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Researchers have discovered that the "runner's high", usually attributed to endorphins – the body's self-produced opiates – may actually be caused by endocannabinoids, self-produced chemicals similar to those found in marijuana.

A new study by researchers at the University of Heidelberg medical school in Germany found that mice showed elevated levels of both endorphins and endocannabinoids after running, an activity they engage in for fun. The researchers also observed that mice were less sensitive to pain, less anxious and more tranquil after running, shown by their willingness to spend time in lighted areas of their cages rather than retreating to dark corners.


When the team used drugs to block the animals' endocannabinoid receptors, the mice were no longer relaxed after running, proving to be just as anxious as before their runs and very sensitive to pain. Blocking opioid receptors, on the other hand, didn't affect the creatures' post-run tranquillity.

However, internal opioid receptors appear to play at least some role in the motivation to get on a treadmill or pound the pavement. A paper by a team from the University of Missouri describes findings which show that chemically activating the dopamine-releasing mu-opiod receptors of rats bred to love running makes them less inclined to exercise, demonstrating a direct link between the receptors and the urge to run. The team also found that shutting off the receptors entirely reduced activity in the rats, although not to the same degree.

"However, when we chemically activated their mu-opioid receptors, those rats drastically reduced their amounts of activity," said Greg Ruegsegger, lead author on the paper from The University of Missouri. "Since exercise and addiction to substances follow this same chemical process in the brain, it stands to reason that activating these receptors in people with dangerous addictions could provide the same rewards they are craving without the use of dangerous drugs or alcohol."


A final interesting finding of the University of Heidelberg study was just how far the mice needed to run in order to experience the high in the first place, averaging over three miles (5km) a day on their wheels. Study lead Johannes Fuss told the New York Times that "reduced sensations of pain and less anxiety through long-distance running would have been a benefit" to humans, too, as we also evolved to move and cover long distances.

This story was originally published in October 2015.