NH> NH2 NH2\ COOH HO \ COOH HO \
— —>
N H N H H N
Tryptophan 5-hydroxytryptophan SerotoninThe two steps are catalyzed by the enzymes tryptophan hydroxylase
(1st step) and aromatic amino-acid decarboxylase (2nd step). Humans
cannot make tryptophan from smaller molecules. It is one of the so-
called essential amino acids, because it is necessary to get it from the
food we eat. Nine of the twenty building-block amino acids are essen-
tial in the adult human diet. Plants can make all the amino acids.
During the 1940s, serotonin was identified as a molecule found in
blood and having effects on constriction and dilation of blood vessels.
Its name reflects this role: sero (blood serum) + tonin (vascular muscle
tone). Serotonin receptors are located on blood vessels throughout the
body. Only later was serotonin found to also be a neurotransmitter in
the brain. The serotonergic neurons in the vertebrate animal brain are
located in several clusters of cells in the brainstem (Fig. 7.3), collec-
tively called the raphe nuclei (Greek rhaphe = seam, suture).