Yup. Dopamine, serotonin, and norepinephrine have all been shown to be produced by microbes in the gut, including fungi in some cases. The gut-serotonin angle h

Yup. Dopamine, serotonin, and norepinephrine have all been shown to be produced by microbes in the gut, including fungi in some cases. The gut-serotonin angle has been largely ignored because it does not cross the blood brain barrier, but this seems like an oversight due to oversimplification and a myopic view of the vast interconnectedness of the body's various systems. After all, tryptophan does cross the BBB and indeed it must do so in order to supply the body with precursor for endogenous serotonin production in the brain. To say nothing of serotonins many roles outside of the brain that are increasingly being uncovered (all 15 serotonin receptor types are expressed throughout the body, not just the brain). [Above is fairly well established in medical literature, the following is speculative:] Gut microbes may be out-competing "us" for dietary tryptophan, or, alternatively, inflammatory conditions in the GI may trigger the body to send the majority of free tryptophan down the kynurenine pathway, with little left over to make it up to the brain for endogenous neurotransmitter production.