Flustered Rats Stuck in a Rut

30 July 2009 (All day)


Bad news. Chronic stress may rewire our brains to act out of habit and ignore changes around us.

Add another ill effect to the negative consequences of stress. In addition to making us more irritable, forgetful, and unhealthy, stress also rearranges wiring in the brain, leading to bad decision-making, according to a new study in rats.

Rats, like humans, aren't too hard to stress out. Stick them in an enclosed space or make them share a cage with a dominant comrade and the rodents get fairly unnerved. And that, researchers have found, leads to bad choices.

Scientists at the University of Minho in Portugal and the U.S. National Institutes of Health (NIH) in Bethesda, Maryland, compared stressed and unstressed rats' responses to two tests. In the first test, they taught the rats to hit a lever to score one of two possible treats: a sip of a sugary solution or a food pellet. The scientists then changed the game, providing the rats with all of the snacks they wanted before giving them the option to press the lever. Satiated, the unstressed rats hit the lever significantly less. But the stressed rats continued pressing at the same rate.

For the second test, the scientists trained the rodents to use two levers, one for each treat. After the rats learned the rules, the researchers picked one treat to dispense randomly, whether or not the rat hit the lever. The relaxed animals hit that treat's lever less often, while the stressed rats continued to hit both levers with equal frequency.

In both experiments, the stressed rats acted out of habit and didn't respond to the changes around them--they were stuck in a rut.

When the scientists studied a region of the rats' brains called the dorsal striatum, they also found striking differences between the two groups. In stressed rats, neurons in the dorsomedial striatum, an area associated with goal-directed behavior (for example, pressing a lever to get a specific treat), had shrunk, making fewer connections to other cells. Meanwhile neurons in the dorsolateral striatum, an area that controls habits (such as pressing the same lever regardless of outcome), had grown and formed more branches. The researchers conclude in tomorrow's issue of Science that chronic stress rewires brain areas involved in the switch between goal-directed and habitual actions. Rui Costa, an NIH neuroscientist and co-author of the study, says that "those changes in the brain bias your behavior tremendously for a while after the stress."

The study provides an animal equivalent to "a frequent, maladaptive feature of human behavior during stress: We fall into doing the same thing ... instead of trying something new," says Robert Sapolsky, a neuroscientist at the Stanford School of Medicine in Palo Alto, California. John Morrison, a neuroscientist at Mount Sinai School of Medicine in New York City, says that the study is significant because it highlights how stress acts differently on specific brain circuits. "And we need to understand that specificity" to help design treatments for stress disorders, he says.

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