It is possible that we know more about the cause of skin cancer than about any other type of cancer. The link between sun exposure (UV radiation) and the development of skin cancer is solidly established. And yet, the incidence of this disease is increasing year after year with a steepest slope than any other type of cancer in the word. Skin cancer alone accounts for almost half of all cancers in United States (a), and in Spain its incidence has almost doubled in the last four years (b).
Arguably, there are a number of cultural patterns behind this sun-seeking tendency, such as cosmetic reasons to have a better-looking tanned skin. But what if there were a more physiological dependency for the sun, some sort of addiction, with the similar reward and withdrawal symptoms as those observed with drugs?
Scientific research has assessed the effects of ultraviolet radiation (UVR) on human brain activity1. When frequent salon bed tanners were irradiated with UV light, an increase in cerebral blood flow in brain regions associated with the experience of reward was detected. These changes, not observed when the light was non-UV, suggest that there is a biological mechanism to encourage tanning.
The production of endogenous opioids in our body is considered a biological response to relieve stress, raise our ability to ignore pain and increase euphoria. For decades these molecules have been known to mediate some of the effects of addiction, and chemically similar compounds such as those derived from opium (morphine and heroine) are amongst the most addictive drugs. Endogenous opioids are released when doing exercise, at stressful situations or even when we laugh2, but it was only recently that they were also linked to sun exposure. An experiment, which consisted on the administration of opioid antagonists (inhibitors) to frequent tanners, showed that after blocking opioid action the subjects stopped feeling the need for UV exposure and, also, they experienced withdrawal symptoms once the treatment was interrupted3.
However, studies carried out in humans fall short in numbers and have not been overly conclusive. Recently, a study performed in mice provided the most complete molecular picture for this process so far4. Researchers gave mice a daily dose of UV light for 6 weeks, which was equivalent to the exposure of a fair-skinned person sunbathing in Florida for 30 minutes per day. They found that after this treatment the levels of the opioid β-endorphin were increased in the skin. Notably, this molecule was produced as part of the same chemical mechanism that generates the pigment that causes skin tanning.
Since β-endorphin is associated with pleasure sensation, animals became more tolerant to pain after sun exposure and didn´t respond quickly if poked with a pin or sit on a warm plate. The more they were irradiated with UV, the less sensitive they became.Finally, all these effects where absent in mice lacking the β-endorphin gene, indicating the importance of this molecule in the UV light-induced dependency.
Overall, the study provides a clear demonstration of opioid action in the brain and supports the idea that UV exposure might cause addiction. One caveat is that, although mice were shaved for the experiments, they actually are nocturnal animals. Whether this behaviour resembles that of humans is still to be elucidated. Also, an addiction would be characterised by an insistent seeking of the UV exposure, which was not shown in this study.
Besides the important cultural component in sun seeking behaviours (mostly seen in Western countries), there is enough evidence to consider that endorphins promote this process through some sort of addition. After millions of years of evolution, the pleasures we feel are usually associated with an experience that is biologically beneficial. Hence, it is not a coincidence that sex, which might be the ultimate pleasure, is also the very reason of our existence. That evolution might have favoured sun-seeking behaviours makes sense given that we get most of our vitamin D from sunlight exposure (5). However, UV radiation is one of the most (if not the most) ubiquitous carcinogens and this behaviour is proving to be counterproductive in the long run. No one said that evolution was perfect anyway.
- Harrington, C. R. et al. Activation of the mesostriatal reward pathway with exposure to ultraviolet radiation (UVR) vs. sham UVR in frequent tanners: a pilot study. Addict. Biol.17, 680–6 (2012) ↩
- Dunbar, R. I. M. et al. Social laughter is correlated with an elevated pain threshold. Proc. Biol. Sci.279, 1161–7 (2012) ↩
- Kaur, M. et al. Induction of withdrawal-like symptoms in a small randomized, controlled trial of opioid blockade in frequent tanners. J. Am. Acad. Dermatol.54, 709–11 (2006) ↩
- Fell, G. L., Robinson, K. C., Mao, J., Woolf, C. J. & Fisher, D. E. Skin β-Endorphin Mediates Addiction to UV Light. Cell157, 1527–34 (2014) ↩