HTR1A and major depressive disorder: Evidence of abnormal 5-HT (5-hydroxytyptamine refers to G protein coupled receptors and ligand-gated ion channels, also known as serotonin receptors) function in MDD is building, including for 5-HT1A specifically (1A is a subtype of 5-HT receptor which is the most widespread 5-HT receptor, including within cortex and medial temporal structures).