HTR4 and major depressive disorder: Brain 5-HT4R density is particularly high in striatum and limbic regions, including hippocampus, and to a lesser extent in the cortex (Marner et al., 2010; Beliveau et al., 2017), which are brain regions thought to be both involved in MDD (Price and Drevets, 2010) and also in regulating the HPA axis (Pruessner et al., 2010).