氨基酸调控畜禽采食的研究进展

余健剑 束 刚 江青艳

(华南农业大学动物科学学院,广州 510642)

在动物生产中,采食量是影响动物生产性能的重要因素。提高采食量有利于发挥动物的生产潜能,而一些不良因素如疾病或热应激等往往会导致动物采食量降低,生产性能下降。研究表明,饲粮中蛋白质或氨基酸水平对动物采食的影响极为重要,下丘脑的采食调控中枢可以通过神经与体液通路监测饲粮中摄入的蛋白质和氨基酸水平[1-2],从而调节动物的食欲。近年来的一系列研究发现,一些氨基酸可以通过中枢和外周机制调节畜禽的采食量。

1 亮氨酸对畜禽采食的调控作用

亮氨酸属于支链氨基酸,是必需氨基酸之一,也是唯一的生酮氨基酸。脑室注射亮氨酸研究结果表明,亮氨酸具有明显的摄食调节效应。Cota等[2]对大鼠中枢注射 1.1μg的 L-亮氨酸,观察到大鼠采食量显著减少,同时促食欲神经肽 Y(NPY)和刺鼠相关蛋白(AgRP)的表达量显著下降。Morrison等[3]给大鼠饲喂低蛋白质饲粮,发现采食量显著高于对照组,AgRPmRNA表达水平亦显著上调;中枢注射 10μg剂量的 L-亮氨酸能强烈抑制采食,下丘脑 AgRPmRNA表达量显著降低。Blouet等[4]研究也表明,在大鼠和小鼠下丘脑内侧基底部注射 L-亮氨酸均能显著降低采食量、体增重以及动物采食的频率,并认为此效应是通过激活下丘脑内侧基底部的阿片 -促黑素细胞皮质素原(POMC)神经元、室旁核神经元、孤束核饱感效应器神经元而实现的。

尽管脑室注射亮氨酸具有强烈的食欲调节作用,但饲粮中添加亮氨酸对哺乳动物和禽类的研究报道效应并不明显。López等[5]报道,饲粮中添加 L-亮氨酸对泌乳期大鼠的采食量、产热量、体增重均无显著影响,但是却显著抑制了下丘脑促食欲肽 NPY和 AgRPmRNA的表达。Y in等[6]运用低蛋白质饲粮饲喂 21日龄的断奶仔猪,发现添加 0.27%和 0.55%的L-亮氨酸对 2周内各组猪的采食量没有明显改变,但添加 0.55%的L-亮氨酸组的日增重分别比对照组和 0.27%组显著提高61%和 41%。对禽类的研究表明,在含有支链氨基酸缬氨酸(0.60%)和异亮氨酸(0.82%)的基础饲粮中添加 5%的亮氨酸能减少鸡的采食量和体增重,降低饲料利用效率。而当缬氨酸和异亮氨酸含量分别增加至 0.80%和 1.07%时,亮氨酸的抑制采食效应消失[7]。

亮氨酸对采食的影响可能有 2方面的作用通路。一方面,亮氨酸是激活哺乳动物雷帕霉素靶蛋白(mTOR)信号通路最有效的氨基酸[8]。由于mTOR是一种充当代谢感受器的丝氨酸 /苏氨酸蛋白激酶,它对氨基酸和生长因子都非常敏感。因此,Cota等[2]和 Morrison等[3]都认为亮氨酸可以直接激活下丘脑 mTOR信号通路发挥食欲调节活性。另一方面,肌肉氧化亮氨酸产物的 α-酮异己酸的能力在绝食时可提高 3～5倍,泌乳期大鼠乳腺中支链氨基酸转氨酶活力和亮氨酸氧化率均上升[9]。亮氨酸转氨基产物 α-酮异己酸通过促进胰岛素的分泌[10],抑制胰高血糖素分泌,从而抑制糖原异生,降低血糖浓度,从而间接参与食欲的调节,这可能正是 López等[5]和 Yin等[6]给哺乳动物(大鼠和仔猪)长期饲喂 L-亮氨酸过程中亮氨酸并未表现出明显的食欲抑制效应的主要原因。

2 色氨酸对畜禽采食的调控作用

色氨酸是哺乳动物的必需氨基酸和生糖氨基酸。饲养试验表明,色氨酸对动物择食以及采食量均有重要影响,仔猪会选择性地采食高浓度色氨酸的饲粮[11]。Burgoon等[12]研究表明,给 6～16 kg仔猪饲喂低色氨酸(0.130%)的饲粮,其采食量比饲喂含足够色氨酸(0.205%)的饲粮低40%。Eder等[13]也证实,新生仔猪对色氨酸缺陷型饲粮的采食量明显比对照组的少,甚至只有对照组的 38%～45%[14-15]。对母猪的研究也有类似的趋势[16]。此外,席鹏彬等[17]研究表明,与0.11%色氨酸缺乏组相比,0.14%～0.23%色氨酸添加组公、母鸡的日采食量分别提高 4.5%～19.5%和 2.2% ～9.0%,下丘脑 5-羟色胺(5-HT)浓度分别提高 37.9%～83.6%和 7.3%～38.7%。

给啮齿动物和禽类腹腔注射色氨酸均能够显著抑制采食。大鼠腹腔注射 100 mg/kg BW的色氨酸使 1、2、12 h的采食量分别减少 45%、33%、11%[18-19]。Lacy等[20]研究发现,给自由采食的鸡腹腔注射色氨酸能呈剂量依赖型地减少采食量,但是对禁食 24 h的鸡没有影响。对禁食 24 h的鸡脑室注射 5-HT则抑制其采食[21]。

色氨酸不仅参与动物机体蛋白质的合成和代谢网络调节,也是 5-HT、褪黑激素、色胺、烟酰胺腺嘌呤二核苷酸(NAD)、烟酰胺腺嘌呤二核苷酸磷酸(NADP)和烟酸等的前体物,这些代谢产物对动物的采食具有非常重要的影响。目前认为,色氨酸对采食量的调控有生长素(ghrelin)和 5-HT 2条相互独立的途径[22]。口服色氨酸可提高仔猪的采食量和血浆 ghrelin水平,并增加胃肠黏膜 ghrelin mRNA的表达,而口服 5-HT则可降低仔猪的采食量,并且不影响血清中的 ghrelin水平。色氨酸发挥作用的另一个途径被认为和 5-HT有关[23]。大量研究表明,饲粮中增加色氨酸的量会增加采食量并伴随着脑内 5-HT的增加[24-25]。也有人认为色氨酸缺乏会导致肉仔鸡增重和采食抑制,原因可能是当色氨酸严重缺乏时,大脑中 5-HT耗竭,导致采食量急剧下降[26-27]。

3 组氨酸对采食调控的影响

组氨酸是碱性的半必需氨基酸,高组氨酸饲粮不利于动物采食。Hitomi-Ohmura等[28]报道,大鼠饲粮添加 5%组氨酸,其采食量显著降低。Kasaoka等[29]也发现,大鼠饲粮添加 4.5%组氨酸能够显著降低采食量。另外,给大鼠饲喂 5%组氨酸饲粮,不仅能降低采食量,而且还可以减少采食的次数[30]。Goto等[31]大鼠采食高组氨酸饲粮,经6 h后,抑制采食的效应才达到显著水平,此时的血液葡萄糖浓度也显著升高。

外周及中枢注射组氨酸同样能够抑制哺乳动物的采食量。Schwartz等[32]报道,大鼠腹腔注射组氨酸使脑内组胺浓度升高。脑室注射组胺能广泛抑制大鼠、猫和山羊的采食行为[33-35]。脑室注射组氨酸对大鼠采食产生明显的抑制效应[36]。Lecklin等[37]对大鼠脑室注射 100～800 nmol组氨酸,结果显著降低大鼠的采食量。

现有研究表明,组氨酸可能主要通过合成组胺而发挥生物学功能。组胺能神经元广泛分布于脑内各个区域,哺乳动物的 3种组胺受体(H1、H2、H3)在脑内绝大部分区域均有分布[38]。研究证明,下丘脑组胺通过 H1受体调控动物的采食行为[39-40]。组胺是由组氨酸合成所得,它能激活位于下丘脑的组胺能神经元而抑制采食[41]。这种抑制采食的效应可能是因为下丘脑的组氨酸脱羧酶(HDC)催化组氨酸转变为组胺而产生的。多项研究也支持以上结论,饲粮添加组氨酸抑制大鼠采食,作用机制可能就是激活了下丘脑的组胺能神经元[29,42]。

4 谷氨酸对采食调控的影响

谷氨酸是哺乳动物中枢神经系统内含量最高的兴奋性氨基酸,它与快速兴奋性突触传递、中枢神经系统发育调节等多种机能有关[43]。脑内含有大量的谷氨酸,绝大部分都是在细胞内尤其是神经末梢。谷氨酸不仅是中枢系统内的信号物质,还在外周器官和组织中充当传递信号的角色[44-47]。谷氨酸需要通过其受体发挥传递信号作用,动物禁食期间,内源性的谷氨酸释放于下丘脑外侧区,而采食期间谷氨酸释放会迅速衰减[48]。

中枢注射谷氨酸对哺乳动物和禽类的采食调控效应截然相反。Duva等[49-50]对饱食后的大鼠脑室注射谷氨酸或者谷氨酸受体激动剂,发现大鼠会产生强烈但短暂的采食行为。但 Zeni等[51]用鸽子作为试验动物模型的研究结果表明,禁食24 h的鸽子脑室注射谷氨酸,能够呈剂量依赖型地减少采食量和采食持续的时间,但对自由采食的鸽子却无此效应。Zendehdel等[52]研究也发现,3周龄肉鸡禁食 24 h后,脑室注射 300和600μmol的谷氨酸,能够剂量依赖性地减少鸡的采食量。

5 其他氨基酸对采食调控的影响

除上述 4种氨基酸以外,目前关于其他氨基酸影响动物采食调控的研究相对较少。Ng等[18]对大鼠腹腔注射 100 mg/kg体重的酪氨酸,发现分别显著减少 1(37%)和 2 h(38%)的采食量。当动物适应了赖氨酸缺乏型的饲粮后,极低浓度的赖氨酸改变都能明显刺激动物对饲粮的选择以及采食量。赖氨酸缺陷型动物会选择相对含量较高的赖氨酸饲粮以尽量维持自身的生存状态[53]。6日龄蛋鸡中枢注射 8μmo l的 L-精氨酸显著抑制其采食量[54]。此外,在断奶仔猪饲粮添加过量的蛋氨酸(4%),试验期间仔猪采食量显著降低[55]。Even等[56]研究表明,饲喂苏氨酸缺乏型饲粮的大鼠的采食量明显降低。Obeid等[57]在大鼠饲粮中添加谷氨酰胺,发现在为期 3周的试验期中,前 2周大鼠的采食量显著高于对照组。

6 小 结

营养素反馈调节动物采食的机制比较复杂。深入认识各种氨基酸对动物采食的调控作用及其机制,对配制氨基酸平衡饲粮具有重要的参考价值。但目前某些氨基酸(如亮氨酸)对哺乳动物和禽类的采食调控作用刚好相反。即使对于同一物种,氨基酸通过饲喂、外周注射和中枢注射的效应也不尽相同。尽管已有研究表明 m TOR通路、磷酸腺苷活化蛋白激酶(AMPK)通路等信号通路可能参与了氨基酸对摄食行为的调控,但目前仍不能完全解释其详细的作用机制,今后有必要在深入揭示畜禽采食调控的差异及其分子机制的基础上,从更广泛的作用途径和信号通路去揭示氨基酸对畜禽采食的调控作用及其机制。

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