Photolabeling of benzodiazepine receptors spares [3H]propyl beta-carboline binding.

When the benzodiazepine receptor in mouse cerebellar, striatal, and hippocampal membranes was photoaffinity labeled with nonradioactive flunitrazepam, specific [3H]diazepam binding determined with either unlabeled diazepam or ethyl beta-carboline-3-carboxylate (betaCCE) as displacer declined greater than 80%. In contrast, specific propyl beta-carboline-3-carboxylate (PrCC) binding in these regions determined with betaCCE as displacer was basically unaltered after photolabeling. Photolabeling lowered specific [3H]PrCC binding with diazepam as displacer to an intermediate extent in the three regions. In cerebellum photolabeling had little effect on either the Kd or Bmax for specific [3H]PrCC binding determined with betaCCE as displacer but significantly lowered the Bmax for specific [3H]PrCC binding determined with diazepam as displacer. These results do not support the idea that [3H]PrCC and [3H]diazepam have a common recognition site on the benzodiazepine receptor. Instead, they suggest that the benzodiazepine receptor is a multicomponent complex.