In studying how ultraviolet radiation affects skin, scientists may have stumbled across humankind's original addiction: sunlight.
A new study in the journal Cell shows that UV radiation, even in modest amounts, prompts production of the "feel-good" hormone beta-endorphin that numbs mice to pain. When scientists blocked beta-endorphin with the drug Naloxone (used to combat heroin overdoses in hospital emergency rooms), UV-exposed mice experienced withdrawal symptoms: shaking, teeth-chattering and jumpiness.
It's the first time scientists have thoroughly mapped out the pathway in mice from UV exposure, to rising beta-endorphin levels in the blood, to physical dependence on sunlight. The findings add to a field of research that examines extreme tanning as a possible addiction in humans, and could explain why people love to sunbathe -- even though it's clear that UV rays damage skin cells, age skin prematurely and cause skin cancer.
In addition, the findings could pave the way for more research on battling opiate addiction in general, since beta-endorphins are opioids -- our body's own natural version of opiate-like drugs.
"The concept that we would have a pathway in our skin that is organically guiding us to to seek out the most common carcinogen in the world almost seems like a joke," said senior study author Dr. David Fisher, chairman of the dermatology department at Massachusetts General Hospital and a professor at Harvard Medical School. "From a scientific perspective, this raises an interesting prospect that perhaps UV-seeking was the original addiction."
While it wasn't the focus of the research, Fisher and his team speculate that vitamin D synthesis is the evolutionary reason human beings seek the sun. Vitamin D is vital to calcium absorption and bone health, and when you're a child, lack of vitamin D can be fatal.
"It's plausible that a behavioral motivation to seek the sun to preserve vitamin D may have produced many years ago, say 100,000 years ago, as an evolutionary advantage," Fisher said. Now, of course, physicians recommend people take vitamin D supplements to guard against deficiencies.
THE 'FEEL-GOOD' HORMONE
The study started off with exposing shaved mice to UV rays for six weeks. Then, in a series of experiments, Fisher and other researchers measured the rodents' blood levels of beta-endorphins, tested their pain thresholds and injected them with Naloxone.
They found that beta-endorphin levels increased significantly in UV-exposed mice, and that these mice had a much higher pain threshold than non-UV-exposed mice (echoing the numbing effects of opiate drugs like heroin and morphine in humans). Also, injecting UV-exposed mice with the overdose drug Naloxone seemed to induce withdrawal symptoms, suggesting they had become physically dependent on UV rays.
To test if the UV rays would alter the rodents' behavior, Fisher placed the UV-exposed mice in two cages connected by a tunnel, one dark and one brightly lit. Researchers wanted to see if the mice, which are nocturnal, would go against their habits to run to the lit cage if they had been exposed to UV rays.
Researchers induced withdrawal symptoms in the mice using Naloxone while they were trapped in the dark cage. The next day, the UV-exposed mice went against their nocturnal nature and crossed over to the lit cages, because they associated the dark cages with something they wanted to avoid.
The important thing to note in all this is that the mice were exposed to UV rays meant to approximate "midday sun exposure in Florida during the summer for [a] fair-skinned person of average tanning ability" -- in other words, a fairly moderate, normal amount of sun for many North Americans. That was deliberate, because researchers didn't want to use a dose that would lead to sunburn or inflammation. "We wanted it to be as much about UV radiation as possible, and not a second consequence of injury to skin," Fisher said.
TREATING TANNING 'ADDICTS'
Dr. Robin Hornung, a dermatologist and skin cancer expert in Everett, Washington, praised the methodology of Fisher's study, and said the findings echo what she sees in her everyday practice: patients who seem to be addicted to tanning.
Understanding how tanning may be an addiction could help more doctors understand that facts alone may not be enough to get patients to quit sunning, Hornung said. Instead, it could be better for doctors to approach tanners similarly to how they approach smokers: by tailoring the approach based on whether patients are contemplating, preparing, or taking action to change behavior. Push them too hard, and doctors might end up alienating their patients.
"I have known patients who have left their dermatologist for another doctor as they felt criticized about their addiction," Hornung wrote. "It is important to establish good rapport and trust with the patient, not to chastise and alienate, and to ultimately take a more comprehensive approach to changing their behavior."
Dr. Bryon Adinoff, chief of the Division on Addictions at the UT Southwestern Medical Center and Director of Mental Health Research at the VA North Texas Health Care System, said the new findings are "relatively conclusive that UVR [UV radiation] increases endorphin in rodent models." However, he cautioned against using the word "addiction" to describe tanning, as research has yet to demonstrate UV addiction biologically.
"Their approach shows a mechanism about how UVR could become addictive, but it did not demonstrate that it did become addictive," Adinoff told HuffPost. "They demonstrated it induces physiological dependence, but anyone taking opioids for a few days, whether its for chronic pain or acute injury, is going to develop some physiological dependence."
And physiological dependence is not necessarily addiction. Adinoff's past UV studies showed that the rays could activate reward regions in the brain, similar to the way any other pleasurable stimuli would activate those regions. But that's different than being addicted, he explained: "People who go to tanning salons or the beach don't necessarily become addicts, just like how people can drink alcohol and not become addicted."