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.
Merck Sharp and Dohme Research Laboratories, Neuroscience Research Centre, Terlings Park, Eastwick Road, Harlow, Essex CM20 2QR, England, UK. john_atack@merck.com
John R
JR
Atack
Luanda
L
Alder
Susan M
SM
Cook
Alison J
AJ
Smith
Ruth M
RM
McKernan
D000818:Animals; D001569:Benzodiazepines / Q000378:metabolism; D001665:Binding Sites; D001667:Binding, Competitive; D001921:Brain / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D005442:Flumazenil / Q000494:pharmacology; D018755:GABA Agonists / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D018757:GABA Modulators / Q000494:pharmacology; D007093:Imidazoles / Q000494:pharmacology*; D046911:Macromolecular Substances; D008297:Male; D051379:Mice; D021122:Protein Subunits / Q000187:drug effects; D011869:Radioligand Assay / Q000379:methods; D011963:Receptors, GABA-A / Q000737:chemistry / Q000378:metabolism*; D011994:Recombinant Proteins / Q000187:drug effects / Q000378:metabolism; D013116:Spinal Cord / Q000187:drug effects; D013997:Time Factors; D014316:Tritium
D018755:GABA Agonists; D018756:GABA Antagonists; D018757:GABA Modulators; D007093:Imidazoles; C105490:L 655,708; D046911:Macromolecular Substances; D021122:Protein Subunits; D011963:Receptors, GABA-A; D011994:Recombinant Proteins; D014316:Tritium; D001569:Benzodiazepines; D005442:Flumazenil