The quantal nature of transmission and spontaneous potentials at the Torpedo electromotor junction

Miniature and stimulus evoked electroplaque potentials (mEpPs and EpPs) were recorded in Torpedo electrocytes intracellularly and extracellularly. The quantal release parameters of EpPs and the time course of quantal EpCs were estimated in normal and low Ca2+-high Mg2+ solutions. Amplitude-frequency distribution of mEpPs showed Gaussian or uneven character with an average mean value of 0.3 +/- 0.08 mV (S.D.). The mean coefficient of variation of mEpPs was 26.8 +/- 7.2% (n = 6). Tetrodotoxin reversibly blocked the stimulus evoked EpP but hardly influenced the amplitude-frequency histogram of spontaneous EpPs in 10(-8)-10(-6) M concentration. The quantum content of stimulus evoked EpPs varied between 100-400 in normal solution which decreased in low Ca2+-high Mg2+ solution and the quantal release conformed to binomial statistics and allowed determination of the parameters p and n. Frequency of the spontaneous discharges varied highly from electrocyte to electrocyte but an analysis of the time intervals showed randomness for the events. The decay phase of quantal current composed of non-exponential and exponential sections which was characteristic with 0.75 +/- 0.16 msec (mean, S.D., at 20 degrees C) time constant of exponential decay. Although, two types of mEpCs could be differentiated having significantly slower and faster time courses. Neostigmine prolonged the time constant of decay of mEpCs in dose-dependent manner with a factor of 2 in 10(-6) M and of 4 in 10(-5) M concentrations (at about 20 degrees C).