Vesicle number does not predict postsynaptic measures of miniature synaptic activity frequency in cultured cortical neurons

We tested the hypothesis that heterogeneity in the frequency of miniature s ynaptic activity reflects differences in the number of vesicles present in presynaptic terminals. Using imaging techniques, we measured dendritic mini ature synaptic calcium transients attributed to the spontaneous release of single transmitter quanta. Following imaging, the identified neurons were p rocessed for serial transmission electron microscopy. At sites of quantal C a2+ transients mediated by N-methyl-D-aspartate receptors, we confirmed the presence of excitatory synapses and measured the total number of vesicles and the number of docked vesicles. We observed no correlation between the frequency of spontaneous miniature a ctivity and either the total vesicle number or the number of docked vesicle s. We conclude that the presynaptic vesicle complement as measured by ultra structural analysis does not necessarily determine the frequency of spontan eous activity at synapsis mediated by N-methyl-D-aspartate receptors. (C) 2 000 IBRO. published by Elsevier Science Ltd. All rights reserved.