BDNF and major depressive disorder: As an essential neurotrophic factor in the central nervous system, BDNF is closely implicated in the pathophysiology of not only depression but also many other neurological and psychiatric disorders such as Parkinson’s disease, Alzheimer’s disease, stroke, and schizophrenia (Björkholm and Monteggia, 2016; Amidfar et al., 2020; Han and Deng, 2020; Palasz et al., 2020; Karantali et al., 2021; Gao L. et al., 2022; Ali et al., 2024).