BDNF and major depressive disorder: In major depressive disorder (MDD), a severe, chronic, and life-threatening disease [110,111], intranasal administration of BDNF fused with cell-penetrating peptides packaged in adenovirus-associated virus (BDNF-HA2TAT/AAV) produced antidepressant effects in the forced swimming test of chronic mild stress (CMS) mice by increasing hippocampal BDNF levels [112].