Their effects include the increase of indoleamine 2,3-dioxygenase (IDO), which metabolizes TRP into kynurenine (KYN), causing its level reduction in serum and, as a consequence, that of 5-HT levels, whose deficiency plays a significant role in the pathomechanism of depression and other psychiatric disorders [47, 48]. Here, IDO1 is linked to depressive disorder.