Clinical neurology has long operated on the premise that the adult central nervous system has limited capacity for self-repair following damage. However, several neural systems have now been described which exhibit neuronal production in adulthood. Among these, the songbird vocal control nucleus, HVc, is the best characterized. Its high rate of neuronal production, well-describing anatomy and physiology, functional dependence on circulating gonadal steroids, and association with a well-defined behavioral endpoint - song-have made this an appealing system within which to study neuronal generation in adulthood. I have recently developed a preparation by which explants of the adult songbird HVc may be maintained in long-term culture, under conditions which permit sustained neuronal production, migration, and differentiation in vitro. Using this system, I propose to study factors regulating adult neuronal production and migration in vitro, by addressing the following issues: 1. By what humoral means is the neurogenic activity of the ventricular zone precursor population constrained? What factors influence the in vitro rate of neuronal mitogenesis by adult avian ventricular zone precursor cells? This study will pursue the hypotheses that fetal serum harbors anti-mitotic factors which suppress neuronal mitogenesis in vitro, and that serum-stimulated astrocytes and ependymal cells may release neuronal differentiation agents with anti-mitogenic activity. 2. What is the ontogeny of the ventricular zone precursor cells? To what extent do these cells remain pluripotential? To what differentiated cell types do they give rise? This work will address the postulate that newly generated neurons and guide cells are derived from a pluripotential neuroepithelial precursor cell. By retroviral-insertion of the lacZ marker gene into ventricular zone progenitor cells in vitro, with complementary video analysis, the ontogeny and fate of the neuronal and guide cell precursors will be defined. 3. By what cellular and molecular adhesive mechanisms are newly produced neurons able to migrate into adult avian brain tissue? These experiments will test the hypothesis that the guide fiber network of the adult avian brain is dynamic in its cytoarchitecture, and that its geometry may be influenced by the regulated turnover and fiber extension of its constituent ependymal cells. Complementary experiments will examine the idea that specific neuroependymal adhesive molecules determine both the migration efficacy and fate of newly generated neurons within the adult avian forebrain. The species-specific, regionally-limited nature of adult neurogenesis may reflect specific molecular constraints upon neuronal mitogenesis and migration in non-neurogenic species and brain regions. The circumvention of these constraints by directed pharmacotherapy may thus provide a strategy for brain repair in otherwise non-neurogenic species. This work is intended to provide the data base and rationale by which potential therapeutic modalities such as induced neurogenesis and site-directed neuronal migration may be developed, in the belief that these techniques will become powerful options for the reconstitution of the damaged adult brain.
