Ethanol Enhances Agonist-Induced Fast Desensitization in Nicotinic Acetylcholine Receptors

Biochemistry. 1994 Aug 9;33(31):9085-91. doi: 10.1021/bi00197a009.

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)

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Acetylcholine / pharmacology*
  • Animals
  • Cell Membrane / metabolism
  • Drug Synergism
  • Electric Organ / metabolism*
  • Ethanol / pharmacology*
  • Kinetics
  • Mathematics
  • Models, Theoretical
  • Receptors, Nicotinic / drug effects*
  • Synaptic Vesicles / metabolism
  • Time Factors
  • Torpedo

Substances

  • Receptors, Nicotinic
  • Ethanol
  • Acetylcholine