POMC and major depressive disorder: Although our results failed to confirm the causal relationship between them, on the basis of previous studies that showed that hormones secreted from the HPA axis increased pro-inflammatory cytokines, we suggest that CMS triggers depression-like behavior via the effects of hormones secreted from the HPA axis, such as corticotropin-releasing factor, ACTH, or glucocorticoids, which upset the equilibrium of circulatory cytokines in an animal model of depression.