HTR1A and major depressive disorder: The effects of chronic SSRI treatment on hippocampal neurogenesis and anxiety- or depression-like behavior are blocked using 5-HT1A antagonists (Klempin et al., 2010) or by knockout of 5-HT1A receptors, globally or specifically on granule cells of the hippocampus (Santarelli et al., 2003; Samuels et al., 2015) (Figure 2).