BDNF and major depressive disorder: Some of the neurobiological factors theoretically associated with depression are the activation of the inflammatory system, hypothalamic-pituitary-adrenal axis disturbances, dysfunctional neuroanatomic circuits (particularly the default mode network), abnormal neural activity, neurotransmitter dysfunction, polymorphisms in the 5-HTT promoter region (5HTTLPR) and interactions between brain derived neurotrophic factor (BDNF); and neurotrophic tyrosine kinase receptor 2 (NTRK2) polymorphisms (2).