THERMODYNAMICS OF GAMMA-AMINOBUTYRIC-ACID TYPE-A RECEPTOR-BINDING DIFFERENTIATE AGONISTS FROM ANTAGONISTS

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