Vol. 58, Issue 4, 756-762, October 2000

Characterization of the Interaction of Zopiclone with -Aminobutyric Acid Type A Receptors

Martin Davies, J. Glen Newell, Jason M. C. Derry, Ian L. Martin, and Susan M. J. Dunn

Department of Pharmacology (M.D., J.G.N., J.M.C.D., S.M.J.D.) and Division of Neuroscience (S.M.J.D.), Faculty of Medicine, University of Alberta, Edmonton, Alberta, Canada; and Pharmaceutical and Biological Sciences, Aston University, Aston Triangle, Birmingham, United Kingdom (I.L.M.)

Zopiclone is a cyclopyrrolone that is used clinically as a hypnotic. Although this drug is known to interact with neuronal -aminobutyric acid type A receptors, its binding site(s) within the receptor oligomer has been reported to be distinct from that of the classical benzodiazepines. After photoaffinity labeling with flunitrazepam, receptors in rat cerebellar membranes showed differentially reduced affinity for flunitrazepam and zopiclone by 50- and 3-fold, respectively. Because histidine 101 of the -subunit is a major site of photolabeling, we have made specific substitutions of this residue and studied the consequences on the binding properties of zopiclone and diazepam using recombinant 122-receptors transiently expressed in tsA201 cells. Both compounds showed similar binding profiles with receptors containing mutated -subunits, suggesting a similar interaction with the residue at position 101. At 123-receptors, flunitrazepam affinity was dramatically decreased by approximately 36-fold, whereas the affinity for zopiclone was decreased only 3-fold, suggesting a differential contribution of the -subunit to the binding pocket. Additionally, we used electrophysiological techniques to examine the contribution of the -subunit isoform in the receptor oligomer to ligand recognition using recombinant receptors expressed in Xenopus oocytes. Both compounds are agonists at 122- and 123-receptors, with flunitrazepam being more potent but less efficacious. In summary, these data suggest that histidine 101 of the 1-subunit plays a similar role in ligand recognition for zopiclone, diazepam, and flunitrazepam.