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OBJECTIVE mTOR inhibitor rapamycin has been proven to prevent epilepsy in a rat seizure model induced by kainic acid and in a tuberous sclerosis complex mouse model.However, as a currently used anti-tumor and immune-depressant drug, rapamycin may have some adverse effect in the long-term treatment of epilepsy.In the present study, we examined the effect of long-term administration of low dose of rapamycin on development and immune function in young SD rats.METHODS SD rats of two weeks of age were randomly divided into control, 0.1 mg/kg, 0.3 mg/kg and 1.0 mg/kg rapamycin group.Body weight was monitored every other day.The relative weight of thymus and spleen to body weight in different groups was calculated after 4 weeks of treatment.Western blotting analysis was used to investigate the inhibition effect of rapamycin on mTOR pathway.The function of learning and memory was detected by Morris water maze and open field.The content of IL-1β, IL-2,IFN-γ and TNF-α in serum and cerebral cortex was measured by ELISA.Immuno-fluorescent technique was used to detect the expression of the microtubule-associated protein, Doublecortin (DCX), in dentate gyrus in rats.RESULTS Rapamycin at the dose of 0.1 mg/kg,0.3 mg/kg and 1.0 mg/kg significantly inhibited mTOR pathway and showed dose-dependent manner which reflected by decrease of S6 phosphorylation.After 4 weeks of treatment, the body weight of the rats was decreased to some extent in all rapamycin-treated groups,and 1.0 mg/kg group showed obvious decrease of body weight after 14 d as compared to control group.The ratio of spleen and thymus to body weight was decreased markedly in 1.0 mg/kg group.No significant differences were noticed in escape latency and the crossing number in 0.1 mg/kg group and 0.3 mg/kg group, while in 1.0 mg/kg group were obviously lower than the control group.We also assayed the content of 4 cytokines to analyze the effect of rapamycin on immune function.No significant difference was shown in the serum and brain content of IL-1β, IL-2, IFN-γ and TNF-α in 0.1 mg/kg group and 0.3 mg/kg group.However, all the four cytokine contents in serum and brain in 1.0mg/kg group were significant decreased.Additionally, the expression of DCX in dentate gyrus in 0.1 mg/kg and 0.3 mg/kg had no obvious difference with control group, while the expression of DCX in 1.0mg/kg group was significantly less than the control group.CONCLUSIONS Long-term administration of low dose of rapamycin has no obvious side effect on development and immune function, however, a relatively high dose of 1.0 mg/kg inhibits immune function significantly, which provides data for implication of clinic use of rapamycin for treatment of epilepsy in young patients.