Responses to repeated oral irritation by capsaicin, cinnamaldehyde and ethanol in PROP tasters and non-tasters.
Both increases (sensitization) and decreases (desensitization) in oral irritation have been reported in response to repeated short-term stimulation by compounds such as capsaicin, zingerone and menthol. It is unclear why one irritant would show sensitization and another desensitization, and this is further complicated by substantial inter-individual variation in response patterns. These variations may be the result of individual differences such as that represented by sensitivity to 6-n-propylthiouracil (PROP), which has been associated with variation in the overall intensity of irritation. In addition, comparisons between irritants have almost always involved inter-study comparisons, entailing different subject groups and frequently different methods. In the studies reported here, responses to three irritants-capsaicin, cinnamaldehyde and ethanol-were examined as a function of PROP taster status. A common core of subjects also received all three irritants, allowing an assessment of the extent to which different response patterns between irritants seen previously were the result of different properties of the irritants themselves. Over a series of ten stimuli presented at 1 min intervals, PROP taster status differentiated subject responses on the basis of overall intensity, but not the pattern of responses over repeated stimulation. The group response to ethanol and cinnamaldehyde was desensitization, a pattern also shown by most of the individual subjects. In contrast, the group response to capsaicin was neither clear sensitization nor desensitization, reflecting much greater individual variability in response patterns. It is suggested that the time course to a single irritant stimulus largely determines between irritant response variations, while the inter-stimulus interval (ISI) used for a given irritant will have critical values for showing predominantly sensitization or desensitization.