Effect Of Cortisol And Chitosan On Immune Function And Growth Of Yellow Catfish

Method of incorporation of cortisol in the diet, simulating natural chronic stress, was used to study the effects of exogenous cortisol on the non-specific immunity and growth of adult yellow catfish (Pelteobagrus fulvidraco). Cortisol was incorporated into diet in a concentration of 10mg/kg, 100mg/kg. The cortisol containing diet was administrated at 3% of the body weight per day for six weeks. The result is of far reaching importance for understanding the mechanism of the stress hormone cortisol on the growth and immunity of yellow catfish. In addition, through intraperitoneal injection, we investigated the impact of chitosan as an immunostimulants on the non-specific immune function of yellow catfish, which aims to answer the question whether chronic stress in aquaculture could be alleviated through the impact of chitosan on immune mechanism.The average serum cortisol of control groups was (8.09±1.83)ng/ml and there was no statistic difference among control groups, which proves that the environment is quiet and satisfactory. Compared with the control groups, the serum cortisol concentration of 10mg/kg and 100mg/kg groups from the second week to the sixth week increased significantly (P＜0.05), and in the second, fifth and sixth weeks, the disparities were great (P＜0.01). During the entire experiment, the increase of serum cortisol in 10mg/kg group was less than that in 100mg/kg group, but the upward trend of the two groups was basically the same. The result demonstrated that the method of incorporation of cortisol in the diet can be used to imitate natural stress response, which is usually characterized with a continuous increase of the serum cortisol.The main effects of long-term oral administration of exogenous cortisol on the adult yellow catfish immunity include: (1) Exogenous cortisol seriously damaged the spleen. From the second feeding week to the sixth week the coefficient of spleen viscera of the cortisol feeding groups decreased significantly (P＜0.05). (2) Exogenous cortisol inhibited the killing mechanism of head kidney phagocytes. From the second week to the sixth week, both the NBT reduction capability and PMA-stimulated respiratory burst were significantly decreased (P＜0.05). (3) From the third week to the sixth week the activity of the serum lysozyme in the cortisol fed groups increased significantly (P＜0.05). (4) Exogenous cortisol inhibited the activity of the serum alternative complement. ACH₅₀ in the 10mg/kg group decreased significantly (P＜0.05) in the second, fifth and sixth weeks, and there was great decrease (P＜0.01) in the 100mg/kg group from the second week to the sixth week. (5) Exogenous cortisol caused a significant drop (P＜0.05) of the number of peripheral leukocyte and the lymphocyte percentage in differential leucocyte counting (DLC) in the second week. The neutrophil percentage in DLC increased during the experiment and had statistical difference (P＜0.05) in the second and forth weeks in the 10mg/kg groups. The monocyte percentage in DLC had an upward trend and had statistical difference (P＜0.05) in the fifth and fourth weeks in the 100mg/kg group. The results indicate that after one week oral administration the stress hormone cortisol has a general inhibitive effect on the immunity of yellow catfish. But the increase of lysozyme activity reminds us that the influence of cortisol on immune function is rather complicated, and that we should never draw conclusion based on the change of one single parameter.The main effects of long-term oral administration of exogenous cortisol on the adult yellow catfish growth include: (1) Exogenous cortisol caused the decrease of the body weight. From the second week to the sixth week, the body weight in 10mg/kg and 100mg/kg groups were less than that in the control group, and there was a significant difference (P＜0.05) in the third week in 10mg/kg group and in the sixth week in 100mg/kg group. (2) Exogenous cortisol caused an obvious increase (P＜0.05) of blood sugar and a decrease of the glycogen content during the whole experimental period, and the glycogen content decreased significantly (P＜0.05) in the second and sixth weeks. (3) Exogenous cortisol reduced the mucous membrane plica of intestine and made the epithelium cells desquamated. The results indicate that exogenous cortisol hinders the growth by damaging the intestine structure, promoting glycogenolysis and gluconeogenesis.Chitosan was injected intraperitoneally at the dose of 2rag per fish to study the effect of the imrnunostimulant on the immunity of yellow catfish. Fish were sampled in the second, forth, sixth and tenth days after injection. The results are as follows: (1) Chitosan increased the phagocytic and bactericidal abilities of the head kidney phagocytes, while had no influence on the chemotaxis of the phagocytes. Both the phagocytosis percentage and phagocytosis index of the head kidney phagocytes increased significantly (P＜0.01) in the forth and sixth day after injection. From the second day to the tenth day after injection the percentage of NBT positive cells in the head kidney increased significantly (P＜0.01). (2) Chitosan increased the activity of serum lysozyme and complement from the second day after injecton to the tenth day, and there was extreme significant deviation in the fourth day (P＜0.01). (3) Chitosan increased the peripheral neutrophil percentage in DLC, but had no effect on the lymphocyte and monocyte percentages. The peripheral neutrophil percentage increased significantly (P＜0.01) in the second, forth and sixth day after injection.