For Your Brain’s Sake, Keep Moving

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Because we can never have enough reasons to keep exercising, a new study with mice finds that physical activity not only increases the number of new neurons in the brain, it also subtly changes the shape and workings of these cells in ways that might have implications for memory and even delaying the onset of dementia.

As most of us have heard, our brains are not composed of static, unchanging tissue. Instead, in most animals, including people, the brain is a dynamic, active organ in which new neurons and neural connections are created throughout life, especially in areas of the brain related to memory and thinking.

This process of creating new neurons, called neurogenesis, can be altered by lifestyle, including physical activity. Many past studies have shown that in laboratory rodents, exercise doubles or even triples the number of new cells produced in adult animals’ brains compared to the brains of animals that are sedentary.

But it has not been clear whether the new brain cells in active animals are somehow different from comparable new neurons in inactive animals or if they are just more numerous.

That question has long interested scientists at the Laboratory of Neurosciences at the National Institute on Aging, who have been examining how running alters the brains and behavior of lab animals.

Last year, in an important study published in NeuroImage, the researchers found for the first time that young brain cells in adult mice that spent a month with running wheels in their cages did seem to be different from those in animals that did not run. For the experiment, the scientists injected a modified rabies vaccine into the animals, where it entered the nervous system and brain. They then tracked and labeled connections between brain cells and learned that compared to the sedentary animals’ brain cells, the runners’ newborn neurons had more and longer dendrites, the snaky tendrils that help to connect the cells into the neural communications network. They also found that more of these connections led to portions of the brain that are important for spatial memory, which is our internal map of where we have been and how we got there.

This type of memory is often diminished in the early stages of dementia.

But these findings, while intriguing, involved animals that had been running for a month, which is the equivalent of years of physical activity by people. The researchers wondered whether such changes in neurons and connections might actually begin earlier and maybe almost immediately after the animals began to exercise.

So for the new study, which was published last month in Scientific Reports, most of the same researchers gathered a group of adult, male mice. (Males were used to avoid accounting for the effects of the female reproductive cycle.) The animals were injected with a substance that marks newborn neurons. Half were then allowed to run for a week on wheels in their cages, while the others remained inactive. Afterward, some were also injected with the modified rabies vaccine to track new synapses and connections between the neurons.

When the scientists then microscopically examined brain tissue, they found that the runners’ brains, as expected, teemed with far more new neurons than did the brains of the sedentary animals, even though the runners had been exercising for only a week.

Interestingly, these neurons also looked unique. They were larger and, as in the study of mice that ran for a month, displayed more and longer dendrites than similar neurons in the other animals. In effect, the young neurons in the runners’ brains appeared to be more mature after only a week of exercise than brain cells from inactive animals.

These young cells were better integrated into the overall brain circuitry, too, with more connections into portions of the brain involved in spatial and other types of memory. Most surprising to the scientists, these cells also proved to be less easily activated by neurochemical messages to fire rapidly, which is usually a hallmark of more mature neurons. They remained calmer and less prone to excitability than new neurons in the inactive animals’ brains.

What these differences in cell structure and connection mean for brain function remains uncertain, though, says Henriette van Praag, a principal investigator at the National Institutes of Health and senior author of this and the earlier study. Neither study was designed to look into whether the running mice thought and remembered differently than mice that were sedentary for most of the day.

But the current study “provides more pieces of evidence that brain cells produced under running conditions are not just quantitatively but qualitatively different” than other neurons, she says, “and these differences are evident very soon” after exercise begins.

Perhaps most important, the new brain cells in the runners tended to integrate into and bulk up portions of the brain that, if damaged by disease, are associated with early memory loss and dementia, she adds.

Of course, this experiment used mice, which are not people. While some past neurological studies with people have hinted that exercise might alter our brain structure in similar ways, she says, that possibility is still theoretical.

Still, she says, “I think it is a very good idea for the sake of the brain to be moving and active.”