Peripheral and central mediators of itch.

Since the majority of itching skin diseases are inflammatory or allergic, it has been assumed that release or activation of inflammatory mediators, stimulating the itch receptors, play an essential role in the pathophysiology of itch. In this review some of the possible mediators are discussed. Histamine induces itch upon intradermal injection, but urticaria is the only itching dermatosis which is significantly relieved by antihistamines. Serotonin is much weaker than histamine in provoking itch upon intradermal injection. Serotonin acting in synergism with prostaglandins may cause itch in polycythaemia vera. Neuropeptides release histamine from skin mast cells, but it remains to be determined whether neurogenic peptides are responsible for clinical pruritus. Prostaglandins enhance pruritus induced by intradermal histamine (and serotonin) but are weak pruritogens per se. Lymphocytes are present in many itching skin diseases and it could be assumed that lymphokines are involved in the pathogenesis of itch. Supporting this theory is the finding that ciclosporin A, an inhibitor of lymphokine production, reduces itch in atopic dermatitis. Central mechanisms are essentially unknown, but there are indications that opioid peptides might be involved in the central transmission of itch.