Serotonin (5-hydroxytryptamine, 5-HT) plays unequivocally a most important role in the normal physiology amongst higher eukaryotes, as do most neurotransmitters. To date, some fifteen serotonin receptors have been cloned and grouped into seven families (5-HT1-5-HT7) based on their signaling mechanisms. The 5-HT1 receptor class comprises 5 different receptors with ranging homology. Apart from 5-HT3, all serotonin receptors belong to the G-Protein Coupled Receptor superfamily and therefore span the membrane seven times.