SENSITIZATION AND DESENSITIZATION TO CAPSAICIN AND MENTHOL IN THE ORAL CAVITY - INTERACTIONS AND INDIVIDUAL-DIFFERENCES

It was reported in a recent study that, like capsaicin, menthol is cap able of producing a desensitization to sensory irritation in the oral cavity. Whereas capsaicin is known to be able to cross-desensitize wit h other chemical irritants, no such information exists for menthol. To address this question, the first experiment was designed to reveal wh ether cross-desensitization would occur between menthol and capsaicin. After a pretest on the tongue tip in which subjects rated the intensi ty of irritation and cold produced by 3.5 ppm capsaicin or 0.3% l-ment hol, five samples of the same stimuli were sipped and swished at 1-min intervals for 5 min. Fifteen minutes later subjects were tested on th e tongue tip with either capsaicin or menthol. The results 1) confirme d self-desensitization for both chemicals, 2) demonstrated cross-desen sitization of menthol by capsaicin, and 3) revealed cross-sensitizatio n of capsaicin by menthol. This series of outcomes suggests that menth ol produces much of its sensory irritation via capsaicin-sensitive pat hways, but that it excites and/or desensitizes those pathways via diff erent mechanisms than does capsaicin. Analysis of the individual data revealed large differences in sensitization and desensitization that w ere significantly correlated across chemicals, which suggests the poss ibility that the perceptual response to repeated exposures to irritant s may be idiosyncratic. Contrary to earlier findings, the first experi ment also revealed apparent self- and cross-desensitization of the men thol sensation of coolness. The latter outcome was investigated in a s econd experiment in which the effect of capsaicin desensitization on t he perception of physical as well as chemical (menthol) cooling was me asured when the stimuli were presented as oral rinses. No desensitizat ion was found for either form of stimulation, which implied the appare nt desensitization of coolness in Experiment 1 may have been due to th e difficulty of discriminating sensations of cold from sensations of c hemical irritation. The overall findings are discussed in terms of the complex sensory and perceptual interactions that take place within th e chemesthetic modality.