Largely on the basis of the observation that most current effective antidepressants and antipsychotics target these systems, the monoaminergic systems (e.g., serotonin, norepinephrine, dopamine) have been extensively studied. Serotonin (5-HT) was given that name because of its activity as an endogenous vasoconstrictor in blood serum (Rapport et al. 1947). It was later acknowledged as being the same molecule (secretin) found in the intestinal mucosa and that is "secreted" by chromaffin cells (Brodie 1900). Following these findings, 5-HT soon became characterized as being a neurotransmitter in the CNS (Bogdansky et al. 1956).