Sports or snacks? How your brain decides

These neuroscience fundamentals are important because many people don't get enough exercise. Most of us have probably already decided once or twice to skip exercise in favor of one of the many temptations of daily life. According to the World Health Organization, 80 percent of adolescents and 27 percent of adults don't get enough exercise. And obesity is increasing at alarming rates not only among adults, but also among children and adolescents.

Focus on orexin

“Despite these statistics, many people resist the ever-present temptation and get enough exercise,” says Denis Buldakov, professor of neuroscience at ETH Zurich. “We wanted to find out what helps make these decisions in the brain.”

In experiments with mice, researchers were able to demonstrate that orexin plays a key role in this process. Orexin is one of more than 100 neurotransmitters active in the brain. Other chemical neurotransmitters, such as serotonin and dopamine, were discovered a long time ago and their role is largely understood. The situation with orexin is different: researchers discovered orexin relatively late, about 25 years ago, and are now gradually elucidating its function. Buldakov is one of the scientists who has devoted his efforts to studying orexin.

Existing knowledge cannot explain the choice

“In neuroscience, dopamine is a common means of explaining why we choose some things and avoid others,” says Buldakov. This brain messenger is essential for our general motivation. “But our current knowledge about dopamine does not easily explain why we decide to exercise instead of eat,” the scientist continues. “Our brain releases dopamine both when we eat and when we exercise, but it does not explain why we choose one over the other.”

To understand what motivates them, the researchers devised a sophisticated behavioral experiment in which mice were given free reign to choose from eight different options over a 10-minute test. The options included a wheel on which the mice could run and a “milkshake bar” where they could enjoy a standard strawberry-flavored milkshake. “Mice like milkshakes for the same reason that humans do: They're high in sugar and fat, and they taste good,” Buldakov says.

Less time spent at the Milkshake Bar

In their experiments, the scientists compared different groups of mice: normal mice and mice in which their orexin system had been blocked, either through drugs or by genetically modifying their cells.

Mice with normal orexin systems spent twice as much time on the wheel and half as much time at the milkshake bar compared to mice with their orexin system blocked. But interestingly, when the scientists gave the mice either the wheel or the milkshakes, the behavior of the two groups didn't differ. “This means that the orexin system's primary role isn't to control how much the mice move or how much they eat,” Burdakov says. “Rather, it seems that the orexin system plays a central role in helping us make the decision to choose one or the other when both options are available.” In the absence of orexin, the decision was heavily skewed in favor of the milkshake, and the mice gave up exercise in favor of eating.

Helping people who are lacking in exercise

The ETH Zurich researchers speculate that orexin may also be involved in this decision in humans. It turns out that the brain functions involved here are practically the same in both species. “Now we will verify our results in humans,” says Daria Peleg-Leibstein, group leader at ETH Zurich, who led the study together with Denis Buldakov. This could include testing patients whose orexin system is limited for genetic reasons. This applies to about one in every 2,000 people. These people suffer from narcolepsy (a sleep disorder). Another possibility is to observe people who are given drugs that block orexin. Such drugs are approved for insomnia patients.

“If we understand how the brain arbitrates between food intake and physical activity, we can develop more effective strategies to combat the global obesity epidemic and its associated metabolic disorders,” says Peleg Leibstein. In particular, it may be possible to develop interventions that help overcome exercise barriers in healthy people and those with limited physical activity. But Buldakov points out that these are important questions for scientists involved in human clinical studies. He and his group are dedicated to basic neuroscience research. Next he wants to elucidate how orexin neurons interact with other parts of the brain when making decisions, such as the decision between exercise and snacking.