(New Your Times) ~ For several decades, the term "runner's high" has been used to describe a feeling of euphoria and calm, experienced after prolonged exercise. These have always been attributed to the brain's production of endorphines.
Endorphines are the sole celebrity peptide; back in the 1980's researchers found increased blood levels of the substance after heavy workouts. They function like the body’s home-brewed opiates, with receptors and actions much like those of pain-relieving morphine. The problem is that endorphines are made up of relatively large molecules which sould not be able to pass through the blood/brain barrier. Finding them in the bloodstream couldn't constitute a direct affect on the mind.
So reserachers started looking for other candidates to explain how runner's high could take effect on the brain. A newly-emerging branch of neuroscience may indicate that an altogether-different neurochemical system within the body and brain, the endocannabinoid system, may be more responsible for that feeling.
The endocannabinoid system was first mapped out when scientists set out to determine just how cannabis acts upon the body. Then, in a groundbreaking 2003 experiment, scientists at the Georgia Institute of Technology found that 50 minutes of hard running on a treadmill or riding a stationary bicycle significantly increased blood levels of endocannabinoid molecules in a group of college students. 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.
Those researchers also found that, with the right stimuli, the body creates its own cannabinoids (the endocannabinoids). These cannabinoids are composed of molecules known as lipids, small enough to cross the blood-brain barrier. Therefore, cannabinoids found in the blood after exercise could be affecting the brain.
Since that time, a flurry of research has been conducted to feel out the role that endocannabinoids play in the body’s reaction to exercise. For instance, rats treated with a drug that blocked endocannabinoid receptors did not experience the increase in new brain cells that usually accompanies running, suggesting that a well-functioning endocannabinoid system may be required for cognitive improvements from exercise.
In another experiment, groups of mice were assigned either to run on wheels or sip a sweetened drink. Researchers saw that both activities lit up and sensitized portions of the animals’ endocannabinoid systems, intimating that the endocannabinoid connection may lend both exercise and dessert their appeal. And perhaps the most intriguing experiment was one where French researchers bred rats with no functioning endocannabinoid receptors; rats usually love to run on cage wheels, but these modified rats ran about half as much as normal rats. Although the intricacies of the endocannabinoid system’s role in motivating and rewarding exercise is not yet understood, it seems obvious, the researchers say, that the cannabinoid-deprived mice were not getting some necessary internal message.
Whether this new science establishes 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. Still, endocannabinoids are a more persuasive candidate, especially given the overlap between the high associated with marijuana and descriptions of the euphoria associated with strenuous exercise.