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It is common knowledge that exercise relieves anxiety, but some recent research brings the concept into question.

Maybe Not, Scientists Say, If You Are a Laboratory Rat

In a recent addition to the otherwise soothing body of scientific research on the beneficial effects of regular exercise on anxiety, researchers at the Central Institute of Mental Health at the University of Heidelberg in Mannheim, Germany report recent experiments involving rodents that suggest exercise may actually increase anxiety rather than relieving it.

When mice have running wheels in their cages, they typically spend nearly every waking minute exercising. A lab rat may run 12 km (7 miles) every day. Exhausted, the little rodents quickly fall asleep at night and no researcher has ever observed nightmares or insomnia in a rat that has access to an exercise wheel in its cage.

Take away the exercise wheel, the German researchers tell us, and a surprising picture emerges.

When the University of Heidelberg research team removed the exercise wheels from their lab rats' cages, the animals began to behave as if they were, well, in a cage they could not escape. The animals froze in place or fled to dark corners of their cages, as if they were experiencing extreme anxiety. The more the rat had used the exercise wheel before it was taken away, the greater the change in its behavior.

Too much exercise seems to make animals a nervous wreck.

These findings fly in the face of numerous previous studies that have investigated the role of exercise in regulating the production of brain chemicals that affect mood, especially serotonin, dopamine, and norepinephrine. The German researchers chose to focus on the effects of exercise on the structure of the brain itself, particularly the hippocampus.

In rats, in humans, and in other animals, the hippocampus is part of the limbic system. Straddling the dividing line between left brain and right brain, this area of the brain is responsible for processing recent memories, inhibiting harmful behaviors, and making distinctions between left and right, forward and backward, and up and down.

Damage to the hippocampus, in rats, in humans, and in other animals, results in excessive activity. In a rat, this may be constant exercise. In a human being, this may be a tremor that begins on one side of the body. Rats and humans alike suffer short-term memory loss after hippocampal tissue is destroyed by disease, injury, or toxic exposure. Exercise, however, helps new brain connections grow.

Certain neurons in the hippocampus known as place cells are activated by movement through the environment. When a rat is resting, these neurons hardly fire at all. When a rat moves through the same space in its environment, these cells may fire up to 40 times a second. Repetitive exercise causes massive activity in this one area of the rat's brain.

But is brain growth necessarily a good thing? The researchers in Heidelberg observed changes in the hippocampus that seem to correlate to "overloading circuits" in the hippocampus when laboratory rats exercise all the time. When the hippocampus is injured, then rats cannot interpret their environment, and freeze in place, or they cannot sort out appropriate ways to respond to their environment, and hide in corners. All of this happens despite the fact their brains have grown new connections to the hippocampus while the animal was on the exercise wheel. Would this also happen to someone who spends half a day every day on a treadmill or elliptical trainer?
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