Benzodiazepine receptors: labeling in intact animals with [3H] flunitrazepam.

[3H]Flumitrazepam appears to label specific benzodiazepine receptors in vitro after i.v. injection in mice. Benzodiazepine potencies in reducing [3H]flunitrazepam binding in vivo correspond to pharmacological potencies and parallel relative affinites for [3H]flunitrazepam binding sites in isolated brain membranes. However, 50% occupation of [3H]-flunitrazepam sites by benzodiazepines in vivo requires brain concentrations of the drugs about 1000 times higher than their Ki values for the binding sites in vitro. In pharmacologically active doses sodium pentobarbital, strychnine, picrotoxin and bicuculline fail to influence [3H] flunitrazepam binding in vivo.