Researchers at Princeton University have found that exercise creates excitable new brain cells — but turns them off when necessary.
Until now, scientists have been somewhat puzzled by two seemingly counterproductive effects of exercise on the brain. On the one hand, exercise is known to prompt the creation of new and very excitable brain cells. Yet, at the same time, exercise seems to create an overall pattern of calm.
For example, neurons are born with certain predispositions. Some, usually the younger ones, are by nature easily excitable. They fire with almost any provocation, which is good news if you desire speed thinking and memory formation.
During moments of stress, however, that effect would be less desirable. Having lots of excitable neurons firing all at once would be counterproductive to daily stress — inducing anxiety rather than calming it.
The mystery was resolved when, in a new mouse study, researchers found that physically active mice also had a large number of new neurons specifically designed to release the neurotransmitter gamma-aminobutyric acid (GABA) — which inhibits brain activity by keeping other neurons from firing easily.
In other words, these are nanny neurons, designed to hush and quiet excited brain activity.
When physically active mice experienced a stressor — exposure to cold water — their brains exhibited a spike in the activity of neurons that turn off excitement in the ventral hippocampus, a brain region shown to regulate anxiety.
For the study, one group of mice was given unlimited access to a running wheel and a second group had no running wheel. Natural runners, mice will run up to 2.5 miles a night when given access to a running wheel. After six weeks, the mice were exposed to cold water for a brief period of time.
In the neurons of sedentary mice, the cold water spiked an increase in short-lived genes that rapidly turn on when a neuron fires. The lack of these genes in the neurons of active mice suggests that their brain cells did not immediately leap into an excited state in response to the stressor.
Instead, in the brains of the physically active mice, there was a boost of activity in inhibitory neurons that are known to keep excitable neurons in check. At the same time, neurons in these mice released more GABA.
From an evolutionary standpoint, a higher likelihood of anxious behavior may have an adaptive advantage for less physically fit creatures. Anxiety often leads to avoidant behavior, and steering clear of potentially dangerous situations would increase survival rates, particularly for those less able to respond with a “fight or flight” reaction, said senior author Elizabeth Gould, Ph.D.
“Understanding how the brain regulates anxious behavior gives us potential clues about helping people with anxiety disorders. It also tells us something about how the brain modifies itself to respond optimally to its own environment,” said Gould.
Source: Princeton University