A four-ligand hypercube model to quantify allosteric interactions within the GABA A 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 (γ-aminobutyric acid), (2) Org 20549 [(2β3α5α)-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 μM, 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.