In a groundbreaking study back in 2014, researchers discovered that when mice exercise, it causes new neurons to be born in a region of the brain called the hippocampus, and this pushes old knowledge out of their brains, making them forget things they had learned before exercising. This obviously came as quite a surprise to scientists, who began to worry that humans may also experience similar memory losses when exercising. Fortunately, however, a new study has revealed that this does not occur in rats, suggesting that it may be a species-specific quirk that only affects some animals.
The hippocampus plays a central role in learning and the formation of memories, and is one of the very few parts of the brain where new neurons are created after we are born. Previous research has shown that people who are physically fit tend to have larger hippocampi than those who do not, suggesting that exercise may stimulate this process, known as neurogenesis.
The discovery that mice who exercised developed more new hippocampal neurons than those who did not exercise therefore came as little surprise to scientists, yet they were astounded to find that when these neurons formed connections with other brain cells, they tended to overwrite existing connections, leading to memory loss.
Exercise has been shown to stimulate neurogenesis in the hippocampus. dreamerb/Shutterstock
However, writing in the Journal of Neuroscience, the authors of this latest study explain that rats are actually more intelligent than mice, and that their brains have more in common with those of humans, which is why they chose to investigate whether or not this same effect occurred in rats.
After training these rats to memorize their way around a maze, the researchers then placed the animals into two groups, one of which was given regular access to a running wheel while the other was forced to remain sedentary for four weeks. At the end of this period, the study authors measured the number of new neurons in the rats’ brains, discovering that those that ran every day had between 1.5 and 2.1 times more new hippocampal neurons than the sedentary rats. This was directly dependent upon the distance that these rats ran over the course of the four weeks, with those that ran the furthest experiencing the highest levels of neurogenesis.
At this stage, the researchers placed the rats back into the maze that they had previously memorized, finding that their ability to remember their way around was completely unaffected by their level of exercise or the number of new hippocampal neurons developed in the intervening period.
Therefore, the authors conclude that exercise “does not impair memory recall ability in a rat model despite substantially increasing neurogenesis,” which is good news, as it means humans may not have to pay for physical fitness with their memories after all.
Though more work is needed in order to determine why hippocampal neurogenesis causes memory loss in some animals but not others, the team suggest that it may have something to do with certain differences in the structure of neurons between species. For instance, rat neurons do not connect in the same way as mouse neurons, and may therefore be able to form without destabilizing existing connections and erasing memories.
Given that human neurons have more in common with those of rats than those of mice, the researchers are hopeful that hitting the gym won’t squeeze our old knowledge out of our brains.
Unlike mice, humans who exercise probably don't lose any memories. Pavel Ilyukhin/Shutterstock