Occipital cortical morphology of the rat: Alterations with age and environment

With the use of Golgi impregnation, this study demonstrated dendritic proliferation in layers II and III in the medial occipital cortex of 444- and 630-day-old male rats. Increases occurred in animals housed in either enriched or standard colony conditions. Specifically, third- and fourth-order basal dendrites increased significantly in frequency from 414 to 444 to 630 days of age. This study approximated a longitudinal study because the different age groups were littermates. The finding of an increase in dendritic branching in old age was not new, but our study was the first to note this increase in the rat's cerebral cortex. In a separate study the cortical morphology was examined in 90- and 630-day-old rats which had been living together in a single enriched environment for 30 days. The brains of these rats were compared with littermates living with their respective age groups in standard colony conditions. The dendritic pattern was similar in those two age groups irrespective of environment. Only the sixth order significantly differed, with the frequency of branching being greater in the 630-day-old animals. An increase in cortical thickness in the enriched animals was apparent compared with controls, but the differences were not significant. Because previous results showed the 60- to 90-day-old enriched rats without old companions developed a significantly thicker cortex than standard control littermates, it is possible that when young rats live with old rats they do not interact with their environment as musch as when living with other young rats.