UV is addictive
Perhaps you've been out in the sunshine recently topping up your tan, or if you're really keen you might have been to a tanning salon. But a note of caution comes from America which suggests that you risk becoming addicted to the ultra violet rays in sunshine. And it also could partly explain the rising rates of skin cancer.
A team at Massachusetts General Hospital have been studying mice and they've found that when they're exposed to UV radiation it stimulates, or upregulates, the way they produce endorphins. They're the so-called feel-good chemicals, produced naturally in our bodies, but also similar to the chemicals found in addictive painkilling drugs including morphine and heroin.
Dr David Fisher explained to Kat Arney how UV light triggers endorphins, and whether you can become addicted to sunshine.
David - The initial clue was simply understanding the molecules that are known to participate in the tanning response. That guided us to test whether endorphin would be upregulated specifically in the skin. This is based on common understanding of how the responses in the skin are triggered by damage such as ultraviolet radiation. So, we first knew that locally in the skin, after ultraviolet radiation, endorphin was upregulated. But we suspected that if it would have an effect on behaviour, the endorphin would probably need to be elevated in the bloodstream in order to make it to the brain where behaviour would be impacted. And so, we could test that in these animals.
And sure enough, after low dose UV radiation for several days, we could see that blood levels of endorphin would rise in a manner that directly paralleled the ultraviolet exposure in the animals. We also could ask whether this elevation in endorphin had a behavioural consequence. And what we noticed is after ultraviolet radiation, the mice became increasingly numb and were not able to feel, slight changes in poking hair-like filaments to the bottom of their feet, or changes in temperature. But that pain sensitivity or that sensory sensitivity was restored if we would give them a drug that blocks opiate signalling.
Kat - This sounds amazing. It sounds like you shine some UV light on these mice and they basically kind of chill out, almost as if they were taking pain-relieving drugs.
David - That was exactly what we saw and in fact, one of the opportunities by doing these studies in experimental animals is that we could then use a variety of genetic tools to study mice in which the gene for endorphin is specifically missing and it's the only thing that's missing. In those mice, if we shine ultraviolet radiation, they did not experience a change in sensation and they did not experience any addiction-like behaviours so that we could really, with a great degree of confidence, conclude from this that endorphin is mediating these behavioural changes, following ultraviolet exposure.
Kat - When you talk about addiction-like behaviour, what does that look like to a mouse?
David - So, the first is the presence of withdrawal symptoms in mice that had received UV radiation and I would emphasise very low doses of UV radiation. We did not want to test this under conditions where there could be sunburning or inflammation in the skin. So, we gave very low doses once a day for several weeks actually so that there was a chronic exposure to ultraviolet radiation. In such mice, if we abruptly gave a drug that block opiates, the same drug that you would give a patient who had overdosed on heroin when they made it to the emergency of a hospital, if we gave that drug to this mice that had only been receiving low dose ultraviolet radiation, the mice started to have withdrawal symptoms. So, they would shake, they would jump, they would have what are considered the animal equivalent of human withdrawal symptoms to an opiate.
Kat - So, they're basically going cold turkey from sunlight.
David - That's exactly what it is.
Kat - Now, we all know that humans aren't mice and even though they seem to be behaving in this interesting way, is there any evidence that this kind of thing might be happening in humans as well?
David - There are data from people who go to indoor tanning beds which have suggested, by using psychiatric questionnaires for example, that there are addiction-like behaviours that are very common in people who find themselves frequently using indoor tanning beds. There are withdrawal-like symptoms that have been associated with disrupting this pathway, this opiate pathway, in people who frequently use tanning beds.
Kat - I'm in London and it is a beautiful day today. But if I go to the park and I'm seeing people out there, soaking up the sun, what are the implications of knowing that they might actually be addicted to this, given that skin cancer is rising so fast?
David - It is beautiful in Boston and I wonder exactly the same thing. It's a big issue I think because what I believe this adds to our knowledge base in terms of how this impacts the public health is that the casual exposure to ultraviolet radiation, I think now reaches a different level because we have evidence that it is feeding into a pathway that is famously organically addictive. If these were drugs that were being given, if it was endorphin that was being injected or heroin or other opiates, I don't think there would be any question that the nature of regulating indoor tanning beds for example should be evaluated in a completely different light than the way it has been up until now.