Pharmacological Properties of GABAA Receptors Containing γ1 Subunits

Abstract

GABA(A) receptors composed of alpha(1), beta(2), gamma(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 alpha(1)beta(2)gamma(1)-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 microM concentrations were able to significantly (>20%) enhance I(GABA) in alpha(1)beta(2)gamma(1) receptors. Methyl-6,7-dimethoxy-4-ethyl-beta-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 microM 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 alpha(1)beta(2)gamma(1) and alpha(1)beta(2)gamma(2S) receptors. The potency and efficiency of these compounds for modulating I(GABA) was smaller for alpha(1)beta(2)gamma(1) than for alpha(1)beta(2)gamma(2S) receptors, and their effects on alpha(1)beta(2)gamma(1) could not be blocked by flumazenil. CGS 20625 displayed the highest efficiency by enhancing at 100 microM I(GABA) (alpha(1)beta(2)gamma(2)) by 775 +/- 17% versus 526 +/- 14% I(GABA) (alpha(1)beta(2)gamma(1)) and 157 +/- 17% I(GABA) (alpha(1)beta(2)) (p < 0.05). These data provide new insight into the pharmacological properties of GABA(A) receptors containing gamma(1) subunits and may aid in the design of specific ligands for this receptor subtype.