BDNF and major depressive disorder: Our study results have suggested that schizophrenic patients have abnormal manifestations of neurotrophy, synaptic transmission, and cellular signaling; BDNF, CREB, PI3K, and AKT are not only associated with schizophrenia, but also correlated with depressive symptoms and impulsive behaviors in schizophrenia to different degrees; the interactions among serum protein factors may aggravate depression and impulse.