A stomach "gripped with fear" or "in knots"; these popular expressions bear their name well. In the last few years, scientists have made many discoveries concerning the relationship between the brain and the gut microbiota. A study led by the Pasteur Institute and involving the CEA-Joliot has looked at the links between microbiota imbalances and psychological disorders, in particular depression.

Using a mouse model (healthy or with mood disorders induced by chronic mild stress), the researchers studied the relationships between microbiota, the metabolism of certain fatty acids, and neurogenesis in the hippocampus, a brain region strongly implicated in the development of depressive symptoms. They showed that the symptoms of stressed mice (decreased neurogenesis in the hippocampus and mood disorders) can be transferred to naive recipient mice by transplanting fecal microbiota. Their analysis reveals that the recipient mice have an altered fatty acid metabolism, which leads to a decrease in endocannabinoid system[1] activity in the brain. The adverse effects of this microbiota transfer can be alleviated by increasing the endogenous cannabinoid levels. In addition, the researchers found that chronic mild stress induces an imbalance in the gut microbiota of mice, characterized by a decrease in the abundance of bacteria in the genus Lactobacilli.

These observations strongly suggest that an imbalance in gut microbiota can be linked to mood disorders through the endocannabinoid system. Moreover, dietary or probiotic interventions could potentially be effective tools to combat stress-related depressive syndromes.

[1] The endocannabinoid system is a major player in the homeostasis (equilibrium) of cells.