[First published August 4 2010]
Maybe it isn't endorphins after all.
What do long-distance running and
marijuana smoking have in common? Quite possibly, more than you’d think.
A growing body of research suggests that the runner’s high and the
cannabis high are more similar than previously imagined.
The nature of the runner’s high is
inconsistent and ephemeral, involving several key neurotransmitters and
hormones, and therefore difficult to measure. Much of the evidence comes
in the form of animal models. Endocannabinoids—the body’s internal
cannabis—“seem to contribute to the motivational aspects of voluntary running in rodents.” Knockout mice lacking the cannabinioid CB1 receptor, it turns out, spend less time wheel running than normal mice.
A Canadian neuroscientist who blogs as NeuroKuz
suggests that “a reduction in CB1 levels could lead to less binding of
endocannabinoids to receptors in brain circuits that drive motivation to
exercise.” NeuroKuz speculates on why this might be the case. Physical
activity and obtaining rewards are clearly linked. The fittest and
fleetest obtain the most food. “A possible explanation for the runner’s
high, or ‘second wind,’ a feeling of intense euphoria associated with
going on a long run, is that our brains are stuck thinking that lots of
exercise should be accompanied by a reward.”
In 2004, the British Journal of Sports Medicine
ran a research review, “Endocannabinoids and exercise,” which seriously
disputed the “endorphin hypothesis” assumed to be behind the runner’s
high. To begin with, other studies have shown that exercise activates the endocannabinoid system.
“In recent years,” according to the
authors, “several prominent endorphin researchers—for example, Dr Huda
Akil and Dr. Solomon Snyder—have publicly criticised the hypothesis as
being ‘overly simplistic,’ being ‘poorly supported by scientific
evidence’, and a ‘myth perpetrated by pop culture.’” The primary
problem is that the opioid system is responsible for respiratory
depression, pinpoint pupils, and other effects distinctly unhelpful to
runners.
The investigators wired up college
students and put them to work in the gym, and found that “exercise of
moderate intensity dramatically increased concentrations of anandamide
in blood plasma.” The researchers break the runner’s high into four
major components. Exercise, they say, “suppresses pain, induces
sedation, reduces stress, and elevates mood.” Some of the parallels
with the cannabis high are not hard to tease out: “Analgesia, sedation
(post-exercise calm or glow), a reduction in anxiety, euphoria, and
difficulties in estimating the passage of time.”
There are cannabinoid receptors in
muscles, skin and the lungs. Intriguingly, the authors suggest that
unlike “other rhythmic endurance activities such as swimming, running is
a weight bearing sport in which the feet must absorb the ‘pounding of
the pavement.’” Swimming, the authors speculate, “may not stimulate
endocannabinoid release to as great an extent as running.” Moreover,
“cannabinoids produce neither the respiratory depression, meiosis, or
strong inhibition of gastrointestinal motility associated with opiates
and opioids. This is because there are few CB1 receptors in the
brainstem and, apparently, the large intestine.”
A big question remains: What about running and the “motor inhibition” characteristic of high-dose cannabis?
(An inhibition that may make cannabis useful in the treatment of
movement disorders like tremors or tics.) Running a marathon is not the
first thing on the minds of most people after getting high on
marijuana. The paper maintains, however, that at low doses,
“cannabinoids tend to produce hyperactivity,” at least in animal models.
The CB1 knockout mice were abnormally inactive, due to the effect of
cannabinoids on the basal ganglia. Practiced, automatic motor skills
like running are controlled in part by the basal ganglia. The authors
predict that “low level skills such as running, which are controlled to
a higher degree by the basal ganglia than high level skills, such as
basketball, hockey, or tennis, may more readily activate the
endocannabinoid system.”
The authors offer other intriguing
bits of evidence. Anandamide, one of the brain’s own cannabinoids, “acts
as a vasodilator and products hypotension, and may thus facilitate
blood flow during exercise.” In addition, “endocannabinoids and
exogenous cannabinoids act as bronchodilators” and could conceivably
facilitate breathing during steady exercise. The authors conclude:
“Compared with the opioid analgesics, the analgesia produced by the
endocannabinoid system is more consistent with exercise induced
analgesia.”
Photo Credit: http://www.madetorun.com/
