Effects of dietary copper on growth, antioxidant capacity and immune responses of juvenile blunt snout bream (Megalobrama amblycephala) as evidenced by pathological examination

The goal of this study was to investigate the effects of dietary Cu on growth, antioxidant capacity and immune responses in juvenile blunt snout bream with histological examination. Fish were fed three practical diets containing graded copper levels ranging from 1.43 mg/kg-9.13 mg/kg for 10 weeks. The control diet (1.43 mg/kg) significantly improved the growth performance of blunt snout bream (P < 0.05). The 5.21 mg/kg dietary copper significantly increased immunoglobin M (IgM) and complement 3 (C3) content and significantly decreased the activity of alanine transaminase (ALT) in plasma (P < 0.05). The diets containing 5.21 and 9.13 mg/kg copper significantly increased albumin (ALB) content and significantly decreased triglyceride (TG) content (P < 0.05). Dietary copper supplementation improved antioxidant capacity and decrease the content of malondialdehyde (MDA) by regulating the activities of catalase (CAT), total superoxide dismutase (T-SOD) and copper zinc superoxide dismutase (Cu/Zn-SOD) (P < 0.05). With respect to the mRNA levels, the 5.21 mg/kg dietary copper significantly lowered the Kelch-like ECH associated protein 1 (Keap1) mRNA levels and significantly increased the nuclear factor erythroid 2-related factor 2 (Nrf2) mRNA levels (P < 0.05). Furthermore, significant improvements in Cu/Zn-SOD, CAT and heat stress protein 70 (HSP70) mRNA levels were observed in fish fed the 5.21 and 9.13 mg/kg dietary coppers. The relative expression of HO-1 was significantly enhanced in fish fed the 5.21 mg/kg dietary copper (P < 0.05). Dietary copper supplementation affected nuclear factor-kappa B (NF-κB) expression, which further decreased the mRNA levels of pro-inflammatory cytokines, including tumour necrosis factor-alpha (TNF-α), interleukin 8 (IL-8) and interleukin 1β (IL-1β), and increased the mRNA levels of anti-inflammatory cytokines, including interleukin 10 (IL-10). With respect to histological examination, there were no obvious pathological differences among the three groups. Overall, our study indicated that suitable dietary copper supplementation could improve growth performance and antioxidant capacity and weaken the inflammatory response in blunt snout bream.