L7 SEROTONINERGIC AND MELATONINERGIC SYSTEMS IN THE SCALP
Slominski A1, Tobin DJ2, Fischer TW1,3, Wortsman J4
1Department of Pathology, University of Tennessee, Memphis, TN, USA
2Department of Biomedical Sciences, University of Bradford, United Kingdom
3Department of Dermatology, Friedrich-Schiller-University, Jena, Germany
4Department of Medicine, Southern Illinois University, Springfield, IL, USA

Mammalian skin can produce serotonin and transform it into melatonin. Pathways for the biodegradation of serotonin and melatonin have also been characterized in human and rodent skin. In situ analyses of the human scalp have shown the expression of main enzymes of this pathway as well as its final metabolic products serotonin and melatonin. Since hair cycle dependence has been demonstrated for the activity of the cutaneous serotoninergic system in the C57BL/6 mouse skin, serotonin and/or melatonin synthesis and metabolism may be related to hairy skin function (phase of the hair cycle). Functional receptors for serotonin and melatonin are indeed expressed in keratinocytes, melanocytes and fibroblasts and these mediate phenotypic actions on cellular proliferation and differentiation. Melatonin receptors have also been detected in human scalp and in the hair follicle compartment. These findings are in agreement with experimental data implicating melatonin in hair growth cycling, pigmentation and melanoma control. In addition, melatonin exerts receptor-independent effects including the activation of pathways protective of oxidative stress and the modification of cellular metabolism. Serotonin in turn is known to have other functions in the skin e.g. pro-edema, vasodilatory, pro-inflammatory and pruritogenic. Thus, it is most probable that these cutaneous serotoninergic and melatoninergic systems would have considerable selectivity of action enabling intra-, auto- or paracrine mechanisms to influence skin function in a highly compartmentalized manner. As for their role in the epidermis, the cutaneous serotoninergic and/or melatoninergic systems could counteract or buffer external (environmental) or internal stresses to preserve the biological integrity of the organ and maintain its homeostasis. It is possible that topically applied melatonin could be used in skin protection against UV induced damage. Moreover, the hair follicle may be an important target for pharmacological intervention by melatonin. In this context, melatonin or its derivatives could be used in the management of alopecia and/or for regulation of hair growth and pigmentation.