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

L-655,708 is an imidazobenzodiazepine possessing 30-70-fold selectivity for the benzodiazepine binding site of GABAA 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 GABAA 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 GABAA 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 GABAA 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 GABAA receptors in vivo and that this ligand can be used to measure receptor occupancy of α5-selective ligands.