Capsaicin as a probe of the relationship between bitter taste and chemesthesis

Previous studies have indicated that capsaicin, traditionally considered to be a pure chemesthetic stimulus, can evoke a bitter taste and might also cross-desensitize the tastes of some bitter and sour tastants. The purpose of the present study was to investigate further the scope and nature of capsaicin's effects on bitter taste. In Experiment 1, subjects rated the taste and burning/stinging of QSO₄ (0.32 and 1.0 mM), saccharin (1.0 and 3.2 mM), urea (3.2 and 10 M), MgCl₂, (0.18 and 0.56 M), PROP (0.32 mM), and sucrose (0.32 and 1.0 M) applied to the tongue tip with cotton swabs before and after 10 applications of 300 μM capsaicin. Capsaicin initially evoked a weak bitterness in some subjects that quickly diminished over repeated exposures. Following capsaicin treatment, the bitterness of QSO₄, urea, MgCl₂, and PROP was reduced, as was the burning sensation produced by MgCl₂ and urea. In Experiment 2, we tested 29 subjects in the circumvallate (CV) region of the tongue using the same general procedure. Capsaicin induced a weak but persistent bitterness in a subset of subjects but failed to desensitize its own bitterness or that of any other tastant. Overall, the results confirm that capsaicin can both stimulate and desensitize bitter taste, but in amounts that vary for different bitter stimuli and between the front and back of the tongue. Possible reasons for these regional and stimulus-dependent differences are discussed.