MODULATION OF IMMATURE NEURONS IN THE NEOCORTICAL LAYER II OF SHEEP KEPT IN DIFFERENT ENVIRONMENTAL CONDITIONS

Cortical immature neurons (cINs) are prenatally generated, nondividing neuronal cells in arrested maturation that can “awake” during adulthood to integrate as new functional neurons in the layer II (neurogenesis without division). While restricted to paleocortex in rodents, the cINs are far more abundant in large-brained, gyrencephalic mammals, also extending into their entire neocortical mantle. Because of this interspecies difference, the possible modulation of these neurons in the neocortex can be studied only in gyrencephalic species. Environmental conditions are known to positively (enriched environment) or negatively (stress) affect the rate of stem cell-driven adult neurogenesis, yet nothing is known regarding the cINs. To address this issue, we investigated three groups of young sheep, including controls (animals within their habitual environment), isolated animals (a condition considered to be perceived as negative in sheep), and animals kept in an enriched environment, across a two month-long experiment. The cINs of the layer II were identified in postmortem fixed brains using immunocytochemistry for the cytoskeletal marker doublecortin and counted to obtain linear densities (number of cells/mm). The counting was performed on a total of 15 brains following the entire layer II perimeter of 180 coronal sections, by considering piriform cortex and neocortex separately. Both conditions resulted in reduced number of immature cells with respect to controls, the drop being significant only in the neocortex. These results show that environmental conditions can influence the availability of neocortical immature neurons, suggesting that detours from routine life can lower their number, likely by accelerating their maturation. On these bases, remarkable structural plasticity might be figured out to occur in the neocortical upper layers of gyrencephalic species as a consequence of lifestyle.