BDNF and major depressive disorder: For this review, we have searched the PubMed primarily for narrative reviews, systematic reviews, meta-analyses, and clinical trials published in English between January 1, 1995, and September 1, 2020, using the search terms “depression” and “depressive” cross-referenced with the terms “neurotrophins”, “brain-derived neurotrophic factor” and “BDNF”.