• Energy Research

BackgroundVentral tegmental area (VTA) GABA neurons have been heavily implicated in alcohol reinforcement and reward. In animals that self‐administer alcohol, VTA GABA neurons exhibit increased excitability that may contribute to alcohol's rewarding effects. The present study investigated the effects of acute and chronic ethanol exposure on glutamate (GLU) synaptic transmission to VTA GABA neurons.MethodsWhole‐cell recordings of evoked, spontaneous, and miniature excitatory postsynaptic currents (eEPSCs, sEPSCs, and mEPSCs, respectively) were performed on identified GABA neurons in the VTA of GAD67‐GFP+ transgenic mice. Three ethanol exposure paradigms were used: acute ethanol superfusion; a single ethanol injection; and chronic vapor exposure.ResultsAcute ethanol superfusion increased the frequency of EPSCs but inhibited mEPSC frequency and amplitude. During withdrawal from a single injection of ethanol, the frequency of sEPSCs was lower than saline controls. There was no difference in α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA)/N‐methyl‐d‐aspartate (NMDA) ratio between neurons following withdrawal from a single exposure to ethanol. However, following withdrawal from chronic ethanol, sEPSCs and mEPSCs had a greater frequency than air controls. There was no difference in AMPA/NMDA ratio following chronic ethanol.ConclusionsThese results suggest that presynaptic mechanisms involving local circuit GLU neurons, and not GLU receptors, contribute to adaptations in VTA GABA neuron excitability that accrue to ethanol exposure, which may contribute to the rewarding properties of alcohol via their regulation of mesolimbic dopamine transmission.

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.

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

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; 1 Hz, 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.

AbstractBackgroundDopamine neuron firing in the ventral tegmental area (VTA) and dopamine release in the nucleus accumbens have been implicated in reward learning. Ethanol is known to increase both dopamine neuron firing in the VTA and dopamine levels in the nucleus accumbens. Despite this, some discrepancies exist between the dose of ethanol required to enhance firingin vivoandex vivo. In the present study we investigated the effects of peripheral dopamine 2 subtype receptor antagonism on ethanol's effects on dopamine neurotransmission.MethodsPlasma catecholamine levels were assessed following ethanol administration across four different doses of EtOH. Microdialysis and voltammetry were used to assess the effects of domperidone pretreatment on ethanol‐mediated increases in dopamine release in the nucleus accumbens. A place conditioning paradigm was used to assess conditioned preference for ethanol and whether domperidone pretreatment altered this preference. Open‐field and loss‐of‐righting reflex paradigms were used to assess the effects of domperidone on ethanol‐induced sedation. A rotarod apparatus was used to assess the effects of domperidone on ethanol‐induced motor impairment.ResultsDomperidone attenuated ethanol's enhancement of mesolimbic dopamine release under non‐physiological conditions at intermediate (1.0 and 2.0 g/kg) doses of ethanol. Domperidone also decreased EtOH‐induced sedation at 2.0 g/kg. Domperidone did not alter ethanol conditioned place preference nor did it affect ethanol‐induced motor impairment.ConclusionsThese results show that peripheral dopamine 2 receptors mediate some of the effects of ethanol on nonphysiological dopamine neurotransmission, although these effects are not related to the rewarding properties of ethanol.

AbstractPrevious studies indicate that moderate‐to‐high ethanol (EtOH) concentrations enhance dopamine (DA) neurotransmission in the mesolimbic DA system from the ventral tegmental area (VTA) and projecting to the nucleus accumbens core (NAc). However, voltammetry studies demonstrate that moderate‐to‐high EtOH concentrations decrease evoked DA release at NAc terminals. The involvement of γ‐aminobutyric acid (GABA) receptors (GABAARs), glycine (GLY) receptors (GLYRs) and cholinergic interneurons (CINs) in mediating EtOH inhibition of evoked NAc DA release were examined. Fast scan cyclic voltammetry, electrophysiology, optogenetics and immunohistochemistry techniques were used to evaluate the effects of acute and chronic EtOH exposure on DA release and CIN activity in C57/BL6, CD‐1, transgenic mice and δ‐subunit knockout (KO) mice (δ−/−). Ethanol decreased DA release in mice with an IC50 of 80 mM ex vivo and 2.0 g/kg in vivo. GABA and GLY decreased evoked DA release at 1–10 mM. Typical GABAAR agonists inhibited DA release at high concentrations. Typical GABAAR antagonists had minimal effects on EtOH inhibition of evoked DA release. However, EtOH inhibition of DA release was blocked by the α4β3δ GABAAR antagonist Ro15‐4513, the GLYR antagonist strychnine and by the GABA ρ1 (Rho‐1) antagonist TPMPA (10 μM) and reduced significantly in GABAAR δ−/− mice. Rho‐1 expression was observed in CINs. Ethanol inhibited GABAergic synaptic input to CINs from the VTA and enhanced firing rate, both of which were blocked by TPMPA. Results herein suggest that EtOH inhibition of DA release in the NAc is modulated by GLYRs and atypical GABAARs on CINs containing δ‐ and Rho‐subunits.