Synthesis of serotonin

The predecessor for serotonin is the amino acid tryptophan. The plasma concentration of tryptophan that varies according to dietary intake can modify brain serotonin levels. The Serotonin is hydroxylated by tryptophan hydroxylase to provide 5-hydroxytryptophan (5-HTP) and this reaction is the rate limiting step in serotonin synthesis. The Decarboxylation of 5-HTP by l-aromatic amino acid decarboxylase (the similar enzyme found in catechol-aminergic neurons) provides serotonin, also termed as 5-hydroxytryptamine (5-HT) which is as shown in figure.

             Figure: Synthesis of serotonin from the amino acid tryptophan.

The Serotonin synthesis is matched to the firing frequency of the neuron. The higher firing rates permit increased Ca2+-dependent phosphorylation of tryptophan hydroxylase, the action of which goes up.