BDNF and major depressive disorder: Numerous studies have indicated that CA1 and DG subfield atrophy is related to the putative neurobiological mediation pathways, including the brain-derived neurotrophic factor (BDNF) genotype [63, 64], oxidative stress [65, 66] and hypothalamic-pituitary-adrenal (HPA) axis dysregulation [67, 68], which are closely linked to the pathogenesis of anhedonia in MDD.