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2014 Abstracts

Functional Switch in GABA(A) Receptors on VTA GABA Neurons by Chronic Ethanol

Ashley Nelson, Brigham Young University

Life Sciences

The motivational effects of opiates and ethanol switch from a dopamine (DA)-independent to a DA-dependent pathway when the animal is in a drug-dependent state. A corresponding change occurs in ventral tegmental area (VTA) GABA(A) receptors in opiate-dependent animals, which switch from a GABA-induced hyperpolarization of VTA GABA neurons to a GABA-induced depolarization. The aim of this study was to evaluate VTA GABA neuron excitability, GABA synaptic transmission to VTA GABA neurons and GABA-mediated DA release in the nucleus accumbens (NAc) under ethanol-naïve and dependent conditions. To accomplish these studies, we used standard whole-cell and attached-cell mode electrophysiological techniques to evaluate acute and chronic ethanol effects on VTA GABA neurons in GAD GFP mice, which enabled the visual identification of GABA neurons in slice preparation. In naïve animals, superfusion of ethanol (IC50 = 30 mM) and GABA(A) receptor agonist muscimol (IC50 = 100 nM) decreased VTA GABA neuron firing rate in a dose-dependent manner. Compared to saline-injected controls, in animals made dependent on ethanol by twice daily injections of 2.0 g/kg ethanol, neither ethanol nor muscimol significantly affected VTA GABA neuron firing rate on average. We and others have found that ethanol decreases DA release at terminals, as measured by fast scan cyclic voltammetry. We have recently reported that ethanol inhibition of DA release at terminals in the NAc of ethanol-naïve animals is mediated by GABA, possibly from VTA GABA neurons that project to the NAc. We evaluated the effects of ethanol on DA release in the same ethanol-dependent animals. Compared to controls, superfusion of ethanol did not significantly affect DA release. Together, these findings suggest that VTA GABA neurons undergo a switch in GABA(A) receptor function with chronic ethanol, which results in a corresponding switch in DA release, perhaps resulting from adaptations in VTA GABA neuron input to the NAc.