Abstract

GABA_A receptors composed of α₁, β₂, γ₁ 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 α₁β₂γ₁-GABA_A 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 (I_GABA) 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 μM concentrations were able to significantly (>20%) enhance I_GABA in α₁β₂γ₁ receptors. Methyl-6,7-dimethoxy-4-ethyl-β-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 μM each) had no significant effect, and flunitrazepam and 2-phenyl-4-(4-ethyl-piperidinyl)-quinoline (PK 8165) inhibited I_GABA. The most potent compounds triazolam, clotiazepam, midazolam, and CGS 20625 were investigated in more detail on α₁β₂γ₁ and α₁β₂γ_2S receptors. The potency and efficiency of these compounds for modulating I_GABA was smaller for α₁β₂γ₁ than for α₁β₂γ_2S receptors, and their effects on α₁β₂γ₁ could not be blocked by flumazenil. CGS 20625 displayed the highest efficiency by enhancing at 100 μM I_GABA (α₁β₂γ₂) by 775 ± 17% versus 526 ± 14% I_GABA (α₁β₂γ₁) and 157 ± 17% I_GABA (α₁β₂) (p < 0.05). These data provide new insight into the pharmacological properties of GABA_A receptors containing γ₁ subunits and may aid in the design of specific ligands for this receptor subtype.