Type II and III Taste Bud Cells Preferentially Expressed Kainate Glutamate Receptors in Rats

Dose-Response Relationship between Head and Neck Radiation and Damages to Gustatory Cells in Mice

Objective

To investigate the dose-response relationship between radiation to the head and neck regions and damage observed in mice gustatory cells.

Materials and Methods

A total number of 45 mice (C57BL/6) (aged 8-12 weeks) were enrolled in this study. The head and neck regions of the mice were irradiated at doses of 8 Gy (low-dose group, n = 15), 16 Gy (moderate-dose group, n = 15), and 24 Gy (high-dose group, n = 15). Each time, 3 mice from each group were sacrificed before radiation and then 2-day, 4-day, 7-day, and 14-day post the irradiation, respectively. The immune-histochemical staining method was employed to obtain gustatory papilla tissues and mark gustatory cells. Careful calculation of the numbers of proliferative cells, taste buds, and type II gustatory cells was conducted.

Results

A decrease in the number of Ki-67-marked proliferative cells was noted at 2 days postirradiation (DPI), and the number of cells was recovered to the normal level at 4-DPI in each group. The number of Ki-67-marked proliferative cells was hypercompensation (significantly higher than normal) in the moderate-dose and high-dose groups at 7-DPI and insufficient compensation (significantly lower than normal) in the high-dose group at 14-DPI. There was a significant reduction of taste buds and type II gustatory cells at 2-DPI and is lowest at 4-DPI in the moderate-dose and high-dose groups, while little change was observed in the low-dose group.

Conclusion

Damages to Gustatory Cells after head and neck radiation were dose-related and compensation occurred in 14-DPI and may be insufficient when overdosed.

Taste buds as peripheral chemosensory processors

Taste buds are peripheral chemosensory organs situated in the oral cavity. Each taste bud consists of a community of 50-100 cells that interact synaptically during gustatory stimulation. At least three distinct cell types are found in mammalian taste buds - Type I cells, Receptor (Type II) cells, and Presynaptic (Type III) cells. Type I cells appear to be glial-like cells. Receptor cells express G protein-coupled taste receptors for sweet, bitter, or umami compounds. Presynaptic cells transduce acid stimuli (sour taste). Cells that sense salt (NaCl) taste have not yet been confidently identified in terms of these cell types. During gustatory stimulation, taste bud cells secrete synaptic, autocrine, and paracrine transmitters. These transmitters include ATP, acetylcholine (ACh), serotonin (5-HT), norepinephrine (NE), and GABA. Glutamate is an efferent transmitter that stimulates Presynaptic cells to release 5-HT. This chapter discusses these transmitters, which cells release them, the postsynaptic targets for the transmitters, and how cell-cell communication shapes taste bud signaling via these transmitters.

Evidence for a role of glutamate as an efferent transmitter in taste buds

Background

Glutamate has been proposed as a transmitter in the peripheral taste system in addition to its well-documented role as an umami taste stimulus. Evidence for a role as a transmitter includes the presence of ionotropic glutamate receptors in nerve fibers and taste cells, as well as the expression of the glutamate transporter GLAST in Type I taste cells. However, the source and targets of glutamate in lingual tissue are unclear. In the present study, we used molecular, physiological and immunohistochemical methods to investigate the origin of glutamate as well as the targeted receptors in taste buds.

Results

Using molecular and immunohistochemical techniques, we show that the vesicular transporters for glutamate, VGLUT 1 and 2, but not VGLUT3, are expressed in the nerve fibers surrounding taste buds but likely not in taste cells themselves. Further, we show that P2X2, a specific marker for gustatory but not trigeminal fibers, co-localizes with VGLUT2, suggesting the VGLUT-expressing nerve fibers are of gustatory origin. Calcium imaging indicates that GAD67-GFP Type III taste cells, but not T1R3-GFP Type II cells, respond to glutamate at concentrations expected for a glutamate transmitter, and further, that these responses are partially blocked by NBQX, a specific AMPA/Kainate receptor antagonist. RT-PCR and immunohistochemistry confirm the presence of the Kainate receptor GluR7 in Type III taste cells, suggesting it may be a target of glutamate released from gustatory nerve fibers.

Conclusions

Taken together, the results suggest that glutamate may be released from gustatory nerve fibers using a vesicular mechanism to modulate Type III taste cells via GluR7.