THE SENSORY RESPONSE TO CAPSAICIN DURING REPEATED TOPICAL EXPOSURES -DIFFERENTIAL-EFFECTS ON SENSATIONS OF ITCHING AND PUNGENCY

Changes in sensory irritation were measured during repeated topical ex posures to capsaicin over 2 days. The perceived intensities of itching and pungent sensations, predominantly burning and stinging/pricking, were assessed every 60 sec during 5 applications of capsaicin at inter -stimulus intervals (ISI) of 90 min (Exp. 1) or 15 min (Exp. 2) and in follow-up tests 24 h later. Psychophysical measurements were obtained with a hand-held dynamometer in conjunction with the method of magnit ude production. When the ISI was 90 min, itching and pungency were bot h significantly reduced (i.e., desensitization occurred) by the fifth exposure; however, the reduction occurred more rapidly and dramaticall y for itching. After 24 h, desensitization remained significant only f or itching. When the ISI was 15 min, the sensations on day 1 first int ensified in a manner consistent with sensitization, then declined in a manner consistent with desensitization; compared to pungency, itch ex hibited less sensitization and more desensitization. On day 2, overall intensity was less for both categories of sensation, primarily becaus e of a reduction in sensitization. Marked individual differences were observed in the overall sensitivity to capsaicin, the time course of s ensation, the susceptibility to capsaicin-induced itch, and the rate a nd duration of sensitization and desensitization. The results are disc ussed in terms of current hypotheses about the sensory mechanisms that underlie chemically induced itch and the use of capsaicin as a topica l analgesic and antipruritic.