It’s commonly accepted that regular exercise helps reduce anxiety and stress, but just how it does this has not been clear. What’s also confused scientists is how exercise can help people feel more calm when it actually stimulates the growth of new, more excitable/active neurons in the ventral hippocampus – the main part of the brain that helps to regulate anxiety.

However, scientists at the University of Princeton in the US now think they have the answer to both of these conundrums. Their findings were published in the May 2013 edition of the Journal of Neuroscience*.

Shocking test
The findings were based on a small study of adult mice. One group of mice was given unlimited access to a running wheel, while another group was not and remained sedentary and caged. As natural runners, mice will cover up to 2.5 miles daily on a wheel.

After six weeks, all mice were briefly exposed to cold water to activate the ventral hippocampus. An analysis of brain activity showed that the cells of active and sedentary mice responded in different ways almost as soon as they were faced with this stressful situation.

Brain-altering activity
The scientists discovered four underlying mechanisms at work when the mice were exposed to the cold water.

In the neurons of the sedentary mice, the shock spurred an increase in ‘immediate early genes’ – short-lived genes that are rapidly turned on when a neuron fires. However, the active mice did not have these genes in their neurons, which suggests their brain cells did not immediately leap into an excited state in response to the cold water.

Instead, the brains in active mice showed every sign of controlling reactions above and beyond what was observed in sedentary mice – which is where the three other mechanisms come into play.

In the active mice, there was also a boost of activity in inhibitory neurons when they were exposed to the cold water. These inhibitory neurons keep excitable neurons – those more likely to rapidly respond to stimuli – in check, thereby shutting off excitement in the ventral hippocampus.

Simultaneously, the scientists observed that the brain neurons in the active mice released more of the neurotransmitter gamma-aminobutyric acid (GABA), which dampens neural excitement. Finally, a protein that helps to deliver and release GABA into the brain was also present in higher amounts in the non-sedentary group.

In short, the scientists found that physical activity rewires the brain to cope better with stress.

Treating anxiety
This is one of the first studies to focus in depth on the impact of exercise on the ventral hippocampus. As such, it has helped to pinpoint the important brain cells and regions related to anxiety regulation. The research also shows that the brain can be extremely adaptive, tailoring its own processes according to the lifestyle and surroundings of an organism.

Senior author Elizabeth Gould, Princeton’s Dorman T Warren professor of psychology, says: “Understanding how the brain regulates anxious behaviour gives us potential clues about helping people with anxiety disorders. It also tells us something about how the brain modifies itself to respond optimally to its own environment.”

*Gould E et al. Physical exercise prevents stress-induced activation of granule neurons and enhances local inhibitory mechanisms in the dentate gyrus. J Neurosci, May 2013