By GRETCHEN REYNOLDS
Joubert/Photo Researchers, Inc A neuron in the brain.

Researchers at Princeton University recently made a remarkable
discovery about the brains of rats that exercise. Some of their
neurons respond differently to stress than the neurons of slothful
rats. Scientists have known for some time that exercise stimulates the
creation of new brain cells (neurons) but not how, precisely, these
neurons might be functionally different from other brain cells.

In the experiment, preliminary results of which were presented last
month at the annual meeting of the Society for Neuroscience in
Chicago, scientists allowed one group of rats to run. Another set of
rodents was not allowed to exercise. Then all of the rats swam in cold
water, which they don’t like to do. Afterward, the scientists examined
the animals’ brains. They found that the stress of the swimming
activated neurons in all of the ’ brains. (The researchers could tell
which neurons were activated because the cells expressed specific
genes in response to the stress.) But the youngest brain cells in the
running rats, the cells that the scientists assumed were created by
running, were less likely to express the genes. They generally
remained quiet. The “cells born from running,” the researchers
concluded, appeared to have been “specifically buffered from exposure
to a stressful experience.” The rats had created, through running, a
brain that seemed biochemically, molecularly, calm.

For years, both in popular imagination and in scientific circles, it
has been a given that exercise enhances mood. But how exercise, a
physiological activity, might directly affect mood and anxiety —
psychological states — was unclear. Now, thanks in no small part to
improved research techniques and a growing understanding of the
biochemistry and the genetics of thought itself, scientists are
beginning to tease out how exercise remodels the brain, making it more
resistant to stress. In work undertaken at the University of Colorado,
Boulder, for instance, scientists have examined the role of serotonin,
a neurotransmitter often considered to be the “happy” brain chemical.
That simplistic view of serotonin has been undermined by other
researchers, and the University of Colorado work further dilutes the
idea. In those experiments, rats taught to feel helpless and anxious,
by being exposed to a laboratory stressor, showed increased serotonin
activity in their brains. But rats that had run for several weeks
before being stressed showed less serotonin activity and were less
anxious and helpless despite the stress.

Other researchers have looked at how exercise alters the activity of
dopamine, another neurotransmitter in the brain, while still others
have concentrated on the antioxidant powers of moderate exercise.
Anxiety in rodents and people has been linked with excessive oxidative
stress, which can lead to cell death, including in the brain. Moderate
exercise, though, appears to dampen the effects of oxidative stress.
In an experiment reported at the Society for Neuroscience meeting,
rats whose oxidative-stress levels had been artificially increased
with injections of certain chemicals were extremely anxious when faced
with unfamiliar terrain during laboratory testing. But rats that had
exercised, even if they had received the oxidizing chemical, were
relatively nonchalant under stress. When placed in the unfamiliar
space, they didn’t run for dark corners and hide, like the unexercised
rats. They insouciantly explored.
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“It looks more and more like the positive stress of exercise prepares
cells and structures and pathways within the brain so that they’re
more equipped to handle stress in other forms,” says Michael Hopkins,
a graduate student affiliated with the Neurobiology of Learning and
Memory Laboratory at Dartmouth, who has been studying how exercise
differently affects thinking and emotion. “It’s pretty amazing,
really, that you can get this translation from the realm of purely
physical stresses to the realm of psychological stressors.”

The stress-reducing changes wrought by exercise on the brain don’t
happen overnight, however, as virtually every researcher agrees. In
the University of Colorado experiments, for instance, rats that ran
for only three weeks did not show much reduction in stress-induced
anxiety, but those that ran for at least six weeks did. “Something
happened between three and six weeks,” says Benjamin Greenwood, a
research associate in the Department of Integrative Physiology at the
University of Colorado, who helped conduct the experiments. Dr.
Greenwood added that it was “not clear how that translates” into an
exercise prescription for humans. We may require more weeks of working
out, or maybe less. And no one has yet studied how intense the
exercise needs to be. But the lesson, Dr. Greenwood says, is “don’t
quit.” Keep running or cycling or swimming. (Animal experiments have
focused exclusively on aerobic, endurance-type activities.) You may
not feel a magical reduction of stress after your first jog, if you
haven’t been exercising. But the molecular biochemical changes will
begin, Dr. Greenwood says. And eventually, he says, they become
“profound.”