HTR5A and major depressive disorder: We have the following considerations: (1) Systemic administration of 5-HT1A receptor antagonist in normal rats may mimic the effect of serotonin depletion, as in the depressive condition; and (2) When animal models of depression (i.e., animals of 5-HT deficiency) are involved, further using 5-HT anatogonist in these animals may lead to more complex and confusing results, for example, a ceiling effect may occur, and we may not be able to determine the specific role of 5-HT.