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

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

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

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

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