Glycyrrhizic acid ameliorates the kynurenine pathway in association with its antidepressant effect

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HIGHLIGHTSHMGB1 can activate the rate-limiting enzyme IDO and two downstream enzymes KMO and KYNU of the kynurenine pathway.GZA blocked cytokine activities of CUMS-induced HMGB1 resulting in the down-regulation of the activated KP.GZA could alleviate the depressive behavior, and one of the principal ways is to block cytokine activities of HMGB1.Our previous study implied the role of central high mobility group box 1 (HMGB1) in lipopolysaccharide (LPS)-induced depressive-like behaviors that could partially abrogate by glycyrrhizic acid (GZA). Here, we considered the potential mechanism underlying GZA ameliorating chronic stress-induced depression both in vivo and in vitro. Depression model was established with the 4-week chronic unpredictable mild stress (CUMS) mice. Sucrose preference test, tail suspension test and open field test were performed to reflect depressive-like behaviors. Enzyme activity of indoleamine-2,3-dioxygenase (IDO) was recorded with the ratio of kynurenine (KYN) / tryptophan (Trp). Transcription of gene was evaluated by RT-PCR. Along with depressive-like behaviors, IDO, the rate-limiting enzyme of the kynurenine pathway (KP), was upregulated at the level of mRNA expression, and enzyme activity was also elevated in stressed hippocampi and LPS/HMGB1-treated hippocampus slices. Treatment of mice with GZA, the inhibitor of HMGB1, prevented the activated enzymes in KP and the development of depressive-like behaviors. These experiments demonstrate that GZA may restrain HMGB1 thus improving chronic stress-induced depressive behavior through regulating KP.

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