Tarantulas, chile peppers turn up the same heat

Scientists use research to understand pain

November 10, 2006|By Michael Stroh | Michael Stroh,Sun reporter

What do tarantulas and chile peppers have in common?

Hint: Think pain.

Researchers have found that the searing sting of a tarantula bite and a chile's tongue-blistering burn are both triggered by the same molecular heat sensor.

This may sound like a random bit of research, but it actually has a practical side: Scientists think it will help them piece together how nerve cells register pain - and, more important, how to squelch that pain.

"The big picture is to try to understand mechanisms of pain sensation," says biochemist David Julius of the University of California at San Francisco, who directed the research.

Julius says he was also motivated by what he calls a "curious" question: Why do some spider bites hurt?

He and colleagues rounded up venom from 22 different spiders and scorpions known for their painful bites, and then exposed nerve cells to the toxins.

One sample, obtained from a West Indies tarantula called the Trinidad chevron, reacted strongly with a receptor on the surface of nerve cells called TRPV1.

This receptor "is a molecular thermometer," says Dr. Michael Caterina, an associate professor of biological chemistry at the Johns Hopkins University School of Medicine. "Its normal job is to sense painful heat."

Found in the skin, mouth, lips and tongue, TRPV1 flips on at 107 degrees Fahrenheit. It's your ouch reflex, says Caterina, who studies the body's temperature sensors. TRPV1 tells you when your iron is ready or when a pan is too hot to handle.

But the sensor can also be chemically activated. And in 1997, Julius and his colleagues showed that it responds to capsaicin, the chemical that gives habaneros and other chile peppers their heat.

The scientists isolated a trio of tarantula toxin proteins that also set off TRPV1. When Julius and his colleagues injected the proteins into the paws of genetically engineered mice that lack the receptor, the animals barely flinched.

Normal mice, on the other hand, furiously licked their paws and exhibited other odd behavior to make it clear they were feeling the burn.

The new result adds to the growing knowledge about the body's web of environmental sensors. In the past, Julius and his collaborators pinpointed cold receptors that register the chill of ice cream and menthol, as well as heat receptors that respond to garlic and wasabi, the fiery Japanese-style mustard often used on sushi.

What makes tarantula toxin especially exciting, scientists say, is that it trips TRPV1 in a slightly different way than capsaicin.

Birds, scientists know, are immune to capsaicin and thus can chow down on chile plants without fear of an adverse reaction.

But when Julius and his colleagues tested the tarantula toxin on the bird version of TRPV1, they found it activated the receptor.

This makes evolutionary sense: Chile plants want birds to disseminate their seeds as widely as possible, whereas tarantulas don't want birds to eat them.

Caterina, who was not involved with the research, says that studying the molecular variation between capsaicin and tarantula venom could help companies devise new compounds to relieve chronic pain, a problem estimated to affect about 50 millions Americans.

"It's potentially very valuable information," he says.


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