Benzodiazepine receptor protein identified and visualized in brain tissue by a photoaffinity label

Flunitrazepam, a potent benzodiazepine, reversibly binds to the benzodiazepine receptor with high affinity. When irradiated with UV light, flunitrazepam was irreversibly linked to brain tissue. Incorporation of [3H]flunitrazepam was inhibited by other benzodiazepines with a potency corresponding to their affinity for the benzodiazepine receptor. Photolabeling with flunitrazepam reduced the number of benzodiazepine receptors determined by reversible binding of benzodiazepines, whereas the apparent affinity of the remaining receptors was unchanged. Half-maximal incorporation of flunitrazepam occurred at a concentration similar to the apparent dissociation constant of flunitrazepam. Thus, flunitrazepam appears to be a photoaffinity label for the benzodiazepine receptor. The receptor component photolabeled with flunitrazepam was a protein of molecular weight 50,000. Its location in cerebral and cerebellar cortex slices could be visualized by electron microscopic autoradiography. A predominant localization of benzodiazepine receptors in regions of synaptic constants, including those formed by GABAergic neurons (GABA is gamma-aminobutyric acid), was observed.