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. 2021 Jun 22;12:687757. doi: 10.3389/fphar.2021.687757

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Copyright © 2021 Zhu, Hu, Deng, Tan, Qiu, Zhang, Ni, Lu, Wang, Li, Luo, Huang, Xiao, Liu, Li, Shang and Wen.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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The timeline of key, seminal original discoveries that gave rise to the fields of research on the kynurenine pathway (KP) in mood disorders and the gut-brain axis. Note. TRY: tryptophan, QUIN: quinolinic acid, NMDAR: N-methyl-d-aspartate receptor, CNS: central nervous system, IDO: indoleamine 2,3-dioxygenase, KYNA: kynurenic acid, IL-1β: interleukin-1β, LPS: lipopolysaccharide, HPA: hypothalamic-pituitary-adrenal, GF: germ-free, GABA: gamma-aminobutyric acid, TNF-α: tumor necrosis factor-α, BCG: bacille Calmette-Guérin.