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 affinities for [³H]flunitrazepam binding sites in isolated brain membranes. However, 50% occupation of [³H]-flunitrazepam sites by benzodiazzepines in vivo requires brain concentrations of the drugs about 1000 times higher than their K_i 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.