HTR1A and major depressive disorder: Multimodal imaging studies show that in MDD patients, an increase in raphe 5-HT1A binding potential is correlated with reduced cortical thickness values and fewer 5-HT tracts projecting to the cortex, while increased terminal 5-HT1A receptors correlate with increased gray matter volume in several cortical and hippocampal regions (Kraus et al., 2012; Zanderigo et al., 2018).