Kynurenine and serotonin pathways of tryptophan metabolism: The etiology and pathogenesis of depression

In a recent perspective, with highlighted role of serotonin in depression, the tryptophan as a precursor of serotonin also became of interest in the pathogenesis of depression. The depletion of tryptophan has been discussed in the etiology of depression. Many studies proposed that serotonin deficiency of depression was caused by the tryptophan shunt towards formation of kynurenine (kynurenine shunt). The inflammatory response system in depression also became of interest. The connection between immune/ neuroendocrine system and tryptophan metabolism through kynurenine pathway is considered as important pathophysiology of depression. In this chapter, we reviewed the changes of tryptophan metabolism under immunological challenges and physiological conditions like stress.