Enhancement of responses to sequential presentation of oral chemical irritants.

The irritative flavor compounds, capsaicin and piperine, were presented in sequence to the oral cavity of human subjects and rated for perceived intensity of irritation. Marked increases in perceived irritation were reported when the second-presented irritant differed from the first, as compared with control conditions in which the same irritant was repeated. This cross-enhancement is explained most simply by recruitment of additional receptors or fiber types, and implies a greater degree of specificity (narrowness of tuning) of trigeminal receptors than previously suggested by desensitization studies.