| Summary: | The study was to evaluate effects of Eucommia ulmoides Oliver (EUO) bark extracts in a diet on growth performance, plasma biochemical and immunological parameters, antioxidant- and inflammation-related genes expression, and histological properties of Japanese eel, Anguilla japonica. In the present study, extraction conditions such as ethanol concentration, extraction temperature, and extraction time were optimized to achieve high contents of geniposidic acid (GA), and low cytotoxicity to fathead minnow cells in EUO bark extracts, using Box-Behnken design. Optimized extraction conditions were found: 0%/85 ℃/10 h (E0), 20%/25 ℃/5 h (E20), and 40%/25 ℃/5 h (E40). Under these conditions, contents of GA were between 8.8 % and 9.6 % in optimized extracts (OEs) and cytotoxicities of OEs at 250 µg/mL were between 12.4 % and 18.0 %. Experimental diets were constructed to feed 20 mg glutathione/kg body weight per day (reference control), 100 or 500 mg/kg b.w. per day of one of three different OEs, or basal diet (control). Eels were fed those experimental diets for 60 days. OEs fed groups showed improved growth performance and significantly reduced levels of alanine aminotransferase, glucose, cortisol, and alkaline phosphatase. Eels fed with OEs diet also showed increased lysozyme activity and myeloperoxidase (MPO) activity compared to those fed control diet. Especially, eels fed with E0 and E20 showed significant increases in MPO activities (P < 0.05). Expression levels of antioxidant related genes such as Cu/Zn superoxide dismutase (SOD) and MnSOD in the liver were found to be significantly higher in the eels fed OEs diet than the eels fed control diet (P < 0.05). In histological analyses, eels fed OEs showed that there was no evidence of histopathological symptoms. Altogether, results of the current study indicate that EUO bark extract can potentially be applied as a feed additive candidate in Japanese eel farms due to its growth promotion, hepatoprotective, antioxidative, stress relieving, and immunomodulating activities.
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