It is not known which of the 5-HT1A heteroreceptor complexes plays a leading role in reducing depression: 5-HT1A–FGFR1, 5-HT1A–5-HT2A, GalR–5-HT1A, or GalR1–GalR2–5-HT1A heteroreceptor complexes. Here, HTR1A is linked to major depressive disorder.