Ventral Tegmental Area GABAergic Neurons Research Articles

Distinct dynamics and intrinsic properties in ventral tegmental area populations mediate reward association and motivation

Ventral tegmental area (VTA) dopamine neurons regulate reward-related associative learning and reward-driven motivated behaviors, but how these processes are coordinated by distinct VTA neuronal subpopulations remains unresolved. Here, we compare the contribution of two primarily dopaminergic and largely non-overlapping VTA subpopulations, all VTA dopamine neurons and VTA GABAergic neurons of the mouse midbrain, to these processes. We find that the dopamine subpopulation that projects to the nucleus accumbens (NAc) core preferentially encodes reward-predictive cues and prediction errors. In contrast, the subpopulation that projects to the NAc shell preferentially encodes goal-directed actions and relative reward anticipation. VTA GABA neuron activity strongly contrasts VTA dopamine population activity and preferentially encodes reward outcome and retrieval. Electrophysiology, targeted optogenetics, and whole-brain input mapping reveal multiple convergent sources that contribute to the heterogeneity among VTA dopamine subpopulations that likely underlies their distinct encoding of reward-related associations and motivation that defines their functions in these contexts.

Neuronal implementation of the temporal difference learning algorithm in the midbrain dopaminergic system.

The temporal difference learning (TDL) algorithm has been essential to conceptualizing the role of dopamine in reinforcement learning (RL). Despite its theoretical importance, it remains unknown whether a neuronal implementation of this algorithm exists in the brain. Here, we provide an interpretation of the recently described signaling properties of ventral tegmental area (VTA) GABAergic neurons and show that a circuitry of these neurons implements the TDL algorithm. Specifically, we identified the neuronal mechanism of three key components of the TDL model: a sustained state value signal encoded by an afferent input to the VTA, a temporal differentiation circuit formed by two types of VTA GABAergic neurons the combined output of which computes momentary reward prediction (RP) as the derivative of the state value, and the computation of reward prediction errors (RPEs) in dopamine neurons utilizing the output of the differentiation circuit. Using computational methods, we also show that this mechanism is optimally adapted to the biophysics of RPE signaling in dopamine neurons, mechanistically links the emergence of conditioned reinforcement to RP, and can naturally account for the temporal discounting of reinforcement. Elucidating the implementation of the TDL algorithm may further the investigation of RL in biological and artificial systems.

Lateral preoptic area glutamate neurons relay nociceptive information to the ventral tegmental area

The ventral tegmental area (VTA) has been proposed to play a role in pain, but the brain structures modulating VTA activity in response to nociceptive stimuli remain unclear. Here, we demonstrate that the lateral preoptic area (LPO) glutamate neurons relay nociceptive information to the VTA. These LPO glutamatergic neurons synapsing on VTA neurons respond to nociceptive stimulation and conditioned stimuli predicting nociceptive stimulation and also mediate aversion. In contrast, LPO GABA neurons synapsing in the VTA mediate reward. By ultrastructural quantitative synaptic analysis, exvivo electrophysiology, and functional neuroanatomy we identify a complex circuitry between LPO glutamatergic and GABAergic neurons and VTA dopaminergic, GABAergic, and glutamatergic neurons. We conclude that LPO glutamatergic neurons play a causal role in the processing of nociceptive stimuli and in relaying information about nociceptive stimuli. The pathway from LPO glutamatergic neurons to the VTA represents an unpredicted interface between peripheral nociceptive information and the limbic system.

IPAC integrates rewarding and environmental memory during the acquisition of morphine CPP.

The association between rewarding and drug-related memory is a leading factor for the formation of addiction, yet the neural circuits underlying the association remain unclear. Here, we showed that the interstitial nucleus of the posterior limb of the anterior commissure (IPAC) integrated rewarding and environmental memory information by two different receiving projections from ventral tegmental area (VTA) and nucleus accumbens shell region (NAcSh) to mediate the acquisition of morphine conditioned place preference (CPP). A projection from the VTA GABAergic neurons (VTAGABA) to the IPAC lateral region GABAergic neurons (IPACLGABA) mediated the effect of morphine rewarding, whereas the pathway from NAcSh dopamine receptor 1-expressing neurons (NAcShD1) to the IPAC medial region GABAergic neurons (IPACMGABA) was involved in the acquisition of environmental memory. These findings demonstrated that the distinct IPAC circuits VTAGABA→IPACLGABA and NAcShD1R→IPACMGABA were attributable to the rewarding and environmental memory during the acquisition of morphine CPP, respectively, and provided the circuit-based potential targets for preventing and treating opioid addiction.

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

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

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

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

GABAergic Neurons in the Dorsal-Intermediate Lateral Septum Regulate Sleep-Wakefulness and Anesthesia in Mice.

The γ-aminobutyric acid-mediated (GABAergic) inhibitory system in the brain is critical for regulation of sleep-wake and general anesthesia. The lateral septum contains mainly GABAergic neurons, being cytoarchitectonically divided into the dorsal, intermediate, and ventral parts. This study hypothesized that GABAergic neurons of the lateral septum participate in the control of wakefulness and promote recovery from anesthesia. By employing fiber photometry, chemogenetic and optogenetic neuronal manipulations, anterograde tracing, in vivo electrophysiology, and electroencephalogram/electromyography recordings in adult male mice, the authors measured the role of lateral septum GABAergic neurons to the control of sleep-wake transition and anesthesia emergence and the corresponding neuron circuits in arousal and emergence control. The GABAergic neurons of the lateral septum exhibited high activities during the awake state by in vivo fiber photometry recordings (awake vs. non-rapid eye movement sleep: 3.3 ± 1.4% vs. -1.3 ± 1.2%, P < 0.001, n = 7 mice/group; awake vs. anesthesia: 2.6 ± 1.2% vs. -1.3 ± 0.8%, P < 0.001, n = 7 mice/group). Using chemogenetic stimulation of lateral septum GABAergic neurons resulted in a 100.5% increase in wakefulness and a 51.2% reduction in non-rapid eye movement sleep. Optogenetic activation of these GABAergic neurons promoted wakefulness from sleep (median [25th, 75th percentiles]: 153.0 [115.9, 179.7] s to 4.0 [3.4, 4.6] s, P = 0.009, n = 5 mice/group) and accelerated emergence from isoflurane anesthesia (514.4 ± 122.2 s vs. 226.5 ± 53.3 s, P < 0.001, n = 8 mice/group). Furthermore, the authors demonstrated that the lateral septum GABAergic neurons send 70.7% (228 of 323 cells) of monosynaptic projections to the ventral tegmental area GABAergic neurons, preferentially inhibiting their activities and thus regulating wakefulness and isoflurane anesthesia depth. The results uncover a fundamental role of the lateral septum GABAergic neurons and their circuit in maintaining awake state and promoting general anesthesia emergence time.

Ventral pallidal GABAergic neurons control wakefulness associated with motivation through the ventral tegmental pathway

The ventral pallidum (VP) regulates motivation, drug addiction, and several behaviors that rely on heightened arousal. However, the role and underlying neural circuits of the VP in the control of wakefulness remain poorly understood. In the present study, we sought to elucidate the specific role of VP GABAergic neurons in controlling sleep–wake behaviors in mice. Fiber photometry revealed that the population activity of VP GABAergic neurons was increased during physiological transitions from non-rapid eye movement (non-REM, NREM) sleep to either wakefulness or REM sleep. Moreover, chemogenetic and optogenetic manipulations were leveraged to investigate a potential causal role of VP GABAergic neurons in initiating and/or maintaining arousal. In vivo optogenetic stimulation of VP GABAergic neurons innervating the ventral tegmental area (VTA) strongly promoted arousal via disinhibition of VTA dopaminergic neurons. Functional in vitro mapping revealed that VP GABAergic neurons, in principle, inhibited VTA GABAergic neurons but also inhibited VTA dopaminergic neurons. In addition, optogenetic stimulation of terminals of VP GABAergic neurons revealed that they promoted arousal by innervating the lateral hypothalamus, but not the mediodorsal thalamus or lateral habenula. The increased wakefulness chemogenetically evoked by VP GABAergic neuronal activation was completely abolished by pretreatment with dopaminergic D1 and D2/D3 receptor antagonists. Furthermore, activation of VP GABAergic neurons increased exploration time in both the open-field and light–dark box tests but did not modulate depression-like behaviors or food intake. Finally, chemogenetic inhibition of VP GABAergic neurons decreased arousal. Taken together, our findings indicate that VP GABAergic neurons are essential for arousal related to motivation.

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

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

Ventral tegmental area GABAergic neurons induce anxiety-like behaviors and promote palatable food intake

The obesity epidemic is a global problem and a great challenge for public health. Overconsumption of food, especially palatable food, is the leading cause of obesity. The precise neural circuits underlying food overconsumption remain unclear and require further characterization. In the present study, we showed that Ca2+ signals of GABAergic neurons within the ventral tegmental area (VTA) increased after the onset of food intake, especially high-fat or high-sugar chow. Optogenetic activation of VTA GABAergic neurons evoked immediate eating of palatable food and significantly increased palatable food intake in satiated mice. Photoinhibition of VTA GABAergic neurons suppressed palatable food intake. Surprisingly, photoactivation of VTA GABAergic neurons suppressed the intake of standard chow in fasted mice, but did not reduce the duration of eating of standard chow. Moreover, we found that photoactivation of these neurons drove a series of anxiety-like behaviors in the open field, elevated plus maze, and marble-burying test. Additionally, we found that VTA GABAergic neurons sent abundant projections to the lateral hypothalamus and photoactivation of GABAergic VTA terminals in the lateral hypothalamus induced overconsumption of palatable food, but not anxiety-like behaviors. Taken together, our results illustrate that GABAergic VTA neurons are a key node in the neural circuitry underlying anxiety-like behavior and over-feeding of palatable food, and that over-excitation of GABAergic VTA neurons may underlie clinical diseases related to anxiety and obesity.

The projections from the anterior cingulate cortex to the nucleus accumbens and ventral tegmental area contribute to neuropathic pain-evoked aversion in rats

Although the anterior cingulate cortex (ACC) plays a vital role in neuropathic pain-related aversion, the underlying mechanisms haven't been fully studied. The mesolimbic dopamine system encodes reward and aversion, and participates in the exacerbation of chronic pain. Therefore, we investigated whether the ACC modulates aversion to neuropathic pain via control of the mesolimbic dopamine system, in a rat model of chronic constriction injury (CCI) to the sciatic nerve. Using anterograde and retrograde tracings, we confirmed that a subgroup of ACC neurons projected to the nucleus accumbens (NAc) and ventral tegmental area (VTA), which are two crucial nodes of the mesolimbic dopamine system. Combining electrophysiology in juvenile rats 7 days post-CCI, we found that the NAc/VTA-projecting neurons were hyperexcitable after CCI. Chemogenetic inhibition of these projections induced conditioned place preference in young adult rats 10–14 days post-CCI, without modulating the evoked pain threshold, whereas activation of these projections in sham rats mimicked aversive behavior. Furthermore, the function of the ACC projections was probably mediated by NAc D2-type medium spiny neurons and VTA GABAergic neurons. Taken together, our findings suggest that projections from the ACC to the NAc and VTA mediate neuropathic pain-related aversive behavior.

Projection-Specific Potentiation of Ventral Pallidal Glutamatergic Outputs after Abstinence from Cocaine.

The ventral pallidum (VP) is a central node in the reward system that is strongly implicated in reward and addiction. Although the majority of VP neurons are GABAergic and encode reward, recent studies revealed a novel glutamatergic neuronal population in the VP [VP neurons expressing the vesicular glutamate transporter 2 (VPVGluT2)], whose activation generates aversion. Withdrawal from drugs has been shown to induce drastic synaptic changes in neuronal populations associated with reward, such as the ventral tegmental area (VTA) or nucleus accumbens neurons, but less is known about cocaine-induced synaptic changes in neurons classically linked with aversion. Here, we demonstrate that VPVGluT2 neurons contact different targets with different intensities, and that cocaine conditioned place preference (CPP) training followed by abstinence selectively potentiates their synapses on targets that encode aversion. Using whole-cell patch-clamp recordings combined with optogenetics in male and female transgenic mice, we show that VPVGluT2 neurons preferentially contact aversion-related neurons, such as lateral habenula neurons and VTA GABAergic neurons, with minor input to reward-related neurons, such as VTA dopamine and VP GABA neurons. Moreover, after cocaine CPP and abstinence, the VPVGluT2 input to the aversion-related structures is potentiated, whereas the input to the reward-related structures is depressed. Thus, cocaine CPP followed by abstinence may allow VPVGluT2 neurons to recruit aversion-related targets more readily and therefore be part of the mechanism underlying the aversive symptoms seen after withdrawal.SIGNIFICANCE STATEMENT The biggest problem in drug addiction is the high propensity to relapse. One central driver for relapse events is the negative aversive symptoms experienced by addicts during withdrawal. In this work, we propose a possible mechanism for the intensification of aversive feelings after withdrawal that involves the glutamatergic neurons of the ventral pallidum. We show not only that these neurons are most strongly connected to aversive targets, such as the lateral habenula, but also that, after abstinence, their synapses on aversive targets are strengthened, whereas the synapses on other rewarding targets are weakened. These data illustrate how after abstinence from cocaine, aversive pathways change in a manner that may contribute to relapse.

Optogenetic/Chemogenetic Activation of GABAergic Neurons in the Ventral Tegmental Area Facilitates General Anesthesia via Projections to the Lateral Hypothalamus in Mice.

The ventral tegmental area (VTA) reportedly regulates sleep and wakefulness through communication with the lateral hypothalamus (LH). It has also been suggested that adequate anesthesia produced by administration of chloral hydrate, ketamine, or halothane significantly reduces the GABAergic neuronal firing rate within the VTA. However, the exact effects on GABAergic neurons in the VTA and the mechanisms through which these neurons modulate anesthesia through associated neural circuits is still unclear. Here, we used optogenetic and chemogenetic methods to specifically activate or inhibit GABAergic neuronal perikarya in the VTA or their projections to the LH in Vgat-Cre mice. Electroencephalogram (EEG) spectral analyses and burst suppression ratio (BSR) calculations were conducted following administration of 0.8 or 1.0% isoflurane, respectively; and loss of righting reflex (LORR), recovery of righting reflex (RORR), and anesthesia sensitivity were assessed under 1.4% isoflurane anesthesia. The results showed that activation of GABAergic neurons in the VTA increased delta wave power from 40.0 to 46.4% (P = 0.006) and decreased gamma wave power from 15.2 to 11.5% (P = 0.017) during anesthesia maintenance. BSR was increased from 51.8 to 68.3% (P = 0.017). Induction time (LORR) was reduced from 333 to 290 s (P = 0.019), whereas arousal time (RORR) was prolonged from 498 to 661 s (P = 0.007). Conversely, inhibition of VTA GABAergic neurons led to opposite effects. In contrast, optical activation of VTA–LH GABAergic projection neurons increased power of slow delta waves from 44.2 to 48.8% (P = 0.014) and decreased that of gamma oscillations from 10.2 to 8.0%. BSR was increased from 39.9 to 60.2% (P = 0.0002). LORR was reduced from 330 to 232 s (P = 0.002), and RORR increased from 396 to 565 s (P = 0.007). Optical inhibition of the projection neurons caused opposite effects in terms of both the EEG spectrum and the BSR, except that inhibition of this projection did not accelerate arousal time. These results indicate that VTA GABAergic neurons could facilitate the anesthetic effects of isoflurane during induction and maintenance while postponing anesthetic recovery, at least partially, through modulation of their projections to the LH.

Precise Coordination of Three-Dimensional Rotational Kinematics by Ventral Tegmental Area GABAergic Neurons.

The ventral tegmental area (VTA) is a midbrain region implicated in a variety of motivated behaviors. However, the function of VTA GABAergic (Vgat+) neurons remains poorly understood. Here, using three-dimensional motion capture, invivo electrophysiology, calcium imaging, and optogenetics, we demonstrate a novel function of VTAVgat+ neurons. We found three distinct populations of neurons, each representing head angle about a principal axis of rotation: yaw, roll, and pitch. For each axis, opponent cell groups were found that increase firing when the head moves in one direction and decrease firing in the opposite direction. Selective excitation and inhibition of VTAVgat+ neurons generate opposite rotational movements. Thus, VTAVgat+ neurons serve a critical role in the control of rotational kinematics while pursuing a moving target. This general-purpose steering function can guide animals toward desired spatial targets in any motivated behavior.

The Ventral Tegmental Area has calbindin neurons with the capability to co-release glutamate and dopamine into the nucleus accumbens.

The ventral tegmental area (VTA) has three major classes of neurons: dopaminergic (expressing tyrosine hydroxylase; TH), GABAergic (expressing vesicular GABA transporter; VGaT) and glutamatergic (expressing vesicular glutamate transporter 2; VGluT2). While VTA dopaminergic and GABAergic neurons have been further characterized by expression of calcium-binding proteins (calbindin, CB; calretinin, CR or parvalbumin, PV), it is unclear whether these proteins are expressed in rat VTA glutamatergic neurons. Here, by a combination of in situ hybridization (for VGluT2 mRNA detection) and immunohistochemistry (for CB-, CR- or PV-detection), we found that among the total population of VGluT2 neurons, 30% coexpressed CB, 3% coexpressed PV and <1% coexpressed CR. Given that some VGluT2 neurons coexpress TH or VGaT, we examined whether these neurons coexpress CB, and found that about 20% of VGluT2-CB neurons coexpressed TH and about 13% coexpressed VGaT. Because VTA TH-CB neurons are known to target the nucleus accumbens (nAcc), we determined whether VGluT2-CB-TH neurons innervate nAcc, and found that about 80% of VGluT2-CB neurons innervating the nAcc shell coexpressed TH. In summary, (a) CB, PV and CR are detected in subpopulations of VTA-VGluT2 neurons; (b) CB is the main calcium-binding protein present in VTA-VGluT2 neurons; (c) one-third of VTA-VGluT2 neurons coexpress CB; (d) some VTA-VGluT2-CB neurons have the capability to co-release dopamine or GABA, and (e) a subpopulation of VTA glutamatergic-dopaminergic neurons innervates nAcc shell. These findings further provide evidence for molecular diversity among VTA-VGluT2 neurons, neurons that may play a role in specific circuitry and behaviours.

Central Processing of Itch in the Midbrain Reward Center

Itch is an aversive sensation that evokes a desire toscratch. Paradoxically, scratching the itch also produces a hedonic experience. The specific brain circuits processing these different aspects of itch, however, remain elusive. Here, we report that GABAergic (GABA) and dopaminergic (DA) neurons in the ventral tegmental area (VTA) are activated with different temporal patterns during acute and chronic itch. DA neuron activation lags behind GABA neurons and is dependent on scratching of the itchy site. Optogenetic manipulations of VTA GABA neurons rapidly modulated scratching behaviors through encoding itch-associated aversion. Incontrast, optogenetic manipulations of VTA DA neurons revealed their roles in sustainingrecurrent scratching episodes through signaling scratching-induced reward. A similar dichotomy exists for the role of VTA in chronic itch. These findings advance understanding of circuit mechanisms of the unstoppable itch-scratch cycles and shed important insights into chronic itch therapy.

Rostral and Caudal Ventral Tegmental Area GABAergic Inputs to Different Dorsal Raphe Neurons Participate in Opioid Dependence

Both the ventral tegmental area (VTA) and dorsal raphe nucleus (DRN) are involved in affective controland reward-related behaviors. Moreover, the neuronal activities of the VTA and DRN are modulated by opioids. However, the precise circuits from the VTA to DRN and how opioids modulate these circuits remain unknown. Here, we found that neurons projecting from the VTA to DRN are primarily GABAergic. Rostral VTA (rVTA) GABAergic neurons preferentially innervate DRN GABAergic neurons, thus disinhibiting DRN serotonergic neurons. Optogenetic activation of this circuit induces aversion. In contrast, caudal VTA (cVTA) GABAergic neurons mainly target DRN serotonergic neurons, and activation of this circuit promotes reward. Importantly, μ-opioid receptors (MOPs) are selectively expressed at rVTA→DRN GABAergic synapses, and morphine depresses the synaptic transmission. Chronically elevating the activity of the rVTA→DRN pathway specifically interrupts morphine-induced conditioned place preference. This opioid-modulated inhibitory circuit may yield insights into morphine reward and dependence pathogenesis.

GABA and glutamate neurons in the VTA regulate sleep and wakefulness.

We screened for novel circuits in the mouse brain that promote wakefulness. Chemogenetic activation experiments and electroencephalogram recordings pointed to glutamatergic/nitrergic (NOS1) and GABAergic neurons in the ventral tegmental area (VTA). Activating glutamatergic/NOS1 neurons, which were wake- and rapid eye movement (REM) sleep-active, produced wakefulness through projections to the nucleus accumbens and the lateral hypothalamus. Lesioning the glutamate cells impaired the consolidation of wakefulness. By contrast, activation of GABAergic VTA neurons elicited long-lasting non-rapid-eye-movement-like sleep resembling sedation. Lesioning these neurons produced an increase in wakefulness that persisted for at least 4 months. Surprisingly, these VTA GABAergic neurons were wake- and REM sleep-active. We suggest that GABAergic VTA neurons may limit wakefulness by inhibiting the arousal-promoting VTA glutamatergic and/or dopaminergic neurons and through projections to the lateral hypothalamus. Thus, in addition to its contribution to goal- and reward-directed behaviors, the VTA has a role in regulating sleep and wakefulness.

Electrophysiological and neurochemical effects of antidiabetic sulphonylurea tolbutamide in the rat ventral tegmental area and prefrontal cortex

Tolbutamide is an orally administered sulphonylurea, used in the management of non‐insulin‐dependent diabetes mellitus. This class of drug acts on the sulphonyureas receptors(SUR) through blockage of ATP‐regulated K‐channels to release insulin. Some studies have revealed the presence of sulphonylureas binding sites in the substantia nigra. However, there is very little documented data about the presence of these receptors in the ventral tegmental area (VTA) and prefrontal cortex(PFC).Here, using in vivo extracellular single unit recordings, we have investigated the electrophysiological effects of acute intravenous administration of tolbutamide (20mg/kg) on VTA dopamine and PFC pyramidal neurons. We also examined in vitro the effects of this drug on dopamine transporter activity using striatum and tegmental synaptosomes.Adult Sprague Dawley rats were deeply anaesthetised with chloral hydrate (400mg/kg) and urethane (1.5 g/kg) and placed in a stereotaxic instrument for VTA dopamine/GABA and PFC pyramidal neurons recordings, respectively. Appropriately, blood level glucose was monitored with glucometer to avoid hypoglycermia followed by administration with 2ml/kg of 20% glucose subsequent to tolbutamide injection, when necessary. Furthermore, we studied tolbutamide(30μM) effects on striatal synaptosome preparation for dopamine transporter activity through in vitro [3H]‐dopamine radiometric uptake assay technique.In the VTA, administration of tolbutamide induced biphasic effects on the basal activity of most of the dopamine neurons tested: large inhibition (up to 100%) during a relatively short period (1–3 min), caused by hyperpolarisation, followed by a long‐lasting excitatory effect (40% of the baseline activity). Significant increased burst activity was also observed. This initial inhibitory effect was insensitive to a pre‐administration of the dopamine D2 antagonist(eticlopride100μg/kg). Interestingly, presumed GABAergic VTA neurons exhibit only a monophasic significant excitatory effect following tolbutamide administration. In the prefrontal cortex, tolbutamide also induced a monophasic excitatory effect in a dose dependent manner on most of the presumed pyramidal neurons tested. Neurochemical in vitro investigation revealed a statistically significant partial decrease (p&lt;0.05) in specific dopamine synaptosomal uptake at relatively high concentration of tolbutamide (30 μM) an indication of possible direct modulation of dopamine turn‐over.In conclusion, tolbutamide induced a unique biphasic effect in the VTA dopamine neurons. The large but short lasting inhibitory effect is hypothesised to be mediated by GABA interneurons which are found to be immediately activated by the drug. Future works are needed to understand the function of this putative metabolic‐sensitive modulation of dopamine neuronal activity.Support or Funding InformationSelf funded international student, supported by the university bursary department.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Adrenergic Receptor Agonists’ Modulation of Dopaminergic and Non-dopaminergic Neurons in the Ventral Tegmental Area

The ventral tegmental area (VTA) neuronal population consists of dopaminergic (DAergic) and non-DAergic neurons (mainly GABAergic), the activity of which is intertwined with VTA behavioral functions. Both DAergic and GABAergic neurons in the VTA have been shown to express adrenergic receptors (ARs) and respond to AR stimulation. The aim of the present study was to demonstrate the effects of selective AR agonists on DAergic and non-DAergic neuronal activity in the central and lateral parts of the VTA using in vivo electrophysiological recording combined with microiontophoretic drug application in anaesthetized rats. Administration of phenylephrine, a selective α1-AR agonist, while having an inhibitory effect on putative DAergic neurons (11% decrease in firing rate), induced a clear excitatory effect (59% increase in firing rate) on putative non-DAergic neurons. In contrast, application of clonidine, a selective α2-AR agonist, or isoprenaline, a selective β-adrenergic receptor agonist, did not change the firing rate of either DAergic or non-DAergic neurons but influenced the firing pattern of non-DAergic cells only. Our results suggest that noradrenaline modulates activity of VTA neurons in vivo primarily via α1, but also via β- and α2-AR to a lesser extent. Furthermore, we show that α1-AR activation has contrasting effects on putative DAergic and non-DAergic neurons. We hypothesize that the phenylephrine-induced inhibition of putative DAergic neurons results from activation of GABAergic terminals present at the site of drug application. Such a mechanism is further supported by the observed α1-AR-induced excitation of putative GABAergic VTA neurons.