Ethanol enhances gamma-aminobutyric acid responses in a subpopulation of nucleus accumbens neurons: role of metabotropic glutamate receptors

The nucleus accumbens (NAcc) may be a key area in the rewarding effects of abused drugs. We previously showed that low ethanol concentrations decreased both N-methyl-D-aspartate (NMDA)-induced and kainate-induced currents in NAcc core neurons. To explore the effects of ethanol on gamma-aminobutyric acid (GABA) responses in NAcc, we used intracellular voltage-clamp recordings and locally applied GABA in a slice preparation containing the NAcc. Ethanol (11-200 mM) had no effect on resting membrane properties, but 11, 22, 44, 100, and 200 mM ethanol increased GABA currents in 17, 33, 45, 50, and 22% of cells, respectively. Superfusion of low glutamate concentrations that had no direct effect on membrane properties enhanced ethanol potentiation of GABA currents in more than half the NAcc cells. Neither alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate receptor nor NMDA receptor antagonists affected the percentage of cells showing ethanol enhancement of GABA responses or the degree of ethanol enhancement of GABA currents in NAcc neurons. However, in ethanol-sensitive cells, the metabotropic receptor antagonist alpha-methyl-4-carboxyphenylglycine (MCPG) blocked the ethanol enhancement of GABA currents. In addition, the metabotropic receptor agonist trans-1-aminocyclopentane-1,3-dicarboxylic acid enhanced GABA responses in 50% of cells tested, an effect blocked by MCPG. These data suggest that NAcc core neurons possess both ethanol-sensitive and -insensitive GABA receptors and that glutamate can mimic and enhance the ethanol potentiation of GABA currents in many of these neurons. Furthermore, the ethanol potentiation of GABA currents may involve metabotropic glutamate receptors, perhaps via a phosphorylation mechanism that regulates ethanol sensitivity of GABA receptors in some NAcc neurons.