Menthol desensitization of capsaicin irritation. Evidence of a short-term anti-nociceptive effect.
Evidence is presented of a short-term antinociceptive effect of menthol that was discovered in the course of investigating menthol's potential to sensitize the mouth to capsaicin. Previous research had shown that treating the tongue with menthol 15 min before exposure to capsaicin could enhance the irritancy of capsaicin, and we wished to learn if this effect would increase as the time between exposure to menthol and capsaicin decreased. We found instead that when capsaicin followed menthol by only 3.5 min, or when it was presented in mixture with menthol for 2-3 min, sensory irritation was reduced rather than enhanced. We examined the duration of this apparent crossdesensitization in a second experiment by varying the delay between exposure to menthol and a block of three consecutive capsaicin stimuli. Cross-desensitization tended to decline as the interstimulus interval (ISI) increased to 5 min, and even when desensitization was maximal, it was significant only for the first of the three capsaicin stimuli. In the final experiment we investigated how menthol self- and cross-desensitization can influence the perception of menthol-capsaicin mixtures. During a series of five, 90-s stimulations, self- and cross-desensitization became evident at the beginning of the second exposure, but the effect on mixture intensity again diminished rapidly as stimulation continued. We infer from these results that method can transiently desensitize capsaicin-sensitive fibers, but that exposure to capsaicin rapidly overrides the effect. The implications these findings have for menthol's potential as a topical analgesic are discussed.