楊燕飛 黃志力
復(fù)旦大學(xué)基礎(chǔ)醫(yī)學(xué)院藥理學(xué)系,醫(yī)學(xué)神經(jīng)生物學(xué)國家重點(diǎn)實(shí)驗(yàn)室,腦科學(xué)研究院與腦科學(xué)協(xié)同創(chuàng)新中心,上海,200032,中國
睡眠是高度保守的行為,從低等的動(dòng)物到高等的人類都需要睡眠,但睡眠-覺醒的調(diào)控機(jī)制仍是不解之謎。根據(jù)腦電圖(electroencephalogram,EEG)的波幅和頻率,以及肌電圖(electromyogram,EMG)的活動(dòng)度,睡眠被分為快速眼動(dòng)(rapid eye movement,REM)睡眠和非快速眼動(dòng)(non-REM,NREM)睡眠二個(gè)時(shí)相。
目前已知調(diào)控睡眠的系統(tǒng)主要包括:下丘腦腹外側(cè)視前區(qū)(ventrolateral preoptic area,VLPO)神經(jīng)叢、基底前腦(basal forebrain,BF)、視前區(qū)(preoptic area,POA)、腦 干(brainstem,BS)、丘 腦 網(wǎng) 狀 核(thalamic reticular nucleus,TRN)和基底神經(jīng)節(jié)(basal ganglia,BG)的γ-氨基丁酸(gamma aminobutyric acid,GABA)能神經(jīng)元等。主要覺醒調(diào)控系統(tǒng)有:腦干網(wǎng)狀結(jié)構(gòu)(reticular formation of brain stem,BSRF)、中縫核(raphe nucleus,RN)5-羥 色 胺(5-hydroxytryptamine,5-HT)能神經(jīng)元、藍(lán)斑(locus coeruleus,LC)去甲腎上腺素(norepinephrine,NE)能神經(jīng)元、下丘腦外側(cè)(lateral hypothalamus,LH)食欲素(orexin)能神經(jīng)元、結(jié)節(jié)乳頭核(tuberomammillary nucleus,TMN)組胺(histamine)能神經(jīng)元、中腦導(dǎo)水管周圍灰質(zhì)(periaqueductal gray,PAG)多巴胺(dopamine,DA)能神經(jīng)元、腦橋 -中腦(pons-midbrain)乙酰膽堿(acetylcholine,ACh)能神經(jīng)元和BF膽堿能和非膽堿能神經(jīng)元等。
睡眠和覺醒是通過腦內(nèi)多種神經(jīng)遞質(zhì)和內(nèi)源性睡眠促進(jìn)物質(zhì)共同作用、相互影響而實(shí)現(xiàn)的,同時(shí)也受到晝夜節(jié)律和內(nèi)環(huán)境穩(wěn)態(tài)的調(diào)控[1-3]。睡眠-覺醒行為受大腦中諸多神經(jīng)系統(tǒng)調(diào)節(jié),具有復(fù)雜的神經(jīng)環(huán)路,理解其調(diào)節(jié)機(jī)制具有極大的科學(xué)價(jià)值和臨床意義[4]。特異性控制神經(jīng)元活性技術(shù)的廣泛應(yīng)用,為揭示睡眠-覺醒的機(jī)制提供了更有力的工具。
2005年斯坦福大學(xué)Karl Deisseroth實(shí)驗(yàn)室首次將光敏感視紫紅質(zhì)通道蛋白 2(channelrhodopsin-2,ChR2)應(yīng)用在神經(jīng)元中,開啟了神經(jīng)生物學(xué)的新紀(jì)元[5]。目前主要發(fā)現(xiàn)三類光控蛋白,包括快速興奮性、快速抑制性和光控信號(hào)通路蛋白三大類。ChR2是最常用的興奮性光控蛋白,可被473 nm波長的藍(lán)光激活,使胞外陽離子內(nèi)流,產(chǎn)生內(nèi)向電流,引起神經(jīng)元去極化[6-7]。嗜鹽菌紫質(zhì)(halorhodopsin,NpHR)可以被570 nm波長黃光激活,引發(fā)氯離子內(nèi)流,使細(xì)胞超極化。此外還有其他類型的抑制性的光控蛋白[6-7]。如,古嗜鹽菌紫質(zhì)(arechaerhodopsin,ArcH)是外向的質(zhì)子泵,ArcH可以被566 nm藍(lán)綠光激活,將胞內(nèi)的質(zhì)子泵出,從而產(chǎn)生外向的電流抑制神經(jīng)元的電活動(dòng)[8]。近年來又發(fā)現(xiàn)響應(yīng)632 nm紅光的氯離子通道,通過篩選得到蛋白Jaws[9]。
北卡羅來納大學(xué)Bryan Roth等通過突變膽堿毒蕈堿型受體,創(chuàng)造了特異性被人工配體激活的人工受體(designer receptors exclusively activated by designer drugs,DREADDs)[10-11]。DREADDs 沒 有 基 礎(chǔ) 活 性,不能被內(nèi)源性配體膽堿所活化,但能通過外源給予氧化氯氮平(clozapine oxide,CNO)激活。DREADDs包括興奮型和抑制型二種,興奮型系人源化的hM3Dq和鼠源化的rM3Ds,抑制型為人源的hM4Di。M3受體是Gq型G蛋白偶聯(lián)受體(G protein-coupled receptors,GPCRs),通過磷脂酰肌醇途徑增加胞內(nèi)鈣濃度,從而興奮神經(jīng)元[12-13]。M4受體是 Gi型 GPCR,通過抑制腺苷酸環(huán)化酶或耦聯(lián)內(nèi)向整流鉀離子通道(inwardly rectifying potassium 3,Kir3)使細(xì)胞超極化從而抑制神經(jīng)元[11]。最近Bryan Roth課題組又通過突變阿片肽受體κ受體獲得的 DREADDs(κ-opioid receptor DREADDs,KORD),KORD和hM4Di一樣也是介導(dǎo)內(nèi)向整流鉀離子通道,使細(xì)胞超極化,從而抑制神經(jīng)元活性[14-15]。
運(yùn)用光遺傳學(xué)和藥理遺傳學(xué)方法,在特異性神經(jīng)元上選擇性表達(dá)人工受體,達(dá)到高空間分辨率人工控制神經(jīng)元活性的目的。光遺傳學(xué)在時(shí)間分辨率上更勝一籌,而藥理遺傳學(xué)作用時(shí)間更持久,根據(jù)實(shí)驗(yàn)需求,兩者聯(lián)用,實(shí)現(xiàn)互補(bǔ)。
已知睡眠相關(guān)腦區(qū),包括VLPO[16-17]和面神經(jīng)旁核(parafacial zone,PZ)[18-19]等,但變性這些核團(tuán),對(duì)睡眠量的長期影響都很小,提示可能還有更重要的核團(tuán)存在。控制睡眠和覺醒的核團(tuán)是相互聯(lián)系,具體哪個(gè)腦區(qū)及哪類神經(jīng)元在睡眠調(diào)控中發(fā)揮重要作用,尚不清楚。本課題組長期關(guān)注腺苷對(duì)睡眠的調(diào)控及其作用機(jī)制,揭示了腺苷通過作用于TMN組胺能覺醒系統(tǒng)中A1受體 (adenosine A1receptor,A1R)而發(fā)揮促眠作用[20-21];發(fā)現(xiàn)咖啡因是通過阻斷腺苷 A2A受體(adenosine A2Areceptor,A2AR)發(fā)揮強(qiáng)效促覺醒作用[22-23],但腦內(nèi)A2AR陽性神經(jīng)元是否啟動(dòng)或維持睡眠,仍有待闡明。
背側(cè)紋狀體(dorsal striatum,DS)是BG主要的輸入核團(tuán)[24],其神經(jīng)元密集表達(dá) A2AR[25]。Yuan 等利用藥理遺傳、光遺傳、免疫電鏡和電生理等技術(shù),發(fā)現(xiàn)小鼠DS頭端和中部區(qū)域的A2AR陽性神經(jīng)元,通過控制外側(cè)蒼白球(external globus pallidus,GPe)中的小清蛋白陽性(parvalbumin positive,PV+)神經(jīng)元,調(diào)節(jié)動(dòng)物活動(dòng)期睡眠(Fig.1)[26]。這一調(diào)節(jié)作用,可能為治療帕金森病(Parkinson’s disease,PD)患者的白天嗜睡提供新的思路[26]。
Fig.1 A2AR neurons in the rostral and central striatum control sleep
伏隔核(nucleus accumbens core,NAc)一直被認(rèn)為在獎(jiǎng)賞、成癮和恐懼等活動(dòng)中起重要作用[27]。有研究表明NAc能夠調(diào)節(jié)慢波睡眠(slow wave sleep,SWS)[28],另外阻斷 NAc中的 A2AR 是咖啡因的促覺醒作用的重要機(jī)制[29],但NAc中A2AR陽性神經(jīng)元在調(diào)節(jié)睡眠中發(fā)揮的作用不明確。。
Oishi等利用轉(zhuǎn)基因小鼠和光遺傳學(xué)技術(shù),發(fā)現(xiàn)光刺激NAc中A2AR陽性神經(jīng)元誘發(fā)快速而強(qiáng)烈的SWS,并增加小鼠睡眠量。當(dāng)小鼠出現(xiàn)行為動(dòng)機(jī)時(shí),伏隔核內(nèi)A2AR神經(jīng)元活性明顯被抑制,小鼠覺醒量增加;相反動(dòng)機(jī)行為缺乏時(shí),伏隔核內(nèi)A2AR神經(jīng)元活性增加,小鼠出現(xiàn)睡眠。以上研究結(jié)果表明,NAc內(nèi)A2AR陽性神經(jīng)元直接調(diào)節(jié)動(dòng)機(jī)行為缺乏性睡眠(Fig.2)[28]。
此外,F(xiàn)ang 等發(fā)現(xiàn),小鼠敲除 A2AR,酒精(3 g·kg-1)所引起的促眠作用消失,提示酒精可能通過增加腦內(nèi)腺苷水平,發(fā)揮促眠作用[30]。
嗅覺功能障礙(嗅覺閾值增加和氣味鑒別受損)和REM睡眠障礙癥狀常出現(xiàn)在PD患者運(yùn)動(dòng)癥狀發(fā)生之前[31-32];動(dòng)物實(shí)驗(yàn)發(fā)現(xiàn)切除大鼠嗅球(olfactory bulb,OB)后,動(dòng)物表現(xiàn)出REM睡眠增加[33],此研究提示嗅覺功能的異常和REM睡眠障礙密切相關(guān)。此外,OB中有大量A2AR分布[34],但OB中A2AR對(duì)睡眠調(diào)節(jié)的機(jī)制目前尚不清楚。
Wang等運(yùn)用DREADDs法特異性操縱OB的A2AR陽性神經(jīng)元活性,發(fā)現(xiàn)OB中的A2AR參與REM睡眠的調(diào)控。當(dāng)激活OB中A2AR陽性神經(jīng)元,小鼠REM睡眠降低;抑制該類型神經(jīng)元,小鼠REM睡眠增加。結(jié)合神經(jīng)元特異性示蹤技術(shù)的研究發(fā)現(xiàn)OB中A2AR陽性神經(jīng)元調(diào)控REM睡眠可能與支配下游的梨狀皮層和嗅結(jié)節(jié)有關(guān)[35]。
PZ是近年新發(fā)現(xiàn)的與促眠相關(guān)的核團(tuán)[36],該區(qū)域的神經(jīng)元在睡眠期興奮性上升,當(dāng)化學(xué)毀損該區(qū)域的神經(jīng)元?jiǎng)t導(dǎo)致動(dòng)物覺醒量顯著增加,表明PZ可能參與睡眠行為的調(diào)控[37],但對(duì)PZ-GABA能神經(jīng)元能否啟動(dòng)和維持SWS尚不清楚。Fuller等利用DREADDs法選擇性激活PZ-GABA能神經(jīng)元,研究發(fā)現(xiàn)小鼠NREM睡眠量出現(xiàn)連續(xù)3 h的顯著增加;光遺傳學(xué)實(shí)驗(yàn)的結(jié)果發(fā)現(xiàn)PZ-GABA能神經(jīng)元可能是通過抑制其下游覺醒核團(tuán) -臂旁核(parabrachial nucleus,PB),達(dá)到啟動(dòng)睡眠的作用??傊琍Z的GABA能神經(jīng)元能夠有效地啟動(dòng)SWS,調(diào)節(jié)大腦皮層的腦電活動(dòng)[18]。
VLPO一直被認(rèn)為是腦內(nèi)重要的睡眠中樞,該區(qū)域通過向腦內(nèi)多個(gè)覺醒腦區(qū)進(jìn)行抑制性投射,從而啟動(dòng)睡眠[38]。
Dan等利用神經(jīng)元逆行示蹤技術(shù)鎖定VLPO區(qū)域內(nèi)投射到下丘腦重要覺醒中樞TMN的神經(jīng)元。研究結(jié)果揭示了光激活VLPO投射到TMN的GABA(GABAVLPO-TMN)能神經(jīng)元,可誘發(fā)小鼠進(jìn)入睡眠狀態(tài);激活整個(gè)VLPO區(qū)域其它的GABA能神經(jīng)元?jiǎng)t誘發(fā)小鼠出現(xiàn)覺醒。這一結(jié)果證實(shí)了VLPO在啟動(dòng)睡眠行為中扮演的重要作用,也提示了同一核團(tuán)內(nèi)相同類型的神經(jīng)元對(duì)睡眠-覺醒行為的調(diào)控可能有著完全不同的作用[39]。
黑色素濃縮激素(melanin-concentrating hormone,MCH)是LH區(qū)域內(nèi)稀疏分布的神經(jīng)肽[40]。MCH神經(jīng)元在睡眠期間活躍,因此藥理學(xué)研究認(rèn)為,此類神經(jīng)元可能參與調(diào)控睡眠[41]。
Sonia Jego等在小鼠REM睡眠期,光激活MCH神經(jīng)元,REM睡眠持續(xù)時(shí)間增加;抑制MCH神經(jīng)元,海馬θ節(jié)律的頻率和幅度降低,但不影響REM睡眠持續(xù)時(shí)間。此外,MCH神經(jīng)元與覺醒核團(tuán)TMN存在著功能性連接,激活MCH神經(jīng)元投射到TMN區(qū)域的神經(jīng)末梢,可誘發(fā)TMN區(qū)域組胺能神經(jīng)元出現(xiàn)抑制性突觸后電流,小鼠REM睡眠顯著延長。此研究結(jié)果表明MCH神經(jīng)元參與REM睡眠調(diào)控可能是通過抑制腦內(nèi)覺醒核團(tuán)TMN實(shí)現(xiàn)的[42]。而Tsunematsu等在NREM睡眠期激活MCH神經(jīng)元,誘發(fā)小鼠REM睡眠,在覺醒期間激活無效;抑制MCH神經(jīng)元?jiǎng)t覺醒增加,NREM睡眠降低,不影響REM睡眠。此研究揭示了只激活MCH神經(jīng)元足以觸發(fā)小鼠從NREM到REM睡眠的轉(zhuǎn)變,并在NREM睡眠的起始和維持中起重要作用[43]。綜上所述,LH中MCH神經(jīng)元參與調(diào)控NREM和REM睡眠,但還有待更近一步的研究。
TRN位于丘腦白質(zhì)附近,是大腦皮層和丘腦信息交流的門戶,TRN中大量的GABA能神經(jīng)元在睡眠調(diào)節(jié)中起著重要作用[44]。BF和腦干中廣泛投射的膽堿能神經(jīng)元在調(diào)節(jié)REM睡眠、睡眠-覺醒周期和促進(jìn)覺醒中發(fā)揮重要作用,投射到TRN的膽堿能神經(jīng)元在睡眠中的作用機(jī)制尚不明確。
Fig.2 Optogenetic stimulation of A2AR neurons in the NAc evoked a rapid and robust SWS response
Fig.2 (continued)
Ni等借助膽堿能神經(jīng)元選擇性表達(dá)ChR2的轉(zhuǎn)基因小鼠和光遺傳學(xué)技術(shù),發(fā)現(xiàn)光刺激投射到TRN的膽堿能神經(jīng)元,激活局部GABA能神經(jīng)元,會(huì)促進(jìn)小鼠睡眠并維持NREM睡眠,但REM睡眠不受影響。直接激活TRN中投射的膽堿能,會(huì)縮短入睡潛伏期和產(chǎn)生NREM睡眠[45]。此外,Lewis等研究表明TRN可快速調(diào)整局部皮層的狀態(tài)。光激活TRN,會(huì)迅速引起皮層產(chǎn)生類似于睡眠中的慢波活動(dòng),小鼠的覺醒明顯減少[46]。
未定帶(zona incerta,ZI)是與 LH 相連的腦區(qū),參與調(diào)節(jié)感覺-運(yùn)動(dòng)的整合[47]。刺激丘腦向ZI投射的神經(jīng)纖維可誘發(fā)睡眠樣狀態(tài)[48-49],但ZI中不同類型的神經(jīng)元在睡眠調(diào)控中的作用未知。Liu等研究發(fā)現(xiàn)腹側(cè)未定帶(ventral ZI,VZI)中的一類GABA能神經(jīng)元可促睡眠。這類神經(jīng)元表達(dá)LIM同源因子Lhx6,能直接抑制可激活覺醒的下丘腦分泌素細(xì)胞和GABA能細(xì)胞的活性。小鼠的睡眠期間,選擇性的激活或抑制VZI中Lhx6陽性神經(jīng)元,可直接增加或減少睡眠時(shí)間。此研究表明,VZI中GABA能神經(jīng)元能夠促進(jìn)睡眠[50]。
由Moruzzi和Magoun提出的“網(wǎng)狀結(jié)構(gòu)上行激活系統(tǒng)”是經(jīng)典的覺醒控制模型,電刺激中腦網(wǎng)狀結(jié)構(gòu)引起貓覺醒[51]。上行興奮性沖動(dòng)在丘腦中央髓板內(nèi)核換神經(jīng)元,通過非特異性投射激活整個(gè)大腦皮層。盡管中腦網(wǎng)狀結(jié)構(gòu)-丘腦-皮層通路是激活大腦皮層的重要結(jié)構(gòu),但并非唯一激活系統(tǒng)。目前已知調(diào)控覺醒的核團(tuán)還包括 RN[52]、LC[53]、TMN[54]、LH[55]和 BF[56]等,但這些核團(tuán)調(diào)控覺醒的機(jī)制尚不明確。
BF-膽堿能神經(jīng)元一直被公認(rèn)在皮層激活和覺醒行為中起關(guān)鍵作用[57-58]。在BF中,膽堿能神經(jīng)元僅占總細(xì)胞數(shù)的5%,GABA能和谷氨酸能神經(jīng)元分別占35%和55%[59]。但由于傳統(tǒng)方法對(duì)特定類型神經(jīng)元選擇性操控較差,BF中這三類神經(jīng)元在睡眠-覺醒調(diào)節(jié)中發(fā)揮的確切作用存在爭(zhēng)議。
3.1.1 基底前腦膽堿能神經(jīng)元
Fig.3 Activation or inhibition of basal forebrain (BF) cholinergic neurons slightly promoted/decreased wakefulness,produced low-delta non-rapid eye movement (NREM) sleep,and decreased/ increased electroencephalogram (EEG) power spectrum of NREM sleep
本課題組通過藥理遺傳學(xué)技術(shù)操控BF-膽堿能神經(jīng)元活性,發(fā)現(xiàn)膽堿能神經(jīng)元通過激活大腦皮層來抑制腦電δ波活動(dòng),而不僅是誘導(dǎo)覺醒行為(Fig.3)。DREADDs選擇性激活膽堿能神經(jīng)元,能夠顯著并持續(xù)降低EEG的δ波強(qiáng)度,并產(chǎn)生低δ波的NREM睡眠,增加覺醒的作用短暫;抑制膽堿能神經(jīng)元,能夠顯著增加EEG的δ波,輕微減少覺醒。人源化海腎熒光素酶綠色熒光蛋白(humanized renilla green fluorescent protein,hrGFP)示蹤BF中膽堿能神經(jīng)元的投射,發(fā)現(xiàn)在次級(jí)運(yùn)動(dòng)皮層和扣帶皮層hrGFP陽性神經(jīng)纖維密集分布,而在已知與睡眠相關(guān)的VLPO區(qū)域稀疏。激活BF-膽堿能神經(jīng)元,c-Fos的表達(dá)在次級(jí)運(yùn)動(dòng)皮層和扣帶皮層明顯增加,VLPO中c-Fos表達(dá)降低[60]。此外,Han等利用光遺傳學(xué)技術(shù)特異性激活BF-膽堿能神經(jīng)元,會(huì)誘導(dǎo)小鼠從SWS狀態(tài)向清醒或REM睡眠的轉(zhuǎn)化,但不影響SWS的最終轉(zhuǎn)化方向與概率。在清醒或REM睡眠期間激活膽堿能神經(jīng)元,會(huì)維持皮層的活躍;連續(xù)刺激1 h后,會(huì)導(dǎo)致不活躍期覺醒時(shí)間增加,引起類失眠樣表現(xiàn)[61]。相比之前激活去甲腎上腺素或丘腦分泌素神經(jīng)元誘導(dǎo)小鼠從SWS和REM睡眠轉(zhuǎn)換至覺醒的研究,認(rèn)為在SWS期間光激活膽堿能神經(jīng)元才最有可能誘導(dǎo)覺醒,在REM睡眠時(shí)激活則不能將小鼠喚醒。
3.1.2 基底前腦GABA能和谷氨酸能神經(jīng)元
覺醒行為被認(rèn)為依賴于BF中膽堿能神經(jīng)元,但病變的膽堿能神經(jīng)元僅引起有限的覺醒和睡眠-覺醒周期變化[56]。提示BF中GABA能和谷氨酸能神經(jīng)元也可能參與了睡眠-覺醒調(diào)控。
Fuller等借助藥理遺傳學(xué)手段研究三種類型神經(jīng)元在睡眠-覺醒中的調(diào)控作用。在小鼠清醒狀態(tài)下,激活BF-谷氨酸能和膽堿能神經(jīng)元,均明顯鞏固覺醒狀態(tài),但不影響覺醒量,說明這兩類神經(jīng)元可能在促覺醒方面發(fā)揮的作用較小;激活BF-GABA能神經(jīng)元,產(chǎn)生持續(xù)的覺醒和高頻的皮層節(jié)律,而抑制該類神經(jīng)元?jiǎng)t增加睡眠[62]。Xu等使用光遺傳學(xué)特異性控制BF神經(jīng)元,進(jìn)一步揭示了BF中不同神經(jīng)元在睡眠-覺醒調(diào)節(jié)中的相互協(xié)作作用,并提供了首個(gè)BF中控制睡眠-覺醒的神經(jīng)環(huán)路圖。研究表明激活BF中膽堿能、谷氨酸和PV+-GABA能神經(jīng)元快速誘導(dǎo)覺醒;激活生長抑素陽性(somatostatin positive,SOM+) GABA 能神經(jīng)元促進(jìn)NREM睡眠。促覺醒神經(jīng)元由興奮性谷氨酸能到膽堿能到PV+神經(jīng)元連接構(gòu)成,它們都會(huì)受到SOM+神經(jīng)元的抑制作用[63]。
腹側(cè)被蓋區(qū)(ventral tegmental area,VTA)被認(rèn)為與自主運(yùn)動(dòng)和目標(biāo)導(dǎo)向行為相關(guān)[64]。動(dòng)機(jī)行為高度依賴于覺醒,說明VTA神經(jīng)元可能在睡眠-覺醒行為調(diào)節(jié)中起作用。
Sun等使用DREADDs法證實(shí)了VTA神經(jīng)元在促覺醒中發(fā)揮重要作用。無論是在活動(dòng)期(21:00)還是非活動(dòng)期(9:00)激活VTA神經(jīng)元,小鼠總覺醒率均大幅度增加,NREM和REM睡眠量均明顯下降。抑制VTA神經(jīng)元,不改變NREM睡眠,REM睡眠和覺醒的時(shí)間[65]。Eban等進(jìn)一步揭示了VTA中多巴胺能神經(jīng)元(VTA dopaminergic neuron,DA-VTA)是覺醒所必需的。使用藥理遺傳學(xué)和光遺傳學(xué)技術(shù)特異性操縱DA-VTA神經(jīng)元,當(dāng)DA-VTA神經(jīng)元受抑制時(shí),即使是面對(duì)顯著的行為刺激,覺醒也會(huì)受到抑制;在小鼠睡眠前,抑制DA-VTA神經(jīng)元,會(huì)促進(jìn)睡眠相關(guān)的筑巢行為。相反,光激活DA-VTA神經(jīng)元會(huì)啟動(dòng)和維持覺醒,抑制睡眠和睡眠相關(guān)的筑巢行為。此外,DA-VTA神經(jīng)元不同的投射,對(duì)覺醒調(diào)節(jié)有差異[66]。以上研究表明VTA神經(jīng)元在睡眠-覺醒行為中起調(diào)節(jié)作用。
腦橋被蓋核(pedunculopontine tegmental,PPT) /背外側(cè)被蓋核(laterodorsal tegmental,LDT)神經(jīng)元支配的促覺醒區(qū)域,包括VTA、LH、BF、額葉皮層和丘腦核[67]。PPT核調(diào)節(jié)覺醒和REM睡眠一直被認(rèn)為主要與PPT核膽堿能神經(jīng)元有關(guān)。光激活PPT和LDT的膽堿能神經(jīng)元,增加小鼠REM睡眠的概率[68];睡眠中電刺激PPT核會(huì)導(dǎo)致快速覺醒;損毀貓的PPT核會(huì)減少REM睡眠[69]。但PPT核內(nèi)還有谷氨酸能和GABA能神經(jīng)元,發(fā)揮的作用尚不明確。
Scammell等利用藥理遺傳學(xué)來確定PPT核內(nèi)這三種類型神經(jīng)元在調(diào)節(jié)大腦皮層活動(dòng)和睡眠-覺醒行為中扮演的角色,并繪制出每個(gè)神經(jīng)元的投射。該研究揭示了PPT-谷氨酸能神經(jīng)元強(qiáng)烈地促進(jìn)覺醒,PPT-膽堿能神經(jīng)元抑制皮層節(jié)律,PPT-GABA能神經(jīng)元略微減少REM睡眠。認(rèn)為PPT-谷氨酸和膽堿能神經(jīng)元協(xié)同作用產(chǎn)生覺醒。此外,PPT神經(jīng)元具有不同的投射,對(duì)皮層活動(dòng)和睡眠-覺醒也有不同的影響[70]。研究PPT-谷氨酸能神經(jīng)元促覺醒本質(zhì)以及PPT核內(nèi)各種類型神經(jīng)元的輸入和投射,也是重要的課題。
促睡眠核團(tuán)VLPO[71]的突觸傳入主要來自于與覺醒相關(guān)的 LH[55],LH 的病變通常會(huì)引起嗜睡[72]。然而,在LH內(nèi),這些投射到VLPO神經(jīng)元的種類及在覺醒網(wǎng)絡(luò)中的功能仍存在很多疑問。
Venner等認(rèn)為LH中向VLPO投射的神經(jīng)元可能含有GABA,而這些神經(jīng)元在睡眠-覺醒中發(fā)揮調(diào)節(jié)作用。利用藥理遺傳學(xué)方法操控Vgat-ires-Cre小鼠LH的GABA能神經(jīng)元,證實(shí)了LH中向VLPO投射神經(jīng)元含有GABA,此類神經(jīng)元在睡眠-覺醒中發(fā)揮調(diào)節(jié)作用。LH中向VLPO投射的神經(jīng)元表達(dá)囊泡GABA轉(zhuǎn)運(yùn) 體(vesicle GABA transporter,VGAT),這 是 GABA釋放神經(jīng)元的標(biāo)記物。當(dāng)激活LH-VGAT神經(jīng)元可促進(jìn)覺醒,抑制LH-VGAT陽性神經(jīng)元?jiǎng)t會(huì)誘發(fā)睡眠,說明LH-VGAT陽性神經(jīng)元是促睡眠核團(tuán)VLPO的主要抑制來源。此外,LH投射到VLPO的這些LH-VGAT陽性神經(jīng)元,也投射到TMN,腹側(cè)中腦導(dǎo)水管周圍灰質(zhì)和 LC[73]。
終紋床核(bed nucleus of the stria terminalis,BNST)在應(yīng)激反應(yīng)、恐懼和焦慮中起重要作用,提高警覺水平和覺醒應(yīng)答,以應(yīng)對(duì)與壓力和危險(xiǎn)相關(guān)的環(huán)境因素[74],但BNST是否參與睡眠-覺醒的調(diào)節(jié)有待進(jìn)一步研究。
Kodani等人發(fā)現(xiàn)BNST中表達(dá)谷氨酸脫羧酶67(glutamate decarboxylase 67,Gad67)的GABA能神經(jīng)元在小鼠睡眠-覺醒的調(diào)節(jié)中起至關(guān)重要的作用。在NREM睡眠期間,光刺激Gad67-Cre小鼠BNSTGABA能神經(jīng)元,小鼠立即轉(zhuǎn)向清醒,orexin系統(tǒng)不參與調(diào)節(jié);在REM睡眠期間,光刺激BNST-GABA能神經(jīng)元,對(duì)小鼠睡眠-覺醒狀態(tài)無影響。使用藥理遺傳學(xué)技術(shù)長時(shí)間刺激BNST-GABA能神經(jīng)元,會(huì)引起持續(xù)的覺醒狀態(tài),而這種持續(xù)覺醒狀態(tài)可通過提前給予食欲素受體拮抗劑消除。順行追蹤實(shí)驗(yàn)揭示了,BNSTGABA能神經(jīng)元投射到與覺醒密切關(guān)聯(lián)的腦區(qū),包括POA、LH、中腦導(dǎo)水管周圍灰質(zhì)和PB。綜上所述,BNST-GABA能神經(jīng)元在NREM睡眠至覺醒的過渡中發(fā)揮重要的作用且orexin神經(jīng)元不相關(guān),當(dāng)BNST中GABA能神經(jīng)元長期受激發(fā)時(shí),則會(huì)調(diào)動(dòng)orexin系統(tǒng)保持清醒[75]。
特異性控制神經(jīng)元活性技術(shù)以及各種轉(zhuǎn)基因動(dòng)物的出現(xiàn),突破了傳統(tǒng)藥理學(xué)研究的局限,為睡眠-覺醒的研究提供了極大的便利。
最新研究發(fā)現(xiàn):PZ-GABA能神經(jīng)元能夠有效地啟動(dòng)SWS;激活VLPO至TMN的GABA能神經(jīng)元可誘發(fā)小鼠進(jìn)入睡眠狀態(tài),整體激活VLPO-GABA能神經(jīng)元?jiǎng)t誘發(fā)小鼠覺醒;LH中MCH神經(jīng)元的激活可維持REM睡眠,并在NREM睡眠的起始和維持中也起重要作用;選擇性激活TRN-膽堿能神經(jīng)元末梢后局部激活GABA能神經(jīng)元,可明顯縮短小鼠入睡潛伏期并增加NREM睡眠量,而REM睡眠不受影響;激活OB-A2AR神經(jīng)元REM睡眠降低,抑制則REM睡眠增加;光刺激NAc中A2AR神經(jīng)元,會(huì)誘發(fā)快速而強(qiáng)烈的SWS反應(yīng)。
對(duì)于覺醒調(diào)節(jié)系統(tǒng),BF中除了膽堿能神經(jīng)元,激活BF-GABA能神經(jīng)元促覺醒;激活VTA神經(jīng)元增加覺醒,其中DA-VTA在維持清醒中起關(guān)鍵性作用;PPT-谷氨酸能神經(jīng)元強(qiáng)烈地促進(jìn)覺醒,PPT-膽堿能神經(jīng)元抑制皮層慢節(jié)律,PPT-GABA能神經(jīng)元略微減少REM睡眠;激活LH-VGAT陽性神經(jīng)元可促覺醒,抑制則增加睡眠;GABA-BNST神經(jīng)元在NREM睡眠至覺醒過渡中發(fā)揮作用,當(dāng)GABA-BNST神經(jīng)元長期受激發(fā)時(shí),則會(huì)調(diào)動(dòng)orexin系統(tǒng)保持清醒。
綜上所述,GABA能、谷氨酸能和及膽堿能神經(jīng)元已成為近年來的研究熱點(diǎn)。因此,未來的挑戰(zhàn)將是揭示這些新發(fā)現(xiàn)的調(diào)節(jié)系統(tǒng)如何協(xié)同來調(diào)節(jié)睡眠和覺醒?內(nèi)穩(wěn)態(tài)和生物節(jié)律因素如何協(xié)同調(diào)節(jié),影響睡眠和覺醒神經(jīng)網(wǎng)絡(luò)?相信隨著新技術(shù)的不斷應(yīng)用,將會(huì)更加深刻而全面地認(rèn)識(shí)到各個(gè)睡眠-覺醒核團(tuán)、不同類型神經(jīng)元及相關(guān)調(diào)控網(wǎng)絡(luò)在睡眠-覺醒中的作用。人工智能及生物物理技術(shù)如能實(shí)現(xiàn)對(duì)這些核團(tuán)的按需性調(diào)控,人為調(diào)控人體睡眠將成為可能。
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