Kinetics of [3H]muscimol binding to the GABAA receptor in bovine brain membranes

The binding of the GABA receptor agonist 3H]muscimol to membrane preparations from bovine cerebral cortex has been investigated in equilibrium and kinetic experiments. Equilibrium binding curves are biphasic and suggest that 3H]muscimol binds to both high-affinity (Kd approximately 10 nM) and low-affinity (Kd approximately 0.5 microM) sites. Binding to each class of sites is inhibited by GABA and by the specific GABAA receptor antagonist bicuculline. The kinetics of 3H]muscimol binding have been measured by using both manual filtration assays and an automated rapid filtration technique which permits the measurement of ligand dissociation on subsecond time scales. Association and dissociation curves are biphasic at all concentrations of 3H]muscimol studied, even under conditions of low receptor saturation when no significant occupancy of the low-affinity sites would be expected. These results cannot be simply explained by the presence of two populations of binding sites in the membrane preparations but suggest the existence of two forms of the monoliganded receptor. Dissociation constants for these two forms have been estimated to be 16 and 82 nM at 23 degrees C. At higher ligand concentrations, kinetic measurements have suggested that the binding of 3H]muscimol to low-affinity sites is accompanied by a slow conformational change of the receptor-ligand complex.