David Linden - Touch

If you conducted this survey where I live, in Baltimore, you’d find that the dominant word used to describe the tactile experience of the habanero, smeared on either the lips or the forearm, would be hot , while for mint it would be cool . Is this merely a convenient turn of phrase, a colloquialism? After all, if we were to use a thermometer to measure the actual temperature of mint or chili peppers, we’d find that they are not literally hot or cool. And Baltimoreans (like many others) often use these words metaphorically—to mean, for example, stylish (“the Tesla Roadster looks so cool”) or sexually attractive (“Rachel Weisz is so hot”). The use of words like cool to mean “stylish” and hot for “sexually attractive” are metaphors that are specific to a particular time and place. People in Shakespeare’s time, for example, appear not to have used either of these linguistic constructions. Are “hot chili peppers” or “cool mint” also local, culturally constructed metaphors, or do they reflect some deeper biological reality? If they are merely cultural constructions, you would expect to find groups of people in your world-traveling survey who don’t describe the tactile sensation of chili peppers as hot or mint as cool.

If, however, your survey did indeed reveal that these figures of speech are widespread, would that constitute proof that hot chilies and cool mint are biologically determined metaphors? Not exactly. To play devil’s advocate, one could imagine that, over many years with widespread communication, the idea of hot chili peppers and cool mint originated in one place and spread around the world through cultural contact. While various species of mint are widely dispersed geographically, chili peppers originated in South America and had been carried only to Central America and the Caribbean before European colonization. They were unknown in Europe, Africa, or Asia before Columbus returned from the New World. Soon after, they were spread by European powers, notably Spain and Portugal, to their other colonies. It’s hard to imagine now, but the foods of places like India and Thailand had no fiery chili pepper before the sixteenth century. 1At present it’s unclear if there are any places left on earth where chili peppers have not been introduced. 2So, to really do this survey properly, you’d also need a time machine to take you back to, say, Thailand in the fifteenth century and do your mint/chili survey there as well.

To my knowledge, this type of ethnographic (not to mention time-travel) survey has yet to be done, but from a biological perspective we can predict how it would turn out. Given what we know about the biology of touch, we’d predict that nearly every person around the world would describe chili peppers as hot and mint as cool, even if he or she were experiencing these tactile sensations for the first time and had never heard others describe them. It appears that the cool-mint and hot-chili-pepper metaphors are biologically hardwired from birth.

The main active ingredient in mint is menthol, while its equivalent in chili peppers is a chemical called capsaicin. Less potent chili peppers, like the Anaheim, have a low concentration of capsaicin, while very strong ones, like the Bhut Jolokia pepper, can produce about one-thousand-fold more. 3So why are we biologically predisposed to perceive menthol as cool and capsaicin as hot? One possibility is that there’s a class of nerve ending in the skin that can sense cooling and a different class that can respond to menthol. The signals conveyed by these distinct fibers could then ultimately converge in the brain: Mint and cooling might feel the same because they activate the same brain region dedicated to the sensation of cooling. In an analogous fashion, separate heat-sensing and capsaicin-sensing nerve fibers could ultimately send their impulses to a heat-sensitive brain region.

This hypothesis, therefore, rests on signal convergence in the somatosensory cortex, and while it’s reasonable and appealing, it’s actually dead wrong. How do we know that? First, we can record electrical signals from single sensory nerve fibers in the arm that respond to both heat and capsaicin, and other single nerve fibers that respond to both menthol and cooling. These show that temperature and chemical signals are present in the neurons that innervate the skin long before any signals reach the brain. We also have some molecular evidence. There are free nerve endings in the epidermal layer of the skin (figure 2.3) that contain a sensor on their outer membrane called TRPV1. This single protein molecule can respond to both heat and capsaicin by opening an ion channel, a pore that lets positive ions flow inside, thereby causing the sensory neuron to fire electrical spikes. Similarly, there are free nerve endings that contain a different sensor, called TRPM8, that can respond to both menthol and cooling. The answer to our puzzle is that the metaphor is not in the culture, or even in the brain region. The metaphor is encoded within the sensor molecules in the nerve endings of the skin.

How did this molecular metaphor develop? How did thermosensors like TRPV1 and TRPM8 become sensitive to plant products like capsaicin and menthol? We can’t know for certain the sequence of evolutionary events that gave rise to these two dual-function sensors. The best guess is that TRPV1 and TRPM8 evolved in some animals as temperature sensors and that certain plants later developed compounds that would activate them in order to deter their consumption by predators. Plants that produced menthol and capsaicin would therefore have a survival and reproductive advantage and become more prevalent in the population of that species. In this scenario it’s plant evolution that initially drove the dual-function properties of the sensors, not animal evolution.

David Julius and his coworkers at the University of California, San Francisco, have studied the molecular properties of TRPV1 and TRPM8 by using genetic tricks to force kidney cells or frog eggs grown in a culture dish to produce great quantities of TRPV1 or TRPM8 while recording the electrical signals that pass across the cell membrane when these sensors are stimulated. 4These studies have revealed that features of these molecules explain aspects of our everyday tactile experiences. For example, the oil of the eucalyptus tree contains a substance called eucalyptol that, like menthol, can activate TRPM8 to produce a cooling sensation. This is why eucalyptus extract is often used in soothing skin creams, mouthwash, and throat lozenges. 5

TRP function can also have an impact on our experience of a summer’s day at the beach. If you’re out in the sun too long, the resulting sunburn will set in motion a cascade of inflammatory processes in your skin, including the production of compounds called prostanoids and bradykinin. These chemicals have the property of reducing the temperature threshold of TRPV1 activation from 109°F to 85°F. As a consequence, when you return home from the beach and step in the shower to rinse off the remaining sand and sunscreen, the water temperature you typically select will now be too hot, and you’ll have to reduce it to avoid a painful burning sensation. 6

Another example involves the bird feeder in your backyard. While mammals have the standard form of TRPV1, activated by both capsaicin and heat, birds are utterly indifferent to capsaicin, as they can’t detect it at all. (Birdwatchers often spike the seeds in their feeders with chili peppers to deter squirrels, raccoons, and other mammals while leaving the birds unaffected.) When the TRPV1 gene is extracted from a bird and expressed artificially in kidney cells, it reveals a bird-variant form of TRPV1 that responds to heat but not capsaicin. Examination of the sequence of bird DNA can pinpoint the change to the exact spot that’s necessary for capsaicin binding, located on the inner surface of the cell’s outer membrane. 7Interestingly, chili pepper plants and birds appear to have reached a satisfying sort of evolutionary détente. When mammals eat chili peppers, they tend to destroy the seeds with their molars. Birds, on the other hand, don’t have molars and so pass most of the seeds through their digestive system intact. When they defecate, they spread viable chili pepper seeds to new locations. It’s a win-win situation for birds and chilies. 8