With the dangers of ultraviolet light exposure so well recognized, why has it been so hard to convince people to avoid time in the sun? A new study from HMS investigators at Massachusetts General Hospital bolsters one theory: ultraviolet (UV) light is addictive.
Using a mouse model, the scientists found that chronic UV exposure raises circulating levels of beta-endorphin and that UV-habituated animals exhibit withdrawal symptoms if beta-endorphin activity is blocked. The study, led by David Fisher, the Edward Wigglesworth Professor of Dermatology at HMS and Mass General, was reported June 19 in Cell.
Part of the skin’s natural response to UV light is the production of a protein called POMC, which is then clipped into several fragments, one of which induces production of the pigment melanin. Processing of a different segment of POMC leads to the generation of beta-endorphin in the skin. Fisher’s study investigated whether this UV-induced beta-endorphin produces opioid-like effects such as pain relief and dependency. The study also examined whether the pathway mediating these effects is initiated by the production of endorphin in the skin.
By exposing the test animals to a daily dose of UV light calculated to induce tanning but not burning of the animals’ skin, the scientists found that, after one week, the levels of beta-endorphin in the animals’ blood rose significantly, remained elevated during the exposure period, then gradually returned to normal after UV exposure was discontinued. Tests conducted at regular intervals during the study period showed that the UV-treated animals were less responsive to light touch or temperature changes than a control group with no UV exposure. The higher the animals’ beta-endorphin levels, the less sensitive they became. But administration of naloxone, which broadly blocks opioid-pathway activity, returned skin sensation to normal in the UV-treated animals.
In UV-habituated animals, naloxone treatment also produced such classic symptoms of opioid withdrawal as trembling, shaking, and teeth chattering. In contrast, a strain of mice in which POMC production was selectively blocked or that lacked the beta-endorphin gene altogether exhibited none of the responses or symptoms seen in UV-exposed normal mice, confirming the presence of a UV-activated opioid pathway in the skin.