Effects of light/dark‐ and dark‐rearing on synaptic morphology in the superior colliculus and visual cortex of the postnatal and adult rat

Serveral synaptic parameters, previously shown to undergo alterations with changes in the internal and external environment, were examined in the visual system of light/dark‐ and dark‐reared postnatal and adult rats. Animals were raised in either 14hr light/10 hr dark (Lt/Dk) or in total darkness (Dk). The specific synaptic parameters in the superficial layers of the superior colliculus (SC), visual cortex (laminae II/III and IV) (VC), and, as contro, the auditory cortex (laminae II/III and IV) (AC), examined during the postnatal period (i.e., postnatal days 7,14,21, and 28) and in the adult (i.e., day 56) included:(1) mean number of synapses, (2) mean synaptic length, (3) percentages of perforated postsynaptic densities, (4) percentages of asymmetric and symmetric synapses, (5) percentages of dendritic, spinous, anmd somatic synapses, and (6) percentages of synapses with positive, negative, or no curvature. Developmental patterns in rats rearded in normal lighting conditions were noted. Specifically, in the SC and VC of Lt/Dk animals, the number of synapses increased up to postnatal day in the mean number of synapses increased; no significant changes in the mean number of synapses between PND =28 and 56 were detected in any of the areas examined. Changes in synaptic length in the SC and VC were not observed during postnatal development or in the adult in any of the aforementioned brain areas. Low percentages of postsynaptic densities (PSDs) were found at all time points and in all brain areas during the postnatal period. Increases in perforated PSDs were seen at PND = 56 compared to PND = 28 in the VC. In the VC and AC, there was decrease in symmetric synapses with age. Asymmetric synapses were prevalent in all brain areas at PND = 28 AND 56. dendritic synapses predominated in the SC, while spinous synapses were the preponderant type in the VC and AC during postnatal development and in the adult, A decrease in the percentage of spinous synapses in the SC was observed at PND = 56 vs. PND = 28. A decrease in the percentage of negatively curved synapses with age and a trend toward a concomitant increase in the percentage of positively curved synapses were seen in all brain areas during development and in the adult. Quantitative analyses of the SC and VC tissues examined from all postnatal animals demonstrated no significant differences between Lt/Dk and Dk animals in all the synaptic parameters measured. On the other hand, similar analyses of adult synapses in the SC and VC showed a significantly smaller mean number of synapses in rats dark‐reared to adulthood as compared to those raised in normal lighting conditions. Dark‐rearing had no effect on synaptic number at any time point in the auditory cortex, indicating that this is a specific light effect. Therefore, the effects of visual deprivation on synaptic numbers occur during early adulthood and not during postnatal development.

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