Researchers from Massachusetts General Hospital (MGH) report that chronic ultraviolet (UV) light exposure raises circulating levels of beta-endorphin in mice and that UV-habituated mice show withdrawal symptoms if beta-endorphin activity is blocked. In other words, spending time in the sun can be addictive.
"Our study identified an organic pathway encoded in skin whereby UV radiation causes the synthesis and release of beta-endorphin and produces opiate-like effects, including addictive behavior," says David E. Fisher, M.D., Ph.D., chair of dermatology and director of the Cutaneous Biology Research Center (CBRC) at MGH, who led the study. "This provides a potential explanation for the 'sun seeking' behavior that may underlie the relentless rise in most forms of skin cancer."
The team’s research ("Skin β-Endorphin Mediates Addiction to UV Light") appears in Cell.
Several studies, particularly those enrolling individuals who use indoor tanning facilities, have found evidence of addiction-like behavior in frequent tanners. For example, frequent tanners were somehow able to tell the difference between tanning beds using UV radiation and those delivering non-UV light. Other studies found that administration of an opioid blocker produced withdrawal-like symptoms in frequent tanners, implying but not proving that something had been regularly activating opioid pathways.
Part of skin's natural response to UV light is production of the POMC protein, which is then clipped into several smaller fragments, one of which induces production of the pigment melanin. Processing of another segment of POMC leads to generation of beta-endorphin in the skin. The current study was designed to investigate 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.
The investigators delivered a daily dose of UV light—equivalent to the exposure of fair-skinned humans to 20 to 30 minutes of midday Florida sun—on the shaved backs of a group of mice for six weeks. The dose was calculated to induce tanning but not burning of the animals' skin. Within a week of the first UV exposure, the animals' blood beta-endorphin levels rose significantly, remaining elevated during the study period and gradually returning 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 would broadly block opioid-pathway activity, returned skin sensation back to normal in the UV-treated animals.
"These UV-induced nociceptive and behavioral effects were absent in β-endorphin knockout mice and in mice lacking p53-mediated POMC induction in epidermal keratinocytes," wrote the investigators. "Although primordial UV addiction, mediated by the hedonic action of β-endorphin and anhedonic effects of withdrawal, may theoretically have enhanced evolutionary vitamin D biosynthesis, it now may contribute to the relentless rise in skin cancer incidence in humans."
"It is possible that a natural mechanism reinforcing UV-seeking behavior may have developed at certain stages of mammalian evolution through its contribution to the synthesis of vitamin D," noted Dr. Fisher. "But such behavioral effects would also carry the carcinogenic risks of UV light that we now recognize. Today's alternative sources of vitamin D, such as inexpensive oral supplements, are both safer and more accurate in maintaining healthy vitamin D levels."