Neurotrophins in adult mouse taste cells in relation to gustatory innervation

Brain-derived neurotrophic factor (BDNF), one of the members of the nerve growth factor family of neurotrophins, is expressed in developing gustatory papillae and is thought to be the neurotrophin that supports gustatory innervation during development. BDNF expression does not cease after development but continues in some taste cells of adult mice. This thesis determines: (a) which taste cell types BDNF is present in, (b) whether other neurotrophins or neurotrophin receptors are also present in the adult taste system and (c) examines the relationship between neurotrophin expression and taste bud innervation.; In mammals, taste cells are situated in taste buds. Taste buds consist of 50–80 elongate, differentiated taste cells and basal cells, which continuously divide and contribute daughter cells to the taste cell population. Morphologically, taste cells are divided into Type I, Type II and Type III taste cells. Type III taste cells contain synapses and accumulate dense-cored vesicles near synapses.; Since there is constant renewal of taste cells throughout adulthood, new synapses must continually be formed in adult taste cells and neurotrophic factors may play a role in this process. We found using immunohistochemical methods, transgenic reporter mouse studies and RT-PCR analysis on isolated taste buds that in addition to BDNF, nerve growth factor (NGF) is also present in adult taste cells. NGF and the receptors to which NGF binds, trkA and p75 are present in Type II taste cells and gustatory nerve fibers. BDNF and the receptors to which BDNF binds, trkB and p75 are present in Type III taste cells with synapses. Following nerve lesion, taste buds are lost and neurotrophins and neurotrophin receptors are no longer expressed in taste buds. However, BDNF expression in regenerating nerve fibers is increased, suggesting that regenerating sensory ganglion upregulate BDNF expression and thus support their own survival.; Expression, nerve lesion and transgenic mouse studies reveal that BDNF and NGF do not act as neurotropic, neurotrophic, proliferative or apoptotic factors in adult taste cells. Further studies will determine if BDNF and NGF are involved in synaptic transmission of gustatory information to the CNS.