The Gut Microbiome and Anxiety

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Introduction to the Gut-Brain Axis

By Laura Grosh

When people think of organs, they would likely not list the gut microbiome. Increasing research, however, is starting to change this assumption. The gut microbiome is a dynamic collection of microbes that live in our intestinal tract, and we are beginning to see that this microbial community is as integral to our health as the organs that may initially come to mind such as the heart or lungs. The gut microbiome is huge; recent estimates state that in a 70 kg person, 0.2 kg are bacteria, many of which reside in the gut. One way that the gut microbiome is vital to us in through their connection to our nervous system. Microbes are directly responsible for making neurotransmitters and neuropeptides, integrating them into the nervous system—but the connection between these seemingly separated systems extends beyond this. The vagus nerve connects the brain and the gut, a direct representation of the gut-brain axis. Autonomic, immune, and endocrine responses further complicate and interact with this connection between the gut microbiome and our nervous system. Research has addressed this connection by performing metagenomic analyses on the gut microbiome, studying model organism lacking commensal bacteria, altering the gut microbiome through probiotics or antibiotics, activating or deactivating the vagus nerve, among many other molecular and genomic manipulations Studying this connection, as complicated as it is, is beginning to uncover the implications of the gut microbiome on mental health. The gut microbiome is increasingly believed to facilitate relationships between stress and anxiety, both in direct and indirect ways. This has direct implications on public health and the field of mental health, and could even increase treatment options for anxiety in the future.

Stress and the HPA axis

One way that the gut microbiome may influence anxiety is through the relationship between the gut microbiome and the HPA axis, stress and inflammation. The HPA axis is a system that modulates stress responses. When a physical or psychological stressor appears, the hypothalamus sends signals to the pituitary gland, which releases adrenocorticotropic hormone. This hormone acts on the adrenal glands to release cortisol, which is the bodies main stress hormone. In an ideal world, cortisone provides negative feedback for both the hypothalamus and pituitary gland. Of course, in the less than perfect body’s we inhabit and in an increasingly stressful world, this can go sometimes go wrong. Our stress response is not the same thing as clinical anxiety, but there are many correlations that make the HPA axis an interesting thing to study when asking questions about anxiety. Dysregulation of the stress response and HPA axis, behaviorally observed as chronic stress, is highly co-morbid with anxiety. To further support the connection between HPA axis and anxiety, patients with anxiety disorders show an increased cortisol response, suggesting an increase in HPA axis activity. Understanding how the gut microbiome influences the HPA axis is the first step in understanding how the gut microbiome affects anxiety.
The influence of the microbiome on stress and the HPA axis may start from birth. Observed in mice, maternal stress during pregnancy and offspring stress in early life both lead to dysbiosis (an abnormal gut microbiome) and altered development of the HPA axis. This is likely because both the gut microbiome and the HPA axis are not yet developed at birth. These findings lead us to believe that the gut microbiome, both at birth and in early development, is connected to the development of the HPA axis. Germ free (GF) mice are a common model organism for studying the relationship between gut microbiota and behavior. GF mice are raised without exposure to microorganisms, giving us insight into what behaviors and processes are impacted without a microbiome. Using GF mice has proven to be extremely useful when studying the gut-brain axis.
Multiple studies have used GF mice to examine HPA development and function. Fascinatingly, not all studies using GF mice yield the same effects on HPA function. One such study, done by Huo et al. in 2017, used a population of GF mice and mice free from specific pathogens (specific pathogen free mice, or SPF mice). After chronic restraint to induce stress, the researchers measured exploratory time, a behavior associated with stress level, as well as level of hormones released by the HPA axis. In the stressed subject groups, they found that GF mice had significantly higher stress hormone levels compared to SPF mice. However, the GF mice exhibited more exploratory behavior than the SPF mice. Other studies, however, have found an increase in anxiety-associated behavior in GF rats compared to SPF rats, and similar dysregulation of HPA axis-related hormones. Even though there is contradictory evidence whether GF animals experience more or less anxiety, it is clear that the microbiome does play a role in anxiety-like behavior by modulating the HPA axis.