FoundationalConceptsNeuroscience

Its ion channel opens in response to capsaicin and also to heat in
the temperature range of 43-50 degrees Celsius (109-122 degrees
Fahrenheit).
The TRPV1 channel is also opened by the binding of piperine, a
molecule found in black pepper, Piper nigrum (see Chapter 12). Like
chili from South America, black pepper from India also has a hot
“taste,” although the overall flavor of black pepper differs from that
of chili, due to the many differing aromatic constituents in the two
unrelated plants. American chili came to be called “pepper” or “chili
pepper” because it shared with black pepper the perceptual quality of
hotness.
What about cold? Are hot and cold sensations on a continuum?
Does the sensation of cold come about when the “hot receptor” is
inhibited? Or by some other mechanism? It turns out there is a sep-
arate signaling pathway for cold. And it was characterized by using
a molecule from mint plants (genus Mentha)—menthol. Menthol is
appreciated for its distinctive flavor qualities, including producing a
perception of a kind of “coolness.”
There is an ionotropic receptor protein, found in the mouth and
elsewhere in the body, in which a calcium channel opens in response
to the binding of menthol. Temperatures somewhat cooler (8-28
degrees Celsius or 46-82 degrees Fahrenheit) than body temperature
(37 degrees Celsius) produce a shape change of this protein and open
the channel. Analogous to the situation with capsaicin and heat, the
same receptor responds to menthol and to cool temperatures. Thus,
menthol is perceived as a coolness of temperature. Like the capsaicin/
hot receptor, the menthol/cool receptor is also a member of the TRP
receptor family and has been given the designation TRPM8.