BackgroundXiao Yao San (XYS) is an herbal prescription which is used in the treatment of depression for thousands of years from Song dynasty in China (960–1127 A.D.), and is the bestselling and most popular herb formula for treating major depression. This study aimed to assess the chronic antidepressant effects of XYS and fluoxetine in depressed mice induced by chronic unpredictable mild stress (CUMS) and its association with  alterations in glutamate/glutamine cycle and glutamate transporters.MethodsMice in the control and model group were given 0.5 ml physiological saline by intragastric administration. Mice in two treatment groups were given XYS (0.25 g/kg/d) and fluoxetine (2.6 mg/kg/d), respectively. The depressive-like behaviors such as forced swim test (FST), sucrose preference test (SPT) and novelty-suppressed feeding (NSF) test were measured after mice exposed to CUMS for 21 days. Body weight, contents of glutamate and glutamine, glutamine/glutamate ratio that is usually thought to reflect glutamate/glutamine cycle, and the protein and mRNA expressions of glutamate transporters (excitatory amino acid transporter 1–2,GLAST/EAAT1 and GLT-1/EAAT2) were measured. The immunoreactivities of GLAST and GLT-1 in the hippocampus were also investigated.ResultsAfter CUMS exposure, mice exhibited depressive-like behaviors, body weight loss, increased glutamate level, decreased glutamine level, elevated glutamine/glutamate ratio, decreased GLT-1 protein expression and mRNA level, and decreased average optical density (AOD) of GLT-1 in the CA1, CA3 and DG in the hippocampus. These abnormalities could be effectively reversed by XYS or fluoxetine treatment. In addition, the study also found that GLAST expression in the hippocampus could not be altered by 21-d CUMS.ConclusionThe studies indicated that XYS may have therapeutic actions on depression-like behaviors induced by CUMS in mice possibly mediated by modulation of glutamate/glutamine cycle and glutamate transporter GLT-1 in the hippocampus.