Ventral Tegmental Area Gamma-aminobutyric Acid Research Articles

Sex differences in VTA GABA transmission and plasticity during opioid withdrawal

The effectiveness of current treatments for opioid use disorder (OUD) varies by sex. Our understanding of the neurobiological mechanisms mediating negative states during withdrawal is lacking, particularly with regard to sex differences. Based on preclinical research in male subjects, opioid withdrawal is accompanied by increased gamma-aminobutyric acid (GABA) release probability at synapses onto dopamine neurons in the ventral tegmental area (VTA). It is unclear, however, if the physiological consequences of morphine that were originally elucidated in male rodents extend to females. The effects of morphine on the induction of future synaptic plasticity are also unknown. Here, we show that inhibitory synaptic long-term potentiation (LTPGABA) is occluded in the VTA in male mice after repeated morphine injections and 1 day of withdrawal, while morphine-treated female mice maintain the ability to evoke LTPGABA and have basal GABA activity similar to controls. Our observation of this physiological difference between male and female mice connects previous reports of sex differences in areas upstream and downstream of the GABA-dopamine synapse in the VTA during opioid withdrawal. The sex differences highlight the mechanistic distinctions between males and females that can be targeted when designing and implementing treatments for OUD.

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

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

Identification of Novel Neurocircuitry Through Which Leptin Targets Multiple Inputs to the Dopamine System to Reduce Food Reward Seeking

BackgroundLeptin reduces the motivation to obtain food by modulating activity of the mesolimbic dopamine (DA) system upon presentation of cues that predict a food reward. Although leptin directly reduces the activity of ventral tegmental area (VTA) DA neurons, the majority of leptin receptor (LepR)-expressing DA neurons do not project to the nucleus accumbens, the projection implicated in driving food reward seeking. Therefore, the precise locus of leptin action to modulate motivation for a food reward is unresolved. MethodsWe used transgenic mice expressing Cre recombinase under the control of the LepR promoter, anatomical tracing, optogenetics-assisted patch-clamp electrophysiology, in vivo optogenetics with fiber photometric calcium measurements, and chemogenetics to unravel how leptin-targeted neurocircuitry inhibits food reward seeking. ResultsA large number of DA neurons projecting to the nucleus accumbens are innervated by local VTA LepR-expressing GABA (gamma-aminobutyric acid) neurons. Leptin enhances the activity of these GABA neurons and thereby inhibits nucleus accumbens–projecting DA neurons. In addition, we find that lateral hypothalamic LepR-expressing neurons projecting to the VTA are inhibited by leptin and that these neurons modulate DA neurons indirectly via inhibition of VTA GABA neurons. In accordance with such a disinhibitory function, optogenetically stimulating lateral hypothalamic LepR projections to the VTA potently activates DA neurons in vivo. Moreover, we found that chemogenetic activation of lateral hypothalamic LepR neurons increases the motivation to obtain a food reward only when mice are in a positive energy balance. ConclusionsWe identify neurocircuitry through which leptin targets multiple inputs to the DA system to reduce food reward seeking.

Chemogenetic Activation of Mesoaccumbal Gamma-Aminobutyric Acid Projections Selectively Tunes Responses to Predictive Cues When Reward Value Is Abruptly Decreased

BackgroundMesolimbic circuits regulate the attribution of motivational significance to incentive cues that predict reward, yet this network also plays a key role in adapting reward-seeking behavior when the contingencies linked to a cue unexpectedly change. Here, we asked whether mesoaccumbal GABA (gamma-aminobutyric acid) projections enhance adaptive responding to incentive cues of abruptly altered reward value, and whether these effects were distinct from global activation of all ventral tegmental area GABA circuits. MethodsWe used a viral targeting system to chemogenetically activate mesoaccumbal GABA projections in male rats during a novel cue-dependent operant value-shifting task, in which the volume of a sucrose reward associated with a predictive cue is suddenly altered, from the beginning and throughout the session. We compared the results with global activation of ventral tegmental area GABA neurons, which will activate local inhibitory circuits and long loop projections. ResultsWe found that activation of mesoaccumbal GABA projections decreases responding to incentive cues associated with smaller-than-expected rewards. This tuning of behavioral responses was specific to cues associated with smaller-than-expected rewards but did not impact measures related to consuming the reward. In marked contrast, activating all ventral tegmental area GABA neurons resulted in a uniform decrease in responding to incentive cues irrespective of changes in the size of the reward. ConclusionsTargeted activation of mesoaccumbal GABA neurons facilitates adaptation in reward-seeking behaviors. This suggests that these projections may play a very specific role in associative learning processes.

Pharmacological Antagonism of T-Type Calcium Channels Constrains Rebound Burst Firing in Two Distinct Subpopulations of GABA Neurons in the Rat Ventral Tegmental Area: Implications for α-Lipoic Acid.

The ventral tegmental area (VTA) is a midbrain region highly involved in motivation and reward. A large body of work has investigated synaptic plasticity and ion channel excitability in this area, which has strong implication in drug abuse. We recently provided electrophysiological and pharmacological evidence that the CaV3.1 isoform of T-type voltage-gated calcium channels contributes to the excitability of VTA dopamine (DA) neurons. However, the role of T-channels in excitability of VTA gamma-amino-butyric acid (GABA) neurons remained unaddressed. Here, with a population study of rat VTA GABA neurons, we provide evidence that T-channels contribute to rebound spiking activity in two phenotypically distinct subpopulations of GABAergic neurons, each with differing electrophysiological characteristics. Additionally, we provide the first study to investigate the effect of α-lipoic acid (ALA) on ion channels in mesolimbic reward circuitry. Taken together, our population study and pharmacology experiments implicate T-channels as a target for therapies aimed at tempering VTA and mesolimbic circuit excitability.

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 GAD65/67, 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.

Adolescent rats are resistant to adaptations in excitatory and inhibitory mechanisms that modulate mesolimbic dopamine during nicotine withdrawal

Adolescent smokers report enhanced positive responses to tobacco and fewer negative effects of withdrawal from this drug than adults, and this is believed to propel higher tobacco use during adolescence. Differential dopaminergic responses to nicotine are thought to underlie these age-related effects, as adolescent rats experience lower withdrawal-related deficits in nucleus accumbens (NAcc) dopamine versus adults. This study examined whether age differences in NAcc dopamine during withdrawal are mediated by excitatory or inhibitory transmission in the ventral tegmental area (VTA) dopamine cell body region. In vivo microdialysis was used to monitor extracellular levels of glutamate and gamma-aminobutyric acid (GABA) in the VTA of adolescent and adult rats experiencing nicotine withdrawal. In adults, nicotine withdrawal produced decreases in VTA glutamate levels (44% decrease) and increases in VTA GABA levels (38% increase). In contrast, adolescents did not exhibit changes in either of these measures. Naïve controls of both ages did not display changes in NAcc dopamine, VTA glutamate, or VTA GABA following mecamylamine. These results indicate that adolescents display resistance to withdrawal-related neurochemical processes that inhibit mesolimbic dopamine function in adults experiencing nicotine withdrawal. Our findings provide a potential mechanism involving VTA amino acid neurotransmission that modulates age differences during withdrawal.

Acute and Chronic Ethanol Modulate Dopamine D2‐Subtype Receptor Responses in Ventral Tegmental Area GABA Neurons

Ventral tegmental area (VTA) gamma-aminobutyric acid (GABA) neurons appear to be critical substrates underlying the acute and chronic effects of ethanol on dopamine (DA) neurotransmission in the mesocorticolimbic system implicated in drug reward. VTA GABA neuron firing rate is reduced by acute ethanol and enhanced by DA via D2 receptor activation. The objective of this study was to evaluate the role of D2 receptors in acute ethanol inhibition of VTA GABA neuron activity, as well as the adaptation of D2 receptors by chronic ethanol consumption. Using electrophysiological methods, we evaluated the effects of intraperitoneal ethanol on DA activation of VTA GABA neurons, the effects of DA antagonists on ethanol inhibition of their firing rate, as well as adaptations in firing rate following chronic ethanol consumption. Using single cell quantitative RT-polymerase chain reaction (PCR), we evaluated the expression of VTA GABA neuron D2 receptors in rats consuming ethanol versus pair-fed controls. In acute ethanol studies, microelectrophoretic activation of VTA GABA neurons by DA was inhibited by acute intraperitoneal ethanol, and intravenous administration of the D2 antagonist eticlopride blocked ethanol suppression of VTA GABA neuron firing rate. In chronic ethanol studies, while there were no signs of withdrawal at 24 hours, or significant adaptation in firing rate or response to acute ethanol, there was a significant down-regulation in the expression of D2 receptors in ethanol-consuming rats versus pair-fed controls. Inhibition of DA activation of VTA GABA neuron firing rate by ethanol, as well as eticlopride block of ethanol inhibition of VTA GABA neuron firing rate, suggests an interaction between ethanol and DA neurotransmission via D2 receptors, perhaps via enhanced DA release in the VTA subsequent to ethanol inhibition of GABA neurons. Down-regulation of VTA GABA neuron D2 receptors by chronic ethanol might result from persistent DA release onto GABA neurons.

Cocaine disinhibits dopamine neurons in the ventral tegmental area via use‐dependent blockade of GABA neuron voltage‐sensitive sodium channels

The aim of this study was to evaluate the effects of cocaine on gamma-aminobutyric acid (GABA) and dopamine (DA) neurons in the ventral tegmental area (VTA). Utilizing single-unit recordings in vivo, microelectrophoretic administration of DA enhanced the firing rate of VTA GABA neurons via D2/D3 DA receptor activation. Lower doses of intravenous cocaine (0.25-0.5 mg/kg), or the DA transporter (DAT) blocker methamphetamine, enhanced VTA GABA neuron firing rate via D2/D3 receptor activation. Higher doses of cocaine (1.0-2.0 mg/kg) inhibited their firing rate, which was not sensitive to the D2/D3 antagonist eticlopride. The voltage-sensitive sodium channel (VSSC) blocker lidocaine inhibited the firing rate of VTA GABA neurons at all doses tested (0.25-2.0 mg/kg). Cocaine or lidocaine reduced VTA GABA neuron spike discharges induced by stimulation of the internal capsule (ICPSDs) at dose levels 0.25-2 mg/kg (IC(50) 1.2 mg/kg). There was no effect of DA or methamphetamine on ICPSDs, or of DA antagonists on cocaine inhibition of ICPSDs. In VTA GABA neurons in vitro, cocaine reduced (IC(50) 13 microm) current-evoked spikes and TTX-sensitive sodium currents in a use-dependent manner. In VTA DA neurons, cocaine reduced IPSCs (IC(50) 13 microm), increased IPSC paired-pulse facilitation and decreased spontaneous IPSC frequency, without affecting miniature IPSC frequency or amplitude. These findings suggest that cocaine acts on GABA neurons to reduce activity-dependent GABA release on DA neurons in the VTA, and that cocaine's use-dependent blockade of VTA GABA neuron VSSCs may synergize with its DAT inhibiting properties to enhance mesolimbic DA transmission implicated in cocaine reinforcement.

Orexin axons in the rat ventral tegmental area synapse infrequently onto dopamine and γ‐aminobutyric acid neurons

Cells in the ventral tegmental area (VTA) facilitate motivated behaviors, and the activity of VTA neurons is regulated by dense projections from the lateral hypothalamic area (LHA). Orexin (Orx) neurons in the lateral and perifornical hypothalamus play important roles in arousal, feeding, and energy metabolism. Orx cells contribute substantially to the LHA projection to the rat midbrain. However, the morphological features of Orx fibers in the VTA and whether they synapse onto dopamine (DA) or gamma-aminobutyric acid (GABA) neurons have not yet been investigated. We utilized immunoperoxidase and immunogold-silver staining to examine the morphological features and synaptic incidence of Orx-labeled axons in the VTA. We then combined immunoperoxidase labeling for Orx with immunogold-silver labeling for GABA or for tyrosine hydroxylase (TH) in DA neurons. Electron microscopic analysis revealed that most Orx-labeled axons in the VTA were passing fibers. The less common Orx varicosities were occasionally apposed to TH- or GABA-labeled dendrites without synapsing. Only a small proportion of Orx-positive axons synapsed onto dendrites or soma. The synapses included both asymmetric and symmetric types and targeted TH- and GABA-labeled profiles with equal frequency. These findings suggest that most Orx fibers in the VTA are axons passing to caudal brainstem structures. However, Orx does mediate some direct synaptic influence on VTA DA and GABA neurons. Additional nonsynaptic effects are suggested by the presence of numerous dense-cored vesicles. These studies have important implications for understanding the mechanisms whereby Orx can alter behavior through regulating VTA DA and GABA cell activity.

Selective ablation of GABA neurons in the ventral tegmental area increases spontaneous locomotor activity.

Gamma-aminobutyric acid (GABA) neurons in the ventral tegmental area (VTA) provide innervation to cortical and subcortical regions of the brain. To solidify the importance of these VTA GABA neurons in behavioral function, we employed the neurotoxin dermorphin-saporin (DS) to selectively lesion VTA GABA neurons prior to assessing spontaneous motor activity. Rats were bilaterally microinfused with DS (1.0 or 2.0 pmol/200 nl/side) or blank-saporin control (BS, 200 nl/side) into the VTA. Seven days later, DS-treated rats exhibited significantly elevated motility in comparison with BS-treated rats; this elevated motility normalized by Day 14 following pretreatment with 1.0 pmol of DS but was sustained on Day 14 after pretreatment with 2.0 pmol of DS. A selective loss of VTA GABA neurons on Day 14 was demonstrated through reduced expression of mRNA for glutamic acid decarboxylase-67 and micro-opioid receptor, but not tyrosine hydroxylase (a dopamine neuron marker), in the VTA. Thus, a dose- and time-related selective loss of VTA GABA neurons was accomplished using this novel neurotoxin. This loss of GABA VTA neurons was associated with hypermotility, further supporting their important regulatory role in the generation of behavior.

Connexin-36 gap junctions mediate electrical coupling between ventral tegmental area GABA neurons

Communication between neurons in the mammalian brain is primarily through chemical synapses; however, evidence is accumulating in support of electrical synaptic transmission between some neuronal types in the mature nervous system. The authors have recently demonstrated that the gap junction (GJ) blocker quinidine suppresses stimulus-induced and dopamine-evoked coupling of gamma amino butyric acid (GABA) neurons in the ventral tegmental area (VTA) of mature rats (Stobbs et al., 2004). The aim of this study was to evaluate the role of connexin-36 (Cx36) GJs in mediating electrical coupling between VTA GABA neurons in P50-80 rats in vivo and P25-50 rats in vitro. Single stimulation of the internal capsule (IC) evoked VTA GABA neuron spike couplets in mature rats when activated antidromically, and multiple poststimulus spike discharges (PSDs) when activated with brief high-frequency stimulation of the IC (ICPSDs). The Cx36 GJ blocker mefloquine (30 mg/kg) suppressed VTA GABA neuron ICPSDs in mature freely behaving rats. VTA GABA neurons recorded via whole-cell patch clamp in the midbrain slice preparation of P25-50 rats showed robust expression of Cx36 transcripts when tested with single-cell quantitative reverse transcription polymerase chain reaction. In P50-80 rats, Cx36 protein immunoreactivity was evident in the VTA and surrounding structures. Dye-coupling between VTA neurons was observed following Neurobiotin labeling of VTA GABA neurons, as well as with the fluorochrome Alexa Fluor 488 using real-time video fluorescent microscopy. Thus, mature VTA GABA neurons appear to be connected by electrical synapses via Cx36 GJs, whose coupling is enhanced by corticotegmental input and by dopamine.

Involvement of 5-HT 1B receptors within the ventral tegmental area in regulation of mesolimbic dopaminergic neuronal activity via GABA mechanisms: a study with dual-probe microdialysis

This study was designed to assess the involvement of 5-HT 1B receptors within the ventral tegmental area (VTA) in the regulation of mesolimbic dopaminergic transmission. Dual-probe microdialysis was performed in freely moving adult Sprague–Dawley rats with one probe within the VTA and the other within the ipsilateral nucleus accumbens (NACC). Drugs were administered into the VTA via retrograde dialysis. Dialysates from both the VTA and the NAC were collected for determination of dopamine (DA) and gamma-aminobutyric acid (GABA) by high-performance liquid chromatography with electrochemical detection. Intra-tegmental infusion of CP 93129 (20, 40, and 80 μM), a 5-HT 1B receptor agonist, increased extracellular DA concentrations in a concentration-dependent manner not only in the NACC but also in the VTA, indicating increased mesolimbic DA neuron activity. Administration of CP 93129 at 80 μM into the VTA also significantly decreased extracellular GABA concentrations in this region. Co-infusion of the 5-HT 1B receptor antagonist SB 216641 (10 μM), but not the 5-HT 1A receptor antagonist WAY 100635 (10 μM) or the 5-HT 1D/1A receptor antagonist BRL 15572 (10 μM), antagonized not only the effects of intra-tegmental CP 93129 (80 μM) on VTA DA and NAC DA but also on VTA GABA. The results suggest that activation of VTA 5-HT 1B receptors increases mesolimbic DA neuron activities. The increased DA neuron activity may be associated, at least in part, with the 5-HT 1B receptor-mediated inhibition of VTA GABA release.

Evidence for a differential medial prefrontal dopamine D 1 and D 2 receptor regulation of local and ventral tegmental glutamate and GABA release : A dual probe microdialysis study in the awake rat

The effects of perfusion with two selective dopamine receptor agonists SKF38393 and pergolide into the medial prefrontal cortex (mPfc) on local and ventral tegmental area (VTA) glutamate and gamma-aminobutyric acid (GABA) release were investigated using dual probe microdialysis in the awake rat. Intracortical SKF38393 (10, 100, 500 μM, 60 min) decreased glutamate and increased GABA release in the mPfc but had no effect on either amino acid neurotransmitter in the VTA. Intracortical perfusion with the selective GABA A receptor antagonist bicuculline (0.1 μM, 140 min) reversed the SKF38393 (100 μM, 60 min)-induced decrease in local glutamate release, while the selective GABA B receptor antagonist CGP35348 (100 μM, 140 min) was without effect. Intracortical pergolide (1 μM, 60 min) was associated with a prolonged reversible decrease in local and VTA glutamate release that was also associated with a decrease in VTA GABA release, which was reversed in the presence of intracortical raclopride (10 μM, 140 min).Taken together, the present findings indicate a differential regulation of glutamate and GABA release in the mPfc and VTA by dopamine D 1 and D 2 receptors in the mPfc whereby (a) activation of the dopamine D 1 receptor in the mPfc decreases local glutamate release possibly via a feed-forward activation of the local GABA interneurons; (b) activation of the dopamine D 2 receptor in the mPfc inhibits both local glutamate release and the excitatory glutamate drive on the VTA.

A dual probe characterization of dialysate amino acid levels in the medial prefrontal cortex and ventral tegmental area of the awake freely moving rat

Dual probe microdialysis was employed to characterize the origins of dialysate glutamate, aspartate and gamma-aminobutyric acid (GABA) in the medial prefrontal cortex (mPfc) and to investigate functional interactions between the mPfc and ventral tegmental area (VTA) in awake, freely moving rats. Perfusion with elevated potassium (K +; KCl, 100 mM, 20 min), low Ca 2+ (0.1 mM, 60 min) or tetrodotoxin (TTX, 10 μM, 100 min) was performed in the mPfc and dialysate levels of glutamate, aspartate and GABA were measured locally and in the VTA. Elevated K + in the mPfc rapidly increased dialysate glutamate and aspartate locally (+90±10 and +41±9% from basal, respectively) and in the VTA (+71±14 and +42±14%, respectively). MPfc GABA was also rapidly increased (+241±62%) while VTA GABA was not affected. Perfusion with low Ca 2+ in the mPfc decreased local glutamate, aspartate and GABA (−26±8; −35±7 and −45±8%, respectively) and decreased only GABA (−40±5%) in the VTA. Intra-mPfc TTX increased glutamate and aspartate locally (+82±23 and +54±27%, respectively) and in the VTA (+84±18 and +38±17%, respectively). In contrast, intra-mPfc TTX decreased local GABA (−33+6%) while VTA GABA levels were not affected. Taken together, these data confirm the influence of the mPfc upon the ipsilateral VTA and provide evidence for two neuronal pools which contribute to basal extracellular mPfc and VTA glutamate, aspartate and GABA levels, the first pool derived from Na +- and Ca 2+-dependent release and the second derived from voltage-dependent reuptake.