PHARMACOLOGICAL MODULATION OF THE DIAZEPAM-INSENSITIVE RECOMBINANT GAMMA-AMINOBUTYRIC ACID(A) RECEPTORS ALPHA-4-ALPHA-2-GAMMA-2 AND ALPHA-6-BETA-2-GAMMA-2

We characterized modulation of the gamma-aminobutyric acid (GABA)-evok ed responses of the diazepam-insensitive alpha 4 beta 2 gamma 2 and al pha 6 beta 2 gamma 2 recombinant GABA(A) receptors. The partial agonis t bretazenil potentiated the responses of both receptors with similar dose dependence but with a higher maximal enhancement at the alpha 4 b eta 2 gamma 2 receptor. The bretazenil-induced potentiation was reduce d by the benzodiazepine antagonist flumazenil. At a high concentration (10 mu M), flumazenil was a weak potentiator of the GABA response. Th e partial agonist imidazenil was inactive. The imidazobenzodiazepine i nverse agonist Ro 15-4513, which is known to bind with high affinity t o the alpha 6 beta 2 gamma 2 receptor, potentiated the GABA responses of the alpha 4 beta 2 gamma 2 and alpha 6 beta 2 gamma 2 receptor subt ypes with similar dose dependence over the concentration range of 0.1- 10 mu M. Methyl-6,7-dimethoxy-4-ethyl-beta-carboline, a beta-carboline inverse agonist, had a similar potentiating effect when tested at a c oncentration of 10 mu M. The alpha 4 beta 2 gamma 2 and alpha 6 beta 2 gamma 2 receptor-mediated currents had equal sensitivities to furosem ide and Zn2+ ions, both of which reduced the GABA-evoked responses. Th e alpha 6 beta 2 gamma 2 receptor but not the alpha 4 beta 2 gamma 2 r eceptor exhibited a low level of spontaneous activity in the absence o f GABA; this resting current could be directly potentiated by Ro 15-45 13, methyl-6,7-dimethoxy-4-ethyl-beta-carboline bretazenil and flumaze nil and was blocked by picrotoxin. Thus, although the alpha 4 beta 2 g amma 2 and alpha 6 beta 2 gamma 2 receptors are insensitive to benzodi azepine binding site full agonists, such as diazepam, they can be modu lated by certain ligands acting as partial and inverse agonists at dia zepam-sensitive receptors and thereby contribute to the respective pha rmacological profiles.