GABAergic and glutamatergic efferents of the mouse ventral tegmental area

Chronic fluoxetine treatment desensitizes serotoninergic inhibition of GABA inputs and the intrinsic excitability of dorsal raphe serotonin neurons

Dorsal raphe serotonin (5-hydroxytryptamine, 5-HT) neurons are spontaneously active and release 5-HT that is critical to normal brain function such mood and emotion. Serotonin reuptake inhibitors (SSRIs) increase the synaptic and extracellular 5-HT level and are effective in treating depression. Treatment of two weeks or longer is often required for SSRIs to exert clinical benefits. The cellular mechanism underlying this delay was not fully understood. Here we show that the GABAergic inputs inhibit the spike firing of raphe 5-HT neurons; this GABAergic regulation was reduced by 5-HT, which was prevented by G-protein-activated inwardly rectifying potassium (Girk) channel inhibitor tertiapin-Q, indicating a contribution of 5-HT activation of Girk channels in GABAergic presynaptic axon terminals. Equally important, after 14 days of treatment of fluoxetine, a widely used SSRI type antidepressant, this 5-HT inhibition of GABAergic inputs was substantially downregulated. Furthermore, the chronic fluoxetine treatment substantially downregulated the 5-HT activation of the inhibitory Girk current in 5-HT neurons. Taken together, our results suggest that chronic fluoxetine administration, by blocking 5-HT reuptake and hence increasing the extracellular 5-HT level, can downregulate the function of 5-HT1B receptors on the GABAergic afferent axon terminals synapsing onto 5-HT neurons, allowing extrinsic, behaviorally important GABA neurons to more effectively influence 5-HT neurons; simultaneously, chronic fluoxetine treatment also downregulate somatic 5-HT autoreceptor-activated Girk channel-mediated hyperpolarization and decrease in input resistance and intrinsic excitability, rendering 5-HT neurons resistant to autoinhibition and leading to increased 5-HT neuron activity, potentially contributing to the antidepressant effect of SSRIs.

Connexin-36-positive gap junctions in ventral tegmental area GABA neurons sustain opiate dependence

Drug dependence is characterized by a switch in motivation wherein a positively reinforcing substance can become negatively reinforcing. Put differently, drug use can transform from a form of pleasure-seeking to a form of relief-seeking. Ventral tegmental area (VTA) GABA neurons form an anatomical point of divergence between two double dissociable pathways that have been shown to be functionally implicated and necessary for these respective motivations to seek drugs. The tegmental pedunculopontine nucleus (TPP) is necessary for opiate conditioned place preferences (CPP) in previously drug-naïve rats and mice, whereas dopaminergic (DA) transmission in the nucleus accumbens (NAc) is necessary for opiate CPP in opiate-dependent and withdrawn (ODW) rats and mice. Here, we show that this switch in functional anatomy is contingent upon the gap junction-forming protein, connexin-36 (Cx36), in VTA GABA neurons. Intra-VTA infusions of the Cx36 blocker, mefloquine, in ODW rats resulted in a reversion to a drug-naïve-like state wherein the TPP was necessary for opiate CPP and where opiate withdrawal aversions were lost. Consistent with these data, conditional knockout mice lacking Cx36 in GABA neurons (GAD65-Cre;Cx36 fl(CFP)/fl(CFP)) exhibited a perpetual drug-naïve-like state wherein opiate CPP was always DA independent, and opiate withdrawal aversions were absent even in mice subjected to an opiate dependence and withdrawal induction protocol. Further, viral-mediated rescue of Cx36 in VTA GABA neurons was sufficient to restore their susceptibility to an ODW state wherein opiate CPP was DA dependent. Our findings reveal a functional role for VTA gap junctions that has eluded prevailing circuit models of addiction.

Phasic dopamine signals are reduced in the spontaneously hypertensive rat and increased by methylphenidate

The spontaneously hypertensive rat (SHR) is a selectively bred animal strain that is frequently used to model attention-deficit hyperactivity disorder (ADHD) because of certain genetically determined behavioural characteristics. To test the hypothesis that the characteristically altered response to positive reinforcement in SHRs may be due to altered phasic dopamine response to reward, we measured phasic dopamine signals in the SHRs and Sprague Dawley (SD) rats using in vivo fast-scan cyclic voltammetry. The effects of the dopamine reuptake inhibitor, methylphenidate, on these signals were also studied. Phasic dopamine signals during the pairing of a sensory cue with electrical stimulation of midbrain dopamine neurons were significantly smaller in the SHRs than in the SD rats. Over repeated pairings, the dopamine response to the sensory cue increased, whereas the response to the electrical stimulation of dopamine neurons decreased, similarly in both strains. However, the final amplitude of the response to the sensory cue after pairing was significantly smaller in SHRs than in the SD rats. Methylphenidate increased responses to sensory cues to a significantly greater extent in the SHRs than in the SD rats, due largely to differences in the low dose effect. At a higher dose, methylphenidate increased responses to sensory cues and electrical stimulation similarly in SHRs and SD rats. The smaller dopamine responses may explain the reduced salience of reward-predicting cues previously reported in the SHR, whereas the action of methylphenidate on the cue response suggests a potential mechanism for the therapeutic effects of low-dose methylphenidate in ADHD.

Mesoaccumbal glutamate neurons drive reward via glutamate release but aversion via dopamine co-release

Ventral tegmental area (VTA) projections to the nucleus accumbens (NAc) drive reward-related motivation. Although dopamine neurons are predominant, a substantial glutamatergic projection is also present, and a subset of these co-release both dopamine and glutamate. Optogenetic stimulation of VTA glutamate neurons not only supports self-stimulation but can also induce avoidance behavior, even in the same assay. Here, we parsed the selective contribution of glutamate or dopamine co-release from VTA glutamate neurons to reinforcement and avoidance. We expressed channelrhodopsin-2 (ChR2) in mouse VTA glutamate neurons in combination with CRISPR-Cas9 to disrupt either the gene encoding vesicular glutamate transporter 2 (VGLUT2) or tyrosine hydroxylase (Th). Selective disruption of VGLUT2 abolished optogenetic self-stimulation but left real-time place avoidance intact, whereas CRISPR-Cas9 deletion of Th preserved self-stimulation but abolished place avoidance. Our results demonstrate that glutamate release from VTA glutamate neurons is positively reinforcing but that dopamine release from VTA glutamate neurons can induce avoidance behavior.

Amygdalo-nigral inputs target dopaminergic and GABAergic neurons in the primate: a view from dendrites and soma

The central nucleus (CeN) of the amygdala is an important afferent to the DA system that mediates motivated learning. We previously found that CeN terminals in nonhuman primates primarily overlap the elongated lateral VTA (parabrachial pigmented nucleus, PBP, A10), and retrorubral field(A8) subregion. Here, we examined CeN afferent contacts on cell somata and proximal dendrites of DA and GABA neurons, and distal dendrites of each, using confocal and electron microscopy (EM) methods, respectively. At the soma/proximal dendrites, the proportion of TH+ and GAD1+ cells receiving at least one CeN afferent contact was surprisingly similar (TH = 0.55: GAD1=0.55 in PBP; TH = 0.56; GAD1 =0.51 in A8), with the vast majority of contacted TH+ and GAD1+ soma/proximal dendrites received 1-2 contacts. Similar numbers of tracer-labeled terminals also contacted TH-positive and GAD1-positive small dendrites and/or spines (39% of all contacted dendrites were either TH- or GAD1-labeled). Overall, axon terminals had more symmetric (putative inhibitory) axonal contacts with no difference in the relative distribution in the PBP versus A8, or onto TH+ versus GAD1+ dendrites/spines in either region. The striking uniformity in the amygdalonigral projection across the PBP-A8 terminal field suggests that neither neurotransmitter phenotype nor midbrain location dictates likelihood of a terminal contact. We discuss how this afferent uniformity can play out in recently discovered differences in DA:GABA cell densities between the PBP and A8, and affect specific outputs.

Significance statement

The amygdala's central nucleus (CeN) channels salient cues to influence both appetitive and aversive responses via DA outputs. In higher species, the broad CeN terminal field overlaps the parabrachial pigmented nucleus ('lateral A10') and the retrorubral field (A8). We quantified terminal contacts in each region on DA and GABAergic soma/proximal dendrites and small distal dendrites. There was striking uniformity in contacts on DA and GABAergic cells, regardless of soma and dendritic compartment, in both regions. Most contacts were symmetric (putative inhibitory) with little change in the ratio of inhibitory to excitatory contacts by region.We conclude that post-synaptic shifts in DA-GABA ratios are key to understanding how these relatively uniform inputs can produce diverse effects on outputs.

The role of the GABAergic cells of the median raphe region in reinforcement-based learning

Learning and memory are important in everyday life as well as in pathological conditions. The median raphe region (MRR) contributes to memory formation; however, its precise role and the neurotransmitters involved have yet to be elucidated. To address this issue, we stimulated the MRR neurons of mice by chemogenetic technique and studied them in the operant conditioning and active avoidance tests. The virus carrier infected a variety of neuron types including both GABAergic and glutamatergic ones. Behavior was not influenced by stimulation. We hypothesize that the lack of effect was due to opposing effects exerted via GABAergic and glutamatergic neurons. Therefore, next we used VGAT-Cre mice that allowed the specific manipulation of MRR-GABAergic neurons. The stimulation did not affect behavior in the learning phase of the operant conditioning task, but increased reward preference and total responses when operant contingencies were reversed. The enhanced responsiveness might be a proclivity to impulsive behavior. Stimulation facilitated learning in the active avoidance test but did not affect reversal learning in this paradigm. Our findings suggest that MRR-GABAergic neurons are involved in both learning and reversal learning, but the type of learning that is affected depends on the task.

Nicotine addiction: More than just dopamine

Despite decades of research and anti-tobacco messaging, nicotine addiction remains an important public health problem leading to hundreds of thousands of deaths each year. While fundamental studies have identified molecular, circuit-level and behavioral mechanisms important for nicotine reinforcement and withdrawal, recent studies have identified additional pathways that are important for both nicotine seeking and aversion. In particular, although dopaminergic mechanisms are necessary for nicotine-dependent reward and drug-seeking, novel glutamate and GABA signaling mechanisms in the mesolimbic system have been identified for their contributions to reward-related behaviors. An additional area of active investigation for nicotine addiction focuses on molecular mechanisms in the habenula-interpeduncular pathway driving nicotine aversion and withdrawal. Across all these domains, sex differences in the molecular basis of nicotine-induced behaviors have emerged that identify important new directions for future research. Recent studies reviewed here highlight additional pathways that could provide therapeutic targets for smoking cessation and problematic nicotine vaping.

Ventral tegmental area glutamate neurons establish a mu-opioid receptor gated circuit to mesolimbic dopamine neurons and regulate opioid-seeking behavior

A two-neuron model of ventral tegmental area (VTA) opioid function classically involves VTA GABA neuron regulation of VTA dopamine neurons via a mu-opioid receptor dependent inhibitory circuit. However, this model predates the discovery of a third major type of neuron in the VTA: glutamatergic neurons. We found that about one-quarter of VTA neurons expressing the mu-opioid receptor are glutamate neurons without molecular markers of GABA co-release. Glutamate-Mu opioid receptor neurons are largely distributed in the anterior VTA. The majority of remaining VTA mu-opioid receptor neurons are GABAergic neurons that are mostly within the posterior VTA and do not express molecular markers of glutamate co-release. Optogenetic stimulation of VTA glutamate neurons resulted in excitatory currents recorded from VTA dopamine neurons that were reduced by presynaptic activation of the mu-opioid receptor ex vivo, establishing a local mu-opioid receptor dependent excitatory circuit from VTA glutamate neurons to VTA dopamine neurons. This VTA glutamate to VTA dopamine pathway regulated dopamine release to the nucleus accumbens through mu-opioid receptor activity in vivo. Behaviorally, VTA glutamate calcium-related neuronal activity increased following oral oxycodone consumption during self-administration and response-contingent oxycodone-associated cues during abstinent reinstatement of drug-seeking behavior. Further, chemogenetic inhibition of VTA glutamate neurons reduced abstinent oral oxycodone-seeking behavior in male but not female mice. These results establish 1) a three-neuron model of VTA opioid function involving a mu-opioid receptor gated VTA glutamate neuron pathway to VTA dopamine neurons that controls dopamine release within the nucleus accumbens, and 2) that VTA glutamate neurons participate in opioid-seeking behavior.

GABAergic modulation of sleep-wake states

Benzodiazepine, a classical medication utilized in the treatment of insomnia, operates by augmenting the activity of the GABAA receptor. This underscores the significance of GABAergic neurotransmission in both the initiation and maintenance of sleep. Nevertheless, an increasing body of evidence substantiates the notion that GABA-mediated neurotransmission also assumes a vital role in promoting wakefulness in specific neuronal circuits. Despite the longstanding belief in the pivotal function of GABA in regulating the sleep-wake cycle, there exists a dearth of comprehensive documentation regarding the specific regions within the central nervous system where GABAergic neurons are crucial for these functions. In this review, we delve into the involvement of GABAergic neurons in the regulation of sleep-wake cycles, with particular focus on those located in the preoptic area (POA) and ventral tegmental area (VTA). Recent research, including our own, has further underscored the importance of GABAergic neurotransmission in these areas for the regulation of sleep-wake cycles.

Atypical antipsychotics antagonize GABAA receptors in the ventral tegmental area GABA neurons to relieve psychotic behaviors

Psychosis is an abnormal mental condition that can cause patients to lose contact with reality. It is a common symptom of schizophrenia, bipolar disorder, sleep deprivation, and other mental disorders. Clinically, antipsychotic medications, such as olanzapine and clozapine, are very effective in treatment for psychosis. To investigate the neural circuit mechanism that is affected by antipsychotics and identify more selective therapeutic targets, we employed a strategy by using these effective antipsychotics to identify antipsychotic neural substrates. We observed that local injection of antipsychotics into the ventral tegmental area (VTA) could reverse the sensorimotor gating defects induced by MK-801 injection in mice. Using in vivo fiber photometry, electrophysiological techniques, and chemogenetics, we found that antipsychotics could activate VTA gamma-aminobutyric acid (GABA) neurons by blocking GABAA receptors. Moreover, we found that the VTAGABA nucleus accumbens (NAc) projection was crucially involved in such antipsychotic effects. In summary, our study identifies a novel therapeutic target for the treatment of psychosis and underscores the utility of a 'bedside-to-bench' approach for identifying neural circuits that influence psychotic disorders.

Rewarding-unrewarding prediction signals under a bivalent context in the primate lateral hypothalamus

Animals can expect rewards under equivocal situations. The lateral hypothalamus (LH) is thought to process motivational information by producing valence signals of reward and punishment. Despite rich studies using rodents and non-human primates, these signals have been assessed separately in appetitive and aversive contexts; therefore, it remains unclear what information the LH encodes in equivocal situations. To address this issue, macaque monkeys were conditioned under a bivalent context in which reward and punishment were probabilistically delivered, in addition to appetitive and aversive contexts. The monkeys increased approaching behavior similarly in the bivalent and appetitive contexts as the reward probability increased. They increased avoiding behavior under the bivalent and aversive contexts as the punishment probability increased, but the mean frequency was lower under the bivalent context than under the aversive context. The population activity correlated with these mean behaviors. Moreover, the LH produced fine prediction signals of reward expectation, uncertainty, and predictability consistently in the bivalent and appetitive contexts by recruiting context-independent and context-dependent subpopulations of neurons, while it less produced punishment signals in the aversive and bivalent contexts. Further, neural ensembles encoded context information and "rewarding-unrewarding" and "reward-punishment" valence. These signals may motivate individuals robustly in equivocal environments.

Differential role of GABAergic and cholinergic ventral pallidal neurons in behavioral despair, conditioned fear memory and active coping

The ventral pallidum (VP), a major component of the reward circuit, is well-associated with appetitive behaviors. Recent evidence suggests that this basal forebrain nucleus may have an overarching role in affective processing, including behavioral responses to aversive stimuli. We investigated this by utilizing selective immunotoxin lesions and a series of behavioral tests in adult male Wistar rats. We made bilateral GAT1-Saporin, 192-IgG-Saporin or PBS (vehicle) injections into the VP to respectively eliminate GABAergic and cholinergic neurons, and tested the animals in the forced swim test (FST), open field test (OFT), elevated plus maze (EPM), Morris water maze (MWM) and cued fear conditioning. Both GAT1-Saporin and 192-IgG-Saporin injections reduced behavioral despair without altering general locomotor activity. During the acquisition phase of cued fear conditioning, this antidepressant effect was accompanied by reduced freezing and increased darting in the 192-IgG-Saporin group, and increased jumping in the GAT1-Saporin group. In the extinction phase, cholinergic lesions impaired fear memory irrespective of the context, while GABAergic lesions reduced memory durability only during the early phases of extinction in a novel context. In line with this, selective cholinergic, but not GABAergic, lesions impaired spatial memory in the MWM. We observed no consistent effect in anxiety-like behavior assessed in the OFT and EPM. These findings indicate that both the GABAergic and cholinergic neuronal groups of the VP may contribute to emotion regulation through modulation of behavioral despair and acquired fear by suppressing active coping and promoting species-specific passive behaviors.

Cav1.3 L-type Ca2+ channel-activated CaMKII/ERK2 pathway in the ventral tegmental area is required for cocaine conditioned place preference

We have previously demonstrated that pharmacological blockade of ventral tegmental area (VTA) Cav1.3 L-type calcium channels (LTCCs) using Cav1.2 dihydropyridine insensitive (Cav1.2DHP-/-) mutant mice attenuates cocaine conditioned place preference (CPP). However, the molecular mechanisms by which Cav1.3 channels mediate the effects of cocaine in the VTA remain largely unknown. In this study using Cav1.2DHP-/- male mice, we find that cocaine place preference increases CaM kinase IIα, ERK2, and CREB phosphorylation in the VTA, proteins strongly linked to cocaine behaviors. To further explore the causal role of these intracellular signaling proteins in cocaine preference, the CaM kinase II inhibitor, KN93 was directly injected into the VTA of male mice before each cocaine conditioning session. We found that KN93 attenuates conditioned preference for cocaine compared to vehicle treated mice and decreased VTA ERK2 and CREB phosphorylation. Additionally, blockade of the ERK pathway with the MEK inhibitor, U0126 or knockdown of ERK2 using siRNA, attenuated cocaine preference and VTA CREB phosphorylation but not CaMKIIα phosphorylation, suggesting that ERK is activated downstream of CaMKIIα. Examination of postsynaptic density (PSD) GluA1 subunit of AMPA receptors in the nucleus accumbens (NAc) that we have previously shown to be upregulated following long withdrawal periods, was blunted by KN93, U0126 and ERK2 siRNA when examined 30 days following cocaine CPP. Taken together, these findings demonstrate that Cav1.3 channels in the VTA are required for cocaine reward behavior and activation of the CaMKIIα/ERK/CREB signaling pathway in the VTA is necessary for long-lasting changes in the NAc. This article is part of the Special Issue on 'L-type calcium channel mechanisms in neuropsychiatric disorders'.

Ventral pallidal regulation of motivated behaviors and reinforcement

The interconnected nuclei of the ventral basal ganglia have long been identified as key regulators of motivated behavior, and dysfunction of this circuit is strongly implicated in mood and substance use disorders. The ventral pallidum (VP) is a central node of the ventral basal ganglia, and recent studies have revealed complex VP cellular heterogeneity and cell- and circuit-specific regulation of reward, aversion, motivation, and drug-seeking behaviors. Although the VP is canonically considered a relay and output structure for this circuit, emerging data indicate that the VP is a central hub in an extensive network for reward processing and the regulation of motivation that extends beyond classically defined basal ganglia borders. VP neurons respond temporally faster and show more advanced reward coding and prediction error processing than neurons in the upstream nucleus accumbens, and regulate the activity of the ventral mesencephalon dopamine system. This review will summarize recent findings in the literature and provide an update on the complex cellular heterogeneity and cell- and circuit-specific regulation of motivated behaviors and reinforcement by the VP with a specific focus on mood and substance use disorders. In addition, we will discuss mechanisms by which stress and drug exposure alter the functioning of the VP and produce susceptibility to neuropsychiatric disorders. Lastly, we will outline unanswered questions and identify future directions for studies necessary to further clarify the central role of VP neurons in the regulation of motivated behaviors. Significance: Research in the last decade has revealed a complex cell- and circuit-specific role for the VP in reward processing and the regulation of motivated behaviors. Novel insights obtained using cell- and circuit-specific interrogation strategies have led to a major shift in our understanding of this region. Here, we provide a comprehensive review of the VP in which we integrate novel findings with the existing literature and highlight the emerging role of the VP as a linchpin of the neural systems that regulate motivation, reward, and aversion. In addition, we discuss the dysfunction of the VP in animal models of neuropsychiatric disorders.

Molecular, Circuit, and Stress Response Characterization of Ventral Pallidum Npas1-Neurons

Altered activity of the ventral pallidum (VP) underlies disrupted motivation in stress and drug exposure. The VP is a very heterogeneous structure composed of many neuron types with distinct physiological properties and projections. Neuronal PAS 1-positive (Npas1+) VP neurons are thought to send projections to brain regions critical for motivational behavior. While Npas1+ neurons have been characterized in the globus pallidus external, there is limited information on these neurons in the VP. To address this limitation, we evaluated the projection targets of the VP Npas1+ neurons and performed RNA-sequencing on ribosome-associated mRNA from VP Npas1+ neurons to determine their molecular identity. Finally, we used a chemogenetic approach to manipulate VP Npas1+ neurons during social defeat stress (SDS) and behavioral tasks related to anxiety and motivation in Npas1-Cre mice. We used a similar approach in females using the chronic witness defeat stress (CWDS). We identified VP Npas1+ projections to the nucleus accumbens, ventral tegmental area, medial and lateral habenula, lateral hypothalamus, thalamus, medial and lateral septum, and periaqueductal gray area. VP Npas1+ neurons displayed distinct translatome representing distinct biological processes. Chemogenetic activation of hM3D(Gq) receptors in VP Npas1+ neurons increased susceptibility to a subthreshold SDS and anxiety-like behavior in the elevated plus maze and open field while the activation of hM4D(Gi) receptors in VP Npas1+ neurons enhanced resilience to chronic SDS and CWDS. Thus, the activity of VP Npas1+ neurons modulates susceptibility to social stressors and anxiety-like behavior. Our studies provide new information on VP Npas1+ neuron circuitry, molecular identity, and their role in stress response.SIGNIFICANCE STATEMENT The ventral pallidum (VP) is a structure connected to both reward-related and aversive brain centers. It is a key brain area that signals the hedonic value of natural rewards. Disruption in the VP underlies altered motivation in stress and substance use disorder. However, VP is a very heterogeneous area with multiple neuron subtypes. This study characterized the projection pattern and molecular signatures of VP Neuronal PAS 1-positive (Npas1+) neurons. We further used tools to alter receptor signaling in VP Npas1+ neurons in stress to demonstrate a role for these neurons in stress behavioral outcomes. Our studies have implications for understanding brain cell type identities and their role in brain disorders, such as depression, a serious disorder that is precipitated by stressful events.

α2δ-1 protein drives opioid-induced conditioned reward and synaptic NMDA receptor hyperactivity in the nucleus accumbens

Glutamate NMDA receptors (NMDARs) in the nucleus accumbens (NAc) are critically involved in drug dependence and reward. α2δ-1 is a newly discovered NMDAR-interacting protein that promotes synaptic trafficking of NMDARs independently of its conventional role as a calcium channel subunit. However, it remains unclear how repeated opioid exposure affects synaptic NMDAR activity and α2δ-1-NMDAR interaction in the NAc. In this study, whole-cell patch-clamp recordings showed that repeated treatment with morphine in mice markedly increased the NMDAR-mediated frequency of miniature excitatory postsynaptic currents (mEPSCs) and amplitude of puff NMDAR currents in medium spiny neurons in the NAc core region. Morphine treatment significantly increased the physical interaction of α2δ-1 with GluN1 and their synaptic trafficking in the NAc. In Cacna2d1 knockout mice, repeated treatment with morphine failed to increase the frequency of mEPSCs and amplitude of puff NMDAR currents in the NAc core. Furthermore, inhibition of α2δ-1 with gabapentin or disruption of the α2δ-1-NMDAR interaction with the α2δ-1 C terminus-interfering peptide blocked the morphine-elevated frequency of mEPSCs and amplitude of puff NMDAR currents in the NAc core. Correspondingly, systemically administered gabapentin, Cacna2d1 ablation, or microinjection of the α2δ-1 C terminus-interfering peptide into the NAc core attenuated morphine-induced conditioned place preference and locomotor sensitization. Our study reveals that repeated opioid exposure strengthens presynaptic and postsynaptic NMDAR activity in the NAc via α2δ-1. The α2δ-1-bound NMDARs in the NAc have a key function in the rewarding effect of opioids and could be targeted for treating opioid use disorder and addiction.

Median raphe serotonergic neurons projecting to the interpeduncular nucleus control preference and aversion

Appropriate processing of reward and aversive information is essential for survival. Although a critical role of serotonergic neurons in the dorsal raphe nucleus (DRN) in reward processing has been shown, the lack of rewarding effects with selective serotonin reuptake inhibitors (SSRIs) implies the presence of a discrete serotonergic system playing an opposite role to the DRN in the processing of reward and aversive stimuli. Here, we demonstrated that serotonergic neurons in the median raphe nucleus (MRN) of mice process reward and aversive information in opposite directions to DRN serotonergic neurons. We further identified MRN serotonergic neurons, including those projecting to the interpeduncular nucleus (5-HT^MRN→IPN), as a key mediator of reward and aversive stimuli. Moreover, 5-HT receptors, including 5-HT_2A receptors in the interpeduncular nucleus, are involved in the aversive properties of MRN serotonergic neural activity. Our findings revealed an essential function of MRN serotonergic neurons, including 5-HT^MRN→IPN, in the processing of reward and aversive stimuli.

Neural circuits provide insights into reward and aversion

Maladaptive changes in the neural circuits associated with reward and aversion result in some common symptoms, such as drug addiction, anxiety, and depression. Historically, the study of these circuits has been hampered by technical limitations. In recent years, however, much progress has been made in understanding the neural mechanisms of reward and aversion owing to the development of technologies such as cell type-specific electrophysiology, neuronal tracing, and behavioral manipulation based on optogenetics. The aim of this paper is to summarize the latest findings on the mechanisms of the neural circuits associated with reward and aversion in a review of previous studies with a focus on the ventral tegmental area (VTA), nucleus accumbens (NAc), and basal forebrain (BF). These findings may inform efforts to prevent and treat mental illnesses associated with dysfunctions of the brain's reward and aversion system.

Activity of a direct VTA to ventral pallidum GABA pathway encodes unconditioned reward value and sustains motivation for reward

Dopamine signaling from the ventral tegmental area (VTA) plays critical roles in reward-related behaviors, but less is known about the functions of neighboring VTA GABAergic neurons. We show here that a primary target of VTA GABA projection neurons is the ventral pallidum (VP). Activity of VTA-to-VP-projecting GABA neurons correlates consistently with size and palatability of the reward and does not change following cue learning, providing a direct measure of reward value. Chemogenetic stimulation of this GABA projection increased activity of a subset of VP neurons that were active while mice were seeking reward. Optogenetic stimulation of this pathway improved performance in a cue-reward task and maintained motivation to work for reward over days. This VTA GABA projection provides information about reward value directly to the VP, likely distinct from the prediction error signal carried by VTA dopamine neurons.

Pharmacological modulation of ventral tegmental area neurons elicits changes in trigeminovascular sensory processing and is accompanied by glycemic changes: Implications for migraine

BACKGROUND

Imaging migraine premonitory studies show increased midbrain activation consistent with the ventral tegmental area, an area involved in pain modulation and hedonic feeding. We investigated ventral tegmental area pharmacological modulation effects on trigeminovascular processing and consequent glycemic levels, which could be involved in appetite changes in susceptible migraine patients.

METHODS

Serotonin and pituitary adenylate cyclase-activating polypeptide receptors immunohistochemistry was performed in ventral tegmental area parabrachial pigmented nucleus of male Sprague Dawley rats. In vivo trigeminocervical complex neuronal responses to dura mater nociceptive electrical stimulation, and facial mechanical stimulation of the ophthalmic dermatome were recorded. Changes in trigeminocervical complex responses following ventral tegmental area parabrachial pigmented nucleus microinjection of glutamate, bicuculline, naratriptan, pituitary adenylate cyclase-activating polypeptide-38 and quinpirole were measured, and blood glucose levels assessed pre- and post-microinjection.

RESULTS

Glutamatergic stimulation of ventral tegmental area parabrachial pigmented nucleus neurons reduced nociceptive and spontaneous trigeminocervical complex neuronal firing. Naratriptan, pituitary adenylate cyclase-activating polypeptide-38 and quinpirole inhibited trigeminovascular spontaneous activity, and trigeminocervical complex neuronal responses to dural-evoked electrical and mechanical noxious stimulation. Trigeminovascular sensory processing through modulation of the ventral tegmental area parabrachial pigmented nucleus resulted in reduced circulating glucose levels.

CONCLUSION

Pharmacological modulation of ventral tegmental area parabrachial pigmented nucleus neurons elicits changes in trigeminovascular sensory processing. The interplay between ventral tegmental area parabrachial pigmented nucleus activity and the sensory processing by the trigeminovascular system may be relevant to understand associated sensory and homeostatic symptoms in susceptible migraine patients.

A preclinical study of deep brain stimulation in the ventral tegmental area for alleviating positive psychotic-like behaviors in mice

Deep brain stimulation (DBS) is a clinical intervention for the treatment of movement disorders. It has also been applied to the treatment of psychiatric disorders such as depression, anorexia nervosa, obsessive-compulsive disorder, and schizophrenia. Psychiatric disorders including schizophrenia, bipolar disorder, and major depression can lead to psychosis, which can cause patients to lose touch with reality. The ventral tegmental area (VTA), located near the midline of the midbrain, is an important region involved in psychosis. However, the clinical application of electrical stimulation of the VTA to treat psychotic diseases has been limited, and related mechanisms have not been thoroughly studied. In the present study, hyperlocomotion and stereotyped behaviors of the mice were employed to mimic and evaluate the positive-psychotic-like behaviors. We attempted to treat positive psychotic-like behaviors by electrically stimulating the VTA in mice and exploring the neural mechanisms behind behavioral effects. Local field potential recording and in vivo fiber photometry to observe the behavioral effects and changes in neural activities caused by DBS in the VTA of mice. Optogenetic techniques were used to verify the neural mechanisms underlying the behavioral effects induced by DBS. Our results showed that electrical stimulation of the VTA activates local gamma-aminobutyric acid (GABA) neurons, and dopamine (DA) neurons, reduces hyperlocomotion, and relieves stereotyped behaviors induced by MK-801 (dizocilpine) injection. The results of optogenetic manipulation showed that the activation of the VTA GABA neurons, but not DA neurons, is involved in the alleviation of hyperlocomotion and stereotyped behaviors. We visualized changes in the activity of specific types in specific brain areas induced by DBS, and explored the neural mechanism of DBS in alleviating positive psychotic-like behaviors. This preclinical study not only proposes new technical means of exploring the mechanism of DBS, but also provides experimental justification for the clinical treatment of psychotic diseases by electrical stimulation of the VTA.

Characterizing and TRAPing a Social Stress-Activated Neuronal Ensemble in the Ventral Tegmental Area

Social stress is a major contributor to neuropsychiatric issues such as depression, substance abuse and eating disorders. The ventral tegmental area (VTA) is involved in the effects of stress on cognitive and emotional processes perturbed in these disorders. However, the VTA is a cellularly heterogeneous brain area and it remains unclear which of its neuronal populations make up the social stress-sensitive ensemble. The current study characterizes the molecular, topographical and functional properties of VTA social stress-activated cells. First, we used immunohistochemical analysis of Fos protein, a marker of recent increased neuronal activity, to show that acute social stress activates a mainly neuronal ensemble in the VTA (VTA_{Social stress} neurons). Topographical analysis showed that this ensemble, which comprises ∼11% of all VTA neurons, occurs across VTA subregions. Further analysis showed that approximately half of the VTA_{Social stress} neurons express the dopamine synthesis rate-limiting enzyme tyrosine hydroxylase (TH). In a minority of cases this occurred with coexpression of vesicular glutamate transporter 2 (Vglut2). Also part of the ensemble were VTA cells expressing just Vglut2 without TH, and cells expressing the vesicular GABA transporter (VGAT) without TH. Next, using targeted recombination in active populations (TRAP2), we showed that VTA_{Social stress} neurons can be permanently tagged and made tractable for future functional investigations. Using a combination of TRAP2 and patch-clamp electrophysiology we demonstrate that VTA_{Social stress} neurons exhibit higher excitability than their non-TRAPed neighbor cells. Overall, this study shows that acute social stress activates an ensemble of neurons throughout the VTA, comprising distinct molecular identities, and with specific electrophysiological features. It also identifies TRAP2 as a tool to make this ensemble tractable for future functional studies.

Unbiased Stereological Estimates of Dopaminergic and GABAergic Neurons in the A10, A9, and A8 Subregions in the Young Male Macaque

The ventral midbrain is the primary source of dopamine- (DA) expressing neurons in most species. GABA-ergic and glutamatergic cell populations are intermixed among DA-expressing cells and purported to regulate both local and long-range dopamine neuron activity. Most work has been conducted in rodent models, however due to evolutionary expansion of the ventral midbrain in primates, the increased size and complexity of DA subpopulations warrants further investigation. Here, we quantified the number of DA neurons, and their GABA-ergic complement in classic DA cell groups A10 (midline ventral tegmental area nuclei [VTA] and parabrachial pigmented nucleus [PBP]), A9 (substantia nigra, pars compacta [SNc]) and A8 (retrorubral field [RRF]) in the macaque. Because the PBP is a disproportionately expanded feature of the A10 group, and has unique connectional features in monkeys, we analyzed A10 data by dividing it into 'classic' midline nuclei and the PBP. Unbiased stereology revealed total putative DA neuron counts to be 210,238 ± 17,127 (A10 = 110,319 ± 9649, A9 = 87,399 ± 7751 and A8 = 12,520 ± 827). Putative GABAergic neurons were fewer overall, and evenly dispersed across the DA subpopulations (GAD67 = 71,215 ± 5663; A10 = 16,836 ± 2743; A9 = 24,855 ± 3144 and A8 = 12,633 ± 3557). Calculating the GAD67/TH ratio for each subregion revealed differential balances of these two cell types across the DA subregions. The A8 subregion had the highest complement of GAD67-positive neurons compared to TH-positive neurons (1:1), suggesting a potentially high capacity for GABAergic inhibition of DA output in this region.

Anatomy and Function of Ventral Tegmental Area Glutamate Neurons

The ventral tegmental area (VTA) is well known for regulating reward consumption, learning, memory, and addiction behaviors through mediating dopamine (DA) release in downstream regions. Other than DA neurons, the VTA is known to be heterogeneous and contains other types of neurons, including glutamate neurons. In contrast to the well-studied and established functions of DA neurons, the role of VTA glutamate neurons is understudied, presumably due to their relatively small quantity and a lack of effective means to study them. Yet, emerging studies have begun to reveal the importance of glutamate release from VTA neurons in regulating diverse behavioral repertoire through a complex intra-VTA and long-range neuronal network. In this review, we summarize the features of VTA glutamate neurons from three perspectives, namely, cellular properties, neural connections, and behavioral functions. Delineation of VTA glutamatergic pathways and their interactions with VTA DA neurons in regulating behaviors may reveal previously unappreciated functions of the VTA in other physiological processes.

Control of Behavioral Arousal and Defense by a Glutamatergic Midbrain-Amygdala Pathway in Mice

In response to external threatening signals, animals evolve a series of defensive behaviors that depend on heightened arousal. It is believed that arousal and defensive behaviors are coordinately regulated by specific neurocircuits in the central nervous system. The ventral tegmental area (VTA) is a key structure located in the ventral midbrain of mice. The activity of VTA glutamatergic neurons has recently been shown to be closely related to sleep–wake behavior. However, the specific role of VTA glutamatergic neurons in sleep–wake regulation, associated physiological functions, and underlying neural circuits remain unclear. In the current study, using an optogenetic approach and synchronous polysomnographic recording, we demonstrated that selective activation of VTA glutamatergic neurons induced immediate transition from sleep to wakefulness and obviously increased the amount of wakefulness in mice. Furthermore, optogenetic activation of VTA glutamatergic neurons induced multiple defensive behaviors, including burrowing, fleeing, avoidance and hiding. Finally, viral-mediated anterograde activation revealed that projections from the VTA to the central nucleus of the amygdala (CeA) mediated the wake- and defense-promoting effects of VTA glutamatergic neurons. Collectively, our results illustrate that the glutamatergic VTA is a key neural substrate regulating wakefulness and defensive behaviors that controls these behaviors through its projection into the CeA. We further discuss the possibility that the glutamatergic VTA-CeA pathway may be involved in psychiatric diseases featuring with excessive defense.

Activation of LH GABAergic inputs counteracts fasting-induced changes in tVTA/RMTG neurons

KEY POINTS

While dopamine neuronal activity changes with motivational state, it is unknown if fasting influences tVTA/RMTg GABAergic neurons, a major inhibitory input to VTA dopamine neurons. In unfasted mice, there were sex differences in inhibitory synaptic transmission onto tVTA/RMTg GABAergic neurons. Activation of LH GABAergic neurons decreases firing of tVTA/RMTg GABAergic neurons through a monosynaptic input. An acute fast decreased the excitability of tVTA/RMTg GABAergic neurons. An acute fast decreases inhibitory synaptic transmission of the LH GABA input to tVTA/RMTg GABAergic neurons in both male and female mice.

ABSTRACT

Dopamine neurons in the ventral tegmental area (VTA) are strongly innervated by GABAergic neurons in the 'tail of the VTA' (tVTA), also known as the rostralmedial tegmental nucleus (RMTg). Disinhibition of dopamine neurons through firing of the GABAergic neurons projecting from the lateral hypothalamus (LH) leads to reward seeking and consumption through dopamine release in the nucleus accumbens. VTA dopamine neurons respond to changes in motivational state, yet less is known of whether tVTA/RMTg GABAergic neurons or the LH GABAergic neurons that project to them are also affected by changes in motivational state, such as fasting. An acute 16 h overnight fast decreased the excitability of tVTA/RMTg GABAergic neurons of male and female mice. In addition, fasting decreased synaptic strength at LH GABA to tVTA/RMTg GABAergic synapses, indicated by reduced amplitude of optically evoked currents, decreased readily releasable pool (RRP) size and replenishment. Optical stimulation of LH GABA terminals suppressed evoked action potentials of tVTA/RMTg GABAergic neurons in unfasted mice, but this effect decreased following fasting. Furthermore, during fasting, LH GABA inputs to tVTA/RMTg neurons maintained functional connectivity during depolarization, as depolarization block was reduced following fasting. Taken together, inhibitory synaptic transmission from LH GABA inputs onto tVTA/RMTg GABAergic neurons decreases following fasting, however ability to functionally inhibit tVTA/RMTg GABAergic neurons is preserved, allowing for possible disinhibition of dopamine neurons and subsequent foraging. Abstract figure legend  The inhibitory synaptic input is represented by the downward arrows. Following fasting, there was a decrease in inhibitory synaptic strength in both males and females. The action potentials represent the excitability, which also decreases in both males and females following fasting. Because both the LH GABA input and excitability of tVTA/RMTg GABA neurons have reduced activity following fasting, we predict that disinhibition of dopamine neurons with stimulation of LH inputs is preserved. This article is protected by copyright. All rights reserved.

Whole-brain opto-fMRI map of mouse VTA dopaminergic activation reflects structural projections with small but significant deviations

Ascending dopaminergic projections from neurons located in the Ventral Tegmental Area (VTA) are key to the etiology, dysfunction, and control of motivation, learning, and addiction. Due to the evolutionary conservation of this nucleus and the extensive use of mice as disease models, establishing an assay for VTA dopaminergic signaling in the mouse brain is crucial for the translational investigation of motivational control as well as of neuronal function phenotypes for diseases and interventions. In this article we use optogenetic stimulation directed at VTA dopaminergic neurons in combination with functional Magnetic Resonance Imaging (fMRI), a method widely used in human deep brain imaging. We present a comprehensive assay producing the first whole-brain opto-fMRI map of dopaminergic activation in the mouse, and show that VTA dopaminergic system function is consistent with its structural VTA projections, diverging only in a few key aspects. While the activation map predominantly highlights target areas according to their relative projection densities (e.g., strong activation of the nucleus accumbens and low activation of the hippocampus), it also includes areas for which a structural connection is not well established (such as the dorsomedial striatum). We further detail the variability of the assay with regard to multiple experimental parameters, including stimulation protocol and implant position, and provide evidence-based recommendations for assay reuse, publishing both reference results and a reference analysis workflow implementation.

Systemic oxytocin increases glutamate efflux in the nucleus accumbens core of cocaine-experienced male and female rats but only increases dopamine efflux in males

Oxytocin attenuates cocaine-seeking when administered both systemically and directly into the nucleus accumbens core. This effect is blocked by intra-accumbens antagonism of mGlu2/3 and, together with our finding that intra-accumbens oxytocin increases glutamate concentrations in this brain region, indicates that pre-synaptic regulation of glutamate release by oxytocin influences cocaine relapse. However, mGlu2/3 receptors also regulate dopamine release in the nucleus accumbens. Here we aimed to determine whether systemic oxytocin increases glutamate and dopamine concentrations in the nucleus accumbens core of cocaine-experienced and cocaine-naïve male and female rats. A subset of rats self-administered cocaine (0.5 mg/kg/infusion) and then underwent extinction training for 2-3 weeks. Rats were implanted with microdialysis probes in the accumbens core and samples were collected for a baseline period, and following saline (1 mL/kg), and oxytocin (1 mg/kg, IP) injections. Locomotion was assessed during microdialysis. In cocaine-experienced rats, oxytocin increased glutamate concentrations in the accumbens core to the same extent in males and females but only increased dopamine concentrations in male rats. Oxytocin did not alter glutamate levels in cocaine-naïve rats. Oxytocin did not produce sedation. These results extend previous findings that systemic oxytocin increases nucleus accumbens dopamine in a sex-specific manner in cocaine-experienced rats. These data are the first to find that systemic oxytocin increases nucleus accumbens glutamate after cocaine experience, providing a mechanism of action by which oxytocin attenuates the reinstatement of cocaine seeking in both male and female rats.

A Whole-Brain Connectivity Map of VTA and SNc Glutamatergic and GABAergic Neurons in Mice

The glutamatergic and GABAergic neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) mediated diverse brain functions. However, their whole-brain neural connectivity has not been comprehensively mapped. Here we used the virus tracers to characterize the whole-brain inputs and outputs of glutamatergic and GABAergic neurons in VTA and SNc. We found that these neurons received similar inputs from upstream brain regions, but some quantitative differences were also observed. Neocortex and dorsal striatum provided a greater share of input to VTA glutamatergic neurons. Periaqueductal gray and lateral hypothalamic area preferentially innervated VTA GABAergic neurons. Specifically, superior colliculus provided the largest input to SNc glutamatergic neurons. Compared to input patterns, the output patterns of glutamatergic and GABAergic neurons in the VTA and SNc showed significant preference to different brain regions. Our results laid the anatomical foundation for understanding the functions of cell-type-specific neurons in VTA and SNc.

Positive modulation of N-methyl-D-aspartate receptors in the mPFC reduces the spontaneous recovery of fear

N-methyl-D-aspartate receptor (NMDAR) modulators have recently received increased attention as potential therapeutics for posttraumatic stress disorder (PTSD). Here, we tested a novel NMDAR-positive modulator, NYX-783, in the following two rodent models of PTSD: an auditory fear-conditioning model and a single-prolonged stress (SPS) model. We examined the ability of NYX-783 to reduce subsequent fear-based behaviors by measuring enhanced fear extinction and reduced spontaneous recovery (spontaneous return of fear) in male mice. NYX-783 administration significantly reduced spontaneous recovery in both PTSD models and enhanced fear extinction in the SPS model. Furthermore, NYX-783 increased the NMDA-induced inward currents of excitatory and inhibitory neurons in the infralimbic medial prefrontal cortex (IL mPFC) and that the GluN2B subunit of NMDARs on pyramidal neurons in the IL mPFC is required for its effect on spontaneous recovery. The downstream expression of brain-derived neurotrophic factor was required for NYX-783 to achieve its behavioral effect. These results elucidate the cellular targets of NYX-783 and the molecular mechanisms underlying the inhibition of spontaneous recovery. These preclinical findings support the hypothesis that NYX-783 may have therapeutic potential for PTSD treatment and may be particularly useful for inhibiting spontaneous recovery.

Mapping the subcortical connectivity of the human default mode network

The default mode network (DMN) mediates self-awareness and introspection, core components of human consciousness. Therapies to restore consciousness in patients with severe brain injuries have historically targeted subcortical sites in the brainstem, thalamus, hypothalamus, basal forebrain, and basal ganglia, with the goal of reactivating cortical DMN nodes. However, the subcortical connectivity of the DMN has not been fully mapped, and optimal subcortical targets for therapeutic neuromodulation of consciousness have not been identified. In this work, we created a comprehensive map of DMN subcortical connectivity by combining high-resolution functional and structural datasets with advanced signal processing methods. We analyzed 7 Tesla resting-state functional MRI (rs-fMRI) data from 168 healthy volunteers acquired in the Human Connectome Project. The rs-fMRI blood-oxygen-level-dependent (BOLD) data were temporally synchronized across subjects using the BrainSync algorithm. Cortical and subcortical DMN nodes were jointly analyzed and identified at the group level by applying a novel Nadam-Accelerated SCAlable and Robust (NASCAR) tensor decomposition method to the synchronized dataset. The subcortical connectivity map was then overlaid on a 7 Tesla 100 µm ex vivo MRI dataset for neuroanatomic analysis using automated segmentation of nuclei within the brainstem, thalamus, hypothalamus, basal forebrain, and basal ganglia. We further compared the NASCAR subcortical connectivity map with its counterpart generated from canonical seed-based correlation analyses. The NASCAR method revealed that BOLD signal in the central lateral nucleus of the thalamus and ventral tegmental area of the midbrain is strongly correlated with that of the DMN. In an exploratory analysis, additional subcortical sites in the median and dorsal raphe, lateral hypothalamus, and caudate nuclei were correlated with the cortical DMN. We also found that the putamen and globus pallidus are negatively correlated (i.e., anti-correlated) with the DMN, providing rs-fMRI evidence for the mesocircuit hypothesis of human consciousness, whereby a striatopallidal feedback system modulates anterior forebrain function via disinhibition of the central thalamus. Seed-based analyses yielded similar subcortical DMN connectivity, but the NASCAR result showed stronger contrast and better spatial alignment with dopamine immunostaining data. The DMN subcortical connectivity map identified here advances understanding of the subcortical regions that contribute to human consciousness and can be used to inform the selection of therapeutic targets in clinical trials for patients with disorders of consciousness.

Neural Substrates for Regulation of Sleep and General Anesthesia

General anesthesia has been successfully used in clinics for over 170 years, but its mechanisms of effect remain unclear. Behaviorally, general anesthesia is similar to sleep as it produces a reversible transition between wakefulness and the state of being unaware of one’s surroundings. A discussion regarding the common circuits of sleep and general anesthesia has been ongoing as an increasing number of sleep-arousal regulatory nuclei are reported to participate in the consciousness shift occurring during general anesthesia. Recently, with progress in research technology, both positive and negative evidence for overlapping neural circuits between sleep and general anesthesia has emerged. This article provides a review of the latest evidence on the neural substrates for sleep and general anesthesia regulation by comparing the roles of pivotal nuclei in sleep and anesthesia.

The circadian clock in the mouse habenula is set by catecholamines

Circadian rhythms are those variations in behavioral and molecular processes of organisms that follow roughly 24 h cycles in the absence of any external cue. The hypothalamic suprachiasmatic nucleus (SCN) harbors the principal brain pacemaker driving circadian rhythms. The epithalamic habenula (Hb) contains a self-sustained circadian clock functionally coupled to the SCN. Anatomically, the Hb projects to the midbrain dopamine (DA) and serotonin (5-HT) systems, and it receives inputs from the forebrain, midbrain, and brainstem. The SCN is set by internal signals such as 5-HT or melatonin from the raphe nuclei and pineal gland, respectively. However, how the Hb clock is set by internal cues is not well characterized. Hence, in the present study, we determined whether DA, noradrenaline (NA), 5-HT, and the neuropeptides orexin (ORX) and vasopressin influence the Hb circadian clock. Using PER2::Luciferase transgenic mice, we found that the amplitude of the PER2 protein circadian oscillations from Hb explants was strongly affected by DA and NA. Importantly, these effects were dose-and region (rostral vs. caudal) dependent for NA, with a main effect in the caudal part of the Hb. Furthermore, ORX also induced a significant change in the amplitude of PER2 protein oscillations in the caudal Hb. In conclusion, catecholaminergic (DA, NA) and ORXergic transmission impacts the clock properties of the Hb clock likely contributing to the circadian regulation of motivated behaviors. Accordingly, pathological conditions that lead in alterations of catecholamine or ORX activity (drug intake, compulsive feeding) might affect the Hb clock and conduct to circadian disturbances.

O-GlcNAcylation in Ventral Tegmental Area Dopaminergic Neurons Regulates Motor Learning and the Response to Natural Reward

Protein O-GlcNAcylation is a post-translational modification that links environmental stimuli with changes in intracellular signal pathways, and its disturbance has been found in neurodegenerative diseases and metabolic disorders. However, its role in the mesolimbic dopamine (DA) system, especially in the ventral tegmental area (VTA), needs to be elucidated. Here, we found that injection of Thiamet G, an O-GlcNAcase (OGA) inhibitor, in the VTA and nucleus accumbens (NAc) of mice, facilitated neuronal O-GlcNAcylation and decreased the operant response to sucrose as well as the latency to fall in rotarod test. Mice with DAergic neuron-specific knockout of O-GlcNAc transferase (OGT) displayed severe metabolic abnormalities and died within 4-8 weeks after birth. Furthermore, mice specifically overexpressing OGT in DAergic neurons in the VTA had learning defects in the operant response to sucrose, and impaired motor learning in the rotarod test. Instead, overexpression of OGT in GABAergic neurons in the VTA had no effect on these behaviors. These results suggest that protein O-GlcNAcylation of DAergic neurons in the VTA plays an important role in regulating the response to natural reward and motor learning in mice.

Central Nervous System Associated With Light Perception and Physiological Responses of Birds

Environmental light that animal receives (i.e., photoperiod and light intensity) has recently been shown that it affects avian central nervous system for the physiological responses to the environment by up or downregulation of dopamine and serotonin activities, and this, in turn, affects the reproductive function and stress-related behavior of birds. In this study, the author speculated on the intriguing possibility that one of the proposed avian deep-brain photoreceptors (DBPs), i.e., melanopsin (Opn4), may play roles in the dual sensory-neurosecretory cells in the hypothalamus, midbrain, and brain stem for the behavior and physiological responses of birds by light. Specifically, the author has shown that the direct light perception of premammillary nucleus dopamine-melatonin (PMM DA-Mel) neurons is associated with the reproductive activation in birds. Although further research is required to establish the functional role of Opn4 in the ventral tegmental area (VTA), dorsal raphe nucleus, and caudal raphe nucleus in the light perception and physiological responses of birds, it is an exciting prospect because the previous results in birds support this hypothesis that Opn4 in the midbrain DA and serotonin neurons may play significant roles on the light-induced welfare of birds.

Ventral pallidum cellular and pathway specificity in drug seeking

The ventral pallidum (VP) is central to the reinforcing effects across a variety of drugs and relapse to drug seeking. Emerging studies from animal models of reinstatement reveal a complex neurobiology of the VP that contributes to different aspects of relapse to drug seeking. This review builds on classical understanding of the VP as part of the final common pathway of relapse but also discusses the properties of the VP as an independent structure. These include VP neural anatomical subregions, cellular heterogeneity, circuitry, neurotransmitters and peptides. Collectively, this review provides a current understanding of the VP from molecular to circuit level architecture that contributes to both the appetitive and aversive symptoms of drug addiction. We show the complex neurobiology of the VP in drug seeking, emphasizing its critical role in addiction, and review strategic approaches that target the VP to reduce relapse rates.

Diverse midbrain dopaminergic neuron subtypes and implications for complex clinical symptoms of Parkinson's disease

Parkinson’s disease (PD), the most common degenerative movement disorder, is clinically manifested with various motor and non-motor symptoms. Degeneration of midbrain substantia nigra pas compacta (SNc) dopaminergic neurons (DANs) is generally attributed to the motor syndrome. The underlying neuronal mechanisms of non-motor syndrome are largely unexplored. Besides SNc, midbrain ventral tegmental area (VTA) DANs also produce and release dopamine and modulate movement, reward, motivation, and memory. Degeneration of VTA DANs also occurs in postmortem brains of PD patients, implying an involvement of VTA DANs in PD-associated non-motor symptoms. However, it remains to be established that there is a distinct segregation of different SNc and VTA DAN subtypes in regulating different motor and non-motor functions, and that different DAN subpopulations are differentially affected by normal ageing or PD. Traditionally, the distinction among different DAN subtypes was mainly based on the location of cell bodies and axon terminals. With the recent advance of single cell RNA sequencing technology, DANs can be readily classified based on unique gene expression profiles. A combination of specific anatomic and molecular markers shows great promise to facilitate the identification of DAN subpopulations corresponding to different behavior modules under normal and disease conditions. In this review, we first summarize the recent progress in characterizing genetically, anatomically, and functionally diverse midbrain DAN subtypes. Then, we provide perspectives on how the preclinical research on the connectivity and functionality of DAN subpopulations improves our current understanding of cell-type and circuit specific mechanisms of the disease, which could be critically informative for designing new mechanistic treatments.

Dysfunction of ventral tegmental area GABA neurons causes mania-like behavior

The ventral tegmental area (VTA), an important source of dopamine, regulates goal- and reward-directed and social behaviors, wakefulness, and sleep. Hyperactivation of dopamine neurons generates behavioral pathologies. But any roles of non-dopamine VTA neurons in psychiatric illness have been little explored. Lesioning or chemogenetically inhibiting VTA GABAergic (VTAVgat) neurons generated persistent wakefulness with mania-like qualities: locomotor activity was increased; sensitivity to D-amphetamine was heightened; immobility times decreased on the tail suspension and forced swim tests; and sucrose preference increased. Furthermore, after sleep deprivation, mice with lesioned VTAVgat neurons did not catch up on lost sleep, even though they were starting from a sleep-deprived baseline, suggesting that sleep homeostasis was bypassed. The mania-like behaviors, including the sleep loss, were reversed by valproate, and re-emerged when treatment was stopped. Lithium salts and lamotrigine, however, had no effect. Low doses of diazepam partially reduced the hyperlocomotion and fully recovered the immobility time during tail suspension. The mania like-behaviors mostly depended on dopamine, because giving D1/D2/D3 receptor antagonists reduced these behaviors, but also partially on VTAVgat projections to the lateral hypothalamus (LH). Optically or chemogenetically inhibiting VTAVgat terminals in the LH elevated locomotion and decreased immobility time during the tail suspension and forced swimming tests. VTAVgat neurons help set an animal's (and perhaps human's) mental and physical activity levels. Inputs inhibiting VTAVgat neurons intensify wakefulness (increased activity, enhanced alertness and motivation), qualities useful for acute survival. In the extreme, however, decreased or failed inhibition from VTAVgat neurons produces mania-like qualities (hyperactivity, hedonia, decreased sleep).

Acetaldehyde Excitation of Lateral Habenular Neurons via Multiple Cellular Mechanisms

Acetaldehyde (ACD), the first metabolite of ethanol, is implicated in several of ethanol's actions, including the reinforcing and aversive effects. The neuronal mechanisms underlying ACD's aversive effect, however, are poorly understood. The lateral habenula (LHb), a regulator of midbrain monoaminergic centers, is activated by negative valence events. Although the LHb has been linked to the aversive responses of several abused drugs, including ethanol, little is known about ACD. We, therefore, assessed ACD's action on LHb neurons in rats. The results showed that intraperitoneal injection of ACD increased cFos protein expression within the LHb and that intra-LHb infusion of ACD induced conditioned place aversion in male rats. Furthermore, electrophysiological recording in brain slices of male and female rats showed that bath application of ACD facilitated spontaneous firing and glutamatergic transmission. This effect of ACD was potentiated by an aldehyde dehydrogenase (ALDH) inhibitor, disulfiram (DS), but attenuated by the antagonists of dopamine (DA) receptor (DAR) subtype 1 (SCH23390) and subtype 2 (raclopride), and partly abolished by the pretreatment of DA or DA reuptake blocker (GBR12935; GBR). Moreover, application of ACD initiated a depolarizing inward current (I _ACD) and enhanced the hyperpolarizing-activated currents in LHb neurons. Bath application of Rp-cAMPs, a selective cAMP-PKA inhibitor, attenuated ACD-induced potentiation of EPSCs and I _ACD Finally, bath application of ZD7288, a selective blocker of hyperpolarization-activated cyclic nucleotide-gated channels, attenuated ACD-induced potentiation of firing, EPSCs, and I _ACD These results show that ACD exerts its aversive property by exciting LHb neurons via multiple cellular mechanisms, and new treatments targeting the LHb may be beneficial for alcoholism.SIGNIFICANCE STATEMENT Acetaldehyde (ACD) has been considered aversive peripherally and rewarding centrally. However, whether ACD has a central aversive property is unclear. Here, we report that ACD excites the lateral habenula (LHb), a brain region associated with aversion and negative valence, through multiple cellular and molecular mechanisms. Intra-LHb ACD produces significant conditioned place aversion. These results suggest that ACD's actions on the LHb neurons might contribute to its central aversive property and new treatments targeting the LHb may be beneficial for alcoholism.

Ventral pallidum DRD3 potentiates a pallido-habenular circuit driving accumbal dopamine release and cocaine seeking

Drugs of abuse induce persistent remodeling of reward circuit function, a process thought to underlie the emergence of drug craving and relapse to drug use. However, how circuit-specific, drug-induced molecular and cellular plasticity can have distributed effects on the mesolimbic dopamine reward system to facilitate relapse to drug use is not fully elucidated. Here, we demonstrate that dopamine receptor D3 (DRD3)-dependent plasticity in the ventral pallidum (VP) drives potentiation of dopamine release in the nucleus accumbens during relapse to cocaine seeking after abstinence. We show that two distinct VP DRD3+ neuronal populations projecting to either the lateral habenula (LHb) or the ventral tegmental area (VTA) display different patterns of activity during drug seeking following abstinence from cocaine self-administration and that selective suppression of elevated activity or DRD3 signaling in the LHb-projecting population reduces drug seeking. Together, our results uncover how circuit-specific DRD3-mediated plasticity contributes to the process of drug relapse.

Neurochemical Signaling of Reward and Aversion to Ventral Tegmental Area Glutamate Neurons

Ventral tegmental area (VTA) glutamate neurons signal and participate in reward and aversion-based behaviors. However, the neurochemical mechanisms that underlie how these neurons contribute to motivated behaviors is unknown. We used a combination of optical sensors to identify how distinct neurochemical inputs to VTA glutamate neurons participate in motivated behavior within female and male transgenic mice. Activity of glutamate inputs to VTA glutamate neurons increased for both reward-predicting and aversion-predicting cues and aversive outcomes, but subpopulations of glutamate inputs were increased or decreased by reward. For both reward and aversion-based cues and outcomes, activity of GABA inputs to VTA glutamate neurons mostly decreased. GCaMP recordings showed overall population increases in VTA glutamate neuron intracellular calcium during reward and aversion-based cues and outcomes. Electrophysiological recordings of VTA VGluT2 neurons showed that glutamate receptor activation increases firing while loss of excitation via glutamate receptor blockade decreases firing. GABA-A receptor activation decreased VTA glutamate neuron firing but GABA-A receptor blockade did not significantly change VTA glutamate neuron firing. Electrophysiological recordings in coordination with our sensor data suggest that glutamate inputs strongly regulate VTA glutamate neuron participation in diverse motivated behaviors.SIGNIFICANCE STATEMENT Glutamate and GABA are the primary excitatory and inhibitory neurotransmitters of the nervous system. However, identifying how these neurotransmitters regulate motivated behavior has remained challenging because of a lack of tools (1) capable of measuring neurotransmission at the temporal scale of motivated behaviors and (2) capable of capturing chemical signaling onto genetically-distinct neuronal populations. We have overcome these obstacles by implementing genetically-encoded fluorescent indicators to monitor both glutamate and GABA input dynamics exclusively to ventral tegmental area (VTA) glutamate neurons during reward and aversion-based behaviors. We identify that glutamate and GABA inputs to VTA glutamate neurons differentially and dynamically signal reward and aversion-based cues and outcomes. This research provides foundational evidence that links distinct neurotransmitters to motivated behaviors regulated by VTA glutamate neurons.

Genetically Targeted Connectivity Tracing Excludes Dopaminergic Inputs to the Interpeduncular Nucleus from the Ventral Tegmentum and Substantia Nigra

The “habenulopeduncular system” consists of the medial habenula (MHb) and its principal target of innervation, the interpeduncular nucleus (IP). Neurons in the ventral MHb (MHbV) express acetylcholine along with glutamate, and both the MHb and IP are rich in nicotinic acetylcholine receptors. Much of the work on this system has focused on nicotinic mechanisms and their clinical implications for nicotine use, particularly because the IP expresses the α5 nicotinic receptor subunit, encoded by the CHRNA5 gene, which is genetically linked to smoking risk. A working model has emerged in which nicotine use may be determined by the balance of reinforcement mediated in part by nicotine effects on dopamine reward pathways, and an aversive “brake” on nicotine consumption encoded in the MHb-IP pathway. However, recent work has proposed that the IP also receives direct dopaminergic input from the ventral tegmental area (VTA). If correct, this would significantly impact the prevailing model of IP function. Here, we have used Chrna5^Cre mice to perform rabies virus-mediated retrograde tracing of global inputs to the IP. We have also used Cre-dependent adeno-associated virus (AAV) anterograde tracing using Slc6a3^Cre (DAT^Cre) mice to map VTA dopaminergic efferents, and we have examined tract-tracing data using other transgenic models for dopaminergic neurons available in a public database. Consistent with the existing literature using non-genetic tracing methods, none of these experiments show a significant anatomic connection from the VTA or substantia nigra (SN) to the IP, and thus do not support a model of direct dopaminergic input to the habenulopeduncular system.

Distinct Signaling by Ventral Tegmental Area Glutamate, GABA, and Combinatorial Glutamate-GABA Neurons in Motivated Behavior

Ventral tegmental area (VTA) neurons play roles in reward and aversion. We recently discovered that the VTA has neurons that co-transmit glutamate and GABA (glutamate-GABA co-transmitting neurons), transmit glutamate without GABA (glutamate-transmitting neurons), or transmit GABA without glutamate (GABA-transmitting neurons). However, the functions of these VTA cell types in motivated behavior are unclear. To identify the functions of these VTA cell types, we combine recombinase mouse lines with INTRSECT2.0 vectors to selectively target these neurons. We find that VTA cell types have unique signaling patterns for reward, aversion, and learned cues. Whereas VTA glutamate-transmitting neurons signal cues predicting reward, VTA GABA-transmitting neurons signal cues predicting the absence of reward, and glutamate-GABA co-transmitting neurons signal rewarding and aversive outcomes without signaling learned cues related to those outcomes. Thus, we demonstrate that genetically defined subclasses of VTA glutamate and GABA neurons signal different aspects of motivated behavior.

Root et al. examine ventral tegmental area (VTA) neurons that release glutamate without GABA, GABA without glutamate, or both glutamate and GABA. Cell types have differential projection densities and unique neuronal activity profiles related to cues predicting rewarding, nonreward, or aversive outcomes and in the receipt of predicted rewards.

Incentive motivation: 'wanting' roles of central amygdala circuitry

The central nucleus of amygdala (CeA) mediates positively-valenced reward motivation as well as negatively-valenced fear. Optogenetic or neurochemical stimulation of CeA circuitry can generate intense incentive motivation to pursue and consume a paired natural food, sex, or addictive drug reward, and even create maladaptive 'wanting what hurts' such as attraction to a shock rod. Evidence indicates CeA stimulations selectively amplify incentive motivation ('wanting') but not hedonic impact ('liking') of the same reward. Further, valence flips can occur for CeA contributions to motivational salience. That is, CeA stimulation can promote either incentive motivation or fearful motivation, even in the same individual, depending on situation. These findings may carry implications for understanding CeA roles in neuropsychiatric disorders involving aberrant motivational salience, ranging from addiction to paranoia and anxiety disorders.

Dopamine Neurons That Cotransmit Glutamate, From Synapses to Circuits to Behavior

Discovered just over 20 years ago, dopamine neurons have the ability to cotransmit both dopamine and glutamate. Yet, the functional roles of dopamine neuron glutamate cotransmission and their implications for therapeutic use are just emerging. This review article encompasses the current body of evidence investigating the functions of dopamine neurons of the ventral midbrain that cotransmit glutamate. Since its discovery in dopamine neuron cultures, further work in vivo confirmed dopamine neuron glutamate cotransmission across species. From there, growing interest has led to research related to neural functioning including roles in synaptic signaling, development, and behavior. Functional connectome mapping reveals robust connections in multiple forebrain regions to various cell types, most notably to cholinergic interneurons in both the medial shell of the nucleus accumbens and the lateral dorsal striatum. Glutamate markers in dopamine neurons reach peak levels during embryonic development and increase in response to various toxins, suggesting dopamine neuron glutamate cotransmission may serve neuroprotective roles. Findings from behavioral analyses reveal prominent roles for dopamine neuron glutamate cotransmission in responses to psychostimulants, in positive valence and cognitive systems and for subtle roles in negative valence systems. Insight into dopamine neuron glutamate cotransmission informs the pathophysiology of neuropsychiatric disorders such as addiction, schizophrenia and Parkinson Disease, with therapeutic implications.

Central amygdala circuits in valence and salience processing

Behavioral responses to environmental stimuli are dictated by the affective valence of the stimulus, good (positive valence) or bad (negative valence). These stimuli can innately elicit an affective response that promotes approach or avoidance behavior. In addition to innately valenced stimuli, valence can also be assigned to initially neutral stimuli through associative learning. A stimulus of a given valence can vary in salience depending on the strength of the stimulus, the underlying state of the animal, and the context of the stimulus presentation. Salience endows the stimulus with the ability to direct attention and elicit preparatory responses to mount an incentive-based motivated behavior. The central nucleus of the amygdala (CeA) has emerged as an early integration point for valence and salience detection to engage preparatory autonomic responses and behavioral posturing in response to both aversive and appetitive stimuli. There are numerous cell types in the CeA that are involved in valence and salience processing through a variety of connections, and we will review the recent progress that has been made in identifying these circuit elements and their roles in these processes.

Activation of VTA GABA neurons disrupts reward seeking by altering temporal processing

The role of ventral tegmental area (VTA) dopamine in reward, cue processing, and interval timing is well characterized. Using a combinatorial viral approach to target activating DREADDs (Designer Receptors Exclusively Activated by Designer Drugs, hM3D) to GABAergic neurons in the VTA of male rats, we previously showed that activation disrupts responding to reward-predictive cues. Here we explored how VTA GABA neurons influence the perception of time in two fixed interval (FI) tasks, one where the reward or interval is not paired with predictive cues (Non-Cued FI), and another where the start of the FI is signaled by a constant tone that continues until the rewarded response is emitted (Cued FI). Under vehicle conditions in both tasks, responding was characterized by "scalloping" over the 30 s FI, in which responding increased towards the end of the FI. However, when VTA GABA neurons were activated in the Non-Cued FI, the time between the end of the 30 s interval and when the rats made a reinforced response increased. Additionally, post-reinforcement pauses and overall session length increased. In the Cued FI task, VTA GABA activation produced erratic responding, with a decrease in earned rewards. Thus, while both tasks were disrupted by VTA GABA activation, responding that is constrained by a cue was more sensitive to this manipulation, possibly due to convergent effects on timing and cue processing. Together these results demonstrate that VTA GABA activity disrupts the perception of interval timing, particularly when the timing is set by cues.

Dopamine Cell Therapy: From Cell Replacement to Circuitry Repair

Cell therapy for Parkinson's disease (PD) is aimed to replace the degenerated midbrain dopamine (mDA) neurons and restore DA neurotransmission in the denervated forebrain targets. A limitation of the intrastriatal grafting approach, which is currently used in clinical trials, is that the mDA neurons are implanted into the target area, in most cases the putamen, and not in the ventral midbrain where they normally reside. This ectopic location of the cells may limit their functionality due to the lack of appropriate afferent regulation from the host. Homotopic transplantation, into the substantia nigra, is now being pursued in rodent PD models as a way to achieve more complete circuitry repair. Intranigral grafts of mDA neurons, derived from human embryonic stem cells, have the capacity to re-establish the nigrostriatal and mesolimbic pathways in their entirety and restore dense functional innervations in striatal, limbic and cortical areas. Tracing of host afferent inputs using the rabies tracing technique shows that the afferent connectivity of grafts implanted in the nigra matches closely that of the intrinsic mDA system, suggesting a degree of circuitry reconstruction that exceeds what has been achieved before. This approach holds great promise, but to match the larger size of the human brain, and the 10 times greater distance between substantia nigra and its forebrain targets, it may be necessary to find ways to improve the growth capacity of the grafted mDA neurons, pointing to a combined approach where growth promoting factors are used to enhance the performance of mDA neuron grafts.

Oxytocin and Addiction: Potential Glutamatergic Mechanisms

Recently, oxytocin (OXT) has been investigated for its potential therapeutic role in addiction. OXT has been found to diminish various drug-seeking and drug-induced behaviors. Although its behavioral effects are well-established, there is not much consensus on how this neuropeptide exerts its effects. Previous research has given thought to how dopamine (DA) may be involved in oxytocinergic mechanisms, but there has not been as strong of a focus on the role that glutamate (Glu) has. The glutamatergic system is critical for the processing of rewards and the disruption of glutamatergic projections produces the behaviors seen in drug addicts. We introduce the idea that OXT has direct effects on Glu transmission within the reward processing pathway. Thus, OXT may reduce addictive behaviors by restoring abnormal drug-induced changes in the glutamatergic system and in its interactions with other neurotransmitters. This review offers insight into the mechanisms through which a potentially viable therapeutic target, OXT, could be used to reduce addiction-related behaviors.