SCCs were first characterized in the nasal cavity of rodents by Finger et al. (2003)
who demonstrated that SCCs express T2R “bitter” taste receptors and that the ethmoid nerve responds when the bitter stimuli cyclocheximide, quinine, or denatonium is presented in the nasal cavity. Therefore it was hypothesized that SCCs mediate trigeminal responses to bitter tasting compounds. Our results partially support this hypothesis by demonstrating that isolated TRPM5-GFP+ or gustducin-GFP+ cells in the nasal cavity respond to the bitter compound denatonium. However, out of all the bitter stimuli tested, SCCs reliably responded only to denatonium through a PLC cascade. The ethmoid responses to denatonium in the rat nasal cavity recorded by Finger et al. (2003)
were not as large as those for cycloheximide and quinine so it was surprising that denatonium evoked the best responses in SCCs. It is possible that trigeminal responses to cycloheximide and quinine are mediated by activation of trigeminal nerve fibers themselves. Liu and Simon (1998)
demonstrated that cultured rat trigeminal ganglion cells are able to respond to several bitter stimuli; quinine and strychnine elicited calcium increases in 22% and 18% of cultured trigeminal ganglion cells respectively, while only 5% responded to denatonium. Therefore, denatonium responses may be more specific to SCCs and not mediated by broadly responsive nerve fibers.
Lin et al. (2007)
recently demonstrated that the anterior nasal epithelium and TRPM5-expressing nasal SCCs respond to high concentrations of several odorants. We did not observe broad responsiveness of SCCs to multiple irritating stimuli. One possible explanation for the apparent discrepancy between the findings of Lin et al. (2007)
and the current study is that the larger population of TRPM5-expressing SCCs exhibits a broad detection range for irritants while the gustducin-expressing subpopulation of SCCs is specifically tuned to detect a narrow range of relevant stimuli.
We find that cycloheximide does not stimulate most SCCs. Cycloheximide was ineffective at stimulating most SCCs with concentrations ranging from 0.1 mM to as high as 20 mM. At this extremely high concentration, cells showed slow large decreases in intracellular calcium or died. This correlates with in situ hybridization results by Finger et al. (2003)
which failed to demonstrate the mouse T2R responsible for detecting cycloheximide (mT2R5) in nasal SCCs. Therefore it is possible that cycloheximide acts through some nonspecific mechanism that activates nerve fibers in the rat nasal cavity. It is also possible that rat SCCs express different T2R receptors than mice. Since the nerve recordings were made in rat and in situ hybridization performed on mouse tissue by Finger et al. (2003)
it is not known if mice and rats should be expected to express similar nasal T2Rs.
Other T2R mediated compounds tested are also ineffective at stimulating SCC receptor pathways. Phenylthiocarbamide (PTC) and the related 6-propyl-2-thiouracil (PROP) cause increases in intracellular calcium in SCCs but this effect appears to be through some nonspecific mechanism since all epithelial cells respond in a similar manner and the response is not blocked by a PLC inhibitor. Responses to PTC and PROP are also slower than what is usually reported for receptor mediated events. Salicin, which is known to activate a human T2R, did not stimulate SCCs.
Since SCCs do not respond to many bitter compounds and seem to be relatively narrowly tuned, they may express a limited number of T2Rs. It is possible the T2Rs expressed by SCCs are not activated by many classical bitter tastants because they have evolved to detect other classes of compounds. For example, a human T2R, hTAS2R46, responds to a broad array of substances including lactones and diterpenoids, as well as denatonium (Brockhoff et al. 2007
) but does not respond to unrelated bitter-tasting substances such as ouabain or salicin.
In conclusion, the principal finding of our study is that not all nasal SCCs are broad detectors of trigeminal irritants, but rather may be “tuned” to detect a narrow range of noxious compounds in the nasal cavity. The only tested compound that both TRPM5-GFP and Gustducin-GFP cells responded to through a PLC-mediated signalling cascade was denatonium benzoate. While SCCs likely mediate trigeminal nerve responses to denatonium, the mechanisms by which the trigeminal nerve responds to other bitter compounds, e.g. cycloheximide, remain enigmatic.