Psychiatry and the Gut: The Role of Microbiota in Mood Disorders

Psychiatry and the Gut: The Role of Microbiota in Mood Disorders
Psychiatry and the Gut: The Role of Microbiota in Mood Disorders

The gut microbiome can be defined as “the community of bacteria and their genetic material living in the gut.”1 These “commensal” bacteria are estimated to number 100 trillion in the human gut alone.2 The microbiome also contains archaea, protozoa, fungi, and viruses, which have co-evolved with the human host.

Human beings have a “symbiotic relationship” with gut microbiota.3 We provide them with a habitat and nutrition, while they confer beneficial effects on a wide range of domains, including the immune, gastrointestinal, endocrine, and neurological systems. They perform vital functions, such as food processing, digestion of complex indigestible polysaccharides, synthesis of vitamins, and inhibition of pathogens.

“In the past decade, significant progress has been made in recognizing the importance of gut microbiota in brain function,” Jane Foster, PhD, an associate professor in the Department of Psychiatry and Behavioral Neurosciences at McMaster University, Hamilton, Canada, told Psychiatry Advisor.

“Recent clinical and preclinical studies are demonstrating that gut microbiota play a central role in contributing to normal, healthy homeostasis, and that their disruption contributes to risk of mood and other [central nervous system] disorders.”

The Brain-Gut-Microbiota Axis

There is a bidirectional communication network between the gut and the brain, Foster said. Gut microbiota are components of the “brain-gut-microbiota axis” (BGM). The scaffolding of the BGM includes the CNS, neuroendocrine, and neuroimmune systems, the sympathetic and parasympathetic arms of the autonomic nervous system, and the enteric nervous system and microbiota.  

“This is the network through which the brain influences motor, sensory, and secretory functions of the gut. In turn, visceral messages from the gut influence brain function,” Foster said. For example, the vagus nerve has efferent and afferent divisions that play a critical role in enabling signals to travel to and from the gut.

Activation of the vagus nerve has anti-inflammatory effects. Many positive effects of gut microbiota and probiotics on brain function depend on vagal activation. But other independent mechanisms are also at play.

Gut, Inflammation, the Immune System, and the Stress Response

Microbiota affect circulating levels of pro- and anti-inflammatory cytokines that directly impact brain function, especially in the hypothalamus, according to Foster. Pro-inflammatory cytokines stimulate the release of corticotrophin releasing hormone (CRH), which is the dominant peptide regulator of the hypothalamic-pituitary-adrenal axis (HPA).4

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