Exercise appears to alter the brain in ways that can reduce some of the characteristics of autism in mice.

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In a mouse model, exercise helped remove surplus connections between neurons, which is a hallmark of autism.

In mice bred to model autism spectrum disorder (ASD), scientists at the University of Tokyo in Japan found that exercise spurred the removal of surplus connections in brain circuits and diminished autism-like behavior.

In a Cell Reports paper on their work, they explain how they showed that "voluntary wheel running ameliorates the abnormalities in sociability, repetitiveness, and anxiety" in ASD mice.

The findings lend weight to the idea that it may be possible to reverse some of the behavioral characteristics of ASD.

Senior study author Ryuta Koyama, an associate professor in the Graduate School of Pharmaceutical Sciences at the University of Tokyo, points out that they did not force the mice to exercise; the voluntary nature of the physical activity was an important feature of the study.

In addition, the researchers did not explore different types of exercise and are not suggesting that people should now start using exercise as a treatment for ASD.

"If children are interested," says Koyama, "they should be encouraged to do any exercise they want, but this research does not reveal how different activities might affect the brains of children with ASD."

ASD, brain connectivity, and exercise

ASD encompasses a wide range of subtypes that share some common features to varying degrees. These features include repetitive behavior and difficulties with verbal and nonverbal communication and social interaction.

No two individuals on the autism spectrum will have the same combination or degree of symptoms. Each will possess a unique cluster of challenges and strengths. Some people with ASD can live independently while others will need a lot of help with daily living.

According to the Centers for Disease Control and Prevention (CDC), around 1 in 59 children in the United States have ASD, with boys four times more likely to have it than girls.

Surplus connections between nerve cells, or neurons, in the brain is a hallmark of ASD.

The brain naturally removes excess connections between neurons during childhood. However, children with ASD end up with a surplus.

A wealth of research has shown that physical activity can result in significant brain alteration in healthy people and even delay some of the brain degeneration that accompanies aging.

Scientists running small studies involving children with ASD have also noticed that exercise seems to lessen repetitive behavior and difficulties with social interaction.

More recently, researchers found that connections in brain circuits persist for longer periods in people with ASD than in those without the disorder. Also, symptom severity appears to go up in line with the duration of connection.

Exercise and removal of weak connections

The new study from Japan now suggests that by triggering the removal of redundant connections, exercise could promote some "fine-tuning" of the brain in ASD.

Speculating on the results that they found in the ASD mice, Koyama suggests that: "Exercise likely enhanced the contrast between active and inactive neuronal connections so that the weak ones could be more easily targeted for removal."

Mice bred as a model for ASD have many of the enduring characteristics of the condition. For instance, they do not interact as much with mice they have not met before, and they engage in repetitive grooming. They also display signs of anxiety, such as delaying eating their food when they are in new surroundings.

For the new study, the team put running wheels in the cages of 4-week-old mice with ASD. They then observed them until the mice reached adulthood at the age of 8 weeks.

Koyama says that some of the mice "exercised very intensely," while others "exercised just a little."

The team found that after just 30 days, the behavior of ASD mice that had exercised was similar to that of healthy mice without ASD.

The researchers then examined the brains of the mice with ASD. They paid particular attention to specific structures in the hippocampus, an area that is important for memory and making new nerve cells in the adult brain.

They found evidence that microglia, immune cells that remove dead cells and other waste materials in the brain, had eliminated the less active connections between nerve cells while leaving the active ones intact.

To show that it was the microglia that were doing the pruning, the team then tested the effect of preventing microglia activity in immature mice without ASD. After only 5 days without microglia activity, the density of brain connections was different from that of mice with intact microglia activity.

"Voluntary exercise can be a safe, drug-free way to improve overall health, and now we have a better understanding of how exercise can positively change brain structure in ASD."

Ryuta Koyama