Wasabi Receptors Unite!

Woohoo!!!!! A ‘wasabi’ receptor!!!

Harking back to a discussion with a colleague from India, we debated the merits and preferences for tasting ‘heat’. By that I mean hot peppers and wasabi. Our preferences were the converse: he preferred peppers and I, wasabi. We also agreed on a similar tendency in that while exhibiting a high tolerance for our preferred ‘heat’ generator, we had a relative high pain sensitivity and intolerance for the other.

We debated that perhaps tolerance may be a function of conditioning: his typical diet incorporated foods prepared with copious amounts of ‘high octane’ peppers. Whereas I tend to prefer Asian foods that are traditionally accompanied by wasabi, or even smearing wasabi on other foods such as ham or even eggs. However, living in Texas, I also eat a fair amount of the typical Southwestern fare often doused with hot peppers. Regardless, I haven’t been able to overreach food with a rating of one by the Scoville chile heat index (100-1,000 Scovlille Units), except for an occasional exploration of a meal that may rate a two if accompanied by plenty of neutralizing chips and iced tea.

The magic ingredient of chili peppers that endows that heated kick is capsaicin, a oily substance found in the veins of a wide variety of peppers. Capsaicin delivers a burning and painful sensation in the mouth and on the skin due to its interaction with sensory neurons. The chemical binds to receptors, classified as TRPV1, that are also stimulated by heat and physical abrasion. When stimulated, these receptors permit ions to pass through the cell membrane and the neuron ultimately signals the brain. The result is the sensation of burning.

Typical of the negative feedback control by most receptors, chronic exposure to capsaicin depletes the neuron of neurotransmitters, leading to a reduction in pain sensation and neurogenic inflammation (a process called ‘desensitization’). After capsaicin is removed and following a short refractory time the neurons recover. This may explain why people who traditionally eat food containing hot peppers have a higher degree of tolerance.

Nevertheless, this does not explain why the differential tolerance, and intolerance, of chili peppers and wasabi. The pain-associated chemicals in wasabi, a Japanese horseradish, are similar to those found in hot mustards: isothiocyanates. The wonderful sinus clearing vapors of wasabi are the primary contrast to the heat of hot chili peppers. Yet both produce painful sensations in the mouth and on the skin when topically applied, exciting sensory nerve fibers.

Whereas hot chili peppers titillated my colleague, I covet the rush through the sinuses and tearing aftermath of wasabi. Both of us exhibited a greater degree of sensitivity (and dislike) for the other source of painful sensation. Could the sensations and tolerance be more than just dietary conditioning? I ventured to posit that perhaps there is a receptor for wasabi!

A recent study in Cell now reports that different receptors are involved in the pain sensations of capsaicin and mustard thiocyanates. While both receptors affect neurons on the pain pathway, TRPV1 receptors are associated with capsaicin and TRPA1 receptors with other plant chemicals specifically found in garlic and various mustards. This implies that they segregate at the molecular level yet elicit similar physiological overlapping responses (irritation and inflammation).

Belonging to a group of neuron receptors in our skin, and on our mouth and tongue, they are part of our natural defense system. Transient receptor potential (TRP) ion channels are like molecular thermometers that detect temperature, mechanical abrasion and irritating chemicals. They help us sense temperature and irritants through the skin and, as we can attest, in spicy and hot foods. Even volatile odors, as evidenced by eating wasabi and the mucus membranes in the sinus cavity and lungs. Ultimately, a message is sent to the brain: “Danger, Danger!” In fact these irritant chemicals are a component of the plant’s natural defense system.

On the other hand, both capsaicin and mustards are used in alternative medicine as topical ointments to relieve pain. Applications to the skin are left on until the area is numb. This is induced by overwhelming the local neurons and depleting their neurotransmitters that create the painful sensation and blocking neurogenic inflammation. Another medicinal use that I discovered serendipitously is to clear the sinuses of mucus from upper respiratory infections.

Alternatively, I just plain enjoy the rush of that volatile wasabi up through my nose and sinuses. You can keep the capsaicin, though.

So, yes, Dr. Sri; there really is a ‘wasabi’ receptor after all.

Cell, 124: 1269-1282, March 2006. TRPA1 mediates the inflammatory actions of environmental irritants and proalgesic agents. Bautista D, Jordt S, Nikai T, et al.