Oral irritation by mustard oil: self-desensitization and cross-desensitization with capsaicin

We investigated the temporal pattern of oral irritation elicited by sequential application of mustard oil (allyl-isothiocyanate), and whether it exhibits self-desensitization and cross-desensitization with capsaicin. Mustard oil (0.125%, 40 micro l) was sequentially applied to one side of the tongue at 1 min intervals, and subjects rated the intensity of the irritant sensation elicited by each stimulus. Ratings successively declined across trials, indicating desensitization. In contrast, sequential application of capsaicin (10 ppm) elicited irritation that increased in intensity across trials (sensitization). To test for self-desensitization by mustard oil, a 10 min hiatus was imposed following the series of unilateral mustard oil stimuli, after which mustard oil was applied to both sides of the tongue. In a two-alternative forced-choice paradigm, subjects chose which side had stronger irritation and also independently rated the irritant intensity on each side. A significant majority of subjects chose the side not previously receiving mustard oil as more intense, and assigned significantly higher intensity ratings to that side, indicating self-desensitization. In two additional sessions, the same paradigm was used to show mustard oil cross-desensitization of irritation elicited by capsaicin, and capsaicin cross-desensitization of irritation from mustard oil. In a final session, sequential application of mustard oil at faster (20 s) intervals initially evoked a sensitizing pattern followed by desensitization. The temporal patterns of oral irritation exhibited by mustard oil, and its reciprocal cross-desensitization with capsaicin, are similar to those of menthol and nicotine.