A Study of the Action of Picrotoxin on the Inhibitory Neuromuscular Junction of the Crayfish

J Physiol. 1969 Nov;205(2):377-91. doi: 10.1113/jphysiol.1969.sp008972.

Abstract

1. The effect of picrotoxin on the neuromuscular junction of the crayfish (Cambarus clarkii) was investigated. The potential changes were recorded intracellularly and extracellularly with micro-electrodes. The membrane conductance of the muscle fibre was also measured.2. Picrotoxin depressed the amplitudes of the inhibitory junctional potentials and the potential changes produced by iontophoretically applied gamma-aminobutyric acid (GABA), but had no appreciable effect on the excitatory junctional potentials and the potential changes produced by L-glutamate.3. The presynaptic action of GABA and the neural transmitter was depressed by picrotoxin. The presynaptic action of beta-guanidinopropionic acid was also depressed by picrotoxin.4. The increase in the membrane conductance produced by the addition of GABA in the bath fluid was depressed by picrotoxin. The dose-response relation showed that picrotoxin depressed the conductance increase produced by GABA in a non-competitive manner. The action of picrotoxin on the conductance increase produced by GABA was more effective in low Cl- solution.5. The analysis of the dose-response curves showed that the action of picrotoxin was well expressed by the Michaelis-Menten equation, but the slope of the dose-response curve of GABA was steeper than this relation. It is proposed that the conductance of the junctional membrane was increased by the combination of two molecules of GABA with a receptor, and the attachment of one molecule of picrotoxin to a specific site depressed the conductance increase.

MeSH terms

  • Aminobutyrates / antagonists & inhibitors
  • Aminobutyrates / pharmacology
  • Animals
  • Chlorides / pharmacology
  • Crustacea
  • Depression, Chemical
  • Glutamates / pharmacology
  • Guanidines / antagonists & inhibitors
  • Iontophoresis
  • Kinetics
  • Membrane Potentials / drug effects
  • Neuromuscular Junction / drug effects*
  • Picrotoxin / pharmacology*
  • Propionates / antagonists & inhibitors
  • Receptors, Drug

Substances

  • Aminobutyrates
  • Chlorides
  • Glutamates
  • Guanidines
  • Propionates
  • Receptors, Drug
  • Picrotoxin