Abstract

Previous studies with cloned gamma-aminobutyric acid type A receptors expressed in human embryonic kidney cells have indicated that the alpha 1 beta 2 gamma 2 and alpha 1 gamma 2 (but not alpha 1 beta 2) subtypes have benzodiazepine sites. We found in this study that even the beta 2 gamma 2 subtype displays gamma-aminobutyric acid-induced Cl- currents that are potentiated by triazolam (a triazolobenzodiazepine). The maximal efficacy of the drug among the subtypes was highest with the alpha 1 beta 2 gamma 2 subtype, followed by the alpha 1 gamma 2 and beta 2 gamma 2 subtypes. These observations led us to compare the ability of several benzodiazepine site agonists of diverse chemical structures to potentiate Cl- currents with these subtypes. With the alpha 1 gamma 2 subtype, diazepam, alpidem, zolpidem, Cl-218872, zopiclone, U-79098 (an imidazoquinoxaline derivative), and U-90167 (a diimidazoquinazoline derivative) at 5 microM potentiated Cl- currents to essentially similar levels (slightly lower for a few ligands), compared with those with the alpha 1 beta 2 gamma 2 subtype. With the beta 2 gamma 2 subtype, the type 1 ligands zolpidem, alpidem, and Cl-218872 showed no or very low levels of potentiation, whereas less selective ligands such as diazepam, zopiclone, U-78098, and U-90167 displayed levels of Cl- current potentiation comparable to those observed with the subtypes containing the alpha 1 and gamma 2 subunits. These data indicate that, in the presence of gamma 2, beta 2 may substitute for alpha 1 in forming the benzodiazepine site of limited sensitivity to the type 1 ligands. It appears that individual ligands for benzodiazepine sites have their own sets of interacting domains, which are distributed in alpha 1 and gamma 2, and the agonistic activity of type 1 ligands may be more dependent on the alpha 1-specific domains than is that of less selective ligands.