Fig. 2.

Apelin/APJ system is involved in depression. Apelin/APJ system inhibits depression by agonizing BDNF/Trk through PI3K/Akt and ERK pathways. Apelin/APJ system protects the HPA axis and thus suppresses depression. LPS promotes NF-κB nuclear translocation and promotes STAT3-mediated rise in the pro-inflammatory factor iNOS and fall in the anti-inflammatory factor Arg-1, thereby promoting inflammation, and apelin reverses this effect. PI3K phosphatidylinositol-3-kinase, Akt protein kinase B, ERK extracellular regulated protein kinases, HPA axis hypothalamic–pituitary–adrenal axis, STAT3 signal transducer and activator of transcription 3, iNOS inducible nitric oxide synthase, Arg-1 Arginase 1