Effect of ethanol treatment on rate and equilibrium constants for [3H] muscimol binding to rat brain membranes: alteration of two affinity states of the GABAA receptor.

Equilibrium binding curves were biphasic in control and ethanol-treated rats. [3H]Muscimol binds to sites of high (KDA of approximately 10 nM) and low (KDB of approximately 0.3-0.4 microM) affinity. Chronic ethanol treatment produced a decrease in BmaxA value, and the hyperbolic binding profiles were progressively affected by the chronic and in vitro ethanol treatments, with most of this effect corresponding to the high-affinity site. IC50 and Ki values were calculated for several competing ligands, using membranes from both control and ethanol-treated animals. The association and dissociation curves were also biphasic, using a radioligand concentration precluding a significant occupancy of the low-affinity sites, which suggests the existence of two forms or affinity states of the monoliganded receptor. Chronic ethanol treatment did not produce changes in the values of the dissociation rate constants (fast and slow phases). By contrast, we report for the first time a decrease in the values of the association rate constants, with this decrease being higher for the slow phase. Consequently, the dissociation equilibrium constants are two times higher in chronically ethanol-treated animals for both phases.