GABA and enkephalin projection from the nucleus accumbens and ventral pallidum to the ventral tegmental area

Sex differences in the structure and function of the vasopressin system in the ventral pallidum are associated with the sex-specific regulation of social play behavior in juvenile rats

Vasopressin (AVP) regulates various social behaviors, often in sex-specific ways, including social play behavior, a rewarding behavior displayed primarily by juveniles. Here, we examined whether and how AVP acting in the brain's reward system regulates social play behavior in juvenile rats. Specifically, we focused on AVP signaling in the ventral pallidum (VP), a brain region that is a part of the reward system. First, we examined the organization of the VP-AVP system in juvenile rats and found sex differences, with higher density of both AVP-immunoreactive fibers and AVP V1a receptor (V1aR) binding in males compared to females while females show a greater number of V1aR-expressing cells compared to males. We further found that, in both sexes, V1aR-expressing cells co-express a GABA marker to a much greater extent (approx. 10 times) than a marker for glutamate. Next, we examined the functional involvement of V1aR-expressing VP cells in social play behavior. We found that exposure to social play enhanced the proportion of activated V1aR-expressing VP cells in males only. Finally, we showed that infusion of a specific V1aR antagonist into the VP increased social play behaviors in juvenile male rats while decreasing these behaviors in juvenile female rats. Overall, these findings reveal structural and functional sex differences in the AVP-V1aR system in the VP that are associated with the sex-specific regulation of social play behavior.

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.

Somatodendritic Release of Cholecystokinin Potentiates GABAergic Synapses Onto Ventral Tegmental Area Dopamine Cells

BACKGROUND

Neuropeptides are contained in nearly every neuron in the central nervous system and can be released not only from nerve terminals but also from somatodendritic sites. Cholecystokinin (CCK), among the most abundant neuropeptides in the brain, is expressed in the majority of midbrain dopamine neurons. Despite this high expression, CCK function within the ventral tegmental area (VTA) is not well understood.

METHODS

We confirmed CCK expression in VTA dopamine neurons through immunohistochemistry and in situ hybridization and detected optogenetically induced CCK release using an enzyme-linked immunosorbent assay. To investigate whether CCK modulates VTA circuit activity, we used whole-cell patch clamp recordings in mouse brain slices. We infused CCK locally in vivo and tested food intake and locomotion in fasted mice. We also used in vivo fiber photometry to measure Ca²⁺ transients in dopamine neurons during feeding.

RESULTS

Here we report that VTA dopamine neurons release CCK from somatodendritic regions, where it triggers long-term potentiation of GABAergic (gamma-aminobutyric acidergic) synapses. The somatodendritic release occurs during trains of optogenetic stimuli or prolonged but modest depolarization and is dependent on synaptotagmin-7 and T-type Ca²⁺ channels. Depolarization-induced long-term potentiation is blocked by a CCK₂ receptor antagonist and mimicked by exogenous CCK. Local infusion of CCK in vivo inhibits food consumption and decreases distance traveled in an open field test. Furthermore, intra-VTA-infused CCK reduced dopamine cell Ca²⁺ signals during food consumption after an overnight fast and was correlated with reduced food intake.

CONCLUSIONS

Our experiments introduce somatodendritic neuropeptide release as a previously unknown feedback regulator of VTA dopamine cell excitability and dopamine-related behaviors.

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.

The role of enkephalinergic systems in substance use disorders

Enkephalin, an endogenous opioid peptide, is highly expressed in the reward pathway and may modulate neurotransmission to regulate reward-related behaviors, such as drug-taking and drug-seeking behaviors. Drugs of abuse also directly increase enkephalin in this pathway, yet it is unknown whether or not changes in the enkephalinergic system after drug administration mediate any specific behaviors. The use of animal models of substance use disorders (SUDs) concurrently with pharmacological, genetic, and molecular tools has allowed researchers to directly investigate the role of enkephalin in promoting these behaviors. In this review, we explore neurochemical mechanisms by which enkephalin levels and enkephalin-mediated signaling are altered by drug administration and interrogate the contribution of enkephalin systems to SUDs. Studies manipulating the receptors that enkephalin targets (e.g., mu and delta opioid receptors mainly) implicate the endogenous opioid peptide in drug-induced neuroadaptations and reward-related behaviors; however, further studies will need to confirm the role of enkephalin directly. Overall, these findings suggest that the enkephalinergic system is involved in multiple aspects of SUDs, such as the primary reinforcing properties of drugs, conditioned reinforcing effects, and sensitization. The idea of dopaminergic-opioidergic interactions in these behaviors remains relatively novel and warrants further research. Continuing work to elucidate the role of enkephalin in mediating neurotransmission in reward circuitry driving behaviors related to SUDs remains crucial.

Choice-selective sequences dominate in cortical relative to thalamic inputs to NAc to support reinforcement learning

How are actions linked with subsequent outcomes to guide choices? The nucleus accumbens, which is implicated in this process, receives glutamatergic inputs from the prelimbic cortex and midline regions of the thalamus. However, little is known about whether and how representations differ across these input pathways. By comparing these inputs during a reinforcement learning task in mice, we discovered that prelimbic cortical inputs preferentially represent actions and choices, whereas midline thalamic inputs preferentially represent cues. Choice-selective activity in the prelimbic cortical inputs is organized in sequences that persist beyond the outcome. Through computational modeling, we demonstrate that these sequences can support the neural implementation of reinforcement-learning algorithms, in both a circuit model based on synaptic plasticity and one based on neural dynamics. Finally, we test and confirm a prediction of our circuit models by direct manipulation of nucleus accumbens input neurons.

Trends in big data analyses by multicenter collaborative translational research in psychiatry

The underlying pathologies of psychiatric disorders, which cause substantial personal and social losses, remain unknown, and their elucidation is an urgent issue. To clarify the core pathological mechanisms underlying psychiatric disorders, in addition to laboratory-based research that incorporates the latest findings, it is necessary to conduct large-sample-size research and verify reproducibility. For this purpose, it is critical to conduct multicenter collaborative research across various fields, such as psychiatry, neuroscience, molecular biology, genomics, neuroimaging, cognitive science, neurophysiology, psychology, and pharmacology. Moreover, collaborative research plays an important role in the development of young researchers. In this respect, the Enhancing Neuroimaging Genetics through Meta-Analysis (ENIGMA) consortium and Cognitive Genetics Collaborative Research Organization (COCORO) have played important roles. In this review, we first overview the importance of multicenter collaborative research and our target psychiatric disorders. Then, we introduce research findings on the pathophysiology of psychiatric disorders from neurocognitive, neurophysiological, neuroimaging, genetic, and basic neuroscience perspectives, focusing mainly on the findings obtained by COCORO. It is our hope that multicenter collaborative research will contribute to the elucidation of the pathological basis of psychiatric disorders.

Dopamine Supersensitivity: A Novel Hypothesis of Opioid-Induced Neurobiological Mechanisms Underlying Opioid-Stimulant Co-use and Opioid Relapse

Emergent harms presented by the co-use of opioids and methamphetamine highlight the broader public health challenge of preventing and treating opioid and stimulant co-use. Development of effective therapeutics requires an understanding of the physiological mechanisms that may be driving co-use patterns, specifically the underlying neurobiology of co-use and how they may facilitate (or be leveraged to prevent) continued use patterns. This narrative review summarizes largely preclinical data that demonstrate clinically-meaningful relationships between the dopamine and opioid systems with direct implications for opioid and stimulant co-use. Synthesized conclusions of this body of research include evidence that changes in the dopamine system occur only once physical dependence to opioids develops, that the chronicity of opioid exposure is associated with the severity of changes, and that withdrawal leaves the organism in a state of substantive dopamine deficit that persists long after the somatic or observed signs of opioid withdrawal appear to have resolved. Evidence also suggests that dopamine supersensitivity develops soon after opioid abstinence and results in increased response to dopamine agonists that increases in magnitude as the abstinence period continues and is evident several weeks into protracted withdrawal. Mechanistically, this supersensitivity appears to be mediated by changes in the sensitivity, not quantity, of dopamine D2 receptors. Here we propose a neural circuit mechanism unique to withdrawal from opioid use with implications for increased stimulant sensitivity in previously stimulant-naïve or inexperienced populations. These hypothesized effects collectively delineate a mechanism by which stimulants would be uniquely reinforcing to persons with opioid physical dependence, would contribute to the acute opioid withdrawal syndrome, and could manifest subjectively as craving and/or motivation to use that could prompt opioid relapse during acute and protracted withdrawal. Preclinical research is needed to directly test these hypothesized mechanisms. Human laboratory and clinical trial research is needed to explore these clinical predictions and to advance the goal of developing treatments for opioid-stimulant co-use and/or opioid relapse prevention and withdrawal remediation.

Reciprocal midbrain-extended amygdala circuit activity in preclinical models of alcohol use and misuse

Alcohol dependence is characterized by a shift in motivation to consume alcohol from positive reinforcement (i.e., increased likelihood of future alcohol drinking based on its rewarding effects) to negative reinforcement (i.e., increased likelihood of future alcohol drinking based on alcohol-induced reductions in negative affective symptoms, including but not limited to those experienced during alcohol withdrawal). The neural adaptations that occur during this transition are not entirely understood. Mesolimbic reinforcement circuitry (i.e., ventral tegmental area [VTA] neurons) is activated during early stages of alcohol use, and may be involved in the recruitment of brain stress circuitry (i.e., extended amygdala) during the transition to alcohol dependence, after chronic periods of high-dose alcohol exposure. Here, we review the literature regarding the role of canonical brain reinforcement (VTA) and brain stress (extended amygdala) systems, and the connections between them, in acute, sub-chronic, and chronic alcohol response. Particular emphasis is placed on preclinical models of alcohol use.

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.

Neuropeptide CART modulates dopamine turnover in the nucleus accumbens: Insights into the anatomy of rewarding circuits

Neuropeptide cocaine- and amphetamine-regulated transcript (CART) is known to influence the activity of the canonical mesolimbic dopaminergic pathway and modulate reward seeking behaviour. CART neurons of the lateral hypothalamus (LH) send afferents to the ventral tegmental area (VTA) and paraventricular thalamic nucleus (PVT) and these nuclei, in turn, send secondary projections to nucleus accumbens. We try to dissect the precise sites of CART's action in these circuits in promoting reward. Rats were implanted with bipolar electrode targeted at the lateral hypothalamus-medial forebrain bundle (LH-MFB) and trained to press the lever through intracranial self-stimulation (ICSS) protocol. CART (55-102) administered directly into posterior VTA (pVTA) or PVT of the conditioned rats significantly increased the number of lever presses, indicating reward-promoting activity of the peptide. Concomitant increase in dopamine (DA) and 3, 4-dihydroxyphenylacetic acid (DOPAC) efflux was noted in the microdialysate collected from the nucleus accumbens shell (AcbSh). On the other hand, immunoneutralization of endogenous CART with CART antibodies injected directly in the pVTA or PVT reduced the lever press activity as well as DA and DOPAC efflux in the AcbSh. Injection of CART (1-39) in pVTA or PVT was ineffective. We suggest that CART cells in the LH-MFB area send afferents to (a) pVTA and influence dopaminergic neurons projecting to AcbSh and (b) PVT, from where the secondary neurons may feed into the AcbSh. Excitation of the CARTergic pathway to the pVTA as well as the PVT seems to promote DA release in the AcbSh and contribute to the generation of reward.

Neonatal programming with sex hormones: Effect on expression of dopamine D1 receptor and neurotransmitters release in nucleus accumbens in adult male and female rats

Steroid sex hormones produce physiological effects in reproductive and non-reproductive tissues, such as the brain. In the brain, sex hormones receptors are expressed in cortical, limbic and midbrain areas modulating memory, arousal, fear and motivation between other behaviors. One neurotransmitters system regulated by sex hormones is dopamine (DA), where during adulthood, sex hormones promote neurophysiological and behavioral effects on DA systems such as tuberoinfundibular (prolactin secretion), nigrostriatal (motor circuit regulation) and mesocorticolimbic (driving of motivated behavior). However, the long-term effects induced by neonatal exposure to sex hormones on DA release induced by D1 receptor activation and its expression in nucleus accumbens (NAcc) have not been fully studied. To answer this question, neurochemical, cellular and molecular techniques were used. The data show sex differences in NAcc DA extracellular levels induced by D1 receptor activation and protein content of this receptor in male and female control rats. In addition, neonatal programming with a single dose of TP increases the NAcc protein content of D1 receptors of adult male and female rats. Our results show new evidence related with sex differences that could explain the dependence to drug of abuse in males and females, which may be associated with increased reinforcing effects of drugs of abuse.

Role of the nucleus accumbens in functional recovery from spinal cord injury

Post brain damage depression impedes functional recovery. On the other hand, higher motivation facilitates functional recovery after damage to the central nervous system, but the neural mechanism of psychological effects on functional recovery is unclear. The nucleus accumbens (NAcc), a motivation center, has not been considered directly involved in motor function. Recently, it was demonstrated that the NAcc makes a direct contribution to motor performance after spinal cord injury by facilitating motor cortex activity. In this perspective, we first summarize our investigation of role of NAcc in motor control during the recovery course after spinal cord injury, followed by a discussion of the current knowledge regarding the relationship between the recovery and NAcc after neuronal damage.

Opposing Regulation of Cocaine Seeking by Glutamate and GABA Neurons in the Ventral Pallidum

Projections from the nucleus accumbens to the ventral pallidum (VP) regulate relapse in animal models of addiction. The VP contains GABAergic (VP_GABA) and glutamatergic (VP_Glu) neurons, and a subpopulation of GABAergic neurons co-express enkephalin (VP_Penk). Rabies tracing reveals that VP_Glu and VP_Penk neurons receive preferential innervation from upstream D1- relative to D2-expressing accumbens neurons. Chemogenetic stimulation of VP_Glu neurons inhibits, whereas stimulation of VP_GABA and VP_Penk neurons potentiates cocaine seeking in mice withdrawn from intravenous cocaine self-administration. Calcium imaging reveals cell type-specific activity patterns when animals learn to suppress drug seeking during extinction training versus engaging in cue-induced cocaine seeking. During cued seeking, VP_GABA neurons increase their overall activity, and VP_Penk neurons are selectively activated around nose pokes for cocaine. In contrast, VP_Glu neurons increase their spike rate following extinction training. These data show that VP subpopulations differentially encode and regulate cocaine seeking, with VP_Penk and VP_GABA neurons facilitating and VP_Glu neurons inhibiting cocaine seeking.

Heinsbroek et al. show that glutamate and GABA neurons in ventral pallidum differentially regulate cued cocaine seeking. Calcium activity in glutamate neurons increases when mice refrain from cocaine seeking. Activating glutamate neurons inhibits cocaine seeking. Calcium activity increases in GABA neurons during cocaine seeking, and activating GABA or enkephalin neurons induces cocaine seeking.

Ventral pallidum neurons dynamically signal relative threat

The ventral pallidum (VP) is anatomically poised to contribute to threat behavior. Recent studies report a VP population that scales firing increases to reward but decreases firing to aversive cues. Here, we tested whether firing decreases in VP neurons serve as a neural signal for relative threat. Single-unit activity was recorded while male rats discriminated cues predicting unique foot shock probabilities. Rats' behavior and VP single-unit firing discriminated danger, uncertainty, and safety cues. Two populations of VP neurons dynamically signaled relative threat, decreasing firing according to foot shock probability during early cue presentation, but disproportionately decreasing firing to uncertain threat as foot shock drew near. One relative threat population increased firing to reward, consistent with a bi-directional signal for general value. The second population was unresponsive to reward, revealing a specific signal for relative threat. The results reinforce anatomy to reveal the VP as a neural source of a dynamic, relative threat signal.

Progress in opioid reward research: From a canonical two-neuron hypothesis to two neural circuits

Opioid abuse and related overdose deaths continue to rise in the United States, contributing to the national opioid crisis in the USA. The neural mechanisms underlying opioid abuse and addiction are still not fully understood. This review discusses recent progress in basic research dissecting receptor mechanisms and circuitries underlying opioid reward and addiction. We first review the canonical GABA-dopamine neuron hypothesis that was upheld for half a century, followed by major findings challenging this hypothesis. We then focus on recent progress in research evaluating the role of the mesolimbic and nigrostriatal dopamine circuitries in opioid reward and relapse. Based on recent findings that activation of dopamine neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) is equally rewarding and that GABA neurons in the rostromedial tegmental nucleus (RMTg) and the substantia nigra pars reticula (SNr) are rich in mu opioid receptors and directly synapse onto midbrain DA neurons, we proposed that the RTMg→VTA → ventrostriatal and SNr → SNc → dorsostriatal pathways may act as the two major neural substrates underlying opioid reward and abuse. Lastly, we discuss possible integrations of these two pathways during initial opioid use, development of opioid abuse and maintenance of compulsive opioid seeking.

Insulin restores the neurochemical effects of nicotine in the mesolimbic pathway of diabetic rats

This study examined whether insulin modulates the neurochemical effects of nicotine in the mesolimbic pathway of diabetic rats. The rats received vehicle or streptozotocin (STZ) to induce hypoinsulinemia. A subset of STZ-treated rats was implanted with insulin pellets that rapidly normalized glucose levels. Two-weeks later, dialysis probes were implanted into the nucleus accumbens (NAc) and ipsilateral ventral tegmental area (VTA). The next day, dialysate samples were collected during baseline and then following systemic administration of nicotine. Samples were also collected following intra-VTA administration of the gamma-aminobutyric acid (GABA)A receptor antagonist, bicuculline. Dopamine, GABA, glutamate, and acetylcholine (ACh) levels were assessed using liquid chromatography/mass spectrometry (LC/MS). The results revealed that vehicle-treated rats displayed a nicotine-induced increase in NAc dopamine levels. In contrast, STZ-treated rats did not display any changes in NAc dopamine following nicotine administration, an effect that was likely related to a concomitant increase in GABA and decrease in glutamate levels in both the NAc and VTA. Intra-VTA administration of bicuculline increased NAc dopamine in vehicle-treated rats, and this effect was absent in STZ-treated rats. Vehicle-treated rats displayed a nicotine-induced increase in ACh levels in the NAc (but not VTA), an effect that was lower in the NAc of STZ-treated rats. Insulin supplementation normalized the neurochemical effects of nicotine in the NAc and VTA of STZ-treated rats, suggesting that insulin modulates the neurochemical effects of nicotine in the mesolimbic pathway of diabetic rats.

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.

The Response of Ventral Tegmental Area Dopaminergic Neurons to Bupropion: Excitation or Inhibition?

Introduction:

Antidepressants can modulate brain monoamines by acting on pre-synaptic and postsynaptic receptors. Autoreceptors can reduce the monoamines effect on the somatodendritic or pre-synaptic regions despite its postsynaptic counter effects. The direct effect of some antidepressants is related to its temporal and spatial bioavailability in the vicinity of these receptors (still a matter of controversies). This research evaluated the direct effect of acute bupropion on the Ventral Tegmental Area (VTA) dopaminergic neuronal firing rate.

Methods:

Male Wistar rats were divided into intracerebroventricular and microiontophoretic groups with 14 subgroups (n=5 in each subgroup). Amounts of 1, 0.5, 0.1, 0.01, 0.001, and 0.0001 mol of bupropion (5 μL/3 min) were microinfused to the first group and then the ejected amounts of bupropion at -500, -300, -150, -50 nA of electrical currents (1 mol, pH=4.5, 5 min) were applied to the second group. The control and sham subgroups were studied in each group, too. The units with stable firing rates were extracted, and the effect of bupropion was evaluated statistically with a P value less than 0.05 as the level of significance.

Results:

The highest amount of bupropion in the intracerebroventricular application could excite 42% of the neurons and inhibit 56% of them, but the highest amount of microiontophoretic application of bupropion could inhibit 97.5% of the neurons. The neuronal response to bupropion was dose-dependent in all treated groups.

Conclusion:

The dual effects of intracerebroventricular bupropion on the VTA dopaminergic neurons but solo inhibitory effect of its microiontophoretic application reflect the intra-VTA and extra-VTA heterogenic cellular and molecular control over the dopaminergic outflow that can be mediated by different receptors. The dopamine autoreceptors on the VTA dopaminergic neurons have complex modulatory effects on the dopaminergic response.

Dissociable roles of ventral pallidum neurons in the basal ganglia reinforcement learning network

Reinforcement learning models treat the basal ganglia (BG) as an actor-critic network. The ventral pallidum (VP) is a major component of the BG limbic system. However, its precise functional roles within the BG circuitry, particularly in comparison to the adjacent external segment of the globus pallidus (GPe), remain unexplored. We recorded the spiking activity of VP neurons, GPe cells (actor) and striatal cholinergic interneurons (critic) while monkeys performed a classical conditioning task. Here, we report that VP neurons can be classified into two distinct populations. The persistent population displayed sustained activation following visual cue presentation, was correlated with monkeys' behavior and showed uncorrelated spiking activity. The transient population displayed phasic synchronized responses that were correlated with the rate of learning and the reinforcement learning model's prediction error. Our results suggest that the VP is physiologically different from the GPe and identify the transient VP neurons as a BG critic.

Alpha-1 Adrenergic Receptors Modulate Glutamate and GABA Neurotransmission onto Ventral Tegmental Dopamine Neurons during Cocaine Sensitization

The ventral tegmental area (VTA) plays an important role in the reward and motivational processes that facilitate the development of drug addiction. Presynaptic α1-AR activation modulates glutamate and Gamma-aminobutyric acid (GABA) release. This work elucidates the role of VTA presynaptic α1-ARs and their modulation on glutamatergic and GABAergic neurotransmission during cocaine sensitization. Excitatory and inhibitory currents (EPSCs and IPSCs) measured by a whole cell voltage clamp show that α1-ARs activation increases EPSCs amplitude after 1 day of cocaine treatment but not after 5 days of cocaine injections. The absence of a pharmacological response to an α1-ARs agonist highlights the desensitization of the receptor after repeated cocaine administration. The desensitization of α1-ARs persists after a 7-day withdrawal period. In contrast, the modulation of α1-ARs on GABA neurotransmission, shown by decreases in IPSCs' amplitude, is not affected by acute or chronic cocaine injections. Taken together, these data suggest that α1-ARs may enhance DA neuronal excitability after repeated cocaine administration through the reduction of GABA inhibition onto VTA dopamine (DA) neurons even in the absence of α1-ARs' function on glutamate release and protein kinase C (PKC) activation. α1-AR modulatory changes in cocaine sensitization increase our knowledge of the role of the noradrenergic system in cocaine addiction and may provide possible avenues for therapeutics.

Repeated restraint stress potentiates methylphenidate and modafinil-induced behavioral sensitization in rats

Stress increases the susceptibility of drug abuse and drugs of abuse impair behavioral tolerance. It has been shown that stress exposure enhances the sensitivity to the reinforcing properties of drugs, augments locomotor sensitization effects of drugs of abuse and impairs behavioral tolerance. Previously, it has been shown that long-term administration of psychostimulants (Methylphenidate and Modafinil) induced locomotor sensitization effect that was more pronounced after 13 days of drug administration and was greater at high dose. The present study is designed to investigate the relationship between restraint stress and psychostimulants (Methylphenidate and Modafinil) that induced sensitization. Methylphenidate (10 mg/kg/day twice a day), modafinil (75 mg/kg/day once daily), and saline (0.9% NaCl; 1 ml/kg/day) were administered orally to treated and control animals. Rats were exposed to immobilization stress for 30 days (until locomotor sensitization produced) to monitor any change in drug-induced behavioral sensitization. The motor activity was compared daily by using familiar environment of home cage and weekly by novel environment of open field. The results show that the methylphenidate and modafinil-induced locomotor sensitization is enhanced and impaired behavioral tolerance in repeated restrained rats. It shows that the psychostimulants like methylphenidate and modafinil produce greater locomotor sensitization in stressful environment, suggesting addictive effects of stress and psychostimulants (methylphenidate/modafinil) on dopaminergic neurotransmission. These finding may be helpful to develop potential pharmacotherapies for the patients with co-occurring depression and substance abuse/dependence disorder.

Role of nociceptin/orphanin FQ and nociceptin opioid peptide receptor in depression and antidepressant effects of nociceptin opioid peptide receptor antagonists

Nociceptin/orphanin FQ (N/OFQ) and its receptor, nociceptin opioid peptide (NOP) receptor, are localized in brain areas implicated in depression including the amygdala, bed nucleus of the stria terminalis, habenula, and monoaminergic nuclei in the brain stem. N/OFQ inhibits neuronal excitability of monoaminergic neurons and monoamine release from their terminals by activation of G protein-coupled inwardly rectifying K⁺ channels and inhibition of voltage sensitive calcium channels, respectively. Therefore, NOP receptor antagonists have been proposed as a potential antidepressant. Indeed, mounting evidence shows that NOP receptor antagonists have antidepressant-like effects in various preclinical animal models of depression, and recent clinical studies again confirmed the idea that blockade of NOP receptor signaling could provide a novel strategy for the treatment of depression. In this review, we describe the pharmacological effects of N/OFQ in relation to depression and explore the possible mechanism of NOP receptor antagonists as potential antidepressants.

Preproenkephalin-expressing ventral pallidal neurons control inhibitory avoidance learning

The ventral pallidum (VP) is a critical component of the basal ganglia neurocircuitry regulating learning and decision making; however, its precise role in controlling associative learning of environmental stimuli conditioned to appetitive or aversive outcomes is still unclear. Here, we investigated the expression of preproenkephalin, a polypeptide hormone previously shown to be expressed in nucleus accumbens neurons controlling aversive learning, within GABAergic and glutamatergic VP neurons. Next, we explored the behavioral consequences of chemicogenetic inhibition or excitation of preproenkephalin-expressing VP neurons on associative learning of reward- or aversion-paired stimuli in autoshaping and inhibitory avoidance tasks, respectively. We reveal for the first time that preproenkephalin is expressed predominantly in GABAergic rather than glutamatergic VP neurons, and that excitation of these preproenkephalin-expressing VP neurons was sufficient to impair inhibitory avoidance learning. These findings indicate the necessity for inhibition of preproenkephalin-expressing VP neurons for avoidance learning, and suggest these neurons as a potential therapeutic target for psychiatric disorders associated with maladaptive aversive learning.

The lateral preoptic area and ventral pallidum embolden behavior

While recently completing a study of the effects of stimulating the lateral preoptic area (LPO) and ventral pallidum (VP) on locomotion and other movements, we also noticed LPO and VP effects on motivational drive and threat tolerance. Here, we have investigated these latter effects by testing conditioned place preference (CPP), behavior on the elevated plus maze (EPM) and the willingness of sated rats to occupy a harshly lit open field center to acquire sweet pellets, a measure of threat tolerance, following infusions of vehicle or bicuculline (bic) into the LPO and VP. LPO-bic infusions robustly increased total locomotion, and, in direct proportion, occupancy of both the harshly lit field center and open arms of the EPM. LPO bic also generated CPP, but did not increase sweet pellet ingestion. These effects were attenuated by dopamine D1 and D2 receptor antagonists, whether given individually or as a cocktail and systemically or infused bilaterally into the nucleus accumbens. VP-bic infusions did not increase total locomotion, but preferentially increased field center occupancy. VP-bic-infused rats compulsively ingested sweet pellets and did so even under the spotlight, whereas harsh illumination suppressed pellet ingestion in the control groups. VP bic produced CPP and increased open arm occupancy on the EPM. These effects were attenuated by pretreatment with dopamine receptor antagonists given systemically or as bilateral infusions into the VP, except for % distance in the field center (by D1 or D2 antagonists) and pellet ingestion (by D1 antagonist). Thus, boldness generated in association with LPO activation is tightly tied to locomotor activation and, as is locomotion itself, strongly DA dependent, whereas that accompanying stimulation of the VP is independent of locomotor activation and, at least in part, DA signaling. Furthermore, respective emboldened behaviors elicited from neither LPO nor VP could clearly be attributed to goal pursuit. Rather, emboldening of behavior seems more to be a fixed action response not fundamentally different than previously for reported locomotion, pivoting, backing, gnawing, and eating elicited by basal forebrain stimulation.

Amino acid modulation of dopamine in the nucleus accumbens mediates sex differences in nicotine withdrawal

The aversive effect of nicotine withdrawal is greater in female versus male rats, and we postulate that this sex difference is mediated in the nucleus accumbens (NAc). Nicotine withdrawal induces decreases in NAc dopamine and increases in acetylcholine (ACh) levels in male rats. To our knowledge, these neurochemical markers of nicotine withdrawal have not been compared in female versus male rats. Given the role of amino acids in modulating NAc dopaminergic and cholinergic transmission, concomitant measures of gamma-aminobutyric acid (GABA) and glutamate levels were also compared across sex. Rats received continuous nicotine exposure for 14 days, and then NAc dialysate was collected during baseline and following administration of the nicotinic receptor antagonist mecamylamine to precipitate withdrawal. Chronic nicotine exposure was associated with larger increases in baseline dopamine, GABA and glutamate levels in the NAc of female versus male rats, whereas baseline ACh was only increased in male rats. During withdrawal, both sexes displayed equivalent increases in NAc ACh levels. As expected, male rats displayed decreases in dopamine, coupled with increases in GABA and decreases in glutamate levels, suggesting the possibility of increased inhibitory tone in the NAc during withdrawal. Relative to males, female rats displayed larger decreases in NAc dopamine and related increases in GABAergic transmission. As female rats also showed elevated glutamate levels that persist during withdrawal, it is suggested that sex differences may arise from increased glutamatergic drive of inhibitory tone in the NAc. The findings provide a potential mechanism whereby the aversive effects of nicotine withdrawal are enhanced in female rats.

Importance of kynurenine 3-monooxygenase for spontaneous firing and pharmacological responses of midbrain dopamine neurons: Relevance for schizophrenia

Kynurenine 3-monooxygenase (KMO) is an essential enzyme of the kynurenine pathway, converting kynurenine into 3-hydroxykynurenine. Inhibition of KMO increases kynurenine, resulting in elevated levels of kynurenic acid (KYNA), an endogenous N-methyl-d-aspartate and α*7-nicotinic receptor antagonist. The concentration of KYNA is elevated in the brain of patients with schizophrenia, possibly as a result of a reduced KMO activity. In the present study, using in vivo single cell recording techniques, we investigated the electrophysiological characteristics of ventral tegmental area dopamine (VTA DA) neurons and their response to antipsychotic drugs in a KMO knock-out (K/O) mouse model. KMO K/O mice exhibited a marked increase in spontaneous VTA DA neuron activity as compared to wild-type (WT) mice. Furthermore, VTA DA neurons showed clear-cut, yet qualitatively opposite, responses to the antipsychotic drugs haloperidol and clozapine in the two genotypes. The anti-inflammatory drug parecoxib successfully lowered the firing activity of VTA DA neurons in KMO K/O, but not in WT mice. Minocycline, an antibiotic and anti-inflammatory drug, produced no effect in this regard. Taken together, the present data further support the usefulness of KMO K/O mice for studying distinct aspects of the pathophysiology and pharmacological treatment of psychiatric disorders such as schizophrenia.

GABAergic Neurons as Putative Neurochemical Substrate Mediating Aversive Effects of Nicotine

Nicotine, the main addictive component of tobacco smoke, has both rewarding and aversive properties. Recent studies have suggested that GABAergic neurons, one of the main neurochemical components of the reward-addiction circuitry, may also play a role in the aversive responses to nicotine. In the present study of transgenic mice expressing Green Fluorescent Protein (GFP) in Glutamate Decarboxylase 67 (GAD67) neurons, we hypothesized that a subpopulation of GABAergic neurons in the Ventral Tegmental Area (VTA) are the targets of aversive doses of nicotine in the CNS. We tested this hypothesis using c-Fos immunohistochemical techniques to identify GAD67-GFP positive cells within the VTA, that are activated by a single intraperitoneal (i.p.) injection of a low (40 ug/kg) or a high (2 mg/kg) dose of nicotine. We also assessed the anatomical location of GAD67-GFP positive cells with respect to tyrosine hydroxylase (TH) Immunoreactive (IR) dopaminergic cells in VTA. Consistent with our previous studies low- and high-dose nicotine both induced c-Fos activation of various intensities at multiple sites in VTA. Double labeling of c-Fos activated cells with GAD67-GFP positive cells identified a subpopulation of GABAergic neurons in Substantia Nigra Compact part Medial tier (SNCM) that were activated by high- but not by low-dose nicotine. Of 217 GABAergic cells counted at this site, 48.9% exhibited nicotine induced c-fos immunoreactivity. GAD67-GFP positive cells in other regions of VTA were not activated by the nicotine doses tested. Double labeling of GAD67-GFP positive cells with TH IR cells showed that the GABAergic neurons that were activated by high-dose nicotine were located in close proximity to the dopaminergic neurons of substantia nigra compact part and VTA. Dose-dependent activation of GAD67-GFP positive neurons in SNCM, by a nicotine dose known to produce aversive responses, implies that GABAergic neurons at these sites may be an important component of the nicotine aversive circuitry.

Nucleus Accumbens Subnuclei Regulate Motivated Behavior via Direct Inhibition and Disinhibition of VTA Dopamine Subpopulations

Mesolimbic dopamine (DA) neurons play a central role in motivation and reward processing. Although the activity of these mesolimbic DA neurons is controlled by afferent inputs, little is known about the circuits in which they are embedded. Using retrograde tracing, electrophysiology, optogenetics, and behavioral assays, we identify principles of afferent-specific control in the mesolimbic DA system. Neurons in the medial shell subdivision of the nucleus accumbens (NAc) exert direct inhibitory control over two separate populations of mesolimbic DA neurons by activating different GABA receptor subtypes. In contrast, NAc lateral shell neurons mainly synapse onto ventral tegmental area (VTA) GABA neurons, resulting in disinhibition of DA neurons that project back to the NAc lateral shell. Lastly, we establish a critical role for NAc subregion-specific input to the VTA underlying motivated behavior. Collectively, our results suggest a distinction in the incorporation of inhibitory inputs between different subtypes of mesolimbic DA neurons.

Intravenous morphine self-administration alters accumbal microRNA profiles in the mouse brain

A significant amount of evidence indicates that microRNAs (miRNAs) play an important role in drug addiction. The nucleus accumbens (NAc) is a critical part of the brain's reward circuit and is involved in a variety of psychiatric disorders, including depression, anxiety, and drug addiction. However, few studies have examined the expression of miRNAs and their functional roles in the NAc under conditions of morphine addiction. In this study, mice were intravenously infused with morphine (0.01, 0.03, 0.3, 1 and 3 mg/kg/infusion) and showed inverted U-shaped response. After morphine self-administration, NAc was used to analyze the functional networks of altered miRNAs and their putative target mRNAs in the NAc following intravenous self-administration of morphine. We utilized several bioinformatics tools, including Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway mapping and CyTargetLinker. We found that 62 miRNAs were altered and exhibited differential expression patterns. The putative targets were related to diverse regulatory functions, such as neurogenesis, neurodegeneration, and synaptic plasticity, as well as the pharmacological effects of morphine (receptor internalization/endocytosis). The present findings provide novel insights into the regulatory mechanisms of accumbal molecules under conditions of morphine addiction and identify several novel biomarkers associated with morphine addiction.

Effects of Injection of Gamma-Aminobutyric Acid Agonists into the Nucleus Accumbens on Naloxone-Induced Morphine Withdrawal

Aims: This study was to investigate the effects of local administration of gamma-aminobutyric acid (GABA) agonists into the nucleus accumbens (NAc) on naloxone-induced morphine withdrawal symptoms. Methods: Bilateral guide cannulas were stereotaxically implanted in the shell or core regions of the NAc of Sprague-Dawley rats. After a recovery period, 3 morphine pellets, each consisting of 75 mg morphine base, were placed subcutaneously on the first and third days of the study with the rats under mild ether anaesthesia. The GABA agonists, baclofen hydrochloride or muscimol hydrobromide, were injected into the NAc, and morphine withdrawal was induced by naloxone on the fifth day. Results: Administration of baclofen to the shell or core regions of the NAc of Sprague-Dawley rats led to statistically significant decreases in both behavioural and locomotor activity parameters during the morphine withdrawal period, compared to the control group. However, there were no statistically significant changes in locomotor activity or withdrawal behavioural parameters, with the exception of wet dog shakes, between control and muscimol-treated groups. Conclusion: These findings show that GABAergic conduction in the NAc is effective on the morphine withdrawal symptoms, and that both the shell and core regions of the NAc are associated with this effect.

Two delta opioid receptor subtypes are functional in single ventral tegmental area neurons, and can interact with the mu opioid receptor

The mu and delta opioid receptors (MOR and DOR) are highly homologous members of the opioid family of GPCRs. There is evidence that MOR and DOR interact, however the extent to which these interactions occur in vivo and affect synaptic function is unknown. There are two stable DOR subtypes: DPDPE sensitive (DOR1) and deltorphin II sensitive (DOR2); both agonists are blocked by DOR selective antagonists. Robust motivational effects are produced by local actions of both MOR and DOR ligands in the ventral tegmental area (VTA). Here we demonstrate that a majority of both dopaminergic and non-dopaminergic VTA neurons express combinations of functional DOR1, DOR2, and/or MOR, and that within a single VTA neuron, DOR1, DOR2, and MOR agonists can differentially couple to downstream signaling pathways. As reported for the MOR agonist DAMGO, DPDPE and deltorphin II produced either a predominant K⁺ dependent hyperpolarization or a Ca_v2.1 mediated depolarization in different neurons. In some neurons DPDPE and deltorphin II produced opposite responses. Excitation, inhibition, or no effect by DAMGO did not predict the response to DPDPE or deltorphin II, arguing against a MOR-DOR interaction generating DOR subtypes. However, in a subset of VTA neurons the DOR antagonist TIPP-Ψ augmented DAMGO responses; we also observed DPDPE or deltorphin II responses augmented by the MOR selective antagonist CTAP. These findings directly support the existence of two independent, stable forms of the DOR, and show that MOR and DOR can interact in some neurons to alter downstream signaling.

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

The nucleus accumbens is a major input structure of the basal ganglia and integrates information from cortical and limbic structures to mediate goal-directed behaviors. Chronic exposure to several classes of drugs of abuse disrupts plasticity in this region, allowing drug-associated cues to engender a pathologic motivation for drug seeking. A number of alterations in glutamatergic transmission occur within the nucleus accumbens after withdrawal from chronic drug exposure. These drug-induced neuroadaptations serve as the molecular basis for relapse vulnerability. In this review, we focus on the role that glutamate signal transduction in the nucleus accumbens plays in addiction-related behaviors. First, we explore the nucleus accumbens, including the cell types and neuronal populations present as well as afferent and efferent connections. Next we discuss rodent models of addiction and assess the viability of these models for testing candidate pharmacotherapies for the prevention of relapse. Then we provide a review of the literature describing how synaptic plasticity in the accumbens is altered after exposure to drugs of abuse and withdrawal and also how pharmacological manipulation of glutamate systems in the accumbens can inhibit drug seeking in the laboratory setting. Finally, we examine results from clinical trials in which pharmacotherapies designed to manipulate glutamate systems have been effective in treating relapse in human patients. Further elucidation of how drugs of abuse alter glutamatergic plasticity within the accumbens will be necessary for the development of new therapeutics for the treatment of addiction across all classes of addictive substances.

Involvement of lateral septum in alcohol's dopamine-elevating effect in the rat

Drugs of abuse share the ability to increase extracellular dopamine (DA) levels in the mesolimbic DA system. This effect has been linked to positive and reinforcing experiences of drug consumption and is presumed to be of importance for continued use, as well as for the development of dependence and addiction. Previous rat studies from our lab have implicated a neuronal circuitry involving glycine receptors in nucleus accumbens (nAc) and, secondarily, nicotinic acetylcholine receptors in the ventral tegmental area (VTA) in ethanol's (EtOH) DA-elevating effect. The work presented here, performed in male Wistar rats, suggests that the lateral septum (LS), which has previously been associated with different aspects of EtOH-related behaviour, is involved as well. In vivo microdialysis methodology demonstrated that blocking the generation of action potentials in LS using tetrodotoxin prevented a DA increase in nAc after accumbal EtOH perfusion. Retrograde tracing and polymerase chain reaction (PCR) were used to identify and characterize cells projecting to VTA from nAc/LS and from LS to nAc. Based on the PCR results, cells projecting from both LS/nAc to anterior VTA and from LS to nAc were mainly GABAergic neurons expressing glycine receptors, and these cells are presumed to be involved in mediating the DA-elevating effect of EtOH. These results provide further evidence implicating LS in the reinforcing effects of EtOH. Additional studies are needed to investigate LS involvement in EtOH consumption behaviour and its potential role in the development of dependence and addiction.

Cannabidiol Modulates Fear Memory Formation Through Interactions with Serotonergic Transmission in the Mesolimbic System

Emerging evidence suggests that the largest phytochemical component of cannabis, cannabidiol (CBD), may possess pharmacotherapeutic properties in the treatment of neuropsychiatric disorders. CBD has been reported to functionally interact with both the mesolimbic dopamine (DA) and serotonergic (5-HT) receptor systems. However, the underlying mechanisms by which CBD may modulate emotional processing are not currently understood. Using a combination of in vivo electrophysiological recording and fear conditioning in rats, the present study aimed to characterize the behavioral, neuroanatomical, and pharmacological effects of CBD within the mesolimbic pathway, and its possible functional interactions with 5-HT and DAergic transmission. Using targeted microinfusions of CBD into the shell region of the mesolimbic nucleus accumbens (NASh), we report that intra-NASh CBD potently blocks the formation of conditioned freezing behaviors. These effects were challenged with DAergic, cannabinoid CB1 receptor, and serotonergic (5-HT_1A) transmission blockade, but only 5-HT_1A blockade restored associative conditioned freezing behaviors. In vivo intra-ventral tegmental area (VTA) electrophysiological recordings revealed that behaviorally effective doses of intra-NASh CBD elicited a predominant decrease in spontaneous DAergic neuronal frequency and bursting activity. These neuronal effects were reversed by simultaneous blockade of 5-HT_1A receptor transmission. Finally, using a functional contralateral disconnection procedure, we demonstrated that the ability of intra-NASh CBD to block the formation of conditioned freezing behaviors was dependent on intra-VTA GABAergic transmission substrates. Our findings demonstrate a novel NAcVTA circuit responsible for the behavioral and neuronal effects of CBD within the mesolimbic system via functional interactions with serotonergic 5-HT_1A receptor signaling.

Selective Enhancement of Dopamine Release in the Ventral Pallidum of Methamphetamine-Sensitized Mice

Drugs of abuse induce sensitization, which is defined as enhanced response to additional drug following a period of withdrawal. Sensitization occurs in both humans and animal models of drug reinforcement and contributes substantially to the addictive nature of drugs of abuse, because it is thought to represent enhanced motivational wanting for drug. The ventral pallidum, a key member of the reward pathway, contributes to behaviors associated with reward, such as sensitization. Dopamine inputs to the ventral pallidum have not been directly characterized. Here we provide anatomical, neurochemical, and behavioral evidence demonstrating that dopamine terminals in the ventral pallidum contribute to reward in mice. We report subregional differences in dopamine release, measured by ex vivo fast-scan cyclic voltammetry: rostral ventral pallidum exhibits increased dopamine release and uptake compared with caudal ventral pallidum, which is correlated with tissue expression of dopaminergic proteins. We then subjected mice to a methamphetamine-sensitization protocol to investigate the contribution of dopaminergic projections to the region in reward related behavior. Methamphetamine-sensitized animals displayed a 508% and 307% increase in baseline dopamine release in the rostral and caudal ventral pallidum, respectively. Augmented dopamine release in the rostral ventral pallidum was significantly correlated with sensitized locomotor activity. Moreover, this presynaptic dopaminergic plasticity occurred only in the ventral pallidum and not in the ventral or dorsal striatum, suggesting that dopamine release in the ventral pallidum may be integrally important to drug-induced sensitization.

Diversity of Dopaminergic Neural Circuits in Response to Drug Exposure

Addictive substances are known to increase dopaminergic signaling in the mesocorticolimbic system. The origin of this dopamine (DA) signaling originates in the ventral tegmental area (VTA), which sends afferents to various targets, including the nucleus accumbens, the medial prefrontal cortex, and the basolateral amygdala. VTA DA neurons mediate stimuli saliency and goal-directed behaviors. These neurons undergo robust drug-induced intrinsic and extrinsic synaptic mechanisms following acute and chronic drug exposure, which are part of brain-wide adaptations that ultimately lead to the transition into a drug-dependent state. Interestingly, recent investigations of the differential subpopulations of VTA DA neurons have revealed projection-specific functional roles in mediating reward, aversion, and stress. It is now critical to view drug-induced neuroadaptations from a circuit-level perspective to gain insight into how differential dopaminergic adaptations and signaling to targets of the mesocorticolimbic system mediates drug reward. This review hopes to describe the projection-specific intrinsic characteristics of these subpopulations, the differential afferent inputs onto these VTA DA neuron subpopulations, and consolidate findings of drug-induced plasticity of VTA DA neurons and highlight the importance of future projection-based studies of this system.

Neonatal finasteride administration decreases dopamine release in nucleus accumbens after alcohol and food presentation in adult male rats

Endogenous levels of the neurosteroid (NS) allopregnanolone (AlloP) during neonatal stages are crucial for the correct development of the central nervous system (CNS). In a recent work we reported that the neonatal administration of AlloP or finasteride (Finas), an inhibitor of the enzyme 5α-reductase needed for AlloP synthesis, altered the voluntary consumption of ethanol and the ventrostriatal dopamine (DA) levels in adulthood, suggesting that neonatal NS manipulations can increase alcohol abuse vulnerability in adulthood. Moreover, other authors have associated neonatal NS alterations with diverse dopaminergic (DAergic) alterations. Thus, the aim of the present work is to analyse if manipulations of neonatal AlloP alter the DAergic response in the nucleus accumbens (NAcc) during alcohol intake in rats. We administered AlloP or Finas from postnatal day (PND) 5 to PND9. At PND98, we measured alcohol consumption using a two-bottle free-choice model (ethanol 10% (v/v)+glucose 3% (w/v), and glucose 3% (w/v)) for 12 days. On the last day of consumption, we measured the DA and 3,4-dihydroxyphenylacetic acid (DOPAC) release in NAcc in response to ethanol intake. The samples were obtained by means of in vivo microdialysis in freely moving rats, and DA and DOPAC levels were determined by means of high-performance liquid chromatography analysis (HPLC). The results revealed that neonatal Finas increased ethanol consumption in some days of the consumption phase, and decreased the DA release in the NAcc in response to solutions (ethanol+glucose) and food presentation. Taken together, these results suggest that neonatal NS alterations can affect alcohol rewarding properties.

Modulation of nucleus accumbens connectivity by alcohol drinking and naltrexone in alcohol-preferring rats: A manganese-enhanced magnetic resonance imaging study

The nonselective opioid receptor antagonist naltrexone is now used for the treatment of alcoholism, yet naltrexone's central mechanism of action remains poorly understood. One line of evidence suggests that opioid antagonists regulate alcohol drinking through interaction with the mesolimbic dopamine system. Hence, our goal here was to examine the role of the nucleus accumbens connectivity in alcohol reinforcement and naltrexone's actions using manganese-enhanced magnetic resonance imaging (MEMRI). Following long-term free-choice drinking of alcohol and water, AA (Alko Alcohol) rats received injections of MnCl2 into the nucleus accumbens for activity-dependent tracing of accumbal connections. Immediately after the accumbal injections, rats were imaged using MEMRI, and then allowed to drink either alcohol or water for the next 24h. Naltrexone was administered prior to the active dark period, and the second MEMRI was performed 24h after the first scan. Comparison of signal intensity at 1 and 24h after accumbal MnCl2 injections revealed an ipsilateral continuum through the ventral pallidum, bed nucleus of the stria terminalis, globus pallidus, and lateral hypothalamus to the substantia nigra and ventral tegmental area. Activation was also seen in the rostral part of the insular cortex and regions of the prefrontal cortex. Alcohol drinking resulted in enhanced activation of these connections, whereas naltrexone suppressed alcohol-induced activity. These data support the involvement of the accumbal connections in alcohol reinforcement and mediation of naltrexone's suppressive effects on alcohol drinking through their deactivation.

To Act or Not to Act: Endocannabinoid/Dopamine Interactions in Decision-Making

Decision-making is an ethologically adaptive construct that is impaired in multiple psychiatric disorders. Activity within the mesocorticolimbic dopamine system has been traditionally associated with decision-making. The endocannabinoid system through its actions on inhibitory and excitatory synapses modulates dopamine activity and decision-making. The aim of this brief review is to present a synopsis of available data obtained when the endocannabinoid system is manipulated and dopamine activity recorded. To this end, we review research using different behavioral paradigms to provide further insight into how this ubiquitous signaling system biases dopamine-related behaviors to regulate decision-making.

Brain activation induced by voluntary alcohol and saccharin drinking in rats assessed with manganese-enhanced magnetic resonance imaging

The neuroanatomical and neurochemical basis of alcohol reward has been studied extensively, but global alterations of neural activity in reward circuits during chronic alcohol use remain poorly described. Here, we measured brain activity changes produced by long-term voluntary alcohol drinking in the alcohol-preferring AA (Alko alcohol) rats using manganese-enhanced magnetic resonance imaging (MEMRI). MEMRI is based on the ability of paramagnetic manganese ions to accumulate in excitable neurons and thereby enhance the T1-weighted signal in activated brain areas. Following 6 weeks of voluntary alcohol drinking, AA rats were allowed to drink alcohol for an additional week, during which they were administered manganese chloride (MnCl2 ) with subcutaneous osmotic minipumps before MEMRI. A second group with an identical alcohol drinking history received MnCl2 during the abstinence week following alcohol drinking. For comparing alcohol with a natural reinforcer, MEMRI was also performed in saccharin-drinking rats. A water-drinking group receiving MnCl2 served as a control. We found that alcohol drinking increased brain activity extensively in cortical and subcortical areas, including the mesocorticolimbic and nigrostriatal dopamine pathways and their afferents. Remarkably similar activation maps were seen after saccharin ingestion. Particularly in the prelimbic cortex, ventral hippocampus and subthalamic nucleus, activation persisted into early abstinence. These data show that voluntary alcohol recruits an extensive network that includes the ascending dopamine systems and their afferent connections, and that this network is largely shared with saccharin reward. The regions displaying persistent alterations after alcohol drinking could participate in brain networks underlying alcohol seeking and relapse.

Differential roles of dopamine D1 and D2 receptor-containing neurons of the nucleus accumbens shell in behavioral sensitization

The nucleus accumbens (Nac) mediates the reinforcing and motor stimulating properties of psychostimulants. It receives dopaminergic afferents from the ventral midbrain and is divided into two distinct subregions: shell and core. Each of these contains two subtypes of medium spiny neurons, which express either dopamine D1 (D1R) or D2 (D2R) receptors. However, functional dissociation between the two subtypes in psychostimulant response remains to be elucidated. We performed selective ablation of each subtype in the Nac shell in mice, using immunotoxin-mediated cell targeting, and examined the behavioral sensitization evoked by repeated administration of methamphetamine. The D1R cell-ablated mice exhibited delayed induction of sensitized locomotion compared to control mice, whereas the D2R cell-ablated mice showed a mildly enhanced rate of induction of sensitization. In vivo microdialysis revealed a marked blockade of the increase in extracellular dopamine in the Nac of the D1R cell-ablated animals in response to methamphetamine, indicating that the observed delay in behavioral sensitization in these mice involves an impairment in accumbal dopamine release. Our results reveal differential roles of D1R- and D2R-containing accumbal shell neurons in the development of behavioral sensitization to psychostimulants. Behavioral sensitization, enhanced motility by repetitive psychostimulant administration, is a model of drug addiction. Here, we show that the nucleus accumbens (Nac) shell neurons containing dopamine D1 receptor (D1R) or D2 receptor (D2R) play distinct roles in behavioral sensitization triggered by methamphetamine, and that D1R-containing neurons enhance the induction of behavioral sensitization at the early phase, whereas D2R-containing neurons act to suppress the rate of development of the behavior.

GIRK Channels Modulate Opioid-Induced Motor Activity in a Cell Type- and Subunit-Dependent Manner

G-protein-gated inwardly rectifying K(+) (GIRK/Kir3) channel activation underlies key physiological effects of opioids, including analgesia and dependence. GIRK channel activation has also been implicated in the opioid-induced inhibition of midbrain GABA neurons and consequent disinhibition of dopamine (DA) neurons in the ventral tegmental area (VTA). Drug-induced disinhibition of VTA DA neurons has been linked to reward-related behaviors and underlies opioid-induced motor activation. Here, we demonstrate that mouse VTA GABA neurons express a GIRK channel formed by GIRK1 and GIRK2 subunits. Nevertheless, neither constitutive genetic ablation of Girk1 or Girk2, nor the selective ablation of GIRK channels in GABA neurons, diminished morphine-induced motor activity in mice. Moreover, direct activation of GIRK channels in midbrain GABA neurons did not enhance motor activity. In contrast, genetic manipulations that selectively enhanced or suppressed GIRK channel function in midbrain DA neurons correlated with decreased and increased sensitivity, respectively, to the motor-stimulatory effect of systemic morphine. Collectively, these data support the contention that the unique GIRK channel subtype in VTA DA neurons, the GIRK2/GIRK3 heteromer, regulates the sensitivity of the mouse mesolimbic DA system to drugs with addictive potential.

The ventral pallidum: Subregion-specific functional anatomy and roles in motivated behaviors

The ventral pallidum (VP) plays a critical role in the processing and execution of motivated behaviors. Yet this brain region is often overlooked in published discussions of the neurobiology of mental health (e.g., addiction, depression). This contributes to a gap in understanding the neurobiological mechanisms of psychiatric disorders. This review is presented to help bridge the gap by providing a resource for current knowledge of VP anatomy, projection patterns and subregional circuits, and how this organization relates to the function of VP neurons and ultimately behavior. For example, ventromedial (VPvm) and dorsolateral (VPdl) VP subregions receive projections from nucleus accumbens shell and core, respectively. Inhibitory GABAergic neurons of the VPvm project to mediodorsal thalamus, lateral hypothalamus, and ventral tegmental area, and this VP subregion helps discriminate the appropriate conditions to acquire natural rewards or drugs of abuse, consume preferred foods, and perform working memory tasks. GABAergic neurons of the VPdl project to subthalamic nucleus and substantia nigra pars reticulata, and this VP subregion is modulated by, and is necessary for, drug-seeking behavior. Additional circuits arise from nonGABAergic neuronal phenotypes that are likely to excite rather than inhibit their targets. These subregional and neuronal phenotypic circuits place the VP in a unique position to process motivationally relevant stimuli and coherent adaptive behaviors.