Mechanism of action of benzodiazepines on GABAA receptors

Authors


  • Note: We use B (instead of D) for diazepam to avoid confusion with the general meaning of KD.

Department of Physiology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229, U.S.A. E-mail: weissd@uthscsa.edu

Abstract

  • 1Wild-type and mutant α1β2γ2 GABAA receptors were expressed in Xenopus laevis oocytes and examined using the two-electrode voltage clamp.
  • 2Dose–response relationships for GABA were compared in the absence and presence of 1 μM diazepam (DZP) or methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM). The dose–current relationships yielded EC50's (concentration for half-maximal activation) of 41.0±3.0, 21.7±2.7, and 118.3±6.8 μM for GABA, GABA plus DZP, and GABA plus DMCM, respectively.
  • 3DZP- and DMCM-mediated modulation were examined in GABAA receptors in which the β-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 EC50's of 0.078±0.005, 0.12±0.03, and 0.14±0.004 μM for GABA, GABA plus 1 μM DZP, and GABA plus 1 μM DMCM.
  • 4DZP- and DMCM-mediated modulations were examined in GABAA receptors in which the α-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 βL259S. In this case, the leftward and rightward shifts for DZP and DMCM were still present with EC50's=0.24±0.03, 0.14±0.02, and 1.2±0.04 μM for GABA, GABA plus 1 μM DZP, and GABA plus 1 μM DMCM, respectively.
  • 5Oocytes expressing ultrahigh levels of wild-type GABAA receptors exhibited currents in response to 1 μM 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 (β2Y205S). The EC50's for DZP-mediated activation were 72.0±2.0 and 115±6.2 nM, respectively, similar to the EC50 for DZP-mediated enhancement of the wild-type GABA-activated current (64.8±3.7 nM).
  • 6Our 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.

British Journal of Pharmacology (2006) 148, 984–990. doi:10.1038/sj.bjp.0706796

Ancillary