An age-dependent novel hyperinnervation of circumvallate papillae by tyrosine hydroxylase-containing nerve fibers in NGF-overexpressing transgenic mice

Immunohistochemical detection of neurotrophin-3 and -4, and their receptors in mouse taste bud cells

Neurotrophin-3 (NT3) and neurotrophin-4 (NT4) affect the survival and maintenance of central and peripheral neurons. Using an immunohistochemical method, we examined whether the taste bud cells in the circumvallate papillae of normal mice expressed NT3, NT4, and their respective receptors TrkC and TrkB, and if so, what type of cells in the taste buds expressed them. Double immunostaining for either of them and PGP 9.5, NCAM, or gustducin was used to determine which cell types expressed which neurotrophins and receptors. Normal taste bud cells expressed NT3, NT4, and the TrkB receptor, but not TrkC. The percentage of NT3-immunoreactive cells among all taste bud cells was 89.0%, that of NT4-immunoreactive cells, 58.6%, and that of TrkB-immunoreactive cells, 80.8%. Almost none of the NT4-immunoreactive cells were reactive with anti-PGP 9.5 or the anti-NCAM antibody, but they could be stained with anti-gustducin, revealing that NT4-immunoreactive cells were contained only in the type-II--and possibly type-I--cell population. On the other hand, NT3-, and TrkB-immunoreactive cells included type-III cells, together with type-II, -I, and basal cells, because they were positive for PGP 9.5 and gustducin. We conclude that NT4 may exert trophic actions on all types of taste bud cells by binding to their TrkB receptors, and NT3 may also have a similar, though negligible role.

Differential expression of brain-derived neurotrophic factor and neurotrophin 3 mRNA in lingual papillae and taste buds indicates roles in gustatory and somatosensory innervation

Although many studies have demonstrated the dependency of taste bud function and/or survival on intact innervation, relatively few have dealt with the development of taste bud innervation. Using in situ hybridization histochemistry, we show that brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT3) mRNA are expressed in a specific pattern in the taste buds, tongue papillae, and lingual epithelium during development and that expression persists into adulthood. BDNF mRNA is expressed in a fraction of the taste cells of the developing and adult taste buds in rats, showing different labeling intensities among the labeled cells. NT3 and mRNA seems to be located in areas other than those where BDNF mRNA is expressed, mainly in the superior epithelial surfaces of circumvallate papillae, the outer surface epithelium of foliate papilae, the superior surface and the lateral epithelium of the fungiform papillae, and the epithelium of the filiform papillae. NT3 mRNA labeling is also observed among muscle and connective tissue of the tongue. The morphological appearance, expression of NT3 mRNA, and ramification of nerve fibers in defined epithelial structures in the posterior wall of the anterior filiform papillae suggest the existence of a mechanosensory apparatus in these papillae. Nerve growth factor and neurotrophin 4 probes did not give rise to selective labeling in tongue, although their presence cannot be totally excluded. Based on present and prior studies, we suggest that BDNF is needed during initiation and for maintenance of gustatory innervation of taste buds and gustatory papillae and that NT3 is mainly needed for somatosensory innervation of the tongue.