BDNF and major depressive disorder: In the 21st century, another popular hypothesis of depression suggests that neurotrophic factors (especially brain-derived neurotrophic factor [BDNF]) and adult neurogenesis are closely implicated in not only the pathophysiology of depression but also the behavioral responses to antidepressants (Adlam and Zaman, 2013; Micheli et al., 2018; Castrén and Monteggia, 2021).