When membranes of rat cerebellum are exposed to UV light in the presence of flunitrazepam this ligand can be incorporated into one of the assumed 4 benzodiazepine binding sites of the GABA-benzodiazepine receptor complex. This irreversible incorporation of flunitrazepam, in contrast to reversible binding of this substance, leads to conformational changes of the remaining 3 benzodiazepine binding sites which result in a decreased affinity of benzodiazepine agonists, but not of benzodiazepine antagonists. The investigation of the affinity of drugs for [3H]benzodiazepine antagonist binding before and after photoaffinity labelling of benzodiazepine receptors with flunitrazepam can therefore be used as a sensitive and simple test to distinguish between agonists and antagonists in vitro.
