Gut microbiome transplant can cause depression

American scientists transplanted the intestinal bacteria of a stress-vulnerable rat to a stress-resistant animal and observed the development of depression in the recipient. The results of the experiments can serve as a basis for the creation of a new group of drugs for mental disorders. Scientists report on the study in the pages of the journal Molecular Psychiatry .  

Stress can lead to neuroinflammation in the hippocampus, the brain region responsible for the formation of emotions, the transition of short-term memory to long-term and spatial navigation. Neuroinflammation is triggered by stress hormones (corticosteroids), to which hippocampal cells carry an increased number of receptors on their membranes. In response to the accumulation of stress hormones, pro-inflammatorymolecules of cytokines – indicators of inflammation – and mobilization of cells of the immune system. This process can lead to partial degeneration of hippocampal cells and cause the development of Alzheimer’s disease, post-traumatic stress disorder, and chronic depression. It is noteworthy that in patients with mental disorders, the intestinal microflora – microorganisms that live in the gastrointestinal tract in symbiosis with the carrier – differs from that in healthy people. By the way, the human intestine contains on average about 50 trillion microorganisms.

Dr. Sima Bhatnagar , a neurologist at the Children’s Hospital of Philadelphia, and her colleagues found that stress affects the gut microflora. To select rats for further experiments, the researchers simulated the following situations: a forced swim test and a social defeat test. In the experimental model of the latter, a situation was created in the course of which one animal acquired a dominant status, and the other — the status of a defeated one. Based on the passive or active approach of rats to deal with stressful situations, the scientists appropriately classified the animals as vulnerable or resistant.

Feces of the vulnerable rats had higher proportions of intestinal bacteria such as Clostridia . By transplanting gut bacteria from a vulnerable rat to a resistant rat, the scientists showed that the recipient animals began to exhibit depressive-type behavior. Moreover, after the transplant, the recipients developed neuroinflammation in the hippocampus. 

“People are already taking over-the-counter probiotics as a supplement,” says Dr. Bhatnagar . “If positive behavioral effects from specific bacteria are confirmed, we can pave the way for a probiotic treatment for human mental disorders such as depression.”

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