The new ‘5-HT’ hypothesis of depression: cell-mediated immune activation induces indoleamine 2,3-dioxygenase, which leads to lower plasma tryptophan and an increased synthesis of detrimental tryptophan catabolites (TRYCATs), both of which contribute to the onset of depression - Maes, M (Prog Neuropsychopharmacol Biol Psychiatry, 2011)
doi: 10.1016/j.pnpbp.2010.12.017