A four-ligand hypercube model to quantify allosteric interactions within the GABAA receptor complex.
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.