Effect of Environmental Enrichment on Morphology of Deep Layer III and Layer V Pyramidal Cells of Occipital Cortex in Oldest-old Rat - A Quantitative Golgi Cox Study

Dendrites and dendritic spine density regress extensively during aging in rats housed under standard conditions (SC), which can be ameliorated by housing in the enriched environment (EE). This event is particularly pronounced on neurons where high rates of plasticity are conceivable, such as on projection neurons of archicortical regions of dentate gyms'. However; effects of EE on neocortical projection neurons are still poorly understood. Therefore, we investigated the effect of EE housing on a deep layer III (L3) and layer V pyramidal cell (L5) morphology in the associative occipital neocortex of male Sprague-Dawley rats at 24 months of age. Rats were randomly distributed in two groups and reared under either SC (n=5) or EE conditions (n=6) for 26 days. In depth quantitative analysis of dendritic tree morphology and spine density on occipital projection neurons, from Golgi-Cox stained sections, showed similar trend in both EE occipital layers L3 and L5. Significant increase was found in total number of dendritic segments (L3 - 37.5 %, L5 -33 %) and in dendritic diameter of intermediate segments (for more than 20 %), while increase in total spine number was around the level of significance (p > 0.55; L3 - 30 %, L5 - 64 %). These findings suggest an outgrowth of new dendritic segments. When compared to archicortical region of dentate gyrus, effects of aging in the associative occipital cortex were less pronounced. Taken together; these findings suggest that structures being more affected by the aging process are more susceptible to the environmental enrichment in old age.