Anion Regulation of Agonist and Inverse Agonist Binding to Benzodiazepine Receptors

Binding of the benzodiazepine inverse agonist 3H]methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate ( 3H]DMCM) and the agonist 3H]flunitrazepam ( 3H]FNZ) was compared in rat cortical membranes. Halide ions enhanced 3H]DMCM binding three- to fourfold, increasing both the apparent affinity and the number of binding sites for this radioligand. The effect was present at both 0 and 37 degrees C. In contrast, the magnitude of halide stimulation of 3H]FNZ binding was much smaller, resulting solely from an increase in the apparent affinity for this radioligand, and was not observed at 37 degrees C. The potencies but not the efficacies of a series of anions to stimulate both 3H]DMCM and 3H]FNZ binding to benzodiazepine receptors were highly correlated with their relative permeabilities through gamma-aminobutyric acid (GABA)-gated chloride channels. Two stress paradigms (10 min of immobilization or ambient-temperature swim stress), previously shown to increase significantly the magnitude of halide-stimulated 3H]FNZ binding, did not significantly affect 3H]DMCM binding. Phospholipase A2 treatment of cortical membrane preparations was equipotent in preventing the stimulatory effect of chloride on both 3H]DMCM and 3H]FNZ binding. These data strongly suggest that anions modify the binding of 3H]DMCM and 3H]FNZ by acting at a common anion binding site that is an integral component of the GABA/benzodiazepine receptor chloride channel complex.