Photoaffinity ligands for the [3H]TBOB binding site of the GABAA receptor

Two types of 1-substituted-4-t-butyl-2,6,7-trioxabicyclo[2.2.2]octanes are examined as candidate photoaffinity ligands to irreversibly inhibit the [3H]TBOB binding site of the GABAA receptor complex present in bovine brain membranes. The 1-substituted-(4-bromo-6-diazocyclohexa -1,4-dien-3-onyl) derivative (IC50 1.1 microM) dissociates completely in the dark, but on irradiation produces over 40% photoirreversible inhibition of [3H]TBOB binding. This photoinactivation can be partially (64%) protected using a potent and completely dissociating TBOB analog. The 1-substituted-(4-azidophenyl) derivative (IC50 149 nM) also dissociates completely in the dark and at 2.2 microM gives 20% photoirreversible inhibition which is completely protectable. The corresponding 1-(4-azido-3,5-dichlorophenyl) analog and its 4-(1-methylpropyl) isomer are 16- and 8-fold more potent inhibitors, respectively, than the azidophenyl derivative but these dichloro probes are not examined further because they do not dissociate in the dark by an apparently non-covalent mechanism. Thus, the bromodiazocyclohexadienonyl and azidophenyl compounds photoirreversibly and therefore covalently modify the [3H]TBOB binding site of the GABAA receptor complex.