Endocannabinoids are a likelier intoxicant, these scientists believed. Similar in chemical structure to cannabis, the cannabinoids made by our bodies surge in number during pleasant activities, such as orgasms, and also when we run, studies show. They can cross the blood-brain barrier, too, making them viable candidates to cause any runner’s high.
A few past experiments had strengthened that possibility. In one notable 2012 study, researchers coaxed dogs, people and ferrets to run on treadmills, while measuring their blood levels of endocannabinoids. Dogs and humans are cursorial, meaning possessed of bones and muscles well adapted to distance running. Ferrets are not; they slink and sprint but rarely cover loping miles, and they did not produce extra cannabinoids while treadmill running. The dogs and people did, though, indicating that they most likely were experiencing a runner’s high and it could be traced to their internal cannabinoids.
That study did not rule out a role for endorphins, however, as Dr. Johannes Fuss realized. The director of the Human Behavior Laboratory at the University Medical Center Hamburg-Eppendorf in Germany, he and his colleagues had long been interested in how various activities affect the inner workings of the brain, and after reading the ferret study and others, thought they might look more closely into the runner’s high.
They began with mice, which are eager runners. For a 2015 study, they chemically blocked the uptake of endorphins in the animals’ brains 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 twitchy as they had been at the start, suggesting that they had felt no runner’s high. But when their endorphins were blocked, their behavior after running was calmer, relatively more blissed-out. They seemed to have developed that familiar, mild buzz, even though their endorphin systems had been inactivated.
Mice emphatically are not people, though. So, for the new study, which was published in February in Psychoneuroendocrinology, Dr. Fuss and his colleagues set out to replicate the experiment, to the extent possible, in humans. Recruiting 63 experienced runners, male and female, they invited them to the lab, tested their fitness and current emotional states, drew blood and randomly assigned half to receive naloxone, a drug that blocks the uptake of opioids, and the rest, a placebo. (The drug they had used to block endocannabinoids in mice is not legal in people, so they could not repeat that portion of the experiment.)