The pathophysiology of MDD is not fully understood, but an overlap of multiple factors are involved, including decreased monoaminergic neurotransmitters (serotonin, dopamine, and norepinephrine), deregulation in the hypothalamic–pituitary–adrenal (HPA) axis, decreased neurotrophic factors, particularly brain-derived neurotrophic factor (BDNF), increased in glutamate (the major excitatory neurotransmitter in the central nervous system (CNS)), and increased inflammation and oxidative stress [22,23,24,25]. Here, BDNF is linked to major depressive disorder.