Photoaffinity labelling of the benzodiazepine receptor cannot be used to predict ligand efficacy

Irreversible attachment of flunitrazepam to brain membranes occurs on exposure to UV light, resulting in a loss of the number of binding sites detected by saturation binding of [3H]flunitrazepam, but not of [3H]ethyl-beta-carboline-3-carboxylate (beta CCE). Changes in the affinity of ligands for the benzodiazepine receptor produced by photoaffinity labelling, as indicated by the relative IC50 values for the displacement of [3H]beta CCE in photoaffinity labelled versus control membranes, appear to be related to their chemical structure rather than to their pharmacological profile.