Sun tanning addictive via β-endorphins
A study in mice may provide the explanation of why it is so hard to discourage people from sunbathing when the dangers of ultraviolet (UV) light are so clear. It appears as though UV light can actually be addictive, since chronic exposure raised circulating levels of β-endorphins in mice. UV-habituated mice even demonstrated withdrawal symptoms and associated behaviour if β-endorphin activity was blocked.
Upon exposure to UV-light, the epidermal keratinocytes normally synthesize proopiomelanocortin (POMC), which is then processed to melanocyte-stimulating hormone that mediates tanning. Another POMC-derived peptide, β-endorphin, is co-ordinately synthesised in the skin. β-endorphin is an endogenous opioid. This study demonstrates that the UV-induced β-endorphin exerted pain-relieving and dependency effects.
Shaved mice received a daily dose of UV-light comparable to 20-30 minutes of midday Florida sun on fair-skinned human skin, for 6 weeks. This dose was meant to tan, but not burn the animal’s skin. Already in the first week, the animals’ blood β-endorphin levels rose significantly, and remained elevated throughout the study period. When UV exposure was stopped, β-endorphin levels gradually returned to normal.
UV-treated animals were less responsive to mechanical and thermal analgesic testing than control mice who were not exposed to UV-light. The responsiveness was inversely correlated to β-endorphin levels. When treated with naloxone, which blocks opioid activity, skin sensitivity of UV-exposed mice returned to normal.
Naloxone treatment also yielded classic symptoms of opioid withdrawal in UV-habituated animals, such as trembling, shaking and teeth chattering. Mice trained to associated the effects of naloxone with an environment they naturally prefer, invariable choose to enter the less-preferred area where they had not experienced the naloxone-induced withdrawal symptoms.
Mice with selectively blocked POMC-activity in skin did not show any of the responses to UV-light as seen in normal mice.
These results thus show that a UV-activated opioid pathway is active in skin, reinforcing UV-seeking behaviour. At some point in time, this may have been beneficial. Nowadays, however, addictive features of exposure to UV-light, in addition to it being an established carcinogen, may contribute to the strong rise in skin cancer incidence in humans.
Skin β-Endorphin Mediates Addiction to UV Light.
Fell GL, et al., Cell. 2014 Jun 19;157(7):1527-34
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