The variance of successive peaks in synaptic amplitude histograms: effects of inter-site differences in quantal size

Variability in the measured amplitude of evoked synaptic events can arise from several factors, including: measurement noise, trial-to-trial variation in the amplitude of the response at a single release site, or variation between different release sites (inter-site variation) in the mean amplitude of the single quantal response. Classic (linear) models of variability include only the first two of these factors, although differences in the number of postsynaptic receptors or the degree of electrotonic attenuation for different release sites could cause substantial inter-site variations in quantal size. In this paper, the effect of inter-site variation on the variance of successive histogram peaks has been determined analytically and verified by computational studies. This effect is minimal at the edges of the histogram and contributes maximally to central peaks. Linear approximations to the variance of successive histogram peaks may therefore result in very poor fits to measured data if substantial inter-site variation in quantal size is involved. Our computational results indicate that for synaptic contacts with high release probabilities and substantial inter-site variation, the variance of histogram peaks will decrease with increasing quantal content.