Eugenol and carvacrol are derived from the oils of clove and oregano, respectively, and impart warming, numbing, and irritant sensations in the oral cavity by interacting with the heat-sensitive ion channel TRPV3. Here we show that these agents induce oral irritation in a manner that exhibits self-desensitization and cross-desensitization of capsaicin-evoked irritancy, and enhancement of warmth and heat pain, but not cold sensation.
Eugenol and carvacrol, from the spices clove and oregano, respectively, are agonists of TRPV3, which is implicated in transduction of warmth and possibly heat pain. We investigated the temporal dynamics of lingual irritation elicited by these agents, and their effects on innocuous warmth and heat pain, using a half-tongue method in human subjects. The irritant sensation elicited by both eugenol and carvacrol decreased across repeated applications at a 1-minute interstimulus interval (self-desensitization) which persisted for at least 10 minutes. Both agents also cross-desensitized capsaicin-evoked irritation. Eugenol and carvacrol significantly increased the magnitude of perceived innocuous warmth (44 °C) for >10 minutes, and briefly (<5 minutes) enhanced heat pain elicited by a 49 °C stimulus. Similar albeit weaker effects were observed when thermal stimuli were applied after the tongue had been desensitized by repeated application of eugenol or carvacrol, indicating that the effect is not due solely to summation of chemoirritant and thermal sensations. Neither chemical affected sensations of innocuous cool or cold pain. A separate group of subjects was asked to subdivide eugenol and carvacrol irritancy into subqualities, the most frequently reported being numbing and warmth, with brief burning, stinging/pricking, and tingle, confirming an earlier study. Eugenol, but not carvacrol, reduced detection of low-threshold mechanical stimuli. Eugenol and carvacrol enhancement of innocuous warmth may involve sensitization of thermal gating of TRPV3 expressed in peripheral warm fibers. The brief heat hyperalgesia following eugenol may involve a TRPV3-mediated enhancement of thermal gating of TRPV1 expressed in lingual polymodal nociceptors.
Department of Neurobiology, Physiology and Behavior, University of California, Davis, CA, USA
*Corresponding author. Address: Department of Neurobiology, Physiology and Behavior, University of California, Davis, 1 Shields Avenue, Davis, CA 95616, USA. Tel.: +1 530 752 7767; fax: +1 530 752 5582.
1Current address: Department of Neurosurgery, The Johns Hopkins University, Baltimore, MD, USA.
Submitted March 23, 2013; revised May 29, 2013; accepted June 17, 2013.