Gut-brain-crosstalk- the vagus nerve and the microbiota-gut-brain axis in depression. A narrative review

This narrative review summarizes key aspects of vagus nerve function as a main player in the microbiota-gut-brain axis in depression. Gut microbes as well as their metabolites signal directly to the vagus nerve via specialized enteroendocrine cells called “neuropods”. Some gut microbes, such as Lactobacillus rhamnosus, do not exhibit anti-anxiety effects in animal models when the vagus nerve is cut. Furthermore, vagus nerve function correlates with gut microbiota diversity, and short-chain fatty acid producers, such as Lactobacillales and Ruminococcaceae, are more abundant in individuals with better vagus function. The vagus nerve is activated by several neurotransmitters produced by the gut microbiota and regulates a cholinergic anti-inflammatory pathway (CAIP), which attenuates inflammation and decreases intestinal permeability, which may be relevant in the inflammatory subtype of depression. Psychobiotics, pre-, pro and synbiotics, with effects on behavior, work via the vagus nerve, and could be used to enhance its function. More research on gut microbiome interactions with the vagus nerve is necessary to develop targeted psychobiotics to improve vagus nerve function for optimal mental health.