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 GABA(A) receptors containing an alpha 5 rather than alpha 1, alpha 2 or alpha 3 subunit. In the present study, [H-3]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 [H-3]Ro 15-1788, the pharmacology of mouse in vivo [H-3]L-655,708 binding was consistent with selective in vivo labelling of a5 subunit-containing GABA(A) receptors. Thus, diazepam was equipotent at inhibiting in vivo [H-3]L-655,708 and [H-3]Ro 15-1788 binding; zolpidem, which has very low affinity for alpha 5-containing GABA(A) receptors, gave no inhibition of in vivo [H-3]L-655,708 binding despite inhibiting in vivo [H-3]Ro 15-1788 binding; and L-655,708 was more potent at inhibiting the in vivo binding of [H-3]L-655,708 compared to [H-3]Ro 15-1788. This pharmacological specificity of in Vivo [H-3]L-655,708 binding was confirmed autoradiographically. Hence, the anatomical distribution of in vivo [H-3]L-655,708 binding was comparable to the distribution of alpha 5-containing GABA(A) receptors identified in vitro. Moreover, this distribution was distinct from that identified using [H-3]Ro 15-1788. These data therefore suggest that [H-3]L-655,708 can be used to identify a5-containing GABAA receptors in vivo and that this ligand can be used to measure receptor occupancy of alpha 5-selective ligands. (c) 2005 Elsevier Ltd. All rights reserved.