Is the quantum of transmitter release composed of subunits?

Abstract

THE quantal theory of transmitter release proposed by del Castillo and Katz¹ states that the endplate potential is built up of unit potentials (quanta), each of which is the size of the spontaneously occurring miniature endplate potentials (m.e.p.ps). It has recently been proposed that unit potentials and m.e.p.ps are built up of 2–15 still smaller subunits^2–4. The basis for this proposal has been drawn in part from two observations. First, some preparations show a distinct class of small-mode m.e.p.ps of amplitude typically about 1/7 that of the average m.e.p.p. Second, amplitude histograms of m.e.p.ps display multiple peaks whose amplitudes sometimes appear to be integer multiples of the small-mode m.e.p.p. amplitude^2–4. We now present evidence suggesting that the peaks on histograms of m.e.p.p. amplitudes may arise from random variation in the data due to a limited sample size. The data do not support the proposal that m.e.p.ps and unit potentials are composed of 2–15 subunits.