Wednesday January 20 2010
Changes in the brains of mice have limited implications for humans
“Running stimulates the brain to grow fresh grey matter and has a big impact on mental ability,” reported The Guardian.
The newspaper does not mention until very late in the story that this research was carried out on rodents. Mice that were given an exercise wheel performed better in a series of tasks, and their autopsies showed they had greater nerve cell development in the brain.
However, the tasks the mice performed have little relation to human memory processes, and a few days on a running wheel is very different to human exercise. Also, only 20 mice were tested, therefore it is possible that any differences in the running group could be due to chance alone.
Despite these limitations, there is no doubt that regular exercise and a healthy balanced diet is beneficial to health, and this is likely to include improvements in physical and mental wellbeing.
Where did the story come from?
The study was carried out by David Creer from the Laboratory of Neurosciences, Intramural Research Program, National Institute on Aging, Baltimore, and colleagues from Cambridge University. The research was supported by the Intramural Research Program of the National Institutes of Health, National Institute on Aging, and the study was published in the peer-reviewed medical journal PNAS.
All the news reports have overinflated these findings and put considerable emphasis on the relevance to humans without considering the limitations of this animal study.
What kind of research was this?
This was an experimental study in mice. The researchers say that previous studies have suggested that regular exercise may improve nerve development and signal transmission between nerve cells. In particular, such changes have been noted in the area of the brain involved in learning and memory - the hippocampus. However, the specific mechanisms through which exercise may influence information processing in the brain are unknown. This is what the study aimed to examine.
Animal studies such as this can improve our understanding of physiological processes in animals that may also be applicable to humans. However, mice are clearly very different from humans, and the findings have limited direct implications for human health.
What did the research involve?
This experiment involved exercising adult and elderly mice to see how running affected their orientation and spatial awareness.
The research was in 20 three-month-old mice and eight 22-month-old mice, which were tested using a system of stimuli, trained responses and reward. This system involved a chamber with clear walls separating it into compartments, a framework of spaced metal bars on the floor, and an ‘operant chamber’ fitted with an infrared touch screen, a food pellet receptacle and dispenser, a light source, tone generator, and several ‘windows’ by which stimuli could be presented. The mouse’s presence at the touch screen was detected by infrared sensors.
After a period of adaptation in which they became accustomed to the chamber, the mice were put through a ‘training’ stage. This consisted of a light stimulus being projected onto the screen. When this stimulus was switched on, a pellet was dispensed any time the mice touched the screen. After the mice mastered this, they progressed to a ‘must touch’ stage where the light stimulus had to be touched to induce pellet delivery. The mice then progressed to a ‘must initiate’ stage, where, after eating the delivered pellet, they had to initiate the next light stimulus by touching the pellet receptacle.
After one month of these training periods, 20 mice were divided into ‘control’ and ‘run’ groups. The run group had a running wheel installed. All mice received injections of bromodeoxyuridine (BrdU) for five days - this would label newly developing nerve cells in later histological analysis. Both groups then underwent 60 further tests in the touch chamber. These involved two light sensors presented in six possible positions, and different pellet-dispensing responses being used in the different tests. After the full testing period, brain sections were examined for newly developing cells and new blood vessel formation.
What were the basic results?
The researchers found different responses between the adult and elderly mice. Access to the running wheel enhanced the ability of adult mice (three months old) to better perform in the series of ‘two stimuli’ tests. Improved performance was found to correspond with increased nerve cell development.
However, the older mice (22 months old) did not show improved performance or nerve cell development when allowed to run.
In younger mice, the improved performance and nerve cell development were only found when the two stimuli were presented close together and not far apart. This suggests that when stimuli are obviously distinct, running has no effect on nerve cell development or improved test performance.
How did the researchers interpret the results?
The researchers conclude that their findings suggest that newly developed nerve cells may improve the brain’s ability to distinguish between fine spatial information, and that exercise may enhance these changes.
This scientific study in mice has little current application to human health. Animal studies such as this can improve our understanding of physiological processes in animals that may also be applicable to humans.
However, mice are very different from humans, and these findings have limited direct implications for human health. Though an improved ability at distinguishing between two closely positioned light sources suggests improved spatial learning in mice, this is probably not comparable to human thought processes. It is a big jump to say that this would relate to improved memory in humans. In addition, the intensive running by the mice has little similarity to human exercise patterns.
Importantly, the fact that only 20 mice were involved in the tests means that any difference between running and control groups could be due to chance alone. The sample sizes were further reduced by subgroup analyses in the two age conditions (young and elderly), so any within-group differences may be even more biased by small samples.
Whatever the limitations of this study and its relevance to humans, there is no doubt that regular exercise combined with a healthy balanced diet is beneficial to health, and that those benefits are likely to include improvements in physical and mental wellbeing.