BDNF and major depressive disorder: Surprisingly, the administration of fluoxetine during critical P21–P42 adolescence but not during the P60–P81 period normalized reduced 5-HT fiber density in the prefrontal cortex of adult BDNF Met/Met mice, indicating that altered BDNF-expression-mediated 5-HT dysfunction during early adolescence may be important in the pathophysiology of mood disorders, including depression and anxiety disorders, during adolescence (Szapacs et al., 2004; Duman, 2017).