The pungency of wasabi, horseradish, brussels sprouts, and mustards comes from compounds called isothiocyanates. Applying those same compounds to a person's skin can cause pain and inflammation.
Isothiocyanates activate pain-signaling neurons by triggering a cell-surface protein that lets ions into cells, David Julius of University of California, San Francisco and his colleagues report in the Jan. 15 Nature. Julius' group had previously shown that capsaicin, the compound that gives chili peppers their spicy kick, activates a different but related ion channel (SN: 11/8/97, p. 297). Curiously, the active ingredient in marijuana triggers the same ion channel that the isothiocyanates do, the researchers found.