Exercise is well known for its cognitive benefits, thought to occur because it causes neurogenesis—or the creation of new neurons— in the hippocampus, a key brain region for learning, memory, and mood regulation.
So it was a surprise when a 2014 study in the journal Science showed that exercise caused mice to forget what they’d already learned.
“It stunned the field of hippocampal neurogenesis,” says Ashok K. Shetty, professor in the molecular and cellular medicine department at Texas A&M University. “It was a very well-done study, so it caused some concern that exercise might in some way be detrimental for memory.”
Animal models in the exercise group of the previous study showed far more neurogenesis than the control group, but the additional neurons seemed to erase memories that were formed before they started the exercise regimen. To test this, researchers removed the extra neurons, and the mice suddenly were able to remember again.
“The mice who exercised had a large number of new neurons,” Shetty says, “but somehow that seemed to break down the old connections, making them forget what they knew.”
Researchers decided to replicate the study using rats instead of mice. Rats are thought to be more like humans physiologically, with more-similar neuronal workings. The rats showed no such degradation in memories.
“We had completely contradictory findings from the 2014 study,” says Maheedhar Kodali, a postdoctoral fellow at the Institute for Regenerative Medicine and the first author of the current study in the Journal of Neuroscience. “Now we need to study other species to fully understand this phenomenon.”
Shetty and his team trained the rats to complete a task over the course of four days, followed by several days of memory consolidation by performing the task over and over again. Then, half the trained rats were put into cages with running wheels for several weeks, while the control group remained sedentary.
The rats who ran further over the course of that time had much greater neurogenesis in their hippocampus, and all rats who had access to a wheel (and therefore ran at least some), had greater neurogenesis than the sedentary group. On an average, they ran about 48 miles in four weeks, and neuron formation doubled in the hippocampus of these animals.
“This is pretty clear evidence that exercise greatly increases neurogenesis in the hippocampus, which has functional implications,” Kodali says.
“Neurogenesis is important for maintaining normal mood function, as well as for learning and creating new memories.” This connection may help explain why exercise is an effective antidepressant.
Importantly, despite differing levels of increased neurogenesis, both moderate runners and brisk runners (those who ran further than average) in the study showed the same ability as the sedentary runners to recall the task they learned before they began to exercise. This means even a large amount of running doesn’t interfere with the recall of memory.
“Exercise is not at all harmful,” Shetty says. “It doesn’t cause any memory problems, and there are many studies proving its benefits for making new memories and maintaining good mood. Now, our study showed that exercise does not interfere with memory recall ability. Keep exercising, and don’t worry about losing your old memories.”
Source: Texas A&M University