The Roles of BDNF in the Pathophysiology of Major Depression and in Antidepressant TreatmentAnimal studies have demonstrated that stress reduces BDNF expression or activity in the hippocampus and that this reduction can be prevented by treatment with antidepressant drugs. A similar change in BDNF activity occurs in the brain of patients with major depression disorder (MDD). Recently, clinical studies have indicated that serum or plasma BDNF levels are decreased in untreated MDD patients. Antidepressant treatment for at least four weeks can restore the decreased BDNF function up to the normal value. Therefore, MDD is associated with impaired neuronal plasticity. Suicidal behavior can be a consequence of severe impaired neuronal plasticity in the brain. Antidepressant treatment promotes increased BDNF activity as well as several forms of neuronal plasticity, including neurogenesis, synaptogenesis and neuronal maturation. BDNF could also play an important role in the modulation of neuronal networks. Such a neuronal plastic change can positively influence mood or recover depressed mood.