Several groups have characterized the pharmacology of ␣ 4 -or ␣ 6 ␤ 3 ␦-containing GABA A receptors expressed in different cell systems. We have previously demonstrated that the pharmacological profiles of a series of GABA A receptor agonists are highly dependent on the ␣ subunit and little on the ␤ and ␥ subunits, so to further understand the contribution of the different subunits in the GABA A receptor complex, we characterized a series of full agonists, partial agonists, and antagonists at ␣ 4 ␤ 3 , ␣ 4 ␤ 3 ␦, and ␣ 6 ␤ 3 ␦ receptors expressed in Xenopus oocytes. Little or no difference was seen when the compounds were compared at ␣␤-and ␣␤␦-containing receptors, whereas a significant reduction in both potency and relative efficacy was observed compared with ␣␤␥-containing receptors described in the literature. These data clearly confirm that the presence of the ␦ subunit in heterotrimeric receptors is a strong determinant of the increased pharmacological activity of compounds with agonist activity. The very similar agonist pharmacology of ␣␤-and ␣␤␦-containing receptors, which is significantly different from that of ␣␤␥-containing receptors, shows that whereas the presence of a ␥ subunit impairs the response to an agonist stimulation of the ␣␤ receptor complex, the ␦ subunit does not affect this in any way. Taken together, these data are well in line with the idea that ␣ 4 ␤ 3 ␦ may contribute to the pharmacological action of exogenously applied agonists and may explain why systemically active compounds such as gaboxadol and muscimol in vivo appear to act as selective extrasynaptic GABA A agonists.

GABA A receptors (GABA A Rs) are usually formed by ␣, ␤, and ␥ or ␦ subunits. Recently, ␦-containing GABA A Rs expressed in Xenopus oocytes were found to be sensitive to low concentrations of ethanol (1-3 mM). Our objective was to replicate and extend the study of the effect of ethanol on the function of ␣ 4 ␤ 3 ␦ GABA A Rs. We independently conducted three studies in two systems: rat and human GABA A Rs expressed in Xenopus oocytes, studied through two-electrode voltage clamp; and human GABA A Rs stably expressed in the fibroblast L(tk Ϫ ) cell line, studied through patch-clamp electrophysiology. In all cases, ␣ 4 ␤ 3 ␦ GABA A Rs were only sensitive to high concentrations of ethanol (100 mM in oocytes, 300 mM in the cell line). Expression of the ␦ subunit in oocytes was assessed through the magnitude of the maximal GABA currents and sensitivity to zinc. Of the three rat combinations studied, ␣ 4 ␤ 3 was the most sensitive to ethanol, isoflurane, and 5␣-pregnan-3␣,21-diol-20-one (THDOC); ␣ 4 ␤ 3 ␦ and ␣ 4 ␤ 3 ␥ 2S were very similar in most aspects, but ␣ 4 ␤ 3 ␦ was more sensitive to GABA, THDOC, and lanthanum than ␣ 4 ␤ 3 ␥ 2S GABA A Rs. Ethanol at 30 mM did not affect tonic GABA-mediated currents in dentate gyrus reported to be mediated by GABA A Rs incorporating ␣ 4 and ␦ subunits. We have not been able to replicate the sensitivity of ␣ 4 ␤ 3 ␦ GABA A Rs to low concentrations of ethanol in four different laboratories in independent studies. This suggests that as yet unidentified factors may play a critical role in the ethanol effects on ␦-containing GABA A Rs.