BDNF and depressive disorder: The discovery that most classical antidepressants, such as SSRIs, norepinephrine reuptake inhibitors (NERI), or monoamine oxidase inhibitors (MAOs) under chronic administration not only increase BDNF expression and signaling, but are also strong inducers of adult neurogenesis [43,204,205], finally led to the neurogenesis hypothesis of depression, whereby BDNF is a central player (see Section 2.1).