What would we be without the sun? Would we even “be”? The sun provides sustenance for existence. We maintain life through its energy and derive pleasure from its rays. The nature of this pleasure, however, may be darker than we can perceive: it may actually be addictive.
In a 2014 paper in Cell, researchers at Harvard Medical School found that mice exposed to a daily dose of UVB (a type of UV light present in sunlight) for six weeks showed behavior similar to that produced by opiates like morphine or heroin. These mice, however, were only given doses of UVB that resembled the UV exposure a light-skinned, Floridian tanner might receive in 20 to 30 minutes during the middle of a summer day.
Compared to mice who were not exposed to UV rays, these UV-treated mice exhibited decreased sensitivity to mechanical and thermal pain as measured by reaction times to painful stimuli. Specifically, UV-treated mice exposed to a hotplate or poked in their paws with fibers at increasing levels of force showed a delayed response to the irritation. In addition, researchers observed tail rigidity in these mice which, along with their decreased perception of pain, is a typical result of the administration of opiates like heroin.
An explanation for this behavior came as researchers found blood plasma levels of β-endorphin up to 30 to 50 percent higher in the UV-light treated mice than in controls. Since β-endorphin is a feel-good, pain-killing molecule produced naturally by the body, it seemed to explain the higher pain thresholds observed in these mice. In fact, β-endorphin’s form and function are known to be mimicked by substances such as heroin and morphine. The molecule is also implicated in reward and reinforcement mechanisms, making it likely that a substance triggering an increase in β-endorphin could become addictive.
In this case, the increased β-endorphin levels of the UV-exposed mice were due to UV-induced DNA damage of the skin. In response to the DNA damage, skin cells are known to “activate” a gene whose protein product is then cleaved into smaller components, one of which is β-endorphin. But to prove that the behaviors exhibited by the UV treated mice were really due to elevated levels of circulating β-endorphin, the researchers needed to test whether blocking the β-endorphin receptor would change, offset, or inhibit the mice’s symptoms.
To do so, the researchers administered the drug Naloxone to the experimental animals. Typically used to treat an opioid overdose, Naloxone competes with opioids (in this case, β-endorphin) for the opioid receptor, and functions to block its activation. As expected, upon administration of Naloxone, the UVB treated mice no longer exhibited an elevated threshold for pain, and their tails resumed their normally limp state. In a separate but parallel experiment, mice engineered with a nonfunctional form of the gene necessary to produce β-endorphin were also unaffected by the UVB doses.
Though researchers did not explore this aspect of addiction in their experiments, they speculate that the steady rise of skin cancer in humans by an approximate three percent per year might demonstrate the presence of sunlight-seeking behavior.”
Researchers also noted that the administration of Naloxone produced classic opioid withdrawal symptoms such as paw tremors and teeth chattering in the UVB treated mice. Since withdrawal symptoms are classically associated with drug dependence, these findings indicate that chronic UV exposure can cause a measurable dependence to UV light. This observation therefore strengthened the idea that excessive UV exposure has features of addiction likely mediated by elevated levels of β-endorphin.
Addiction itself, however, is also characterized by compulsive behaviors and active “seeking” of the high’s source. Though researchers did not explore this aspect of addiction in their experiments, they speculate that the steady rise of skin cancer in humans by an approximate three percent per year might demonstrate the presence of sunlight-seeking behavior.
Whether or not the behavior observed by the researchers can really be generalized to human beings, this study suggests exciting new questions about our responses to sunlight. The sun, as it shines down on us every day, may be having much more complex effects on us than we can imagine.
- Fell GL, Robinson KC, Mao J, Woolf CJ, Fisher DE. Skin β-Endorphin Mediates Addiction to UV Light. Cell, 157 (2014), pp. 1527-1534
Atrin Toussi is a Brevia Primary Research Co-editor. She can be reached at firstname.lastname@example.org.