GABAA receptors composed of a1, ß2, ?1 subunits are expressed in only a few areas of the brain and thus represent interesting drug targets. The pharmacological properties of this receptor subtype, however, are largely unknown. In the present study, we expressed a1ß 2?1-GABAA receptors in Xenopus laevis oocytes and analyzed their modulation by 21 ligands from 12 structural classes making use of the two-microelectrode voltage-clamp method and a fast perfusion system. Modulation of GABA-induced chloride currents (IGABA) was studied at GABA concentrations eliciting 5 to 10% of the maximal response. Triazolam, clotiazepam, midazolam, 2-(4-methoxyphenyl)-2,3,5,6,7,8,9,10- octahydro-cyclohepta-(b)pyrazolo[4,3-d]pyridin-3-one (CGS 20625), 2-(4-chlorophenyl)-pyrazolo[4,3-c]quinolin-3-one (CGS 9896), diazepam, zolpidem, and bretazenil at 1 œM concentrations were able to significantly (>20%) enhance IGABA in a1ß2? 1 receptors. Methyl-6,7-dimethoxy-4-ethyl-ß-carboline-3- carboxylate, 3-methyl-6-[3-trifluoromethyl-phenyl]-1,2,4-triazolo[4,3-b] pyridazine (Cl 218,872), clobazam, flumazenil, 5-(6-ethyl-7-methoxy-5- methylimidazo[1,2-a]pyrimidin-2-yl)-3-methyl-[1,2,4]-oxadiazole (Ru 33203), 2-phenyl-4-(3-ethyl-piperidinyl)-quinoline (PK 9084), flurazepam, ethyl-7-methoxy-11,12,13,13a-tetrahydro-9-oxo-9H-imidazo[1,5-a]pyrrolo[2,1-c] [1,4]benzodiazepine-1-carboxylate (L-655,708), 2-(6-ethyl-7-methoxy-5- methylimidazo[1,2-a]pyrimidin-2-yl)-4-methyl-thiazole (Ru 33356), and 6-ethyl-7-methoxy-5-methylimidazo[ 1,2-a]pyrimidin-2-yl)phenylmethanone (Ru 32698) (1 œM each) had no significant effect, and flunitrazepam and 2-phenyl-4-(4-ethyl-piperidinyl)-quinoline (PK 8165) inhibited IGABA. The most potent compounds triazolam, clotiazepam, midazolam, and CGS 20625 were investigated in more detail on a1ß2? 1 and a1ß2?2S receptors. The potency and efficiency of these compounds for modulating I GABA was smaller for a1ß2? 1 than for a1ß2?2S receptors, and their effects on a1ß2? 1 could not be blocked by flumazenil. CGS 20625 displayed the highest efficiency by enhancing at 100 œM IGABA (a 1ß2?2) by 775 ‘ 17% versus 526 ‘ 14% IGABA (a1ß2? 1) and 157 ‘ 17% IGABA (a1ß 2) (p < 0.05). These data provide new insight into the pharmacological properties of GABAA receptors containing ?1 subunits and may aid in the design of specific ligands for this receptor subtype. Copyright Œ 2006 The American Society for Pharmacology and Experimental Therapeutics.