In water, ammonia nitrogen, a harmful environmental factor in aquaculture, exists in two chemi-cal forms(the unionized NH3 and ionized NH+4).The unionized NH3 is considered more toxic to fish than NH+4 since unionized NH3 can easily diffuse across the epithelial membranes of aquatic animals and lead to higher concentrations of ammonia in plasma and tissue.Under intensive fish farming conditions,massive organic wastes such as unconsumed feed and feces make ambient ammonia concentration easier to get to a high level.Addition-ally,the a much higher level(>20 mg/L) at times due to ineffective or nonexistent sewage treatment or other reasons.In fish,the immunosuppressive effects of ammonia have been proven to contribute to disease out-breaks.Therefore,it is urgently necessary to clarify the mechanism of ammonia nitrogen' s effect on immunity.Under ammonia nitrogen exposure,the cortisol secretion of the head-kidney increases, and the immune re-sponse is down-regulated.Generally,high cortisol levels(COR) often inhibit immune function.Consequently, we hypothesize that ammonia exposure might affect fish immunity by increasing COR levels.The aim of this study to clarify the effects of COR on the immune function of fish exposed to ammonia nitrogen.The juvenile Megalobrama amblycephala[(2.63±0.33) g] were randomly assigned to five different treatment groups(am-monia nitrogen group,COR group, Mifepristone(RU486: a glucocorticoid receptor blocker) group, RU486 + ammonia nitrogen group and control group) for 48 h.The contents of serum COR,immune parameters, and mRNA expression of TLR(Toll-like receptors) pathway-related molecules were determined.Our present results showed that the contents of serum COR increased significantly in ammonia nitrogen and COR groups compared with the control group,indicating that stresses were caused by ammonia nitrogen and COR.Similar-ly,the significantly decreased serum lysozyme and C3 levels were detected in ammonia nitrogen and COR groups.The significantly down-regulation expression of tnf-α,il-1β, il-6 and the genes of TLR/MyD88 sig-naling pathway(trl1, tlr2, tlr3, tlr4, tlr5, myd88, traf6, nf-κb1 and nf-κb2) were detected in ammonia nitro-gen and COR groups.In addition,the significant effects of ammonia nitrogen on immunity were eliminated by RU486, which indicated that the immune function of Megalobrama amblycephala was inhibited by acute ammonia exposure through COR/TLR/MyD88 signaling pathways.In conclusion,the immune function of M.amblycepha-la was inhibited by acute ammonia nitrogen exposure through increasing COR secretion.These results provided a scientific basis for managing and preventing the effects of ammonia nitrogen through breeding new varieties and developing stress-reducing measures.
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