Benzodiazepine binding site heterogeneity in the purified GABAA receptor

The displacement of [3H]flunitrazepam binding activity by ethyl-beta-carboline-3-carboxylate (beta CCE) was studied in both membrane-bound and purified GABAA receptors from adult bovine cerebral cortex, hippocampus and cerebellum. It was found that the best fit for the displacement of benzodiazepine binding in the cerebellar membranes was a single site with IC50 = 0.55 +/- 0.21 nM, whereas the best fit for cortical and hippocampal membranes was a two-site model with respective values of IC50 = 0.2 +/- 0.09 nM (high affinity), IC50 = 21 +/- 6 nM (low affinity) (cortex) and IC50 = 0.25 +/- 0.05 nM, IC50 = 20 +/- 2 nM (hippocampus). These same properties were retained in the purified GABAA receptor from the three brain regions. Thus, we have demonstrated that binding site heterogeneity as defined by the displacement of beta CCE is preserved in purified GABAA receptors and we suggest that this provides evidence for the existence of GABAA receptor isoforms.