NGF and major depressive disorder: In certain regions of the brain in a depression model, e.g., the prefrontal cortex and hippocampus, NGF downregulation is accompanied by the suppression of the cAMP/cAMP response element binding protein (CREB), PI3K/AKT, and MAPK/ERK pathways (Li et al. 2023; Xia et al. 2023).