Chronic Unpredictable Stress Alters Brain Tryptophan Metabolism and Impairs Working Memory in Mice without Causing Depression-Like Behaviour.

Chronic stress is a well-known risk factor in major depressive disorder and disrupts the kynurenine and serotonin pathways of tryptophan metabolism. Here, we characterize the temporal central and peripheral changes in tryptophan metabolism and concomitant depressive-like behavioural phenotype induced during the progression of chronic unpredictable stress (CUS). Mice were exposed to 0, 10, 20, or 30 days of CUS followed by a panel of behavioural assays to determine depressive-like phenotypes. Immediately after behavioural testing, plasma and brain tissue were collected for metabolic analysis. While anhedonia-like and anxiety-like behaviours were unaffected by stress, nesting behaviour and cognitive deficits became apparent in response to CUS exposure. While CUS caused a transient reduction in circulating quinolinic acid, no other tryptophan metabolites significantly changed in response to CUS. In the brain, tryptophan, kynurenine, picolinic acid, and 5-hydroxyindoleacetic acid concentrations were significantly elevated in CUS-exposed mice compared with non-stress control animals, while kynurenic acid, xanthurenic acid, and serotonin decreased in CUS-exposed mice. Metabolic turnover of serotonin to the major metabolite 5- hydroxyindoleacetic acid was markedly increased in response to CUS. These results suggest that CUS impairs hippocampal-dependent working memory and enhances nascent nesting behaviour in C57BL/6J male mice, and these behaviours are associated with increased brain kynurenine pathway metabolism leading to accumulation of picolinic acid and a significant reduction in serotonin levels.