Ventral Tegmental Area GABA Neurons Research Articles

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dopaminergic (DA) transmission in the nucleus accumbens (NAc) is necessary for opiate CPP in opiate-dependent and withdrawn (ODW) rats and mice.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.

CAMP-mediated upregulation of HCN channels in VTA dopamine neurons promotes cocaine reinforcement

Chronic cocaine exposure induces enduring neuroadaptations that facilitate motivated drug taking. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are known to modulate neuronal firing and pacemaker activity in ventral tegmental area (VTA) dopamine neurons. However, it remained unknown whether cocaine self-administration affects HCN channel function and whether HCN channel activity modulates motivated drug taking. We report that rat VTA dopamine neurons predominantly express Hcn3-4 mRNA, while VTA GABA neurons express Hcn1–4 mRNA. Both neuronal types display similar hyperpolarization-activated currents (Ih), which are facilitated by acute increases in cAMP. Acute cocaine application decreases voltage-dependent activation of Ih in VTA dopamine neurons, but not in GABA neurons. Unexpectedly, chronic cocaine self-administration results in enhanced Ih selectively in VTA dopamine neurons. This differential modulation of Ih currents is likely mediated by a D2 autoreceptor-induced decrease in cAMP as D2 (Drd2) mRNA is predominantly expressed in dopamine neurons, whereas D1 (Drd1) mRNA is barely detectable in the VTA. Moreover, chronically decreased cAMP via Gi-DREADD stimulation leads to an increase in Ih in VTA dopamine neurons and enhanced binding of HCN3/HCN4 with tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b), an auxiliary subunit that is known to facilitate HCN channel surface trafficking. Finally, we show that systemic injection and intra-VTA infusion of the HCN blocker ivabradine reduces cocaine self-administration under a progressive ratio schedule and produces a downward shift of the cocaine dose-response curve. Our results suggest that cocaine self-administration induces an upregulation of Ih in VTA dopamine neurons, while HCN inhibition reduces the motivation for cocaine intake.

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

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

Netrin-1 regulates the balance of synaptic glutamate signaling in the adult ventral tegmental area.

The axonal guidance cue netrin-1 serves a critical role in neural circuit development by promoting growth cone motility, axonal branching, and synaptogenesis. Within the adult mouse brain, expression of the gene encoding (Ntn1) is highly enriched in the ventral midbrain where it is expressed in both GABAergic and dopaminergic neurons, but its function in these cell types in the adult system remains largely unknown. To address this, we performed viral-mediated, cell-type specific CRISPR-Cas9 mutagenesis of Ntn1 in the ventral tegmental area (VTA) of adult mice. Ntn1 loss-of-function in either cell type resulted in a significant reduction in excitatory postsynaptic connectivity. In dopamine neurons, the reduced excitatory tone had a minimal phenotypic behavioral outcome; however, reduced glutamatergic tone on VTA GABA neurons induced behaviors associated with a hyperdopaminergic phenotype. Simultaneous loss of Ntn1 function in both cell types largely rescued the phenotype observed in the GABA-only mutagenesis. These findings demonstrate an important role for Ntn1 in maintaining excitatory connectivity in the adult midbrain and that a balance in this connectivity within two of the major cell types of the VTA is critical for the proper functioning of the mesolimbic system.

Ethanol blocks a novel form of iLTD, but not iLTP of inhibitory inputs to VTA GABA neurons.

The ventral tegmental area (VTA) is an essential component of the mesocorticolimbic dopamine (DA) circuit that processes reward and motivated behaviors. The VTA contains DA neurons essential in this process, as well as GABAergic inhibitory cells that regulate DA cell activity. In response to drug exposure, synaptic connections of the VTA circuit can be rewired via synaptic plasticity-a phenomenon thought to be responsible for the pathology of drug dependence. While synaptic plasticity to VTA DA neurons as well as prefrontal cortex to nucleus accumbens GABA neurons are well studied, VTA GABA cell plasticity, specifically inhibitory inputs to VTA GABA neurons, is less understood. Therefore, we investigated the plasticity of these inhibitory inputs. Using whole cell electrophysiology in GAD67-GFP mice to identify GABA cells, we observed that these VTA GABA cells experience either inhibitory GABAergic long-term potentiation (iLTP) or inhibitory long-term depression (iLTD) in response to a 5 Hz stimulus. Paired pulse ratios, coefficient of variance, and failure rates suggest a presynaptic mechanism for both plasticity types, where iLTP is NMDA receptor-dependent and iLTD is GABAB receptor-dependent-this being the first report of iLTD onto VTA GABA cells. As illicit drug exposure can alter VTA plasticity, we employed chronic intermittent exposure (CIE) to ethanol (EtOH) vapor in male and female mice to examine its potential impact on VTA GABA input plasticity. Chronic EtOH vapor exposure produced measurable behavioral changes illustrating dependence and concomitantly prevented previously observed iLTD, which continued in air-exposed controls, illustrating the impact of EtOH on VTA neurocircuitry and suggesting physiologic mechanisms at play in alcohol use disorder and withdrawal states. Taken together, these novel findings of unique GABAergic synapses exhibiting either iLTP or iLTD within the mesolimbic circuit, and EtOH blockade specifically of iLTD, characterize inhibitory VTA plasticity as a malleable, experience-dependent system modified by EtOH.

Role of α6-Nicotinic Receptors in Alcohol-Induced GABAergic Synaptic Transmission and Plasticity to Cholinergic Interneurons in the Nucleus Accumbens.

The prevailing view is that enhancement of dopamine (DA) transmission in the mesolimbic system, consisting of DA neurons in the ventral tegmental area (VTA) that project to the nucleus accumbens (NAc), underlies the reward properties of ethanol (EtOH) and nicotine (NIC). We have shown previously that EtOH and NIC modulation of DA release in the NAc is mediated by α6-containing nicotinic acetylcholine receptors (α6*-nAChRs), that α6*-nAChRs mediate low-dose EtOH effects on VTA GABA neurons and EtOH preference, and that α6*-nAChRs may be a molecular target for low-dose EtOH. However, the most sensitive target for reward-relevant EtOH modulation of mesolimbic DA transmission and the involvement of α6*-nAChRs in the mesolimbic DA reward system remains to be elucidated. The aim of this study was to evaluate EtOH effects on GABAergic modulation of VTA GABA neurons and VTA GABAergic input to cholinergic interneurons (CINs) in the NAc. Low-dose EtOH enhanced GABAergic input to VTA GABA neurons that was blocked by knockdown of α6*-nAChRs. Knockdown was achieved either by α6-miRNA injected into the VTA of VGAT-Cre/GAD67-GFP mice or by superfusion of the α-conotoxin MII[H9A;L15A] (MII). Superfusion of MII blocked EtOH inhibition of mIPSCs in NAc CINs. Concomitantly, EtOH enhanced CIN firing rate, which was blocked by knockdown of α6*-nAChRs with α6-miRNA injected into the VTA of VGAT-Cre/GAD67-GFP mice. The firing rate of CINs was not enhanced by EtOH in EtOH-dependent mice, and low-frequency stimulation (LFS; 1Hz, 240 pulses) caused inhibitory long-term depression at this synapse (VTA-NAc CIN-iLTD) which was blocked by knockdown of α6*-nAChR and MII. Ethanol inhibition of CIN-mediated evoked DA release in the NAc was blocked by MII. Taken together, these findings suggest that α6*-nAChRs in the VTA-NAc pathway are sensitive to low-dose EtOH and play a role in plasticity associated with chronic EtOH.

Invariant inhibition to calculate prediction errors?

The ventral tegmental area (VTA) has a pivotal role in motivated behavior. Much of the research on the VTA has focused on the mesocorticolimbic dopamine projections and their role in the computation of a ‘reward prediction error’ (RPE) for reward-guided learning. In a recent study, Zhou et al. report that VTA GABA neurons, the axons of which innervate the ventral pallidum (VP), have a unique role in signaling reward value to the basal ganglia and guiding reward seeking.

Ventral tegmental area astrocytes modulate cocaine reward by tonically releasing GABA

Addictive drugs increase ventral tegmental area (VTA) dopamine (DA) neuron activity through distinct cellular mechanisms, one of which involves disinhibition of DA neurons by inhibiting local GABA neurons. How drugs regulate VTA GABA neuron activity and drive addictive behaviors remains poorly understood. Here, we show that astrocytes control VTA GABA neuron activity in cocaine reward via tonic inhibition in mice. Repeated cocaine exposure potentiates astrocytic tonic GABA release through volume-regulated anion channels (VRACs) and augments tonic inhibition of VTA GABA neurons, thus downregulating their activities and disinhibiting nucleus accumbens (NAc) projecting DA neurons. Attenuation of tonic inhibition by either deleting Swell1 (Lrrc8a), the obligatory subunit of VRACs, in VTA astrocytes or disrupting δ subunit of GABAA receptors in VTA GABA neurons reduces cocaine-evoked changes in neuron activity, locomotion, and reward behaviors in mice. Together, our findings reveal the critical role of astrocytes in regulating the VTA local circuit and cocaine reward.

Chronic Δ9-tetrahydrocannabinol impact on plasticity, and differential activation requirement for CB1-dependent long-term depression in ventral tegmental area GABA neurons in adult versus young mice.

The ventral tegmental area (VTA) mediates incentive salience and reward prediction error through dopamine (DA) neurons that are regulated by local VTA GABA neurons. In young mice, VTA GABA cells exhibit a form of synaptic plasticity known as long-term depression (LTD) that is dependent on cannabinoid 1 (CB1) receptors preceded by metabotropic glutamate receptor 5 (mGluR5) signaling to induce endocannabinoid production. This LTD was eliminated following chronic (7-10 consecutive days) exposure to the marijuana derived cannabinoid Δ9 -tetrahydrocannabinol (THC). We now examine the mechanism behind THC-induced elimination of LTD in adolescents as well as plasticity induction ability in adult versus young male and female mice using whole-cell electrophysiology experiments of VTA GABA cells. Chronic THC injections in adolescents resulted in a loss of CB1 agonist-mediated depression, illustrating chronic THC likely desensitizes or removes synaptic CB1. We noted that seven days withdrawal from chronic THC restored LTD and CB1 agonist-induced depression, suggesting reversibility of THC-induced changes. Adult mice continue to express functional mGluR5 and CB1, but require a doubling of the synaptic stimulation compared to young mice to induce LTD, suggesting a quantitative difference in CB1-dependent plasticity between young and adult mice. One potential rationale for this difference is changes in AMPA and NMDA glutamate receptors. Indeed, AMPA/NMDA ratios were increased in in adults compared to young mice. Lastly, we performed quantitative reverse-transcription PCR and identified that CB1, DAGLα, and GluA1 levels increased following chronic THC exposure. Collectively, our data demonstrate the first age-dependent GABA neuron plasticity in the VTA, which could have implications for decreased THC dependence capacity in adults, as well as the mechanism behind chronic THC-induced synaptic alterations in young mice.

Function of Excitatory Periaqueductal Gray Synapses in the Ventral Tegmental Area following Inflammatory Injury.

Manipulating the activity of ventral tegmental area (VTA) dopamine (DA) neurons can drive nocifensive reflexes, and their firing rates are reduced following noxious stimuli. However, the pain-relevant inputs to the VTA remain incompletely understood. In this study, we used male and female mice in combination with identified dopamine and GABA neurons in the VTA that receive excitatory inputs from the periaqueductal gray (PAG), a nexus of ascending pain information. We tested whether PAG-VTA synapses undergo functional plasticity in response to a pain model using optical stimulation in conjunction with slice electrophysiology. We found that acute carrageenan inflammation does not significantly affect the strength of excitatory PAG synapses onto VTA DA neurons. However, at the PAG synapses on VTA GABA neurons, the subunit composition of NMDA receptors is altered; the complement of NR2D subunits at synaptic sites appears to be lost. Thus, our data support a model in which injury initially alters synapses on VTA GABA neurons. Over time, these alterations may increase tonic inhibition of VTA DA neurons leading to their reduced firing.

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.

The serotonin 2A receptor agonist TCB-2 attenuates heavy alcohol drinking and alcohol-induced midbrain inhibitory plasticity.

Disruption of neuronal chloride ion (Cl- ) homeostasis has been linked to several pathological conditions, including substance use disorder, yet targeted pharmacotherapies are lacking. In this study, we explored the potential of serotonin 2A receptor (5-HT2A R) agonism to reduce alcohol consumption in male wild-type C57Bl/6J mice and to ameliorate alcohol-induced inhibitory plasticity in the midbrain. We found that administration of the putative 5-HT2A R agonist TCB-2 attenuated alcohol consumption and preference but did not alter water or saccharin consumption. We hypothesized that the selective behavioural effects of TCB-2 on alcohol drinking were due, at least in part, to effects of the agonist on ventral tegmental area (VTA) neurocircuitry. Alcohol consumption impairs Cl- transport in VTA GABA neurons, which acts as a molecular adaptation leading to increased alcohol self-administration. Using ex vivo electrophysiological recordings, we found that exposure to either intermittent volitional alcohol drinking or an acute alcohol injection diminished homeostatic Cl- transport in VTA GABA neurons. Critically, in vivo TCB-2 administration normalized Cl- transport in the VTA after alcohol exposure. Thus, we show a potent effect of alcohol consumption on VTA inhibitory circuitry, in the form of dysregulated Cl- homeostasis that is reversible with agonism of 5-HT2A Rs. Our results provide insight into the potential therapeutic action of 5-HT2A R agonists for alcohol abuse.

Morphine Exposure Reduces Nicotine-Induced Upregulation of Nicotinic Receptors and Decreases Volitional Nicotine Intake in a Mouse Model.

IntroductionNicotine addiction remains a primary health concern as tobacco smoking remains the number one cause of preventable death in America. At the same time, America is still facing the threat of the opioid epidemic. While the prevalence of smoking combustible cigarettes or electronic nicotine delivery systems in the United States varies between 12% and 35%, the smoking rates among the opioid use dependent (OUD) population is 74%–97%. We examined changes in brain reward mechanisms in which co-use of nicotine and opioids may result in enhanced reward and reinforcement.Aims and MethodsAdult male and female α4-mCherryα6-GFP mice (C57BL/6J) were used in conditioned place preference (CPP) and microscopy assays to examine reward-related behavior and nicotinic acetylcholine receptor (nAChR) upregulation following treatments with saline, nicotine, morphine, or nicotine plus morphine. Following this, separate mice were trained in e-Vape self-administration assays to examine morphine’s impact on nicotine reinforcement.ResultsWe observed that nicotine and morphine coexposure in a CPP assay did not produce enhanced reward-related behavior when compared with nicotine or morphine alone. In parallel we observed coexposure reduced nicotine-induced upregulation of nAChRs on ventral tegmental area dopamine and GABA neurons. Additionally, we observed that concurrent morphine exposure reduced nicotine (plus menthol) vapor self-administration in male and female mice.ConclusionsWhile nicotine use is high among OUD individuals, our CPP assays suggest coexposure not only fails to enhance reward-related behavior but also reduces nicotine-induced changes in ventral tegmental area neurobiology. Our self-administration assays suggest that morphine exposure during nicotine acquisition reduces nicotine reinforcement-related behavior.ImplicationsWhile some may postulate that the co-use of opioids and nicotine may be driven by reward-related mechanisms, our data indicate that opioid exposure may hinder nicotine intake due to reduced upregulation of nAChRs critical for nicotine reward and reinforcement. Thus, the high co-use in OUD individuals may be a result of other mechanisms and this warrants further investigations into nicotine and opioid co-use.

Ventral tegmental area GABA neurons mediate stress-induced blunted reward-seeking in mice

Decreased pleasure-seeking (anhedonia) forms a core symptom of depression. Stressful experiences precipitate depression and disrupt reward-seeking, but it remains unclear how stress causes anhedonia. We recorded simultaneous neural activity across limbic brain areas as mice underwent stress and discovered a stress-induced 4 Hz oscillation in the nucleus accumbens (NAc) that predicts the degree of subsequent blunted reward-seeking. Surprisingly, while previous studies on blunted reward-seeking focused on dopamine (DA) transmission from the ventral tegmental area (VTA) to the NAc, we found that VTA GABA, but not DA, neurons mediate stress-induced blunted reward-seeking. Inhibiting VTA GABA neurons disrupts stress-induced NAc oscillations and rescues reward-seeking. By contrast, mimicking this signature of stress by stimulating NAc-projecting VTA GABA neurons at 4 Hz reproduces both oscillations and blunted reward-seeking. Finally, we find that stress disrupts VTA GABA, but not DA, neural encoding of reward anticipation. Thus, stress elicits VTA-NAc GABAergic activity that induces VTA GABA mediated blunted reward-seeking.

Serum- and glucocorticoid-inducible kinase 1 activity in ventral tegmental area dopamine neurons regulates cocaine conditioned place preference but not cocaine self-administration.

Drugs of abuse regulate the activity of the mesolimbic dopamine (DA) system, and drug-induced changes in ventral tegmental area (VTA) cellular activity and gene regulation are linked to behavioral outputs associated with addiction. Previous work from our lab determined that VTA serum- and glucocorticoid-inducible kinase 1 (SGK1) transcription and catalytic activity were increased by repeated cocaine administration; however, it was unknown if these biochemical changes contributed to cocaine-elicited behaviors. Using transgenic and viral-mediated manipulations, we investigated the role of VTA SGK1 catalytic activity in regulating cocaine conditioned place preference and self-administration. We showed intra-VTA infusion of a catalytically inactive SGK1 mutant (K127Q) significantly decreased cocaine conditioned place preference (CPP). Further, we found that K127Q expression in VTA DA neurons significantly decreased cocaine CPP, while this same manipulation in VTA GABA neurons had no effect. However, blunted VTA DA SGK1 catalytic activity did not alter cocaine self-administration. Altogether, these studies identify the specific VTA cells critical for SGK1-mediated effects on cocaine CPP but not self-administration.

Ventral tegmental area GABA, glutamate, and glutamate-GABA neurons are heterogeneous in their electrophysiological and pharmacological properties.

The ventral tegmental area (VTA) contains dopamine neurons intermixed with GABA-releasing (expressing vesicular GABA transporter, VGaT), glutamate-releasing (expressing vesicular glutamate transporter 2, VGluT2), and glutamate-GABA co-releasing (co-expressing VGluT2 and VGaT) neurons. By delivering INTRSECT viral vectors into the VTA of double vglut2-Cre/vgat-Flp transgenic mice, we targeted specific VTA cell populations for ex vivo recordings. We found that VGluT2+ VGaT- and VGluT2+ VGaT+ neurons on average had relatively hyperpolarized resting membrane potential, greater rheobase, and lower spontaneous firing frequency compared to VGluT2- VGaT+ neurons, suggesting that VTA glutamate-releasing and glutamate-GABA co-releasing neurons require stronger excitatory drive to fire than GABA-releasing neurons. In addition, we detected expression of Oprm1mRNA (encoding µ opioid receptors, MOR) in VGluT2+ VGaT- and VGluT2- VGaT+ neurons, and that the MOR agonist DAMGO hyperpolarized neurons with these phenotypes. Collectively, we demonstrate the utility of the double transgenic mouse to access VTA glutamate, glutamate-GABA, and GABA neurons to determine their electrophysiological properties. SIGNIFICANT STATEMENT: Some physiological properties of VTA glutamate-releasing and glutamate-GABA co-releasing neurons are distinct from those of VTA GABA-releasing neurons. µ-opioid receptor activation hyperpolarizes some VTA glutamate-releasing and some GABA-releasing neurons.

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

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

Projection-specific dopamine neurons in the ventral tegmental area participated in morphine-induced hyperalgesia and anti-nociceptive tolerance in male mice.

Long-term morphine use is associated with serious side effects, such as morphine-induced hyperalgesia and analgesic tolerance. Previous investigations have documented the association between dopamine (DA) neurons in the ventral tegmental area (VTA) and pain. However, whether VTA DA neurons are implicated in morphine-induced hyperalgesia and analgesic tolerance remains elusive. Initially, we observed behavioural effects of lidocaine administration into VTA or ablation of VTA DA neurons on morphine-induced hyperalgesia and anti-nociceptive tolerance. Subsequently, c-Fos expression in nucleus accumbens (NAc) shell-projecting and medial prefrontal cortex (mPFC)-projecting VTA DA neurons after chronic morphine treatment was respectively investigated. Afterwards, the effects of chemogenetic manipulation of NAc shell-projecting or mPFC-projecting DA neurons on morphine-induced hyperalgesia and anti-nociceptive tolerance were observed. Additionally, effects of chemogenetic manipulation of VTA GABA neurons on c-Fos expression in VTA DA neurons were investigated. Lidocaine injection into VTA relieved established hyperalgesia and anti-nociceptive tolerance whereas ablation of VTA DA neurons prevented the development of morphine-induced hyperalgesia and anti-nociceptive tolerance. Chronic morphine treatment increased c-Fos expression in NAc shell-projecting DA neurons, rather than in mPFC-projecting DA neurons. Chemogenetic manipulation of NAc shell-projecting DA neurons had influence on morphine-induced hyperalgesia and tolerance. However, chemogenetic manipulation of mPFC-projecting DA neurons had no significant effects on morphine-induced hyperalgesia and anti-nociceptive tolerance. Chemogenetic manipulation of VTA GABA neurons affected the c-Fos expression in VTA DA neurons. These findings revealed the involvement of NAc shell-projecting VTA DA neurons in morphine-induced hyperalgesia and anti-nociceptive tolerance, and may shed new light on the clinical management of morphine-induced hyperalgesia and analgesic tolerance. This study demonstrated that NAc shell-projecting DA neurons rather than mPFC-projecting DA neurons in the VTA were implicated in morphine-induced hyperalgesia and anti-nociceptive tolerance. Our findings may pave the way for the discovery of novel therapies for morphine-induced hyperalgesia and analgesic tolerance.

Acute cocaine exposure occludes long-term depression in ventral tegmental area GABA neurons

The ventral tegmental area (VTA) in the midbrain is essential in incentive salience of reward behavior. Drugs of abuse increase midbrain dopamine cell activity and/or dopamine levels, and can alter endogenous VTA glutamate plasticity, leading to addiction or dependence. VTA dopamine cells are regulated by local inhibitory GABA cells, which exhibit a form of pre-synaptic cannabinoid receptor 1-dependent long-term depression of their glutamatergic inputs. Our current aim was to determine cocaine's influence on VTA GABA cell glutamate plasticity and circuity. Using whole cell voltage-clamp electrophysiology in VTA slices of GAD67-GFP knock-in mice, we recorded excitatory inputs on VTA GABA cells. Acute and chronic injections of cocaine were sufficient to occlude long-term depression. The plasticity could be reversed to the naïve state however, as long-term depression was again observed following a 7-day abstinence from acute cocaine exposure. Furthermore, chronic cocaine decreased AMPA/NMDA ratios at glutamate synapses onto VTA GABA cells, compared to vehicle injection controls, the opposite change noted in dopamine cells. Collectively, our data suggest the cellular mechanism of cocaine-mediated synaptic modification that may result in dependence/withdrawal could involve changes in glutamate input to VTA GABA circuitry in addition to VTA dopamine cells. Therefore VTA GABA cells may also play a role, possibly in a synergistic manner with the dopamine circuit, in cocaine-induced changes to the VTA reward pathway than previously known.

Basal forebrain mediates prosocial behavior via disinhibition of midbrain dopamine neurons

Sociability is fundamental for our daily life and is compromised in major neuropsychiatric disorders. However, the neuronal circuit mechanisms underlying prosocial behavior are still elusive. Here we identify a causal role of the basal forebrain (BF) in the control of prosocial behavior via inhibitory projections that disinhibit the midbrain ventral tegmental area (VTA) dopamine (DA) neurons. Specifically, BF somatostatin-positive (SST) inhibitory neurons were robustly activated during social interaction. Optogenetic inhibition of these neurons in BF or their axon terminals in the VTA largely abolished social preference. Electrophysiological examinations further revealed that SST neurons predominantly targeted VTA GABA neurons rather than DA neurons. Consistently, optical inhibition of SST neuron axon terminals in the VTA decreased DA release in the nucleus accumbens during social interaction, confirming a disinhibitory action. These data reveal a previously unappreciated function of the BF in prosocial behavior through a disinhibitory circuitry connected to the brain's reward system.