Depressive disorders are partially attributed to chronic inflammation associated with the tryptophan (Trp)-kynurenine (Kyn) pathway. Recent evidence suggests that anti-inflammatory agents may reduce the risk of depression. The present study aimed to elucidate the potential of the citrus flavonoid hesperidin, which exhibits anti-inflammatory activity, in suppressing the Trp-Kyn pathway in the brain, using a lipopolysaccharide (LPS)-induced inflammation mouse model. Dietary hesperidin was found to suppress activation of the Trp-Kyn pathway in the prefrontal cortex. In addition, it reduced systemic LPS-induced signs of illness, such as low skin temperature and enhanced leukocyte count in the blood. However, dietary supplementation with hesperidin did not improve body weight loss, food intake, water intake, or splenic increases in leukocyte numbers in the LPS model. Collectively, the results suggest that dietary hesperidin can partially regulate central and peripheral events linked to inflammation in LPS mouse models.

Identification of early biomarkers of stress is important for preventing mood and anxiety disorders. Saliva is an easy-to-collect and non-invasive diagnostic target. The aim of this study was to characterize the changes in salivary whole microRNAs (miRNAs) and metabolites in mice subjected to subchronic and mild social defeat stress (sCSDS). In this study, we identified seven upregulated and one downregulated miRNAs/PIWI-interacting RNA (piRNA) in the saliva of sCSDS mice. One of them, miR-208b-3p, which is reported as a reliable marker for myocardial infarction, was upregulated in the saliva of sCSDS mice. Histological analysis showed frequent myocardial interstitial fibrosis in the heart of such mice. In addition, gene ontology and pathway analyses suggested that the pathways related to energy metabolism, such as the oxidative phosphorylation and the pentose phosphate pathway, were significantly related to the miRNAs affected by sCSDS in saliva. In contrast, salivary metabolites were not significantly changed in the sCSDS mice, which is consistent with our previous metabolomic study on the plasma of sCSDS mice. Taken in the light of previous studies, the present study provides novel potential stress biomarkers for future diagnosis using saliva.

The kynurenine (Kyn) pathway plays crucial roles in several inflammation-induced disorders such as depression. In this study, we measured Kyn and other related molecules in the blood plasma, brain, and urine of male C57BL/6J mice (B6) fed non-purified (MF) and semi-purified (AIN-93G and AIN-93M) standard rodent diets. Mice fed MF had increased plasma Kyn levels compared with those on AIN93-based diets, as well as decreased hippocampal Kyn levels compared with those fed AIN-93G. Previous studies showed that branched chain amino acids (BCAAs) suppress peripheral blood Kyn transportation to the brain, but plasma BCAA levels were not significantly different between the diet groups in our study. Urine metabolome analysis revealed that feed ingredients affected the excretion of many metabolites, and MF-fed mice had elevated excretion of kynurenic and quinolinic acids, pivotal metabolites in the Kyn pathway. Collectively, the level of critical metabolites in the Kyn pathway in the central and peripheral tissues was strongly affected by feed ingredients. Therefore, feed selection is a critical factor to ensure the reproducibility of experimental data in studies involving rodent models.

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