Thermodynamics of gamma-aminobutyric acid type A receptor binding differentiate agonists from antagonists

G Maksay

Department of Molecular Pharmacology, Hungarian Academy of Sciences, Budapest.

Specific binding of the gamma-aminobutyric acid (GABA)A antagonist [3H]SR 95531 to synaptosomal membranes of rat whole brain was examined between 0 degrees and 37 degrees. Scatchard analysis revealed two (high and low affinity) populations of [3H]SR 95531 binding sites. The Kd values increased with increasing temperature. Ki values for GABAA agonists and antagonists were determined from the displacement of [3H]SR 95531 binding at a low concentration (1.8 nM) of [3H]SR 95531, which binds predominantly to high affinity sites. For most compounds van't Hoff plots (--In Ki, i.e., In Ka, versus 1/T) were linear between 0 degrees and 37 degrees. Curvilinear van't Hoff plots for the antagonists R 5135 and bicuculline methiodide can be attributed to their hydrophobic binding interactions. The enthalpy changes of binding (delta H degrees) were positive for the agonists (muscimol, isoguvacine, GABA, 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol hydrochloride, and imidazole-4-acetic acid) and negative for the antagonists (pitrazepin, bicuculline methiodide, R 5135, SR 95531, and SR 95103). Separation of the enthalpic and entropic components of the Gibbs free energy changes of binding (delta G degrees) revealed that binding of the antagonists is driven by both the enthalpic and entropic terms, whereas that of the agonists is driven entirely by entropy changes. A plot of the entropic term (-T delta S degrees) versus the enthalpic term (delta H degrees) showed separate patterns for GABAA agonists and antagonists, with the partial agonists [5-(4- piperidyl)isoxazol-3-ol, imidazole-4-acetic acid, and 4,5,6,7- tetrahydroisoxazolo[4,5-c]pyridin-3-ol hydrochloride] between them. It is proposed that the entropic term is partly determined by a transition from antagonist to agonist conformation of the GABAA binding sites.

Volume 46, Issue 2, pp. 386-390, 08/01/1994
Copyright © 1994 by American Society for Pharmacology and Experimental Therapeutics

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