For decades, scientists have associated the runner's high - a feeling of euphoria that one gets after doing aerobic exercises - with endorphins. The belief was that the level of endorphins in the blood increases after a workout and causes one's mood to lift.
Apparently, this is not true, according to a group of German scientists who did a study about runner's high. The scientists found that endorphins were actually too large to pass through the blood-brain barrier, making it impossible for endorphins to influence mood after an exercise, The Washington Post reported.
The researchers found that the brain's endocannabinoid system could be involved in producing the runner's high, particularly though anandamide, an endocannabinoid that is present at elevated levels in the blood after an aerobic workout. Anandamide reportedly travels from the blood into the brain and causes the runner's high.
Endocannabinoids are like the body's internal marijuana. They influence a person's mood, tolerance for pain, appetite and memory. The body's endocannabinoid system is the one that is affected by the tetrahydrocannabinol (THC) in marijuana.
Lead study author Johannes Fuss at the University Medical Center Hamburg-Eppendorf was surprised that "no one had investigated the effects of endocannabinoids on behavior after running."
To test the effect of cannabinoid receptors after an aerobic activity, the researchers took mice and allowed them to run on running wheels. After the runs, the mice exhibited reduced anxiety and greater tolerance for pain.
Next, the researchers gave them drugs that blocked their endocannabinoid system and made them run on the wheels. The mice showed the same signs of anxiety after running as they had before running, and they appeared to be more sensitive to pain.
"We thus show for the first time to our knowledge that cannabinoid receptors are crucial for main aspects of a runner's high," the researchers wrote.
"The authors have moved the field forward by providing such a complete view of how this key reward system is involved in allowing exercise to improve psychological state and pain sensitivity," David A. Raichlen, an expert in human brain evolution and exercise at the University of Arizona who was not involved in the study, told Scientific American.
The study was published in the Oct. 5 online issue of the Proceedings of the National Academy of Sciences.
