BDNF and major depressive disorder: It is known that neurotrophic factors are able to promote the growth, proliferation, differentiation, and survival of neurons; neurotrophic factors such as BDNF, GDNF, and NGF are dysregulated in MDD patients and are important in antidepressant drug mechanisms (Saavedra et al., 2008; Allen et al., 2013; Song et al., 2017; Sun et al., 2019; Castrén and Monteggia, 2021).