Researchers have discovered that the burinng pain of arthritis is similar to the pain associated with eating chili peppers. “The receptor activated by chili peppers in the mouth and other tissues also increases in the terminals of sensory neurons in the skin after inflammation, and this contributes to pain hypersensitivity,” says Clifford Woolf, director of the Neural Plasticity Research Group in the Department of Anesthesia and Critical Care at Massachusetts General Hospital (MGH). A receptor is a protein that transports a chemical signal into a cell. Woolf and study lead author Ru-Rong Ji, also of the MGH Neural Plasticity Research Group, found that the increased production of the receptor following inflammation is mediated by a signal molecule called p38, located within sensory neurons. The chili pepper receptor responds to capsaicin, the chemical that is responsible for the “hot” in peppers. It also responds to actual heat and to low pH, a condition that occurs with inflammation. “With these findings, we’re starting to understand why patients with arthritis or other inflammatory conditions are likely to have increased pain and sensitivity to heat,” says Woolf, who also is Richard J. Kitz Professor of Anaesthesia Research at Harvard Medical School.