Comprehensive growth performance, immune function, plasma biochemistry, gene expressions and cell death morphology responses to a daily corticosterone injection course in broiler chickens

The massive meat production of broiler chickens make them continuously exposed to potential stressors that stimulate releasing of stress-related hormones like corticosterone (CORT) which is responsible for specific pathways in biological mechanisms and physiological activities. Therefore, this research was conducted to evaluate a wide range of responses related to broiler performance, immune function, plasma biochemistry, related gene expressions and cell death morphology during and after a 7-day course of CORT injection. A total number of 200 one-day-old commercial Cobb broiler chicks were used in this study. From 21 to 28 d of age, broilers were randomly assigned to one of 2 groups with 5 replicates of 20 birds each; the first group received a daily intramuscular injection of 5 mg/kg BW corticosterone dissolved in 0.5 ml ethanol:saline solution (CORT group), while the second group received a daily intramuscular injection of 0.5 ml ethanol:saline only (CONT group). Growth performance, including body weight (BW), daily weight gain (DG), feed intake (FI) and feed conversion ratio (FC), were calculated at 0, 3 and 7 d after the start of the CORT injections. At the same times, blood samples were collected in each group for hematological (TWBC’s and H/L ratio), T- and B-lymphocytes proliferation and plasma biochemical assays (total protein, TP; free triiodothyronine hormone, fT3; aspartate amino transaminase, AST; and alanine amino transaminase, ALT). The liver, thymus, bursa of Fabricius and spleen were dissected and weighed, and the mRNA expression of insulin-like growth factor 1 gene (IGF-1) in liver and cell-death-program gene (caspase-9) in bursa were analyzed for each group and time; while the apoptotic/necrotic cells were morphologically detected in the spleen. From 28 to 35 d of age, broilers were kept for recovery period without CORT injection and the same sampling and parameters were repeated at the end (at 14 d after initiation of the CORT injection). In general, all parameters of broiler performance were negatively affected by the CORT injection. In addition, CORT treatment decreased the plasma concentration of fT3 and the mRNA expression of hepatic IGF-1. A significant increase in liver weight accompanied by an increase in plasma TP, AST and ALT was observed with CORT treatment, indicating an incidence of liver malfunction by CORT. Moreover, the relative weights of thymus, bursa and spleen decreased by the CORT treatment with low counts of TWBC’s and low stimulation of T & B cells while the H/L ratio increased; indicating immunosuppressive effect for CORT treatment. Furthermore, high expression of caspase-9 gene occurred in the bursa of CORT-treated chickens, however, it was associated with a high necrotic vs. low apoptotic cell death pathway in the spleen. Seven days after termination of the CORT treatment in broilers, most of these aspects remained negatively affected by CORT and did not recover to its normal status. The current study provides a comprehensive view of different physiological modulations in broiler chickens by CORT treatment and may set the potential means to mount a successful defense against stress in broilers and other animals as well.

肉鸡大规模集约化肉用生产模式使其持续暴露于各类潜在应激源，这些应激源会刺激应激相关激素（如皮质酮（corticosterone, CORT））的释放，而该激素参与调控生物机制与生理活动中的特定信号通路。为此，本研究旨在评估为期7天的皮质酮注射干预期间及干预后，肉鸡生长性能、免疫功能、血浆生化指标、相关基因表达及细胞死亡形态学等多维度响应变化。本研究共选用200羽1日龄商品代科宝（Cobb）肉鸡雏鸡。试验于肉鸡21至28日龄期间开展，将其随机分为2组，每组设5个重复，每个重复20羽鸡。其中试验组（CORT组）每日以肌肉注射方式给予5 mg/kg体重（body weight, BW）的皮质酮，溶媒为0.5 ml乙醇-生理盐水混合液；对照组（CONT组）每日仅肌肉注射0.5 ml乙醇-生理盐水混合液。于皮质酮注射开始后的第0、3、7天，计算各组肉鸡的生长性能指标，包括体重（BW）、日增重（daily weight gain, DG）、采食量（feed intake, FI）及料重比（feed conversion ratio, FC）。同期采集各组血液样本，用于血液学检测（总白细胞计数（total white blood cells, TWBCs）及异嗜性粒细胞与淋巴细胞比值（heterophil-to-lymphocyte ratio, H/L））、T、B淋巴细胞增殖检测及血浆生化指标测定，包括总蛋白（total protein, TP）、游离三碘甲状腺原氨酸（free triiodothyronine hormone, fT3）、天冬氨酸氨基转移酶（aspartate aminotransferase, AST）与丙氨酸氨基转移酶（alanine aminotransferase, ALT）。剖取各组肉鸡的肝脏、胸腺、法氏囊（bursa of Fabricius）及脾脏并称重，同时按不同时间点分析各组肝脏中胰岛素样生长因子1（insulin-like growth factor 1, IGF-1）基因、法氏囊中细胞死亡程序基因半胱氨酸天冬氨酸蛋白酶9（caspase-9）的mRNA表达水平；并通过形态学方法检测脾脏中的凋亡/坏死细胞。于28至35日龄期间，将肉鸡置于无皮质酮注射的恢复期饲养，于恢复期结束时（即皮质酮注射开始后第14天）重复进行相同的采样与指标检测。总体而言，皮质酮注射对肉鸡所有生长性能指标均产生了负面影响。此外，皮质酮处理降低了血浆fT3浓度与肝脏IGF-1的mRNA表达水平。皮质酮处理组肉鸡的肝脏重量显著升高，同时血浆TP、AST与ALT水平上升，提示皮质酮可引发肝功能异常。此外，皮质酮处理使肉鸡胸腺、法氏囊及脾脏的相对重量降低，同时TWBCs计数与T、B淋巴细胞增殖刺激水平下降，而H/L比值升高，表明皮质酮处理具有免疫抑制效应。进一步而言，皮质酮处理组肉鸡的法氏囊中caspase-9基因表达上调，而脾脏中则以坏死性细胞死亡为主、凋亡性细胞死亡为辅。肉鸡皮质酮注射结束7天后，多数上述指标仍受皮质酮的负面影响，未恢复至正常水平。本研究全面阐明了皮质酮处理对肉鸡的各类生理调控效应，可为肉鸡及其他动物的应激防御策略提供潜在研究方向。