Allosteric modulation by benzodiazepine receptor ligands of the GABAA receptor channel expressed in Xenopus oocytes

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Abstract
Chick brain mRNA was isolated and injected into Xenopus oocytes. This led to the expression in the surface membrane of functional GABA- activated channels with properties reminiscent of vertebrate GABAA channels. The GABA-induced current was analyzed quantitatively under voltage-clamp conditions. Picrotoxin inhibited this current in a concentration-dependent manner with IC50 = 0.6 microM. The allosteric modulation of GABA currents by a number of drugs acting at the benzodiazepine binding site was characterized quantitatively. In the presence of the benzodiazepine receptor ligands diazepam and clorazepate, GABA responses were enhanced, and in the presence of the convulsant beta-carboline compound methyl 6,7-dimethoxy-4-ethyl-beta- carboline-3-carboxylate (DMCM), they were depressed. Maximal stimulation of the response elicited by 10 microM GABA was 160% with diazepam and 90% with clorazepate, and maximal inhibition was 42% with DMCM, 30% with methyl beta-carboline-3-carboxylate (beta-CCM), 15% with ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo [1,5a][1,4]benzodiazepine-3-carboxylate (Ro 15–1788), and 12% with ethyl beta-carboline-3-carboxylate (beta-CCE). Half-maximal stimulation was observed with 20 nM diazepam and 390 nM clorazepate, respectively, and half-maximal inhibition with 6 nM DMCM. beta-CCM had a similar effect to DMCM, whereas beta-CCE and Ro 15–1788 showed only small inhibition at low concentrations (less than 1 microM). All the tested carboline compounds and Ro 15–1788 showed a biphasic action and stimulated GABA current at concentrations higher than 1 microM.