Studies investigating the relationship between 5-HT1A and 5-HT1B receptors showed that 5-HT1A/1B knockouts had increased extracellular serotonin in the hippocampus, suggesting that the pairing of SSRI with 5-HT1A/1B antagonist might prove to be a potent treatment for anxiety and depression [18]. This evidence concerns the gene HTR1A and major depressive disorder.