<prompt> With help of pyridoxine, serotonin is synthesized from which of the following precursor amino acid?TyrosineTryptophanPhenylalanineGlycine  <response>: Serotonin is synthesized from the essential amino acid tryptophan. The rate-limiting step is the conversion of the amino acid to 5-hydroxytrptophan by tryptophan hydroxylase. This is then conveed to serotonin by the aromatic L-amino acid decarboxylase. 5-hydroxyindoleacetic acid 5-HIAA. This substance is the principal urinary metabolite of serotonin. There are seven classes of 5-HT receptors from 5-HT1 through 5-HT7 receptors. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. 2012. Chapter 7. Neurotransmitters Neuromodulators. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks Eds, Ganongs Review of Medical Physiology, 24e. 