Fig. 4

a ROC curve. b Three tryptophan metabolic pathways in FS group. 1. Tryptophan-serotonin pathway, elevation in plasma tryptophan and Htr5a expression in FS group indicated that the serotonin pathway was more active than that in the FNS group. 2. Tryptophan-kynurenine pathway, changes in Ido1, Ido2, Ifngr2, Il1β, and Il10 indicated that the kynurenine pathway of tryptophan metabolism was inhibited in FS macaques. Increased KA indicated that the metabolism of kynurenine in the FS macaques was mainly along the KA pathway over the more neurotoxic quinolinic pathway. 3. Tryptophan-AhR ligand, tryptophan can be metabolized to AhR ligand (IAA) through gut microbiota. IAA: indole-3-acetic acid, KA: kynurenic acid