By Fernando Luna Vera
Ph.D. Candidate, Chemistry Department, VCU
Science Museum of Virginia Volunteer
“Can you please pass me the hot sauce?” a friend of mine asked. “This one?” I replied, holding up and showing him a warm spinach dip cup. “No! The spicy one,” he said. As I passed it to him, I mentally wondered an almost childish question, “Why do we call it hot sauce if it is not really hot…nor is it even served warm!” Appreciating and feeling the taste of food involves a complex mechanism that uses the sense of taste, smell and touch. This rise of sensations and perceptions sparked by food requires hundreds of chemical signals and our brain acting as traffic officer to control them.
After you bite a spicy taco your body can recognize that familiar, pungency sensation thanks to a well equipped network of sensors called neurons. Neurons, as do all animal cells, contain a boundary layer called a membrane, where specific receptors are allocated. These receptors are like the geometric figures on the surface of a shape sorter toy which recognizes specific shapes. Certain neurons, called nociceptors, have the specialized job of sensing pain. These kinds of neurons contain a specific receptor for capsaicin, the molecule found in high concentration within chili peppers. One can image then, capsaicin molecules traveling to the tongue and getting caught later by the nociceptors, which immediately after recognizing them, trigger an electrical signal that travels to the brain and makes us aware of the irritating sensation of the hot sauce. That specific capsaicin receptor is called TRPV-1.
But why does our brain read the signal produced by capsaicin as an increment in temperature? An experiment performed in 2000 by scientist of UCLA helped us to better understand this outcome. By using genetic techniques, they “knocked out” the gene that produces the capsaicin receptor (TRPV-1) from a group of mice and compared it with other group that still had the TRPV-1. After exposing the two groups to capsaicin, the one lacking TRPV-1 showed to be insensitive to the irritant substance, as expected. However, surprisingly the same group showed a high insensibility to temperatures above 43ºC, which is when pain is normally sensed. This result implied that the same receptor for the chili peppers irritant molecule is the same receptor for sensing high temperature. So when neurons bind capsaicin, the brain interprets the signal produced as an increase in temperature, like something “hot” is touching your tongue.
Additionally, neurons possess certain receptors called TRM8, which are activated by low temperatures (> 12 ºC). These receptors also happen to be sensitive to menthol, the compound found in high concentration within peppermint and used in products like mouthwashes and toothpaste. By then using the same mechanism for associating capsaicin and hot temperatures, the menthol bond to a TRM8 receptor sends a signal that tricks the brain; therefore, by just the taste of mint, makes you feel cool!
Sven-Eric Jordt, David D McKemy and David Julius, Current Opinion in Neurobiology, 2003, 13:487–492.
M. J. Caterina, A. Lefßer, A. B. Malmberg, W. J. Martin, J. Trafton, K. R. Petersen-Zeitz, M. Koltzenburg, A. I. Basbaum, D. Julius, Science, 2000, 288, 306-313