HTR1A and major depressive disorder: We also suggested that under depression, the ratio of 5-HT1A/5-HT1A homo- to 5-HT1A/5-HT7 heterodimers in presynaptic neurons may shift towards 5-HT1A/5-HT1A homodimers, leading to a delay in 5-HT1A autoreceptor internalization following SSRI treatment which could result in antidepressant resistance.