Properties of miniature post-synaptic currents at the Torpedo marmorata nerve-electroplate junction

The post-synaptic conductance changes induced by spontaneous release of neurotransmitter on the Torpedo marmorata electroplates were studied by focal extracellular recording methods. It was found that miniature post-synaptic currents (m.p.s.c.s) recorded on the innervated and non-innervated faces of the electrocyte are opposite in sense and the distribution of amplitudes is bimodal. The bimodal distribution could reflect the presence of two populations of m.p.s.c.s (smaller and giant m.p.s.c.s). In the smaller m.p.s.c.s the distribution of the ratio (amplitude/decay time constant) shows two populations that probably represent areas with different receptor densities. The value of the mean open-channel lifetime estimated from the decay time constant of the m.p.s.c. is 0.92 +/- 0.67 ms (n = 319, temperature 20 +/- 1 degree C). Prostigmine (10 microM) increases the amplitude, decay time constant and time to peak of the m.p.s.c. A decrease of temperature increases m.p.s.c. decay time, the activation energies found ranging from 8.0 to 12.6 kcal x mol-1. Increasing the external K+ concentration produces a decrease in the decay time constant. High concentrations of divalent ions, particularly Ca2+, increase the decay time constant of the m.p.s.c. Alkaline or acid bathing solution produces a decrease in the decay time constant. An almost total substitution of external Cl- by non-permeant SO42- does not modify the time course of the m.p.s.c.