Differential rearing effects on rat visual cortex synapses. II. Synaptic morphometry.

An array of morphological measurements was made upon spine synapses in the upper 4 layers of occipital cortex of rats reared for 30 days after weaning in complex (EC), social (SC) or isolated (IC) environments. The mean length of the synaptic contact zone (post-synaptic density plus interpolated non-impregnated regions) was greater in layer IV of EC rats than in IC rats. SC rats were intermediate, not differing from other groups. There were no differences in these measures in other layers, nor were there differences in the mean area or perimeter of presynaptic terminals or postsynaptic processes, the relative frequency of headed vs sessile shaped spines, the length of the apposition between pre- and postsynaptic processes, or the ratio of perimeter to area (inverse roundness) of postsynaptic processes. Cleft width was greater in regions of the contact zone where postsynaptic density was present than in regions where it was absent (perforations), but, aside from the previously described differences in the frequency of perforated synapses, there were no group differences in cleft width. The maximum length of synaptic contact zones and the maximum area of presynaptic terminals was greater in EC than in IC rats in layer IV, but not in other layers, with SC rats again intermediate. These results support previous findings of larger layer IV synaptic contacts in EC rats and suggest that the size of some synaptic components can change without changes in others, a population of very large synapses is seen in layer IV of EC rats that is not seen in IC rats, and perforations may be unlikely sites of synapse splitting, given that membranes are more closely apposed in these regions, rather than pulling apart.