Figure 2

Peripheral and central kynurenine pathway interaction. As indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) are found at only very low concentrations within the brain, concentrations of within-brain tryptophan (TRP) and kynurenines are largely dependent on those from the periphery. TRP from the periphery competes with other large neutral amino acids (LNAA) for passage across the blood brain barrier (BBB) to be used in the synthesis of serotonin (5-HT). Kynurenine (KYN) and 3-hydroxykynurenine (3-HK) are also capable of crossing the BBB whereby they participate in the production of kynurenic acid (KYNA) and quinolinic acid (QUIN) which do not easily cross it.