Kynurenines Increase MRS Metabolites in Basal Ganglia and Decrease Resting State Connectivity in Frontostriatal Reward Circuitry in Depression

Abstract

Inflammation is associated with depressive symptoms including anhedonia in patients with major depression. Nevertheless, the mechanisms by which peripheral inflammatory signals are communicated to the brain to influence central nervous system (CNS) function has yet to be fully elucidated. Based on laboratory animal studies, molecules of the kynurenine pathway (KP), which is activated by inflammation, can readily enter the brain, and generate metabolites that can alter neuronal and glial function, leading to behavioral changes. We therefore examined the relationship between KP metabolites in the plasma and cerebrospinal fluid (CSF) and brain chemistry and neural network function using multi-modal neuroimaging in 49 unmedicated, depressed subjects. CNS measures included 1) biochemical markers of glial dysfunction including glutamate (Glu) and myo-inositol (mI) in the left basal ganglia (LBG) using magnetic resonance spectroscopy (MRS); 2) local activity coherence (regional homogeneity, ReHo) and functional connectivity using resting-state functional magnetic resonance imaging; and 3) anhedonia from the Inventory for Depressive Symptoms-Self Reported. Plasma quinolinic acid (QA) was associated with increases and kynurenic acid (KYNA) and KYNA/QA with decreases in LBG Glu. Plasma kynurenine/tryptophan and CSF 3-hydroxy kynurenine (3HK) were associated with increases in LBG mI. Plasma and CSF KP were associated with decreases in ReHo in LBG and dorsomedial prefrontal cortex (DMPFC), and impaired functional connectivity between these two brain regions. DMPFC-BG connectivity mediated the effect of plasma and CSF KP metabolites on anhedonia. These findings highlight the contribution of KP metabolites to glial and neuronal dysfunction and ultimately behavior in depression.