Peripheral and central kynurenine pathway abnormalities in major depression

Area of Science:

• Neuroscience
• Immunology
• Metabolic Psychiatry

Background:

• Major depressive disorder (MDD) is linked to immune system dysfunction and altered tryptophan metabolism via the kynurenine pathway (KP).
• Pro-inflammatory cytokines may shift KP metabolism towards neurotoxic products, a hypothesis primarily tested peripherally.
• Previous research suggests MDD is associated with brain structure changes and metabolic disturbances like increased BMI.

Purpose of the Study:

• To investigate kynurenine pathway (KP) metabolite levels in both plasma and cerebrospinal fluid (CSF) of individuals with MDD.
• To examine the relationship between KP metabolite abnormalities, brain structure volumes (hippocampus, amygdala), and body mass index (BMI) in MDD.

Main Methods:

• Measured KP metabolite levels (picolinic acid (PIC), kynurenic acid (KYNA), quinolinic acid (QUIN)) in plasma and CSF of MDD patients and healthy controls.
• Assessed brain-structure volumes (hippocampal, amygdalar) using neuroimaging techniques.
• Correlated KP metabolite levels with brain volumes and body mass index (BMI) in the MDD group.

Main Results:

• MDD patients showed altered plasma KP metabolites: lower picolinic acid (PIC), lower kynurenic/quinolinic acid (KYNA/QUIN) ratio, and lower PIC/QUIN ratio, but increased QUIN levels.
• Cerebrospinal fluid (CSF) analysis revealed lower PIC levels in MDD patients.
• Confirmed reduced hippocampal and amygdalar volumes in MDD, positively correlated with plasma KYNA/QUIN ratio. Increased BMI in MDD correlated inversely with plasma/CSF PIC and PIC/QUIN, and positively with plasma QUIN.

Conclusions:

• Findings partially support previous peripheral KP alterations in MDD and extend them to the CSF.
• Novelly demonstrates that kynurenine pathway (KP) metabolite abnormalities are associated with metabolic disturbances (BMI) in depression.
• The direct relationship between KP metabolites and depression-associated brain atrophy may be less pronounced than previously hypothesized.