BDNF and Cognitive impairment: Because individuals with depressive disorders display cognitive impairments (Airaksinen et al., 2004), it is suggested that downregulation of BDNF/TrkB system is the major mechanism for the pathogenesis of MDD, while stress including increased levels of glucocorticoids caused by overactivation of hypothalamic–pituitary–adrenal (HPA) axis, affects expression/function of the BDNF/TrkB system (Numakawa et al., 2014).