Microbiota-gut-brain axis and psychiatric disorders

The human intestine contains an astonishing number of microorganisms. It is estimated that these microorganisms contain 150 times the number of genes in the human genome. The types and distribution of microbes in the intestines have been confirmed to be related to depression, autism, or stress, and other emotions. In the relevant literature, it is mentioned that "microbes in the stomach and intestines can influence the development, function and even behavior of the brain through neuronal, endocrine, and immune pathways, etc."; the transmission of these pathways include: the production of neurotransmitters, the metabolism of tryptophan, the regulation of immune activity in the central nervous system, the production of short-chain fatty acids, the production of intestinal hormones and the production of branched-chain amino acids. Similarly, the state of the brain (stress level, happy or not) will also use these pathways to affect gastrointestinal motility, permeability, digestion, and the number and types of microorganisms, which are inseparable from each other. This also shows that this imbalance in communication will affect the development of some mental disorders. The term microbiota-gut-brain axis came into being to describe the role of intestinal bacteria, intestinal epidermal cells, and the brain in the interaction.
The enteric nervous system is composed of 500 million neurons and is the largest nervous system outside the brain. There are dozens of neurotransmitters in the intestine, which shows that the intestine is independent, can think, feel, and express emotions, like the second brain, its important performance is the same as the brain. For example, there are two important pleasure neurotransmitters in the human body that affect our minds and emotions: dopamine and serotonin. When the brain secretes a large amount of dopamine, it can make us show creativity, and it is the propellant of human development and progress. Serotonin brings happiness, balance, and stability to the human body, participating in emotion, sleep, appetite, body temperature regulation, and other functions in the brain. The happy channel transmitter that affects our sense of happiness: serotonin, the brain secretes only 5%, and the intestine secretes 95% of the human body; and dopamine, 50% is synthesized in the intestine. And the production of these substances is greatly related to the intestinal flora, which may indicate that the intestinal flora has a great influence on mood. At present, there are some evidences in clinical trials showing that some mental disorders are related to the intestinal flora, and the related symptoms can be significantly improved after supplementing with probiotics. For example, supplementing with Bifidobacterium spp., Lactobacillus spp. can improve the cognition of patients with Alzheimer’s disease regarding functional and metabolic status; supplementation of Bifidobacterium can improve symptoms of anxiety and depression in patients with schizophrenia; supplementation of Lactobacillus spp., Bifidobacterium lactis can reduce the rate of individual readmission after a manic episode in patients with bipolar disorder; supplementation of Lactobacillus, Bifidobacterium Streptococcus can reduce epilepsy in patients with the frequency of seizures, improving the quality of life. All these indicate that the bacteria-gut-brain axis is of great significance to some mental disorders.
Although there is evidence that certain types of microorganisms can improve related symptoms in patients with mental disorders, it also illustrates the role of the bacteria-gut-brain axis. However, in all studies, the exact mechanism and signaling pathways of the activation of the gut microbiota and its metabolites have not yet been fully elucidated; this requires more epidemiological studies with considerable sample size and consideration of multiple influencing factors ( genetics, lifestyle, environment), in order to better understand the role of the gut microbiota in mental disorders.

  1. Lami KF, Oliveira VF, Batista KZS. Gut-brain axis and immunoneuroendocrine
modulation in neurological and psychiatric disorders: A systematic review. Research, Society and Development, v. 10, n. 4, e28110414185, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i4.14185.
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  2. Updated on August 20, 2020