Biologist Andrew Naylor arrived at this by studying rats running in a treadmill. Rats that were allowed to run the treadmill for nine days acquired five times as many new stem cells in the part of the brain called the hippocampus compared with rats in a control group that did not run at all. Four weeks after their training, that is four weeks after the treadmill had been removed from the cage, about a third of the new cells were still there and had developed into functioning nerve cells.
The hippocampus is a part of the brain that is important for the regulation of stress reactions, but also for memory and learning. Andrew Naylor could confirm that these rats actually were better learners after their exercise by a running memory and learning test on them. The test had the rats swim in a round basin that had one place where they could get up out of the pool. The rats that had exercised learned more quickly than those who hadn’t where they should swim to get up out of the water.
A third group of rats were allowed to run on the treadmill as much as they wanted for 24 days. For some reason these rats increased their amount of running from six kilometers a day to as much as 20 km a day.
“Just why these rats ran more and more, I can’t say, but they seem to like it. Maybe they develop an addiction of some sort. I plan to study this in the future,” says Andrew Naylor. But even if the rats run a lot because they like it, this had no positive impact on the number of cells in the brain. On the contrary, they developed only half as many new cells in the hippocampus as the ones that didn’t exercise at all.
“What’s interesting is that if the over-exercising rats were left to rest for a few weeks, more than half of their newly-formed cells survived, which is more than twice as many as in rats who didn’t exercise. But since there were fewer new cells from the beginning in the over-exercised rats, the ultimate result was largely the same number of cells in both groups anyway. In other words, the brain seems to find a way to compensate for the fact that fewer cells are formed when rats run too much.”
How come physical exercise in moderation is good for the brain but too much is not? This probably has to do with endorphin, the body’s own morphine, and the stress hormone corticosterone.
In physical exercise endorphin is released, and Andrew Naylor has shown that it is endorphin that stimulates the new generation of cells in the hippocampus: when he blocked the effect of endorphin in the rats, the increase in new cell formation did not take place. In rats that exercise too much it is probably the effect of corticosterone that takes over. In previous studies it has been shown that coricosterone, which is released with stress, and stress itself both decrease the new formation of cells in the hippocampus in rats. In humans the effect of long-term stress is a shrinking of the hippocampus, which may be the result of decreased generation of new cells.
“It isn’t easy to translate what is moderate exercise for a rat into what is just right for us humans. There are recommendations that say it is optimal for your health to exercise for 35 minutes four to five days per week at 60 percent of you maximum pulse. It is hard to say whether this is also optimal for mental health. There is research that indicates that elite athletes can be negatively affected by long and intensive periods of training. We everyday exercisers don’t train like that, but if the physical stress is too great, the same thing can no doubt happen to us,” says Andrew Naylor.
Title of dissertation: Differential effects of voluntary running on hippocampal plasticity in the adult rat brain.