Photoaffinity Labeling of Benzodiazepine Receptor Proteins with the Partial Inverse Agonist [3H]Ro 15–4513: A Biochemical and Autoradiographic Study

Photolabeling of the benzodiazepine receptor, which to date has been done with benzodiazepine agonists such as flunitrazepam, can also be achieved with Ro 15-4513, a partial inverse agonist of the benzodiazepine receptor. 3H]Ro 15-4513 specifically and irreversibly labeled a protein with an apparent molecular weight of 51,000 (P51) in cerebellum and at least two proteins with apparent molecular weights of 51,000 (P51) and 55,000 (P55) in hippocampus. Photolabeling was inhibited by 10 microM diazepam but not by 10 microM Ro 5-4864. The BZ1 receptor-selective ligands CL 218872 and beta-carboline-3-carboxylate ethyl ester preferentially inhibited irreversible binding of 3H]Ro 15-4513 to protein P51. Not only these biochemical results but also the distribution and density of 3H]Ro 15-4513 binding sites in rat brain sections were similar to the findings with 3H]flunitrazepam. Thus, the binding sites for agonists and inverse agonists appear to be located on the same proteins. In contrast, whereas 3H]flunitrazepam is known to label only 25% of the benzodiazepine binding sites in brain membranes, all binding sites are photolabeled by 3H]Ro 15-4513. Thus, all benzodiazepine receptor sites are associated with photolabeled proteins with apparent molecular weights of 51,000 and/or 55,000. In cerebellum, an additional protein (MW 57,000) unrelated to the benzodiazepine receptor was labeled by 3H]Ro 15-4513 but not by 3H]flunitrazepam. In brain sections, this component contributed to higher labeling by 3H]Ro 15-4513 in the granular than the molecular layer.