Cross-desensitization of capsaicin-evoked oral irritation by high but not low concentrations of nicotine in human subjects.
We previously showed that sequential application of a relatively low nicotine concentration (7.4 mM) to the tongue, followed by a rest period, induced self-desensitization, i.e. a reduction of oral irritation elicited by subsequent nicotine, but not cross-desensitization of capsaicin-evoked irritation. We presently investigated if cross-desensitization of capsaicin-evoked irritation might be induced by higher concentrations of nicotine. Nicotine (74 or 300 mM) was applied once, unilaterally to the tongue of human subjects. When the irritant sensation had completely subsided, capsaicin (33 microM) was applied bilaterally. In a 2-alternative forced-choice (2-AFC) test, subjects indicated which side of the tongue had a stronger irritant sensation. They additionally rated the intensity of irritation on each side separately. Pretreatment with 74 mM nicotine did not induce cross-desensitization, since no significant difference was observed in intensity ratings or in choice of the treated vs. non-treated side in the 2-AFC. However, 300 mM nicotine did induce cross-desensitization, since a significant majority of subjects chose the non-treated side as having a stronger capsaicin-evoked irritant intensity and assigned significantly higher ratings to that side. These psychophysical findings are discussed in terms of possible neural mechanisms of desensitization.
Section of Neurobiology, Physiology and Behavior, University of California, Davis, CA 95616, USA.
J M
JM
Dessirier
H K
HK
Chang
M
M
O'Mahony
E
E
Carstens
D000328:Adult; D002211:Capsaicin / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009538:Nicotine / Q000378:metabolism / Q000494:pharmacology*; D009619:Nociceptors / Q000187:drug effects / Q000378:metabolism; D010146:Pain / Q000139:chemically induced* / Q000188:drug therapy* / Q000503:physiopathology; D014059:Tongue / Q000187:drug effects* / Q000473:pathology / Q000503:physiopathology
D009538:Nicotine; D002211:Capsaicin