Enhancement of Muscimol Binding and Gating by Allosteric Modulators of the GABA Receptor: Relating Occupancy to State Functions

Muscimol is a psychoactive isoxazole derived from the mushroom and a potent orthosteric agonist of the GABA receptor. The binding of [H]muscimol has been used to evaluate the distribution of GABA receptors in the brain, and studies of modulation of [H]muscimol binding by allosteric GABAergic modulators such as barbiturates and steroid anesthetics have provided insight into the modes of action of these drugs on the GABA receptor. It has, however, not been feasible to directly apply interaction parameters derived from functional studies to describe the binding of muscimol to the receptor. Here, we employed the Monod-Wyman-Changeux concerted transition model to analyze muscimol binding isotherms. We show that the binding isotherms from recombinant 13 GABA receptors can be qualitatively predicted using electrophysiological data pertaining to properties of receptor activation and desensitization in the presence of muscimol. The model predicts enhancement of [H]muscimol binding in the presence of the steroids allopregnanolone and pregnenolone sulfate, although the steroids interact with distinct sites and either enhance (allopregnanolone) or reduce (pregnenolone sulfate) receptor function. We infer that the concerted transition model can be used to link radioligand binding and electrophysiological data. SIGNIFICANCE STATEMENT: The study employs a three-state resting-active-desensitized model to link radioligand binding and electrophysiological data. We show that the binding isotherms can be qualitatively predicted using parameters estimated in electrophysiological experiments and that the model accurately predicts the enhancement of [H]muscimol binding in the presence of the potentiating steroid allopregnanolone and the inhibitory steroid pregnenolone sulfate.