TPH2 and major depressive disorder: Concerning potential mechanisms for a lower stress/despair response of female Tph2+/− mutants in the modified swim test, we hypothesize that this might also be due to the suppression of the expression of 5-HT1A and 5-HT2A receptors in the brain, whose roles in stress response, major depressive disorder, and consolidation of aversive memories are well established [70,103,104,105].