Welsh John P, Yuen Genevieve, Placantonakis Dimitris G, Vu Toan Q, Haiss Florent, O'Hearn Elizabeth, Molliver Mark E, Aicher Sue A
Neurological Sciences Institute, Oregon Health and Sciences University, Beaverton, Oregon, USA.
Adv Neurol. 2002;89:331-59.
The experiments strongly suggested that the reason why Purkinje cells die so easily after global brain ischemia relates to deficiencies in aldolase C and EAAT4 that allow them to survive pathologically intense synaptic input from the inferior olive after the restoration of blood flow. This conclusion is based on: (a) the remarkably tight correspondence between the regional absence of aldolase C and EAAT4 in Purkinje cells and the patterned loss of Purkinje cells after a bout of global brain ischemia; (b) the necessity of the olivocerebellar pathway for the ischemic death of Purkinje cells; and (c) the build-up of pathologically synchronous and high-frequency burst activity within the inferior olive during recovery from ischemia. Indeed, the correspondence between the absence of aldolase C and EAAT4 to sensitivity to ischemia could be demonstrated for zones of Purkinje cells as small as two neurons. A second finding was that Purkinje cells are not uniformly sensitive to transient ischemia, since they die most frequently in zones where aldolase C and EAAT4 are absent. One implication of the experiment is that factors beyond the unique synaptic and membrane properties of Purkinje cells play an important role in determining this neuron's high sensitivity to ischemia. The data strongly imply that two properties of Purkinje cells that make them susceptible to ischemic death are their reduced capability to sequester glutamate and reduced ability to generate energy during anoxia. The patterned death of Purkinje cells is sufficient to induce a form of audiogenic myoclonus, as determined with a neurotoxic dose of ibogaine. Ibogaine-induced myoclonus is recognized behaviorally as a reduced ability to habituate to a startle stimulus and resembles the myoclonic jerk of rats during recovery from a prolonged bout of global brain ischemia. Commonalities of ischemia and ibogaine-induced neurodegeneration are the intricately striped Purkinje cell loss in the posterior lobe and a nearly complete deafferentation of the lateral aspect of the fastigial nucleus from the cerebellar cortex, in particular the dorsolateral protuberance. Thus, the data point strongly to a cerebellar contribution to audiogenic myoclonus. Single-neuron electrophysiology experiments in monkeys have demonstrated that the evoked activity in the deep cerebellar nuclei occurs too late to initiate the startle response (60) and electromyography of the postischemic myoclonus of rats corroborates this view (see Chapter 31) (20). However, the nearly complete loss of GABAergic terminals in the dorsolateral protuberance after Purkinje cell death would be expected to dramatically increase its tonic firing and the background excitation of the brain-stem structures that it innervates. The fastigial nucleus innervates a large number of autonomic and motor structures in the brainstem and diencephalon, including the ventrolateral nucleus of the thalamus and the gigantocellular reticular nucleus in the medulla--structures that have been implicated in human posthypoxic myoclonus (6, 7). We propose that the posthypoxic myoclonic jerk of rats is, at least in part, due to disinhibition of the fastigial nucleus produced by patterned Purkinje cell death in the vermis. The argument is as follows: the loss of GABAergic inhibition in the fastigial nucleus after ischemia leads to diaschisis of the motor thalamus and reticular formation which, in turn, is responsible for enhanced motor excitability and myoclonus. That the audiogenic myoclonus after global brain ischemia in the rat gradually resolves over a period of 2 to 3 weeks is consistent with this view, as restoration of background excitability after CNS damage in rats has been documented to occur within this time-frame (61). Our view brings together the physiologic finding that posthypoxic myoclonus appears to originate in the sensory-motor cortices and/or reticular formation with the consistent anatomical finding of Purkinje cell loss after ischemia, and explains the puzzle of Marsden's unique cases of myoclonus associated with coeliac disease (1). Moreover, our argument is consistent with findings both in rats (62, 63) and humans (64) that damage to the vermis impairs the long-term habituation of the startle reflex. It remains to be determined whether the pathologically enhanced startle responses after vermal damage resemble brain-stem reticular or cortical myoclonus at the electrophysiologic level of analysis. What is the purpose of the regional expression of aldolase C and EAAT4 in Purkinje cells? The close correspondence between the spatial distribution of aldolase C and the parasagittal anatomy of the cerebellum (48) has led to the view that aldolase C may help specify connectivity during development. While the present experiments do not address this issue, they underscore the fact that aldolase plays a fundamental role in metabolism. Because Purkinje cells have a repressed expression of aldolase A (31), whatever role the absence of aldolase C may play during development comes at the price of metabolic frailty later in adulthood. From another point of view, aldolase C and EAAT4 appear to confer upon Purkinje cells the ability to survive their own climbing fiber. Indeed, climbing fibers form a distributed synapse that synchronously releases glutamate (or aspartate) at all levels of the dendritic tree simultaneously (65, 66). Such synchronous activation triggers calcium influx throughout the Purkinje cell dendrites at a magnitude that is unparalleled in the nervous system (12), and, thus, places an extraordinarily high metabolic demand on the Purkinje cell. The apparently reduced level of aldolase in a subpopulation of Purkinje cells provides the condition for energy failure and death during anoxia so long as the climbing fibers are intact or when climbing fiber activation is pharmacologically enhanced under normoxic conditions, such as after ibogaine (53-56). Lastly, the argument that diaschisis produced by patterned cerebellar degeneration leads to thalamo-cortical and reticular hyperexcitability agrees with C. David Marsden and his colleagues' bold demonstration of an inhibitory influence of cerebellar cortex on motor cortex in humans (67). Our anatomic data indicate that the spatially distinct zones of Purkinje cells, which are killed by global brain ischemia, may be the origin of such inhibition.
实验有力地表明，浦肯野细胞在全脑缺血后极易死亡的原因与醛缩酶C和EAAT4的缺乏有关，这使得它们在血流恢复后无法承受来自下橄榄核的病理性强烈突触输入。这一结论基于以下几点：（a）浦肯野细胞中醛缩酶C和EAAT4的区域缺失与全脑缺血发作后浦肯野细胞的模式性丢失之间存在显著的紧密对应关系；（b）橄榄小脑通路对浦肯野细胞缺血性死亡的必要性；（c）缺血恢复过程中下橄榄核内病理性同步高频爆发活动的积累。事实上，对于小至两个神经元的浦肯野细胞区域，都能证明醛缩酶C和EAAT4的缺失与对缺血的敏感性之间的对应关系。第二个发现是，浦肯野细胞对短暂缺血的敏感性并不一致，因为它们最常在缺乏醛缩酶C和EAAT4的区域死亡。该实验的一个启示是，除了浦肯野细胞独特的突触和膜特性之外，其他因素在决定该神经元对缺血的高敏感性方面起着重要作用。数据强烈表明，浦肯野细胞易发生缺血性死亡的两个特性是其摄取谷氨酸的能力降低以及在缺氧期间产生能量的能力降低。浦肯野细胞的模式性死亡足以诱发一种听觉性肌阵挛，这是通过给予神经毒性剂量的伊波加因确定的。伊波加因诱发的肌阵挛在行为上表现为对惊吓刺激的习惯化能力降低，类似于大鼠在长时间全脑缺血恢复过程中的肌阵挛抽搐。缺血和伊波加因诱发的神经退行性变的共同之处在于，后叶浦肯野细胞呈复杂的条纹状丢失，以及小脑顶核外侧部分与小脑皮质，特别是背外侧隆起几乎完全脱失传入神经。因此，数据有力地表明小脑对听觉性肌阵挛有影响。对猴子进行的单神经元电生理实验表明，小脑深部核团中的诱发活动发生得太晚，无法启动惊吓反应（60），大鼠缺血后肌阵挛的肌电图证实了这一观点（见第31章）（20）。然而，浦肯野细胞死亡后，背外侧隆起中几乎完全丧失的GABA能终末预计会显著增加其紧张性放电以及它所支配的脑干结构的背景兴奋性。顶核支配脑干和间脑中大量的自主神经和运动结构，包括丘脑腹外侧核和延髓中的巨细胞网状核——这些结构与人类缺氧后肌阵挛有关（6,7）。我们提出，大鼠缺氧后肌阵挛性抽搐至少部分是由于蚓部浦肯野细胞的模式性死亡导致顶核去抑制所致。论证如下：缺血后顶核中GABA能抑制的丧失导致运动丘脑和网状结构的远隔性机能障碍，进而导致运动兴奋性增强和肌阵挛。大鼠全脑缺血后的听觉性肌阵挛在2至3周内逐渐消退，这与该观点一致，因为已证明大鼠中枢神经系统损伤后背景兴奋性的恢复在这个时间范围内发生（61）。我们的观点将缺氧后肌阵挛似乎起源于感觉运动皮层和/或网状结构这一生理学发现与缺血后浦肯野细胞丢失这一一致的解剖学发现结合在一起，并解释了马斯登与腹腔疾病相关的独特肌阵挛病例之谜（1）。此外，我们的观点与大鼠（62,63）和人类（64）的研究结果一致，即蚓部损伤会损害惊吓反射的长期习惯化。在电生理分析水平上，蚓部损伤后病理性增强的惊吓反应是否类似于脑干网状或皮层肌阵挛，仍有待确定。浦肯野细胞中醛缩酶C和EAAT regional expression的目的是什么？醛缩酶C的空间分布与小脑矢状旁解剖结构之间的密切对应关系（48）导致了这样一种观点，即醛缩酶C可能有助于在发育过程中确定连接性。虽然目前的实验没有涉及这个问题，但它们强调了醛缩酶在代谢中起基本作用这一事实。由于浦肯野细胞中醛缩酶A的表达受到抑制（31），醛缩酶C在发育过程中可能起的任何作用都以成年后期代谢脆弱为代价。从另一个角度来看，醛缩酶C和EAAT4似乎赋予浦肯野细胞在其自身攀缘纤维作用下存活的能力。事实上，攀缘纤维形成分布式突触，在树突树的所有层面同时同步释放谷氨酸（或天冬氨酸）（65,66）。这种同步激活会触发整个浦肯野细胞树突中的钙内流，其幅度在神经系统中是无与伦比的（12），因此，对浦肯野细胞提出了极高的代谢需求。只要攀缘纤维完整或在常氧条件下通过药理学方法增强攀缘纤维激活，如在伊波加因作用后（53 - 56），浦肯野细胞亚群中醛缩酶水平明显降低就为缺氧期间的能量衰竭和死亡提供了条件。最后，小脑模式性变性产生的远隔性机能障碍导致丘脑 - 皮质和网状结构过度兴奋这一观点与C. 大卫·马斯登及其同事关于人类小脑皮质对运动皮质有抑制作用的大胆证明一致（67）。我们的解剖学数据表明，全脑缺血导致死亡的浦肯野细胞在空间上不同的区域可能是这种抑制的起源。
