BDNF and major depressive disorder: A variety of studies have implicated impaired BDNF/TrkB signaling and Erk1/2/MAPK pathway in the pathogenesis and treatment of depression (Bjorkholm and Monteggia, 2016; Castren and Kojima, 2017; Wang and Mao, 2019; Chen et al., 2020; Castren and Monteggia, 2021).