The cutaneous serotoninergic/melatoninergic system: securing a place under the sun

It was recently discovered that mammalian skin can produce serotonin and transform it into melatonin. Pathways for the biosynthesis and biodegradation of serotonin and melatonin have been characterized in human and rodent skin and in their major cellular populations. Moreover, receptors for serotonin and melatonin receptors are expressed in keratinocytes, melanocytes, and fibroblasts and these mediate phenotypic actions on cellular proliferation and differentiation. Melatonin exerts receptor-independent effects, including activation of pathways protective of oxidative stress and the modification of cellular metabolism. While serotonin is known to have several roles in skin - e.g., pro-edema, vasodilatory, proinflammatory, and pruritogenic - melatonin has been experimentally implicated in hair growth cycling, pigmentation physiology, and melanoma control. Thus, the widespread expression of a cutaneous seorotoninergic/melatoninergic system(s) indicates considerable selectivity of action to facilitate intra-, auto-, or paracrine mechanisms that define and influence skin function in a highly compartmentalized manner. Notably, the cutaneous melatoninergic system is organized to respond to continuous stimulation in contrast to the pineal gland, which ( being insulated from the external environment) responds to discontinuous activation by the circadian clock. Overall, the cutaneous serotoninergic/melatoninergic system could counteract or buffer external ( environmental) or internal stresses to preserve the biological integrity of the organ and to maintain its homeostasis.