BDNF and major depressive disorder: So far, many hypotheses have been proposed for the pathogenesis of depression, including heritability, neurotransmitter systems, brain-derived neurotrophic factor (BDNF), and overactivity of the hypothalamic pituitary-adrenal axis (4), etc. Whereas existing antidepressants are mainly based on the monoamine hypothesis, a proportion of depressed patients (20% to 30%) do not respond to pharmacological treatment.