Chili Peppers Produce a Hot Clue to the Puzzle of Pain

Scientists have discovered a chemical button that chili peppers push to cause eye-watering, burning pain.

The same button appears to help you realize you’re touching something hot.

Further work might help scientists develop new treatments for some kinds of pain, including sunburn, heartburn and rheumatism, experts said.

Chili peppers burn in your mouth because they contain a substance called capsaicin (pronounced cap-SAY-uh-sin).

Capsaicin causes pain by grabbing onto structures called receptors on the surface of nerve cells. Despite more than a decade of looking, scientists hadn’t found a capsaicin receptor until now.

The discovery is reported in today’s issue of the journal Nature by David Julius, an associate professor of cellular and molecular pharmacology at UC San Francisco, and colleagues. Julius said other capsaicin receptors probably exist, and he and his colleagues are looking for them.

Capsaicin can cause pain in the eyes, on the skin and deep inside the body, as well as in the mouth. That’s why spicy foods cause heartburn.

So with the receptor in hand, scientists may be able to find new treatments for heartburn as well other pain from internal organs, said Dr. Bill Willis, a pain expert at the University of Texas Medical Branch at Galveston, who did not participate in the new study.

Another scientist familiar with the findings, Bob LaMotte of the Yale University School of Medicine, said drugs that block the capsaicin receptor might bring relief for some forms of itching, sunburns and rheumatism.

Julius said the finding will let scientists explore whether the receptor plays any role in chronic pain from such problems as back injuries, burns and complications from diabetes. If so, scientists may be able to devise new treatments, he said.

Capsaicin can relieve pain if nerves are exposed to it continuously--it is already used in skin ointments, for example--so drugs that act at the receptor level might be good painkillers, Julius said.

The receptor is like a pipe through the surface of a nerve cell. It can open and close.

When it is exposed to capsaicin or high heat, the new research indicates, it lets electrically charged particles of sodium and calcium rush into the cell. That lets the nerve cell fire off an “ouch” message that eventually reaches the brain.

To study the receptor’s behavior, the scientists equipped human kidney cells with it and did lab tests to see how the cells reacted.

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In one set of experiments, they saw that a few seconds after these cells were heated to about 115 degrees Fahrenheit, they showed an in-rushing of calcium particles.

The cells also generated electrical currents that resembled those they produced when exposed to capsaicin.

That suggests that the capsaicin receptor helps the body feel pain from dangerously high heat, which is probably why animals have it in the first place, Julius said.

It is not clear whether the heat is sensed by the receptor or by something else in cells.

The receptor might not be involved in all heat-related pain, the researchers wrote, but Julius said he believes it will turn out to be important.