- 學位論文
大腦皮質-視丘下核路徑在巴金森動作障礙之角色
The Role of Cortico-subthalamic Pathway in Parkinsonian Motor Deficits
摘要
巴金森氏病是最常見的動作障礙疾病，在症狀學上以動作不能做為主要表現。在病理學上，中腦於黑質緻密部的多巴胺神經元退化是其動作障礙的主因，並伴隨了兩個電生理異常表現：視丘下核過多的叢集式放電，及大腦—視丘下核間的不正常共振。在基底神經核的動作控制理論上，巴金森氏病也是最重要的疾病模式之一。中腦的多巴胺神經元退化，使興奮性的直接路徑受到過度壓制，而抑制性的間接路徑則受到過度活化，進而造成總體性的抑制效果而引發動作困難。雖然以直接—間接路徑為主的病理機轉為現今主流，仍有許多臨床現象及動作實驗結果無法解釋。在大腦—基底神經核的迴路中除了直接及間接路徑，還有一個較少被提及的大腦皮質—視丘下核路徑，或被稱之為「超直接路徑」。此一路徑由大腦直接支配視丘下核而越過了紋狀體—這個多巴胺調控直接及間接路徑的必經之路。然而，超直接路徑和巴金森氏病的關聯性仍屬未知。因此，我博士學程的研究課題即是超直接路徑在巴金森動作障礙的角色。 在以鼠類模式為基礎的實驗中，我們發現阻斷視丘下核的NMDA受體可以成功的治療巴金森的動作困難，並同時矯正兩大電生理異常。相對於NMDA受體，AMPA受體在動作行為或電生理表現上，並沒有顯著的效應。由此可知，經由NMDA受體的傳遞的超直接路徑在巴金森氏病中扮演了關鍵性的角色。在進一步的實驗中我們發現，於大腦刺激超直接路徑，可以在巴金森鼠的視丘下核引發過多的叢集式放電，以及異常的放電時間。這兩個超直接路徑機轉可直接導致巴金森氏病的兩大電生理異常，亦可被NMDA受體抑制劑所矯治。雖然此機轉在巴金森氏病有其必要性，因果關係的建立需要更強的證據。因此，我們進一步利用新興的光遺傳學方法，在沒有多巴胺缺乏的正常鼠上，選擇性的刺激超直接路徑。我們發現，模擬巴金森鼠之放電模式來刺激超直接路徑，可以立刻誘發正常鼠產生巴金森動作障礙。由此可知，依靠NMDA受體的超直接路徑異常，是造成巴金森動作困難的充要條件及核心機轉。為更深入檢驗超直接路徑機轉在巴金森氏病的角色，我們選用MK-801來干預巴金森鼠的行為。MK-801只抑制已開啟的NMDA受體，且結合受體所需時間為超直接路徑傳遞時間的數千倍。依前述的超直接路徑機轉，我們預測MK-801雖亦為NMDA受體抑制劑，但應無療效。實驗結果證實我們的推論，並對超直接路徑調控巴金森動作障礙的細部原理提供了進一步的佐證。 新發現的超直接路徑機轉，並不牽涉到紋狀體及直接-間接路徑的多巴胺調控。因此，了解多巴胺治療在巴金森氏病的原理成為一重要課題。我們進一步發現，多巴胺在巴金森氏病的療效可以歸因於視丘下核的調節。將多巴胺受體的促效劑注入視丘下核，就足以治療巴金森動作障礙而不需矯治紋狀體或直接-間接路徑的多巴胺缺乏。這個結果撼動了多巴胺治療的基礎原理，並提供了新的治療方向及契機。 由於超直接路徑越過了紋狀體，不論是電生理機轉，NMDA受體相關的療效，甚或多巴胺的治療機轉，都不需要紋狀體的直接調控。而中腦於黑質緻密部的多巴胺神經元，主要支配紋狀體及視丘下核。因此，我們進一步解析視丘下核的多巴胺去神經化，是否就足以產生巴金森動作障礙。實驗結果證實，選擇性損害支配視丘下核的多巴胺神經元，並不足使老鼠產生巴金森症狀。因此，紋狀體的多巴胺調控，在巴金森的致症機轉上仍有其角色。 總體而言，這一系統研究得出以下結論：巴金森氏病的動作障礙乃歸因於超直接路徑的異常，而多巴胺治療的原理亦歸因於視丘下核的調控。雖然超直接路徑的異常是巴金森症狀的直接原因及充要條件，紋狀體的多巴胺調節對巴金森的病態變化(即超直接路徑的異常)的成因仍具有關鍵性的角色。
並列摘要
Parkinson’s disease (PD) is the most prevalent movement disorder, and the prototypical disease model showing hypokinetic movements. Electrophysiologically, excessive subthalamic bursts and cortico-subthalamic beta synchronization are two specific abnormalities in PD. The symptomatic pathogenesis has been ascribable to the imbalance between direct and indirect pathways in the basal ganglia circuitry. However, there are still electrophysiological and clinical observations remains unexplained. Here we found that inhibition of NMDAergic cortico-subthalamic transmission rescues both motor deficits and electrophysiological abnormalities in 6-OHDA parkinsonian rats. Stimulation of premotor cortex further characterizes that NMDAergic cortico-subthalamic transmission in PD causes excessive burst generation in subthalamic nucleus (STN) and time-locked transmission between cortex and STN, which explain the firing pattern and synchronization-based abnormalities, respectively. Moreover, by mimicking the over-activation and synchronization of cortico-subthalamic transmission with optogenetic stimulation, we can instantaneously and reversibly induce parkinsonian symptoms in normal mice. The results clearly show that deranged NMDAergic cortico-subthalamic transmission is both essential and sufficient, hence causative, for hypokinetic motor deficits. Furthermore, subthalamic microinfusion of apomorphine, a dopaminergic agent commonly used in the treatment of PD, readily remedies parkinsonian motor deficits without correcting the dopaminergic deficiency in striatum. The behavioral results also show that NMDAergic intervention in STN does not cause paradoxical movements, which are widely seen in the standard dopaminergic treatment of PD animals, indicating that we target on the hypokinetic-specific mechanism. We further challenge this new model with MK-801, which is also a NMDA receptor blocker but only work on open channel with binding rate 1,000 times slower than the cortico-subthalamic transmission time. Our model successfully predicts the failure of the electrophysiological and behavioral effects in PD. To further explore the generating mechanism of the cortico-subthalamic derangement, we applied a rat model of subthalamic 6-OHDA microinfusion, which selectively deprived dopaminergic innervation in STN but preserved those in striatum. This manipulation failed to generate parkinsonian behaviors. In conclusion, our results show that deranged cortico-subthalamic transmission is the causative mechanism of parkinsonian motor behaviors. Although striatal dopamine pathology is not the direct pathophysiology of the motor deficits, it may still play some roles, with or without the dopaminergic denervation in STN, in the pathogenesis of cortico-subthalamic mechanism. Our works clearly impact the current concepts of parkinsonian motor control as well as related therapies, and significantly contribute to the revisit of the true essential elements of motor control theory in the basal ganglia circuitry.