Benzodiazepine agonist and inverse agonist coupling in GABA A receptors antagonized by increased atmospheric pressure

Abstract

Past work found that exposure to 12 times normal atmospheric pressure (ATA) of helium–oxygen gas (heliox) selectively antagonizes (uncouples) and differentiates allosteric coupling in GABA A receptors initiated by benzodiazepines versus neurosteroids. The present study tested the hypothesis that pressure can differentiate coupling initiated by a spectrum of benzodiazepine receptor ligands by measuring the effects of pressure on benzodiazepine ligand modulation of GABA-activated 36Cl − uptake in mouse brain membranes. 12 ATA completely antagonized allosteric modulation by: benzodiazepine receptor agonists diazepam and flunitrazepam; Type-1 selective benzodiazepine receptor agonist zolpidem and the benzodiazepine receptor partial inverse agonist ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-α][1,4]benzodiazepine-3-carboxylate (Ro15-4513). The similar, non-competitive-like characteristics of pressure antagonism of these ligands suggest common structural/functional elements underlying their coupling. Pressure also antagonized allosteric modulation by the benzodiazepine receptor inverse agonist methyl 6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM), but the antagonism was not complete and appeared to be surmountable (competitive-like) suggesting unexpected differences in coupling for DMCM versus Ro15-4513. These studies represent the first attempt to use pressure as a tool to dissect benzodiazepine receptor coupling. The results suggest that there is a common, pressure antagonism sensitive structural/functional element underlying coupling for benzodiazepine receptor ligands and that coupling for the full inverse benzodiazepine receptor agonist DMCM differs from coupling for benzodiazepine receptor agonists and benzodiazepine receptor partial inverse agonists.