Hot stuff: a receptor for spicy foods.

For people with painful memories of biting down on a chili pepper or touching the side of a boiling pot, it's probably cold comfort that scientists have discovered a cell surface protein that links the two burning sensations. Yet the finding may lead to help someday for millions of people desperate for pain relief.

Both extreme heat and capsaicin, the agent that puts the hot in hot peppers, trigger pain-sensing nerves by activating a cell surface protein, or receptor, that allows calcium ions to rush into the cells, David Julius of the University of California, San Francisco and his colleagues report in the Oct. 23 Nature.

As aficionados of spicy food can appreciate, scientists have wondered whether capsaicin's fiery taste had anything to do with how the body normally senses heat. "if you put capsaicin on your skin, you'll feel a tingling and a burning," notes Julius.

To identify receptors that recognize capsaicin's presence and therefore trigger the burning feeling, researchers isolated genes active in sensory nerve cells that connect to the spinal cord. They added small groups of the genes to non-neuronal cells, observing which ones then took in calcium when exposed to capsaicin. Eventually, the investigators closed in on one gene that made the cells sensitive to the spicy compound.

This gene encodes a kind of protein called an ion channel, and capsaicin isn't the only thing that will open it. Increasing cell temperature from 22 [degrees] C to 45 [degrees] C also activates the receptor, Julius' group discovered.

Capsaicin interests physicians because, paradoxically, prolonged exposure to it can relieve pain (SN: 11/14/92, p. 333). Scientists remain uncertain whether this analgesic action results because pain nerves gradually become less sensitive or because an overabundance of calcium ions kills them.

Identifying capsaicin's receptor may help researchers design an improved form of the analgesic that would help people with chronic pain by inhibiting or destroying pain-sensing nerves. "What would be ideal is, instead of activating this receptor [as capsaicin does], to block it painlessly," says David E. Clapham of Harvard Medical School in Boston.