There are three pathways in tryptophan (TRP) metabolism. Serotonin (5-hydroxytryptamine; 5-HT) pathway is important in mood, anxiety, memory, and cognition and is impaired in depression. Kynurenine (KYN) pathways are involved in immunity, inflammation, muscles movement, and mental health. We investigated changes in TRP metabolites in plasmas of stressed rats and in depressive patients. TRP metabolite levels in 5-HT and KYN pathways in various brain areas and plasma were increased soon after electric foot shock given to rats but returned to normal 24 h later. Plasma levels of 5-HT were very low or undetectable in patients of monopolar depression. 5-hydroxyindole acetic acid (5-HIAA)/TRP ratios or KYN/TRP ratios were not different between healthy controls and depressive patients, indicating 5-HT quickly being degraded into 5-HIAA in patients of depression but KYN levels were not changed in depression. These results indicate that TRP metabolism changes upon stress application and in patients of depression.
Part of the book: Melatonin