The neurotropic theory is a newer hypothesis for MDD, and it proposes that antidepressant treatment leads to acute restoration of monoamine levels in the brain, followed by changes in neuroplasticity via increased synaptic contacts and dendritic arborisation.44 The neurotropic hypothesis implies that neurotropic factors are the key factors in antidepressant action.29 These include the nerve growth factor (NGF) and the neurokine or neuropoetin superfamilies. This evidence concerns the gene NGF and major depressive disorder.