HTR4 and major depressive disorder: In comparison, our group also found 7-8% lower 5-HT4R global binding in unmedicated depressed individuals compared to healthy controls, and intriguingly, this gap was only evident in those who remitted after eight weeks of antidepressant treatment with a selective reuptake inhibitor (SSRI), indicating that those responding to the treatment may have a serotonergic subtype of MDD (26).