Mechanism Of Action Of Benzodiazepines On Gaba A Receptors
Wild-type and mutant Î±1Î²2Î³2 GABA A receptors were expressed in Xenopus laevis oocytes and examined using the two-electrode voltage clamp. Dose-response relationships for GABA were compared in the absence and presence of 1 Î¼M diazepam (DZP) or methyl-6,7-dimethoxy-4-ethyl-beta- carboline-3-carboxylate (DMCM). The dose-current relationships yielded EC 50's (concentration for half-maximal activation) of 41.0 Â± 3.0, 21.7 Â± 2.7, and 118.3 Â± 6.8 Î¼M for GABA, GABA plus DZP, and GABA plus DMCM, respectively. DZP- and DMCM-mediated modulation were examined in GABA A receptors in which the Î²-subunit carries the L259S mutation. This mutation has been shown to produce spontaneous opening and impart a leftward shift in the dose-response relationship. In this case, neither DZP nor DMCM produced a significant alteration in the GABA dose-response relationship with GABA EC 50's of 0.078 Â± 0.005, 0.12 Â± 0.03, and 0.14 Â± 0.004 Î¼M for GABA, GABA plus 1 Î¼M DZP, and GABA plus 1 Î¼M DMCM. DZP- and DMCM-mediated modulations were examined in GABA A receptors in which the Î±-subunit carries the L263S mutation. This mutation also produced spontaneous opening and a leftward shift of the GABA dose-response relation, but to a lesser extent than that of Î²L259S. In this case, the leftward and rightward shifts for DZP and DMCM were still present with EC 50's = 0.24 Â± 0.03, 0.14 Â± 0.02, and 1.2 Â± 0.04 Î¼M for GABA, GABA plus 1 Î¼M DZP, and GABA plus 1 Î¼M DMCM, respectively. Oocytes expressing ultrahigh levels of wild-type GABA A receptors exhibited currents in response to 1 Î¼M DZP alone, whereas DMCM decreased the baseline current. The DZP-mediated activation currents were determined in wild-type receptors as well as receptors in which the GABA binding site was mutated (Î²2Y205S). The EC 50's for DZP-mediated activation were 72.0 Â± 2.0 and 115 Â± 6.2 nM, respectively, similar to the EC 50 for DZP-mediated enhancement of the wild-type GABA-activated current (64.8 Â± 3.7 nM). Our results support a mechanism in which DZP increases the apparent affinity of the receptor, not by altering the affinity of the closed state, but rather by shifting the equilibrium towards the high-affinity open state. Â© 2006 Nature Publishing Group. All rights reserved.
British Journal of Pharmacology
Digital Object Identifier (DOI)
Campo-Soria, Claudia; Chang, Yongchang; and Weiss, David S., "Mechanism Of Action Of Benzodiazepines On Gaba A Receptors" (2006). Neurobiology. 59.