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Stress Linked to Habitual Behavior in Rats

In a new Portuguese and American study, when exposed to daily chronic stress, rats quickly started making poor decisions, relying on habit instead of actively thinking about whether to press a lever to get a food pellet. This finding could help us understand more about the relationship between stress and behavior patterns in humans, including why some people turn to alcohol and drugs when their stress levels rise.

"Have you ever meant to stop by the grocery store on the way home after a bad day at work, and instead just forgot and went straight home?" said Rui Costa of the National Institute on Alcohol Abuse and Alcoholism, one of the study authors. "You do the automatic thing instead of doing what you had planned."

For three weeks, rats were exposed to a standard regimen of randomly timed stresses, such as being confined in a tube or forced to swim for ten minutes. (Scientists are being sensitive about being accused of cruelty to animals, so these practices are established by the National Institutes of Health and its European counterparts.)

They were also given a standardized test in which they would press a lever to get a sugar treat. After three weeks, the rats were significantly worse at it than they had been at the start of the experiment.

Unlike other rats, said Costa, they pressed the levers in the same pattern they always had—even if they had already eaten several treats. They appeared to follow habits instead of making an active decision about whether they were hungry.

The researchers also found that stress created brain changes in the rats. Parts of the brain believed to control “goal-directed behavior” shrank slightly, but an area thought to be important in forming habits (the dorsolateral striatum) actually grew.

"I'm not at all surprised that decision-making was affected," said Dr. Sudeepta Varma, a clinical assistant professor of psychiatry at New York University's school of medicine. "One of the symptoms of Post Traumatic Stress Disorder is an effect on memory and concentration," she said. "Even after they've undergone treatment, these areas lag. They can't find their keys, they forget appointments, things like that."

Dr. C. Rober Cloninger of Washington University School of Medicine joined other psychiatrists in reminding people that stress can be controlled. "Chronic stress 'hijacks' or short-circuits access to the neocortex, so stressed rats and humans become creatures of habit with little flexibility or initiative in setting goals," he wrote in an e-mail to ABC News. "Consequently in human beings who are stressed, the first step in treatment is always to reduce stress and help them learn ways to relax."

Costa added that in a way, we are often like the rats, stuck on a treadmill, running because we are used to it. “The person burns out from chronic stress," he said. "We fall into routines that require less energy."

Posted on July 30th, 2009
Posted in Research

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