Contradictory Results on New Neurons and Old Memories

The adult brain adds new neurons at a very slow pace. Exercise increases that pace, so it is a place to start when trying to determine the likely outcome of therapies that greatly increase the generation of new neurons. As they are developed, such therapies will likely come to have an important place in the near future toolkit of rejuvenation therapies. The generation of new neurons diminishes with age, and is an important part of the plasticity of the brain, determining the ability to learn, change, and heal minor damage. At this point, many of the possible outcomes of a greater supply of new neurons remain debated, with studies still taking place. The contradictory animal data covering effects on memory noted here is just one example.

Research has found that exercise causes more new neurons to be formed in a critical brain region, and contrary to an earlier study, these new neurons do not cause the individual to forget old memories. Exercise is well known for its cognitive benefits, thought to occur because it causes neurogenesis, or the creation of new neurons, in the hippocampus, which is a key brain region for learning, memory and mood regulation. Therefore, it was a surprise in 2014 when a research study found that exercise caused mice to forget what they'd already learned. "It was a very well-done study, so it caused some concern that exercise might in some way be detrimental for memory."

The animal models in the exercise group - in the previous study - showed far more neurogenesis than the control group, but contrary to what one might think, these additional neurons seemed to erase memories that were formed before they started the exercise regimen. To test this, the 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, but somehow that seemed to break down the old connections, making them forget what they knew."

Researchers decided to replicate this earlier research, using rats instead of mice. Rats are thought to be more like humans physiologically, with more-similar neuronal workings. They found that - luckily for runners everywhere - these animal models showed no such degradation in memories. "We had completely contradictory findings from the 2014 study. Now we need to study other species to fully understand this phenomenon." The researchers trained their animal models 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 animal models 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.

Importantly, despite differing levels of increased neurogenesis, both moderate runners and brisk runners (those who ran further than average) in the new 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 (akin to people who perform significant amount of exercise on a daily basis) doesn't interfere with the recall of memory.



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