Entropy as the predominant driving force of binding to human recombinant alpha(x)beta(3)gamma(2) GABA(A) receptors

In order to study the correlation of the thermodynamic driving forces of bi nding with the efficacies of displacing ligands, the specific binding of [H -3]SR 95531 [2-(3-carboxypropyl)3-amino-6-p-methoxyphenylpyridazinium bromi de], a GABA(A) receptor antagonist, was studied in cell lines stably expres sing human alpha (1)beta (3)gamma (3) and alpha (2)beta (3)gamma (2) GABA(A ) receptors. Displacing potencies for the agonists with different efficacie s (muscimol, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) and pipe ridine-4-sulfonic acid) and for antagonists (SR 95531 and 5-(4-piperidyl)is othiazol-3-ol) were determined at 0 degreesC, 20 degreesC and 37 degreesC. Displacing potencies were temperature-nearly independent for alpha (1)beta (3)gamma (2) receptors. At alpha (2)beta (3)gamma (2), receptor binding of the antagonists was exothermic, endothermic for the agonists THIP and piper idine-4-sulfonic acid and isothermic for muscimol. The free energy incremen ts of displacement for the binding of the antagonist [H-3]SR 95531 versus t he agonist [H-3]muscimol approach saturation as a function of the efficacie s of the displacers only for alpha (1)beta (3)gamma (2) receptors. This sug gests that. for binding to alpha (1)beta (3)gamma (2) GABA(A) receptors, di splacement is an efficacy-dependent interaction predominantly driven by ent ropic increases. (C) 2001 Elsevier Science B.V. All rights reserved.