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Endocannabinoids are probably a more likely intoxicant, according to these scientists. Similar in chemical structure to cannabis, the cannabinoids made by our bodies increase in number during pleasurable activities, such as orgasms, and also when we run, according to studies. They can also cross the blood brain barrier, making them viable candidates for causing any runner’s high.
Some past experiences had reinforced this possibility. In a remarkable 2012 study, researchers persuaded dogs, people and ferrets to run on treadmills while measuring their blood endocannabinoid levels. Dogs and humans are cursive – that is, they have bones and muscles that are well suited to running. Ferrets are not; they slink and sprint but rarely run oblique miles, and they did not produce additional cannabinoids during treadmill running. Dogs and people did, however, indicating that they were most likely suffering from a runner’s high and that this could be attributed to their internal cannabinoids.
This study did not rule out a role for endorphins, however, as Dr Johannes Fuss realized. Director of the Human Behavior Laboratory at Hamburg-Eppendorf University Medical Center in Germany, he and his colleagues had long been interested in how various activities affect the internal functioning of the brain, and after reading the ferret study and others, thought they could take a closer look at the runner’s high.
They started with mice, which are keen runners. For a 2015 study, they chemically blocked the uptake of endorphins in the brains of animals and let them run, then did the same with the uptake of endocannabinoids. When their endocannabinoid system was turned off, the animals ended their runs just as anxious and nervous as they had been at the start, suggesting they hadn’t felt any runner defiance. But when their endorphins were blocked, their post-run behavior was calmer, relatively happier. They seemed to have developed that familiar, gentle hum, even though their endorphins systems had been inactivated.
However, mice are not human beings. So for the new study, which was published in February in Psychoneuroendocrinology, Dr Fuss and his colleagues set out to replicate the experiment, where possible, in humans. Recruiting 63 experienced runners, male and female, they invited them to the lab, tested their current fitness and emotional state, drew blood and randomly assigned half to receive naloxone, a drug that blocks absorption. opioids, and the rest, a placebo. (The drug they had used to block endocannabinoids in mice is not legal in humans, so they couldn’t repeat this part of the experiment.)
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