Detection of sub-miniature endplate potentials by harmonic analysis

In phrenic nerve-diaphragm preparations from young rats and mice, long-term recordings of miniature endplate potentials (MEPPs) from single synapses were performed. Amplitude histograms obtained from these experiments were analysed with respect to the presence of multiple peaks which are believed to reflect quantal transmitter release. This was accomplished by subjecting MEPP amplitude histograms as well as matched random number histograms to Fourier analysis; harmonics in the MEPP amplitude distribution were considered valid only if they exceeded those of the random number distribution. In 9 out of 11 experiments, substantial evidence for the existence of preferred amplitudes was obtained; in two cases, however, it was completely missing. Applying our method to a series of promising MEPP amplitude histograms selected from the literature, we found a similar relationship between positive and negative results. We also noted a fairly constant relationship between the size of the elementary sub-MEPPs and the mean amplitude of the bell-MEPPs, named so according to the Gaussian shape of their amplitude distribution.