Lab mice that feel the need for speed

Study: Selective breeding yields superfast yet compulsive rodents that could shed light on human addictions.

Medicine & Science

December 08, 2003|By KNIGHT RIDDER/TRIBUNE

Like horse breeders hoping to win the Kentucky Derby, researchers at the University of Wisconsin-Madison spent eight years selectively breeding mice, creating a super-race of furry white critters that are born to run.

The mice will spend most of their nights happily running on wheels, logging six or seven miles - nearly three times the distance covered by normal mice.

As the exercise addicts of the rodent world, their brains might hold secrets to similar addictive behavior in people, according to the researchers, whose work was published in the journal Behavioral Neuroscience.

"There may be a genetic predisposition to exercise addiction," said Justin Rhodes, the study's lead author, now a postdoctoral fellow at Oregon Health and Science University.

Just how addictive exercise is remains unresolved. But the research suggests that exercise addiction has neurological underpinnings that are similar to other forms of addictive behavior, such as compulsive eating and drug addiction.

The researchers found that the same areas of the mouse brains that were activated when they were denied a chance to run also are known to fire up in drug-addicted mice that are denied their next fix.

"It definitely suggests a link between addictive behavior and running," said Stephen Gammie, the study's senior author and an assistant professor of zoology at the University of Wisconsin.

For years, it has been known that chronic runners can suffer from a type of withdrawal syndrome when they can't run for extended periods. Researchers think that it is due to lower levels of endorphins, the brain's own morphine like painkiller.

"It might be that chronic running does induce some state similar to drug addiction, although probably less so," Gammie said.

The Wisconsin researchers found some drastic differences in the brains of running mice compared with the brains of normal mice.

For the study, the mice were allowed to run all they wanted for six days. On the seventh day, they were not allowed to run.

When the mice were euthanized, researchers looked for levels of a protein that indicates brain cell activation. Among the super runners, the levels of that protein were about 42 percent to 63 percent higher, Rhodes said.

And the higher levels were found in parts of the brain that are activated when drug-addicted mice are denied drugs such as cocaine, alcohol, nicotine and morphine.

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