Taste is an important sense in survival and growth of animals. The growth and maintenance of taste buds, the receptor organs of taste sense, are under the regulation of various neurotrophic factors. But the distribution aspect of neurotrophic factors and their receptors in distinct taste cell types are not clearly known. The present research was designed to characterize mRNA expression pattern of neurotrophic factors and their receptors in distinct type of taste cells. In male 45-60 day-old Sprague-Dawley rats, epithelial tissues with and without circumvallate and folliate papillaes were dissected and homogenized, and mRNA expressions for neurotrophic factors and their receptors were determined by RT-PCR. The mRNA expressions of brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3), receptor tyrosine kinase B (TrkB), exclusion of nerve growth factor (NGF), neurotrophin-4/5 (NT4/5), receptor tyrosine kinase A (TrkA), receptor tyrosine kinase C (TrkC), and p75NGFR were observed in some population of taste cell. In support of this result and to characterize which types of taste cells express NT3, BDNF, or TrkB, we examined mRNA expressions of NT3, BDNF, or TrkB in the PLCβ2 (a marker of Type II cell)- and/or SNAP25 (a marker of Type III cell)-positive taste cells by a single taste cell RT-PCR and found that the ratio of positively stained cell numbers were 17.4, 6.5, 84.1, 70.3, and 1.4 % for PLCβ2, SNAP25, NT3, BDNF, and TrkB, respectively. In addition, all of PLCβ2- and SNAP25-positive taste cells expressed NT3 mRNA, except for one taste bud cell. The ratios of NT3 mRNA expressions were 100% and 91.7% in the SNAP25- and PLCβ2-positive taste cells, respectively. However, two TrkB-positive taste cells co-expressed neither PLCβ2 nor SNAP 25. The results suggest that the most of type II or type III cells express BDNF and NT3 mRNA, but the expression is shown to be less in type I taste cells.
Glutamate-induced cobalt uptake reveals non-NMDA glutamate receptors (GluRs) in rat taste bud cells. Previous studies suggest that glutamate-induced cobalt uptake in taste cells occurs mainly via kainate type GluRs. Cobaltstained cells were immunoreactive against GluR6 and KA1 subunits of GluRs. However, the functions of those type of receptors are not known yet. It is important question which types of taste cells are cobalt-stained when stimulated by glutamate and whether they express these kinds of GluRs. Circumvallate and foliate papilla of Sprague-Dawley rats (45-60 days old) were used. A cobalt-staining technique combined with immunohistochemistry against specific markers for taste bud cell types, such as blood group H antigen (BGH), α-gustducin (Gus), or neural cell adhesion molecule (NCAM) was employed. We also performed double labeling of GluR6 or KA1 subunits of GluR with each specific marker for taste bud cell types. Lots of cobaltstained taste bud cells expressed Gus-like immunoreactivity, and subsets of the cobalt stained cells appeared NCAM- or BGH-like immunoreactivity. Stimulation with 1 mM glutamate significantly increased the number of cobaltstained cells in Gus-like immunoreactive cells, but not in NCAM- or BGH-like immunoreactive cells. In the double labeling experiments, GluR6 and KA1 subunits of GluRs were mainly expressed with Gus. These results suggest that kainate glutamate receptors preferentially expressed in type II taste bud cells in rat.