BDNF and major depressive disorder: Another hypothesis posits that the susceptibility to depression results from dysfunction of the mechanisms underlying the plasticity of neuronal networks [4,5] and the abnormal expression of genes that encode neurotrophic factors such as the brain-derived neurotrophic factor (BDNF) and its tyrosine kinase receptor B (trkB) in neural circuits which are modulated by monoaminergic inputs [5,6].