余健劍 束 剛 江青艷

(華南農(nóng)業(yè)大學動物科學學院,廣州 510642)

在動物生產(chǎn)中,采食量是影響動物生產(chǎn)性能的重要因素。提高采食量有利于發(fā)揮動物的生產(chǎn)潛能,而一些不良因素如疾病或熱應激等往往會導致動物采食量降低,生產(chǎn)性能下降。研究表明,飼糧中蛋白質(zhì)或氨基酸水平對動物采食的影響極為重要,下丘腦的采食調(diào)控中樞可以通過神經(jīng)與體液通路監(jiān)測飼糧中攝入的蛋白質(zhì)和氨基酸水平[1-2],從而調(diào)節(jié)動物的食欲。近年來的一系列研究發(fā)現(xiàn),一些氨基酸可以通過中樞和外周機制調(diào)節(jié)畜禽的采食量。

1 亮氨酸對畜禽采食的調(diào)控作用

亮氨酸屬于支鏈氨基酸,是必需氨基酸之一,也是唯一的生酮氨基酸。腦室注射亮氨酸研究結果表明,亮氨酸具有明顯的攝食調(diào)節(jié)效應。Cota等[2]對大鼠中樞注射 1.1μg的 L-亮氨酸,觀察到大鼠采食量顯著減少,同時促食欲神經(jīng)肽 Y(NPY)和刺鼠相關蛋白(AgRP)的表達量顯著下降。Morrison等[3]給大鼠飼喂低蛋白質(zhì)飼糧,發(fā)現(xiàn)采食量顯著高于對照組,AgRPmRNA表達水平亦顯著上調(diào);中樞注射 10μg劑量的 L-亮氨酸能強烈抑制采食,下丘腦 AgRPmRNA表達量顯著降低。Blouet等[4]研究也表明,在大鼠和小鼠下丘腦內(nèi)側基底部注射 L-亮氨酸均能顯著降低采食量、體增重以及動物采食的頻率,并認為此效應是通過激活下丘腦內(nèi)側基底部的阿片 -促黑素細胞皮質(zhì)素原(POMC)神經(jīng)元、室旁核神經(jīng)元、孤束核飽感效應器神經(jīng)元而實現(xiàn)的。

盡管腦室注射亮氨酸具有強烈的食欲調(diào)節(jié)作用,但飼糧中添加亮氨酸對哺乳動物和禽類的研究報道效應并不明顯。López等[5]報道,飼糧中添加 L-亮氨酸對泌乳期大鼠的采食量、產(chǎn)熱量、體增重均無顯著影響,但是卻顯著抑制了下丘腦促食欲肽 NPY和 AgRPmRNA的表達。Y in等[6]運用低蛋白質(zhì)飼糧飼喂 21日齡的斷奶仔豬,發(fā)現(xiàn)添加 0.27%和 0.55%的L-亮氨酸對 2周內(nèi)各組豬的采食量沒有明顯改變,但添加 0.55%的L-亮氨酸組的日增重分別比對照組和 0.27%組顯著提高61%和 41%。對禽類的研究表明,在含有支鏈氨基酸纈氨酸(0.60%)和異亮氨酸(0.82%)的基礎飼糧中添加 5%的亮氨酸能減少雞的采食量和體增重,降低飼料利用效率。而當纈氨酸和異亮氨酸含量分別增加至 0.80%和 1.07%時,亮氨酸的抑制采食效應消失[7]。

亮氨酸對采食的影響可能有 2方面的作用通路。一方面,亮氨酸是激活哺乳動物雷帕霉素靶蛋白(mTOR)信號通路最有效的氨基酸[8]。由于mTOR是一種充當代謝感受器的絲氨酸 /蘇氨酸蛋白激酶,它對氨基酸和生長因子都非常敏感。因此,Cota等[2]和 Morrison等[3]都認為亮氨酸可以直接激活下丘腦 mTOR信號通路發(fā)揮食欲調(diào)節(jié)活性。另一方面,肌肉氧化亮氨酸產(chǎn)物的 α-酮異己酸的能力在絕食時可提高 3～5倍,泌乳期大鼠乳腺中支鏈氨基酸轉氨酶活力和亮氨酸氧化率均上升[9]。亮氨酸轉氨基產(chǎn)物 α-酮異己酸通過促進胰島素的分泌[10],抑制胰高血糖素分泌,從而抑制糖原異生,降低血糖濃度,從而間接參與食欲的調(diào)節(jié),這可能正是 López等[5]和 Yin等[6]給哺乳動物(大鼠和仔豬)長期飼喂 L-亮氨酸過程中亮氨酸并未表現(xiàn)出明顯的食欲抑制效應的主要原因。

2 色氨酸對畜禽采食的調(diào)控作用

色氨酸是哺乳動物的必需氨基酸和生糖氨基酸。飼養(yǎng)試驗表明,色氨酸對動物擇食以及采食量均有重要影響,仔豬會選擇性地采食高濃度色氨酸的飼糧[11]。Burgoon等[12]研究表明,給 6～16 kg仔豬飼喂低色氨酸(0.130%)的飼糧,其采食量比飼喂含足夠色氨酸(0.205%)的飼糧低40%。Eder等[13]也證實,新生仔豬對色氨酸缺陷型飼糧的采食量明顯比對照組的少,甚至只有對照組的 38%～45%[14-15]。對母豬的研究也有類似的趨勢[16]。此外,席鵬彬等[17]研究表明,與0.11%色氨酸缺乏組相比,0.14%～0.23%色氨酸添加組公、母雞的日采食量分別提高 4.5%～19.5%和 2.2% ～9.0%,下丘腦 5-羥色胺(5-HT)濃度分別提高 37.9%～83.6%和 7.3%～38.7%。

給嚙齒動物和禽類腹腔注射色氨酸均能夠顯著抑制采食。大鼠腹腔注射 100 mg/kg BW的色氨酸使 1、2、12 h的采食量分別減少 45%、33%、11%[18-19]。Lacy等[20]研究發(fā)現(xiàn),給自由采食的雞腹腔注射色氨酸能呈劑量依賴型地減少采食量,但是對禁食 24 h的雞沒有影響。對禁食 24 h的雞腦室注射 5-HT則抑制其采食[21]。

色氨酸不僅參與動物機體蛋白質(zhì)的合成和代謝網(wǎng)絡調(diào)節(jié),也是 5-HT、褪黑激素、色胺、煙酰胺腺嘌呤二核苷酸(NAD)、煙酰胺腺嘌呤二核苷酸磷酸(NADP)和煙酸等的前體物,這些代謝產(chǎn)物對動物的采食具有非常重要的影響。目前認為,色氨酸對采食量的調(diào)控有生長素(ghrelin)和 5-HT 2條相互獨立的途徑[22]。口服色氨酸可提高仔豬的采食量和血漿 ghrelin水平,并增加胃腸黏膜 ghrelin mRNA的表達,而口服 5-HT則可降低仔豬的采食量,并且不影響血清中的 ghrelin水平。色氨酸發(fā)揮作用的另一個途徑被認為和 5-HT有關[23]。大量研究表明,飼糧中增加色氨酸的量會增加采食量并伴隨著腦內(nèi) 5-HT的增加[24-25]。也有人認為色氨酸缺乏會導致肉仔雞增重和采食抑制,原因可能是當色氨酸嚴重缺乏時,大腦中 5-HT耗竭,導致采食量急劇下降[26-27]。

3 組氨酸對采食調(diào)控的影響

組氨酸是堿性的半必需氨基酸,高組氨酸飼糧不利于動物采食。Hitomi-Ohmura等[28]報道,大鼠飼糧添加 5%組氨酸,其采食量顯著降低。Kasaoka等[29]也發(fā)現(xiàn),大鼠飼糧添加 4.5%組氨酸能夠顯著降低采食量。另外,給大鼠飼喂 5%組氨酸飼糧,不僅能降低采食量,而且還可以減少采食的次數(shù)[30]。Goto等[31]大鼠采食高組氨酸飼糧,經(jīng)6 h后,抑制采食的效應才達到顯著水平,此時的血液葡萄糖濃度也顯著升高。

外周及中樞注射組氨酸同樣能夠抑制哺乳動物的采食量。Schwartz等[32]報道,大鼠腹腔注射組氨酸使腦內(nèi)組胺濃度升高。腦室注射組胺能廣泛抑制大鼠、貓和山羊的采食行為[33-35]。腦室注射組氨酸對大鼠采食產(chǎn)生明顯的抑制效應[36]。Lecklin等[37]對大鼠腦室注射 100～800 nmol組氨酸,結果顯著降低大鼠的采食量。

現(xiàn)有研究表明,組氨酸可能主要通過合成組胺而發(fā)揮生物學功能。組胺能神經(jīng)元廣泛分布于腦內(nèi)各個區(qū)域,哺乳動物的 3種組胺受體(H1、H2、H3)在腦內(nèi)絕大部分區(qū)域均有分布[38]。研究證明,下丘腦組胺通過 H1受體調(diào)控動物的采食行為[39-40]。組胺是由組氨酸合成所得,它能激活位于下丘腦的組胺能神經(jīng)元而抑制采食[41]。這種抑制采食的效應可能是因為下丘腦的組氨酸脫羧酶(HDC)催化組氨酸轉變?yōu)榻M胺而產(chǎn)生的。多項研究也支持以上結論,飼糧添加組氨酸抑制大鼠采食,作用機制可能就是激活了下丘腦的組胺能神經(jīng)元[29,42]。

4 谷氨酸對采食調(diào)控的影響

谷氨酸是哺乳動物中樞神經(jīng)系統(tǒng)內(nèi)含量最高的興奮性氨基酸,它與快速興奮性突觸傳遞、中樞神經(jīng)系統(tǒng)發(fā)育調(diào)節(jié)等多種機能有關[43]。腦內(nèi)含有大量的谷氨酸,絕大部分都是在細胞內(nèi)尤其是神經(jīng)末梢。谷氨酸不僅是中樞系統(tǒng)內(nèi)的信號物質(zhì),還在外周器官和組織中充當傳遞信號的角色[44-47]。谷氨酸需要通過其受體發(fā)揮傳遞信號作用,動物禁食期間,內(nèi)源性的谷氨酸釋放于下丘腦外側區(qū),而采食期間谷氨酸釋放會迅速衰減[48]。

中樞注射谷氨酸對哺乳動物和禽類的采食調(diào)控效應截然相反。Duva等[49-50]對飽食后的大鼠腦室注射谷氨酸或者谷氨酸受體激動劑,發(fā)現(xiàn)大鼠會產(chǎn)生強烈但短暫的采食行為。但 Zeni等[51]用鴿子作為試驗動物模型的研究結果表明,禁食24 h的鴿子腦室注射谷氨酸,能夠呈劑量依賴型地減少采食量和采食持續(xù)的時間,但對自由采食的鴿子卻無此效應。Zendehdel等[52]研究也發(fā)現(xiàn),3周齡肉雞禁食 24 h后,腦室注射 300和600μmol的谷氨酸,能夠劑量依賴性地減少雞的采食量。

5 其他氨基酸對采食調(diào)控的影響

除上述 4種氨基酸以外,目前關于其他氨基酸影響動物采食調(diào)控的研究相對較少。Ng等[18]對大鼠腹腔注射 100 mg/kg體重的酪氨酸,發(fā)現(xiàn)分別顯著減少 1(37%)和 2 h(38%)的采食量。當動物適應了賴氨酸缺乏型的飼糧后,極低濃度的賴氨酸改變都能明顯刺激動物對飼糧的選擇以及采食量。賴氨酸缺陷型動物會選擇相對含量較高的賴氨酸飼糧以盡量維持自身的生存狀態(tài)[53]。6日齡蛋雞中樞注射 8μmo l的 L-精氨酸顯著抑制其采食量[54]。此外,在斷奶仔豬飼糧添加過量的蛋氨酸(4%),試驗期間仔豬采食量顯著降低[55]。Even等[56]研究表明,飼喂蘇氨酸缺乏型飼糧的大鼠的采食量明顯降低。Obeid等[57]在大鼠飼糧中添加谷氨酰胺,發(fā)現(xiàn)在為期 3周的試驗期中,前 2周大鼠的采食量顯著高于對照組。

6 小 結

營養(yǎng)素反饋調(diào)節(jié)動物采食的機制比較復雜。深入認識各種氨基酸對動物采食的調(diào)控作用及其機制,對配制氨基酸平衡飼糧具有重要的參考價值。但目前某些氨基酸(如亮氨酸)對哺乳動物和禽類的采食調(diào)控作用剛好相反。即使對于同一物種,氨基酸通過飼喂、外周注射和中樞注射的效應也不盡相同。盡管已有研究表明 m TOR通路、磷酸腺苷活化蛋白激酶(AMPK)通路等信號通路可能參與了氨基酸對攝食行為的調(diào)控,但目前仍不能完全解釋其詳細的作用機制,今后有必要在深入揭示畜禽采食調(diào)控的差異及其分子機制的基礎上,從更廣泛的作用途徑和信號通路去揭示氨基酸對畜禽采食的調(diào)控作用及其機制。

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