Phasic firing in dopaminergic neurons is sufficient for behavioral conditioning

Morphine selectively disinhibits glutamatergic input from mPFC onto dopamine neurons of VTA, inducing reward

Ventral tegmental area (VTA) dopamine (DA) neurons presynaptic glutamate release plays a very important role in the mechanism of morphine. Previously, a study from our lab found that morphine disinhibited glutamatergic input onto the VTA-DA neurons, which was an important mechanism for the morphine-induced increase in the VTA-DA neuron firing and related behaviors (Chen et al., 2015). However, what source of glutamatergic inputs is disinhibited by morphine remains to be elucidated. Using optogenetic strategy combined with whole-cell patch-clamp, qRT-PCR, immunofluorescence and chemical genetic approach combined with behavioral methods, our results show that: 1) morphine promotes glutamate release from glutamatergic terminals of medial prefrontal cortex (mPFC) neurons projecting to VTA-DA neurons but does not on those from glutamatergic terminals of the lateral hypothalamus (LH) neurons projecting to VTA-DA neurons; 2) different response of glutamatergic neurons projecting to VTA-DA neurons from the mPFC or the LH to morphine is related to the expression of GABA_B receptors at terminals of these neurons; 3) inhibition of projection neurons from the mPFC to the VTA significantly reduces morphine-induced locomotor activity increase and conditioned place preference but inhibition of projection neurons from the LH to the VTA does not. These results suggest that morphine selectively promotes glutamate release of the glutamatergic input from mPFC onto VTA-DA neurons by removing the inhibition of the GABA_B receptors in this glutamatergic input from mPFC.

Phasic dopamine reinforces distinct striatal stimulus encoding in the olfactory tubercle driving dopaminergic reward prediction

The learning of stimulus-outcome associations allows for predictions about the environment. Ventral striatum and dopaminergic midbrain neurons form a larger network for generating reward prediction signals from sensory cues. Yet, the network plasticity mechanisms to generate predictive signals in these distributed circuits have not been entirely clarified. Also, direct evidence of the underlying interregional assembly formation and information transfer is still missing. Here we show that phasic dopamine is sufficient to reinforce the distinctness of stimulus representations in the ventral striatum even in the absence of reward. Upon such reinforcement, striatal stimulus encoding gives rise to interregional assemblies that drive dopaminergic neurons during stimulus-outcome learning. These assemblies dynamically encode the predicted reward value of conditioned stimuli. Together, our data reveal that ventral striatal and midbrain reward networks form a reinforcing loop to generate reward prediction coding.

It is not entirely understood how network plasticity produces the coding of predicted value during stimulus-outcome learning. Here, the authors reveal a reinforcing loop in distributed limbic circuits, transforming sensory stimuli into reward prediction coding broadcasted by dopamine neurons to the brain.

Dopamine Suppresses Synaptic Responses of Fan Cells in the Lateral Entorhinal Cortex to Olfactory Bulb Input in Mice

The lateral entorhinal cortex (LEC) is involved in odor discrimination, odor-associative multimodal memory, and neurological or neuropsychiatric disorders. It receives direct axonal projections from both olfactory bulb (OB) output neurons and midbrain dopaminergic neurons. However, the cellular targets in LEC receiving direct synaptic input from OB output neuron, the functional characteristics of these synapses, and whether or how dopamine (DA) modulates the OB-LEC pathway remain undetermined. We addressed these questions in the present study by combing optogenetic and electrophysiological approaches with four major findings: (1) selective activation of OB input elicited glutamate-mediated monosynaptic responses in all fan cells, the major output neurons in layer II of the LEC; (2) this excitatory synaptic transmission exhibited robust paired-pulse facilitation (PPF), a presynaptically derived short-term synaptic plasticity; (3) DA dramatically attenuated the strength of the OB input-fan cell synaptic transmission via activation of D1 receptors; and (4) DA altered the PPF of this transmission but neither intrinsic properties of postsynaptic neurons nor the kinetic profile of postsynaptic responses, suggesting that presynaptic mechanisms underlie the DA inhibitory actions. This study for the first time demonstrates the FCs in the LEC layer II as the postsynaptic target of direct OB input and characterizes DA modulation of the OB input-fan cell pathway. These findings set the foundation for future studies to examine the synaptic transmission from the OB output neuron axon terminals to other potential cell types in the LEC and to pinpoint the pathophysiological mechanisms underlying olfactory deficits associated with DA-relevant neurological and neuropsychiatric disorders.

CCKergic Tufted Cells Differentially Drive Two Anatomically Segregated Inhibitory Circuits in the Mouse Olfactory Bulb

Delineation of functional synaptic connections is fundamental to understanding sensory processing. Olfactory signals are synaptically processed initially in the olfactory bulb (OB) where neural circuits are formed among inhibitory interneurons and the output neurons mitral cells (MCs) and tufted cells (TCs). TCs function in parallel with but differently from MCs and are further classified into multiple subpopulations based on their anatomic and functional heterogeneities. Here, we combined optogenetics with electrophysiology to characterize the synaptic transmission from a subpopulation of TCs, which exclusively express the neuropeptide cholecystokinin (CCK), to two groups of spatially segregated GABAergic interneurons, granule cells (GCs) and glomerular interneurons in mice of both sexes with four major findings. First, CCKergic TCs receive direct input from the olfactory sensory neurons (OSNs). This monosynaptic transmission exhibits high fidelity in response to repetitive OSN input. Second, CCKergic TCs drive GCs through two functionally distinct types of monosynaptic connections: (1) dendrodendritic synapses onto GC distal dendrites via their lateral dendrites in the superficial external plexiform layer (EPL); (2) axodendritic synapses onto GC proximal dendrites via their axon collaterals or terminals in the internal plexiform layer (IPL) on both sides of each bulb. Third, CCKergic TCs monosynaptically excite two subpopulations of inhibitory glomerular interneurons via dendrodendritic synapses. Finally, sniff-like patterned activation of CCKergic TCs induces robust frequency-dependent depression of the dendrodendritic synapses but facilitation of the axodendritic synapses. These results demonstrated important roles of the CCKergic TCs in olfactory processing by orchestrating OB inhibitory activities.SIGNIFICANCE STATEMENT Neuronal morphology and organization in the olfactory bulb (OB) have been extensively studied, however, the functional operation of neuronal interactions is not fully understood. We combined optogenetic and electrophysiological approaches to investigate the functional operation of synaptic connections between a specific population of excitatory output neuron and inhibitory interneurons in the OB. We found that these output neurons formed distinct types of synapses with two populations of spatially segregated interneurons. The functional characteristics of these synapses vary significantly depending on the presynaptic compartments so that these output neurons can dynamically rebalance inhibitory feedback or feedforward to other neurons types in the OB in response to dynamic rhythmic inputs.

Mapping the Brain-Wide Network Effects by Optogenetic Activation of the Corpus Callosum

Optogenetically driven manipulation of circuit-specific activity enables causality studies, but its global brain-wide effect is rarely reported. Here, we applied simultaneous functional magnetic resonance imaging (fMRI) and calcium recording with optogenetic activation of the corpus callosum (CC) connecting barrel cortices (BC). Robust positive BOLD was detected in the ipsilateral BC due to antidromic activity, spreading to the ipsilateral motor cortex (MC), and posterior thalamus (PO). In the orthodromic target, positive BOLD was reliably evoked by 2 Hz light pulses, whereas 40 Hz light pulses led to reduced calcium, indicative of CC-mediated inhibition. This presumed optogenetic CC-mediated inhibition was further elucidated by pairing light pulses with whisker stimulation at varied interstimulus intervals. Whisker-induced positive BOLD and calcium signals were reduced at intervals of 50/100 ms. The calcium-amplitude-modulation-based correlation with whole-brain fMRI signal revealed that the inhibitory effects spread to contralateral BC, ipsilateral MC, and PO. This work raises the need for fMRI to elucidate the brain-wide network activation in response to optogenetic stimulation.

Distinct signals in medial and lateral VTA dopamine neurons modulate fear extinction at different times

Dopamine (DA) neurons are thought to encode reward prediction error (RPE), in addition to other signals, such as salience. While RPE is known to support learning, the role of salience in learning remains less clear. To address this, we recorded and manipulated VTA DA neurons in mice during fear extinction. We applied deep learning to classify mouse freezing behavior, eliminating the need for human scoring. Our fiber photometry recordings showed DA neurons in medial and lateral VTA have distinct activity profiles during fear extinction: medial VTA activity more closely reflected RPE, while lateral VTA activity more closely reflected a salience-like signal. Optogenetic inhibition of DA neurons in either region slowed fear extinction, with the relevant time period for inhibition differing across regions. Our results indicate salience-like signals can have similar downstream consequences to RPE-like signals, although with different temporal dependencies.

Dopaminergic modulation of the exploration/exploitation trade-off in human decision-making

Involvement of dopamine in regulating exploration during decision-making has long been hypothesized, but direct causal evidence in humans is still lacking. Here, we use a combination of computational modeling, pharmacological intervention and functional magnetic resonance imaging to address this issue. Thirty-one healthy male participants performed a restless four-armed bandit task in a within-subjects design under three drug conditions: 150 mg of the dopamine precursor L-dopa, 2 mg of the D2 receptor antagonist haloperidol, and placebo. Choices were best explained by an extension of an established Bayesian learning model accounting for perseveration, directed exploration and random exploration. Modeling revealed attenuated directed exploration under L-dopa, while neural signatures of exploration, exploitation and prediction error were unaffected. Instead, L-dopa attenuated neural representations of overall uncertainty in insula and dorsal anterior cingulate cortex. Our results highlight the computational role of these regions in exploration and suggest that dopamine modulates how this circuit tracks accumulating uncertainty during decision-making.

Heterogeneity in striatal dopamine circuits: Form and function in dynamic reward seeking

The striatal dopamine system has long been studied in the context of reward learning, motivation, and movement. Given the prominent role dopamine plays in a variety of adaptive behavioral states, as well as diseases like addiction, it is essential to understand the full complexity of dopamine neurons and the striatal systems they target. A growing number of studies are uncovering details of the heterogeneity in dopamine neuron subpopulations. Here, we review that work to synthesize current understanding of dopamine system heterogeneity across three levels, anatomical organization, functions in behavior, and modes of action, wherein we focus on signaling profiles and local mechanisms for modulation of dopamine release. Together, these studies reveal new and emerging dimensions of the striatal dopamine system, informing its contribution to dynamic motivational and decision-making processes.

Dynamic functional connectivity between nucleus accumbens and the central executive network relates to chronic cannabis use

The neural mechanisms of drug cue‐reactivity regarding the temporal fluctuations of functional connectivity, namely the dynamic connectivity, are sparsely studied. Quantifying the task‐modulated variability in dynamic functional connectivity at cue exposure can aid the understanding. We analyzed changes in dynamic connectivity in 54 adult cannabis users and 90 controls during a cannabis cue exposure task. The variability was measured as standard deviation in the (a) connectivity weights of the default mode, the central executive, and the salience networks and two reward loci (amygdalae and nuclei accumbens); and (b) topological indexes of the whole brain (global efficiency, modularity and network resilience). These were compared for the main effects of task conditions and the group (users vs. controls), and correlated with pre‐ and during‐scan subjective craving. The variability of connectivity weights between the central executive network and nuclei accumbens was increased in users throughout the cue exposure task, and, was positively correlated with during‐scan craving for cannabis. The variability of modularity was not different by groups, but positively correlated with prescan craving. The variability of dynamic connectivity during cannabis cue exposure task between the central executive network and the nuclei accumbens, and, the level of modularity, seem to relate to the neural underpinning of cannabis use and the subjective craving.

Acute Stress Enhances Associative Learning via Dopamine Signaling in the Ventral Lateral Striatum

Acute stress transiently increases vigilance, enhancing the detection of salient stimuli in one's environment. This increased perceptual sensitivity is thought to promote the association of rewarding outcomes with relevant cues. The mesolimbic dopamine system is critical for learning cue-reward associations. Dopamine levels in the ventral striatum are elevated following exposure to stress. Together, this suggests that the mesolimbic dopamine system could mediate the influence of acute stress on cue-reward learning. To address this possibility, we examined how a single stressful experience influenced learning in an appetitive pavlovian conditioning task. Male rats underwent an episode of restraint prior to the first conditioning session. This acute stress treatment augmented conditioned responding in subsequent sessions. Voltammetry recordings of mesolimbic dopamine levels demonstrated that acute stress selectively increased reward-evoked dopamine release in the ventral lateral striatum (VLS), but not in the ventral medial striatum. Antagonizing dopamine receptors in the VLS blocked the stress-induced enhancement of conditioned responding. Collectively, these findings illustrate that stress engages dopamine signaling in the VLS to facilitate appetitive learning.SIGNIFICANCE STATEMENT Acute stress influences learning about aversive and rewarding outcomes. Dopamine neurons are sensitive to stress and critical for reward learning. However, it is unclear whether stress regulates reward learning via dopamine signaling. Using fast-scan cyclic voltammetry as rats underwent pavlovian conditioning, we demonstrate that a single stressful experience increases reward-evoked dopamine release in the ventral lateral striatum. This enhanced dopamine signal accompanies a long-lasting increase in conditioned behavioral responding. These findings highlight that the ventral lateral striatum is a node for mediating the effect of stress on reward processing.

Frequency- and State-Dependent Network Effects of Electrical Stimulation Targeting the Ventral Tegmental Area in Macaques

The ventral tegmental area (VTA) is a midbrain structure at the heart of the dopaminergic system underlying adaptive behavior. Endogenous firing rates of dopamine cells in the VTA vary from fast phasic bursts to slow tonic activity. Artificial perturbations of the VTA, through electrical or optogenetic stimulation methods, generate different and sometimes even contrasting behavioral outcomes depending on stimulation parameters such as frequency, amplitude, and pulse width. Here, we investigate the global functional effects of electrical stimulation frequency (10, 20, 50, and 100 Hz) of the VTA in rhesus monkeys. We stimulated 2 animals with chronic electrodes, either awake or anesthetized, while concurrently acquiring whole-brain functional magnetic resonance imaging (fMRI) signals. In the awake state, activity as a function of stimulation frequency followed an inverted U-shape in many cortical and subcortical structures, with highest activity observed at 20 and 50 Hz and lower activity at 10 and 100 Hz. Under anesthesia, the hemodynamic responses in connected brain areas were slightly positive at 10 Hz stimulation, but decreased linearly as a function of higher stimulation frequencies. A speculative explanation for the remarkable frequency dependence of stimulation-induced fMRI activity is that the VTA makes use of different frequency channels to communicate with different postsynaptic sites.

Learning from Action: Reconsidering Movement Signaling in Midbrain Dopamine Neuron Activity

Animals infer when and where a reward is available from experience with informative sensory stimuli and their own actions. In vertebrates, this is thought to depend upon the release of dopamine from midbrain dopaminergic neurons. Studies of the role of dopamine have focused almost exclusively on their encoding of informative sensory stimuli; however, many dopaminergic neurons are active just prior to movement initiation, even in the absence of sensory stimuli. How should current frameworks for understanding the role of dopamine incorporate these observations? To address this question, we review recent anatomical and functional evidence for action-related dopamine signaling. We conclude by proposing a framework in which dopaminergic neurons encode subjective signals of action initiation to solve an internal credit assignment problem.

Control of food approach and eating by a GABAergic projection from lateral hypothalamus to dorsal pons

Stimulation of lateral hypothalamic (LH) neurons produces eating in sated animals and increases activity of dopamine neurons. The present study shows that the activation of dopamine neurons failed to induce eating. Rather, food approach and eating were observed via activation of LH fibers that project through the VTA, continuing caudally and terminating in a brainstem region medial to the locus coeruleus (LC). We found that activation of GABA neurons in this peri-LC region is both necessary and sufficient for LH stimulation-induced eating, whereas their role in normal homeostatic feeding appears negligible. These findings suggest that this circuit orchestrates just one of the multiple aspects of eating: a compulsive consumption of food in the absence of a physiological stimulus of hunger.

Electrical or optogenetic stimulation of lateral hypothalamic (LH) GABA neurons induces rapid vigorous eating in sated animals. The dopamine system has been implicated in the regulation of feeding. Previous work has suggested that a subset of LH GABA neurons projects to the ventral tegmental area (VTA) and targets GABA neurons, inhibiting them and thereby disinhibiting dopaminergic activity and release. Furthermore, stimulation-induced eating is attenuated by dopamine lesions or receptor antagonists. Here we explored the involvement of dopamine in LH stimulation-induced eating. LH stimulation caused sated mice to pick up pellets of standard chow with latencies that varied based on stimulation intensity; once food was picked up, animals ate for the remainder of the 60-s stimulation period. However, lesion of VTA GABA neurons failed to disrupt this effect. Moreover, direct stimulation of VTA or substantia nigra dopamine cell bodies failed to induce food approach or eating. Looking further, we found that some LH GABA fibers pass through the VTA to more caudal sites, where they synapse onto neurons near the locus coeruleus (LC). Similar eating was induced by stimulation of LH GABA terminals or GABA cell bodies in this peri-LC region. Lesion of peri-LC GABA neurons blocked LH stimulation-induced eating, establishing them as a critical downstream circuit element for LH neurons. Surprisingly, lesions did not alter body weight, suggesting that this system is not involved in the hunger or satiety mechanisms that govern normal feeding. Thus, we present a characterization of brain circuitry that may promote overeating and contribute to obesity.

Optofluidic control of rodent learning using cloaked caged glutamate

Glutamate is the major excitatory neurotransmitter in the brain, and photochemical release of glutamate (or uncaging) is a chemical technique widely used by biologists to interrogate its physiology. A basic prerequisite of these optical probes is bio-inertness before photolysis. However, all caged glutamates are known to have strong antagonism toward receptors of γ-aminobutyric acid, the major inhibitory transmitter. We have developed a caged glutamate probe that is inert toward these receptors at concentrations that are effective for photolysis with violet light. Pharmacological tests in vitro revealed that attachment of a fifth-generation (G5) dendrimer (i.e., cloaking) to the widely used 4-methoxy-7-nitro-indolinyl(MNI)-Glu probe prevented such off-target effects while not changing the photochemical properties of MNI-Glu significantly. G5-MNI-Glu was used with optofluidic delivery to stimulate dopamine neurons of the ventral tegmental area of freely moving mice in a conditioned place-preference protocol so as to mediate Pavlovian conditioning.

Glutamatergic basolateral amygdala to anterior insular cortex circuitry maintains rewarding contextual memory

Findings have shown that anterior insular cortex (aIC) lesions disrupt the maintenance of drug addiction, while imaging studies suggest that connections between amygdala and aIC participate in drug-seeking. However, the role of the BLA → aIC pathway in rewarding contextual memory has not been assessed. Using a cre-recombinase under the tyrosine hydroxylase (TH+) promoter mouse model to induce a real-time conditioned place preference (rtCPP), we show that photoactivation of TH+ neurons induced electrophysiological responses in VTA neurons, dopamine release and neuronal modulation in the aIC. Conversely, memory retrieval induced a strong release of glutamate, dopamine, and norepinephrine in the aIC. Only intra-aIC blockade of the glutamatergic N-methyl-D-aspartate receptor accelerated rtCPP extinction. Finally, photoinhibition of glutamatergic BLA → aIC pathway produced disinhibition of local circuits in the aIC, accelerating rtCPP extinction and impairing reinstatement. Thus, activity of the glutamatergic projection from the BLA to the aIC is critical for maintenance of rewarding contextual memory.

Aversion hot spots in the dopamine system

Through the development of optogenetics and other viral vector-based technologies, our view of the dopamine system has substantially advanced over the last decade. In particular, progress has been made in the reclassification of dopamine neurons based on subtypes displaying specific projections, which are associated with different features at the anatomical, molecular and behavioral level. Together, these discoveries have raised the possibility that individual groups of dopamine cells make a unique contribution to the processing of reward and aversion. Here, we review recent studies that have identified non-canonical dopamine pathways that are excited in response to aversive stimuli, including dopamine projections to the ventromedial shell of the nucleus accumbens, prefrontal cortex, tail of the striatum, and amygdala.

Dopaminergic and Prefrontal Basis of Learning from Sensory Confidence and Reward Value

Deciding between stimuli requires combining their learned value with one's sensory confidence. We trained mice in a visual task that probes this combination. Mouse choices reflected not only present confidence and past rewards but also past confidence. Their behavior conformed to a model that combines signal detection with reinforcement learning. In the model, the predicted value of the chosen option is the product of sensory confidence and learned value. We found precise correlates of this variable in the pre-outcome activity of midbrain dopamine neurons and of medial prefrontal cortical neurons. However, only the latter played a causal role: inactivating medial prefrontal cortex before outcome strengthened learning from the outcome. Dopamine neurons played a causal role only after outcome, when they encoded reward prediction errors graded by confidence, influencing subsequent choices. These results reveal neural signals that combine reward value with sensory confidence and guide subsequent learning.

Plasticity in the Brainstem: Prenatal and Postnatal Experience Can Alter Laterodorsal Tegmental (LDT) Structure and Function

The brainstem has traditionally been considered an area of the brain with autonomous control of mostly homeostatic functions such as heart rate, respiration, and the sleep and wakefulness state, which would preclude the necessity to exhibit the high degree of synaptic or cellular mechanisms of plasticity typical of regions of the brain responsible for flexible, executive control, such as the medial prefrontal cortex or the hippocampus. The perception that the brainstem does not share the same degree of flexibility to alter synaptic strength and/or wiring within local circuits makes intuitive sense, as it is not easy to understand how "soft wiring" would be an advantage when considering the importance of faithful and consistent performance of the homeostatic, autonomic functions that are controlled by the brainstem. However, many of the molecular and cellular requirements which underlie strengthening of synapses seen in brain regions involved in higher-level processing are present in brainstem nuclei, and recent research suggest that the view of the brainstem as "hard wired," with rigid and static connectivity and with unchanging synaptic strength, is outdated. In fact, information from studies within the last decades, including work conducted in our group, leads us to propose that the brainstem can dynamically alter synaptic proteins, and change synaptic connections in response to prenatal or postnatal stimuli, and this would likely alter functionality and output. This article reviews recent research that has provided information resulting in our revision of the view of the brainstem as static and non-changing by using as example recent information gleaned from a brainstem pontine nucleus, the laterodorsal tegmentum (LDT). The LDT has demonstrated mechanisms underlying synaptic plasticity, and plasticity has been exhibited in the postnatal LDT following exposure to drugs of abuse. Further, exposure of the brain during gestation to drugs of abuse results in alterations in development of signaling pathways in the LDT. As the LDT provides a high degree of innervation of mesoaccumbal and mesocortical circuits involved in salience, as well as thalamocortical circuits involved in control of arousal and orientation, changes in synaptic strength would be expected to alter output, which would significantly impact behavioral state, motivated behavior and directed attention. Further, alterations in developmental trajectory within the LDT following prenatal exposure to drugs of abuse would be expected to impact on later life expression of motivation and arousal.

Transient and sustained effects of dopamine and serotonin signaling in motivation-related behavior

Pharmacological studies of antidepressants and atypical antipsychotics have suggested a role of dopamine and serotonin signaling in depression. However, depressive symptoms and treatment effects are difficult to explain based simply on brain-wide decrease or increase in the concentrations of these molecules. Recent animal studies using advanced neuronal manipulation and observation techniques have revealed detailed dopamine and serotonin dynamics that regulate diverse aspects of motivation-related behavior. Dopamine and serotonin transiently modulate moment-to-moment behavior at timescales ranging from sub-second to minutes and also produce persistent effects, such as reward-related learning and stress responses that last longer than several days. Transient and sustained effects often exhibit specific roles depending on the projection sites, where distinct synaptic and cellular mechanisms are required to process the neurotransmitters for each transient and sustained timescale. Therefore, it appears that specific aspects of motivation-related behavior are regulated by distinct synaptic and cellular mechanisms in specific brain regions that underlie the transient and sustained effects of dopamine and serotonin signaling. Recent clinical studies have implied that subjects with depressive symptoms show impaired transient and sustained signaling functions; moreover, they exhibit heterogeneity in depressive symptoms and neuronal dysfunction. Depressive symptoms may be explained by the dysfunction of each transient and sustained signaling mechanism, and distinct patterns of impairment in the relevant mechanisms may explain the heterogeneity of symptoms. Thus, detailed understanding of dopamine and serotonin signaling may provide new insight into depressive symptoms.

Opposite Consequences of Tonic and Phasic Increases in Accumbal Dopamine on Alcohol-Seeking Behavior

Despite many years of work on dopaminergic mechanisms of alcohol addiction, much of the evidence remains mostly correlative in nature. Fortunately, recent technological advances have provided the opportunity to explore the causal role of alterations in neurotransmission within circuits involved in addictive behaviors. Here, we address this critical gap in our knowledge by integrating an optogenetic approach and an operant alcohol self-administration paradigm to assess directly how accumbal dopamine (DA) release dynamics influences the appetitive (seeking) component of alcohol-drinking behavior. We show that appetitive reward-seeking behavior in rats trained to self-administer alcohol can be shaped causally by ventral tegmental area-nucleus accumbens (VTA-NAc) DA neurotransmission. Our findings reveal that phasic patterns of DA release within this circuit enhance a discrete measure of alcohol seeking, whereas tonic patterns of stimulation inhibit this behavior. Moreover, we provide mechanistic evidence that tonic-phasic interplay within the VTA-NAc DA circuit underlies these seemingly paradoxical effects.

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VTA-NAc DA transmission can bidirectionally modulate motivated behavior

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Optogenetic increases in phasic DA release in the NAc enhance alcohol seeking

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Optogenetic increases in tonic DA release in the NAc inhibit alcohol seeking

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Phasic DA release can be decreased by the concurrent tonic activation

Short-term plasticity in glomerular inhibitory circuits shapes olfactory bulb output

Short-term plasticity is a fundamental synaptic property thought to underlie memory and neural processing. The glomerular microcircuit comprises complex excitatory and inhibitory interactions and transmits olfactory nerve signals to the excitatory output neurons, mitral/tufted cells (M/TCs). The major glomerular inhibitory interneurons, short axon cells (SACs) and periglomerular cells (PGCs), both provide feedforward and feedback inhibition to M/TCs and have reciprocal inhibitory synapses between each other. Olfactory input is episodically driven by sniffing. We hypothesized that frequency-dependent short-term plasticity within these inhibitory circuits could influence signals sent to higher-order olfactory networks. To assess short-term plasticity in glomerular circuits and MC outputs, we virally delivered channelrhodopsin-2 (ChR2) in glutamic acid decarboxylase-65 promotor (GAD2-cre) or tyrosine hydroxylase promoter (TH-cre) mice and selectively activated one of these two populations while recording from cells of the other population or from MCs. Selective activation of TH-ChR2-expressing SACs inhibited all recorded GAD2-green fluorescent protein(GFP)-expressing presumptive PGC cells, and activation of GAD2-ChR2 cells inhibited TH-GFP-expressing SACs, indicating reciprocal inhibitory connections. SAC synaptic inhibition of GAD2-expressing cells was significantly facilitated at 5-10 Hz activation frequencies. In contrast, GAD2-ChR2 cell inhibition of TH-expressing cells was activation-frequency independent. Both SAC and PGC inhibition of MCs also exhibited short-term plasticity, pronounced in the 5-20 Hz range corresponding to investigative sniffing frequency ranges. In paired SAC and olfactory nerve electrical stimulations, the SAC to MC synapse was able to markedly suppress MC spiking. These data suggest that short-term plasticity across investigative sniffing ranges may differentially regulate intra- and interglomerular inhibitory circuits to dynamically shape glomerular output signals to downstream targets.NEW & NOTEWORTHY Short-term plasticity is a fundamental synaptic property that modulates synaptic strength based on preceding activity of the synapse. In rodent olfaction, sensory input arrives episodically driven by sniffing rates ranging from quiescent respiration (1-2 Hz) through to investigative sniffing (5-10 Hz). Here we show that glomerular inhibitory networks are exquisitely sensitive to input frequencies and exhibit plasticity proportional to investigative sniffing frequencies. This indicates that olfactory glomerular circuits are dynamically modulated by episodic sniffing input.

Dopamine transients do not act as model-free prediction errors during associative learning

Dopamine neurons are proposed to signal the reward prediction error in model-free reinforcement learning algorithms. This term represents the unpredicted or 'excess' value of the rewarding event, value that is then added to the intrinsic value of any antecedent cues, contexts or events. To support this proposal, proponents cite evidence that artificially-induced dopamine transients cause lasting changes in behavior. Yet these studies do not generally assess learning under conditions where an endogenous prediction error would occur. Here, to address this, we conducted three experiments where we optogenetically activated dopamine neurons while rats were learning associative relationships, both with and without reward. In each experiment, the antecedent cues failed to acquire value and instead entered into associations with the later events, whether valueless cues or valued rewards. These results show that in learning situations appropriate for the appearance of a prediction error, dopamine transients support associative, rather than model-free, learning.

Risky decision-making predicts dopamine release dynamics in nucleus accumbens shell

The risky decision-making task (RDT) measures risk-taking in a rat model by assessing preference between a small, safe reward and a large reward with increasing risk of punishment (mild foot shock). It is well-established that dopaminergic drugs modulate risk-taking; however, little is known about how differences in baseline phasic dopamine signaling drive individual differences in risk preference. Here, we used in vivo fixed potential amperometry in male Long-Evans rats to test if phasic nucleus accumbens shell (NACs) dopamine dynamics are associated with risk-taking. We observed a positive correlation between medial forebrain bundle-evoked dopamine release in the NACs and risky decision-making, suggesting that risk-taking is associated with elevated dopamine sensitivity. Moreover, "risk-taking" subjects were found to demonstrate greater phasic dopamine release than "risk-averse" subjects. Risky decision-making also predicted enhanced sensitivity to the dopamine reuptake inhibitor nomifensine, and elevated autoreceptor function. Importantly, this hyperdopaminergic phenotype was selective for risky decision-making, as delay discounting performance was not predictive of phasic dopamine release or dopamine supply. These data identify phasic NACs dopamine release as a possible therapeutic target for alleviating the excessive risk-taking observed across multiple forms of psychopathology.

Nucleus accumbens medium spiny neurons subtypes signal both reward and aversion

Deficits in decoding rewarding (and aversive) signals are present in several neuropsychiatric conditions such as depression and addiction, emphasising the importance of studying the underlying neural circuits in detail. One of the key regions of the reward circuit is the nucleus accumbens (NAc). The classical view on the field postulates that NAc dopamine receptor D1-expressing medium spiny neurons (D1-MSNs) convey reward signals, while dopamine receptor D2-expressing MSNs (D2-MSNs) encode aversion. Here, we show that both MSN subpopulations can drive reward and aversion, depending on their neuronal stimulation pattern. Brief D1- or D2-MSN optogenetic stimulation elicited positive reinforcement and enhanced cocaine conditioning. Conversely, prolonged activation induced aversion, and in the case of D2-MSNs, decreased cocaine conditioning. Brief stimulation was associated with increased ventral tegmenta area (VTA) dopaminergic tone either directly (for D1-MSNs) or indirectly via ventral pallidum (VP) (for D1- and D2-MSNs). Importantly, prolonged stimulation of either MSN subpopulation induced remarkably distinct electrophysiological effects in these target regions. We further show that blocking κ-opioid receptors in the VTA (but not in VP) abolishes the behavioral effects induced by D1-MSN prolonged stimulation. In turn, blocking δ-opioid receptors in the VP (but not in VTA) blocks the behavioral effects elicited by D2-MSN prolonged stimulation. Our findings demonstrate that D1- and D2-MSNs can bidirectionally control reward and aversion, explaining the existence of controversial studies in the field, and highlights that the proposed striatal functional opposition needs to be reconsidered.

Dysfunction of homeostatic control of dopamine by astrocytes in the developing prefrontal cortex leads to cognitive impairments

Astrocytes orchestrate neural development by powerfully coordinating synapse formation and function and, as such, may be critically involved in the pathogenesis of neurodevelopmental abnormalities and cognitive deficits commonly observed in psychiatric disorders. Here, we report the identification of a subset of cortical astrocytes that are competent for regulating dopamine (DA) homeostasis during postnatal development of the prefrontal cortex (PFC), allowing for optimal DA-mediated maturation of excitatory circuits. Such control of DA homeostasis occurs through the coordinated activity of astroglial vesicular monoamine transporter 2 (VMAT2) together with organic cation transporter 3 and monoamine oxidase type B, two key proteins for DA uptake and metabolism. Conditional deletion of VMAT2 in astrocytes postnatally produces loss of PFC DA homeostasis, leading to defective synaptic transmission and plasticity as well as impaired executive functions. Our findings show a novel role for PFC astrocytes in the DA modulation of cognitive performances with relevance to psychiatric disorders.

Ventromedial hypothalamus glucose-inhibited neurones: A role in glucose and energy homeostasis?

The ventromedial hypothalamus (VMH) plays a complex role in glucose and energy homeostasis. The VMH is necessary for the counter-regulatory response to hypoglycaemia (CRR) that increases hepatic gluconeogenesis to restore euglycaemia. On the other hand, the VMH also restrains hepatic glucose production during euglycaemia and stimulates peripheral glucose uptake. The VMH is also important for the ability of oestrogen to increase energy expenditure. This latter function is mediated by VMH modulation of the lateral/perifornical hypothalamic area (lateral/perifornical hypothalamus) orexin neurones. Activation of VMH AMP-activated protein kinase (AMPK) is necessary for the CRR. By contrast, VMH AMPK inhibition favours decreased basal glucose levels and is required for oestrogen to increase energy expenditure. Specialised VMH glucose-sensing neurones confer the ability to sense and respond to changes in blood glucose levels. Glucose-excited (GE) neurones increase and glucose-inhibited (GI) neurones decrease their activity as glucose levels rise. VMH GI neurones, in particular, appear to be important in the CRR, although a role for GE neurones cannot be discounted. AMPK mediates glucose sensing in VMH GI neurones suggesting that, although activation of these neurones is important for the CRR, it is necessary to silence them to lower basal glucose levels and enable oestrogen to increase energy expenditure. In support of this, we found that oestrogen reduces activation of VMH GI neurones in low glucose by inhibiting AMPK. In this review, we present the evidence underlying the role of the VMH in glucose and energy homeostasis. We then discuss the role of VMH glucose-sensing neurones in mediating these effects, with a strong emphasis on oestrogenic regulation of glucose sensing and how this may affect glucose and energy homeostasis.

Neural reinforcement: re-entering and refining neural dynamics leading to desirable outcomes

How do organisms learn to do again, on-demand, a behavior that led to a desirable outcome? Dopamine-dependent cortico-striatal plasticity provides a framework for learning behavior's value, but it is less clear how it enables the brain to re-enter desired behaviors and refine them over time. Reinforcing behavior is achieved by re-entering and refining the neural patterns that produce it. We review studies using brain-machine interfaces which reveal that reinforcing cortical population activity requires cortico-basal ganglia circuits. Then, we propose a formal framework for how reinforcement in cortico-basal ganglia circuits acts on the neural dynamics of cortical populations. We propose two parallel mechanisms: i) fast reinforcement which selects the inputs that permit the re-entrance of the particular cortical population dynamics which naturally produced the desired behavior, and ii) slower reinforcement which leads to refinement of cortical population dynamics and more reliable production of neural trajectories driving skillful behavior on-demand.

Dopamine metabolism by a monoamine oxidase mitochondrial shuttle activates the electron transport chain

Monoamine oxidase (MAO) metabolizes cytosolic dopamine (DA), thereby limiting auto-oxidation, but is also thought to generate cytosolic hydrogen peroxide (H₂O₂). We show that MAO metabolism of DA does not increase cytosolic H₂O₂ but leads to mitochondrial electron transport chain (ETC) activity. This is dependent upon MAO anchoring to the outer mitochondrial membrane and shuttling electrons through the intermembrane space to support the bioenergetic demands of phasic DA release.

Phasic dopamine responses to a food-predictive cue are suppressed by the glucagon-like peptide-1 receptor agonist Exendin-4

Phasic dopamine activity is evoked by reliable predictors of food reward and plays a role in cue-triggered, goal-directed behavior. While this important signal is modulated by physiological state (e.g. hunger, satiety), the mechanisms by which physiological state is integrated by dopamine neurons is only beginning to be elucidated. Activation of central receptors for glucagon-like peptide-1 (GLP-1R) via long-acting agonists (e.g., Exendin-4) suppresses food intake and food-directed motivated behavior, in part, through action in regions with dopamine cell bodies, terminals, and/or neural populations that directly target the mesolimbic dopamine system. However, the effects of GLP-1R activation on cue-evoked, phasic dopamine signaling remain unknown. Here, in vivo fiber photometry was used to capture real-time signaling dynamics selectively from dopamine neurons in the ventral tegmental area of male and female transgenic (tyrosine hydroxylase-Cre; TH:Cre+) rats trained to associate an audio cue with the brief availability of a sucrose solution. Cue presentation evoked a brief spike in dopamine activity. Administration of Exendin-4 (Ex4; 0, 0.05, 0.1 μg) to the lateral ventricle both dose-dependently suppressed sucrose-directed behaviors and the magnitude of cue-evoked dopamine activity. Moreover, the amplitude of cue evoked dopamine activity was significantly correlated with subsequent sucrose-directed behaviors. While female rats exhibited overall reduced dopamine responses to the sucrose-paired cue relative to males, there was no significant interaction with Ex4. Together, these findings support a role for central GLP-1Rs in modulating a form of dopamine signaling that influences approach behavior and provide a potential mechanism whereby GLP-1 suppresses food-directed behaviors.

Leaky Optoelectrical Fiber for Optogenetic Stimulation and Electrochemical Detection of Dopamine Exocytosis from Human Dopaminergic Neurons

In Parkinson's disease, the degeneration of dopaminergic neurons in substantia nigra leads to a decrease in the physiological levels of dopamine in striatum. The existing dopaminergic therapies effectively alleviate the symptoms, albeit they do not revert the disease progression and result in significant adverse effects. Transplanting dopaminergic neurons derived from stem cells could restore dopamine levels without additional motor complications. However, the transplanted cells disperse in vivo and it is not possible to stimulate them on demand to modulate dopamine release to prevent dyskinesia. In order to address these issues, this paper presents a multifunctional leaky optoelectrical fiber for potential neuromodulation and as a cell substrate for application in combined optogenetic stem cell therapy. Pyrolytic carbon coated optical fibers are laser ablated to pattern micro-optical windows to permit light leakage over a large area. The pyrolytic carbon acts as an excellent electrode for the electrochemical detection of dopamine. Human neural stem cells are genetically modified to express the light sensitive opsin channelrhodopsin-2 and are differentiated into dopaminergic neurons on the leaky optoelectrical fiber. Finally, light leaking from the micro-optical windows is used to stimulate the dopaminergic neurons resulting in the release of dopamine that is detected in real-time using chronoamperometry.

β2* nAChRs on VTA dopamine and GABA neurons separately mediate nicotine aversion and reward

Evidence shows that the neurotransmitter dopamine mediates the rewarding effects of nicotine and other drugs of abuse, while nondopaminergic neural substrates mediate the negative motivational effects. β2* nicotinic acetylcholine receptors (nAChR) are necessary and sufficient for the experience of both nicotine reward and aversion in an intra-VTA (ventral tegmental area) self-administration paradigm. We selectively reexpressed β2* nAChRs in VTA dopamine or VTA γ-amino-butyric acid (GABA) neurons in β2^-/- mice to double-dissociate the aversive and rewarding conditioned responses to nicotine in nondependent mice, revealing that β2* nAChRs on VTA dopamine neurons mediate nicotine's conditioned aversive effects, while β2* nAChRs on VTA GABA neurons mediate the conditioned rewarding effects in place-conditioning paradigms. These results stand in contrast to a purely dopaminergic reward theory, leading to a better understanding of the neurobiology of nicotine motivation and possibly to improved therapeutic treatments for smoking cessation.

The neuroscience of unmet social needs

John Cacioppo has compared loneliness to hunger or thirst in that it signals that one needs to act and repair what is lacking. This paper reviews Cacioppo's and others' contributions to our understanding of neural mechanisms underlying social motivation in humans and in other social species. We focus particularly on the dopaminergic reward system and try to integrate evidence from animal models and human research. In rodents, objective social isolation leads to increased social motivation, mediated by the brains' mesolimbic dopamine system. In humans, social rejection can lead to either increased or decreased social motivation, and is associated with activity in the insular cortex; while chronic loneliness is typically associated with decreased social motivation but has been associated with altered dopaminergic responses in the striatum. This mixed pattern of cross-species similarities and differences may arise from the substantially different methods used to study unmet social needs across species, and suggests the need for more direct and deliberate cross-species comparative research in this critically important domain.

Frontostriatal and Dopamine Markers of Individual Differences in Reinforcement Learning: A Multi-modal Investigation

Prior studies have shown that dopamine (DA) functioning in frontostriatal circuits supports reinforcement learning (RL), as phasic DA activity in ventral striatum signals unexpected reward and may drive coordinated activity of striatal and orbitofrontal regions that support updating of action plans. However, the nature of DA functioning in RL is complex, in particular regarding the role of DA clearance in RL behavior. Here, in a multi-modal neuroimaging study with healthy adults, we took an individual differences approach to the examination of RL behavior and DA clearance mechanisms in frontostriatal learning networks. We predicted that better RL would be associated with decreased striatal DA transporter (DAT) availability and increased intrinsic functional connectivity among DA-rich frontostriatal regions. In support of these predictions, individual differences in RL behavior were related to DAT binding potential in ventral striatum and resting-state functional connectivity between ventral striatum and orbitofrontal cortex. Critically, DAT binding potential had an indirect effect on reinforcement learning behavior through frontostriatal connectivity, suggesting potential causal relationships across levels of neurocognitive functioning. These data suggest that individual differences in DA clearance and frontostriatal coordination may serve as markers for RL, and suggest directions for research on psychopathologies characterized by altered RL.

Neurocircuitry of Reward and Addiction: Potential Impact of Dopamine-Glutamate Co-release as Future Target in Substance Use Disorder

Dopamine-glutamate co-release is a unique property of midbrain neurons primarily located in the ventral tegmental area (VTA). Dopamine neurons of the VTA are important for behavioral regulation in response to rewarding substances, including natural rewards and addictive drugs. The impact of glutamate co-release on behaviors regulated by VTA dopamine neurons has been challenging to probe due to lack of selective methodology. However, several studies implementing conditional knockout and optogenetics technologies in transgenic mice have during the past decade pointed towards a role for glutamate co-release in multiple physiological and behavioral processes of importance to substance use and abuse. In this review, we discuss these studies to highlight findings that may be critical when considering mechanisms of importance for prevention and treatment of substance abuse.

Contemporary strategies for dissecting the neuronal basis of neurodevelopmental disorders

Great efforts in clinical and basic research have shown progress in understanding the neurobiological mechanisms of neurodevelopmental disorders, such as autism, schizophrenia, and attention-deficit hyperactive disorders. Literature on this field have suggested that these disorders are affected by the complex interaction of genetic, biological, psychosocial and environmental risk factors. However, this complexity of interplaying risk factors during neurodevelopment has prevented a complete understanding of the causes of those neuropsychiatric symptoms. Recently, with advances in modern high-resolution neuroscience methods, the neural circuitry analysis approach has provided new solutions for understanding the causal relationship between dysfunction of a neural circuit and behavioral alteration in neurodevelopmental disorders. In this review we will discuss recent progress in developing novel optogenetic and chemogenetic strategies to investigate neurodevelopmental disorders.

Age-dependent differences on neurochemistry and behavior in rats raised with low and high levels of maternal care

In laboratory rats, naturally-occurring variations in maternal care have been used to study the neurobehavioral consequences of maternal nursing and to model the early-life adversity associated with many psychiatric disorders. This study aimed to determine the role of maternal care on behavior and monoamine concentrations at the prepubertal and young adulthood ages. We observed the licking/grooming (LG) behavior of Sprague-Dawley (SD) dams and assigned the litter to either low (LLG) or high (HLG) LG groups. Behavioral testing in the male offspring consisted of the open-field test, the elevated plus-maze, and the forced swimming test. Afterward, neurotransmitters contents were measured in the prefrontal cortex, the nucleus accumbens, the amygdala, and the hippocampus. We found that at the prepubertal stage, the effects of maternal care were only noticeable in the elevated plus-maze and the serotonin concentration in the nucleus accumbens. At adulthood, body weight and monoamines contents increased substantially in LLG rats. Specifically, they showed higher serotonin contents with a reduced turnover in almost all brain regions, followed by higher contents of norepinephrine and dopamine, especially in the nucleus accumbens. Changes in monoamines concentrations seem to be independent of the behavioral phenotype shaped by variations in maternal care, as behavioral effects were somewhat weak in both experiments. If higher monoamines contents in LLG rats represent an adaptive mechanism to deal with further adverse events, the behavioral paradigms used here were insufficiently challenging to bring out noticeable differences, at least in SD rats.

Learning, memory and consolidation mechanisms for behavioral control in hierarchically organized cortico-basal ganglia systems

This article aims to provide a synthesis on the question how brain structures cooperate to accomplish hierarchically organized behaviors, characterized by low‐level, habitual routines nested in larger sequences of planned, goal‐directed behavior. The functioning of a connected set of brain structures—prefrontal cortex, hippocampus, striatum, and dopaminergic mesencephalon—is reviewed in relation to two important distinctions: (a) goal‐directed as opposed to habitual behavior and (b) model‐based and model‐free learning. Recent evidence indicates that the orbitomedial prefrontal cortices not only subserve goal‐directed behavior and model‐based learning, but also code the “landscape” (task space) of behaviorally relevant variables. While the hippocampus stands out for its role in coding and memorizing world state representations, it is argued to function in model‐based learning but is not required for coding of action–outcome contingencies, illustrating that goal‐directed behavior is not congruent with model‐based learning. While the dorsolateral and dorsomedial striatum largely conform to the dichotomy between habitual versus goal‐directed behavior, ventral striatal functions go beyond this distinction. Next, we contextualize findings on coding of reward‐prediction errors by ventral tegmental dopamine neurons to suggest a broader role of mesencephalic dopamine cells, viz. in behavioral reactivity and signaling unexpected sensory changes. We hypothesize that goal‐directed behavior is hierarchically organized in interconnected cortico‐basal ganglia loops, where a limbic‐affective prefrontal‐ventral striatal loop controls action selection in a dorsomedial prefrontal–striatal loop, which in turn regulates activity in sensorimotor‐dorsolateral striatal circuits. This structure for behavioral organization requires alignment with mechanisms for memory formation and consolidation. We propose that frontal corticothalamic circuits form a high‐level loop for memory processing that initiates and temporally organizes nested activities in lower‐level loops, including the hippocampus and the ripple‐associated replay it generates. The evidence on hierarchically organized behavior converges with that on consolidation mechanisms in suggesting a frontal‐to‐caudal directionality in processing control.

Re-examining the role of ventral tegmental area dopaminergic neurons in motor activity and reinforcement by chemogenetic and optogenetic manipulation in mice

The precise contributions of ventral tegmental area (VTA) dopaminergic (DAergic) neurons to reward-related behaviors are a longstanding hot topic of debate. Whether the activity of VTA DAergic neurons directly modulates rewarding behaviors remains uncertain. In the present study, we investigated the fundamental role of VTA DAergic neurons in reward-related movement and reinforcement by employing dopamine transporter (DAT)-Cre transgenic mice expressing hM3Dq, hM4Di or channelrhodopsin 2 (ChR2) in VTA DAergic neurons through Cre-inducible adeno-associated viral vector transfection. On the one hand, locomotion was tested in an open field to examine motor activity when VTA DAergic neurons were stimulated or inhibited by injection of the hM3Dq or hM4Di ligand clozapine-N-oxide (CNO), respectively. CNO injection to selectively activate or inhibit VTA DAergic neurons significantly increased or decreased locomotor activity, respectively, compared with vehicle injection, indicating that VTA DAergic neuron stimulation is directly involved in the regulation of motor activity. On the other hand, we used the optical intracranial self-stimulation (oICSS) model to investigate the causal link between reinforcement and VTA DAergic neurons. Active poking behavior but not inactive poking behavior was significantly escalated in a frequency- and pulse duration-dependent manner. In addition, microdialysis revealed that the concentration of dopamine (DA) in the nucleus accumbens (NAc) was enhanced by selective optogenetic activation of VTA DAergic neurons. Furthermore, systemic administration of a DA D1 receptor antagonist significantly decreased oICSS reinforcement. Our research profoundly demonstrates a direct regulatory role of VTA DAergic neurons in movement and reinforcement and provides meaningful guidance for the development of novel treatment strategies for neuropsychiatric diseases related to the malfunction of the reward system.

Recent advances in understanding the role of phasic dopamine activity

The latest animal neurophysiology has revealed that the dopamine reward prediction error signal drives neuronal learning in addition to behavioral learning and reflects subjective reward representations beyond explicit contingency. The signal complies with formal economic concepts and functions in real-world consumer choice and social interaction. An early response component is influenced by physical impact, reward environment, and novelty but does not fully code prediction error. Some dopamine neurons are activated by aversive stimuli, which may reflect physical stimulus impact or true aversiveness, but they do not seem to code general negative value or aversive prediction error. The reward prediction error signal is complemented by distinct, heterogeneous, smaller and slower changes reflecting sensory and motor contributors to behavioral activation, such as substantial movement (as opposed to precise motor control), reward expectation, spatial choice, vigor, and motivation. The different dopamine signals seem to defy a simple unifying concept and should be distinguished to better understand phasic dopamine functions.

Central leptin signaling transmits positive valence

Hunger resulting from food deprivation is associated with negative affect. This is supported by recent evidence showing that hunger-sensitive neurons drive feeding through a negative valence teaching signal. However, the complementary hypothesis that hormonal signals of energy surfeit counteract this negative valence, or even transmit positive valence, has received less attention. The adipose-derived hormone leptin signals in proportion to fat mass, is an indicator of energy surplus, and reduces food intake. Here, we showed that centrally-delivered leptin reduced food intake and conditioned a place preference in food-restricted as well as ad libitum fed rats. In contrast, leptin did not reduce food intake nor condition a place preference in obese rats, likely due to leptin resistance. Despite a well-known role for hindbrain leptin receptor signaling in energy balance control, hindbrain leptin delivery did not condition a place preference in food-restricted rats, suggesting that leptin acting in midbrain or forebrain sites mediates place preference conditioning. Supporting the hypothesis that leptin signaling induces a positive affective state, leptin also decreased the threshold for ventral tegmental area brain stimulation reward. Together, these data suggest that leptin signaling is intrinsically preferred, and support the view that signals of energy surfeit are associated with positive affect. Harnessing the positive valence of signals such as leptin may attenuate the negative affect associated with hunger, providing a compelling new approach for weight loss maintenance.

Ventral midbrain stimulation induces perceptual learning and cortical plasticity in primates

Practice improves perception and enhances neural representations of trained visual stimuli, a phenomenon known as visual perceptual learning (VPL). While attention to task-relevant stimuli plays an important role in such learning, Pavlovian stimulus-reinforcer associations are sufficient to drive VPL, even subconsciously. It has been proposed that reinforcement facilitates perceptual learning through the activation of neuromodulatory centers, but this has not been directly confirmed in primates. Here, we paired task-irrelevant visual stimuli with microstimulation of a dopaminergic center, the ventral tegmental area (VTA), in macaques. Pairing VTA microstimulation with a task-irrelevant visual stimulus increased fMRI activity and improved classification of fMRI activity patterns selectively for the microstimulation-paired stimulus. Moreover, pairing VTA microstimulation with a task-irrelevant visual stimulus improved the subject’s capacity to discriminate that stimulus. This is the first causal demonstration of the role of neuromodulatory centers in VPL in primates.

Practice can improve the perception of stimuli used to achieve a task (perceptual learning). Here, the authors show in monkeys that perceptual learning can be produced even for irrelevant stimuli if the stimuli are paired with stimulation of a dopaminergic centre, the ventral tegmental area (VTA).

Striatal circuits for reward learning and decision-making

The striatum is essential for learning which actions lead to reward and for implementing those actions. Decades of experimental and theoretical work have led to several influential theories and hypotheses about how the striatal circuit mediates these functions. However, owing to technical limitations, testing these hypotheses rigorously has been difficult. In this Review, we briefly describe some of the classic ideas of striatal function. We then review recent studies in rodents that take advantage of optical and genetic methods to test these classic ideas by recording and manipulating identified cell types within the circuit. This new body of work has provided experimental support of some longstanding ideas about the striatal circuit and has uncovered critical aspects of the classic view that are incorrect or incomplete.

Nicotine and alcohol: the role of midbrain dopaminergic neurons in drug reinforcement

Nicotine and alcohol addiction are leading causes of preventable death worldwide and continue to constitute a huge socio-economic burden. Both nicotine and alcohol perturb the brain's mesocorticolimbic system. Dopamine (DA) neurons projecting from the ventral tegmental area (VTA) to multiple downstream structures, including the nucleus accumbens, prefrontal cortex, and amygdala, are highly involved in the maintenance of healthy brain function. VTA DA neurons play a crucial role in associative learning and reinforcement. Nicotine and alcohol usurp these functions, promoting reinforcement of drug taking behaviors. In this review, we will first describe how nicotine and alcohol individually affect VTA DA neurons by examining how drug exposure alters the heterogeneous VTA microcircuit and network-wide projections. We will also examine how coadministration or previous exposure to nicotine or alcohol may augment the reinforcing effects of the other. Additionally, this review briefly summarizes the role of VTA DA neurons in nicotine, alcohol, and their synergistic effects in reinforcement and also addresses the remaining questions related to the circuit-function specificity of the dopaminergic system in mediating nicotine/alcohol reinforcement and comorbidity.

Cerebellar modulation of the reward circuitry and social behavior

The cerebellum has been implicated in a number of nonmotor mental disorders such as autism spectrum disorder, schizophrenia, and addiction. However, its contribution to these disorders is not well understood. In mice, we found that the cerebellum sends direct excitatory projections to the ventral tegmental area (VTA), one of the brain regions that processes and encodes reward. Optogenetic activation of the cerebello-VTA projections was rewarding and, in a three-chamber social task, these projections were more active when the animal explored the social chamber. Intriguingly, activity in the cerebello-VTA pathway was required for the mice to show social preference in this task. Our data delineate a major, previously unappreciated role for the cerebellum in controlling the reward circuitry and social behavior.

Biophysical Properties of Somatic and Axonal Voltage-Gated Sodium Channels in Midbrain Dopaminergic Neurons

Spiking activities of midbrain dopaminergic neurons are critical for key brain functions including motor control and affective behaviors. Voltage-gated Na⁺ channels determine neuronal excitability and action potential (AP) generation. Previous studies on dopaminergic neuron excitability mainly focused on Na⁺ channels at the somatodendritic compartments. Properties of axonal Na⁺ channels, however, remain largely unknown. Using patch-clamp recording from somatic nucleated patches and isolated axonal blebs from the axon initial segment (AIS) of dopaminergic neurons in mouse midbrain slices, we found that AIS channel density is approximately 4–9 fold higher than that at the soma. Similar voltage dependence of channel activation and inactivation was observed between somatic and axonal channels in both SNc and VTA cells, except that SNc somatic channels inactivate at more hyperpolarized membrane potentials (V_m). In both SNc and VTA, axonal channels take longer time to inactivate at a subthreshold depolarization V_m level, but are faster to recover from inactivation than somatic channels. Moreover, we found that immunosignals of Nav1.2 accumulate at the AIS of dopaminergic neurons. In contrast, Nav1.1 and Nav1.6 immunosignals are not detectible. Together, our results reveal a high density of Na⁺ channels at the AIS and their molecular identity. In general, somatic and axonal channels of both SNc and VTA dopaminergic neurons share similar biophysical properties. The relatively delayed inactivation onset and faster recovery from inactivation of axonal Na⁺ channels may ensure AP initiation at high frequencies and faithful signal conduction along the axon.

Cell-type and projection-specific dopaminergic encoding of aversive stimuli in addiction

Drug addiction is a major public health concern across the world for which there are limited treatment options. In order to develop new therapies to correct the behavioral deficits that result from repeated drug use, we need to understand the neural circuit dysfunction that underlies the pathophysiology of the disorder. Because the initial reinforcing effects of drugs are dependent on increases in dopamine in reward-related brain regions such as the mesolimbic dopamine pathway, a large focus of addiction research has centered on the dysregulation of this system and its control of positive reinforcement and motivation. However, in addition to the processing of positive, rewarding stimuli, there are clear deficits in the encoding and valuation of information about potential negative outcomes and how they control decision making and motivation. Further, aversive stimuli can motivate or suppress behavior depending on the context in which they are encountered. We propose a model where rewarding and aversive information guides the execution of specific motivated actions through mesocortical and mesolimbic dopamine acting on D1- and D2- receptor containing neuronal populations. Volitional drug exposure alters the processing of rewarding and aversive stimuli through remodeling of these dopaminergic circuits, causing maladaptive drug seeking, self-administration in the face of negative consequences, and drug craving. Together, this review discusses the dysfunction of the circuits controlling different types of aversive learning as well as how these guide specific discrete behaviors, and provides a conceptual framework for how they should be considered in preclinical addiction models.

The NeuroD6 Subtype of VTA Neurons Contributes to Psychostimulant Sensitization and Behavioral Reinforcement

Reward-related behavior is complex and its dysfunction correlated with neuropsychiatric illness. Dopamine (DA) neurons of the ventral tegmental area (VTA) have long been associated with different aspects of reward function, but it remains to be disentangled how distinct VTA DA neurons contribute to the full range of behaviors ascribed to the VTA. Here, a recently identified subtype of VTA neurons molecularly defined by NeuroD6 (NEX1M) was addressed. Among all VTA DA neurons, less than 15% were identified as positive for NeuroD6. In addition to dopaminergic markers, sparse NeuroD6 neurons expressed the vesicular glutamate transporter 2 (Vglut2) gene. To achieve manipulation of NeuroD6 VTA neurons, NeuroD6(NEX)-Cre-driven mouse genetics and optogenetics were implemented. First, expression of vesicular monoamine transporter 2 (VMAT2) was ablated to disrupt dopaminergic function in NeuroD6 VTA neurons. Comparing Vmat2^{lox/lox;NEX-Cre} conditional knock-out (cKO) mice with littermate controls, it was evident that baseline locomotion, preference for sugar and ethanol, and place preference upon amphetamine-induced and cocaine-induced conditioning were similar between genotypes. However, locomotion upon repeated psychostimulant administration was significantly elevated above control levels in cKO mice. Second, optogenetic activation of NEX-Cre VTA neurons was shown to induce DA release and glutamatergic postsynaptic currents within the nucleus accumbens. Third, optogenetic stimulation of NEX-Cre VTA neurons in vivo induced significant place preference behavior, while stimulation of VTA neurons defined by Calretinin failed to cause a similar response. The results show that NeuroD6 VTA neurons exert distinct regulation over specific aspects of reward-related behavior, findings that contribute to the current understanding of VTA neurocircuitry.