Lack of efficacy of topical capsaicin in serotonin-induced itch

Capsaicin (CAP) has been demonstrated to be an effective topical inhibitor of cutaneous vasodilatation, pain and pruritus induced by a variety of chem ical and physical stimuli. In a previous study, we showed a significantly i nhibitory effect of topical CAP treatment on histamine-induced itch and cut aneous vascular reactions in healthy subjects compared to atopic eczema pat ients, As serotonin is proposed to play a pathophysiological role in some t ypes of pruritus (e.g. uremic and hepatic pruritus) and CAP has been descri bed to be successful in hemodialysis-related pruritus, we investigated the a nti pruritic effect of topical CAP on serotonin-induced reactions in 10 h ealthy volunteers in comparison to untreated skin (UPS) and placebo substan ce (vehicle)-treated skin (VS). On the first day, serotonin iontophoresis w as performed in untreated skin. One week later, the treatments started, usi ng either CAP 0.05% liniment or a placebo liniment (vehicle) 3 times daily over a 5-day period. On day 6, serotonin was applied by iontophoresis withi n the pretreated skin. After another 1-week break, the treatments were perf ormed vice versa on the corresponding infrascapular region. Weal and flare areas were planimetrically evaluated. Itch sensations were documentated by the volunteer on a scale over a 24-min followup period. The examination als o comprised alloknesis, which stands for induction of perifocal sensations by usually non-itching stimuli. In CAP-treated skin, serotonin-induced whea ls were significantly larger post-application compared to non-pretreated sk in. Wheals were significantly larger in VS than in UPS. Comparison of serot onin-induced flares in the different study arms did not reveal any signific ant differences. Itch sensations were not significantly reduced by topical CAP application. The areas of alloknesis were smaller in capsaicin-treated skin compared to VS and UPS, but did not reach significant value. In conclu sion, topical CAP application is not effective in serotonin-induced itching in healthy volunteers. Serotonin is most unlikely to play a role in the me chanism of action of CAP. Copyright (C) 2000 S. Karger AG. Basel.