Kynurenic / Quinolinic Ratio

There is literature regarding a low kynurenic/quinolinic ratio association with neurotoxicity and major depressive disorder. The literature supports the association between a low kynurenic acid (KA) to quinolinic acid (QA) ratio and neurotoxicity, as well as its link to major depressive disorder (MDD). A reduced KA/QA ratio has been observed in both currently depressed and remitted patients with MDD compared to healthy controls. This suggests a persistent abnormality in the kynurenine metabolic pathway in MDD that may worsen with additional depressive episodes. The imbalance between KA and QA is significant because:

1. KA is considered neuroprotective:
   • It acts as an antagonist of excitatory amino acid receptors
   • It can protect neurons from excitotoxic damage
2. QA is considered neurotoxic:
   • It is an N-methyl D-aspartate glutamatergic receptor (NMDAR) agonist
   • It can cause glutamatergic excitotoxicity
   • It is toxic to brain cells at concentrations above 150 nM

The KA/QA ratio has been negatively correlated with anhedonia in depressed patients, and positively correlated with hippocampal and amygdalar volumes in MDD subjects. This suggests that the imbalance may contribute to structural brain changes observed in depression. The activation of the kynurenine pathway, which leads to this imbalance, is associated with inflammation. Interferon-alpha, an important contributor to inflammation in autoimmune diseases like lupus, induces the enzyme that catalyzes the breakdown of tryptophan into kynurenine, potentially leading to increased QA production.

Research indicates that targeting this pathway could be a potential avenue for developing biomarkers or therapeutic interventions for MDD and other conditions associated with cognitive dysfunction.

Potential treatment options:

Potential treatment options to increase the kynurenic acid (KA) to quinolinic acid (QA) ratio in major depressive disorder (MDD) could include:

1. Anti-inflammatory interventions: Since inflammation is associated with activation of the kynurenine pathway and reduced KA/QA ratios, targeting inflammation may help improve this ratio. Options could include:
   • Non-steroidal anti-inflammatory drugs (NSAIDs)
   • Omega-3 fatty acid supplementation
   • Specialized pro-resolving mediators (SPMs)
2. Tryptophan supplementation: Increasing tryptophan availability could potentially boost KA production, as tryptophan is a precursor in the kynurenine pathway.
3. Kynurenine aminotransferase (KAT) enzyme activators: KAT enzymes are responsible for converting kynurenine to KA. Enhancing their activity could increase KA production relative to QA.
4. Indoleamine 2,3-dioxygenase (IDO) inhibitors: IDO is an enzyme that catalyzes the first step in the kynurenine pathway. Inhibiting IDO could potentially reduce the production of neurotoxic metabolites like QA.
5. Exercise interventions: Regular physical activity has been shown to have anti-inflammatory effects and may positively influence kynurenine metabolism.
6. Dietary interventions: A diet rich in anti-inflammatory foods and nutrients that support tryptophan metabolism could potentially help improve the KA/QA ratio.
7. Stress reduction techniques: Chronic stress can contribute to inflammation and dysregulation of the kynurenine pathway. Stress management strategies like mindfulness meditation or cognitive-behavioral therapy may be beneficial.

It's important to note that these potential treatments are based on the current understanding of the kynurenine pathway in MDD, and further research is needed to establish their efficacy and safety in clinical practice.

References:

1. Savitz, J., et al. (2015). Reduction of kynurenic acid to quinolinic acid ratio in both the depressed and remitted phases of major depressive disorder. Brain, Behavior, and Immunity, 46, 55-59.
2. Aarsland, T.I., et al. (2015). Serum concentrations of kynurenines in adult patients with attention-deficit hyperactivity disorder (ADHD): A case-control study. Behavioral and Brain Functions, 11, 36.
3. Kandemir, H., & Taneli, F. (2019). The possible role of the kynurenine pathway and the Cytokine levels in the adolescents with major depression. Klinik Psikofarmakoloji Bulteni, 29, 271-273.
4. Myint, A.M. (2012). Kynurenines: from the perspective of major psychiatric disorders. FEBS Journal, 279(8), 1375-1385.
5. Guillemin, G.J. (2012). Quinolinic acid, the inescapable neurotoxin. FEBS Journal, 279(8), 1356-1365.

Because of the specific inflammatory component of quinolinic acid, as well as the potentially protective role of kynurenic acid peripherally, laboratories measure the ratio of kynurenic acid to quinolinic acid. This ratio can act as a measure of disturbed kynurenine pathway metabolism. It suggests that tryptophan is catabolized via the kynurenine pathway, rather than the serotonin pathway.