BDNF and depressive disorder: In subjects with depression, the expression of brain-derived neurotrophic factor (BDNF), which is required for neurogenesis, eventually decreases; this is associated with high GC levels, and signaling for the promotion of neurogenesis, including the phosphorylation of the signaling molecules cAMP response element-binding protein (CREB) and extracellular regulated kinase (ERK), in the hippocampus is altered, which may negatively affect the HPA axis activity.