Electrophysiological characterization of GABAergic neurons in the ventral tegmental area

The distribution of neurotransmitters in the brain circuitry: Mesolimbic pathway and addiction

Understanding the central nervous system (CNS) circuitry and its different neurotransmitters that underlie reward is essential to improve treatment for many common health issues, such as addiction. Here, we concentrate on understanding how the mesolimbic circuitry and neurotransmitters are organized and function, and how drug exposure affects synaptic and structural changes in this circuitry. While the role of some reward circuits, like the cerebral dopamine (DA)/glutamate (Glu)/gamma aminobutyric acid (GABA)ergic pathways, in drug reward, is well known, new research using molecular-based methods has shown functional alterations throughout the reward circuitry that contribute to various aspects of addiction, including craving and relapse. A new understanding of the fundamental connections between brain regions as well as the molecular alterations within these particular microcircuits, such as neurotrophic factor and molecular signaling or distinct receptor function, that underlie synaptic and structural plasticity evoked by drugs of abuse has been made possible by the ability to observe and manipulate neuronal activity within specific cell types and circuits. It is exciting that these discoveries from preclinical animal research are now being applied in the clinic, where therapies for human drug dependence, such as deep brain stimulation and transcranial magnetic stimulation, are being tested. Therefore, this chapter seeks to summarize the current understanding of the important brain regions (especially, mesolimbic circuitry) and neurotransmitters implicated in drug-related behaviors and the molecular mechanisms that contribute to altered connectivity between these areas, with the postulation that increased knowledge of the plasticity within the drug reward circuit will lead to new and improved treatments for addiction.

Understanding How Acute Alcohol Impacts Neural Encoding in the Rodent Brain

Alcohol impacts neural circuitry throughout the brain and has wide-ranging effects on the biophysical properties of neurons in these circuits. Articulating how these wide-ranging effects might eventually result in altered computational properties has the potential to provide a tractable working model of how alcohol alters neural encoding. This chapter reviews what is currently known about how acute alcohol influences neural activity in cortical, hippocampal, and dopaminergic circuits as these have been the primary focus of understanding how alcohol alters neural computation. While other neural systems have been the focus of exhaustive work on this topic, these brain regions are the ones where in vivo neural recordings are available, thus optimally suited to make the link between changes in neural activity and behavior. Rodent models have been key in developing an understanding of how alcohol impacts the function of these circuits, and this chapter therefore focuses on work from mice and rats. While progress has been made, it is critical to understand the challenges and caveats associated with experimental procedures, especially when performed in vivo, which are designed to answer this question and if/how to translate these data to humans. The hypothesis is discussed that alcohol impairs the ability of neural circuits to acquire states of neural activity that are transiently elevated and characterized by increased complexity. It is hypothesized that these changes are distinct from the traditional view of alcohol being a depressant of neural activity in the forebrain.

Ventral tegmental area interneurons revisited: GABA and glutamate projection neurons make local synapses

The ventral tegmental area (VTA) contains projection neurons that release the neurotransmitters dopamine, GABA, and/or glutamate from distal synapses. VTA also contains GABA neurons that synapse locally on to VTA dopamine neurons, synapses widely credited to a population of so-called VTA interneurons. Interneurons in cortex, striatum, and elsewhere have well-defined morphological features, physiological properties, and molecular markers, but such features have not been clearly described in VTA. Indeed, there is scant evidence that local and distal synapses originate from separate populations of VTA GABA neurons. In this study we tested whether several markers expressed in non-dopamine VTA neurons are selective markers of interneurons, defined as neurons that synapse locally but not distally. Challenging previous assumptions, we found that VTA neurons genetically defined by expression of parvalbumin, somatostatin, neurotensin, or mu-opioid receptor project to known VTA targets including nucleus accumbens, ventral pallidum, lateral habenula, and prefrontal cortex. Moreover, we provide evidence that VTA GABA and glutamate projection neurons make functional inhibitory or excitatory synapses locally within VTA. These findings suggest that local collaterals of VTA projection neurons could mediate functions prior attributed to VTA interneurons. This study underscores the need for a refined understanding of VTA connectivity to explain how heterogeneous VTA circuits mediate diverse functions related to reward, motivation, or addiction.

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.

NAc-DBS corrects depression-like behaviors in CUMS mouse model via disinhibition of DA neurons in the VTA

Major depressive disorder (MDD) is characterized by diverse debilitating symptoms that include loss of motivation and anhedonia. If multiple medications, psychotherapy, and electroconvulsive therapy fail in some patients with MDD, their condition is then termed treatment-resistant depression (TRD). MDD can be associated with abnormalities in the reward-system-dopaminergic mesolimbic pathway, in which the nucleus accumbens (NAc) and ventral tegmental area (VTA) play major roles. Deep brain stimulation (DBS) applied to the NAc alleviates the depressive symptoms of MDD. However, the mechanism underlying the effects of this DBS has remained elusive. In this study, using the chronic unpredictable mild stress (CUMS) mouse model, we investigated the behavioral and neurobiological effects of NAc-DBS on the multidimensional depression-like phenotypes induced by CUMS by integrating behavioral, in vivo microdialysis coupled with high-performance liquid chromatography-electrochemical detector (HPLC-ECD), calcium imaging, pharmacological, and genetic manipulation methods in freely moving mice. We found that long-term and repeated, but not single, NAc-DBS induced robust antidepressant responses in CUMS mice. Moreover, even a single trial NAc-DBS led to the elevation of the γ-aminobutyric acid (GABA) neurotransmitter, accompanied by the increase in dopamine (DA) neuron activity in the VTA. Both the inhibition of the GABAA receptor activity and knockdown of the GABAA-α1 gene in VTA-GABA neurons blocked the antidepressant effect of NAc-DBS in CUMS mice. Our results showed that NAc-DBS could disinhibit VTA-DA neurons by regulating the level of GABA and the activity of VTA-GABA in the VTA and could finally correct the depression-like behaviors in the CUMS mouse model.

The lateral habenula: A hub for value-guided behavior

The habenula is an evolutionarily highly conserved diencephalic brain region divided into two major parts, medial and lateral. Over the past two decades, studies of the lateral habenula (LHb), in particular, have identified key functions in value-guided behavior in health and disease. In this review, we focus on recent insights into LHb connectivity and its functional relevance for different types of aversive and appetitive value-guided behavior. First, we give an overview of the anatomical organization of the LHb and its main cellular composition. Next, we elaborate on how distinct LHb neuronal subpopulations encode aversive and appetitive stimuli and on their involvement in more complex decision-making processes. Finally, we scrutinize the afferent and efferent connections of the LHb and discuss their functional implications for LHb-dependent behavior. A deepened understanding of distinct LHb circuit components will substantially contribute to our knowledge of value-guided behavior.

Activation of nucleus accumbens projections to the ventral tegmental area alters molecular signaling and neurotransmission in the reward system

The nucleus accumbens (NAc) and the ventral tegmental area (VTA) are integral brain regions involved in reward processing and motivation, including responses to drugs of abuse. Previously, we have demonstrated that activation of NAc-VTA afferents during the acquisition of cocaine conditioned place preference (CPP) reduces the rewarding properties of cocaine and diminished the activity of VTA dopamine neurons. In the current study, we examined the impact of enhancing these inhibitory inputs on molecular changes and neurotransmission associated with cocaine exposure. Our results unveiled significant reductions in extracellular signal-regulated kinase (ERK) levels in the VTA and medial prefrontal cortex (mPFC) of both cocaine-treated groups compared with the saline control group. Furthermore, optic stimulation of NAc-VTA inputs during cocaine exposure decreased the expression of GluA1 subunit of AMPA receptor in the VTA and mPFC. Notably, in the NAc, cocaine exposure paired with optic stimulation increased ERK levels and reduced GluA1 phosphorylation at Ser845 as compared with all other groups. Additionally, both cocaine-treated groups exhibited decreased levels of GluA1 phosphorylation at Ser831 in the NAc compared with the saline control group. Moreover, cocaine exposure led to reduced ERK, GluA1, and GluA1 phosphorylation at Ser845 and Ser831 in the mPFC. Augmentation of GABAergic tone from the NAc during cocaine conditioning mitigated changes in GluA1 phosphorylation at Ser845 in the mPFC but reduced ERK, GluA1, and GluA1 phosphorylation at Ser831 compared with the saline control group. Interestingly, enhancing GABAergic tone during saline conditioning decreased GluA1 phosphorylation at Ser831 compared with the saline control group in the mPFC. Our findings highlight the influence of modulating inhibitory inputs from the NAc to the VTA on molecular signaling and glutamatergic neurotransmission in cocaine-exposed animals. Activation of these inhibitory inputs during cocaine conditioning induced alterations in key signaling molecules and AMPA receptor, providing valuable insights into the neurobiological mechanisms underlying cocaine reward and cocaine use disorder. Further exploration of these pathways may offer potential therapeutic targets for the treatment of substance use disorder.

Cognitive performance in aged rats is associated with differences in distinctive neuronal populations in the ventral tegmental area and altered synaptic plasticity in the hippocampus

Introduction

Deterioration of cognitive functions is commonly associated with aging, although there is wide variation in the onset and manifestation. Albeit heterogeneity in age-related cognitive decline has been studied at the cellular and molecular level, there is poor evidence for electrophysiological correlates. The aim of the current study was to address the electrophysiological basis of heterogeneity of cognitive functions in cognitively Inferior and Superior old (19-20 months) rats in the ventral tegmental area (VTA) and the hippocampus, having Young (12 weeks) rats as a control. The midbrain VTA operates as a hub amidst affective and cognitive facets, processing sensory inputs related to motivated behaviours and hippocampal memory. Increasing evidence shows direct dopaminergic and non-dopaminergic input from the VTA to the hippocampus.

Methods

Aged Superior and Inferior male rats were selected from a cohort of 88 animals based on their performance in a spatial learning and memory task. Using in vivo single-cell recording in the VTA, we examined the electrical activity of different neuronal populations (putative dopaminergic, glutamatergic and GABAergic neurons). In the same animals, basal synaptic transmission and synaptic plasticity were examined in hippocampal slices.

Results

Electrophysiological recordings from the VTA and hippocampus showed alterations associated with aging per se, together with differences specifically linked to the cognitive status of aged animals. In particular, the bursting activity of dopamine neurons was lower, while the firing frequency of glutamatergic neurons was higher in VTA of Inferior old rats. The response to high-frequency stimulation in hippocampal slices also discriminated between Superior and Inferior aged animals.

Discussion

This study provides new insight into electrophysiological information underlying compromised cerebral ageing. Further understanding of brain senescence, possibly related to neurocognitive decline, will help develop new strategies towards the preservation of a high quality of life.

Heterogeneity of mesencephalic dopaminergic neurons: From molecular classifications, electrophysiological properties to functional connectivity

The mesencephalic dopamine (DA) system is composed of neuronal subtypes that are molecularly and functionally distinct, are responsible for specific behaviors, and are closely associated with numerous brain disorders. Existing research has made significant advances in identifying the heterogeneity of mesencephalic DA neurons, which is necessary for understanding their diverse physiological functions and disease susceptibility. Moreover, there is a conflict regarding the electrophysiological properties of the distinct subsets of midbrain DA neurons. This review aimed to elucidate recent developments in the heterogeneity of midbrain DA neurons, including subpopulation categorization, electrophysiological characteristics, and functional connectivity to provide new strategies for accurately identifying distinct subtypes of midbrain DA neurons and investigating the underlying mechanisms of these neurons in various diseases.

Activation of RMTg projections to the VTA reverse cocaine-induced molecular adaptation in the reward system

The rostromedial tegmental nucleus (RMTg) plays a crucial role in regulating reward-related behavior by exerting inhibitory control over the ventral tegmental area (VTA). This modulation of dopamine neuron activity within the VTA is essential for maintaining homeostasis in the reward system. Recently we have shown that activation of RMTg projections to the VTA during the acquisition of cocaine-conditioned place preference (CPP) reduces the rewarding properties of cocaine and decreases VTA dopamine neuron activity. By inhibiting dopamine neurons in the VTA, we hypothesized that RMTg projections hold the potential to restore reward system homeostasis disrupted by repeated cocaine use, and attenuate molecular adaptations in the reward system, including alterations in signaling pathways. Our study demonstrates that enhancing the GABAergic inputs from the RMTg to the VTA can mitigate cocaine-induced molecular changes in key regions, namely the VTA, nucleus accumbens (NAc), and prefrontal cortex (PFC). Specifically, we found that cocaine-induced alteration in the phosphorylation state of ERK (pERK) and GluA1 on serine 845 (S845) and serine 831 (S831), that play a major role in plasticity by controlling the activity and trafficking of AMPA receptors, were significantly reversed following optic stimulation of RMTg afferents to the VTA. These findings highlight the therapeutic potential of targeting the RMTg-VTA circuitry for mitigating cocaine reward. Ultimately, this research may pave the way for novel therapeutic interventions that restore balance in the reward system and alleviate the detrimental effects of cocaine.

The Atypical Dopamine Transporter Inhibitor CE-158 Enhances Dopamine Neurotransmission in the Prefrontal Cortex of Male Rats: A Behavioral, Electrophysiological, and Microdialysis Study

BACKGROUND

Dopamine plays a key role in several physiological functions such as motor control, learning and memory, and motivation and reward. The atypical dopamine transporter inhibitor S,S stereoisomer of 5-(((S)-((S)-(3-bromophenyl)(phenyl)methyl)sulfinyl)methyl)thiazole (CE-158) has been recently reported to promote behavioral flexibility and restore learning and memory in aged rats.

METHODS

Adult male rats were i.p. administered for 1 or 10 days with CE-158 at the dose of 1 or 10 mg/kg and tested for extracellular dopamine in the medial prefrontal cortex by means of intracerebral microdialysis and single unit cell recording in the same brain area. Moreover, the effects of acute and chronic CE-158 on exploratory behavior, locomotor activity, prepulse inhibition, working memory, and behavioral flexibility were also investigated.

RESULTS

CE-158 dose-dependently potentiated dopamine neurotransmission in the medial prefrontal cortex as assessed by intracerebral microdialysis. Moreover, repeated exposure to CE-158 at 1 mg/kg was sufficient to increase the number of active pyramidal neurons and their firing frequency in the same brain area. In addition, CE-158 at the dose of 10 mg/kg stimulates exploratory behavior to the same extent after acute or chronic treatment. Noteworthy, the chronic treatment at both doses did not induce any behavioral alterations suggestive of abuse potential (e.g., motor behavioral sensitization) or pro-psychotic-like effects such as disruption of sensorimotor gating or impairments in working memory and behavioral flexibility as measured by prepulse inhibition and Y maze.

CONCLUSIONS

Altogether, these findings confirm CE-158 as a promising pro-cognitive agent and contribute to assessing its preclinical safety profile in a chronic administration regimen for further translational testing.

Interleukin-10 enhances activity of ventral tegmental area dopamine neurons resulting in increased dopamine release

Dopamine transmission from the ventral tegmental area (VTA) to the nucleus accumbens (NAc) regulates important aspects of motivation and is influenced by the neuroimmune system. The neuroimmune system is a complex network of leukocytes, microglia and astrocytes that detect and remove foreign threats like bacteria or viruses and communicate with each other to regulate non-immune (e.g neuronal) cell activity through cytokine signaling. Inflammation is a key regulator of motivational states, though the effects of specific cytokines on VTA circuitry and motivation are largely unknown. Therefore, electrophysiology, neurochemical, immunohistochemical and behavioral studies were performed to determine the effects of the anti-inflammatory cytokine interleukin-10 (IL-10) on mesolimbic activity, dopamine transmission and conditioned behavior. IL-10 enhanced VTA dopamine firing and NAc dopamine levels via decreased VTA GABA currents in dopamine neurons. The IL-10 receptor was localized on VTA dopamine and non-dopamine cells. The IL-10 effects on dopamine neurons required post-synaptic phosphoinositide 3-kinase activity, and IL-10 appeared to have little-to-no efficacy on presynaptic GABA terminals. Intracranial IL-10 enhanced NAc dopamine levels in vivo and produced conditioned place aversion. Together, these studies identify the IL-10R on VTA dopamine neurons as a potential regulator of motivational states.

Whole-Body Vibration Prevents Neuronal, Neurochemical, and Behavioral Effects of Morphine Withdrawal in a Rat Model

Peripheral mechanoreceptor-based treatments such as acupuncture and chiropractic manipulation have shown success in modulating the mesolimbic dopamine (DA) system originating in the ventral tegmental area (VTA) of the midbrain and projecting to the nucleus accumbens (NAc) of the striatum. We have previously shown that mechanoreceptor activation via whole-body vibration (WBV) ameliorates neuronal and behavioral effects of chronic ethanol exposure. In this study, we employ a similar paradigm to assess the efficacy of WBV as a preventative measure of neuronal and behavioral effects of morphine withdrawal in a Wistar rat model. We demonstrate that concurrent administration of WBV at 80 Hz with morphine over a 5-day period significantly reduced adaptations in VTA GABA neuronal activity and NAc DA release and modulated expression of δ-opioid receptors (DORs) on NAc cholinergic interneurons (CINs) during withdrawal. We also observed a reduction in behavior typically associated with opioid withdrawal. WBV represents a promising adjunct to current intervention for opioid use disorder (OUD) and should be examined translationally in humans.

Effects of treadmill exercise and chronic stress on anxiety-like behavior, neuronal activity, and oxidative stress in basolateral amygdala in morphine-treated rats

The basolateral amygdala (BLA), which is sensitive to stress, is necessary for reward-seeking behavior and addiction. Regular exercise can produce various positive effects by affecting the BLA. Therefore, we aimed to investigate the effects of chronic stress and treadmill running (TR) on anxiety-like behavior, neuronal activity, lipid peroxidation (measured by malondialdehyde (MDA) levels, a marker for oxidative stress), and total thiol in BLA, in morphine-treated rats. Male Wistar rats were restricted in restraint stress and/or ran on the treadmill and treated with morphine (5 mg/kg) for 21 days. Anxiety-like behavior was evaluated using an elevated plus maze (EPM) and open field tests (OFTs), on day 22. On day 23, neuronal activity in BLA was assessed via single-unit recording. Finally, MDA and total thiol were assessed in BLA. Our results showed that chronic administration of morphine (5 mg/kg) did not affect anxiety-like behavior. However, the morphine-treated rats, subjected to chronic stress and exercise, showed fewer anxiety-like behaviors. Morphine increased BLA's MDA levels but it was prevented by TR. Glutamatergic and GABAergic basal neuronal activities were low in morphine-treated rats but after acute morphine application, there was a significant decrease in GABAergic neuronal activities in the morphine-exercise-stress (Mor-Exe-St) group. The results of this study showed that in morphine-treated rats, stress and exercise or their combination could have either co-directional or opposite effects to the chronic effects of morphine. These results indicate the existence of common pathways similar to endogenous opioids.

Nociceptive Stimuli Activate the Hypothalamus-Habenula Circuit to Inhibit the Mesolimbic Reward System and Cocaine-Seeking Behaviors

Nociceptive signals interact with various regions of the brain, including those involved in physical sensation, reward, cognition, and emotion. Emerging evidence points to a role of nociception in the modulation of the mesolimbic reward system. The mechanism by which nociception affects dopamine (DA) signaling and reward is unclear. The lateral hypothalamus (LH) and the lateral habenula (LHb) receive somatosensory inputs and are structurally connected with the mesolimbic DA system. Here, we show that the LH-LHb pathway is necessary for nociceptive modulation of this system using male Sprague Dawley rats. Our extracellular single-unit recordings and head-mounted microendoscopic calcium imaging revealed that nociceptive stimulation by tail pinch excited LHb and LH neurons, which was inhibited by chemical lesion of the LH. Tail pinch increased activity of GABA neurons in ventral tegmental area, decreased the extracellular DA level in the nucleus accumbens ventrolateral shell in intact rats, and reduced cocaine-increased DA concentration, which was blocked by disruption of the LH. Furthermore, tail pinch attenuated cocaine-induced locomotor activity, 22 and 50 kHz ultrasonic vocalizations, and reinstatement of cocaine-seeking behavior, which was inhibited by chemogenetic silencing of the LH-LHb pathway. Our findings suggest that nociceptive stimulation recruits the LH-LHb pathway to inhibit mesolimbic DA system and drug reinstatement.SIGNIFICANCE STATEMENT The LHb and the LH have been implicated in processing nociceptive signals and modulating DA release in the mesolimbic DA system. Here, we show that the LH-LHb pathway is critical for nociception-induced modulation of mesolimbic DA release and cocaine reinstatement. Nociceptive stimulation alleviates extracellular DA release in the mesolimbic DA system, cocaine-induced psychomotor activities, and reinstatement of cocaine-seeking behaviors through the LH-LHb pathway. These findings provide novel evidence for sensory modulation of the mesolimbic DA system and drug addiction.

Mechanical Stimulation Alters Chronic Ethanol-Induced Changes to VTA GABA Neurons, NAc DA Release and Measures of Withdrawal

Therapeutic activation of mechanoreceptors (MStim) in osteopathy, chiropractic and acupuncture has been in use for hundreds of years with a myriad of positive outcomes. It has been previously shown to modulate the firing rate of neurons in the ventral tegmental area (VTA) and dopamine (DA) release in the nucleus accumbens (NAc), an area of interest in alcohol-use disorder (AUD). In this study, we examined the effects of MStim on VTA GABA neuron firing rate, DA release in the NAc, and behavior during withdrawal from chronic EtOH exposure in a rat model. We demonstrate that concurrent administration of MStim and EtOH significantly reduced adaptations in VTA GABA neurons and DA release in response to a reinstatement dose of EtOH (2.5 g/kg). Behavioral indices of EtOH withdrawal (rearing, open-field crosses, tail stiffness, gait, and anxiety) were substantively ameliorated with concurrent application of MStim. Additionally, MStim significantly increased the overall frequency of ultrasonic vocalizations, suggesting an increased positive affective state.

Modafinil Administration to Preadolescent Rat Impairs Non-Selective Attention, Frontal Cortex D2 Expression and Mesolimbic GABA Levels

The misuse of psychostimulants is an increasing behavior among young people, highlighting in some countries the abuse of modafinil (MOD) as a neuropotentiator. However, several clinical trials are investigating MOD as an alternative pharmacological treatment for attentional deficit and hyperactivity disorder (ADHD) in children and adolescents. On the other hand, the early use of psychostimulants and the misdiagnosis rates in ADHD make it crucial to investigate the brain effects of this type of drug in young healthy individuals. The aim of this work was to evaluate the effects of chronic MOD treatment on neurochemicals (γ-aminobutyric acid and glutamate), dopamine receptor 2 (D₂) expression and behavior (non-selective attention "NSA") in the mesocorticolimbic system of young healthy Sprague-Dawley rats. Preadolescent male rats were injected with MOD (75 mg/kg, i.p.) or a vehicle for 14 days (from postnatal day 22 to 35). At postnatal day 36, we measured the GLU and GABA contents and their extracellular levels in the nucleus accumbens (NAc). In addition, the GLU and GABA contents were measured in the ventral tegmental area (VTA) and D₂ protein levels in the prefrontal cortex (PFC). Chronic use of MOD during adolescence induces behavioral and neurochemical changes associated with the mesocorticolimbic system, such as a reduction in PFC D₂ expression, VTA GABA levels and NSA. These results contribute to the understanding of the neurological effects of chronic MOD use on a young healthy brain.

A non-canonical GABAergic pathway to the VTA promotes unconditioned freezing

Freezing is a conserved defensive behaviour that constitutes a major stress-coping mechanism. Decades of research have demonstrated a role of the amygdala, periaqueductal grey and hypothalamus as core actuators of the control of fear responses, including freezing. However, the role that other modulatory sites provide to this hardwired scaffold is not known. Here, we show that freezing elicited by exposure to electrical foot shocks activates laterodorsal tegmentum (LDTg) GABAergic neurons projecting to the VTA, without altering the excitability of cholinergic and glutamatergic LDTg neurons. Selective chemogenetic silencing of this inhibitory projection, but not other LDTg neuronal subtypes, dampens freezing responses but does not prevent the formation of conditioned fear memories. Conversely, optogenetic-activation of LDTg GABA terminals within the VTA drives freezing responses and elicits bradycardia, a common hallmark of freezing. Notably, this aversive information is subsequently conveyed from the VTA to the amygdala via a discrete GABAergic pathway. Hence, we unveiled a circuit mechanism linking LDTg-VTA-amygdala regions, which holds potential translational relevance for pathological freezing states such as post-traumatic stress disorders, panic attacks and social phobias.

Acupuncture reduces cocaine psychomotor responses by activating the rostromedial tegmental nucleus

The rostromedial tegmental nucleus (RMTg), a GABAergic afferent to midbrain dopamine (DA) neurons, has emerged as an integral player in both rewarding and nociceptive responses. While previous studies have demonstrated that acupuncture modulates DA transmission in the mesolimbic reward system originating in the ventral tegmental area (VTA) and projecting to the nucleus accumbens (NAc) and can reduce drug self-administration, the central links between peripheral acupuncture signals and brain reward systems are not well-characterized. Thus, we hypothesised that acupuncture would elicit inhibitory signals from RMTg neurons to brain reward systems. Acupuncture reduced acute cocaine-induced locomotor activity and DA release in a point-specific manner, which was blocked by optogenetic silencing or chemical lesion of the RMTg. The acupuncture effect was mimicked by chemical activation of the RMTg. Acupuncture activated RMTg GABA neurons. In addition, the inhibitory effects of acupuncture on acute cocaine-induced locomotor activity were prevented by electrolytic lesions of the lateral habenula (LHb) or fasciculus retroflexus (FR), areas known to project to the RMTg. These findings suggest that acupuncture recruits the RMTg to reduce the psychomotor responses enhanced by acute cocaine.

Reactivating a positive feedback loop VTA-BLA-NAc circuit associated with positive experience ameliorates the attenuated reward sensitivity induced by chronic stress

Both genetic predisposition and life events, particularly life stress, are thought to increase the risk for depression. Reward sensitivity appears to be attenuated in major depressive disorder (MDD), suggesting deficits in reward processing in these patients. We identified the VTA-BLA-NAc circuit as being activated by sex reward, and the VTA neurons that respond to sex reward are mostly dopaminergic. Acute or chronic reactivation of this circuit ameliorates the reward insensitivity induced by chronic restraint stress. Our histological and electrophysiological results show that the VTA neuron subpopulation responding to restraint stress, predominantly GABAergic neurons, inhibits the responsiveness of VTA dopaminergic neurons to reward stimuli, which is probably the mechanism by which stress modulates the reward processing neural circuits and subsequently disrupts reward-related behaviours. Furthermore, we found that the VTA-BLA-NAc circuit is a positive feedback loop. Blocking the projections from the BLA to the NAc associated with sex reward increases the excitability of VTA GABAergic neurons and decreases the excitability of VTA dopaminergic neurons, while activating this pathway decreases the excitability of VTA GABAergic neurons and increases the excitability of VTA dopaminergic neurons, which may be the cellular mechanism by which the VTA-BLA-NAc circuit associated with sex reward ameliorates the attenuated reward sensitivity induced by chronic stress.

GABAergic projections to the ventral tegmental area govern cocaine-conditioned reward

Elevated dopamine (DA) levels in the reward system underlie various drug-related behaviors, including addiction. As a major DA source in the reward system, the ventral tegmental area (VTA) is highly regulated by GABAergic inputs projected from different brain regions. It was previously shown that cocaine exposure reduces GABA_A -mediated inhibitory postsynaptic currents (IPSCs) in VTA DA neurons; however, the specific GABAergic input underlying this inhibitory effect remains unknown. Here, using optogenetics, we separately activate and characterize different GABAergic afferents innervating the VTA, focusing on the rostromedial tegmental nucleus (RMTg) and the nucleus accumbens (NAc). GABA_A -mediated IPSCs were recorded from VTA DA neurons, and the effect of DA-induced inhibition was measured in an afferent-specific manner. In addition, to examine the effect of enhanced GABAergic tone on the rewarding properties of cocaine, we exogenously activated the different GABAergic inputs during the acquisition phase of cocaine conditioned place preference (CPP). We found that acute cocaine exposure strongly attenuates GABA_A -mediated IPSCs in VTA DA neurons from both inhibitory sources. Furthermore, exogenous light activation of both RMTg and NAc afferents in the VTA during the acquisition of cocaine-CPP significantly reduced the rewarding properties of cocaine. This behavioral observation was correlated with the reduction in the neuronal activity of VTA DA neurons as measured by the expression of c-fos. Together, these results emphasize the critical role of these GABAergic inputs to the VTA in modulating and potentially interrupting cocaine reward.

NMDA receptor membrane dynamics tunes the firing pattern of midbrain dopaminergic neurons

KEY POINTS

NMDA receptors (NMDARs) expressed by dopamine neurons of the ventral tegmental area (VTA) play a central role in glutamate synapse plasticity, neuronal firing and adaptative behaviours. The NMDAR surface dynamics shapes synaptic adaptation in hippocampal networks, as well as associative memory. We investigated the basic properties and role of the NMDAR surface dynamics on cultured mesencephalic and VTA dopamine neurons in rodents. Using a combination of single molecule imaging and electrophysiological recordings, we demonstrate that NMDARs are highly diffusive at the surface of mesencephalic dopamine neurons. Unexpectedly, the NMDAR membrane dynamics per se regulates the firing pattern of VTA dopaminergic neurons, probably through a functional interplay between NMDARs receptors and small-conductance calcium-dependent potassium (SK) channels.

ABSTRACT

Midbrain dopaminergic (DA) neurons play a central role in major physiological brain functions, and their dysfunctions have been associated with neuropsychiatric diseases. The activity of midbrain DA neurons is controlled by ion channels and neurotransmitter receptors, such as the glutamate NMDA receptor (NMDAR) and small-conductance calcium-dependent potassium (SK) channels. However, the cellular mechanisms through which these channels tune the firing pattern of midbrain DA neurons remain unclear. Here, we investigated whether the surface dynamics and distribution of NMDARs tunes the firing pattern of midbrain DA neurons. Using a combination of single molecule imaging and electrophysiological recordings, we report that NMDARs are highly diffusive at the surface of cultured midbrain DA neurons from rodents and humans. Reducing acutely the NMDAR membrane dynamics, which leaves the ionotropic function of the receptor intact, robustly altered the firing pattern of midbrain DA neurons without altering synaptic glutamatergic transmission. The reduction of NMDAR surface dynamics reduced apamin (SK channel blocker)-induced firing change and the distribution of SK3 channels in DA neurons. Together, these data show that the surface dynamics of NMDAR, and not solely its ionotropic function, tune the firing pattern of midbrain DA neurons partly through a functional interplay with SK channel function.

Nociceptin/orphanin FQ neurons in the Arcuate Nucleus and Ventral Tegmental Area Act via Nociceptin Opioid Peptide Receptor Signaling to Inhibit Proopiomelanocortin and A10 Dopamine Neurons and Thereby Modulate Ingestion of Palatable Food

The neuropeptide nociceptin/orphanin FQ (N/OFQ) inhibits neuronal activity via its cognate nociceptin opioid peptide (NOP) receptor throughout the peripheral and central nervous systems, including those areas involved in the homeostatic and hedonic regulation of energy homeostasis. We thus tested the hypothesis that N/OFQ neurons in the hypothalamic arcuate nucleus (ARC) and ventral tegmental area (VTA) act via NOP receptor signaling to inhibit nearby anorexigenic proopiomelanocortin (POMC) and A10 dopamine neuronal excitability, respectively, and thereby modulate ingestion of palatable food. Electrophysiologic recordings were performed in slices prepared from transgenic male and ovariectomized (OVX) female N/OFQ-cre/enhanced green fluorescent protein-POMC, N/OFQ-cre and tyrosine hydroxylase-cre animals to see if optogenetically-stimulated peptide release from N/OFQ neurons could directly inhibit these neuronal populations. Binge-feeding behavioral experiments were also conducted where animals were exposed to a high-fat-diet (HFD) for one hour each day for five days and monitored for energy intake. Photostimulation of ARC and VTA N/OFQ neurons produces an outward current in POMC and A10 dopamine neurons receiving input from these cells. This is associated with a hyperpolarization and decreased firing. These features are also sex hormone- and diet-dependent; with estradiol-treated slices from OVX females being less sensitive, and obese males being more sensitive, to N/OFQ. Limited access to HFD causes a dramatic escalation in consumption, such that animals eat 25-45% of their daily intake during that one-hour exposure. Moreover, the NOP receptor-mediated regulation of these energy balance circuits are engaged, as N/OFQ injected directly into the VTA or ARC respectively diminishes or potentiates this binge-like increase in a manner heightened by diet-induced obesity or dampened by estradiol in females. Collectively, these findings provide key support for the idea that N/OFQ regulates appetitive behavior in sex-, site- and diet-specific ways, along with important insights into aberrant patterns of feeding behavior pertinent to the pathogenesis of food addiction.

In silico Hierarchical Clustering of Neuronal Populations in the Rat Ventral Tegmental Area Based on Extracellular Electrophysiological Properties

The ventral tegmental area (VTA) is a heterogeneous brain region, containing different neuronal populations. During in vivo recordings, electrophysiological characteristics are classically used to distinguish the different populations. However, the VTA is also considered as a region harboring neurons with heterogeneous properties. In the present study, we aimed to classify VTA neurons using in silico approaches, in an attempt to determine if homogeneous populations could be extracted. Thus, we recorded 291 VTA neurons during in vivo extracellular recordings in anesthetized rats. Initially, 22 neurons with high firing rates (>10 Hz) and short-lasting action potentials (AP) were considered as a separate subpopulation, in light of previous studies. To segregate the remaining 269 neurons, presumably dopaminergic (DA), we performed in silico analyses, using a combination of different electrophysiological parameters. These parameters included: (1) firing rate; (2) firing rate coefficient of variation (CV); (3) percentage of spikes in a burst; (4) AP duration; (5) Δt₁ duration (i.e., time from initiation of depolarization until end of repolarization); and (6) presence of a notched AP waveform. Unsupervised hierarchical clustering revealed two neuronal populations that differed in their bursting activities. The largest population presented low bursting activities (<17.5% of total spikes in burst), while the remaining neurons presented higher bursting activities (>17.5%). Within non-high-firing neurons, a large heterogeneity was noted concerning AP characteristics. In conclusion, this analysis based on conventional electrophysiological criteria clustered two subpopulations of putative DA VTA neurons that are distinguishable by their firing patterns (firing rates and bursting activities) but not their AP properties.

Heterogeneous somatostatin-expressing neuron population in mouse ventral tegmental area

The cellular architecture of the ventral tegmental area (VTA), the main hub of the brain reward system, remains only partially characterized. To extend the characterization to inhibitory neurons, we have identified three distinct subtypes of somatostatin (Sst)-expressing neurons in the mouse VTA. These neurons differ in their electrophysiological and morphological properties, anatomical localization, as well as mRNA expression profiles. Importantly, similar to cortical Sst-containing interneurons, most VTA Sst neurons express GABAergic inhibitory markers, but some of them also express glutamatergic excitatory markers and a subpopulation even express dopaminergic markers. Furthermore, only some of the proposed marker genes for cortical Sst neurons were expressed in the VTA Sst neurons. Physiologically, one of the VTA Sst neuron subtypes locally inhibited neighboring dopamine neurons. Overall, our results demonstrate the remarkable complexity and heterogeneity of VTA Sst neurons and suggest that these cells are multifunctional players in the midbrain reward circuitry.

VTA GABA Neurons at the Interface of Stress and Reward

The ventral tegmental area (VTA) is best known for its robust dopaminergic projections to forebrain regions and their critical role in regulating reward, motivation, cognition, and aversion. However, the VTA is not only made of dopamine (DA) cells, as approximately 30% of cells in the VTA are GABA neurons. These neurons play a dual role, as VTA GABA neurons provide both local inhibition of VTA DA neurons and long-range inhibition of several distal brain regions. VTA GABA neurons have increasingly been recognized as potent mediators of reward and aversion in their own right, as well as potential targets for the treatment of addiction, depression, and other stress-linked disorders. In this review article, we dissect the circuit architecture, physiology, and behavioral roles of VTA GABA neurons and suggest critical gaps to be addressed.

Mechanical stimulation of cervical vertebrae modulates the discharge activity of ventral tegmental area neurons and dopamine release in the nucleus accumbens

Background:

Growing evidence suggests that mechanical stimulation modulates substrates in the supraspinal central nervous system (CNS) outside the canonical somatosensory circuits.

Objective/Methods:

We evaluate mechanical stimulation applied to the cervical spine at the C7-T1 level (termed “MStim”) on neurons and neurotransmitter release in the mesolimbic dopamine (DA) system, an area implicated in reward and motivation, utilizing electrophysiological, pharmacological, neurochemical and immunohistochemical techniques in Wistar rats.

Results:

Low frequency (45–80 Hz), but not higher frequency (115 Hz), MStim inhibited the firing rate of ventral tegmental area (VTA) GABA neurons (52.8% baseline; 450 s) while increasing the firing rate of VTA DA neurons (248% baseline; 500 s). Inactivation of the nucleus accumbens (NAc), or systemic or in situ antagonism of delta opioid receptors (DORs), blocked MStim inhibition of VTA GABA neuron firing rate. MStim enhanced both basal (178.4% peak increase at 60 min) and evoked DA release in NAc (135.0% peak increase at 40 min), which was blocked by antagonism of DORs or acetylcholine release in the NAc. MStim enhanced c-FOS expression in the NAc, but inhibited total expression in the VTA, and induced translocation of DORs to neuronal membranes in the NAc.

Conclusion:

These findings demonstrate that MStim modulates neuron firing and DA release in the mesolimbic DA system through endogenous opioids and acetylcholine in the NAc. These findings demonstrate the need to explore more broadly the extra-somatosensory effects of peripheral mechanoreceptor activation and the specific role for mechanoreceptor-based therapies in the treatment of substance abuse.

Time-varying Effects of GABRG1 and Maladaptive Peer Behavior on Externalizing Behavior from Childhood to Adulthood: Testing Gene × Environment × Development Effects

Engagement in externalizing behavior is problematic. Deviant peer affiliation increases risk for externalizing behavior. Yet, peer effects vary across individuals and may differ across genes. This study determines gene × environment × development interactions as they apply to externalizing behavior from childhood to adulthood. A sample (n = 687; 68% male, 90% White) of youth from the Michigan Longitudinal Study was assessed from ages 10 to 25. Interactions between γ-amino butyric acid type A receptor γ1 subunit (GABRG1; rs7683876, rs13120165) and maladaptive peer behavior on externalizing behavior were examined using time-varying effect modeling. The findings indicate a sequential risk gradient in the influence of maladaptive peer behavior on externalizing behavior depending on the number of G alleles during childhood through adulthood. Individuals with the GG genotype are most vulnerable to maladaptive peer influences, which results in greater externalizing behavior during late childhood through early adulthood.

Alpha-synuclein is strategically positioned for afferent modulation of midbrain dopamine neurons and is essential for cocaine preference

Alpha-synuclein (α-syn) is an abundant neuroprotein elevated in cocaine addicts, linked to drug craving, and recruited to axon terminals undergoing glutamatergic plasticity - a proposed mechanism for substance abuse. However, little is known about normal α-syn function or how it contributes to substance abuse. We show that α-syn is critical for preference of hedonic stimuli and the cognitive flexibility needed to change behavioral strategies, functions that are altered with substance abuse. Electron microscopic analysis reveals changes in α-syn targeting of ventral tegmental area axon terminals that is dependent upon the duration of cocaine exposure. The dynamic changes in presynaptic α-syn position it to control neurotransmission and fine-tune the complex afferent inputs to dopamine neurons, potentially altering functional dopamine output. Cocaine also increases postsynaptic α-syn where it is needed for normal ALIX function, multivesicular body formation, and cocaine-induced exosome release indicating potentially similar α-syn actions for vesicle release pre- and post-synaptically.

Delta-9-tetrahydrocannabinol potentiates fear memory salience through functional modulation of mesolimbic dopaminergic activity states

Chronic or acute exposure to delta-9-tetrahydrocannabinol (THC), the main psychoactive compound in cannabis, has been associated with numerous neuropsychiatric side-effects, including dysregulation of emotional processing and associative memory formation. Clinical and preclinical evidence suggests that the effects of THC are due to the ability to modulate mesolimbic dopamine (DA) activity states in the nucleus accumbens (NAc) and ventral tegmental area (VTA). Nevertheless, the mechanisms by which THC modulates mesolimbic DA function and emotional processing are not well understood. Using an olfactory associative fear memory procedure combined with in vivo neuronal electrophysiology, we examined the effects of direct THC microinfusions targeting the shell region of the NAc (NASh) and examined how THC may modulate the processing of fear-related emotional memory and concomitant activity states of the mesolimbic DA system. We report that intra-NASh THC dose-dependently potentiates the emotional salience of normally subthreshold fear conditioning cues. These effects were dependent upon intra-VTA transmission through GABAergic receptor mechanisms and intra-NASh DAergic transmission. Furthermore, doses of intra-NASh THC that potentiated fear memory salience were found to modulate intra-VTA neuronal network activity by increasing the spontaneous firing and bursting frequency of DAergic neurones whilst decreasing the activity levels of a subpopulation of putative GABAergic VTA neurones. These findings demonstrate that THC can act directly in the NASh to modulate mesolimbic activity states and induce disturbances in emotional salience and memory formation through modulation of VTA DAergic transmission.

Conventional and Novel Pharmacological Approaches to Treat Dopamine-Related Disorders: Focus on Parkinson's Disease and Schizophrenia

The dopaminergic system integrated by cell groups distributed in several brain regions exerts a modulatory role in brain. Particularly important for this task are the mesencephalic dopamine neurons, which from the substantia nigra and ventral tegmental area project to the dorsal striatum and the cortical/subcortical limbic systems, respectively. Dopamine released from these neurons operates mainly via the short distance extrasynaptic volume transmission and activates five different dopaminergic receptor subtypes modulating synaptic GABA and glutamate transmission. To accomplish this task dopaminergic neurons keep mutual modulating interactions with neurons of other neurotransmitter systems, including allosteric receptor-receptor interactions in heteroreceptor complexes. As a result of its modulatory role dopaminergic mechanisms are involved in either the etiology or physiopathology of many brain diseases such as Parkinsońs disease and schizophrenia. The aim of this work is to review some novel and conventional approaches that either have been used or are currently employed to treat these diseases. Particular attention is paid to the approaches derived from the knowledge recently acquired in the realm of receptor-receptor interactions taking place through multiple dopamine heteroreceptor complexes in the plasma membrane. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Lack of correlation between the activity of the mesolimbic dopaminergic system and the rewarding properties of pregabalin in mouse

RATIONALE

Pregabalin is a psychoactive drug indicated in the treatment of epilepsy, neuropathic pain, and generalized anxiety disorders. Pregabalin acts on different neurotransmission systems by inactivating the alpha2-delta subunit of voltage-gated calcium channels. In light of this pharmacological property, the hypothesis has been raised that pregabalin may regulate the mesolimbic dopamine pathway and thereby display a potential for misuse or abuse as recently observed in humans. Although some preclinical data support this possibility, the rewarding properties of gabapentinoid are still a matter for debate.

OBJECTIVE

The aim of this work was to evaluate the rewarding properties of pregabalin and to determine its putative mechanism of action in healthy mice.

RESULTS

Pregabalin alone (60 mg/kg; s.c.) produced a rewarding effect in the conditioned place preference (CPP) test albeit to a lower extent than cocaine (30 mg/kg; s.c.). Interestingly, when assessing locomotor activity in the CPP, the PGB60 group, similarly to the cocaine group, showed an increased locomotor activity. In vivo single unit extracellular recording showed that pregabalin had mixed effects on dopamine (DA) neuronal activity in the ventral tegmental area since it decreased the activity of 50% of neurons and increased 28.5% of them. In contrast, cocaine decreased 75% of VTA DA neuronal activity whereas none of the neurons were activated. Intracerebal microdialysis was then conducted in awake freely mice to determine to what extent such electrophysiological parameters influence the extracellular DA concentrations ([DA]ext) in the nucleus accumbens. Although pregabalin failed to modify this parameter, cocaine produced a robust increase (800%) in [DA]ext.

CONCLUSIONS

Collectively, these electrophysiological and neurochemical experiments suggest that the rewarding properties of pregabalin result from a different mode of action than that observed with cocaine. Further experiments are warranted to determine whether such undesirable effects can be potentiated under pathological conditions such as neuropathic pain, mood disorders, or addiction and to identify the key neurotransmitter system involved.

Dopamine neurons projecting to medial shell of the nucleus accumbens drive heroin reinforcement

The dopamine (DA) hypothesis posits the increase of mesolimbic dopamine levels as a defining commonality of addictive drugs, initially causing reinforcement, eventually leading to compulsive consumption. While much experimental evidence from psychostimulants supports this hypothesis, it has been challenged for opioid reinforcement. Here, we monitor genetically encoded DA and calcium indicators as well as cFos in mice to reveal that heroin activates DA neurons located in the medial part of the VTA, preferentially projecting to the medial shell of the nucleus accumbens (NAc). Chemogenetic and optogenetic manipulations of VTA DA or GABA neurons establish a causal link to heroin reinforcement. Inhibition of DA neurons blocked heroin self-administration, while heroin inhibited optogenetic self-stimulation of DA neurons. Likewise, heroin occluded the self-inhibition of VTA GABA neurons. Together, these experiments support a model of disinhibition of a subset of VTA DA neurons in opioid reinforcement.

Dynamical ventral tegmental area circuit mechanisms of alcohol-dependent dopamine release

A large body of data has identified numerous molecular targets through which ethanol (EtOH) acts on brain circuits. Yet how these multiple mechanisms interact to result in dysregulated dopamine (DA) release under the influence of alcohol in vivo remains unclear. In this manuscript, we delineate potential circuit-level mechanisms responsible for EtOH-dependent dysregulation of DA release from the ventral tegmental area (VTA) into its projection areas. For this purpose, we constructed a circuit model of the VTA that integrates realistic Glutamatergic (Glu) inputs and reproduces DA release observed experimentally. We modelled the concentration-dependent effects of EtOH on its principal VTA targets. We calibrated the model to reproduce the inverted U-shape dose dependence of DA neuron activity on EtOH concentration. The model suggests a primary role of EtOH-induced boost in the I_h and AMPA currents in the DA firing-rate/bursting increase. This is counteracted by potentiated GABA transmission that decreases DA neuron activity at higher EtOH concentrations. Thus, the model connects well-established in vitro pharmacological EtOH targets with its in vivo influence on neuronal activity. Furthermore, we predict that increases in VTA activity produced by moderate EtOH doses require partial synchrony and relatively low rates of the Glu afferents. We propose that the increased frequency of transient (phasic) DA peaks evoked by EtOH results from synchronous population bursts in VTA DA neurons. Our model predicts that the impact of acute ETOH on dopamine release is critically shaped by the structure of the cortical inputs to the VTA.

Glutamate Transmission to Ventral Tegmental Area GABA Neurons Is Altered by Acute and Chronic Ethanol

BACKGROUND

Ventral tegmental area (VTA) GABA neurons have been heavily implicated in alcohol reinforcement and reward. In animals that self-administer alcohol, VTA GABA neurons exhibit increased excitability that may contribute to alcohol's rewarding effects. The present study investigated the effects of acute and chronic ethanol exposure on glutamate (GLU) synaptic transmission to VTA GABA neurons.

METHODS

Whole-cell recordings of evoked, spontaneous, and miniature excitatory postsynaptic currents (eEPSCs, sEPSCs, and mEPSCs, respectively) were performed on identified GABA neurons in the VTA of GAD67-GFP+ transgenic mice. Three ethanol exposure paradigms were used: acute ethanol superfusion; a single ethanol injection; and chronic vapor exposure.

RESULTS

Acute ethanol superfusion increased the frequency of EPSCs but inhibited mEPSC frequency and amplitude. During withdrawal from a single injection of ethanol, the frequency of sEPSCs was lower than saline controls. There was no difference in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/N-methyl-d-aspartate (NMDA) ratio between neurons following withdrawal from a single exposure to ethanol. However, following withdrawal from chronic ethanol, sEPSCs and mEPSCs had a greater frequency than air controls. There was no difference in AMPA/NMDA ratio following chronic ethanol.

CONCLUSIONS

These results suggest that presynaptic mechanisms involving local circuit GLU neurons, and not GLU receptors, contribute to adaptations in VTA GABA neuron excitability that accrue to ethanol exposure, which may contribute to the rewarding properties of alcohol via their regulation of mesolimbic dopamine transmission.

α6 subunit-containing nicotinic receptors mediate low-dose ethanol effects on ventral tegmental area neurons and ethanol reward

Dopamine (DA) neuron excitability is regulated by inhibitory GABAergic synaptic transmission and modulated by nicotinic acetylcholine receptors (nAChRs). The aim of this study was to evaluate the role of α6 subunit-containing nAChRs (α6*-nAChRs) in acute ethanol effects on ventral tegmental area (VTA) GABA and DA neurons. α6*-nAChRs were visualized on GABA terminals on VTA GABA neurons, and α6*-nAChR transcripts were expressed in most DA neurons, but only a minority of VTA GABA neurons from GAD67 GFP mice. Low concentrations of ethanol (1-10 mM) enhanced GABA_A receptor (GABA_A R)-mediated spontaneous and evoked inhibition with blockade by selective α6*-nAChR antagonist α-conotoxins (α-Ctxs) and lowered sensitivity in α6 knock-out (KO) mice. Ethanol suppression of VTA GABA neuron firing rate in wild-type mice in vivo was significantly reduced in α6 KO mice. Ethanol (5-100 mM) had no effect on optically evoked GABA_A R-mediated inhibition of DA neurons, and ethanol enhancement of VTA DA neuron firing rate at high concentrations was not affected by α-Ctxs. Ethanol conditioned place preference was reduced in α6 KO mice compared with wild-type controls. Taken together, these studies indicate that relatively low concentrations of ethanol act through α6*-nAChRs on GABA terminals to enhance GABA release onto VTA GABA neurons, in turn to reduce GABA neuron firing, which may lead to VTA DA neuron disinhibition, suggesting a possible mechanism of action of alcohol and nicotine co-abuse.

Glutamatergic Ventral Pallidal Neurons Modulate Activity of the Habenula-Tegmental Circuitry and Constrain Reward Seeking

BACKGROUND

The ability to appropriately integrate and respond to rewarding and aversive stimuli is essential for survival. The ventral pallidum (VP) plays a critical role in processing both rewarding and aversive stimuli. However, the VP is a heterogeneous structure, and how VP subpopulations integrate into larger reward networks to ultimately modulate these behaviors is not known. We identify a noncanonical population of glutamatergic VP neurons that play a unique role in responding to aversive stimuli and constraining inappropriate reward seeking.

METHODS

Using neurochemical, genetic, and electrophysiological approaches, we characterized glutamatergic VP neurons (n = 4-8 mice/group). We performed patch clamp and in vivo electrophysiology recordings in the lateral habenula, rostromedial tegmental nucleus, and ventral tegmental area to determine the effect of glutamatergic VP neuron activation in these target regions (n = 6-10 mice/group). Finally, we selectively optogenetically stimulated glutamatergic VP neurons in a real-time place preference task and ablated these neurons using a virally expressed caspase to determine their necessity for reward seeking.

RESULTS

Glutamatergic VP neurons exhibit little overlap with cholinergic or gamma-aminobutyric acidergic markers, the canonical VP subtypes, and exhibit distinct membrane properties. Glutamatergic VP neurons innervate and increase firing activity of the lateral habenula, rostromedial tegmental nucleus, and gamma-aminobutyric acidergic ventral tegmental area neurons. While nonselective optogenetic stimulation of the VP induced a robust place preference, selective activation of glutamatergic VP neurons induced a place avoidance. Viral ablation of glutamatergic VP neurons increased reward responding and abolished taste aversion to sucrose.

CONCLUSIONS

Glutamatergic VP neurons constitute a noncanonical subpopulation of VP neurons. These glutamatergic VP neurons increase activity of the lateral habenula, rostromedial tegmental nucleus, and gamma-aminobutyric acidergic ventral tegmental area neurons and adaptively constrain reward seeking.

Synaptic function and plasticity in identified inhibitory inputs onto VTA dopamine neurons

Ventral tegmental area (VTA) dopaminergic neurons are key components of the reward pathway, and their activity is powerfully controlled by a diverse array of inhibitory GABAergic inputs. Two major sources of GABAergic nerve terminals within the VTA are local VTA interneurons and neurons in the rostromedial tegmental nucleus (RMTg). Here, using optogenetics, we compared synaptic properties of GABAergic synapses on VTA dopamine neurons using selective activation of afferents that originate from these two cell populations. We found little evidence of co-release of glutamate from either input, but RMTg-originating synaptic currents were reduced by strychnine, suggesting co-release of glycine and GABA. VTA-originating synapses displayed a lower initial release probability, and at higher frequency stimulation, short-term depression was more marked in VTA- but not RMTg-originating synapses. We previously reported that nitric oxide (NO)-induced potentiation of GABAergic synapses on VTA dopaminergic cells is lost after exposure to drugs of abuse or acute stress; in these experiments, multiple GABAergic afferents were simultaneously activated by electrical stimulation. Here we found that optogenetically-activated VTA-originating synapses on presumptive dopamine neurons also exhibited NO-induced potentiation, whereas RMTg-originating synapses did not. Despite providing a robust inhibitory input to the VTA, RMTg GABAergic synapses are most likely not those previously shown by our work to be persistently altered by addictive drugs and stress. Our work emphasises the idea that dopamine neuron excitability is controlled by diverse inhibitory inputs expected to exert varying degrees of inhibition and to participate differently in a range of behaviours.

The Role of Ventral Tegmental Area Gamma-Aminobutyric Acid in Chronic Neuropathic Pain after Spinal Cord Injury in Rats

Spinal cord injury (SCI) frequently results in chronic neuropathic pain (CNP). However, the understanding of brain neural circuits in CNP modulation is unclear. The present study examined the changes of ventral tegmental area (VTA) putative GABAergic and dopaminergic neuronal activity with CNP attenuation in rats. SCI was established by T10 clip compression injury (35 g, 1 min) in rats, and neuropathic pain behaviors, in vivo extracellular single-cell recording of putative VTA gamma-aminobutyric acid (GABA)/dopamine neurons, extracellular GABA level, glutamic acid decarboxylase (GAD), and vesicular GABA transporters (VGATs) were measured in the VTA, respectively. The results revealed that extracellular GABA level was significantly increased in the CNP group (50.5 ± 18.9 nM) compared to the sham control group (10.2 ± 1.7 nM). In addition, expression of GAD_65/67, c-Fos, and VGAT exhibited significant increases in the SCI groups compared to the sham control group. With regard to neuropathic pain behaviors, spontaneous pain measured by ultrasound vocalizations (USVs) and evoked pain measured by paw withdrawal thresholds showed significant alteration, which was reversed by intravenous (i.v.) administration of morphine (0.5-5.0 mg/kg). With regard to in vivo electrophysiology, VTA putative GABAergic neuronal activity (13.6 ± 1.7 spikes/sec) and putative dopaminergic neuronal activity (2.4 ± 0.8 spikes/sec) were increased and decreased, respectively, in the SCI group compared to the sham control group. These neuronal activities were reversed by i.v. administration of morphine. The present study suggests that chronic increase of GABAergic neuronal activity suppresses dopaminergic neuronal activity in the VTA and is responsible for negative emotion and motivation for attenuation of SCI-induced CNP.

VTA mTOR Signaling Regulates Dopamine Dynamics, Cocaine-Induced Synaptic Alterations, and Reward

Mechanistic target of rapamycin (mTOR) regulates long-term synaptic plasticity, learning, and memory by controlling dendritic protein synthesis. The mTOR inhibitor rapamycin has been shown to attenuate the behavioral effects of drugs of abuse, including cocaine. Using viral vectors to selectively delete mTOR in the ventral tegmental area (VTA) in adult male mTOR^loxP/loxP mice, we investigated the role of mTOR in regulating neuronal morphology, basal synaptic transmission, dopamine dynamics, and cocaine-induced synaptic plasticity and rewarding effects. We find that targeted deletion of mTOR in the VTA had no significant effects on soma size and dendritic morphology of VTA neurons but significantly decreased dopamine release and reuptake in the nucleus accumbens (NAc) shell, a major target region. Western blot analysis revealed that mTOR deletion led to decreases in phosphorylated tyrosine hydroxylase (pTH-Ser40) levels in the VTA and dopamine transporter expression in the NAc. mTOR deletion had no significant effects on basal excitatory transmission in VTA dopamine neurons but caused an increase in GABAergic inhibition because of an increase in VTA GABAergic neuron firing. Furthermore, mTOR deletion attenuated conditioned place preference to cocaine and cocaine-induced potentiation of excitation and reduction of GABAergic inhibition in VTA dopamine neurons. Taken together, these results suggest that loss of mTOR in the VTA shifts the balance of excitatory and inhibitory synaptic transmission and decreases dopamine release and reuptake in the NAc. In addition, VTA mTOR signaling regulates cocaine-cue associative learning and cocaine-induced synaptic plasticity in VTA dopamine neurons.

Ventral Tegmental Area GABA Neurons Are Resistant to GABA(A) Receptor-Mediated Inhibition During Ethanol Withdrawal

The neural mechanisms underlying alcohol dependence are not well-understood. GABAergic neurons in the ventral tegmental area (VTA) are a relevant target for ethanol. They are inhibited by ethanol at physiologically-relevant levels in vivo and display marked hyperexcitability during withdrawal. In the present study, we examined the effects of the GABA(A) receptor agonist muscimol on VTA neurons ex vivo following withdrawal from acute and chronic ethanol exposure. We used standard cell-attached mode electrophysiology in the slice preparation to evaluate the effects of muscimol on VTA GABA neuron firing rate following exposure to acute and chronic ethanol in male CD-1 GAD-67 GFP mice. In the acute condition, the effect of muscimol on VTA neurons was evaluated 24 h and 7 days after a single in vivo dose of saline or ethanol. In the chronic condition, the effect of muscimol on VTA neurons was evaluated 24 h and 7 days after either 2 weeks of twice-daily IP ethanol or saline or following exposure to chronic intermittent ethanol (CIE) vapor or air for 3 weeks. VTA GABA neuron firing rate was more sensitive to muscimol than DA neuron firing rate. VTA GABA neurons, but not DA neurons, were resistant to the inhibitory effects of muscimol recorded 24 h after a single ethanol injection or chronic ethanol exposure. Administration of the NMDA receptor antagonist MK-801 before ethanol injection restored the sensitivity of VTA GABA neurons to muscimol inhibition. Seven days after ethanol exposure, VTA GABA neuron firing rate was again susceptible to muscimol's inhibitory effects in the acute condition, but the resistance persisted in the chronic condition. These findings suggest that VTA GABA neurons exclusively undergo a shift in GABA(A) receptor function following acute and chronic exposure. There appears to be transient GABA(A) receptor-mediated plasticity after a single exposure to ethanol that is mediated by NMDA glutamate receptors. In addition, the resistance to muscimol inhibition in VTA GABA neurons persists in the dependent condition, which may contribute to the the hyperexcitability of VTA GABA neurons and inhibition of VTA DA neurons during withdrawal as well as the motivation to seek alcohol.

Acupuncture reduces relapse to cocaine-seeking behavior via activation of GABA neurons in the ventral tegmental area

There is growing public interest in alternative approaches to addiction treatment and scientific interest in elucidating the neurobiological underpinnings of acupuncture. Our previous studies showed that acupuncture at a specific Shenmen (HT7) points reduced dopamine (DA) release in the nucleus accumbens (NAc) induced by drugs of abuse. The present study was carried out to evaluate the effects of HT7 acupuncture on γ-aminobutyric acid (GABA) neuronal activity in the ventral tegmental area (VTA) and the reinstatement of cocaine-seeking behavior. Using microdialysis and in vivo single-unit electrophysiology, we evaluated the effects of HT7 acupuncture on VTA GABA and NAc DA release and VTA GABA neuronal activity in rats. Using a within-session reinstatement paradigm in rats self-administering cocaine, we evaluated the effects of HT7 stimulation on cocaine-primed reinstatement. Acupuncture at HT7 significantly reduced cocaine suppression of GABA release and GABA neuron firing rates in the VTA. HT7 acupuncture attenuated cocaine-primed reinstatement, which was blocked by VTA infusions of the selective GABA_B receptor antagonist 2-hydroxysaclofen. HT7 stimulation significantly decreased acute cocaine-induced DA release in the NAc, which was also blocked by 2-hydroxysaclofen. HT7 acupuncture also attenuated cocaine-induced sensitization of extracellular DA levels in the NAc. Moreover, HT7 acupuncture reduced both locomotor activity and neuronal activation in the NAc induced by acute cocaine in a needle-penetration depth-dependent fashion. These results suggest that acupuncture may suppress cocaine-induced DA release in the NAc and cocaine-seeking behavior through activation of VTA GABA neurons. Acupuncture may be an effective therapy to reduce cocaine relapse by enhancing GABAergic inhibition in the VTA.

Alterations in Rat Accumbens Endocannabinoid and GABA Content during Fentanyl Treatment: The Role of Ghrelin

The opioid-induced rise of extracellular dopamine, endocannabinoid anandamide and γ-aminobutyric acid (GABA) concentrations triggered by opioids in the nucleus accumbens shell (NACSh) most likely participate in opioid reward. We have previously demonstrated that systemic administration of ghrelin antagonist (JMV2959) significantly decreased morphine-induced dopamine and anandamide (N-arachidonoylethanolamine, AEA) increase in the NACSh. Fentanyl is considered as a µ-receptor-selective agonist. The aim of this study was to test whether JMV2959, a growth hormone secretagogue receptor (GHS-R1A) antagonist, can influence the fentanyl-induced effects on anandamide, 2-arachidonoylglycerol (2-AG) and GABA in the NACSh and specify the involvement of GHS-R1A located in the ventral tegmental area (VTA) and nucleus accumbens (NAC). Using in vivo microdialysis in rats, we have found that pre-treatment with JMV2959 reversed dose dependently fentanyl-induced anandamide increases in the NACSh, resulting in a significant AEA decrease and intensified fentanyl-induced decreases in accumbens 2-AG levels, with both JMV2959 effects more expressed when administered into the NACSh in comparison to the VTA. JMV2959 pre-treatment significantly decreased the fentanyl-evoked accumbens GABA efflux and reduced concurrently monitored fentanyl-induced behavioural stimulation. Our current data encourage further investigation to assess if substances affecting GABA or endocannabinoid concentrations and action, such as GHS-R1A antagonists, can be used to prevent opioid-seeking behaviour.

Optogenetic Investigation of Arousal Circuits

Modulation between sleep and wake states is controlled by a number of heterogeneous neuron populations. Due to the topological proximity and genetic co-localization of the neurons underlying sleep-wake state modulation optogenetic methods offer a significant improvement in the ability to benefit from both the precision of genetic targeting and millisecond temporal control. Beginning with an overview of the neuron populations mediating arousal, this review outlines the progress that has been made in the investigation of arousal circuits since the incorporation of optogenetic techniques and the first in vivo application of optogenetic stimulation in hypocretin neurons in the lateral hypothalamus. This overview is followed by a discussion of the future progress that can be made by incorporating more recent technological developments into the research of neural circuits.

Stressful life events increase aggression and alcohol use in young carriers of the GABRA2 rs279826/rs279858 A-allele

Research of GABRA2 gene in alcohol use and impulse control suggests its role in aggressive behaviour. The purpose of the present study was to examine the effects of GABRA2 genotype and stressful life events on aggressive behaviour, alcohol use frequency and occurrence of alcohol use disorder in a population representative sample of adolescents followed up from third grade to 25 years of age. The sample consisted of the younger cohort of the longitudinal Estonian Children Personality, Behaviour and Health Study. Aggressive behaviour was rated with the activity scale of af Klinteberg, Illinois Bully Scale and Buss-Perry Aggression Questionnaire. Stressful life events and alcohol use were self-reported. Life history of aggression and lifetime occurrence of psychiatric disorders were estimated in a structured interview. The sample was genotyped for GABRA2 rs279826 and rs279858 polymorphisms that are in strong linkage disequilibrium and yielded very similar findings: Higher number of stressful life events reported at age 15 was associated with increased fighting in A-allele carriers, but not in GG homozygotes. At age 25, A-allele carriers with more stressful life events scored higher on physical aggression than those with less stress, and this was also observed regarding life history of aggression. A-allele carriers exposed to higher stress had consumed alcoholic beverages more frequently at age 15, and by age 25, they had alcohol use disorder with higher prevalence. The results of the present study suggest that the GABRA2 genotype interacts with stress in young people with impact on the development of alcohol use and aggressive behaviour.

Dopamine Neurons Change the Type of Excitability in Response to Stimuli

The dynamics of neuronal excitability determine the neuron's response to stimuli, its synchronization and resonance properties and, ultimately, the computations it performs in the brain. We investigated the dynamical mechanisms underlying the excitability type of dopamine (DA) neurons, using a conductance-based biophysical model, and its regulation by intrinsic and synaptic currents. Calibrating the model to reproduce low frequency tonic firing results in N-methyl-D-aspartate (NMDA) excitation balanced by γ-Aminobutyric acid (GABA)-mediated inhibition and leads to type I excitable behavior characterized by a continuous decrease in firing frequency in response to hyperpolarizing currents. Furthermore, we analyzed how excitability type of the DA neuron model is influenced by changes in the intrinsic current composition. A subthreshold sodium current is necessary for a continuous frequency decrease during application of a negative current, and the low-frequency "balanced" state during simultaneous activation of NMDA and GABA receptors. Blocking this current switches the neuron to type II characterized by the abrupt onset of repetitive firing. Enhancing the anomalous rectifier Ih current also switches the excitability to type II. Key characteristics of synaptic conductances that may be observed in vivo also change the type of excitability: a depolarized γ-Aminobutyric acid receptor (GABAR) reversal potential or co-activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) leads to an abrupt frequency drop to zero, which is typical for type II excitability. Coactivation of N-methyl-D-aspartate receptors (NMDARs) together with AMPARs and GABARs shifts the type I/II boundary toward more hyperpolarized GABAR reversal potentials. To better understand how altering each of the aforementioned currents leads to changes in excitability profile of DA neuron, we provide a thorough dynamical analysis. Collectively, these results imply that type I excitability in dopamine neurons might be important for low firing rates and fine-tuning basal dopamine levels, while switching excitability to type II during NMDAR and AMPAR activation may facilitate a transient increase in dopamine concentration, as type II neurons are more amenable to synchronization by mutual excitation.

VTA dopamine neuron plasticity - the unusual suspects

Dopamine neurons in the ventral tegmental area (VTA) are involved in a variety of physiological and pathological conditions, ranging from motivated behaviours to substance use disorders. While many studies have shown that these neurons can express plasticity at excitatory and inhibitory synapses, little is known about how inhibitory inputs and glial activity shape the output of DA neurons and therefore, merit greater discussion. In this review, we will attempt to fill in a bit more of the puzzle, with a focus on inhibitory transmission and astrocyte function. We summarize the findings within the VTA as well as observations made in other brain regions that have important implications for plasticity in general and should be considered in the context of DA neuron plasticity.