Scratch induction in the rat by intradermal serotonin: a model for pruritus.

Eight substances (histamine, compound 48/80, kallikrein, trypsin, papain, substance P, serotonin and platelet activating factor) were injected intradermally (volume 50 microl) into the rostral back (neck) of rats in order to establish an animal model for peripherally elicited pruritus. While serotonin induced excessive scratching at the site of injection, the other substances were weak or inactive. The dose-response relationship of serotonin was sigmoid, EC50=2.1 mg/ml (95% confidence interval: 1.0 to 4.3 mg/ml). Injections of serotonin 1 mg/ml into the caudal back elicited no scratching at all, i.e. neither at the site of injection nor elsewhere, so the experiment indicated no systemic effect of serotonin 1 mg/ml intradermally. Scratching was probably elicited histamine-independently, since histamine itself did not elicit scratching. The intra- and inter-observer variations were 3-4%. We conclude that serotonin is a reproducible local pruritogen eliciting scratching in the rat. The model may be useful in research and development of topical antipruritics of the nonhistaminic type as well as for various other purposes in pruritus research.