HTR4 and major depressive disorder: From the literature, we know there exist a large number of 5-HT receptor subtypes, some of which should be activated by agonists to produce antidepressant effects like postjunctional 5-HT1A and 5-HT4 agonists, while others should be blocked to improve depression, like serotonin 5-HT2A, 5-HT2C and 5-HT7 auto-receptors [37,38,39,40].