The skin as a mirror of the soul: exploring the possible roles of serotonin.

Serotonin (5-hydroxytryptamine; 5-HT) is an important mediator of bidirectional interactions between the neuroendocrine system and the skin. The rate of synthesis of 5-HT from l-tryptophan can be enhanced by brain-derived neuronal growth factor, cytokines, exposure to ultraviolet light and steroids. The major source of 5-HT in the skin are platelets, which, upon aggregation, release this biogenic amine. Moreover, the epidermal and dermal skin express the enzymes required for the transformation of tryptophan to 5-HT, and certain skin cells, such as melanocytes, have been demonstrated to produce 5-HT. In addition, rodent mast cells produce 5-HT, but human mast cells have not yet been fully examined in this respect. Skin cells express functionally active, membrane-bound receptors for 5-HT, as well as proteins that transport 5-HT. The interactions of 5-HT with these various proteins determines the nature, magnitude and duration of serotonergic responses. The immune and vasculature systems in the skin are traditional targets for bioregulation by 5-HT. Moreover, recent findings indicate that keratinocytes, melanocytes and dermal fibroblasts also respond to this amine in various ways. Thus, mammalian skin is both a site for the production of and a target for bioregulation by 5-HT. This indicates that agonists and antagonists directed towards specific 5-HT receptors could be useful in connection with treatment of skin diseases. Based on our increasing knowledge concerning these receptors and their plasticity, future research will focus on the development of serotonergic drugs that exert metabotrophic effects on the cells of the skin without affecting the central nervous system.