Oral irritant properties of piperine and nicotine: Psychophysical evidence for asymmetrical desensitization effects

Using a bipolar rating scale, human subjects rated the intensity of irritation sensation evoked by repeated application of piperine (75 p.p.m.) or nicotine (0.12%) to one side of the dorsal surface of the tongue. The intensity of irritation elicited by repeated application of piperine significantly increased, while irritation elicited by repeated nicotine significantly decreased. We additionally tested if nicotine or piperine desensitized the tongue. After either piperine or nicotine was repeatedly applied to one side of the tongue, a 5 or 10 min rest period ensued, followed by re-application of piperine or nicotine to both sides of the tongue. Subjects were asked to choose which side of the tongue gave rise to a stronger irritation in a two-alternative forced choice (2-AFC) paradigm. In addition, they gave separate ratings of the intensity of irritation on the two sides of the tongue. When piperine was applied bilaterally after unilateral pretreatment with piperine and a 10 min rest period, subjects consistently chose the non-pretreated side to yield stronger irritation a nd assigned significantly higher ratings to that side, indicative of piperine self-desensitization. A similar self-desensitization effect was found when bilateral application of nicotine followed unilateral treatment with nicotine and a 5 min rest period. Unilateral treatment with piperine also reduced nicotine-evoked irritation on the pretreated side (cross-desensitization), but treatment with nicotine did not affect piperine- evoked irritation. This asymmetrical cross-desensitization pattern is similar to that observed between capsaicin and nicotine and constitutes an additional similarity between piperine and capsaicin.