By GRETCHEN REYNOLDS
Gretchen Reynolds on the science of fitness.
In an eye-opening demonstration of nature’s ingenuity, researchers at
Princeton University recently discovered that exercise creates vibrant
new brain cells — and then shuts them down when they shouldn’t be in
action.
For some time, scientists studying exercise have been puzzled by
physical activity’s two seemingly incompatible effects on the brain. On
the one hand, exercise is known to prompt the creation of new and very
excitable brain cells. At the same time, exercise can induce an overall
pattern of calm in certain parts of the brain.
Most of us probably don’t realize that neurons are born with certain
predispositions. Some, often the younger ones, are by nature easily
excited. They fire with almost any provocation, which is laudable if you
wish to speed thinking and memory formation.
But that feature is less desirable during times of everyday stress.
If a stressor does not involve a life-or-death decision and require
immediate physical action, then having lots of excitable neurons firing
all at once can be counterproductive, inducing anxiety.
Studies in animals have shown that physical exercise creates
excitable neurons in abundance, especially in the hippocampus, a portion
of the brain known to be involved in thinking and emotional responses.
But exercise also has been found to reduce anxiety in both people and animals.
How can an activity simultaneously create ideal neurological
conditions for anxiety and leave practitioners with a deep-rooted calm,
the Princeton researchers wondered?
So they gathered adult mice, injected them with a substance that
marks newborn cells in the brain, and for six weeks, allowed half of
them to run at will on little wheels, while the others sat quietly in
their cages.
Afterward, the scientists determined each group’s baseline
nervousness. Given access to cages with open, well-lighted areas, as
well as shadowy corners, the running mice were more willing to
cautiously explore and spend time in open areas, an indication that they
were more confident and less anxious than the sedentary animals.
The researchers also checked the brains of some of the runners and
the sedentary mice to determine how many and what varieties of new
neurons they contained.
As expected, the runners’ brains teemed with many new, excitable
neurons. The sedentary mice’s brains also contained similar, volatile
newborn cells, but not in such profusion.
The runners’ brains, however, also had a notable number of new
neurons specifically designed to release the neurotransmitter GABA,
which inhibits brain activity, keeping other neurons from firing easily.
In effect, these are nanny neurons, designed to shush and quiet
activity in the brain.
In the runners’ brains, there were large new populations of these
cells in a portion of the hippocampus, the ventral region, associated
with the processing of emotions. (The rest of the hippocampus, the
dorsal region, is more involved with thinking and memory.)
What role these nanny neurons were playing in the animals’ brains and subsequent behavior was not altogether clear.
So the scientists next gently placed the remaining mice in ice-cold
water for five minutes. Mice do not enjoy cold water. They find
immersion stressful and anxiety-inducing, although it is not
life-threatening.
Then the scientists checked these animals’ brains. They were looking
for markers, known as immediate early genes, that indicate a neuron has
recently fired.
They found them, in profusion. In both the physically fit and the
sedentary mice, large numbers of the excitable cells had fired in
response to the cold bath. Emotionally, the animals had become fired up
by the stress.
But with the runners, it didn’t last long. Their brains, unlike those
of the sedentary animals, showed evidence that the shushing neurons
also had been activated in large numbers, releasing GABA, calming the
excitable neurons’ activity and presumably keeping unnecessary anxiety
at bay.
In effect, the runners’ brains had responded to the relatively minor
stress of a cold bath with a quick rush of worry and a concomitant,
overarching calm.
What all of this suggests, says Elizabeth Gould, director of the
Gould Lab at Princeton, who wrote the paper with her graduate student
Timothy Schoenfeld, now at the National Institute of Mental Health, and
others, “is that the hippocampus of runners is vastly different from
that of sedentary animals. Not only are there more excitatory neurons
and more excitatory synapses, but the inhibitory neurons are more likely
to become activated, presumably to dampen the excitatory neurons, in
response to stress.” The
findings were published in The Journal of Neuroscience.
It’s important to note, she adds, that this study examined long-term
training responses. The runners’ wheels had been locked for 24 hours
before their cold bath, so they would gain no acute calming effect from
exercise. Instead, the difference in stress response between the runners
and the sedentary animals reflected fundamental remodeling of their
brains.
Of course, as we all know, mice are not men or women. But, Dr. Gould
says, other studies “show that physical exercise reduces anxiety in
humans,” suggesting that similar remodeling takes place in the brains of
people who work out.
“I think it’s not a huge stretch,” she concludes, “to suggest that
the hippocampi of active people might be less susceptible to certain
undesirable aspects of stress than those of sedentary people.