynorphin inhibits reward-seeking behavior and produces aversive conditioning through the Kappa opioid receptor (KOR) and decreases dopamine when administered intra-VTA. Exogenous application of orexin or orexin 1 receptor (OXR1) antagonists in the VTA bidirectionally modulates dopamine-driven motivation and reward-seeking behaviours. However, the effect of endogenous stimulation of LHox/dyn-containing projections to the VTA and the potential contribution of co-released dynorphin on mesolimbic dopamine neuronal activity, dopamine release, and reward-related processes remains uncharacterised. We combined optogenetic, electrophysiological, electrochemical, and behavioural approaches to examine this. We demonstrated a diverse response of LHox/dyn photostimulation on dopamine neuronal firing rate. Photostimulation of LHox/dyn inputs in the VTA inhibited firing of the majority of BLA projecting dopamine neurons. However, photostimulation of LHox/dyn inputs in the VTA both increased and reduced firing of dopamine neurons that project to either the lateral or medial nucleus accumbens (NAc) shell. SB334687, an OXR1 receptor inhibitor or NorBNI, a KOR inhibitor reversed the potentiation or inhibition of firing, respectively. Furthermore, we found that optical stimulation of LHox/dyn inputs in the VTA potentiates mesolimbic dopamine in the NAc, and produces real time and conditioned place preference. LHox/dyn potentiation of NAc dopamine release and real time place preference was completely blocked by an OXR1 antagonist, but not a KOR antagonist. Thus, rewarding effects associated with optical stimulation of LHox/dyn inputs in the VTA are predominantly driven by orexin rather than dynorphin.

Yevgenia Kozorovitskiy
Neurobiology, Northwestern University
Dopamine system dynamics and plasticity in aversive learning
Millions of people struggle with the uncontrollable sadness of depression, with effective treatments remaining out of reach or taking weeks to exert therapeutic effects. In a major paradigmatic shift, the small molecule drug ketamine has been demonstrated to ameliorate depressive symptoms within hours in humans. Remarkably, despite extensive study over the last decade, we still lack a unified framework for understanding the mechanisms of ketamine effects on brain and behavior. Using a model of aversive learning, we examined dopamine (DA) signaling correlates of aversive learning and ketamine effects in mice. Optical readout of Ventral Tegmental Area (VTA) dopamine neuron activity was sufficient to predict behavioral state during aversive learning. A single dose of ketamine normalized blunted DA dynamics, recovering behavioral responses and rescuing cortical synaptic plasticity. We use multilaser 2-photon microscopy in order to probabilistically induce de novo growth of dendritic spines and synapses with high spatiotemporal precision on genetically targeted pyramidal neurons of the medial prefrontal cortex. Using this approach to causally interrogate structural plasticity in aversive learning, we found that ketamine potently increases the potential for spinogenesis in cortical pyramidal cells, and this effect required VTA DA signaling. Ketamine actions on both neuroplasticity and behavior are blocked by chemogenetic inhibition of DA signaling and mimicked by activating VTA DA neurons. Together, these data demonstrate a causal link between neuromodulatory systems regulating cortical function, aversive learning, and plasticity enhancements driven by a therapeutically promising drug.
Neurobiology, Northwestern University
Dopamine system dynamics and plasticity in aversive learning
Millions of people struggle with the uncontrollable sadness of depression, with effective treatments remaining out of reach or taking weeks to exert therapeutic effects. In a major paradigmatic shift, the small molecule drug ketamine has been demonstrated to ameliorate depressive symptoms within hours in humans. Remarkably, despite extensive study over the last decade, we still lack a unified framework for understanding the mechanisms of ketamine effects on brain and behavior. Using a model of aversive learning, we examined dopamine (DA) signaling correlates of aversive learning and ketamine effects in mice. Optical readout of Ventral Tegmental Area (VTA) dopamine neuron activity was sufficient to predict behavioral state during aversive learning. A single dose of ketamine normalized blunted DA dynamics, recovering behavioral responses and rescuing cortical synaptic plasticity. We use multilaser 2-photon microscopy in order to probabilistically induce de novo growth of dendritic spines and synapses with high spatiotemporal precision on genetically targeted pyramidal neurons of the medial prefrontal cortex. Using this approach to causally interrogate structural plasticity in aversive learning, we found that ketamine potently increases the potential for spinogenesis in cortical pyramidal cells, and this effect required VTA DA signaling. Ketamine actions on both neuroplasticity and behavior are blocked by chemogenetic inhibition of DA signaling and mimicked by activating VTA DA neurons. Together, these data demonstrate a causal link between neuromodulatory systems regulating cortical function, aversive learning, and plasticity enhancements driven by a therapeutically promising drug.

Anne Collins
UC Berkeley, Department of Psychology and Helen Wills Neuroscience Institute
One-shot intrinsic reward valuation in humans
Humans continuously need learn to make good choices in our environment – be it using a new video-conferencing set up, or selecting which location in a house is least likely to be interrupted by toddlers during work calls. However, the goals we seek to attain – such as using zoom successfully – are often vaguely defined and previously unexperienced, and in that sense cannot be known by us as being rewarding. How does the brain enable us to immediately encode such one-shot goals as having value as intrinsic reinforcers? We hypothesized that learning to make good choices in such situations leverages classic, dopaminergic reinforcement learning processes, and that executive functions in general, and working memory in particular, play a crucial role in defining the intrinsic reward function for arbitrary, novel outcomes, in such a way that they become reinforcing. I will show results from a novel behavioral protocol, as well as computational and imaging evidence supporting our hypothesis.
UC Berkeley, Department of Psychology and Helen Wills Neuroscience Institute
One-shot intrinsic reward valuation in humans
Humans continuously need learn to make good choices in our environment – be it using a new video-conferencing set up, or selecting which location in a house is least likely to be interrupted by toddlers during work calls. However, the goals we seek to attain – such as using zoom successfully – are often vaguely defined and previously unexperienced, and in that sense cannot be known by us as being rewarding. How does the brain enable us to immediately encode such one-shot goals as having value as intrinsic reinforcers? We hypothesized that learning to make good choices in such situations leverages classic, dopaminergic reinforcement learning processes, and that executive functions in general, and working memory in particular, play a crucial role in defining the intrinsic reward function for arbitrary, novel outcomes, in such a way that they become reinforcing. I will show results from a novel behavioral protocol, as well as computational and imaging evidence supporting our hypothesis.

Mary Kay Lobo
Anatomy and Neurobiology, University of Maryland School of Medicine
Ventral pallidum transcriptome adaptations after cocaine self-administration
The ventral pallidum (VP) is critical for drug intake and seeking behavior including psychostimulants, which enhance dopamine transmission. Repeated exposure to the psychostimulant cocaine alters VP neuronal firing and neurotransmission but there is limited information on the molecular adaptations occurring in VP neurons following cocaine intake. To provide insight into this we performed RNA-seq on VP of mice that underwent cocaine self-administration followed by twenty-four hours of abstinence. We observed differential gene expression in 363 genes between animals that self-administered cocaine vs saline. Gene Ontology analysis uncovered alterations in synaptic and structural plasticity related genes. We then identified a common upstream regulator of these sets of genes, the transcription factor Nr4a1. Nr4a1 was increased in VP, specifically in VP neurons that project to mediodorsal thalamus (MDT), after cocaine self-administration. Consistent with this data, overexpression of Nr4a1 in the VP-MDT neurons enhanced drug seeking behavior after cocaine self-administration. In contrast CRISPR mediated knockdown of Nr4a1 reduces cocaine intake and drug seeking behavior. Altogether, our work provides new information into the molecular adaptations occurring in VP neurons in cocaine self-administration and relapse-like behavior.
Anatomy and Neurobiology, University of Maryland School of Medicine
Ventral pallidum transcriptome adaptations after cocaine self-administration
The ventral pallidum (VP) is critical for drug intake and seeking behavior including psychostimulants, which enhance dopamine transmission. Repeated exposure to the psychostimulant cocaine alters VP neuronal firing and neurotransmission but there is limited information on the molecular adaptations occurring in VP neurons following cocaine intake. To provide insight into this we performed RNA-seq on VP of mice that underwent cocaine self-administration followed by twenty-four hours of abstinence. We observed differential gene expression in 363 genes between animals that self-administered cocaine vs saline. Gene Ontology analysis uncovered alterations in synaptic and structural plasticity related genes. We then identified a common upstream regulator of these sets of genes, the transcription factor Nr4a1. Nr4a1 was increased in VP, specifically in VP neurons that project to mediodorsal thalamus (MDT), after cocaine self-administration. Consistent with this data, overexpression of Nr4a1 in the VP-MDT neurons enhanced drug seeking behavior after cocaine self-administration. In contrast CRISPR mediated knockdown of Nr4a1 reduces cocaine intake and drug seeking behavior. Altogether, our work provides new information into the molecular adaptations occurring in VP neurons in cocaine self-administration and relapse-like behavior.

Bryan Roth
Pharmacology, UNC Chapel Hill Medical School
New insights into dopamine function at the molecular level
In this talk I will present new information generated from high resolution structures of multiple dopamine receptors and show how this information guides the discovery of potent and selective dopaminergic drugs.
Pharmacology, UNC Chapel Hill Medical School
New insights into dopamine function at the molecular level
In this talk I will present new information generated from high resolution structures of multiple dopamine receptors and show how this information guides the discovery of potent and selective dopaminergic drugs.

Aryn Gittis
Biological Sciences and the Neuroscience Institute, Carnegie Mellon University
Cell-Based Strategies To Promote Long-Lasting Motor Recovery Following Dopamine Depletion
Identification of distinct neuronal subpopulations has been essential for understanding brain function, but clinical applications struggle to access specific neurons in heterogeneously mingled populations. Recently, optogenetic protocols targeting neuronal subpopulations in the external globus pallidus (GPe) were shown to provide long-lasting therapeutic effects in dopamine depleted mice. Here, we leverage underlying synaptic differences between Parvalbumin (PV) and Lim homeobox 6 (Lhx6) subpopulations to drive population-specific neuromodulation in the GPe, using brief bursts of electrical stimulation. We then apply these findings to strategically design a clinically appropriate deep brain stimulation (DBS) protocol, which we show induces long-lasting therapeutic effects that far exceed those of conventional DBS, extending for hours beyond stimulation. These results establish the feasibility of transforming knowledge about circuit architecture into quickly translatable therapeutic approaches.
Biological Sciences and the Neuroscience Institute, Carnegie Mellon University
Cell-Based Strategies To Promote Long-Lasting Motor Recovery Following Dopamine Depletion
Identification of distinct neuronal subpopulations has been essential for understanding brain function, but clinical applications struggle to access specific neurons in heterogeneously mingled populations. Recently, optogenetic protocols targeting neuronal subpopulations in the external globus pallidus (GPe) were shown to provide long-lasting therapeutic effects in dopamine depleted mice. Here, we leverage underlying synaptic differences between Parvalbumin (PV) and Lim homeobox 6 (Lhx6) subpopulations to drive population-specific neuromodulation in the GPe, using brief bursts of electrical stimulation. We then apply these findings to strategically design a clinically appropriate deep brain stimulation (DBS) protocol, which we show induces long-lasting therapeutic effects that far exceed those of conventional DBS, extending for hours beyond stimulation. These results establish the feasibility of transforming knowledge about circuit architecture into quickly translatable therapeutic approaches.

Garret Stuber
Center for the Neurobiology of Addiction, Pain, and Emotion, Department of Anesthesiology and Pain Medicine, Department of Pharmacology, University of Washington, Seattle, WA
Molecular and Functional Phenotyping of Habenula Circuitry
The habenula complex is appreciated as a critical regulator of motivated and pathological behavioral states via its output to midbrain nuclei. Despite this, transcriptional definition of cell populations that comprise both the medial (MHb) and lateral habenular (LHb) subregions in mammals remain undefined. To resolve this, we performed single-cell transcriptional profiling and highly multiplexed in situ hybridization experiments of the mouse habenula complex in naïve mice and those exposed to an acute aversive stimulus. Transcriptionally distinct neuronal cell types identified within the MHb and LHb, were spatially defined, differentially engaged by aversive stimuli and had distinct electrophysiological properties. Cell types identified in mice, also displayed a high degree of transcriptional similarity to those previously described in zebrafish, highlighting the well conserved nature of habenular cell types across the phylum. These data identify key molecular targets within habenular cell types, and provide a critical resource for future studies.
Center for the Neurobiology of Addiction, Pain, and Emotion, Department of Anesthesiology and Pain Medicine, Department of Pharmacology, University of Washington, Seattle, WA
Molecular and Functional Phenotyping of Habenula Circuitry
The habenula complex is appreciated as a critical regulator of motivated and pathological behavioral states via its output to midbrain nuclei. Despite this, transcriptional definition of cell populations that comprise both the medial (MHb) and lateral habenular (LHb) subregions in mammals remain undefined. To resolve this, we performed single-cell transcriptional profiling and highly multiplexed in situ hybridization experiments of the mouse habenula complex in naïve mice and those exposed to an acute aversive stimulus. Transcriptionally distinct neuronal cell types identified within the MHb and LHb, were spatially defined, differentially engaged by aversive stimuli and had distinct electrophysiological properties. Cell types identified in mice, also displayed a high degree of transcriptional similarity to those previously described in zebrafish, highlighting the well conserved nature of habenular cell types across the phylum. These data identify key molecular targets within habenular cell types, and provide a critical resource for future studies.

Matt Botvinick
Director of Neuroscience and Team Lead in AGI Research, DeepMind ; Honorary Professor, Gatsby Computational Neuroscience Unit, University College London
Deep reinforcement learning and its neuroscientific implications
The last few years have seen some dramatic developments in artificial intelligence research. What implications might these have for neuroscience? Investigations of this question have, to date, focused largely on deep neural networks trained using supervised learning, in tasks such as image classification. In this talk, I'll discuss another area of recent AI work which has so far received less attention from neuroscientists, but which may have more profound implications: Deep reinforcement learning. Deep RL provides a rich framework for studying the interplay among learning, representation and decision-making, offering to the brain sciences a new set of research tools and a wide range of novel hypotheses. I'll provide a high level introduction to deep RL and survey some of its key implications for research on the brain and behavior, with a particular focus on potential implications for understanding dopaminergic function.
Director of Neuroscience and Team Lead in AGI Research, DeepMind ; Honorary Professor, Gatsby Computational Neuroscience Unit, University College London
Deep reinforcement learning and its neuroscientific implications
The last few years have seen some dramatic developments in artificial intelligence research. What implications might these have for neuroscience? Investigations of this question have, to date, focused largely on deep neural networks trained using supervised learning, in tasks such as image classification. In this talk, I'll discuss another area of recent AI work which has so far received less attention from neuroscientists, but which may have more profound implications: Deep reinforcement learning. Deep RL provides a rich framework for studying the interplay among learning, representation and decision-making, offering to the brain sciences a new set of research tools and a wide range of novel hypotheses. I'll provide a high level introduction to deep RL and survey some of its key implications for research on the brain and behavior, with a particular focus on potential implications for understanding dopaminergic function.

Jochen Roeper
Neurophysiology Goethe University Frankfurt
Pacemaker mechanism and plasticity of DA SN neurons
The mechanisms of pacemaking in dopamine neurons in the substantia nigra (DA SN) are still not clear and particular the role of L-type channels are both controversial and potentially relevant in the context of neuroprotection in Parkinson Disease. Using a mouse model where only Cav1.3 channels are sensitive to the dihydropyridine isradipine we defined the functional role of this low-threshold L-type channel to autonomous pacing in vitro. Dynamic clamp in vitro experiments revealed how Cav1.3 channels also boost high-frequency discharge in the in vivo burst frequnency range of lateral SN DA neurons. Clinically relevant concentrations of isradipine in the low nanomolar range are tested in this setting and compared to systemic effects of isradipine on in vivo firing properties of identified DA SN neurons. Our results demonstated how lateral SN DA neurons can be targeted selectively in vivo. Finally, we identified the acceleration of pacemaker function in DA SN neurons surviving in a partial 6-OHDA lesion model via downregulation of Kv4.3 channels. We demonstrate that DA SN pacing can be targeted in vivo by dihydropyridines and also adapts in surviving neurons after a lesion.
Neurophysiology Goethe University Frankfurt
Pacemaker mechanism and plasticity of DA SN neurons
The mechanisms of pacemaking in dopamine neurons in the substantia nigra (DA SN) are still not clear and particular the role of L-type channels are both controversial and potentially relevant in the context of neuroprotection in Parkinson Disease. Using a mouse model where only Cav1.3 channels are sensitive to the dihydropyridine isradipine we defined the functional role of this low-threshold L-type channel to autonomous pacing in vitro. Dynamic clamp in vitro experiments revealed how Cav1.3 channels also boost high-frequency discharge in the in vivo burst frequnency range of lateral SN DA neurons. Clinically relevant concentrations of isradipine in the low nanomolar range are tested in this setting and compared to systemic effects of isradipine on in vivo firing properties of identified DA SN neurons. Our results demonstated how lateral SN DA neurons can be targeted selectively in vivo. Finally, we identified the acceleration of pacemaker function in DA SN neurons surviving in a partial 6-OHDA lesion model via downregulation of Kv4.3 channels. We demonstrate that DA SN pacing can be targeted in vivo by dihydropyridines and also adapts in surviving neurons after a lesion.

Mark Walton
Department of Experimental Psychology, University of Oxford
What you see is not always what you get: Asymmetric reporting and updating of value by dopamine during inference-guided choice
Accurately estimating future reward associated with different choices is critical to adaptive decision making. Reinforcement learning has provided an influential framework to explain how this might be implemented, with dopamine playing a central role in these accounts. However, standard reinforcement learning is not necessarily the most efficient way of updating preferences in dynamic environments, particularly where environmental change is itself structured and hence predictable given sufficient experience; for example, the best route to work may vary reliably depending on the time of day or day of the week. Here, I’ll describe a series of experiments where we measured and manipulated dopamine in mice performing a multi-step decision making task to better understand the role dopamine plays in shaping behaviour in structured dynamic environments. I’ll show evidence that, while dopamine displays rich signatures of task-related variables in different striatal regions, these may play have a surprisingly limited influence over trial-by-trial choice.
Department of Experimental Psychology, University of Oxford
What you see is not always what you get: Asymmetric reporting and updating of value by dopamine during inference-guided choice
Accurately estimating future reward associated with different choices is critical to adaptive decision making. Reinforcement learning has provided an influential framework to explain how this might be implemented, with dopamine playing a central role in these accounts. However, standard reinforcement learning is not necessarily the most efficient way of updating preferences in dynamic environments, particularly where environmental change is itself structured and hence predictable given sufficient experience; for example, the best route to work may vary reliably depending on the time of day or day of the week. Here, I’ll describe a series of experiments where we measured and manipulated dopamine in mice performing a multi-step decision making task to better understand the role dopamine plays in shaping behaviour in structured dynamic environments. I’ll show evidence that, while dopamine displays rich signatures of task-related variables in different striatal regions, these may play have a surprisingly limited influence over trial-by-trial choice.

Marisela Morales
National Institute of Drug Abuse, NIH
Diverse control of Dorsal Raphe over dopamine neurons in the Ventral Tegmental Area
Alterations in serotonergic function and reward-related processing have been hypothesized in schizophrenia, depression, and drug abuse. We will discuss the ultrastructural and molecular characteristics of the synaptic connectivity between Dorsal Raphe (DR) serotonin and dopamine neurons in the Ventral Tegmental Area. We will present evidence for two types of serotonin axon terminals establishing specific type of synapses on VTA dopamine neurons, and proposed a model by which different types of DR neurons regulate the activity of VTA dopamine neurons. We will also present behavioral data on the participation of DR-VTA pathways in cocaine- seeking behavior.
National Institute of Drug Abuse, NIH
Diverse control of Dorsal Raphe over dopamine neurons in the Ventral Tegmental Area
Alterations in serotonergic function and reward-related processing have been hypothesized in schizophrenia, depression, and drug abuse. We will discuss the ultrastructural and molecular characteristics of the synaptic connectivity between Dorsal Raphe (DR) serotonin and dopamine neurons in the Ventral Tegmental Area. We will present evidence for two types of serotonin axon terminals establishing specific type of synapses on VTA dopamine neurons, and proposed a model by which different types of DR neurons regulate the activity of VTA dopamine neurons. We will also present behavioral data on the participation of DR-VTA pathways in cocaine- seeking behavior.

Ben Hobson
Columbia University
Subcellular mRNA Localization in Dopamine Neurons
Subcellular localization and translation of mRNA is involved in neuronal development and synaptic plasticity, but most studies of local translation have focused on excitatory and inhibitory neurons. Dopamine (DA) neurons produce extensive, unmyelinated axons that travel via the medial forebrain bundle to innervate basal ganglia and cortical targets. In addition to axonal release in the forebrain, DA neurons also exhibit somatodendritic DA release in the midbrain. The molecular mechanisms by which DA neurons regulate neurotransmission across expansive subcellular compartments remain unclear. Therefore, detailed studies of subcellular translation in DA neurons may shed light on molecular mechanisms of dopaminergic function in health and disease. Given the dynamic regulation of DA release across an elaborate cytoarchitecture, we hypothesized that DA neurons might employ local protein synthesis to enable rapid changes in the dendritic and axonal proteome. Here, we employed a combination of subcellular fractionation, synaptosome sorting, fluorescence in situ hybridization, and DA neuron-specific ribosome-bound RNA-sequencing to investigate local protein synthesis in midbrain DA neurons. We identified localization of mRNAs encoding DA transmission proteins in dendrites, but not axons of DA neurons. We show that ribosomes and dopaminergic mRNAs are prominently localized within dendrites of the substantia nigra pars reticulata. Finally, ribosome-bound RNA sequencing revealed dendritic translating mRNAs encoding proteins involved in membrane fusion. Although we employed highly-sensitive sequencing and imaging approaches, we found very limited evidence of axonal mRNA localization and translation in DA neurons. These results demonstrate that DA neurons employ local translation primarily in dendrites, where it may regulate dendritic DA release and reuptake.
Columbia University
Subcellular mRNA Localization in Dopamine Neurons
Subcellular localization and translation of mRNA is involved in neuronal development and synaptic plasticity, but most studies of local translation have focused on excitatory and inhibitory neurons. Dopamine (DA) neurons produce extensive, unmyelinated axons that travel via the medial forebrain bundle to innervate basal ganglia and cortical targets. In addition to axonal release in the forebrain, DA neurons also exhibit somatodendritic DA release in the midbrain. The molecular mechanisms by which DA neurons regulate neurotransmission across expansive subcellular compartments remain unclear. Therefore, detailed studies of subcellular translation in DA neurons may shed light on molecular mechanisms of dopaminergic function in health and disease. Given the dynamic regulation of DA release across an elaborate cytoarchitecture, we hypothesized that DA neurons might employ local protein synthesis to enable rapid changes in the dendritic and axonal proteome. Here, we employed a combination of subcellular fractionation, synaptosome sorting, fluorescence in situ hybridization, and DA neuron-specific ribosome-bound RNA-sequencing to investigate local protein synthesis in midbrain DA neurons. We identified localization of mRNAs encoding DA transmission proteins in dendrites, but not axons of DA neurons. We show that ribosomes and dopaminergic mRNAs are prominently localized within dendrites of the substantia nigra pars reticulata. Finally, ribosome-bound RNA sequencing revealed dendritic translating mRNAs encoding proteins involved in membrane fusion. Although we employed highly-sensitive sequencing and imaging approaches, we found very limited evidence of axonal mRNA localization and translation in DA neurons. These results demonstrate that DA neurons employ local translation primarily in dendrites, where it may regulate dendritic DA release and reuptake.

Robert Philips
University of Alabama at Birmingham
An atlas of transcriptionally defined cell populations in the rat ventral tegmental area
The ventral tegmental area (VTA) is a complex brain region that is essential for reward function but is also implicated in neuropsychiatric diseases including substance abuse. While decades of research on VTA function have focused on the role of dopaminergic neurons, recent evidence has identified critical roles for VTA GABAergic and glutamatergic neurons in reward processes as well. Interestingly, molecular characterization has revealed that subsets of these neurons express genes involved in the transport, synthesis, and vesicular packaging of multiple neurotransmitters, providing evidence for the presence of co-release neurons. However, these studies have largely relied on low-throughput methods, and the molecular architecture of the VTA has not been comprehensively examined. Here, we performed single nucleus RNA-sequencing (snRNA-seq) on 21,600 VTA cells from both male and female Sprague-Dawley rats to generate a transcriptional atlas of the rat VTA. We identified 16 transcriptionally distinct cell types within the VTA, including 7 dissociable neuronal populations. Further subclustering revealed several VTA neuronal populations expressing markers for more than one neurotransmitter system, with one cluster exhibiting high expression levels of genes involved in the synthesis and transport of GABA, glutamate, and dopamine. Finally, snRNA-seq enabled the de novo identification of thousands of marker genes for each transcriptionally distinct population, revealing cluster-specific enrichment of gene-sets implicated in neuropsychiatric and neurodevelopmental disorders, as well as specific phenotypes associated with alcohol and tobacco use. Together, these results highlight the heterogeneity of cellular populations in the VTA and identify novel markers and disease-linked genes enriched in distinct neuronal subtypes.
University of Alabama at Birmingham
An atlas of transcriptionally defined cell populations in the rat ventral tegmental area
The ventral tegmental area (VTA) is a complex brain region that is essential for reward function but is also implicated in neuropsychiatric diseases including substance abuse. While decades of research on VTA function have focused on the role of dopaminergic neurons, recent evidence has identified critical roles for VTA GABAergic and glutamatergic neurons in reward processes as well. Interestingly, molecular characterization has revealed that subsets of these neurons express genes involved in the transport, synthesis, and vesicular packaging of multiple neurotransmitters, providing evidence for the presence of co-release neurons. However, these studies have largely relied on low-throughput methods, and the molecular architecture of the VTA has not been comprehensively examined. Here, we performed single nucleus RNA-sequencing (snRNA-seq) on 21,600 VTA cells from both male and female Sprague-Dawley rats to generate a transcriptional atlas of the rat VTA. We identified 16 transcriptionally distinct cell types within the VTA, including 7 dissociable neuronal populations. Further subclustering revealed several VTA neuronal populations expressing markers for more than one neurotransmitter system, with one cluster exhibiting high expression levels of genes involved in the synthesis and transport of GABA, glutamate, and dopamine. Finally, snRNA-seq enabled the de novo identification of thousands of marker genes for each transcriptionally distinct population, revealing cluster-specific enrichment of gene-sets implicated in neuropsychiatric and neurodevelopmental disorders, as well as specific phenotypes associated with alcohol and tobacco use. Together, these results highlight the heterogeneity of cellular populations in the VTA and identify novel markers and disease-linked genes enriched in distinct neuronal subtypes.

Paul Kramer
NINDS
Axonal membrane potential dynamics during spontaneous and evoked cholinergic transmission onto striatal dopaminergic terminals
Axons of dopamine neurons are locally modulated by striatal cholinergic interneurons through presynaptic nicotinic receptors (nAChRs). However, the nature of cholinergic transmission is unknown. The prevailing view based on electron microscopy is that cholinergic neurons signal largely through non-specific, slow volume transmission. Functional studies examining the regulation of striatal dopamine release show that synchronous activation of cholinergic interneurons can trigger dopamine release through activation of axonal nAChRs alone. Yet, the indirect nature of the methods used so far have been insufficient to allow precise examination of this axo-axonal transmission. To test this, therefore, we obtained patch-clamp recordings of the axonal membrane potential to precisely examine spontaneous and evoked cholinergic transmission onto striatal dopaminergic terminals. Our axonal recordings reveal the presence of clear phasic depolarizations that occur spontaneously at rates of 1-2 events per second, and which were abolished by nicotinic receptor antagonists. The cholinergic-evoked depolarizations had average rise times slightly less than 20 ms indicating the likely presence of a non-classical synapse. Tetrodotoxin-block of spike evoked transmission resulted in a 10-fold reduction in the frequency of spontaneous EPSPs but had no effect on amplitude, confirming that these events represent true miniature events resulting from quantal release. Paired-pulse stimulation of cholinergic interneurons with light-evoked opsins or electrical stimulation showed the marked synaptic depression. Importantly, we also show that the kinetics of nicotinic EPSPs are shaped by acetylcholinesterase, which limits the activation of nicotinic receptors distal to the site of ACh release. Thus, we provide the first comprehensive examination of membrane dynamics during axo-axonic synaptic transmission onto the dopaminergic axons. Although the large and extensively branching axonal arbors of dopamine neurons have led to the idea of global signaling, our data showing synaptic-like depolarizations suggests instead that local signaling within the arbor may also contribute to striatal subregion circuit dynamics.
NINDS
Axonal membrane potential dynamics during spontaneous and evoked cholinergic transmission onto striatal dopaminergic terminals
Axons of dopamine neurons are locally modulated by striatal cholinergic interneurons through presynaptic nicotinic receptors (nAChRs). However, the nature of cholinergic transmission is unknown. The prevailing view based on electron microscopy is that cholinergic neurons signal largely through non-specific, slow volume transmission. Functional studies examining the regulation of striatal dopamine release show that synchronous activation of cholinergic interneurons can trigger dopamine release through activation of axonal nAChRs alone. Yet, the indirect nature of the methods used so far have been insufficient to allow precise examination of this axo-axonal transmission. To test this, therefore, we obtained patch-clamp recordings of the axonal membrane potential to precisely examine spontaneous and evoked cholinergic transmission onto striatal dopaminergic terminals. Our axonal recordings reveal the presence of clear phasic depolarizations that occur spontaneously at rates of 1-2 events per second, and which were abolished by nicotinic receptor antagonists. The cholinergic-evoked depolarizations had average rise times slightly less than 20 ms indicating the likely presence of a non-classical synapse. Tetrodotoxin-block of spike evoked transmission resulted in a 10-fold reduction in the frequency of spontaneous EPSPs but had no effect on amplitude, confirming that these events represent true miniature events resulting from quantal release. Paired-pulse stimulation of cholinergic interneurons with light-evoked opsins or electrical stimulation showed the marked synaptic depression. Importantly, we also show that the kinetics of nicotinic EPSPs are shaped by acetylcholinesterase, which limits the activation of nicotinic receptors distal to the site of ACh release. Thus, we provide the first comprehensive examination of membrane dynamics during axo-axonic synaptic transmission onto the dopaminergic axons. Although the large and extensively branching axonal arbors of dopamine neurons have led to the idea of global signaling, our data showing synaptic-like depolarizations suggests instead that local signaling within the arbor may also contribute to striatal subregion circuit dynamics.

Changliang Liu
Harvard Medical School
An ectopic firing mechanism for broadcasting dopamine signaling
Neurotransmitter release in the brain is chiefly determined by action potentials initiated at the axonal initiation segment (AIS). Ectopic action potentials arise at locations other than the AIS and are usually associated with pathological conditions or network activities. Dopamine neurons reside in the midbrain and innervate the striatum. It is well established that stimulation of striatal cholinergic interneurons can induce dopamine release through nicotinic receptor (nAChR) activation on dopamine axons. However, it remains unknown what happens after nAChR activation and whether such a process represents a bona fide physiological regulation employed by the dopamine system. Here, we show that spontaneous activity of cholinergic interneurons not only induces local dopamine release but also broadcasts dopamine signaling by generating ectopic action potentials in the widely arborized dopamine axons. We further demonstrate that this regulation contributes to movement initiation and dopamine responses evoked by salient stimuli in freely moving animals. Our findings establish that striatal cholinergic innervation serves to initiate ectopic firing in dopamine axons, and that ectopic axonal firing is a new mechanism for controlling dopamine release independent of somatic action potentials.
Harvard Medical School
An ectopic firing mechanism for broadcasting dopamine signaling
Neurotransmitter release in the brain is chiefly determined by action potentials initiated at the axonal initiation segment (AIS). Ectopic action potentials arise at locations other than the AIS and are usually associated with pathological conditions or network activities. Dopamine neurons reside in the midbrain and innervate the striatum. It is well established that stimulation of striatal cholinergic interneurons can induce dopamine release through nicotinic receptor (nAChR) activation on dopamine axons. However, it remains unknown what happens after nAChR activation and whether such a process represents a bona fide physiological regulation employed by the dopamine system. Here, we show that spontaneous activity of cholinergic interneurons not only induces local dopamine release but also broadcasts dopamine signaling by generating ectopic action potentials in the widely arborized dopamine axons. We further demonstrate that this regulation contributes to movement initiation and dopamine responses evoked by salient stimuli in freely moving animals. Our findings establish that striatal cholinergic innervation serves to initiate ectopic firing in dopamine axons, and that ectopic axonal firing is a new mechanism for controlling dopamine release independent of somatic action potentials.

Yvette Fisher
University of California, Berkeley
Dopaminergic modulation of plasticity in the Drosophila compass network
Dopamine can promote associative synaptic plasticity. When dopamine drive tracks reward-prediction error this mechanism is hypothesized to enable reinforcement learning. However, not all dopamine signals encode unexpected rewards—many dopamine neurons are time-locked to locomotion or motor events. A general framework for how motor-locked dopamine signals shape learning is less established. To explore this topic, we investigated the hypothesis that dopamine release makes learning in the Drosophila compass system contingent on locomotor events. Within the Drosophila central complex, ‘compass’ neurons (E-PGs) form a ring attractor network which integrates rotational movements over time to generate an internal estimate of heading direction. We and others have recently found evidence that visual inputs to the compass are plastic. Associative synaptic plasticity is thought to reinforce connectivity patterns to form a stable spatial map in familiar environments and to rapidly update when surroundings change. Intriguingly, axons of dopaminergic ExR2 neurons wrap around this region and position themselves to modulate this synaptic plasticity. Using in vivo multiphoton calcium imaging from the brains of tethered walking flies, we demonstrate that the activity of ExR2 dopamine neurons is mainly locked to body rotations, and their activity scales with rotational speed. Next, using chemo-genetic activation of ExR2 neurons during electrophysiology or calcium imaging from the compass network, we show that dopamine drive can trigger a remapping of the visual world onto compass coordinates. Our data suggests that dopamine promotes associative visual plasticity. We propose that during navigation, motor-locked dopamine signals tune the learning rate between sensory inputs and compass representation. These results suggest a new explanation for why dopamine release should be linked to movement speed: the brain’s learning rate should be linked to the rate at which the relevant external stimuli are sampled during exploration.
University of California, Berkeley
Dopaminergic modulation of plasticity in the Drosophila compass network
Dopamine can promote associative synaptic plasticity. When dopamine drive tracks reward-prediction error this mechanism is hypothesized to enable reinforcement learning. However, not all dopamine signals encode unexpected rewards—many dopamine neurons are time-locked to locomotion or motor events. A general framework for how motor-locked dopamine signals shape learning is less established. To explore this topic, we investigated the hypothesis that dopamine release makes learning in the Drosophila compass system contingent on locomotor events. Within the Drosophila central complex, ‘compass’ neurons (E-PGs) form a ring attractor network which integrates rotational movements over time to generate an internal estimate of heading direction. We and others have recently found evidence that visual inputs to the compass are plastic. Associative synaptic plasticity is thought to reinforce connectivity patterns to form a stable spatial map in familiar environments and to rapidly update when surroundings change. Intriguingly, axons of dopaminergic ExR2 neurons wrap around this region and position themselves to modulate this synaptic plasticity. Using in vivo multiphoton calcium imaging from the brains of tethered walking flies, we demonstrate that the activity of ExR2 dopamine neurons is mainly locked to body rotations, and their activity scales with rotational speed. Next, using chemo-genetic activation of ExR2 neurons during electrophysiology or calcium imaging from the compass network, we show that dopamine drive can trigger a remapping of the visual world onto compass coordinates. Our data suggests that dopamine promotes associative visual plasticity. We propose that during navigation, motor-locked dopamine signals tune the learning rate between sensory inputs and compass representation. These results suggest a new explanation for why dopamine release should be linked to movement speed: the brain’s learning rate should be linked to the rate at which the relevant external stimuli are sampled during exploration.

Alexey Ostroumov
Georgetown University
Enhanced excitability of VTA GABA circuitry facilitates phasic dopamine signaling and reward-related learning
It is widely assumed that ventral tegmental area (VTA) GABA neurons suppress dopamine signaling and reward-related behaviors. However, some evidence suggests that increased GABAergic input onto DA cells correlates with potentiated behavioral responses to rewards. To address this controversy, my laboratory examines how VTA GABA neurons can shape dopamine signaling and acquisition of reward-related behaviors. First, we describe a novel form of experience-dependent inhibitory synaptic plasticity that enhances excitability of VTA GABA circuitry. Specifically, stress and addictive drugs alter synaptic inhibition of VTA GABA neurons via downregulation of KCC2, a chloride transporter that maintains low intracellular chloride concentrations in neurons. Because low intracellular chloride underlies GABAA receptor-mediated inhibition, KCC2 dysfunction reduces synaptic inhibition and can even cause paradoxical GABAergic excitation of VTA GABA neurons. To investigate the role of KCC2 dysfunction at the circuit level, we show that optogenetic excitation of VTA GABA neurons unexpectedly potentiates phasic bursting in dopamine neurons despite attenuating tonic firing. Accordingly, our behavioral studies indicate that both stress and addictive drugs facilitate the learning of cue-reward associations and that this facilitation depends on KCC2 function in the VTA. In summary, our findings suggest a novel mechanism by which enhanced activity of VTA GABA neurons can counterintuitively facilitate dopamine-dependent learning.
Georgetown University
Enhanced excitability of VTA GABA circuitry facilitates phasic dopamine signaling and reward-related learning
It is widely assumed that ventral tegmental area (VTA) GABA neurons suppress dopamine signaling and reward-related behaviors. However, some evidence suggests that increased GABAergic input onto DA cells correlates with potentiated behavioral responses to rewards. To address this controversy, my laboratory examines how VTA GABA neurons can shape dopamine signaling and acquisition of reward-related behaviors. First, we describe a novel form of experience-dependent inhibitory synaptic plasticity that enhances excitability of VTA GABA circuitry. Specifically, stress and addictive drugs alter synaptic inhibition of VTA GABA neurons via downregulation of KCC2, a chloride transporter that maintains low intracellular chloride concentrations in neurons. Because low intracellular chloride underlies GABAA receptor-mediated inhibition, KCC2 dysfunction reduces synaptic inhibition and can even cause paradoxical GABAergic excitation of VTA GABA neurons. To investigate the role of KCC2 dysfunction at the circuit level, we show that optogenetic excitation of VTA GABA neurons unexpectedly potentiates phasic bursting in dopamine neurons despite attenuating tonic firing. Accordingly, our behavioral studies indicate that both stress and addictive drugs facilitate the learning of cue-reward associations and that this facilitation depends on KCC2 function in the VTA. In summary, our findings suggest a novel mechanism by which enhanced activity of VTA GABA neurons can counterintuitively facilitate dopamine-dependent learning.

Gabriela Izowit
Jagiellonian University
Brain state dependent responses of midbrain dopaminergic neurons’ to the aversive stimulus
For a long time, it has been assumed that dopaminergic (DA) neurons of the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) respond to reward and aversive stimuli homogenously across the entire population with either increase or decrease of their activity respectively – coding in this manner information about the value of perceived stimuli. This notion was questioned after the identification of midbrain DA neurons that are excited by both rewarding and aversive stimuli. This resulted in division of midbrain DA neurons into two functionally distinct subpopulations: one encoding the value and the other encoding the salience of the stimuli. Additionally, it has been shown that the general state of the brain modulates the electrical activity of midbrain DA neurons, but it remains unknown whether this factor may also influence signalling of value and salience. Our experiments were aimed at answering this question. For this purpose, we recorded responses of VTA and SNc DA neurons to electrical footshocks across alternating brain states of urethane anaesthetized rats. We identified DA neurons based on electrophysiological criteria combined with the use of either juxtacellular recording-labelling technique or optotagging. Besides the previously described populations of value- and salience-coding neurons, we also observed unidentified so far subpopulation of VTA and SNc DA neurons, that changes its type of response to an aversive stimulus depending on the ongoing brain state. Majority of these neurons were inhibited by footshocks during a REM-like brain state, but with the appearance of nREM-like brain state, they changed their type of response to excitation. Based on our observations, it can be hypothesised that there is a subpopulation of DA neurons that are involved in ‘dual-coding’ of both the value and the salience of the stimulus depending on the general state of the brain.
Jagiellonian University
Brain state dependent responses of midbrain dopaminergic neurons’ to the aversive stimulus
For a long time, it has been assumed that dopaminergic (DA) neurons of the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) respond to reward and aversive stimuli homogenously across the entire population with either increase or decrease of their activity respectively – coding in this manner information about the value of perceived stimuli. This notion was questioned after the identification of midbrain DA neurons that are excited by both rewarding and aversive stimuli. This resulted in division of midbrain DA neurons into two functionally distinct subpopulations: one encoding the value and the other encoding the salience of the stimuli. Additionally, it has been shown that the general state of the brain modulates the electrical activity of midbrain DA neurons, but it remains unknown whether this factor may also influence signalling of value and salience. Our experiments were aimed at answering this question. For this purpose, we recorded responses of VTA and SNc DA neurons to electrical footshocks across alternating brain states of urethane anaesthetized rats. We identified DA neurons based on electrophysiological criteria combined with the use of either juxtacellular recording-labelling technique or optotagging. Besides the previously described populations of value- and salience-coding neurons, we also observed unidentified so far subpopulation of VTA and SNc DA neurons, that changes its type of response to an aversive stimulus depending on the ongoing brain state. Majority of these neurons were inhibited by footshocks during a REM-like brain state, but with the appearance of nREM-like brain state, they changed their type of response to excitation. Based on our observations, it can be hypothesised that there is a subpopulation of DA neurons that are involved in ‘dual-coding’ of both the value and the salience of the stimulus depending on the general state of the brain.

Miriam Matamales
University of New South Wales - Sydney
Local D2- to D1-neuron transmodulation in the striatum and its role in goal-directed learning
One of the most intriguing characteristic of the striatum is the random spatial distribution and high degree of intermingling between its D1-(direct) and D2-(indirect) spiny projection neurons (SPNs). The resulting highly entropic mosaic extends through a homogeneous space and is mostly devoid of histological boundaries. The anatomical organisation of these two principal neuronal populations is actively promoted during development and has been highly conserved throughout evolution, and yet its relationship to function is still not fully understood. In a recent study in my team, by mapping a dopamine-dependent transcriptional activation marker in large ensembles of D1- or D2-SPNs in mice, we demonstrated an extensive and dynamic D2- to D1-SPN transmodulation across the striatum that is necessary for updating previous goal-directed learning. We found evidence that activated D2-SPNs access and modify developing behavioural programs encoded by regionally defined ensembles of transcriptionally active D1-SPNs. This process is slow because it depends on the molecular integration of additive neuromodulatory signals. However, with time, it creates the regional functional boundaries that are necessary to identify and shape specific learning in the striatum. Our work therefore suggests that the striatum takes full advantage of the ‘one-to-one’ structure of the binary mosaic and provides unexpected insights into the peculiar histoanatomical organization of this disordered, borderless environment.
University of New South Wales - Sydney
Local D2- to D1-neuron transmodulation in the striatum and its role in goal-directed learning
One of the most intriguing characteristic of the striatum is the random spatial distribution and high degree of intermingling between its D1-(direct) and D2-(indirect) spiny projection neurons (SPNs). The resulting highly entropic mosaic extends through a homogeneous space and is mostly devoid of histological boundaries. The anatomical organisation of these two principal neuronal populations is actively promoted during development and has been highly conserved throughout evolution, and yet its relationship to function is still not fully understood. In a recent study in my team, by mapping a dopamine-dependent transcriptional activation marker in large ensembles of D1- or D2-SPNs in mice, we demonstrated an extensive and dynamic D2- to D1-SPN transmodulation across the striatum that is necessary for updating previous goal-directed learning. We found evidence that activated D2-SPNs access and modify developing behavioural programs encoded by regionally defined ensembles of transcriptionally active D1-SPNs. This process is slow because it depends on the molecular integration of additive neuromodulatory signals. However, with time, it creates the regional functional boundaries that are necessary to identify and shape specific learning in the striatum. Our work therefore suggests that the striatum takes full advantage of the ‘one-to-one’ structure of the binary mosaic and provides unexpected insights into the peculiar histoanatomical organization of this disordered, borderless environment.

Priscilla Ambrosi
Northwestern University
Striato-nigro-striatal Loop and Spiral Circuits Differ in their Ability to Regulate Dopamine Neuron Function
Habit formation is thought to involve the sequential recruitment of dorsomedial striatum (DMS) and dorsolateral striatum (DLS). As animals are overtrained in instrumental tasks designed to elicit habit, their behavior becomes more inflexible, and dependence of the behavior shifts from DMS to DLS. It has been hypothesized that this DMS-DLS transition is controlled by an ascending spiral circuit, which connects striatal subregions via substantia nigra pars reticulata (SNr) and pars compacta (SNc). Specifically, the ascending spiral hypothesis predicts that there is polysynaptic circuit in the form of DMS-SNr-SNc-DLS that supports disinhibition of DLS-projecting dopamine neurons following DMS activation. In addition, this hypothesis predicts that a descending circuit in the form of DLS-SNr-SNc-DMS does not exist. Anatomical work done in non-human primates weakly support this hypothesis, but functional evidence is lacking. Indeed, multiple studies point to a predominantly closed loop organization of basal ganglia circuits. Using a trans-synaptic vector, projection labeling and intersectional genetics, we tested the predictions of the ascending spiral hypothesis in mice. To assess connectivity in polysynaptic striato-nigro-striatal circuits, we delivered ChR2 to input-defined GABAergic SNr neurons (e.g., DMS- and DLS-targeted) and labeled dopaminergic neurons based on their output (e.g., DMS- and DLS-projecting). Then, using slice electrophysiology and optogenetics, we tested the synaptic connectivity between these sub-populations of SNr and SNc cells. Our data support the existence of a DMS-SNr-SNc-DLS circuit as predicted by the ascending spiral hypothesis but challenge the prediction that this circuit alone can support disinhibition of dopamine neurons. Instead, our data suggest that closed striato-nigro-striatal loops (e.g., DLS-SNr-SNc-DLS) are better suited to support disinhibition. Our findings further diverge from the ascending spiral hypothesis by confirming the existence of a descending spiral of approximately equal strength to the ascending spiral. Future work will explore the functional consequences of our findings on habit formation.
Northwestern University
Striato-nigro-striatal Loop and Spiral Circuits Differ in their Ability to Regulate Dopamine Neuron Function
Habit formation is thought to involve the sequential recruitment of dorsomedial striatum (DMS) and dorsolateral striatum (DLS). As animals are overtrained in instrumental tasks designed to elicit habit, their behavior becomes more inflexible, and dependence of the behavior shifts from DMS to DLS. It has been hypothesized that this DMS-DLS transition is controlled by an ascending spiral circuit, which connects striatal subregions via substantia nigra pars reticulata (SNr) and pars compacta (SNc). Specifically, the ascending spiral hypothesis predicts that there is polysynaptic circuit in the form of DMS-SNr-SNc-DLS that supports disinhibition of DLS-projecting dopamine neurons following DMS activation. In addition, this hypothesis predicts that a descending circuit in the form of DLS-SNr-SNc-DMS does not exist. Anatomical work done in non-human primates weakly support this hypothesis, but functional evidence is lacking. Indeed, multiple studies point to a predominantly closed loop organization of basal ganglia circuits. Using a trans-synaptic vector, projection labeling and intersectional genetics, we tested the predictions of the ascending spiral hypothesis in mice. To assess connectivity in polysynaptic striato-nigro-striatal circuits, we delivered ChR2 to input-defined GABAergic SNr neurons (e.g., DMS- and DLS-targeted) and labeled dopaminergic neurons based on their output (e.g., DMS- and DLS-projecting). Then, using slice electrophysiology and optogenetics, we tested the synaptic connectivity between these sub-populations of SNr and SNc cells. Our data support the existence of a DMS-SNr-SNc-DLS circuit as predicted by the ascending spiral hypothesis but challenge the prediction that this circuit alone can support disinhibition of dopamine neurons. Instead, our data suggest that closed striato-nigro-striatal loops (e.g., DLS-SNr-SNc-DLS) are better suited to support disinhibition. Our findings further diverge from the ascending spiral hypothesis by confirming the existence of a descending spiral of approximately equal strength to the ascending spiral. Future work will explore the functional consequences of our findings on habit formation.

Rudolf Faust
City University of New York - Queens College
Open-loop striato-nigro-striatal circuits from limbic and sensorimotor striatum trigger dopamine release in the associative striatum
Suzanne Haber and colleagues’ ascending spiral hypothesis posits that the limbic, ventromedial striatum (VMS) can influence associative and sensorimotor domains of the dorsal striatum through serial striato-nigro-striatal projection loops. We sought to provide anatomical and functional evidence for this hypothesis by dissecting how striatal afferents to the substantia nigra control dopamine release. Combining optogenetics with fast-scan cyclic voltammetry in vivo and with whole-cell recording ex vivo, we interrogated candidate circuits for region-specific control of dorsal striatal dopamine release. We also combined anterograde and retrograde tracing to elucidate whether rats exhibit a ‘spiraling’ architecture of striato-nigro-striatal loops similar to that described in nonhuman primates. Optogenetic stimulation of striatonigral cell bodies in or axon terminals originating from the VMS or anterior dorsolateral striatum (aDLS) evoked phasic dopamine release in the associative, posterior dorsomedial striatum (pDMS), but not the sensorimotor aDLS. Dual retrograde tracing revealed that separate populations of ventral midbrain dopamine neurons project to VMS, pDMS, and aDLS, consistent with our functional data. Retrograde tracing of monosynaptic inputs to dopamine neurons projecting to pDMS or aDLS revealed that they are innervated by topologically distinct populations of substantia nigra pars reticulata non-dopamine neurons. Furthermore, striatonigral projections from the VMS displayed a higher connection probability with non-dopamine neurons in the substantia nigra than with pDMS-projecting dopamine neurons, indicating that VMS striatonigral neurons may evoke pDMS dopamine release by a disinhibitory mechanism. Together, our results are consistent with the hypothesis that ‘spiraling’ striato-nigro-striatal loops are substrates for region-specific communication between different functional domains of the striatum.
City University of New York - Queens College
Open-loop striato-nigro-striatal circuits from limbic and sensorimotor striatum trigger dopamine release in the associative striatum
Suzanne Haber and colleagues’ ascending spiral hypothesis posits that the limbic, ventromedial striatum (VMS) can influence associative and sensorimotor domains of the dorsal striatum through serial striato-nigro-striatal projection loops. We sought to provide anatomical and functional evidence for this hypothesis by dissecting how striatal afferents to the substantia nigra control dopamine release. Combining optogenetics with fast-scan cyclic voltammetry in vivo and with whole-cell recording ex vivo, we interrogated candidate circuits for region-specific control of dorsal striatal dopamine release. We also combined anterograde and retrograde tracing to elucidate whether rats exhibit a ‘spiraling’ architecture of striato-nigro-striatal loops similar to that described in nonhuman primates. Optogenetic stimulation of striatonigral cell bodies in or axon terminals originating from the VMS or anterior dorsolateral striatum (aDLS) evoked phasic dopamine release in the associative, posterior dorsomedial striatum (pDMS), but not the sensorimotor aDLS. Dual retrograde tracing revealed that separate populations of ventral midbrain dopamine neurons project to VMS, pDMS, and aDLS, consistent with our functional data. Retrograde tracing of monosynaptic inputs to dopamine neurons projecting to pDMS or aDLS revealed that they are innervated by topologically distinct populations of substantia nigra pars reticulata non-dopamine neurons. Furthermore, striatonigral projections from the VMS displayed a higher connection probability with non-dopamine neurons in the substantia nigra than with pDMS-projecting dopamine neurons, indicating that VMS striatonigral neurons may evoke pDMS dopamine release by a disinhibitory mechanism. Together, our results are consistent with the hypothesis that ‘spiraling’ striato-nigro-striatal loops are substrates for region-specific communication between different functional domains of the striatum.

Kamil Pradel
Jagiellonian University
Superior colliculus controls the activity of dopaminergic system in an asymmetrical manner via both direct and indirect pathway
Dopaminergic (DA) neurons of the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) control animals’ orienting and approach toward relevant external stimuli. To optimise the choice of motor actions these neurons perform calculations based on the information from various sources. The information about the stimuli is provided predominantly by the superior colliculus (SC), a brain region processing sensory information from the contralateral body side. Since the direction of movement depends on the asymmetry of dopamine release between left and right striatum, we aimed to investigate the lateralisation of the connection between SC and both midbrain DA system and the rostromedial tegmental nucleus (RMTg) – the main inhibitory input to DA system. To study the anatomy of the aforementioned neuronal pathways we used anterograde, retrograde and transsynaptic tracing methods. To study the physiology of direct and indirect innervation from SC to VTA/SNc and RMTg, we performed single unit and silicon probe electrophysiological recordings in vivo, and multielectrode array recordings ex vivo, all combined with optogenetics. Obtained results revealed that SC innervates preferably ipsilateral SNc/VTA, while innervating predominantly contralateral RMTg. Consequently, unilateral SC activation in vivo excited primarily RMTg neurons located contralaterally, while slightly tending to activate ipsilateral DA neurons. Nonetheless, ex vivo activation of SC terminals within the midbrain excited mainly ipsilateral DA neurons. Moreover, activating RMTg neurons monosynaptically innervated by the contralateral SC, or contralateral SC-originating axon terminals within the RMTg, inhibited midbrain DA neurons. Therefore, sensory information from one body side might directly increase the activity of the ipsilateral DA system, while indirectly (via RMTg) inhibiting the contralateral DA system, thereby causing an imbalance in DA release between left and right striatum. This suggests that the described brain circuit contributes to orienting and approach behaviors based on the direction of incoming sensory stimuli.
Jagiellonian University
Superior colliculus controls the activity of dopaminergic system in an asymmetrical manner via both direct and indirect pathway
Dopaminergic (DA) neurons of the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) control animals’ orienting and approach toward relevant external stimuli. To optimise the choice of motor actions these neurons perform calculations based on the information from various sources. The information about the stimuli is provided predominantly by the superior colliculus (SC), a brain region processing sensory information from the contralateral body side. Since the direction of movement depends on the asymmetry of dopamine release between left and right striatum, we aimed to investigate the lateralisation of the connection between SC and both midbrain DA system and the rostromedial tegmental nucleus (RMTg) – the main inhibitory input to DA system. To study the anatomy of the aforementioned neuronal pathways we used anterograde, retrograde and transsynaptic tracing methods. To study the physiology of direct and indirect innervation from SC to VTA/SNc and RMTg, we performed single unit and silicon probe electrophysiological recordings in vivo, and multielectrode array recordings ex vivo, all combined with optogenetics. Obtained results revealed that SC innervates preferably ipsilateral SNc/VTA, while innervating predominantly contralateral RMTg. Consequently, unilateral SC activation in vivo excited primarily RMTg neurons located contralaterally, while slightly tending to activate ipsilateral DA neurons. Nonetheless, ex vivo activation of SC terminals within the midbrain excited mainly ipsilateral DA neurons. Moreover, activating RMTg neurons monosynaptically innervated by the contralateral SC, or contralateral SC-originating axon terminals within the RMTg, inhibited midbrain DA neurons. Therefore, sensory information from one body side might directly increase the activity of the ipsilateral DA system, while indirectly (via RMTg) inhibiting the contralateral DA system, thereby causing an imbalance in DA release between left and right striatum. This suggests that the described brain circuit contributes to orienting and approach behaviors based on the direction of incoming sensory stimuli.

Vikram Gadagkar
Columbia University
Dopamine Neurons Evaluate Natural Fluctuations in Performance Quality
Many motor skills are learned by comparing ongoing behavior to internal performance benchmarks. Dopamine neurons have been shown to encode performance error in artificial paradigms where error is externally induced, rather than during natural behavior. Here we recorded dopamine neurons in singing birds and examined how spontaneous dopamine spiking activity correlated with natural fluctuations in ongoing song. Antidromically identified basal ganglia-projecting dopamine neurons correlated with recent, and not future, song variations, consistent with a role in evaluation, not production. Furthermore, dopamine spiking was suppressed following the production of outlying vocal variations, consistent with a role for active song maintenance. These data show for the first time that spontaneous dopamine spiking can evaluate natural behavioral fluctuations unperturbed by experimental events.
Columbia University
Dopamine Neurons Evaluate Natural Fluctuations in Performance Quality
Many motor skills are learned by comparing ongoing behavior to internal performance benchmarks. Dopamine neurons have been shown to encode performance error in artificial paradigms where error is externally induced, rather than during natural behavior. Here we recorded dopamine neurons in singing birds and examined how spontaneous dopamine spiking activity correlated with natural fluctuations in ongoing song. Antidromically identified basal ganglia-projecting dopamine neurons correlated with recent, and not future, song variations, consistent with a role in evaluation, not production. Furthermore, dopamine spiking was suppressed following the production of outlying vocal variations, consistent with a role for active song maintenance. These data show for the first time that spontaneous dopamine spiking can evaluate natural behavioral fluctuations unperturbed by experimental events.

Julie Chouinard
OIST
Striatal dopamine in time and space: effects of methylphenidate
Methylphenidate – an effective treatment for attention deficit hyperactivity disorder (ADHD) – is known to be a dopamine reuptake inhibitor at the molecular level. However, its macroscopic effects are incompletely understood. We investigated DA release caused by focal electrical or diffuse optogenetic stimulation in the dorsolateral striatum using both fast-scan cyclic voltammetry (FSCV) and dLight imaging techniques. We first gathered time-course of dopamine concentration changes at single points using FSCV, then added simultaneous imaging to monitor the spatiotemporal distribution of dopamine in two dimensions, using two variants of genetically encoded dopamine sensors: dLight1.3b and dLight2.1. We also engineered a new type of Cre-dependent AAV virus to enhance the more sensitive dLight2.1 version, which was found to provide bright, easy to locate sensor expression, with a better signal to noise ratio at 3 weeks post-injection. When clearance of dopamine is reduced by methylphenidate, the peak concentration of dopamine measured at a single distant point is delayed until after the end of the stimulation. This cannot be explained by a simple time delay due to diffusion time because there is no corresponding delay in onset of dopamine increase at the distant recording site. We hypothesized that the immediacy of the initial increase in dopamine can be explained by electrical current spread causing release at distant sites, while the delayed peak is due to diffusion delays. A two-compartment model of dopamine concentration was implemented, in which electrically-stimulated release, reuptake and diffusion occurred in both compartments. Predictions of the model were compared to the experimental data and produced good quantitative fit. These findings partially support the two-compartment model. The effect of methylphenidate on apparent diffusion distance of dopamine may be important for understanding its effects on processing of reward signals and its mechanisms of action in the treatment of ADHD.
OIST
Striatal dopamine in time and space: effects of methylphenidate
Methylphenidate – an effective treatment for attention deficit hyperactivity disorder (ADHD) – is known to be a dopamine reuptake inhibitor at the molecular level. However, its macroscopic effects are incompletely understood. We investigated DA release caused by focal electrical or diffuse optogenetic stimulation in the dorsolateral striatum using both fast-scan cyclic voltammetry (FSCV) and dLight imaging techniques. We first gathered time-course of dopamine concentration changes at single points using FSCV, then added simultaneous imaging to monitor the spatiotemporal distribution of dopamine in two dimensions, using two variants of genetically encoded dopamine sensors: dLight1.3b and dLight2.1. We also engineered a new type of Cre-dependent AAV virus to enhance the more sensitive dLight2.1 version, which was found to provide bright, easy to locate sensor expression, with a better signal to noise ratio at 3 weeks post-injection. When clearance of dopamine is reduced by methylphenidate, the peak concentration of dopamine measured at a single distant point is delayed until after the end of the stimulation. This cannot be explained by a simple time delay due to diffusion time because there is no corresponding delay in onset of dopamine increase at the distant recording site. We hypothesized that the immediacy of the initial increase in dopamine can be explained by electrical current spread causing release at distant sites, while the delayed peak is due to diffusion delays. A two-compartment model of dopamine concentration was implemented, in which electrically-stimulated release, reuptake and diffusion occurred in both compartments. Predictions of the model were compared to the experimental data and produced good quantitative fit. These findings partially support the two-compartment model. The effect of methylphenidate on apparent diffusion distance of dopamine may be important for understanding its effects on processing of reward signals and its mechanisms of action in the treatment of ADHD.

Katharina Schmack
Cold Spring Harbor Laboratory
Striatal Dopamine Mediates Hallucination-like Perception in Mice
Hallucinations, a central symptom of psychotic disorders, are attributed to excessive dopamine transmission. However, the neural circuit mechanisms by which dopamine produces hallucinations remain elusive, largely because hallucinations have been challenging to study in model organisms. Here, we developed a cross-species computational psychiatry approach to directly relate human and rodent behavior and used this approach to study the neural circuits sustaining hallucination-like perception in mice. Our rationale was that hallucinations can be operationalized as false perceptions of non-existent signals that are experienced with high confidence. We set up analogous auditory detection tasks for humans and mice to measure such hallucination-like perceptions. Hallucination-like percepts, defined as high-confidence false detections, increased after hallucination-related manipulations in mice and correlated with self-reported hallucinations in humans. Using fiber photometry in mice, we found that elevations in dopamine levels before stimulus onset predicted hallucination-like perception in both the ventral striatum and the tail of the striatum. Computational modeling indicated that ventral striatal dopamine reflected reward expectations, whereas tail of striatum dopamine resembled perceptual expectations. In line with this, optogenetic stimulation of dopaminergic activity in the tail of the striatum induced more hallucination-like perceptions that were reversed by the antipsychotic D2 antagonist haloperidol. Our findings support the idea that hallucinations arise as faulty perceptual inferences due to elevated dopamine producing a bias in favor of prior expectations against current sensory evidence. Thereby, our results also yield circuit-level insights into the long-standing dopamine hypothesis of psychosis and provide a rigorous framework for dissecting the neural circuit mechanisms involved in hallucinations. We hope that this approach can guide the development of novel treatments for psychotic disorders.
Cold Spring Harbor Laboratory
Striatal Dopamine Mediates Hallucination-like Perception in Mice
Hallucinations, a central symptom of psychotic disorders, are attributed to excessive dopamine transmission. However, the neural circuit mechanisms by which dopamine produces hallucinations remain elusive, largely because hallucinations have been challenging to study in model organisms. Here, we developed a cross-species computational psychiatry approach to directly relate human and rodent behavior and used this approach to study the neural circuits sustaining hallucination-like perception in mice. Our rationale was that hallucinations can be operationalized as false perceptions of non-existent signals that are experienced with high confidence. We set up analogous auditory detection tasks for humans and mice to measure such hallucination-like perceptions. Hallucination-like percepts, defined as high-confidence false detections, increased after hallucination-related manipulations in mice and correlated with self-reported hallucinations in humans. Using fiber photometry in mice, we found that elevations in dopamine levels before stimulus onset predicted hallucination-like perception in both the ventral striatum and the tail of the striatum. Computational modeling indicated that ventral striatal dopamine reflected reward expectations, whereas tail of striatum dopamine resembled perceptual expectations. In line with this, optogenetic stimulation of dopaminergic activity in the tail of the striatum induced more hallucination-like perceptions that were reversed by the antipsychotic D2 antagonist haloperidol. Our findings support the idea that hallucinations arise as faulty perceptual inferences due to elevated dopamine producing a bias in favor of prior expectations against current sensory evidence. Thereby, our results also yield circuit-level insights into the long-standing dopamine hypothesis of psychosis and provide a rigorous framework for dissecting the neural circuit mechanisms involved in hallucinations. We hope that this approach can guide the development of novel treatments for psychotic disorders.

Stephen Zhang
Beth Israel Deaconess Medical Center
Hypothalamic dopamine neurons control the motivation to mate via persistent cAMP-PKA signaling
Transient neuromodulation can have long-lasting effects on neural circuits and motivational states. We examined dopaminergic mechanisms underlying mating drive and its persistence in male mice. Brief investigation of females primes a male’s interest to mate for tens of minutes, while a single successful mating triggers satiety that gradually recovers over days. We found that both sexual priming and satiety are controlled by specialized anteroventral and preoptic periventricular (AVPV/PVpo) dopamine neurons in the hypothalamus. During investigations of females, dopamine concentration transiently ramps up in the medial preoptic area (MPOA), an area critical for mating behaviors. Optogenetic stimulation of AVPV/PVpo dopamine axons in MPOA recapitulates the priming effects of female exposure. Using in vivo two-photon fluorescence lifetime imaging microscopy (2p FLIM) as well as novel optogenetic tools to track and manipulate intracellular signaling, we show that these priming effects emerge from accumulation of cyclic adenosine monophosphate (cAMP) levels and protein kinase A (PKA) activity that can be sustained for tens of minutes. Dopamine transients in MPOA are abolished following a successful mating, likely ensuring abstinence. Consistent with this idea, inhibiting AVPV/PVpo dopamine neurons selectively demotivates mating, while stimulating these neurons restores the motivation to mate following sexual satiety. Therefore, accumulation or suppression of signals from specialized dopamine neurons regulates mating behaviors across minutes and days.
Beth Israel Deaconess Medical Center
Hypothalamic dopamine neurons control the motivation to mate via persistent cAMP-PKA signaling
Transient neuromodulation can have long-lasting effects on neural circuits and motivational states. We examined dopaminergic mechanisms underlying mating drive and its persistence in male mice. Brief investigation of females primes a male’s interest to mate for tens of minutes, while a single successful mating triggers satiety that gradually recovers over days. We found that both sexual priming and satiety are controlled by specialized anteroventral and preoptic periventricular (AVPV/PVpo) dopamine neurons in the hypothalamus. During investigations of females, dopamine concentration transiently ramps up in the medial preoptic area (MPOA), an area critical for mating behaviors. Optogenetic stimulation of AVPV/PVpo dopamine axons in MPOA recapitulates the priming effects of female exposure. Using in vivo two-photon fluorescence lifetime imaging microscopy (2p FLIM) as well as novel optogenetic tools to track and manipulate intracellular signaling, we show that these priming effects emerge from accumulation of cyclic adenosine monophosphate (cAMP) levels and protein kinase A (PKA) activity that can be sustained for tens of minutes. Dopamine transients in MPOA are abolished following a successful mating, likely ensuring abstinence. Consistent with this idea, inhibiting AVPV/PVpo dopamine neurons selectively demotivates mating, while stimulating these neurons restores the motivation to mate following sexual satiety. Therefore, accumulation or suppression of signals from specialized dopamine neurons regulates mating behaviors across minutes and days.

Belgin Yalcin
Stanford University
Dopaminergic Neuron Activity-regulated Myelination Modulates Reward-related Behavior
Myelin is a significant evolutionary alteration of the vertebrate nervous system and is crucial for motor, sensory and cognitive functions. In the central nervous system, myelin is formed by oligodendrocytes and their lineage dynamics regulate myelination. Myelin can exhibit plasticity and may adapt with neuronal activity in the healthy brain. Even small changes in myelination may alter conduction velocity, which in turn may influence neural circuit function and consequently cognition and behavior. It is not well-understood whether myelin adaptation by neuronal activity is a brain-wide phenomenon or if different neural circuits respond differentially depending on their functional, anatomical or cellular features. Similarly, it remains poorly understood if activity-regulated myelination can become maladaptive in disease contexts and aggravate various pathologies. Here, we show that the dopaminergic neuron activity in ventral tegmental area (VTA), a central component of reward circuitry, regulates oligodendrogenesis and myelination, which in turn modulates drug-evoked reward-related behavior. Optogenetic stimulations of VTA dopaminergic neurons increased proliferation of oligodendroglial precursors in VTA, but not in the long-range axonal projections in nucleus accumbens (NAc). This response is specific to phasic (30 Hz) but not tonic (1Hz) firing of dopaminergic neurons. Strikingly, acute morphine exposure also increased the proliferation of oligodendroglial precursors selectively in VTA, suggesting parallels for activity-regulated oligodendrogenesis in the healthy brain and drug-evoked states. Acquisition of morphine-induced conditioned place preference (CPP) promoted oligodendrogenesis in VTA, the extend of which correlated with the behavior. Genetic blockade of oligodendroglial differentiation abrogated both morphine-induced oligodendrogenesis and CPP acquisition, highlighting oligodendroglial cells as critical players in reward-related behavior. This study demonstrates that activity-induced oligodendrogenesis could be a part of the initial neural circuit modifications evoked by opioids, and it could contribute to progression of drug-seeking behaviors.
Stanford University
Dopaminergic Neuron Activity-regulated Myelination Modulates Reward-related Behavior
Myelin is a significant evolutionary alteration of the vertebrate nervous system and is crucial for motor, sensory and cognitive functions. In the central nervous system, myelin is formed by oligodendrocytes and their lineage dynamics regulate myelination. Myelin can exhibit plasticity and may adapt with neuronal activity in the healthy brain. Even small changes in myelination may alter conduction velocity, which in turn may influence neural circuit function and consequently cognition and behavior. It is not well-understood whether myelin adaptation by neuronal activity is a brain-wide phenomenon or if different neural circuits respond differentially depending on their functional, anatomical or cellular features. Similarly, it remains poorly understood if activity-regulated myelination can become maladaptive in disease contexts and aggravate various pathologies. Here, we show that the dopaminergic neuron activity in ventral tegmental area (VTA), a central component of reward circuitry, regulates oligodendrogenesis and myelination, which in turn modulates drug-evoked reward-related behavior. Optogenetic stimulations of VTA dopaminergic neurons increased proliferation of oligodendroglial precursors in VTA, but not in the long-range axonal projections in nucleus accumbens (NAc). This response is specific to phasic (30 Hz) but not tonic (1Hz) firing of dopaminergic neurons. Strikingly, acute morphine exposure also increased the proliferation of oligodendroglial precursors selectively in VTA, suggesting parallels for activity-regulated oligodendrogenesis in the healthy brain and drug-evoked states. Acquisition of morphine-induced conditioned place preference (CPP) promoted oligodendrogenesis in VTA, the extend of which correlated with the behavior. Genetic blockade of oligodendroglial differentiation abrogated both morphine-induced oligodendrogenesis and CPP acquisition, highlighting oligodendroglial cells as critical players in reward-related behavior. This study demonstrates that activity-induced oligodendrogenesis could be a part of the initial neural circuit modifications evoked by opioids, and it could contribute to progression of drug-seeking behaviors.

Zachary Brodnik
NIH NIDA IRP
Dopamine and GABA release from combinatorial neurons in the zona incerta play distinct but complimentary roles in threat response behavior
While the notion that neurons signal by releasing one unique neurotransmitter is widely accepted, we and others have provided evidence that neurons may co-release multiple neurotransmitters. Tyrosine hydroxylase (TH) positive neurons in the ventral tegmental area have been shown to release the inhibitory neurotransmitter GABA. However, in contrast to GABA neurons distributed throughout the brain, VTA TH-GABA neurons do not have the capability to synthesize GABA utilizing the enzyme glutamic acid decarboxylase (GAD) and lack the GABA vesicular transporter (VGaT). By brain mapping of cellular co-expression we identified a subpopulation of TH-GAD-VGaT neurons in the zona incerta. Based on these anatomical findings, we designed up a multidisciplinary approach to determine the role of zona incerta TH-GAD-VGaT neurons in behavior. (1) By viral based tract tracing and immuno-ultrastructural analysis, we found that axon terminals from these neurons contained both TH- and VGaT-proteins. (2) By ex vivo fast scan cyclic voltammetry and electrophysiology, we found that TH-VGaT axon terminals indeed released both dopamine and GABA. (3) By in vivo fiber photometry, we found that zona incerta TH cells preferentially respond to threatening stimuli. (4) By chemogenetics, we found that excitation of zona incerta TH neurons suppressed threat response behaviors while their inhibition enhanced threat response behaviors. (5) By viral based CRISPR knockout strategy, we found that specific GABA signal ablation from TH-GABA neurons increased the rate of at which threat response behaviors are initiated. In contrast, specific dopamine signal ablation from TH-GABA neurons increased the duration threat response behavior bouts. In summary, we demonstrated that the zona incerta contains neurons that co-release dopamine and GABA, and provide evidence indicating that the release of these two neurotransmitters from the same neuron play distinct but complementary roles in gating the expression of active threat response behaviors.
NIH NIDA IRP
Dopamine and GABA release from combinatorial neurons in the zona incerta play distinct but complimentary roles in threat response behavior
While the notion that neurons signal by releasing one unique neurotransmitter is widely accepted, we and others have provided evidence that neurons may co-release multiple neurotransmitters. Tyrosine hydroxylase (TH) positive neurons in the ventral tegmental area have been shown to release the inhibitory neurotransmitter GABA. However, in contrast to GABA neurons distributed throughout the brain, VTA TH-GABA neurons do not have the capability to synthesize GABA utilizing the enzyme glutamic acid decarboxylase (GAD) and lack the GABA vesicular transporter (VGaT). By brain mapping of cellular co-expression we identified a subpopulation of TH-GAD-VGaT neurons in the zona incerta. Based on these anatomical findings, we designed up a multidisciplinary approach to determine the role of zona incerta TH-GAD-VGaT neurons in behavior. (1) By viral based tract tracing and immuno-ultrastructural analysis, we found that axon terminals from these neurons contained both TH- and VGaT-proteins. (2) By ex vivo fast scan cyclic voltammetry and electrophysiology, we found that TH-VGaT axon terminals indeed released both dopamine and GABA. (3) By in vivo fiber photometry, we found that zona incerta TH cells preferentially respond to threatening stimuli. (4) By chemogenetics, we found that excitation of zona incerta TH neurons suppressed threat response behaviors while their inhibition enhanced threat response behaviors. (5) By viral based CRISPR knockout strategy, we found that specific GABA signal ablation from TH-GABA neurons increased the rate of at which threat response behaviors are initiated. In contrast, specific dopamine signal ablation from TH-GABA neurons increased the duration threat response behavior bouts. In summary, we demonstrated that the zona incerta contains neurons that co-release dopamine and GABA, and provide evidence indicating that the release of these two neurotransmitters from the same neuron play distinct but complementary roles in gating the expression of active threat response behaviors.

Kim Blackwell
George Mason University
Enhancing reinforcement learning models by including direct and indirect pathways
Reward learning requires dopamine inputs to striatal neurons, which then undergo synaptic plasticity. Numerous reinforcement learning models, such as Q learning, make use of the reward prediction error, which resembles dopamine neuron firing, to learn the best action in response to a set of cues. The resemblance of these reinforcement learning models to the basal ganglia breaks down if one considers that there are two subtypes of striatal projection neurons: the direct pathway (promoting actions) and indirect pathway (inhibiting actions). This limitation may contribute to the difficulty of reinforcement learning models to reproduce complex learning paradigms. We present a new reinforcement learning model which utilizes two Q matrices, one representing direct pathway neurons and one representing indirect pathway neurons. Update of the Q matrices utilize the reward prediction error, but the learning rule for the Q matrix corresponding to the indirect pathway neurons is derived from the synaptic plasticity rules of those neurons. When the two Q matrices disagree as to the best action, the action with the highest probability is selected. The model is tested on a range of tasks including extinction, renewal, discrimination learning, sequence leaning and the 2 arm-bandit task. Simulations show that using 2 Q matrices improves sequence learning, and exhibits probability matching similar to rats on the 2 arm-bandit task. Blocking the update rule on the 2nd Q matrix blocks discrimination learning, as observed experimentally. These results suggest that including additional aspects of basal ganglia physiology can improve the performance of reinforcement learning models.
George Mason University
Enhancing reinforcement learning models by including direct and indirect pathways
Reward learning requires dopamine inputs to striatal neurons, which then undergo synaptic plasticity. Numerous reinforcement learning models, such as Q learning, make use of the reward prediction error, which resembles dopamine neuron firing, to learn the best action in response to a set of cues. The resemblance of these reinforcement learning models to the basal ganglia breaks down if one considers that there are two subtypes of striatal projection neurons: the direct pathway (promoting actions) and indirect pathway (inhibiting actions). This limitation may contribute to the difficulty of reinforcement learning models to reproduce complex learning paradigms. We present a new reinforcement learning model which utilizes two Q matrices, one representing direct pathway neurons and one representing indirect pathway neurons. Update of the Q matrices utilize the reward prediction error, but the learning rule for the Q matrix corresponding to the indirect pathway neurons is derived from the synaptic plasticity rules of those neurons. When the two Q matrices disagree as to the best action, the action with the highest probability is selected. The model is tested on a range of tasks including extinction, renewal, discrimination learning, sequence leaning and the 2 arm-bandit task. Simulations show that using 2 Q matrices improves sequence learning, and exhibits probability matching similar to rats on the 2 arm-bandit task. Blocking the update rule on the 2nd Q matrix blocks discrimination learning, as observed experimentally. These results suggest that including additional aspects of basal ganglia physiology can improve the performance of reinforcement learning models.