Agricultural Sciences, College of Science, Health, Engineering and Education, Murdoch University, Murdoch WA, Australia.
AB Vista Asia Pte. Ltd., The Mezzo, Singapore.
J Anim Sci. 2020 Aug 1;98(8). doi: 10.1093/jas/skaa190.
Dietary tryptophan (Trp) is a precursor for serotonin, a neuromediator involved in stress responses. Tryptophan competes with other large neutral amino acids (LNAA: tyrosine, isoleucine, leucine, valine, and phenylalanine) to cross the blood-brain barrier; therefore, the regulation of circulating LNAA can influence Trp availability in the cortex and serotonin biosynthesis. The hypothesis examined in this study was that increased supplementation of dietary Trp and a reduction in LNAA for weaned pigs experimentally infected with enterotoxigenic Escherichia coli (ETEC; F4) will increase Trp availability in plasma and reduce indices of the stress response, which will translate to reduced production losses. At 21 ± 3 d of age (mean ± SEM), 96 male pigs (Large White × Landrace) weighing 6.3 ± 0.98 kg (mean ± SEM) were individually penned and allocated to a 4 × 2 factorial arrangement of treatments, with respective factors being 1) four dietary standardized ileal digestible (SID) Trp and LNAA contents, being HTrpHLNAA (Low Trp-High LNAA; 0.24% SID Trp: 5.4% SID LNAA), HTrpHLNAA (Low Trp-Low LNAA; 0.24% SID Trp: 4.6% SID LNAA), HTrpHLNAA (High Trp-High LNAA; 0.34% SID Trp: 5.4% SID LNAA), and HTrpHLNAA (High Trp-Low LNAA; 0.34% SID Trp: 4.6% SID LNAA), and 2) without/with ETEC infection. Pigs were orally infected with 0.8 mL (3.6 × 109 CFU/mL) ETEC at days 7 and 8 after weaning. Pigs fed diets high in Trp irrespective of the level of LNAA (HTrpHLNAA and HTrpLLNAA) had higher plasma Trp concentrations (P < 0.001) and a Trp:LNAA ratio (P < 0.001) before infection and 6 d after infection. Following infection, noninfected pigs had higher plasma Trp (P = 0.03) and a Trp:LNAA ratio (P = 0.004) compared with pigs infected with ETEC. Plasma cortisol levels after infection were higher in ETEC-infected pigs (P = 0.05) and altering dietary Trp and LNAA concentrations did not influence (P > 0.05) plasma cortisol. Pigs fed diet HTrpLLNAA had higher serum serotonin levels 24 h after infection (P = 0.02) compared with pigs fed diets LTrpLLNAA and HTrpHLNAA. Similarly, pigs fed diet HTrpLLNAA had a higher (P = 0.02) average daily gain during the 3-wk study. Overall, average daily feed intake tended to be higher in pigs fed an HTrpLLNAA diet compared with the other diets (P = 0.08). These results suggest that the increased supplementation of dietary Trp with reduced LNAA increased circulating Trp levels that, in turn, likely caused higher serum serotonin levels, irrespective of infection with ETEC, and improved aspects of post-weaning performance.
饮食色氨酸（Trp）是血清素的前体，血清素是一种参与应激反应的神经递质。色氨酸与其他大中性氨基酸（LNAA：酪氨酸、异亮氨酸、亮氨酸、缬氨酸和苯丙氨酸）竞争穿过血脑屏障；因此，循环 LNAA 的调节可以影响皮质中的 Trp 可用性和血清素的生物合成。本研究检验的假设是，给断奶后感染肠产毒性大肠杆菌（ETEC；F4）的猪补充更多的饮食色氨酸并减少 LNAA，将增加血浆中色氨酸的可用性，并降低应激反应的指标，这将转化为减少生产损失。在 21 ± 3 日龄（平均值 ± SEM）时，96 头雄性大白猪 × 长白猪体重为 6.3 ± 0.98 公斤（平均值 ± SEM），单独圈养并按 4 × 2 因子设计分配处理，各自的因子为 1）四种饮食标准化回肠可消化（SID）色氨酸和 LNAA 含量，分别为 HTrpHLNAA（低 Trp-高 LNAA；0.24% SID Trp：5.4% SID LNAA）、HTrpHLNAA（低 Trp-低 LNAA；0.24% SID Trp：4.6% SID LNAA）、HTrpHLNAA（高 Trp-高 LNAA；0.34% SID Trp：5.4% SID LNAA）和 HTrpHLNAA（高 Trp-低 LNAA；0.34% SID Trp：4.6% SID LNAA），以及 2）是否感染 ETEC。在断奶后第 7 天和第 8 天，猪通过口服感染 0.8 毫升（3.6×109 CFU/mL）ETEC。无论 LNAA 水平如何（HTrpHLNAA 和 HTrpLLNAA），高 Trp 饮食的猪在感染前和感染后 6 天具有更高的血浆 Trp 浓度（P<0.001）和 Trp:LNAA 比值（P<0.001）。感染后，未感染的猪的血浆 Trp（P=0.03）和 Trp:LNAA 比值（P=0.004）均高于感染 ETEC 的猪。感染 ETEC 的猪的血浆皮质醇水平更高（P=0.05），改变饮食 Trp 和 LNAA 浓度不会影响（P>0.05）血浆皮质醇。感染后 24 小时，喂食 HTrpLLNAA 饮食的猪血清素水平更高（P=0.02），而喂食 LTrpLLNAA 和 HTrpHLNAA 饮食的猪则更低。同样，喂食 HTrpLLNAA 饮食的猪在为期 3 周的研究中平均日增重更高（P=0.02）。总体而言，与其他饮食相比，喂食 HTrpLLNAA 饮食的猪的平均日采食量趋于更高（P=0.08）。这些结果表明，增加饮食中色氨酸的补充并减少 LNAA 增加了循环色氨酸水平，这反过来又可能导致更高的血清素水平，而不论是否感染 ETEC，并改善了断奶后的性能。
