A large body of studies have shown that abnormal monoamine transmitters/receptors system, altered stress hormone dynamics, deficient production of growth factors and neurotrophins, dysregulation of pro-inflammatory cytokines, disturbed adult neurogenesis, altered synaptic connectivity, oxidative stress, abnormal miRNA expression and abnormal delivery of gastrointestinal signaling peptides are all able to induce major mood alterations and associated with the pathogenesis of depression (Villanueva, 2013; Salim, 2014; Hodes et al., 2016; Boku et al., 2018). Here, BDNF is linked to major depressive disorder.