Oral irritation by sodium chloride: sensitization, self-desensitization, and cross-sensitization to capsaicin.

Psychophysical methods were used to investigate the irritant sensory properties of concentrated NaCl. The first experiment investigated potential sensitization and desensitization properties. Subjects rated the intensity of the irritation elicited by 10 successive applications of 5 M NaCl on one side of the dorsal surface of the tongue. The mean irritant sensation increased significantly across trials, consistent with sensitization. To test for self- and cross-desensitization effects of unilateral sequential stimulation with NaCl followed by a 10-min rest period, either 5 M NaCl or 10 microM capsaicin was applied bilaterally. In a two-alternative forced-choice (2-AFC) test, subjects indicated which side of the tongue had a stronger irritant sensation. They also rated the intensity of irritation on each side separately. When NaCl was applied bilaterally, the side not previously receiving NaCl was chosen as stronger by a significant majority of subjects and was given significantly higher intensity ratings, consistent with self-desensitization. In contrast, when capsaicin was applied bilaterally, the side that had previously received sequential NaCl was perceived as having a significantly more intense irritation, consistent with cross-sensitization. In a second experiment, the effect of amiloride on NaCl-evoked irritation was studied. One side of the tongue was treated with 1 mM amiloride, after which 5 M NaCl was applied bilaterally and subjects performed the same 2-AFC and rating procedures. Since amiloride significantly reduced the intensity of the irritant sensation, the contribution of amiloride-sensitive ionic currents or the Na+/H+ exchange pump (NHE) are suggested as possible transduction mechanisms in lingual nociceptors mediating NaCl-evoked oral irritation.