Although stroke occurs mainly in the aged population, most animal studies investigating both stroke and neurogenesis are conducted on young-adult brains. Hence, better understanding of how neural stem/progenitor cells (NSCs) are regulated after stroke in the aged brain seems essential. In the past funding period, we study the roles of aging in neurogenesis after stroke. The goal of our research in the next funding period is to focus on the mechanisms underlying regulation of stroke-induced neurogenesis by aging. Specifically, we will explore the role of the Notch pathway in neurogenesis during aging. Previous studies show that Notch signaling pathway plays critical roles during maintenance, proliferation, and differentiation of NSCs in developing brain. Recent evidence shows that Notch1 signaling is conserved in the regulation of adult neurogenesis. Our pilot studies show that Notch1 and its downstream targets are expressed in SVZ cells, and that the number of BrdU-positive (proliferating) cells in the normal adult SVZ is significantly altered after inhibiting or activating the Notch1 pathway. In addition, we find that Notch1 signaling in the SVZ is activated after stroke and that stroke-induced cell proliferation in the SVZ can be blocked by inhibiting the Notch1 pathway in young-adult brain. These results led to our hypothesis that Notch1 signaling is essential for neurogenesis to occur in the adult brain and that changes in Notch1 signaling activity may contribute, directly or indirectly, to the aged-dependent decline in neurogenesis, including that following stroke. To test this hypothesis, we propose to: (1) to examine the temporal profiles of Notch1 signaling molecule expression in the SVZ in response to aging, and to characterize the phenotypes of SVZ cells expressing Notch1 pathway molecules in the young-adult vs. aged rat brain; (2) to investigate the effect of altering the Notch1 pathway on cell proliferation and other signaling pathways in the SVZ of young-adult and aged rat brain in vivo; (3) to examine Notch1 pathway activity in the SVZ of the young-adult and aged rat brain after stroke; (4) to assess the effect of forced activation or blockade of the Notch1 pathway on neurogenesis in SVZ of young-adult and aged rat brain after stroke in vivo. The long-term goal of the proposed experiments is, by studying the mechanisms that regulate stroke- induced neurogenesis in aged brain, to achieve better understanding of the fundamental principles that govern neurogenesis in normal aging and age-related neurological diseases like stroke.