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KA Wafford, PJ Whiting and JA Kemp
Merck, Sharp & Dohme Research Laboratories, Harlow, Essex, UK.
Two forms of the multisubunit gamma-aminobutyric acid (GABA)A receptor were expressed in Xenopus oocytes by injecting mRNA encoding the bovine GABAA receptor subunit cDNAs for alpha 1 beta 1 gamma 2L and alpha 3 beta 1 gamma 2L. The properties of these two combinations were examined by electrophysiological recording of GABA currents using the two- electrode voltage-clamp method. The actions of several benzodiazepine site ligands were compared in terms of their affinity for and efficacy at these two subunit combinations. Flunitrazepam potentiated control GABA responses to a maximum of 77% with alpha 1 beta 1 gamma 2L and 105% with alpha 3 beta 1 gamma 2L, with EC50 values of 29 +/- 11 nM and 23 +/- 10 nM, respectively. Flunitrazepam also produced a greater shift to the left of the GABA concentration-response curve with alpha 3 beta 1 gamma 2L than with alpha 1 beta 1 gamma 2L. Concentration-response curves for the type I benzodiazepine receptor-preferring compounds zolpidem and CL218,872 showed a selectivity for the alpha 1 beta 1 gamma 2L receptor, with respective affinity ratios 7-fold and 17-fold higher, compared with alpha 3 beta 1 gamma 2L. The inverse agonist methyl-6,7-dimethoxy-4-ethyl-beta- carboline-3-carboxylate produced a maximum inhibition of 30% with both receptor combinations and also had a higher affinity for alpha 1 beta 1 gamma 2L than alpha 3 beta 1 gamma 2L. For the first time CL218,872 and FG8205 were shown to be partial agonists of individual receptor combinations, compared with a full agonist such as flunitrazepam. The FG8205 concentration-response curve reached a maximum approximately 60% that of a full agonist with both alpha 1 beta 1 gamma 2L and alpha 3 beta 1 gamma 2L. CL218,872 reached a lower maximum efficacy with alpha 3 beta 1 gamma 2L (30%) than with alpha 1 beta 1 gamma 2L (51%), demonstrating not only that different compounds can have varying levels of partial agonist activity but also that the same compound can have differing degrees of efficacy at different receptor combinations.
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