Endocannabinoids and the runner's high

Throughout most of human history, our hunter-gatherer ancestors had to engage in physical activity to obtain food. But nowadays we can drive to the supermarket, briefly walk through its aisles, check-out, then drive back home. This may seem like a luxury, but evolution hasn’t prepared us for such a drastic shift in behaviour.

A possible explanation for the “runner’s high,” 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. Perhaps our ancestors who were able to achieve the runner’s high while hunting for food ran more often than those who could not achieve the high. These ‘high-achievers’ (no pun intended) would gather more food as a result of their enhanced motivation, and would be more fit to pass on their genes to the next generation.

Anecdotal reports of the runner’s high often come from endurance runners. However, there has been little scientific study of the runner’s high, so it is difficult to speculate about its role or mechanism. The traditional, widely-publicized explanation for the runner’s high is an “endorphin rush” that inhibits pain during vigorous exercise. However, other chemicals that potentially contribute to the high are epinephrine, serotonin, dopamine and endocannabinoids.

In a study recently published in Experimental Neurology, investigators deleted the gene for the cannabinoid receptor CB1 in mice, and examined how this change to the endocannabinoid system affects voluntary running. The mice with CB1 deletions exhibited 30-40% less running activity than mice that did not get deletions. The knockout mice also had reduced hippocampal neurogenesis, or neuron birth that is known to be induced by exercise, but they were able to increase neurogenesis at a regular rate when they exercised.

These findings indicate that the endocannabinoid system is somehow involved in the regulation of voluntary running activity. In particular, a reduction in CB1 levels could lead to less binding of endocannabinoids to receptors in brain circuits that drive motivation to exercise. It appears that the endocannabinoid system does not play a major role in controlling neurogenesis caused by exercise.

It is easy to point to endocannabinoids as a candidate mediator of the runner’s high, since endocannabinoids are the body’s natural tetrahydrocannabinol (THC), the psychoactive ingredient of marijuana. The study described here doesn’t directly speak much to this proposed parallel, but if the motivation to exercise is considered to be related to the runner’s high, then endocannabinoids may be a driving factor to achieve the runner’s high.

Physical activity has been associated with obtaining rewards throughout evolution. Today we might be left with a certain high associated with the prospect of obtaining a reward – a motivational high mediated by endocannabinoids. This ‘pre-runner’s high’ is an anticipation of the runner’s high, so the two experiences cannot necessarily be thought of as separate. That is – of course – assuming that the runner’s high happened often enough in history that our brains continue to develop to anticipate it. But even if the runner’s high was not common throughout our past, the peaceful feeling that almost everyone experiences after an exhausting run or bike ride should be adequate motivation to start moving.

Endocannabinoids have previously been shown to increase in blood levels after exercise, so there is still a possibility that endocannabinoids mediate the runner’s high. It is most likely, however, that many chemicals converge on brain circuits that underlie the experience. Given the newly discovered role of endocannabinoids in motivation for exercise, it would be unsurprising if endocannabinoids played an important part in directly inducing the runner’s high.

So kids out there: don’t smoke weed if you wish to activate your CB1 receptors. Run.

References:

Dubreucq S, Koehl M, Abrous DN, Marsicano G, & Chaouloff F (2010). CB1 receptor deficiency decreases wheel-running activity: consequences on emotional behaviours and hippocampal neurogenesis. Experimental neurology, 224 (1), 106-13 PMID: 20138171

Fuss J, & Gass P (2010). Endocannabinoids and voluntary activity in mice: runner's high and long-term consequences in emotional behaviors. Experimental neurology, 224 (1), 103-5 PMID: 20353785