The effects of fructooligosaccharide on the immune response, antioxidant capability and HSP70 and HSP90 expressions in blunt snout bream (Megalobrama amblycephala Yih) under high heat stress

• Non-specific immunity decreased under high heat stress.
• Antioxidant capacity also decreased under high heat stress.
• HSP70 and HSP90 mRNA expression levels were induced under high heat stress.
• FOS relieved the adverse effects of high heat stress.

This study evaluated the effects of fructooligosaccharide (FOS) on the immune response, antioxidant capability and HSP70 and HSP90 mRNA expressions in blunt snout bream (Megalobrama amblycephala) under high heat stress. A total of 360 fish were randomly distributed into three groups (each with four replicates) and fed with three levels of FOS (0, 0.4% and 0.8%) for 8 weeks. After the feeding trial, 20 fish per tank were exposed to high heat stress at 34 °C (ambient temperature + 8 °C). At 3 and 6 h after stress, both the plasma cortisol and glucose levels of fish fed with 0.4% FOS were significantly lower than those of fish fed with the control diets. This also held true for the lactate levels of fish fed with 0.4% FOS, except that a significant difference was only observed at 3 h. Plasma lysozyme, acid phosphatase (ACP) and alternative complement (ACH50) activities, as well as total protein, immunoglobulin M (IgM) and nitrogen monoxide (NO) contents, increased significantly, with maximum levels attained at 6 h for all parameters except for the ACP activity. Thereafter, these parameters all decreased significantly. In addition, fish fed with 0.4% or 0.8% FOS obtained significantly higher lysozyme, ACP and ACH50 activities as well as total protein, IgM and NO contents at 3 and/or 6 h after stress. The liver superoxide dismutase and catalase activities of fish fed with 0.4% FOS were both significantly higher than those of the control group before and after stress, while the opposite was true for the malondialdehyde content. After stress, the HSP70 and HSP90 expressions of fish fed with 0.4% FOS were both significantly higher than those of fish fed with the control diets at 3, 6 and 12 h (except the HSP90 at 12 h). Similar results were also observed in fish fed with 0.8% FOS except that the differences in HSP70 expression were only significant at 3 and 12 h. The results indicated that the supplementation of 0.4% FOS could increase the non-specific immunity, antioxidant capacity and HSP70 and HSP90 expressions of blunt snout bream and enhance its resistance to high heat stress.