Gut microbes are believed to influence brain disorders, potentially through the vagus nerve, as seen in experimental and observational studies.
The existence of a gut microbiome has been shown to influence behavior. Neuroscientist Jane Foster in 2006 compared two groups of mice that differed by their gut microbiomes: one had a healthy microbiome, the other lacked one altogether. The mice without a gut microbiome were less anxious, exhibiting less fear and greater willingness to enter into open spaces.
Parkinson's disease research has also suggested a link between gut microbes and alterations in brain function. In Parkinson's disease patients, a protein called α-synuclein is misfolded and accumulates in the brain to form Lewy bodies. In 2015, neurologist Robert Friedland proposed that bacterial proteins in the gut promote misshapen α-synuclein in the brain because a similar misfolded protein is also produced in the gut. The message to misfold this protein was suspected to be transmitted from the gut to the brain through the vagus nerve. Subsequent experiments showed that mice fed a particular strain of E. coli that produced a clumping protein in the gut had more accumulation of α-synuclein in the brain as well. Further evidence for the vagus nerve's implication comes from another mice study in which misfolded α-synuclein injections into the gut resulted in brain accumulation unless the vagus nerve was removed first.
Also of note, patients who underwent the popular procedure of having their vagus nerve removed either partially or fully as a treatment for stomach ulcers in the 1970s were found to be less susceptible to Parkinson's disease years later.
Mice studies of ALS have pointed to the gut's potential influence on disease severity. Immunologist Eran Elinav found that symptoms in ALS mice improved when Akkermansia muciniphila bacteria were introduced into their gut, presumably due to a metabolite they produce – nicotinamide, or vitamin B3 – that's believed to travel from the blood into the brain.