Endocannabinoids are a more likely toxicant, these scientists believed. With a chemical structure similar to that of cannabis, the cannabinoids produced by our bodies increase in number during pleasurable activities, such as orgasms, and also when we run, studies show. They can also cross the blood-brain barrier, making them viable candidates to feel cheap in any corridor.
Some previous experiences have reinforced this possibility. In a remarkable 2012 study, researchers persuaded dogs, people and ferrets to run on treadmills, while measuring their blood levels of endocannabinoids. Dogs and humans are cursors, which means that they have bones and muscles well adapted to long-distance running. Ferrets are not; they sneak and run, but rarely cover galloping miles and don’t produce extra cannabinoids while running on the treadmill. Dogs and people did, indicating that they were probably experiencing a runner’s high and that this could be attributed 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 have long been interested in how various activities affect the inner workings of the brain and, after reading the study of the ferret and others, thought they could take a closer look at the emotion of the corridor.
They started with rats, which are anxious runners. For a 2015 study, they chemically blocked the absorption of endorphins in the animals’ brains and let them run, then did the same with the absorption of endocannabinoids. When the endocannabinoid system was turned off, the animals ended their runs as anxious and restless as at first, suggesting that they did not feel any buzz. But when his endorphins were blocked, his post-race behavior was calmer, relatively more ecstatic. They seemed to have developed that mild, familiar tinnitus, although their endorphin systems had been deactivated.
Rats, emphatically, are not people. So, for the new study, which was published in February in Psychoneuroendocrinology, Dr. Fuss and his colleagues decided to replicate the experiment, as far as possible, in humans. Recruiting 63 experienced runners, men and women, they invited them to the lab, tested their current fitness and emotional states, collected blood and randomly distributed half to receive naloxone, a drug that blocks opioid absorption, and the rest, one placebo. (The drug they used to block endocannabinoids in mice is not legal in people, so they couldn’t repeat this part of the experiment.)