Figure 13.3. Chili plant, Capsicum annuum.Amazingly, thermal heat also activates these capsaicin-sensitive
proteins. That is, increasing the temperature of the protein causes
the same kind of shape change as the binding of capsaicin, the same
opening of a Ca** channel, and the same consequent cell excitability.
Chili is experienced as “hot” because actual heat—that is, increases
in temperature—is sensed by exactly the same molecular and cellu-
lar mechanism! Thus, in the world of our perception, chili hot and
fire hot are actually the same quality. However, unlike the heat from
capsaicin, heat due to temperature would also have other effects, in-
cluding potential damage to tissue (that is, burns) if the temperature
is sufficiently high.
These capsaicin/thermal receptor proteins are not just in the
mouth—they are all over the body, where their presence allows us to
sense the temperature of our skin and thus of the surrounding envi-
ronment. The receptors in the skin will also respond to capsaicin. Rub
powdered cayenne onto your skin and it will feel warm, at least, and
perhaps hot. And many have experienced the painful burning feeling
that results when you inadvertently touch your eye after cutting
chilies when cooking.
When these receptor proteins were first characterized in the 1990s,
they were simply called capsaicin receptors. However, it was soon
appreciated that they were members of a large family of ionotropic
proteins originally characterized from the study of vision in the fruit
fly. These proteins were already known as TRP channels, from the
esoteric terminology “transient receptor potential,” a reference to an
electrical response of the insect eye to light. The TRP terminology was
kept, and the capsaicin receptor is now known by the name TRPV1.