Immature neurons in subcortical regions of horse brain

Biomedical research, including neuroscience, commonly employs laboratory rodents as animal models. However, remarkable interspecies differences are emerging in brain plasticity of mammals, which can lead to incorrect interpretation of preclinical data and problems in translation to humans. Neurogenic processes, such as adult neurogenesis, exhibit significant heterogeneity in mammals, both in terms of type (canonical and non-canonical) and origin of the new neurons (stem cell-driven and “without division”). Therefore, comparative studies in neuroplasticity performed in a comparable manner are needed. In the complex landscape of mammalian neuroplasticity, a novel form of “dormant” immature neurons (dINs) is emerging. The dINs are prenatally generated, non-dividing cells that continue to express markers of immaturity (e.g., the cytoskeletal protein doublecortin; DCX) during adulthood. At different time points, these cells can “awake” by committing to maturation and integrating successfully into existing neural networks, thus representing a form of “neurogenesis without division”. In the rodent brain, the dINs are restricted to the paleocortex. Conversely, in other mammalian species, particularly those with a large, highly gyrencephalic brain, they are far more abundant and extended to the entire neocortex. There is also evidence for interspecies heterogeneity in numbers of dINs linked to animal age progression. However, distinguishing them from neurons produced by stem cell-driven adult neurogenesis is tricky, as some markers of immaturity are shared by both cell populations, which has led to confusion about their origin in the past. DCX+ cells considered to be dINs are also present in subcortical regions, such as the amygdala. This thesis is a contribution to a larger project concerning the mapping of DCX+ cell densities in the whole brain of widely different mammals, herein considering analysis in subcortical regions of horses (a species with very large brain size), to be compared to data obtained in other species. The study considered two different age groups (young-adult and middle-aged), and was extended to investigate cell division, searching for possible coexpression with Ki-67 antigen (a marker for cell proliferation). To achieve these objectives, the occurrence, morphology, topographical distribution, and amount of DCX+ cells were investigated in the amygdala of eight horse brains (four animals/age group), by using the same methodology previously employed in order to obtain comparable results. The density of DCX+ cells/area was significantly higher in primate brains (chimpanzee, marmoset) compared to rodents (mouse, naked mole rat). In horses, however, this metric showed no significant difference compared to other mammals, such as herbivores and carnivora. This clearly indicates that the brain size is not the primary driver of phylogenetic variation (as previously thought), and more complex relations must be involved. A significant decrease in the number of DCX+ cells was found by comparing young adult horses to middle age ones, suggesting that these neurons progressively mature losing the DCX staining also in large-brained animals. At the same time, the degree of reduction with age in horses was lower than in rodents, thus confirming the maintenance of a reservoir of dINs in gyrencephalic species. Cell morphology and cell type distribution found in horse amygdala were very similar to those of cortical dINs found, and no evidence