Does basking in the sun make you feel relaxed and happy, like nothing can bother you? There may be a biological reason for that.
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.
"What our study added is the mechanism," Fisher said. "How does it happen."
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 scienitsts. 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 also 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 like paw tremor, teeth chatter, and wet dog shake.
Fisher said there are two reasons it made sense to do this study on mice. "It happens that the response of skin to UV radiation is remarkably similar in mice and humans," he said. "Mice tan, and the same gene regulates pigment in mice and in people."
He added that using a mouse model also allowed the team to ask very specific questions about the mechanisms that make sun exposure addictive. For example, they ran the same tests described above on mice that were missing the beta-endorphin gene. These mice did not exhibit any change in their pain threshold when they were exposed to UV light, nor did they show any withdrawal symptoms when given naloxone.
"That is enormous," Fisher said. "Now we know that UV light and increased beta-endorphins don't just correlate, but that the endorphin is mediating behavior."
The researchers hypothesize that this feel-good feeling we get from the sun may have once been physically beneficial for humans who were perhaps not getting enough vitamin D.
Fisher said he hopes that the study, published in the journal Cell, will help people better understand why sunbathing continues to be so seductive, in spite of all the ways we now know it harms our skin.
"There is real damage that comes from frequent UV exposure," said Fisher. "If people recognize that their behavior of going out and tanning may go beyond a conscious decision, maybe they will be better able to avoid it."
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