Or not. A post by Gretchen Reynolds at the Well blog in the NYTimes points out that endorphins don't cross the blood brain barrier, so can't be responsible for the euphoria of a good run. Instead, neuroscientists are talking now about the 'endocannabinoid system', a neurochemical pathway in the brain, with receptors elsewhere in the body, that is involved in the reduction of pain and anxiety.
An experiment in 2003 found increased levels of endocannabinoid molecules in the blood of student subjects after doing hard exercise.
The endocannabinoid system was first mapped some years before that, when scientists set out to determine just how cannabis, a k a marijuana, acts upon the body. They found that a widespread group of receptors, clustered in the brain but also found elsewhere in the body, allow the active ingredient in marijuana to bind to the nervous system and set off reactions that reduce pain and anxiety and produce a floaty, free-form sense of well-being. Even more intriguing, the researchers found that with the right stimuli, the body creates its own cannabinoids (the endocannabinoids). These cannabinoids are composed of molecules known as lipids, which are small enough to cross the blood-brain barrier, so cannabinoids found in the blood after exercise could be affecting the brain.And new research is showing that, for example, mice love to exercise, and choose to do so -- until their endocannabinoid system is knocked out. So indications are good that this is in fact the system that gets people addicted to running.
We were interested in the report of all this in the Times, though, because it was written in a way that hit many of the buttons we try to hit here on MT.
Whether this accumulating new science establishes, or ever can establish, definitively, that endocannabinoids are behind runner’s high, is uncertain. As Francis Chaouloff, a researcher at the University of Bordeaux in France and lead author of the genetically modified mouse study, pointed out in an e-mail, rodents, although fine models for studying endocannabinoid action, “do not fill questionnaires to express their feelings related to running,” and runners’ high is a subjective human experience.First, a canonical belief was challenged -- endorphins cause runner's high. And, a new explanation was offered based on what seems to be solid experimental evidence that makes sense -- it's endocannabinoids. But, Reynolds cautions that we still don't know for certain that endocannabinoids cause runner's high, we may never know for certain, and we certainly can't know for certain from studying mice. But it's an intriguing possibility.
We love a healthy dose of skepticism -- or reality -- mixed in with some promising research.