Lab mice become addicted to UV light

Does basking in the sun make you feel relaxed and happy? There may be a biological reason for that.

LOS ANGELES TIMESJune 24, 2014 

LOS ANGELES - Researchers found that mice who were regularly exposed to UV light had a higher pain tolerance than mice that were not exposed to UV light. They also experienced withdrawal symptoms when the UV light was taken away.

In other words, the mice's response to the UV radiation was similar to what would happen if the scientists had given them low doses of heroin.

"I know people are thinking, heroin and UV radiation, give me a break," said David Fisher, professor of dermatology at Harvard Medical School and Massachusetts General Hospital, who led the study. "But we think there could be evolutionary reasons for it."

Previous studies had already established that people who obsessively frequent tanning salons can exhibit addictive behaviors, but nobody knew exactly what was triggering that response.

Fisher and his team started their series of experiments by shaving the backs of mice and exposing them to UV radiation five days a week for six weeks. Each exposure was designed to be similar to what fair-skinned people in Florida would experience if they spent 20 to 30 minutes in mid-day summer sun.

Within the first week, the researchers found that the UV-exposed mice had elevated levels of beta-endorphins - a naturally occurring opioid - circulating throughout their bodies.

This wasn't a huge surprise for the scientists. They already knew that beta-endorphins were produced in the skin in response to UV radiation. What they didn't know was whether it was possible for beta-endorphins produced in the skin to affect the mice's behavior.

To find out, they put the mice through a series of tests. They discovered that mice that had been exposed to UV radiation were less sensitive to heat and pain than their non-UV exposed counterparts. When they treated the UV-exposed mice with naloxone, a drug that inhibits opioids in the body, the same mice responded totally normally to these tests of pain.

The researchers found that the UV-exposed mice became addicted to the elevated levels of beta-endorphins. Mice that received naloxone after days of UV exposure showed classic signs of opioid withdrawal such as paw tremor, teeth chatter, and wet dog shake.

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