The Roles of Brain-derived Neurotrophic Factor in the Development of Nasal Chemoreceptor Neurons

Brain-derived neurotrophic factor (BDNF) is one of the neurotrophins, and known to facilitate differentiation, growth and maturation of neurons. BDNF binds to the high-affinity receptor, tyrosine kinase receptor B (TrkB) to initiate signal transduction (Korsching, 1993; Lindsay et al., 1994). Olfactory and vomeronasal receptor neurons (ORNs and VRNs, respectively) are chemoreceptors located in the nasal cavity of most mammalian species (Graziadei, 1977). In the olfactory epithelium (OE) lining the olfactory mucosa (OM), progenitor cells differentiate into immature and mature ORNs; a single dendrite extending from the apical pole of each soma reaches the surface of the OE to form a dendritic ending, and a single axon from the base of the soma runs downward to pass through the basement membrane of the OE to reach the brain. Similar bipolar neurons are also contained in the vomeronasal sensory epithelium (VSE) of the vomeronasal organ (VNO), and designated as VRNs or vomeronasal sensory cells/ neurons (Graziadei, 1977; Takami, 2002). Although several neurotrophic factors including neurotrophins are thought to be involved in the development, maturation, and regeneration of ORNs (Carter and Roskams, 2002; Schwob, 2002), the functional roles of BDNF remain to be understood. In the case of the VRNs, we were the first to report the distribution of BDNF and TrkB at both domestic (Takami et al., 2001) and international (Takami and Nishiyama, 1997b) meetings. In this paper, we present summary of our recent studies concerning BDNF and TrkB in the rat OM and VNO. Our overall research goal is to clarify functional roles of BDNF in the differentiation and maturation of ORNs and VRNs.