The role of adult neurogenesis and oligodendrogenesis in age-related cognitive decline in the non-human primate
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Cognitive aging is a biological process characterized by physical changes in the brain and subsequent alterations in cognitive function. While neurodegenerative diseases result in extensive neuronal death and anatomical abnormalities, normal aging has subtle changes resulting in a range of cognitive abilities. Early studies of cognitive aging focused on changes in the neuronal population, but evidence has demonstrated that forebrain neurons are largely preserved with age. Furthermore, the proliferation of new neurons in the adult brain has generated great speculation regarding the role and contribution of new neurons to cognitive function. Conversely, both imaging and ultrastructural analyses have shown that age-related alterations in white matter and myelin are good predictors of cognitive impairment, suggesting that alterations in connectivity between brain regions may result in cognitive decline. In this dissertation, a rhesus monkey model of normal aging was used to assess the contribution of adult-neurogenesis and oligodendrogenesis to cognitive function. First, cell proliferation and adult neurogenesis were assessed in the subgranular zone of the hippocampal dentate gyrus. Aged animals demonstrated a decline in proliferating cells and neurogenesis but only limited correlations with behavioral impairment. Immature neurons were also identified in temporal lobe cortices, but results indicate these immature cortical neurons are most likely not adult-generated. Moreover, despite an age-related decline in numbers, they persist throughout the lifespan and many differentiate into Calretinin neurons. Further investigation of white matter alterations used immunohistochemistry and diffusion spectrum imaging to correlate oligodendrocyte numbers with white matter connectivity. In the corpus callosum and cingulum bundle, there were no correlations with age, but cognitive impairment was associated with increased oligodendrocyte number and decreased white matter connectivity. These correlations were only present in the anterior aspect of the cingulum bundle, not the posterior cingulum suggesting differential oligodendrocyte responses along the anterior-posterior axis of the brain. Together, these data demonstrate an age-related decline in adult neurogenesis may be only a small contributor to cognitive impairment. Additionally, a reserve pool of immature neurons continues to differentiate in the temporal cortex potentially contributing to local plasticity. Furthermore, cognitive impairment rather than aging has a stronger correlation with oligodendrocytes alterations and connectivity.