Figure 2.

Tryptophan metabolites generated in the gut, periphery and brain. In red—AhR ligands; in italics—metabolites penetrating through the blood–brain barrier. Tryptophan (Trp) and kynurenine (KYN) are also easily transported into the brain. Kynurenines are produced in all three compartments, in contrast to indoles, which are synthesized only in the gut. In the brain, Trp comes from ingested proteins, and KYN originates from Trp metabolism in the gut and peripheral organs and from the conversion of Trp in situ. Other brain kynurenines are produced locally within the brain.