How ultraviolet light touches the brain and endocrine system through skin, and why.

The skin, a self-regulating protective barrier organ, is empowered with sensory and computing capabilities to counteract the environmental stressors to maintain/restore disrupted cutaneous homeostasis. These complex functions are coordinated by a cutaneous neuro-endocrine system that also communicates in a bidirectional fashion with the central nervous, endocrine and immune systems, all acting in concert to control body homeostasis. While ultraviolet energy (UV) has played an important role in the origin and evolution of life, UV absorption by the skin not only triggers mechanisms that defend skin integrity and regulate global homeostasis, but also induces skin pathology (e.g., cancer, aging, autoimmune responses). These effects are secondary to the transduction of UV electromagnetic energy into chemical, hormonal and neural signals, defined by the nature of the chromophores and tissue compartments receiving specific UV-wavelength. UV radiation can upregulate local neuroendocrine axes with UVB being markedly more efficient than UVA. The locally induced cytokines, corticotropin releasing hormone, urocortins, POMC-peptides, enkephalins or others can be released into circulation to exert systemic effects, including activation of the central hypothalamic-pituitary-adrenal axis, opioidogenic effects and immunosuppression, independent of vitamin D synthesis. Similar effects are seen after exposure of the eyes and skin to UV, through which UVB activates hypothalamic paraventricular and arcuate nuclei and exerts very rapid stimulatory effects on the brain. Thus, UV touches the brain and central neuroendocrine system to reset body homeostasis. This invites multiple therapeutic applications of UV radiation, for example in the management of autoimmune and mood disorders, addiction, and obesity.

PMID: 29546369 DOI: 10.1210/en.2017-03230