Chronic ammonia exposure impairs the growth and intestinal function in juvenile Megalobrama amblycephala

This study investigated the effects of chronic ammonia exposure on the growth parameters, intestinal histology, and digestive enzyme activity of aquatic fish. Blunt snout bream, Megalobrama amblycephala juveniles, with 6.55 ± 0.24 g were exposed to 0 (G1), 6.82 (G2), and 13.64 mg/L (G3) NH₄Cl (ammonium chloride) for 56 days. Then, the intestinal enzyme activity and histological characteristics of blunt snout bream were analyzed using modern biological techniques. The results showed that chronic ammonia exposure significantly decreased the final body weight and specific growth rate of blunt snout bream (p < 0.05). The intestinal protease, lipase, amylase, CK, AKP, and γ-GT activities of blunt snout bream after chronic ammonia exposure significantly decreased (p < 0.05). Meanwhile, the intestinal muscularis thickness and microvillus density of fish decreased as the increased ammonia concentration. The intestinal microvillus length of blunt snout bream after chronic ammonia exposure significantly reduced compared to the control group (p < 0.05). Furthermore, chronic ammonia exposure caused a significant down-regulated expression of Occludin, ZO-1, Claudin-3c, and Claudin-7 mRNA in the intestine (p < 0.05) but up-regulated the levels of NF-κB, TNF-α, IL-1β, and IL-6 mRNA (p < 0.05). Overall, chronic ammonia exposure inhibited the growth and compromised the intestinal structure of fish.