吳 德 卓 勇 呂 剛 徐盛玉 林 燕車(chē)煉強(qiáng) 方正鋒 李 勇

(1.四川農(nóng)業(yè)大學(xué)動(dòng)物營(yíng)養(yǎng)研究所，雅安 625014;2.動(dòng)物抗病營(yíng)養(yǎng)教育部重點(diǎn)實(shí)驗(yàn)室，雅安 625014;3.通威股份有限公司，成都 610000;4.廣西商大科技有限公司，南寧 530021)

母豬的繁殖活動(dòng)是一項(xiàng)具有典型周期循環(huán)的系統(tǒng)工程，適時(shí)啟動(dòng)發(fā)情周期是這項(xiàng)工程的樞紐。據(jù)報(bào)道，規(guī)?；i場(chǎng)后備母豬不發(fā)情或發(fā)情推遲的比例達(dá)20% ～30%［1］。同時(shí)，因情期啟動(dòng)失敗導(dǎo)致經(jīng)產(chǎn)母豬淘汰的比例高達(dá)30% ～40%，50%的母豬僅提供30～40頭斷奶仔豬就被淘汰［2-4］。雌性動(dòng)物的情期啟動(dòng)受復(fù)雜的神經(jīng)-內(nèi)分泌系統(tǒng)調(diào)控，情期啟動(dòng)體現(xiàn)在3個(gè)層面［5］:1)下丘腦促性腺激素釋放激素(gonadotropin-releasing hormone，GnRH)神經(jīng)元脈沖分泌;2)垂體腺在GnRH刺激下脈沖分泌促卵泡素(follicle-stimulating hormone，F(xiàn)SH)和促黃體素(luteinizing hormone，LH);3)性腺軸接收來(lái)自FSH和LH的脈沖信號(hào)，刺激排卵，由卵巢分泌的性腺激素(雌二醇)對(duì)下丘腦GnRH神經(jīng)元進(jìn)行正、負(fù)反饋調(diào)控。作為動(dòng)物情期啟動(dòng)的關(guān)鍵因素，GnRH的分泌調(diào)控是深入揭示動(dòng)物情期啟動(dòng)奧秘的關(guān)鍵組成部分。營(yíng)養(yǎng)是動(dòng)物繁殖活動(dòng)的物質(zhì)基礎(chǔ)，但營(yíng)養(yǎng)代謝信號(hào)并不直接作用于GnRH神經(jīng)元，這表明下丘腦存在介導(dǎo)營(yíng)養(yǎng)調(diào)控GnRH分泌的中間信號(hào)途徑。Kisspeptin(最初名為metastin)是由Kiss-1基因編碼的神經(jīng)內(nèi)分泌肽類激素，由Kisspeptin神經(jīng)元分泌，為G蛋白偶聯(lián)受體54(GPR54)的內(nèi)源性配體［9］。大量研究揭示，由下丘腦Kiss-1基因編碼的蛋白質(zhì)Kisspeptin，與其受體GRP54結(jié)合啟動(dòng)的信號(hào)途徑是下丘腦GnRH脈沖分泌的關(guān)鍵信號(hào)［6-33］。下丘腦Kisspeptin神經(jīng)元表達(dá)營(yíng)養(yǎng)代謝信號(hào)如胰島素(insulin)、瘦素(leptin)、胰島素樣生長(zhǎng)因子-1(insulin like growth factor-1，IGF-1)的 受 體［28］，表 明Kisspeptin神經(jīng)元是營(yíng)養(yǎng)調(diào)控動(dòng)物情期啟動(dòng)及卵泡發(fā)育的關(guān)鍵組成部分。

營(yíng)養(yǎng)調(diào)控母豬情期啟動(dòng)主要有2種代謝途徑:1)營(yíng)養(yǎng)改變激素分泌(如瘦素、胰島素、胰島素樣生長(zhǎng)因子)，調(diào)控情期啟動(dòng);2)營(yíng)養(yǎng)改變血液代謝底物(metabolites)濃度，參與情期啟動(dòng)調(diào)控。因此，本文重點(diǎn)討論營(yíng)養(yǎng)調(diào)控母豬情期啟動(dòng)的理論假設(shè)及分子機(jī)制。

1 營(yíng)養(yǎng)調(diào)控后備母豬情期啟動(dòng)的主要理論假設(shè)

總結(jié)前人研究，營(yíng)養(yǎng)調(diào)控后備母豬情期啟動(dòng)主要存在2種理論假設(shè)。

理論假設(shè)一:組織器官發(fā)育理論假說(shuō)。母豬各組織和器官發(fā)育到一定閾值后才能啟動(dòng)情期(圖1)。該理論認(rèn)為母豬體重和體組成是營(yíng)養(yǎng)累積的綜合效應(yīng)，后備母豬在達(dá)到最低閾值的體重或者體組成之后才能啟動(dòng)情期?！白畹椭炯僬f(shuō)”提出雌性動(dòng)物只有沉積一定比例的脂肪才會(huì)進(jìn)入青春期［34］。研究發(fā)現(xiàn)，瘦肉組織沉積對(duì)后備母豬情期啟動(dòng)亦非常重要［35-37］。Oury 等［38］證實(shí)骨骼處于合成代謝時(shí)性腺才能正常發(fā)育。依據(jù)此理論假設(shè)，下丘腦存在響應(yīng)外周組織發(fā)育信號(hào)的組織和細(xì)胞，當(dāng)機(jī)體組織的生長(zhǎng)和發(fā)育達(dá)到一定標(biāo)準(zhǔn)后，觸發(fā)下丘腦中Kisspeptin表達(dá)和GnRH、LH脈沖分泌，動(dòng)物由生長(zhǎng)轉(zhuǎn)向繁殖。

圖1 組織器官發(fā)育與情期啟動(dòng)Fig.1 Tissue organ development and estrus onset

理論假設(shè)二:雌激素回饋理論(gonadostat hypothesis)(圖2)。生長(zhǎng)期體組織尚未發(fā)育完善時(shí)，雌激素主要通過(guò)下丘腦弓狀核(arcuate nucleus，ARC)區(qū)域Kisspeptin神經(jīng)元產(chǎn)生負(fù)反饋效應(yīng)，抑制性腺發(fā)育，防止早熟;當(dāng)體組織發(fā)育完善時(shí)，雌激素通過(guò)下丘腦前腹側(cè)室旁核(anteroventral periventricular nucleus，AVPV)Kisspeptin 神經(jīng)元產(chǎn)生正反饋效應(yīng)，動(dòng)物啟動(dòng)情期，加速卵泡發(fā)育［39］，雌激素回饋理論是確保動(dòng)物體成熟和性成熟同步性的關(guān)鍵。下丘腦ARC是營(yíng)養(yǎng)代謝信號(hào)作用靶點(diǎn)，生長(zhǎng)期動(dòng)物血液中營(yíng)養(yǎng)代謝因子刺激ARC區(qū)域Kisspeptin的分泌，以保證對(duì)雌激素的負(fù)反饋抑制;當(dāng)動(dòng)物體組織逐漸發(fā)育完善，營(yíng)養(yǎng)儲(chǔ)備足夠時(shí)，雌激素負(fù)反饋抑制減弱，正反饋?zhàn)饔眉訌?qiáng)，動(dòng)物性腺加速發(fā)育并啟動(dòng)情期。

圖2 雌激素回饋理論與情期啟動(dòng)Fig.2 Estrogen feedback hypothesis and estrus onset

2 營(yíng)養(yǎng)對(duì)母豬情期啟動(dòng)的影響及分子機(jī)制

2.1 能量

2.1.1 能量負(fù)平衡

能量負(fù)平衡導(dǎo)致后備母豬發(fā)情推遲甚至乏情。Zhou等［40］將已有2個(gè)正常發(fā)情周期的長(zhǎng)大二雜后備母豬分別飼喂1.00和2.86 kg/d飼糧，連續(xù)限飼4個(gè)發(fā)情周期后，母豬出現(xiàn)營(yíng)養(yǎng)性乏情。下丘腦 ARC和 AVPV區(qū)域上 Kiss-1、GPR54和GnRH mRNA表達(dá)量，以及垂體和卵巢上IGF-1R、FSH和LH mRNA表達(dá)量均發(fā)生了顯著變化，表明Kisspeptin/GPR54信號(hào)系統(tǒng)參與營(yíng)養(yǎng)調(diào)控后備母豬情期啟動(dòng)。Castellano等［41］發(fā)現(xiàn)禁食小鼠的情期啟動(dòng)嚴(yán)重紊亂，血液中GnRH和LH等繁殖激素的水平下降，對(duì)小鼠中樞灌注Kisspeptin后，小鼠血液中GnRH和LH的水平顯著提高，并重新表現(xiàn)正常情期循環(huán)。繁殖系統(tǒng)處于養(yǎng)分分配末端，營(yíng)養(yǎng)不足時(shí)養(yǎng)分優(yōu)先用于維持需要，并抑制繁殖活動(dòng)。Owen等［42］研究了營(yíng)養(yǎng)缺乏情況下繁殖活動(dòng)受到抑制的分子機(jī)理，發(fā)現(xiàn)成纖維細(xì)胞生長(zhǎng)因子21(fibroblast growth factor 21，F(xiàn)GF21)是營(yíng)養(yǎng)不足情況下肝臟分泌的關(guān)鍵信號(hào)，營(yíng)養(yǎng)不足時(shí)肝臟分泌的FGF21作用于視交叉上核(suprachiasmatic nucleus，SCN)神經(jīng)元，抑制 Kisspeptin分泌，推遲雌性動(dòng)物的情期啟動(dòng)及卵泡發(fā)育。同時(shí)，F(xiàn)GF21增加外周組織如骨骼肌、脂肪組織的胰島素敏感性，有利于機(jī)體在營(yíng)養(yǎng)不足情況下優(yōu)先保證生存需要［43］。

母豬泌乳期情期循環(huán)終止，卵泡發(fā)育受到抑制，有2個(gè)方面的原因(圖3):1)Kisspeptin神經(jīng)元存在催乳素受體(PRL-R)，泌乳期高濃度催乳素通過(guò)其受體抑制 Kisspeptin分泌［44］，抑制下丘腦-垂體-性腺軸活性;2)泌乳母豬采食量低，泌乳量大，機(jī)體處于分解代謝，血液中瘦素、胰島素樣生長(zhǎng)因子-1濃度降低，情期啟動(dòng)和卵泡發(fā)育受阻。泌乳母豬能量負(fù)平衡導(dǎo)致實(shí)際生產(chǎn)中斷奶母豬不發(fā)情、受胎率低，增加母豬的淘汰率。有學(xué)者將泌乳期血液瘦素、胰島素等濃度恢復(fù)至正常生理水平，但是并未發(fā)現(xiàn)下丘腦Kiss-1基因表達(dá)量及卵泡發(fā)育恢復(fù)［45-46］。上述結(jié)果表明泌乳期乏情是一個(gè)復(fù)雜且多因素綜合作用的結(jié)果，只有動(dòng)物機(jī)體組織恢復(fù)到“標(biāo)準(zhǔn)”體況后動(dòng)物的情期啟動(dòng)才能恢復(fù)。目前有關(guān)營(yíng)養(yǎng)對(duì)泌乳母豬代謝狀態(tài)、雌激素正負(fù)反饋途徑、斷奶-發(fā)情分子機(jī)理的研究較少，待進(jìn)一步探索。

圖3 泌乳或營(yíng)養(yǎng)限制導(dǎo)致母豬生理性乏情機(jī)制Fig.3 Mechanisms of the phvsiological anestrus of sows in lactation or nutrient restriction

動(dòng)物機(jī)體發(fā)展了一套精準(zhǔn)的適應(yīng)性機(jī)制，讓下丘腦能夠準(zhǔn)確地感知外周組織的營(yíng)養(yǎng)代謝狀態(tài)。能量負(fù)平衡時(shí)，外周代謝信號(hào)能夠快速、準(zhǔn)確地傳遞至下丘腦，養(yǎng)分分配轉(zhuǎn)向生存需要，繁殖軸活性抑制，說(shuō)明下丘腦存在一套能量負(fù)平衡響應(yīng)機(jī)制感知并調(diào)控機(jī)體的繁殖活動(dòng)(圖4)。Roland等［47］研究表明，GnRH神經(jīng)元細(xì)胞中的能量感受器腺苷酸活化蛋白激酶(AMP-activated protein kinase，AMPK)能夠感知細(xì)胞內(nèi)葡萄糖濃度，低葡萄糖濃度通過(guò)AMPK途徑抑制GnRH分泌。Zhang等［48］發(fā)現(xiàn)下丘腦ATP敏感型鉀離子通道可能參與機(jī)體能量負(fù)平衡對(duì)LH濃度的調(diào)控。哺乳動(dòng)物雷帕霉素靶蛋白(mammalian target of rapamycin，mTOR)是機(jī)體廣泛表達(dá)的一種蛋白質(zhì)，通過(guò)中樞雷帕霉素處理抑制mTOR活性，下丘腦ARC區(qū)域的Kiss-1基因表達(dá)量受到顯著抑制，卵巢和子宮萎縮，小鼠的初情日齡顯著推遲［49］。Altarejos等［50］通過(guò)基因敲除模型，特異性敲除下丘腦環(huán)AMP響應(yīng)元件結(jié)合蛋白-1轉(zhuǎn)錄調(diào)控因子［cyclic AMP responsive element-binding protein-1(Creb1)-regulated transcription coactivator-1，CRTC-1］，阻斷下丘腦對(duì)外周代謝狀態(tài)響應(yīng)通路，發(fā)現(xiàn)小鼠表現(xiàn)出肥胖且不育。進(jìn)一步研究證實(shí)，CRTC-1對(duì)繁殖活動(dòng)的調(diào)控主要依賴Kisspeptin信號(hào)途徑，當(dāng)瘦素處理增加Kiss-1基因表達(dá)量的同時(shí)，CRTC-1與Kiss-1基因啟動(dòng)子區(qū)域出現(xiàn)非常緊密的結(jié)合［50］。

圖4 下丘腦能量響應(yīng)機(jī)制對(duì)動(dòng)物情期啟動(dòng)的影響Fig.4 Influence of hypothalamic energy sensing mechanism on estrus onset

2.1.2 能量正平衡

本課題組研究表明，后備母豬飼糧中添加脂肪，可提高血液中瘦素濃度，增強(qiáng)下丘腦ARC區(qū)域瘦素信號(hào)途徑，后備母豬的初情日齡提前［51］。瘦素是反映能量代謝的關(guān)鍵代謝信號(hào)，對(duì)母豬的繁殖軸存在非常廣泛的影響［52］。體外培養(yǎng)大鼠的ARC神經(jīng)元，培養(yǎng)基中添加生理水平瘦素可通過(guò)蛋白酪氨酸激酶2(janus kinase 2，JAK2)/磷酸化信號(hào)轉(zhuǎn)導(dǎo)子與激活子3(signal transducer and activator of transcription 3，STAT3)信號(hào)途徑促進(jìn)Kisspeptin表達(dá)，誘導(dǎo)GnRH的體外分泌［53］。隨著瘦素對(duì)Kisspeptin信號(hào)途徑影響研究的深入，發(fā)現(xiàn)瘦素可直接作用于Kiss-1基因促進(jìn)Kisspeptin表達(dá)［54］。

過(guò)度飼喂或肥胖對(duì)會(huì)動(dòng)物繁殖活動(dòng)產(chǎn)生負(fù)面影響。本課題組體外培養(yǎng)下丘腦ARC神經(jīng)元后發(fā)現(xiàn)(圖5)，培養(yǎng)基中瘦素濃度過(guò)高時(shí)會(huì)抑制Kiss-1基因的表達(dá)豐度。高脂飼糧誘導(dǎo)DBA/2J小鼠肥胖［55］，下丘腦ARC神經(jīng)元Kiss-1的基因表達(dá)量顯著下調(diào)，且小鼠表現(xiàn)出不育。目前有關(guān)肥胖對(duì)動(dòng)物情期啟動(dòng)的分子機(jī)理的報(bào)道較少，肥胖者血液中FGF21濃度顯著提高，因此可能通過(guò)FGF21抑制下丘腦Kisspeptin表達(dá)抑制動(dòng)物的下丘腦-垂體-性腺軸活性，但該假設(shè)尚待進(jìn)一步證實(shí)。

2.2 蛋白質(zhì)(氨基酸)

由于遺傳選育更趨向于選擇瘦肉型豬種，后備母豬蛋白質(zhì)沉積對(duì)情期啟動(dòng)更為重要。后備母豬飼喂10%蛋白質(zhì)水平組比14%蛋白質(zhì)水平組的初情日齡推遲18.7 d［56］。研究發(fā)現(xiàn)，后備母豬瘦肉沉積量與初情日齡呈顯著負(fù)相關(guān)關(guān)系［57］。小鼠限制采食量40%誘導(dǎo)營(yíng)養(yǎng)性乏情后，分別給予富含碳水化合物、脂肪、蛋白質(zhì)的飼糧，結(jié)果發(fā)現(xiàn)飼喂富含蛋白質(zhì)飼糧的小鼠能更快地恢復(fù)情期循環(huán)［58］。飼糧中單一氨基酸缺乏同樣導(dǎo)致情期啟動(dòng)紊亂，研究發(fā)現(xiàn)，某一特定的氨基酸如蘇氨酸、賴氨酸、色氨酸、蛋氨酸、纈氨酸缺乏將導(dǎo)致發(fā)情周期紊亂［59］。由于氨基酸或者蛋白質(zhì)缺乏會(huì)導(dǎo)致機(jī)體蛋白質(zhì)處于分解代謝，當(dāng)Kisspeptin神經(jīng)元感知這種分解代謝時(shí)，繁殖軸活性減低，繁殖活動(dòng)受到抑制(圖6)。

圖5 瘦素信號(hào)調(diào)控下丘腦ARC區(qū)域Kisspeptin表達(dá)的機(jī)制Fig.5 Mechanism of Kisspeptin expression in ARC area regulated by leptin signal

氨基酸對(duì)動(dòng)物情期啟動(dòng)的影響可能通過(guò)肝臟內(nèi)分泌胰島素樣生長(zhǎng)因子-1介導(dǎo)。肝臟是動(dòng)物機(jī)體的首要代謝器官，是感知?jiǎng)游锬芰科胶夂痛x狀態(tài)的第1道“門(mén)”。胰島素樣生長(zhǎng)因子-1是肝臟在機(jī)體處于合成代謝時(shí)分泌的代謝激素，血液循環(huán)中的胰島素樣生長(zhǎng)因子-1有70%來(lái)自肝臟。胰島素樣生長(zhǎng)因子-1廣泛參與動(dòng)物的繁殖活動(dòng)，對(duì)卵泡發(fā)育、胚胎存活均表現(xiàn)出積極效果。有關(guān)胰島素樣生長(zhǎng)因子-1對(duì)后備母豬情期啟動(dòng)的研究較少，已有的研究結(jié)果表明高濃度的胰島素樣生長(zhǎng)因子 -1與雌性動(dòng)物的早熟有關(guān)［60］。Roongsitthichai等［61］對(duì)80頭后備母豬按照初情日齡進(jìn)行劃分，發(fā)現(xiàn)初情日齡早于200 d的后備母豬血液中胰島素樣生長(zhǎng)因子-1濃度高于初情日齡晚于200 d的后備母豬［(30.2±1.2)nmol/L vs.(25.4±1.1)nmol/L，P=0.002］。在牛上的研究表明，初情日齡與18月齡時(shí)的血清胰島素樣生長(zhǎng)因子 -1濃度呈顯著負(fù)相關(guān)關(guān)系［62］。Fortes等［63］應(yīng)用單核苷酸多態(tài)性(single nucleotide polymorphism，SNP)技術(shù)，發(fā)現(xiàn)胰島素樣生長(zhǎng)因子-1受體的SNP與動(dòng)物初情日齡顯著相關(guān)。

下丘腦胰島素樣生長(zhǎng)因子-1信號(hào)途徑直接參與雌激素正負(fù)反饋途徑對(duì)Kisspeptin神經(jīng)元的影響。通過(guò)中樞和外周灌注胰島素樣生長(zhǎng)因子-1，6 h后小鼠AVPV區(qū)域的Kiss-1基因的表達(dá)量顯著提高。當(dāng)處理胰島素樣生長(zhǎng)因子-1受體拮抗劑JB-1之后，胰島素樣生長(zhǎng)因子-1對(duì)Kisspeptin神經(jīng)元的激活作用消失，雌激素的正負(fù)反饋調(diào)節(jié)效應(yīng)中斷［64］。胰島素樣生長(zhǎng)因子-1的作用主要通過(guò)其受體介導(dǎo)的信號(hào)途徑發(fā)揮作用，Todd等［65］和Sun等［66］通過(guò)中樞灌注胰島素樣生長(zhǎng)因子-1及其受體拮抗劑JB-1，證實(shí)了胰島素樣生長(zhǎng)因子-1受體是雌激素正反饋?zhàn)饔煤虶nRH神經(jīng)元激活的必需組成部分，下丘腦胰島素樣生長(zhǎng)因子-1信號(hào)途徑減弱，則動(dòng)物的情期循環(huán)受到干擾。

2.3 維生素D3

實(shí)際生產(chǎn)中，光照影響后備母豬的情期啟動(dòng)。光照影響體內(nèi)維生素D3的代謝，因此維生素D3可能參與調(diào)控動(dòng)物的情期啟動(dòng)。下丘腦-垂體-性腺軸均表達(dá)維生素D3受體，已有研究表明維生素D3缺乏動(dòng)物繁殖能力受損，但是機(jī)理不詳［67-88］。最新研究證實(shí)，維生素D3缺乏導(dǎo)致雌性小鼠初情日齡推遲6 d，此外，維生素D3缺乏導(dǎo)致雌性成年小鼠無(wú)法維持正常的情期循環(huán)［69］。維生素D3廣泛參與下丘腦-垂體-性腺軸的細(xì)胞增殖和分化、激素分泌等活動(dòng)，為了進(jìn)一步探索維生素D3影響小鼠情期啟動(dòng)的作用靶點(diǎn)，對(duì)維生素D3缺乏或Cyp27b1基因缺失小鼠進(jìn)行外源促性腺激素處理，發(fā)現(xiàn)小鼠的垂體及卵巢功能并未受損，證實(shí)了下丘腦GnRH分泌抑制是維生素D3影響小鼠情期啟動(dòng)的關(guān)鍵靶點(diǎn)［69］。骨骼代謝與維生素D3密切相關(guān)，因此維生素D3可能通過(guò)影響骨骼代謝影響動(dòng)物情期啟動(dòng)。Patterson等［70］觀察了431頭后備母豬初情啟動(dòng)與骨骼發(fā)育的關(guān)系，發(fā)現(xiàn)317頭180日齡之前啟動(dòng)初情期的后備母豬無(wú)腿病發(fā)生;180日齡后發(fā)情的母豬中，高達(dá)16%的后備母豬因發(fā)生腿病而被淘汰。骨骼分泌蛋白骨鈣素(osteocalcin)已被證明可以調(diào)控動(dòng)物的能量代謝，同時(shí)可作用于雄性動(dòng)物繁殖組織而影響睪丸細(xì)胞的功能［39］，因此也可能參與雌性動(dòng)物情期啟動(dòng)的調(diào)節(jié)，但此領(lǐng)域研究甚少，有待進(jìn)一步證實(shí)。

圖6 蛋白質(zhì)限制推遲情期啟動(dòng)的理論假設(shè)Fig.6 Theory hypothesis of estrus onset delayed by protein restriction

2.4 礦物元素

礦物元素廣泛參與調(diào)控細(xì)胞的增殖和分化、組織器官的發(fā)育。眾多微量元素如鐵、鋅、硒等被發(fā)現(xiàn)廣泛參與動(dòng)物的繁殖活動(dòng)，但是否調(diào)控動(dòng)物的情期啟動(dòng)及卵泡發(fā)育，相關(guān)研究較少。鎂是動(dòng)物骨代謝和骨膠原生長(zhǎng)和發(fā)育的必需礦物元素，其缺乏會(huì)導(dǎo)致生長(zhǎng)和發(fā)育停滯。血液中鎂在生長(zhǎng)發(fā)育期跨過(guò)血腦屏障的效率是成年期的4倍，更有趣的是，鎂十分容易在下丘腦中沉積。Pine等［71］在雌性生長(zhǎng)大鼠第3腦室中灌注氯化鎂，發(fā)現(xiàn)可顯著刺激下丘腦促黃體素釋放激素(luteinizing hormone-releasing hormone，LHRH)基因表達(dá)量及血液LH濃度;此外，生長(zhǎng)大鼠飼糧中添加10 mg/kg氯化鎂，血液中LH、FSH、雌二醇等繁殖激素的濃度顯著提高，且初情日齡顯著提前。通過(guò)體外研究［72］及體內(nèi)研究［73］進(jìn)一步證實(shí)了氯化鎂通過(guò)下丘腦鳥(niǎo)苷酸環(huán)化酶、胰島素樣生長(zhǎng)因子受體、環(huán)氧合酶-2刺激下丘腦分泌LHRH及垂體分泌 LH。Srivastava等［74］進(jìn)一步證實(shí)，在飼糧中添加10 mg/kg的氯化鎂，大鼠血液GnRH濃度顯著提高，并且引起下丘腦情期啟動(dòng)相關(guān)基因Kiss-1表達(dá)顯著上調(diào)，并誘發(fā)大鼠早熟(圖8)。在母豬養(yǎng)殖過(guò)程中，妊娠母豬便秘是困擾母豬健康的一大難題，在母豬飼糧中通常添加2～3 kg/t硫酸鎂以防止便秘。鑒于鎂對(duì)神經(jīng)內(nèi)分泌及繁殖系統(tǒng)的干擾效應(yīng)，因此在母豬飼糧配制時(shí)需足夠謹(jǐn)慎。

圖7 血液鎂干擾下丘腦GnRH分泌及性早熟機(jī)制Fig.7 Mechanism of GnRH secretion and precocious puberty interfered by blood manganese

3 表觀遺傳修飾和內(nèi)分泌干擾因子對(duì)雌性動(dòng)物情期啟動(dòng)的影響

雌性動(dòng)物由生長(zhǎng)向繁殖轉(zhuǎn)換過(guò)程中，表觀遺傳修飾發(fā)揮著決定性作用。Tomikawaa等［75］分別觀察了雌激素對(duì)下丘腦AVPV和ARC區(qū)域Kiss-1基因的表觀遺傳修飾狀態(tài)，發(fā)現(xiàn)雌激素對(duì)AVPV神經(jīng)元的正反饋調(diào)控與Kiss-1基因的表觀遺傳修飾有關(guān)［75］。不同區(qū)域Kiss-1基因表達(dá)模式的改變反映了雌激素受體誘導(dǎo)的正負(fù)反饋調(diào)控敏感性的改變，是觸發(fā)情期啟動(dòng)的關(guān)鍵生理過(guò)程［34］。Lomniczi等［76］對(duì)小鼠注射甲基化抑制劑，小鼠初情期推遲甚至消失。進(jìn)一步研究證實(shí)，小鼠正常情期啟動(dòng)依賴于PcG(polycomb group)蛋白對(duì)Kiss-1基因啟動(dòng)區(qū)域的一系列表觀遺傳修飾，導(dǎo)致Kiss-1基因表達(dá)量上調(diào)，促進(jìn)性腺發(fā)育及初情期來(lái)臨。

值得注意的是，基因的表觀遺傳修飾對(duì)外界環(huán)境十分敏感，尤其是具有內(nèi)分泌干擾效應(yīng)的化學(xué)物質(zhì)。多酚A是廣泛存在于食品、飼料中的一種化學(xué)物質(zhì)，可誘導(dǎo)基因啟動(dòng)子富含雙核苷酸“CG”的區(qū)域(即 CpG島)低甲基化。研究表明［77］，在 小 鼠 飼 糧 中 添 加 低 劑 量 多 酚 A(50 μg/kg)，可通過(guò)影響Kiss-1基因表達(dá)及GnRH神經(jīng)元數(shù)量誘發(fā)性早熟。苯甲雌二醇和甲氧氯具有明顯的類雌激素結(jié)構(gòu)，可顯著干擾動(dòng)物的內(nèi)分泌狀態(tài)并誘發(fā)早熟［76］。研究表明，當(dāng)動(dòng)物在發(fā)育早期被苯甲雌二醇和甲氧氯干擾時(shí)，下丘腦Kiss-1的基因表達(dá)及雌激素受體α(estrogen receptor α，ERα)基因啟動(dòng)區(qū)域的甲基化模式改變，導(dǎo)致情期循環(huán)紊亂，繁殖系統(tǒng)提前衰退(圖8)，繁殖功能提前終止［78］。因此，母豬飼糧中應(yīng)特別關(guān)注重金屬、霉菌毒素、內(nèi)分泌干擾因子對(duì)母豬情期啟動(dòng)及繁殖系統(tǒng)的損害。

4 小結(jié)

情期啟動(dòng)是提高母豬終身繁殖成績(jī)的關(guān)鍵環(huán)節(jié)。體組織的生長(zhǎng)和發(fā)育是營(yíng)養(yǎng)積累的結(jié)果，各器官組織只有在營(yíng)養(yǎng)儲(chǔ)備達(dá)到一定標(biāo)準(zhǔn)后才能激活下丘腦Kisspeptin神經(jīng)元并觸發(fā)下丘腦-垂體-性腺軸活性，推動(dòng)母豬由生長(zhǎng)轉(zhuǎn)向繁殖，或由泌乳轉(zhuǎn)向發(fā)情。雌激素正負(fù)反饋效應(yīng)參與了營(yíng)養(yǎng)調(diào)控情期啟動(dòng)，但機(jī)理不詳，待進(jìn)一步研究。

圖8 毒素與內(nèi)分泌干擾因子干擾繁殖功能的理論假設(shè)Fig.8 Theoryhypothesis of reproduction function interfered by toxins and endocrine disruptors

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