A few years later, in the mid-1950s, investigators discovered serotonin in the brain of laboratory animals. Subsequent experiments demonstrated its precise localization and its effects on electrical and chemical functions of individual nerve cells. Drugs or surgery that modified serotonin-containing areas of an animal’s brain profoundly altered sexual and aggressive behavior as well as sleep, wakefulness, and a diverse array of basic biological functions. The presence and function of serotonin in the brain and in animal behavior clinched its role as the first known neurotransmitter.2
