In this study, PROEs restored the levels of tryptophan and kynurenine in plasma, increased the expression of 5-HT1A mRNA in the cortex, reduced the expression of 5-HT2A mRNA in the cortex, and reduced the expression of IDO1 mRNA in the cortex and duodenum of depression-model rats. Here, HTR2A is linked to major depressive disorder.