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This is DAY TO DAY, and this just in from the NPR science desk. Researchers have figured out why eating a dollop of wasabi makes you feel as those your head might explode. Wasabi is the green paste served with sushi. Lately, real wasabi's become rare. Some restaurants have switched to horseradish and green food coloring. No matter, the kick remains.

Scientists now understand how wasabi causes that almost pain, but as NPR's Joe Palca reports, they're not clear why people would subject themselves to it.

JOE PALCA reporting:

I'm at the Sushi Tower(ph) restaurant in Washington, D.C., and chef Nabou(ph) has just handed me a big glob of wasabi.

Chef NABOU (Sushi Tower): Don't take too much of it. Little piece.

PALCA: Whoa, that's hot!

What's happening is, compounds inside the wasabi called isothyocyanates are stimulating molecules called TRP receptors on the nerve cells in my mouth and tongue. These receptors play a key role in why we feel pain. They are they first step in a cascade of events that leads to a signal to the brain meaning ouch.

David Julius is one of the scientists studying the TRP receptors. He's at the University of California San Francisco. Julius believe that one of these TRP receptors, TRPA1, and only TRPA1, was the receptor that signaled the pain from isothyocyanates, so he bred some mice that didn't have that receptor.

Mr. DAVID JULIUS (Scientist, University of California San Francisco): What we found in this mouse is that it's completely deficient for its ability to respond to these pungent compounds. So this tells us that this TRPA1 is essential for the ability of the sensory nervous system to respond to these compounds.

PALCA: And it turns out that plants make a variety of compounds that stimulate TRP receptors.

Mr. JULIUS: Capsaicin from hot chili peppers or menthol from mint leaves or in this case, a series of compounds called isothyocyanate compounds from mustard plants.

PALCA: Presumably the reason plants started making these compounds was to make them taste bad, so humans or other animals wouldn't eat them. Not a complete success, but it was worth a try. If the TRP receptors' only role in life was to respond to unpleasant compounds found in nature, they wouldn't be all that interesting to study. But Julius has evidence that they're involved in a lot of pain from inflammation, like the kind you get from sunburn and arthritis.

Neuroscientist Steve McMahon agrees TRP receptors may be broadly important in pain. He's at King's College London.

Mr. STEVE McMAHON (Neuroscientist, King's College London): If they were only responsive to mustard oil, for instance, you'd have to say, Great, well, a drug that blocks TRPA1 would be a great analgesic for all those patients who happen to expose themselves to mustard oil. Well, that's not a big clinical problem.

PALCA: He says a better understanding of these TRP receptors could lead to a new class of painkillers for a variety of conditions. Of course, there are some people who do expose themselves to one of the ingredients in mustard oil, the isothyocyanates that are in horseradish or wasabi. Wasabi does cause pain, yet people gobble it up.

Part of the reason veteran sushi eaters don't suffer so much from wasabi's kick is that over time their TRPA1 receptors get less sensitive. Some food scientists think people eat hot foods to show off or because they get an endorphin rush from the pain, and a certain sensation of heat or even burning may enhance the flavor of food. Or, as perverse as it may sound, Steve McMahon says that people may just like pain.

Ms. McMAHON: Many people get some pleasure from mildly pain producing substances, and of course a subset of the population, they seem to get quite a lot of sexual pleasure as well. I mean, so that the associations of injury can give rise to pleasure, yes.

PALCA: Precisely why is still a matter for further inquiry. Joe Palca, NPR News, Washington.

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