A recently published study by a research team at McMaster University in Canada found that gut bacteria may play a key role in the development of depression and anxiety.
This study is far from the first to investigate this gut-brain connection – in fact, journalists and scientists alike have started to refer to the digestive system as the “second brain.” The enteric nervous system, or the several hundred million nerve cells that line the gastrointestinal tract, may hold a surprising amount of power over our more traditional brains. Understanding how naturally occurring gut bacteria can influence the enteric nervous system – and thereby behavior – may help scientists craft treatments for mood disorders, autism and even Parkinson’s disease.
In this study, titled “Microbiota and host determinants of behavioural phenotype in maternally separated mice,” published in the prestigious journal Nature Communications, researchers chose to investigate two groups of mice. One group was healthy and had normal levels of gut bacteria. The other group, however, had no gut bacteria at all. Some mice in each group were subjected to early life stress – specifically, they were separated from their mothers for a set time each day – an animal model known to provoke anxiety and depression.
Once both sets of mice reached adulthood, the researchers examined their behavior and physiology.
Mice that were born with normal amounts of gut bacteria demonstrated symptoms of depression and anxiety as well as heightened levels of corticosterone, a stress hormone. These mice also demonstrated impaired gut function.
Mice that were born without gut bacteria, meanwhile, did not show symptoms of anxiety or depression. Like the healthy mice, however, they experienced disrupted gut function and high levels of stress hormones.
What does this mean?
Even though both sets of mice were treated identically, the mice without gut bacteria demonstrated a psychological resilience to early life stress and failed to develop anxiety or depression. The scientists then wondered: Could depression be transferred through gut bacteria?
Scientists inserted gut bacteria from the stressed, depressed mice with normal gut bacteria into the stressed (but not depressed) mice without gut bacteria. Sure enough, the mice that previously had no gut bacteria developed depression as well, suggesting that depression could be effectively transferred from one organism to another through gut bacteria.
The researchers then inserted gut bacteria from stressed, depressed mice with normal gut bacteria into non-stressed mice without gut bacteria. These mice did not develop depression, suggesting that both external stress and gut bacteria are necessary for an organism to develop depression.
“This suggests that in this model, both host and microbial factors are required for the development of anxiety and depression-like behavior,” explained the senior researcher, Premysl Bercik, M.D., an associate professor of medicine at the Michael G. DeGroote School of Medicine at McMaster University. “Neonatal stress leads to increased stress reactivity and gut dysfunction that changes the gut microbiota which, in turn, alters brain function.”
The future of depression treatment
Research study after research study has confirmed that gut bacteria plays a role in behavioral health. If the results of this study are true, the future of depression treatment might involve probiotic food rather than antidepressants. Of course, mice are not humans, and more research will be necessary before the entire framework of mental health treatment can be tossed out the window. Still, the results show tantalizing promise and add yet another brick to the foundation of future depression treatment.
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Written by Courtney Lopresti, M.S. neuroscience, Sovereign Health Group writer