Mice made feel-good chemical after exposure to ultraviolet light — and missed that light when the treatments ended.
The sun’s ultraviolet rays can cause more than a tan or burn. Indeed, their influence goes more than skin deep — to the brain, a new study finds. It showed that exposure to ultraviolet (UV) light causes mice to make a feel-good chemical. And that chemical may explain why so many people feel compelled to get a tan.
The study also may help explain why people flock to beaches and coasts for relaxation, Steven Feldman told Science News. He studies public health and skin diseases at the Wake Forest University School of Medicine in Winston-Salem, N.C.
“Do you know why people go to the beach on vacation? Why they put [Disney World] in Florida and not in Minnesota, where it’s cooler? Why caves are not more popular as a tourist destination? It’s all because of what these guys studied” in their new research, Feldman says. He did not work on the new study.
High-energy, UV rays come from the sun and the special lights used in tanning booths. Even though people know UV radiation can be dangerous, they continue to risk sunburns for a tan. Rates of skin cancer have been going up. David Fisher wanted to know why. He's an oncologist — a doctor who treats people with cancer — at Massachusetts General Hospital in Boston.
“We know [UV light is] dangerous,” Fisher says, but many people choose not to protect themselves.
Fisher and his colleagues exposed mice to a moderate amount of UV light five days a week, for six weeks. The scientists shaved the mice so the light would reach their skin. The animals received an exposure to UV light that was similar to what a fair-skinned person gets after 20 or 30 minutes in the Florida sun.
After just a week of the UV treatment, mice had higher levels of a molecule in their blood called beta-endorphin than did mice not exposed to this radiation. Similar to drugs such as heroin and opium, beta-endorphin activates feel-good processes in the brain. Previous studies had found beta-endorphin in the skin. This new study showed the molecule also ends up in the blood.
UV light didn’t boost beta-endorphin for very long, though. Levels returned to normal a week after the UV treatments stopped. And the mice didn’t seem to have wanted those treatments to end. In fact, they showed signs of an addiction to this light.
Fisher’s group gave a drug, called naloxone (Naa-LOX-ohn), to the UV-treated mice. Doctors prescribe this drug to people who have been abusing heroin. It blocks the places in the brain where heroin triggers the feel-good response. But when a heroin addict receives naxolone, the drug can set off withdrawal symptoms within minutes.
In the UV-treated mice, the drug seemed to block the feel-good effects of UV exposure. It also caused chattering teeth and shaking in the animals. Those symptoms looked similar to what people go through when they are suffering from withdrawal from heroin.
The new data are “fascinating,” Bryon Adinoff told Science News. This psychiatrist at the Veterans Administration North Texas Health Care System and the University of Texas Southwestern Medical Center in Dallas, had no role in the new study.
Other studies with people have shown that UV rays make people feel good, Adinoff notes.
“There are a lot of people out there who tan, either on the beach or in indoor salon tanners. And many of these folks report symptoms that are consistent with addiction,” he notes. What's more, UV light boosts activity in parts of the human brain associated with addiction, he and his colleagues have shown.
Still, he argues, the new study falls short of proving the mice had become addicted to UV light. “Addiction means craving, loss of control and tolerance,” he explains. And the new study “didn’t look at any of those things.”
Power Words
addiction The uncontrolled use of a habit-forming drug or uncontrolled and unhealthy habit (such as video game playing or phone texting). It results from an illness triggered by brain changes that occur after using some drugs or engaging in some extremely pleasurable activities. Persons will an addiction will feel a compelling need to use a drug (which can be alcohol, the nicotine in tobacco, a prescription drug or an illegal chemical such as cocaine or heroin), even when the user knows that doing so risks severe health or legal consequences. (For instance, even though 35 million Americans try to quit smoking each year, fewer than 15 out of 100 succeed. Most begin smoking again within a week, according to the National Institute on Drug Abuse.) Someone who suffers from this disease is known as an addict.
cancer Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. The development and growth of cancers, also known as malignancies, can lead to tumors, pain and death.
endorphins Any of a group of chemicals secreted within the brain and nervous system. As neurotransmitters, they relay messages within the nervous system. They also activate the feel-good receptors in the body and can raise an individual’s threshold for pain.
heroin A highly addictive and illegal drug derived from morphine, a potent pain killer. People often take heroin as a narcotic — something that dulls the senses, relieves pain and makes them sleepy or unmotivated to do anything other than lay around in a slump.
naloxone A drug that works as an antidote to help people who take dangerous amounts of heroin or some other opiate drug. Naloxone works by binding to the same features on brain-cell surfaces that heroin or related drugs do. This reduces or turns off the pleasurable sensations that opiate drugs would normally produce.
radiation Energy, emitted by a source, that travels through space in waves or as moving subatomic particles. Examples include visible light, infrared energy and microwaves.
ultraviolet A portion of the light spectrum that is close to violet but invisible to the human eye.
withdrawal (in medicine) An almost disease-like syndrome that can develop after animals (including people) attempt to stop using a drug (including alcohol) to which they have become addicted. Shaking, sweating, trouble sleeping, anxiety, diarrhea, vomiting, abdominal cramping, muscle aches and flu-like symptoms can occur and last for days.
