On the elementary conductance event produced by L‐glutamate and quanta of the natural transmitter at the neuromuscular junctions of Maia squinado.

Abstract

The membrane potential of giant muscle fibers of M. squinado was measured with an intracellular wire electrode. On applying L-glutamate to the fiber the cell depolarized and fluctuations of the membrane potential around its mean level, glutamate noise, were seen. The variance of the glutamate voltage noise was proportional to the mean level of depolarization. The noise was probably caused by numerous exponentially decaying elementary voltage events about 5 .times. 10-10 V in amplitude. The miniature excitatory junctional potential (min. e.j.p.) was .apprx. 6000 times the amplitude of the elementary voltage event produced by L-glutamate. The power spectrum of glutamate voltage noise was Lorentzian with a half-power frequency of .apprx. 20 Hz. Min. e.j.p. decayed exponentially with a time constant that coincided with the average lifetime of the elementary glutamate voltage event. When glutamate was applied locally to a spot where extracellular min. e.j.p. could be recorded with a focal glass pipette, extracellular glutamate noise was seen. Glutamate noise could not be detected elsewhere on the fiber. The variance of the extracellular noise was proportional to the mean extracellular potential, and its power spectrum was Lorentzian with a half-power frequency of .apprx. 110 Hz. The extracellular min e.j.p. decay exponentially with a time constant that coincided with average lifetime of the elementary glutamate current event. The decay of the quantal currents flowing at the excitatory junctions was limited by closure of the conductance channels in the post-synaptic membrane and not by relaxation of the transmitter concentration in the synaptic cleft.