Why aren't endorphins addicting?

Beta endorphins: why sunbathing is addicting

Boston - Sunbathing releases beta-endorphins in the skin, which have an analgesic effect, but can be addictive in the long run. In a series of experiments in Cell (2014; 157: 1527-1534) the injection of naloxone triggered acute withdrawal symptoms in mice that had previously received a daily UV dose on their clean-shaven backs for six weeks.

The fact that some people, despite all the warnings of dermatologists, expose their skin to the sun more or less unprotected has long made addiction researchers suspicious. Studies show that many sun worshipers meet the criteria of the CAGE questionnaire and the DSM-IV manual for an addiction disease.

In an earlier study, sunbathers were able to differentiate between real UV light and UV-free light irradiation by feeling (JAAD 2004; 51: 45-51), in another, the injection of naloxone caused discomfort and concentration disorders as well as vomiting in individual test persons (JAAD 2005; 52: 916), which can be interpreted as withdrawal symptoms.

Sunbathing leads to increased beta-endorphin levels in the blood
A team led by David Fisher from the Massachusetts General Hospital in Boston has now investigated the question of an addictive effect in an animal study. The researchers exposed mice with shaved backs to UV radiation daily for six weeks, which in humans corresponds to about 20 to 30 minutes of sunbathing at the geographic altitude of Florida. After just one week, the beta-endorphin levels in the blood of the mice increased. They remained elevated for the remaining five weeks, but returned to normal within seven days of the end of the sun vacation.

Addictive effects already after two weeks
From the second week onwards, the drug's effect was shown in the mice by continually straightening the tail, which is also known as the “straub tail”. This phenomenon disappeared after the researchers injected the animals with the opiate antagonist naloxone. Fisher reports that the animals became restless and shook like a wet dog, their paws trembled, and their teeth chattered. For the researcher, this is a clear sign that the animals were suffering from naloxone-induced withdrawal.

In another experiment, the animals quickly learned to avoid a dark cage where naloxone was administered to them, although the nocturnal animals otherwise seek darkness. The endogenous opiate also induced habituation. In an experiment in which the animals were placed on a 52 degree hot plate, the analgesic effect of a morphine treatment (which was supposed to prevent them from escaping from the plate) was attenuated.

With the help of knockout mice, the researchers were able to study the pathogenesis of the addictive effect more closely. Mice lacking the beta-endorphin gene did not become addicted. There was also no effect if the gene for the tumor suppressor p53 was missing in the keratinocytes. The activation of p53 is one of the skin's defense reactions to UV radiation.

The keratinocytes form proopiomelanocortin under UV radiation. The melanocyte-stimulating hormone that causes the tanning of the skin is split off from it. Another breakdown product is endorphin. Its function in the skin is unknown. Fisher suspects, however, that beta-endorphins trigger addictive behavior. It could ultimately ensure the supply of vitamin D, which is formed in the skin under the influence of UV. © rme / aerzteblatt.de