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The concept of a “runner’s high” strikes some people as a conspiracy theory invented by the athletic apparel industry. You know, Big Sneaker. But others live and breathe the high, hooked on hitting the pavement and passing their days jonesing for the chance to put one foot in front of the other. It feels good. It’s pleasure. Quite literally, running is their drug.

We’ve known for a long time that exercise can be addictive. Just consider how many rehab programs include exercise as an integral part of recovery; they replace an addict’s compulsion to blow lines with one to cross finish lines.

The scientific community has yammered on about endorphins for years, but we’ve actually had a fairly weak grasp on the neurobiological roots of exercise’s addictive properties. Scientists, however, are getting closer to clarity. In a new study, published in the journal Neuropharmacology, researchers from the University of Missouri made headway in understanding how the brain’s reward system (home to the feel-good neurotransmitter dopamine) controls the drive to run.

In the study, researchers selectively bred two groups of rats — one comprised of active wheel-fiends, the other of lazy rodents. Breeding for different activity levels, they found, affected the rats’ brain chemistry. As compared to the lethargic group, the active rats had 400 percent more of the reward receptors involved in regulating dopamine levels.

Then, the researchers tinkered with the rat brains by turning up and shutting off their reward systems. When they activated reward systems, the active rats ceased running around like furry maniacs and settled down. When they shut down the reward systems, the same thing happened. Yes, the same thing.

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To learn why opposite forces had the same effect, we contacted study author Gregory Ruegsegger.

He told us to think of the active rats as drug addicts. When researchers shut down reward systems, the rats simmered down. This makes sense. Without the possibility of a dopamine rush, they no longer felt the drive to run. Previously, they stayed active because moving their furry legs coincided with a dopamine spike, essentially getting them high. No high? No drive.

Conversely, when researchers activated the rats’ already-naturally-heightened reward systems, they simmered down because they had enough dopamine flooding their tiny mesolimbic pathways to feel high. Already high? No drive.

This isn’t the only study looking at the neurochemical basis of “runner’s high.” As the New York Times reported on Tuesday, another research group identified the endocannabinoid system (yes, that cannabis) as the force behind runner’s high. These findings complement Ruegsegger’s research.

“It could be that the reward system acts on the endocannabinoid system,” Ruegsegger said, “and together, they mediate running behavior.”

But what about those lazy rats? They didn’t react much to activation or suppression, and it’s less clear why. With more research, the Missouri team hopes to figure it out. We may be looking at rats on a wheel, but there are important public health implications.

“What motivates someone to be lazy is a bigger problem than an exercise addiction,” Ruegsegger said. “The ultimate goal is to understand why lazy rats are lazy, and apply that to humans. It’s a lot easier to convince people to exercise to stay healthy than it is to treat patients once they’re unhealthy.”