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. This evidence concerns the gene HTR1A and major depressive disorder.