Differences in affinity and efficacy of benzodiazepine receptor ligands at recombinant gamma-aminobutyric acidA receptor subtypes.
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.
Merck, Sharp & Dohme Research Laboratories, Harlow, Essex, UK.
K A
KA
Wafford
P J
PJ
Whiting
J A
JA
Kemp
D000818:Animals; D001569:Benzodiazepines / Q000037:antagonists & inhibitors / Q000378:metabolism; D004594:Electrophysiology; D066298:In Vitro Techniques; D008024:Ligands; D009865:Oocytes; D011963:Receptors, GABA-A / Q000145:classification / Q000187:drug effects / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D014981:Xenopus
D008024:Ligands; D011963:Receptors, GABA-A; D011994:Recombinant Proteins; D001569:Benzodiazepines