Expression and role of neurotrophic factors in taste bud cells.

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Neurotrophic factors affect the survival and maintenance of central and peripheral neurons. Using an i m m unocytochemical method, we examined whether the taste bud cells in the circumvallate papillae of normal mice expressed GDNF (Glial cell line-derived neurotrophic factor), NT3 (Neurotrophin3), NT4 (Neurotrophin4), and their GFR α 1, TrkC, and TrkB receptors. Using double immunostaining for either of them and NCAM, PGP9.5, or gustducin, we additionally sought to determine what type of taste bud cells expressed GDNF, NT3, NT4, or their receptors, because NCAM is reported to be expressed in type-III cells, PGP9.5, in type-III and some type-II cells, and gustducin, in some type-11 cells. Normal taste bud cells expressed GDNF, NT3, NT4, GFα1, and TrkB, however they did not express TrkC.Confocal laser scanning microscopic observations after double immunostaining showed that almost none of the GDNF- and NT3-immunoreactive cells in the taste buds were reactive with anti-NCAM or anti-PGP9.5 antibody, but could be stained with anti-gustduan antibody. On the other hand, almost all anti-PGP9.5- or anti-gustducin-irnmunoreactive cells were positive for NT3, GFRα1, and TrkB. Thus, GDNF- and NT4-immunoreacvtive cells did not include type-III cells, but type-II cells, which are gustducin-immunoreactive; on the other hand, NT3-, GFR α 1-, and TrkB-immunoreactive cells included type-II and -IIIcells, and perhaps type-I cells.We conclude that GDNF, NT3, NT4 in the taste bud cells may exert trophic actions on type-I, - II and -III cells by binding to their GFRα1 and TrkB receptors.