Exercise can protect memory and learning systems from the negative cognitive impact of stress on the brain.
Researchers examined the impact of stress and exercise on synaptic plasticity, which neuroscientists call long-term potentiation (LTP). The hippocampus is the structure in the brain that processes and stores memory. LTP in the hippocampus is the leading theory of how the brain learns, forms memory, and recalls information. When we study or practice, our LTP is strengthened, learning occurs, and memory is stabilized in the brain.
While mice are not people, our brains share sufficient similarities to conclude that rodent research results are likely to apply to humans. Prior rodent studies have shown that chronic intermittent stress impairs the ability to form and recall spatial memory, and it weakens performance on a water maze and novel object recognition tests. Chronic stress also reduces the birth of new brain cells in the hippocampus and can kill existing hippocampal brain cells. Research consistently demonstrates that chronic stress inhibits LTP and has a profound adverse impact on memory.
This study involved 4 groups of mice: sedentary no stress, sedentary with stress, exercised no stress, and exercised with stress. The data confirmed that stress alone significantly reduced hippocampal LTP and exercise alone increased LTP when compared to non-stressed and non-exercised control mice.
The exercise mice were allowed to run as much as they wanted for 4 weeks, and they averaged 3 miles per day of voluntary running. The stressed mice were subjected to 3 days of stressors to produce chronic stress: day one was a 5-minute cold water swim; day two was 30 minutes on an elevated platform; and day three was 60 minutes of restraint in a plastic tube with an electric shock to the tail once per minute. The sedentary with stress mice had less LTP than the sedentary without stress, indicating that 3 days of stress was sufficient to change synaptic connections in the hippocampus. The exercise with stress mice had greater LTP than the sedentary with stress mice, and their LTP levels were very similar to the sedentary no stress mice. This indicates that the exercise had counteracted the negative impacts of the 3 days of stress.
To test memory, the researchers conducted a different set of experiments. These exercise mice were allowed as much running as they wanted for 30 days prior to the experiments, and also through the following 6 weeks of behavioral experiments. During the 6 weeks of testing, these mice were subjected to weekday stressors alternating between a 5-minute cold swim; 30 minutes on an elevated platform; and 30 minutes in a restraining tube with no shocks. During the maze training, food was restricted to nighttime access, to ensure the mice would explore the maze to find the food treat during the weekday training. The exercise mice made significantly fewer errors in the maze training than the sedentary mice did.
The study conclusion stated that if exercise and stress occur at the same time, the exercise is able to neutralize the harmful impact of the stress so that the dorsal hippocampus can experience normal rates of LTP. The maze data showed that exercise had a positive impact on spatial memory when the mice were learning a new task.
Takeaway: Based on the literature and these studies, avoiding stress and exercising are the ideal prescription for improving learning and memory via enhanced hippocampal plasticity. Since stress is unavoidable, exercise can limit the negative impacts stress has on our learning and memory systems.
Roxanne M. Miller, et al., Running Exercise Mitigates the Negative Consequences of Chronic Stress on Dorsal Hippocampal Long-term Potentiation in Male Mice, 149 Neurobiology of Learning and Memory 28-38, Mar. 2018, https://pubmed.ncbi.nlm.nih.gov/29408274/.