FIGURE 1: Bioactive molecules originating from microbial metabolism are thought to modulate emotional behavior through several mechanisms:

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(1) Activation of afferent vagal nerve fibers.

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(2) Stimulation of the mucosal immune system or of circulatory immune cells after translocation from the gut into the circulation.

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(3) Absorption into the bloodstream, and biochemical interaction with a number of distal organs. In the brain, such metabolites may be able to activate receptors on neurons or glia, modulate neuronal excitability, and change expression patterns by means of epigenetic mechanisms.

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