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 ar ise 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) m odels of variability include only the first two of these factors, alth ough differences in the number of postsynaptic receptors or the degree of electrotonic attenuation for different release sites could cause s ubstantial inter-site variations in quantal size. In this paper, the e ffect of intersite variation on the variance of successive histogram p eaks has been determined analytically and verified by computational st udies. This effect is minimal at the edges of the histogram and contri butes maximally to central peaks. Linear approximations to the varianc e 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 cont acts with high release probabilities and substantial inter-site variat ion, the variance of histogram peaks will decrease with increasing qua ntal content.