Scientists used to presume that the mild elation, mental clarity, and pain-killing effects from physical exercise are due to a release of endogenous opioids. But evidence is accumulating that the endocannabinoid system contributes significantly to the so-called “runner’s high.”
A recent article published in Neuroscience by Brazilian researchers examined how exercise influences pain perception. Their results show that activation of cannabinoid (CB2) receptors on immune cells in the spinal cord can numb pain after exercise. In addition to reducing pain, exercise increased the concentration of anandamide, an endogenous cannabinoid, in the spine. And exercise also caused immune cells to produce more CB2 receptors.
But anandamide, often referred to as the “bliss molecule,” has little binding affinity for the CB2 receptor. This suggests that exercise mobilizes endocannabinoid activity at multiple levels. Previous research on exercise has highlighted a role for anandamide in the runner’s high, but this appears to be the first work specifically implicating the immunoregulatory effect of CB2 receptors.
Interestingly, the blissful elevation of endocannabinoid tone only occurred in animals with a muscle injury, while healthy animals did not show a commensurate exercise-induced increase in endocannabinoid activity. Why?
Human research has found that the intensity of exercise is an important variable in this sort of research. When exercise is too strenuous or too mild it does not lead to heightened endocannabinoid activity. It’s possible that the injured animals in this study exercised to a different degree than the healthy ones, which could explain why the effect was only seen in animals with a muscle injury. Alternatively, injury may modify the body’s and the endocannabinoid system’s response to exercise.