HTR4 and major depressive disorder: 5-HT4R density is particularly high in the striatal and limbic areas, which are involved in mood, hedonic health, and cognition (Murphy et al., 2021), and it is a promising treatment target for MDD (Mendez-David et al., 2014a; Segi-Nishida, 2017; Frokjaer, 2023).