Figure 1.

Tyrptophan–Kynurenine pathway. Diagram showing the alteration of tryptophan metabolism by IFN-α. Tryptophan is normally converted in 5-hydroxy tryptophan (5-HTP) and serotonin (5-HT). However, this metabolism is switched to the KYN pathway by IDO, which is induced by immune stimuli such as IFN-α, and it is this pathway that produced the neurotoxin quinolinic acid (QUIN). Copyright © 2002, Elsevier. Adapted with permission from Konsman JP, Parnet P, Dantzer R. Cytokine-induced sickness behaviour: mechanisms and implications. Trends Neurosci. 2002;25(3):154–159.