Photoaffinity Ligands for the [³H]tbob Binding Site of the Gaba_A Receptor

Abstract

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 GABA_A receptor complex present in bovine brain membranes. The 1–substituted-(4–bromo-6–diazocyclohexa-1,4–dien-3–onyl) derivative (lC₅₀ 1.1,μM) 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–azidopnenyl) derivative (lC₅₀ 149 nM) also dissociates completely in the dark and at 2.2 μM 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 GABA_A receptor complex.