Adaptation of gamma-aminobutyric acid type A receptors to alcohol exposure: studies with stably transfected cells.

We studied the adaptation of gamma-aminobutyric acid type A (GABAA) receptor function to chronic ethanol exposure in cells stably transfected with the following GABAA receptor subunits: alpha-1 beta-2 gamma-2L, alpha-1 beta-2 gamma-2S, alpha-1 beta-3 gamma-2S, alpha-1 beta-1, alpha-5 beta-3 gamma-3 and alpha-6 beta-3 gamma-2S. Chronic exposure to ethanol resulted in a decrease in muscimol-stimulated 36Cl- flux and a decrease in modulation of that flux by ethanol, flunitrazepam, methyl-6,7-4-dimethoxy-4-ethyl-beta-carboline-3-carboxylate and pregnanolone without any change in the modulation by pentobarbital or zinc. Direct activation of the GABAA receptor by pentobarbital was enhanced by chronic ethanol treatment. Reduction of the action of muscimol, ethanol and flunitrazepam differed in the duration and amount of ethanol required to see an effect. Reduction of the action of ethanol of alpha-1 beta-2 gamma-2L cells occurred within 15 min and was near-maximal for 25 mM ethanol, whereas reduction of the actions of muscimol and flunitrazepam actions required hours of exposure and higher concentrations of ethanol. Chronic ethanol exposure produced a reduction in the Emax value for the action of muscimol for all six subunit combinations, but quantification of surface receptors by immunolabeling showed no change in GABAA receptor density. The differences in alcohol sensitivity and time courses for different effects of ethanol indicate multiple mechanisms of adaptation of GABAA receptors. Use of stably transfected cells rules out "subunit substitution" as a mechanism for these changes and points to post-translational changes (e.g., phosphorylation, receptor assembly) as the most likely mechanisms. These in vitro findings are compared with results from in vivo studies.