Abstract
GABAA 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-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 μM concentrations were able to significantly (>20%) enhance IGABA 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 IGABA. The most potent compounds triazolam, clotiazepam, midazolam, and CGS 20625 were investigated in more detail on α1β2γ1 and α1β2γ2S receptors. The potency and efficiency of these compounds for modulating IGABA was smaller for α1β2γ1 than for α1β2γ2S receptors, and their effects on α1β2γ1 could not be blocked by flumazenil. CGS 20625 displayed the highest efficiency by enhancing at 100 μM IGABA (α1β2γ2) by 775 ± 17% versus 526 ± 14% IGABA (α1β2γ1) and 157 ± 17% IGABA (α1β2) (p < 0.05). These data provide new insight into the pharmacological properties of GABAA receptors containing γ1 subunits and may aid in the design of specific ligands for this receptor subtype.
- Received July 26, 2005.
- Accepted November 4, 2005.
- The American Society for Pharmacology and Experimental Therapeutics
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