In vivo labelling of α5 subunit-containing GABA A receptors using the selective radioligand [ 3H]L-655,708

Abstract

L-655,708 is an imidazobenzodiazepine possessing 30–70-fold selectivity for the benzodiazepine binding site of GABA A receptors containing an α5 rather than α1, α2 or α3 subunit. In the present study, [ 3H]L-655,708 was used to label mouse brain benzodiazepine binding sites in vivo. When compared to inhibition of in vivo binding of the non-selective ligand [ 3H]Ro 15-1788, the pharmacology of mouse in vivo [ 3H]L-655,708 binding was consistent with selective in vivo labelling of α5 subunit-containing GABA A receptors. Thus, diazepam was equipotent at inhibiting in vivo [ 3H]L-655,708 and [ 3H]Ro 15-1788 binding; zolpidem, which has very low affinity for α5-containing GABA A receptors, gave no inhibition of in vivo [ 3H]L-655,708 binding despite inhibiting in vivo [ 3H]Ro 15-1788 binding; and L-655,708 was more potent at inhibiting the in vivo binding of [ 3H]L-655,708 compared to [ 3H]Ro 15-1788. This pharmacological specificity of in vivo [ 3H]L-655,708 binding was confirmed autoradiographically. Hence, the anatomical distribution of in vivo [ 3H]L-655,708 binding was comparable to the distribution of α5-containing GABA A receptors identified in vitro. Moreover, this distribution was distinct from that identified using [ 3H]Ro 15-1788. These data therefore suggest that [ 3H]L-655,708 can be used to identify α5-containing GABA A receptors in vivo and that this ligand can be used to measure receptor occupancy of α5-selective ligands.