BDNF and major depressive disorder: An aberrant level of BDNF expression is closely associated with the pathophysiology of numerous neurological disorders, including depression (neurotrophic hypothesis of depression) (Dwivedi, 2009; Castrén and Monteggia, 2021; Duman et al., 2021), Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease (Murer et al., 2001; Lee et al., 2005; Peng et al., 2005).