ETHANOL, FLUNITRAZEPAM, AND PENTOBARBITAL MODULATION OF GABA(A) RECEPTORS EXPRESSED IN MAMMALIAN-CELLS AND XENOPUS OOCYTES

GABA(A) receptors composed of human alpha 1 beta 2 gamma 2L, alpha 1 b eta 2 gamma 2S, alpha 1 beta 3 gamma 2S, alpha 6 beta 3 gamma 2S, and alpha 5 beta 3 gamma 3 subunits as well as bovine alpha 1 beta 1 gamma 2L and alpha 1 beta 1 subunits were stably expressed in mammalian L(t k(-)) cells and transiently expressed in Xenopus oocytes, Effects of m uscimol, ethanol, flunitrazepam, and pentobarbital on receptor functio n were compared for the two expression systems using a Cl-36(-) flux a ssay for cells and an electrophysiological assay for oocytes. Muscimol activated all receptors in both expression systems but was more poten t for L(tk(-)) cells than oocytes; this difference ranged from 2.6- to 26-fold, depending upon subunit composition, The most pronounced diff erences between receptors and expression systems were found for ethano l. In L(tk(-)) cells, low (5-50 mM) concentrations of ethanol potentia ted muscimol responses only with receptors containing the gamma 2L sub unit, In oocytes, concentrations of 30-100 mM produced small enhanceme nts for most subunit combinations, Flunitrazepam enhanced muscimol res ponses for all receptors except alpha 6 beta 3 gamma 2S and alpha 1 be ta 1, and this enhancement was similar for both expression systems, Pe ntobarbital also enhanced muscimol responses for all receptors, and th is enhancement was similar for L(tk(-)) cells and oocytes, except for alpha 6 beta 3 gamma 2S where the pentobarbital enhancement was much g reater in oocytes than cells, The alpha 6 beta 3 gamma 2S receptors we re also distinct in that pentobarbital produced direct activation of c hloride channels in both expression systems. Thus, the type of express ion/assay system markedly affects the actions of ethanol on GABA(A) re ceptors and also influences the actions of muscimol and pentobarbital on this receptor, Differences between these expression systems may ref lect posttranslational modifications of receptor subunits.