HTR1A and major depressive disorder: The US Food and Drug Administration (FDA) has so far approved a number of selective serotonin reuptake inhibitors (SSRIs), i.e., paroxetine, fluoxetine, fluvoxamine, sertraline, citalopram, escitalopram (S-form of the optical isomers of citalopram), vilazodone (also a partial agonist of 5-HT1A receptor), and vortioxetine (also an agonist of 5-HT1A receptor and an antagonist of 5-HT3 receptor) for the treatment of depression (Hiemke and Härtter, 2000; Lochmann and Richardson, 2019).