Rat Race Is Bad When Alone

By Katherine Unger
ScienceNOW Daily News
13 March 2006

Can stress be good for the brain? It may depend on the social setting. A new study shows that stressed rats have an increase in brain cell generation, but only when they can hang out with other rats. Brain cell growth in isolated rats under stress, in contrast, was suppressed.

The finding emerged from a study of exercise. Given an exercise wheel, a rat will run a few kilometers each night. Previous research has shown this can boost brain cell growth. But it also elevates levels of stress hormones, which can dampen the growth of neurons. So researchers from Princeton University in New Jersey, led by neurobiologist Elizabeth Gould, wanted to see if the benefits of running could outweigh the negatives.

They first injected rats with a dye that marks new brain cells and then allowed only some of them to work out. To their surprise, they found that rats that exercised in social groups had more new brain cells than nonrunners, while rats that ran but were housed by themselves had comparatively less neuron growth. (All the couch potato rats, whether housed alone or together, had similar levels of neuron generation.)

To figure out how isolation makes a difference in brain cell growth, Gould's team measured levels of corticosterone, a stress hormone, in the blood of rats at two times. When rats were beginning to run, they had higher levels of corticosterone than did those without exercise wheels. But 4 hours later, isolated animals--both runners and nonrunners--had higher hormone levels than did their socially housed counterparts, showing that the effect of isolation overwhelmed the benefits of exercise. And when rats underwent an operation to clamp their adrenal glands so they couldn't secrete corticosterone, all runners had increased neuron growth, indicating that the stress hormone had negated the positive effects of exercise in the earlier experiments, researchers report 12 March in Nature Neuroscience.

Gould says these results don't apply directly to humans, since even people who live alone typically have some type of social interaction. Plus rats, unlike most humans, run without being prompted. So there's no evidence yet to say that joining a running club makes you smarter. But she believes the results indicate that an individual's social context can determine whether stress is harmful or helpful.

"It's a very important observation," says neuroscientist Gerd Kempermann of the Max Delbrück Center for Molecular Medicine in Berlin, Germany. But Kempermann emphasizes that the conditions in these experiments are rarely found in human society. "Unless you're in isolation in a prison, it is very unlikely that you are in a situation like these rats."