Morphological characteristics of perforated synapses in the latter stages of synaptogenesis in rat neocortex: stereological and three-dimensional approaches.

Over recent years attention has been focused on perforated synapses (PSs), on account of their possible involvement in synaptic plasticity in the nervous system. The mechanism invoked has involved the conversion of conventional (non-perforated) synapses into perforated ones, with the latter subsequently splitting to form new daughter synapses. Data challenging this mechanism have emerged from studies using unbiased stereological and three-dimensional reconstruction procedures in early synaptogenesis and also in early- to mid-adulthood. The present study is an attempt to complement these earlier studies by concentrating on the latter part of synaptogenesis and early adulthood in rats (7-60 days postnatal). Using the disector method, the frequency of perforated synapses increases with age from 12% at seven days to 33% at 60 days. Reconstruction of the perforated synapses has led to the description of three sub-types, while quantitation of the reconstructions has highlighted major differences between perforated and non-perforated synapses over this developmental period. For instance, the postsynaptic density of perforated synapses increases in size with age, but remains static in the non-perforated variety; in addition, it is 2-3 times larger in perforated synapses. The proportion of the synaptic contact zone occupied by the postsynaptic density increases in perforated synapses with increasing age, but is static in non-perforated synapses. No evidence has been found to support a perforated synapse splitting model, since perforated synapses are present from early in synaptogenesis, the frequency of non-perforated synapses reaches a peak prior to, but not following, that of perforated synapses, and the size of the postsynaptic density of non-perforated synapses remains constant throughout the period of the study.