Abstract: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 μM diazepam but not by 10 μM Ro 5‐4864. The BZ1receptor‐selective ligands CL 218872 and ß‐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 pro‐teins. 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 granu