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. 2021 Feb 25;13(3):732. doi: 10.3390/nu13030732

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© 2021 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Hypothetical influences on brain diseases by gut bacteria-derived tyrosine and tryptophane metabolites. This figure illustrates the mechanistic effects by which gut microbial metabolites might influence brain functions related to autism spectrum disorder (ASD) and neurodegenerative disorders (NDs): Parkinson’s disease (PD), Alzheimer’s disease (AD), multiple sclerosis (MS) and psychiatric disorders (PsyD). Gut bacteria taking part in metabolite production are listed in black boxes situated under the orange (tyrosine metabolites) and blue boxes (tryptophane metabolites). Arrows accompanied with + or − represent an agonistic (+) or antagonistic (−) effect on a receptor, whereas unaccompanied arrows symbolize an effect described in the white boxes. ↑ = upregulated, ↓ = downregulated or lowered levels of. ROS = reactive oxygen species; NMDA-R = N-methyl-D-aspartate receptor; Trp = tryptophane; AHR = aryl hydrocarbon receptor; 4EPS = 4-ethylphenylsulfate; IPA = indole-propionic acid; IAA = indole-acetic acid; kyn = kynurenine; quin = quino-linate; trypt = tryptamine; i.a. = inter alia.