Estimating the efficiency of benzodiazepines on GABA A receptors comprising γ1 or γ2 subunits

Background and purpose: Heterologous expression of α1, β2 and γ2S(γ1) subunits produces a mixed population of GABA A receptors containing α1β2 or α1β2γ2S(γ1) subunits. GABA sensitivity (lower in receptors containing γ1 or γ2S subunits) and the potentiation of GABA‐activated chloride currents ( I GABA ) by benzodiazepines (BZDs) are dependent on γ2S(γ1) incorporation. A variable γ subunit incorporation may affect the estimation of I GABA potentiation by BZDs. We propose an approach for estimation of BZD efficiency that accounts for mixed population of α1β2 and α1β2γ2S(γ1) receptors. Experimental approach: We investigated the relation between GABA sensitivity ( EC 50 ) and BZD modulation by analysing triazolam‐, clotiazepam‐ and midazolam‐induced potentiation of I GABA in Xenopus oocytes under two‐microelectrode voltage clamp. Key results: Plotting EC 50 versus BZD‐induced shifts of GABA concentration‐response curves (Δ EC 50 (BZD)) of oocytes injected with different amounts of α1, β2 and γ2S(γ1) cRNA (1:1:1–1:1:10) revealed a linear regression between γ2S(γ1)‐mediated reduction of GABA sensitivity ( EC 50 ) and Δ EC 50 (BZD). The slope factors of the regression were always higher for oocytes expressing α1β2γ1 subunit receptors (1.8±0.1 (triazolam), 1.6±0.1 (clotiazepam), 2.3±0.2 (midazolam)) than for oocytes expressing α1β2γ2S receptors (1.4±0.1 (triazolam), 1.4±0.1 (clotiazepam), 1.3±0.1 (midazolam)). Mutant GABA A receptors (α1β2‐R207Cγ2S) with lower GABA sensitivity showed higher drug efficiencies (slope factors=1.1±0.1 (triazolam), 1.1±0.1 (clotiazepam), 1.2±0.1 (midazolam)). Conclusions and implications: Regression analysis enabled the estimation of BZD efficiency when variable mixtures of α1β2 and α1β2γ2S(γ1) receptors are expressed and provided new insights into the γ2S(γ1) dependency of BZD action. British Journal of Pharmacology (2008) 155 , 424–433; doi: 10.1038/bjp.2008.271 ; published online 7 July 2008