title: Photoaffinity labeling of benzodiazepine receptors causes altered agonist-antagonist interactions.
abstract: Previous studies have shown that [3H]flunitrazepam forms irreversible cross-links with brain tissue when exposed to ultraviolet irradiation. Comparison of the amount of [3H]flunitrazepam irreversibly incorporated and the number of benzodiazepine binding sites blocked after photolabeling has indicated that several binding sites are inactivated for each molecule of [3H]flunitrazepam incorporated. To learn the cause of this discrepancy, binding to the benzodiazepine binding sites has been examined using several radiolabeled benzodiazepine antagonists. Binding of a beta-carboline ester, CGS-8216, and Ro 15-1788 was not altered by photolabeling; however, displacement studies revealed that photolabeling converted a homogeneous set of benzodiazepine binding sites into two subsets: one of high affinity (unaltered sites) and one of low affinity. The low affinity sites could be detected by displacement studies of antagonist binding by benzodiazepines, and conversion to a low affinity form accounts for the discrepancy observed after photolabeling using [3H]flunitrazepam as ligand.
mesh_terms: D000818:Animals; D001569:Benzodiazepines / Q000378:metabolism*; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism*; D002243:Carbolines / Q000378:metabolism; D005445:Flunitrazepam / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D051381:Rats; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011963:Receptors, GABA-A
chemical_list: D002243:Carbolines; D008024:Ligands; D011956:Receptors, Cell Surface; D011963:Receptors, GABA-A; D001569:Benzodiazepines; D005445:Flunitrazepam; C024915:beta-carboline-3-carboxylic acid ethyl ester