title: Ethanol enhances agonist-induced fast desensitization in nicotinic acetylcholine receptors.
abstract: 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 86Rb+ influx when the vesicles were subsequently briefly exposed to a saturating concentration of acetylcholine in 86RbCl. Acetylcholine-induced fast desensitization had a maximum observed rate, kdmax, of 6.8 s-1, a half-effect concentration, KD, of 157 microM, and a Hill coefficient of 1.4. Increasing the ethanol concentration up to 1.0 M causes a linear increase in kdmax, such that 1.0 M ethanol doubles the rate. Ethanol (1 M) also decreased KD 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.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Anesthesia, Massachusetts General Hospital, Boston 02114.
authors.forename: G
authors.initials: G
authors.lastname: Wu
authors.forename: K W
authors.initials: KW
authors.lastname: Miller
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology*; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D004357:Drug Synergism; D004557:Electric Organ / Q000378:metabolism*; D000431:Ethanol / Q000494:pharmacology*; D007700:Kinetics; D008433:Mathematics; D008962:Models, Theoretical; D011978:Receptors, Nicotinic / Q000187:drug effects*; D013572:Synaptic Vesicles / Q000378:metabolism; D013997:Time Factors; D014101:Torpedo
chemical_list: D011978:Receptors, Nicotinic; D000431:Ethanol; D000109:Acetylcholine