Sensory properties of citric acid: psychophysical evidence for sensitization, self-desensitization, cross-desensitization and cross-stimulus-induced recovery following capsaicin.

In a first experiment, human subjects used a bipolar scale to rate the irritant sensation elicited by 10 sequentially repeated applications of either 3 ppm capsaicin or 250 mM citric acid on one side of the dorsal surface of the tongue, at 1 min intervals (30 s inter-stimulus interval). Citric acid-evoked irritation significantly increased across trials, consistent with sensitization. With capsaicin there was a large degree of inter- and intra-individual variation in successive ratings with no overall sensitization. Following the sequential stimulation series and a 10 min rest period, self- and cross-desensitization effects were tested in a two-alternative forced choice (2-AFC) paradigm by placing either citric acid or capsaicin on both sides of the tongue and asking subjects to indicate which side of the tongue yielded a stronger irritant sensation. Subjects also gave separate intensity ratings for irritation on each side of the tongue. Capsaicin self-desensitization was confirmed, while cross-desensitization to citric acid was not observed. In addition, citric acid self-desensitization and cross-desensitization to capsaicin were observed. In a second experiment a stronger capsaicin solution (33 ppm) was applied to one side of the tongue using cotton swabs. After the burning sensation elicited by capsaicin had disappeared, citric acid was applied bilaterally and cross-desensitization was observed using the same 2-AFC and rating procedures. This was followed by repeated re-application of citric acid at 1 min intervals to the capsaicin-treated side. The irritant sensation elicited by citric acid increased significantly, indicating a 'cross-stimulus-induced recovery' from capsaicin desensitization. In a final experiment we investigated the effect of the sodium channel blocker amiloride on the perceived irritation elicited by citric acid or capsaicin. Following application of amiloride to one side of the tongue with cotton swabs, either citric acid or capsaicin was applied bilaterally and subjects asked to perform a 2-AFC and intensity ratings. Amiloride significantly, albeit weakly, reduced the irritation elicited by citric acid while it weakly but significantly enhanced capsaicin-evoked irritation. These findings are discussed in terms of involvement of vanilloid and acid-sensitive ion channels in acid-evoked irritation and pain.