The initial spark for clarifying the daring hypothesis that existing endogenous molecules cause mental illnesses took place in a laboratory that dealt with completely different questions about the “biological universe”. Around 1935, the Italian physician and physiologist Vittorio Erspamer (Fig. 8.1) focused on the gastrointestinal mucosa of the rabbit. From extracts of the enterochromaffin cells, he discovered that the activity of a certain substance caused the smooth muscles of the rat intestine and uterus to contract. When he was able to reverse the effects with ergot alkaloids, his interest in the unknown substance increased. Based on various color reactions, Erspamer suspected that his compound could be a phenylalkylamine, a phenol derivative or an indole, which he called enteramine. He was unable to find out more about the chemistry of the compound, which he subsequently found in tissue extracts from amphibians, molluscs, and tunicates, as well as in the spleens of cattle, sheep, pigs, and humans [1, 2].