title: A four-ligand hypercube model to quantify allosteric interactions within the GABAA receptor complex.
abstract: The aim of this study was to investigate the couplings between various binding sites on the GABA(A) receptor complex. We investigated combinations of three test compounds: (1) GABA (gamma-aminobutyric acid), (2) Org 20549 [(2 beta 3 alpha 5 alpha)-21hydroxy-3Hydroxy-2(4morpholinyl)pregnan-20one methane-sulphonate)], a neuroactive steroid and (3) retigabine (D-23129, N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester), a new antiepileptic drug. Receptor-binding assays were conducted using rat brain membranes. [3H]TBOB ([3H]-t-butyl-bicyclo-ortho-benzoate) was the tracer ligand. All three test compounds inhibited the binding of [3H]TBOB with EC(50)'s of 4.0, 98 and 23 microM, respectively. Isobolic analysis of the combination data showed that the three compounds act in synergy in displacing [3H]TBOB. These interactions could be described and quantified by a hypercube model in which each of the three test compounds and [3H]TBOB bind to different, allosterically coupled sites such that each of the test compounds allosterically displaces the tracer [3H]TBOB and allosterically enhances the affinity of any other test compound by a factor 4.4. The simultaneous binding of any two ligands enhances the affinity of the third by a factor 9. These results may contribute to the understanding of individual variability in drug responses and to the discussion about rational polytherapy.
authors.affiliation: NICI/Department of Biological Psychology, University of Nijmegen, Post-box 9104, 6500 HE Nijmegen, The Netherlands. rijn@nici.kun.nl
authors.forename: Clementina M
authors.initials: CM
authors.lastname: van Rijn
authors.forename: Elly
authors.initials: E
authors.lastname: Willems-van Bree
mesh_terms: D000495:Allosteric Site / Q000502:physiology*; D000818:Animals; D001665:Binding Sites; D002219:Carbamates / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D008024:Ligands; D008956:Models, Chemical; D008958:Models, Molecular; D008959:Models, Neurological; D010655:Phenylenediamines / Q000378:metabolism; D016548:Prosencephalon / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D011963:Receptors, GABA-A / Q000737:chemistry / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
chemical_list: D002219:Carbamates; D008024:Ligands; D010655:Phenylenediamines; D011963:Receptors, GABA-A; C101866:ezogabine; D005680:gamma-Aminobutyric Acid