Active and inactive central synapses in cell culture.

Synaptic interactions between pairs of spinal cord (SC) neurons and between dorsal root ganglion neurons and SC neurons were studied in dissociated cell cultures prepared from fetal mouse. Combined injection of horseradish peroxidase into presynaptic neurons and Lucifer yellow into postsynaptic neurons allowed detailed correlation of morphological-physiological analyses of synaptically linked cells. Statistical analysis of trains of evoked EPSPs under conditions of high and of low transmitter output was used to determine the number of physiological release elements, n, involved in a given synaptic connection. When n was compared with the number of boutons subserving a synaptic connection, it was found that in 80% of cases the number of boutons was equal to or greater than the number of release elements. In some cases, the bouton count was more than fivefold greater than n. The simplest explanation is that, in general, one bouton can release no more than one quantum of transmitter and, in a significant proportion of synaptic connections, a large fraction of boutons do not participate in the release process. Theoretical consideration and analysis of the electrotonic structure of some of the neurons studied indicate that the dendritic location of synaptic inputs does not affect our results. Variations in the probability of release, p, may contribute to the apparent disparity between n and bouton number. If so, this variation must be large with many boutons having a very low p, difficult to distinguish experimentally from zero.