Permeability changes produced by L‐glutamate at the excitatory post‐synaptic membrane of the crayfish muscle.

Abstract

Permeability changes produced by L-glutamate at the neuromuscular junction of the crayfish (Cambarus clarkii) were investigated by application of the drug iontophoretically to the voltage-clamped junction and measuring the resulting glutamate current. Reversal potentials were determined by measuring the glutamate current at different membrane potentials. They were +39.1 .+-. 3.6 mV (mean .+-. SE of mean) in normal solution and +16.5 .+-. 2.0 mV in solutions made twice as hypertonic by the addition of sucrose. Decreasing external Na+ concentration shifted the reversal potential in the negative direction; increased Na+ in the positive direction. The relation between the amplitude of the glutamate current and extracellular Na+ concentration was approximately linear. Alteration of the external K+ or Cl- concentration did not affect the amplitude or reversal potential of glutamate current. In Na+-free solution the application of L-glutamate produced a small inward current at the resting potential and its amplitude was augmented by increasing the external Ca2+ concentration. Increasing the Ca2+ concentration in the normal Na+ media produced no appreciable effect on the reversal potential but decreased the amplitude of glutamate current. L-glutamate increases membrane permeability mainly to Na+ and slightly to Ca2+. The time course of glutamate current was shorter than that of the concentration calculated from the diffusion equation and it was simulated more closely by the square of the concentration.