In vivo labelling of alpha5 subunit-containing GABA(A) receptors using the selective radioligand [(3)H]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 alpha5 rather than alpha1, alpha2 or alpha3 subunit. In the present study, [(3)H]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 [(3)H]Ro 15-1788, the pharmacology of mouse in vivo [(3)H]L-655,708 binding was consistent with selective in vivo labelling of alpha5 subunit-containing GABA(A) receptors. Thus, diazepam was equipotent at inhibiting in vivo [(3)H]L-655,708 and [(3)H]Ro 15-1788 binding; zolpidem, which has very low affinity for alpha5-containing GABA(A) receptors, gave no inhibition of in vivo [(3)H]L-655,708 binding despite inhibiting in vivo [(3)H]Ro 15-1788 binding; and L-655,708 was more potent at inhibiting the in vivo binding of [(3)H]L-655,708 compared to [(3)H]Ro 15-1788. This pharmacological specificity of in vivo [(3)H]L-655,708 binding was confirmed autoradiographically. Hence, the anatomical distribution of in vivo [(3)H]L-655,708 binding was comparable to the distribution of alpha5-containing GABA(A) receptors identified in vitro. Moreover, this distribution was distinct from that identified using [(3)H]Ro 15-1788. These data therefore suggest that [(3)H]L-655,708 can be used to identify alpha5-containing GABA(A) receptors in vivo and that this ligand can be used to measure receptor occupancy of alpha5-selective ligands.