19 Regulation of Adult Neurogenesis by Neurotransmitters

Active adult neurogenesis occurs from neuronal progenitor cells (NPCs) in discrete regions of the adult mammalian central nervous system (CNS) (Abrous et al. 2005; Ming and Song 2005; Lledo et al. 2006). The generation of nascent neurons from NPCs in the intact adult CNS is restricted to the subventricular zone (SVZ) of the lateral ventricle and the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG) (Alvarez-Buylla and Lim 2004). Outside of these two regions, proliferating NPCs normally generate only glia cells, but they appear to be able to give rise to neurons after insults (Emsley et al. 2005). Accumulative evidence suggests that continuous neuronal production in the adult brain under physiological conditions is involved in specific brain functions, such as olfaction, learning, and memory (Kempermann et al. 2004a). On the other hand, neural production of NPCs under pathological conditions may contribute to brain repair (Emsley et al. 2005). Functional integration of nascent neurons is achieved by progression through sequential developmental steps that resemble embryonic and fetal neurogenesis, from proliferation and fate specification of NPCs, to differentiation, migration, axonal/dendritic development, and synaptic integration of newborn neurons (Ming and Song 2005). In contrast to developing neurogenesis, adult neurogenesis arises from a significantly different environment and proceeds with concurrent activities of mature neurons within the existing circuit. Adult neurogenesis, a striking form of structural plasticity in the intact adult CNS, is dynamically regulated by many physiological and pathological stimuli (Abrous et al. 2005; Ming and Song 2005). For example, environmental enrichment (Kempermann...