“Neuroscientists tend to think of our brains as separate entities, or black boxes,” says Aggie Mika, a graduate student in Integrative Physiology at CU Boulder. “But it’s clear we need to start thinking about our central nervous system as being part of the whole.” And Mika doesn’t just mean part of our individual bodies—she means part of the conglomeration of cells that live on us and inside us, not all of them human. We are a walking ecosystem, and the brain must be considered as a part of that.
Our bodies are composed of at least as many bacterial cells as human cells. The bacteria that live within us, a collection of microorganisms called the microbiome, help to digest our food, build vital nutrients, and protect us from pathogens. They have evolved along with us, so our relationship with them is symbiotic and mutually advantageous.
Yet most of us still think of ourselves as being under our own control – that is, we think and act only using our own human brain cells and neural circuits. But, what if bacteria contribute not only to our digestion, but also to our behavior?
When Mika joined Monika Fleshner’s lab in the Integrative Physiology department, she had a completely different question in mind. Initially, she was intrigued by the neuroscience behind stress physiology. Stress can make people susceptible to psychiatric disorders such as depression and anxiety, and Mika wanted to study ways of guarding against these negative effects of stress. The Fleshner lab has repeatedly shown that exercise can safeguard mental wellbeing in the face of stress—so maybe all that pacing back and forth we do during finals week is actually helpful!
Ongoing research has revealed that stress has important impacts on the microbiome, and that gut microbes and the brain can communicate more directly than previously assumed. Though she was studying the connection between stress relief and exercise, Mika’s labmates were probing the relationship between the microbiome and behavior. As she learned more, it became clear that there could be a connection between her own areas of study—exercise and stress—and the microbiome.
“There was a huge burst in curiosity and knowledge concerning microbial species’ communication with the brain,” says Mika. “We wanted to know if certain species are altering behavior, and if so, how they do it.”
Mika began by asking the opposing question: how do we alter our bacterial species? Specifically, does exercise change the microbiome? And, if so, are these microbial changes tied to the beneficial effects of exercise on mental health? She began to answer these questions in her recently published paper – the first installment of the full story she hopes to tell one day.
Mika tested the effects of exercise on the microbial make-up of young and adult rats. It turns out that exercising at a young age may have the largest impact on gut bacteria, and contribute the most to the physical and mental benefits of exercise.
She found that young rats have remarkably malleable microbiomes, more so than the adults. In other words, young rats can get a benefit, in terms of good bacteria, from exercise that older rats don’t.
And that may influence the other benefits of exercising at a young age that Mika found: lean muscle mass (a sign of healthier body composition) in the exercising young rats increased compared to sedentary young rats and remained higher even after the rats stopped exercising and grew into adulthood. Exercise in early life also led to increases in certain bacterial species that are associated in the literature with promoting healthy body weight and positively affecting brain function. So Mika’s big question is: does the effect of exercise on gut bacteria lead to more positive mental health?
Studies in humans have already shown that exercise earlier in life can produce long lasting health benefits, and Mika is excited that the results of her study offer an explanation for this phenomenon.
“People who began exercising as children are more likely to be lean, less likely to suffer from cardiovascular disease and less likely to suffer from psychiatric disorders such as depression,” says Mika. “However, we don’t quite know how exercise earlier in life can improve health in such a robust and lasting way. Perhaps increases in these gut bacteria are a missing piece of this puzzle.”
Mika suggests that by knowing the relationship between exercise and “good” bacteria, clinicians can help children who aren’t able to exercise, or children on long-term antibiotics. For example, clinicians could enrich the gut bacteria of patients with health-promoting species—hopefully in an easy-to-swallow form such as yogurt.
Moving forward, Mika is trying to help dispel the “black box” perception of the brain by tying brain function to the rest of our body, including our microbial hitchhikers. She and her labmates are currently trying to figure out how these microbes are communicating with the brain, and there are many mechanistic details in this model that may remain elusive. But the excitement with which she’s attacking this project speaks for itself. Watch out world—Aggie Mika (and her accompanying ecosystem) is going places.
By Alison Gilchrist