BDNF and major depressive disorder: For example, the antagonism of serotonin 2A receptors (5-HT2A) by ketanserin partially blocks the effect of stress on BDNF expression [99, 100], suggesting that serotonin is a key mediator, an idea congruent with the numerous findings about both serotonin and the 5-HT2A receptor in the aetiology and treatment of depression and other affective disorders [101, 102].