BDNF and major depressive disorder: The altered levels in certain growth factors and neurotransmitters such as serotonin, norepinephrine, dopamine, GABA, brain-derived neurotrophic factor (BDNF), glutamate, cannabinoid (CB1) receptors, acetylcholine, and substance P have been proven time and again to cause depression and the protocols for treating depression have been developed to maintain those imbalances (Duman et al., 1997; Svenningsson et al., 2006; Thase, 2007; Hill and Gorzalka, 2009).