Figure 1. Relationship between the kynurenine pathway, N-methyl-d-aspartate receptor, and food intake.
(a) In the kynurenine pathway, tryptophan (TRP) is metabolized into kynurenine (KYN).
(b) The kynurenic pathway metabolism of tryptophan is regulated by the rate-limiting enzyme, indoleamine 2,3-dioxygenase 1 (IDO-1).
(c) IDO-1 activity is inhibited by nitric oxide (NO).
(d) Patients with anorexia nervosa (AN) have elevated NO levels.
(e) Kynurenic acid (KYNA) and quinolinic acid (QUIN) are metabolites of kynurenine.
(f) KYNA is an antagonist of the N-methyl-d-aspartate (NMDA) receptor.
(g) QUIN is an agonist of the NMDA receptor.
(h) NMDA agonists decrease food intake.
(i) NMDA antagonists increase food intake.
From: Elevated Plasma Tryptophan in Patients with Anorexia Nervosa Compared with Hypotrophic Controls
