title: Benzodiazepines modulate GABAA receptors by regulating the preactivation step after GABA binding.
abstract: GABA(A) receptors (GABA(A)Rs) composed of αβγ subunits are allosterically modulated by the benzodiazepines (BDZs). Agonists at the BDZ binding site potentiate submaximal GABA responses by increasing the apparent affinity of GABA(A)Rs for GABA. Although BDZs were initially thought to affect the binding of GABA agonists, recent studies suggest an effect on receptor gating; however, the involvement of preactivation steps in the modulation by BDZs has not been considered. Consequently, we examined whether BDZ agonists could exert their modulatory effect by displacing the equilibrium between resting and preactivated states of recombinant α1β2γ2 GABA(A)Rs expressed in Xenopus oocytes. For GABA and the partial agonists 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol and piperidine-4-sulfonic acid, we examined BDZ modulation using a simple three-step model incorporating agonist binding, receptor preactivation, and channel opening. The model accounted for diazepam modulation simply by increasing the preactivation constant by approximately fourfold. To assess whether BDZs preferentially affected a specific GABA binding site, pentameric concatamers were used. This demonstrated that single GABA-binding site mutant receptors were equally sensitive to modulation by BDZs compared with wild-type counterparts. Overall, our results suggest that BDZs affect the preactivation step to cause a global conformational rearrangement of GABA(A)Rs, thereby modulating receptor function.
authors.affiliation: Department of Neuroscience, Physiology, and Pharmacology, University College London, London WC1E 6BT, United Kingdom.
authors.forename: Marc C
authors.initials: MC
authors.lastname: Gielen
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Lumb
authors.forename: Trevor G
authors.initials: TG
authors.lastname: Smart
mesh_terms: D000818:Animals; D001569:Benzodiazepines / Q000494:pharmacology*; D001665:Binding Sites / Q000187:drug effects; D003975:Diazepam / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D018755:GABA Agonists / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D018757:GABA Modulators / Q000494:pharmacology; D015640:Ion Channel Gating / Q000187:drug effects / Q000235:genetics; D007555:Isoxazoles / Q000494:pharmacology; D007814:Larva; D008564:Membrane Potentials / Q000187:drug effects / Q000235:genetics; D008954:Models, Biological; D009154:Mutation / Q000235:genetics; D009865:Oocytes / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D010880:Piperidines / Q000494:pharmacology; D011485:Protein Binding / Q000187:drug effects / Q000235:genetics; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011963:Receptors, GABA-A / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014981:Xenopus; D005680:gamma-Aminobutyric Acid / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D018755:GABA Agonists; D018756:GABA Antagonists; D018757:GABA Modulators; D007555:Isoxazoles; D010880:Piperidines; D021122:Protein Subunits; D011963:Receptors, GABA-A; D001569:Benzodiazepines; D005680:gamma-Aminobutyric Acid; C023402:piperidine-4-sulfonic acid; C015542:gaboxadol; D003975:Diazepam