• Masum Öztürk, Şermin Yalın Sapmaz, Hasan Kandemir, Fatma Taneli, and Ömer Aydemir.
• Department of Child and Adolescent Psychiatry, Kızıltepe State Hospital, Mardin, Turkey.
• Int. J. Clin. Pract. 2021 Apr 1; 75 (4): e13739.

BackgroundThe biological mechanisms underlying major depressive disorder (MDD) are not yet sufficiently understood. The kynurenine pathway has been proposed to play a key role between peripheral inflammation and alterations in the central nervous system. This is because of reduced usability of tryptophan (TRP) and production of oxygen radicals and highly potent neurotoxic agents in this pathway.ObjectiveIn this study, we aimed to compare the metabolites of the serum kynurenine pathway (tryptophan, kynurenine, quinolinic acid and kynurenic acid) and IFN-γ, IL-6, IL-1β and high-sensitivity C-reactive protein (hsCRP) levels in patients with major depressive disorder and in healthy controls and to evaluate the relationship between cytokine levels and the functioning of the kynurenine pathway.MethodsClinical and biochemical data from the patients were obtained and assessed in a cross-sectional design. Serum samples were analysed for IL-6, IL-1β, interferon (IFN)-γ, tryptophan (TRP), quinolinic acid (QUIN), kynurenic acid (KYNA) and kynurenine (Kyn) levels by the enzyme-linked immunosorbent assay. hsCRP test was analysed by the immunoturbidimetric method.ResultsIn total, 48 adolescent patients with major depressive disorder (no drug use) and 31 healthy controls were included in the study. TRP levels were observed to be significantly lower in patients with MDD than in healthy controls (P = .046); the Kyn/TRP ratio was significantly higher in patients with MDD than in healthy controls (P = .032); the levels of QUIN were significantly higher in patients with MDD than in healthy controls (P = .003). No significant difference was found between the groups in terms of other kynurenine metabolites and cytokines levels.ConclusionThese results suggest that the Kyn and related molecular pathways may play a role in the pathophysiology of MDD. The most important finding was the increased level of QUIN, which has a neurotoxic effect, in the kynurenine pathway.© 2020 John Wiley & Sons Ltd.

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