HTR5A and major depressive disorder: In summary, 5-HT is the biogenic amine most strongly associated with depression, as evidenced by the negative influence of 5-HT depletion on mood, the antidepressant efficacy of SSRIs, the perturbed 5-HT turnover and neuronal sensitivity in MDD patients and animal models, and the numerous associations between 5-HT pathway gene polymorphisms and MDD (Table 1).