TPH2 and major depressive disorder: As shown in the present study, 1) the two isoforms of TPH have a different spatial distribution, with TPH1 expressed predominantly in the peripheral tissues and TPH2 primarily localized in the central serotonergic neurons; 2) Tissues 5-HT concentrations and TPH1/2 expression were decreased in brain tissues in the rat model of depression; 3) Stress inhibited TPH2 expression in the depressive rats possibly due to an increase in the methylation of the TPH2 gene promoter in brain tissues.