Capsaicin sensitization and desensitization on the tongue produced by brief exposures to a low concentration

The intensity of sensations of burning and stinging produced by repeated exposures to capsaicin (at a nominal concentration of 3 ppm) was measured on a localized area of the tongue as a function of both the number of exposures and the time between them. It was discovered in the initial experiment that stimulation at the rate of 1/min (for up to 25 min) resulted in a monotonic increase in the intensity of burning sensations in a manner consistent with the phenomenon of sensitization. However, the insertion of a 15-min delay in stimulation resulted in a reduction in the intensity of the sensations produced by further stimulation, i.e., desensitization occurred. Desensitization was statistically significant even after exposure to as few as 5 stimuli prior to the delay. A subsequent experiment established that the minimum delay necessary to produce desensitization was between 2.5 and 5 min. Hence, paradoxically, the sensory response to capsaicin on the tongue increased as stimulation continued, then decreased after stimulation had ceased.