Time course of self-desensitization of oral irritation by nicotine and capsaicin.

Nicotine contacting mucous membranes elicits irritation that decreases with repeated exposures (self-desensitization). We investigated the time course of nicotine self-desensitization and compared it with that of capsaicin. Nicotine (300 mM, 10 microl) was applied to one-half of the dorsal tongue and vehicle to the other. Following a rest period ranging from 0.5 to 48 h, nicotine (5 microl) was reapplied to each side of the tongue and subjects indicated on which side they experienced stronger irritation and separately rated the intensity of the sensation on each side. After intervals of 0.5, 1, and 24 h, a significant majority of subjects chose the vehicle-treated side as having stronger irritation and assigned significantly higher intensity ratings to that side, indicating self-desensitization. The effect was not present after 48 h. By comparison, 10 parts per million (ppm) (33 microM) capsaicin induced significant self-desensitization at 1 but not 24 h, whereas a higher concentration of capsaicin (100 ppm, 330 microM) induced significant self-desensitization at intervals of 1, 24, and 48 h. These results indicate that initial exposure to nicotine or capsaicin can markedly attenuate irritant sensations elicited by subsequent exposure to these irritants hours to days later.