ETHANOL ENHANCES AGONIST-INDUCED FAST DESENSITIZATION IN NICOTINIC ACETYLCHOLINE-RECEPTORS

The reversible decline of the nicotinic acetylcholine receptor's response to acetylcholine during prolonged exposure to acetylcholine is known as desensitization. Here, we studied ethanol's modulation of fast agonist-induced desensitization of the nicotinic acetylcholine receptor in postsynaptic membrane vesicles from Torpedo using a fast kinetic technique: pulsed quenched flow. Preincubation of the vesicles with various concentrations of acetylcholine at 4 degrees C for times ranging from 80 ms to 1.5 s caused fast desensitization, which was revealed as a decreased Rb-86(+) influx when the vesicles were subsequently briefly exposed to a saturating concentration of acetylcholine in (RbCl)-Rb-86. Acetylcholine-induced fast desensitization had a maximum observed rate, k(d)(max), of 6.8 s(-1), a half-effect concentration, K-D, of 157 mu M, and a Hill coefficient of 1.4. Increasing the ethanol concentration up to 1.0 M causes a linear increase in k(d)(max), such that 1.0 M ethanol doubles the rate. Ethanol (1 M) also decreased K-D 10-fold without changing the Hill coefficient. We consider a modified sequential model to interpret our data. Two acetylcholine molecules bind sequentially to the receptor's resting state to form a pre-open (closed) state, which then opens and, at very high acetylcholine concentrations, is inhibited. A priori fast desensitization might occur from any of these acetylcholine-occupied states. If we assume fast desensitization to occur solely from the pre-open state, our data predict an excessively large action of ethanol on the fast desensitization rate constant (> 200-fold increase in the desensitization rate constant at 1 M ethanol). When we assume fast desensitization to occur from all states, ethanol is seen to have two actions. It shifts the equilibrium between the closed and open states toward the open state, accounting for the shift in the concentration response curve for fast desensitization, and it enhances the fast desensitization rate constant, accounting for the observed increase in the desensitization rate. As a consequence of this dual action, ethanol's net effect depends on both the agonist's concentration and its duration of action. Physiological concentrations of ethanol have their most pronounced effects at low agonist concentrations.