Experts have long wondered why many people tan regularly despite the known risk of skin cancer. Past studies suggest that the motivation is not just vanity—some tanning buffs have symptoms of dependence and withdrawal. Now a study in Cell adds more evidence that tanning is addictive. It showed that mice become dependent on beta-endorphin, a druglike opioid molecule made by the skin under ultraviolet light.
A team at Massachusetts General Hospital scrutinized the opioid system, the reward pathway hijacked by drugs such as heroin, because the researchers had earlier found that beta-endorphin and the skin pigment melanin originate from the same protein. Other studies have also pointed to the opioid system; in one, frequent tanners showed withdrawal symptoms when they took a drug that blocked opioid receptors.
In the new study, shaved mice got a daily dose of UV light long enough to tan but not burn—on a par with 20 to 30 minutes in midday Florida sun for a fair-skinned human. After a few days, levels of beta-endorphin rose in the mice's blood. Then the researchers rated pain tolerance, a marker of opioid dependence, using heat and touch. The UV mice had a pain threshold up to three times higher than mice that had not tanned. As levels of beta-endorphin rose, so did pain tolerance, suggesting the endorphin played a key role.
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When the UV mice received an opioid blocker, their pain threshold reverted to normal, and they showed withdrawal symptoms such as shaking paws and chattering teeth. The mice even modified their behavior to avoid withdrawal: those that received opioid blockers in a dark box preferred to spend time in a white box, despite rodents' natural penchant for darkness.
Humans and mice share these chemical processes, so the researchers believe beta-endorphin may cause addiction in people. Getting sun may be rewarding to the brain because we need vitamin D, explains David Fisher, a co-author of the study and director of the melanoma program at Mass General. Next Fisher hopes to investigate whether this pathway is involved in seasonal affective disorder, possibly providing a new therapeutic target.