Nicholls D G, Budd S L, Ward M W, Castilho R F
Department of Pharmacology and Neuroscience, University of Dundee, U.K.
Biochem Soc Symp. 1999;66:55-67. doi: 10.1042/bss0660055.
Excitotoxicity is the process whereby a massive glutamate release in the central nervous system in response to ischaemia or related trauma leads to the delayed, predominantly necrotic death of neurons. Excitotoxicity is also implicated in a variety of slow neurodegenerative disorders. Mitochondria accumulate much of the post-ischaemic calcium entering the neurons via the chronically activated N-methyl-D-aspartate receptor. This calcium accumulation plays a key role in the subsequent death of the neuron. Cultured cerebellar granule cells demonstrate delayed calcium de-regulation (DCD) followed by necrosis upon exposure to glutamate. DCD is unaffected by the ATP synthase inhibitor oligomycin but is inhibited by the further addition of a respiratory chain inhibitor to depolarize the mitochondria and inhibit mitochondrial calcium accumulation without depleting ATP [Budd and Nicholls (1996) J. Neurochem. 67, 2282-2291]. Mitochondrial depolarization paradoxically decreases the cytoplasmic calcium elevation following glutamate addition, probably due to an enhanced calcium efflux from the cell. Cells undergo immediate calcium de-regulation in the presence of glutamate if the respiratory chain is inhibited; this is due to ATP depletion following ATP synthase reversal and can be reversed by oligomycin. In contrast, DCD is irreversible. Elevated cytoplasmic calcium is not excitotoxic as long as mitochondria are depolarized; alternative substrates do not rescue cells about to undergo DCD, suggesting that glycolytic failure is not involved. Mitochondria in situ remain sufficiently polarized during granule cell glutamate exposure to continue to generate ATP and show a classic mitochondrial state 3-state 4 hyperpolarization on inhibiting ATP synthesis; mitochondrial depolarization follows, and may be a consequence of rather than a cause of DCD. In addition, our studies show no evidence of the mitochondrial permeability transition prior to DCD. The mitochondrial generation of superoxide anions is enhanced during glutamate exposure and a working hypothesis is that DCD may be caused by oxidative damage to calcium extrusion pathways at the plasma membrane.
兴奋毒性是指在中枢神经系统中，因局部缺血或相关创伤而导致大量谷氨酸释放，进而引起神经元延迟性、主要为坏死性死亡的过程。兴奋毒性还与多种缓慢的神经退行性疾病有关。线粒体积累了大量经慢性激活的N-甲基-D-天冬氨酸受体进入神经元的缺血后钙。这种钙积累在随后的神经元死亡中起关键作用。培养的小脑颗粒细胞在暴露于谷氨酸后表现出延迟性钙调节异常（DCD），随后发生坏死。DCD不受ATP合酶抑制剂寡霉素的影响，但在进一步添加呼吸链抑制剂使线粒体去极化并抑制线粒体钙积累而不消耗ATP时受到抑制[Budd和Nicholls（1996年）《神经化学杂志》67卷，2282 - 2291页]。矛盾的是，线粒体去极化会降低添加谷氨酸后细胞质钙的升高，这可能是由于细胞内钙外流增强所致。如果呼吸链受到抑制，细胞在谷氨酸存在下会立即出现钙调节异常；这是由于ATP合酶逆转导致ATP耗竭，可被寡霉素逆转。相比之下，DCD是不可逆的。只要线粒体去极化，细胞质钙升高就不会产生兴奋毒性；替代底物无法挽救即将发生DCD的细胞，这表明糖酵解功能障碍不参与其中。在颗粒细胞暴露于谷氨酸期间，原位线粒体保持足够的极化以继续产生ATP，并在抑制ATP合成时表现出典型的线粒体状态3 - 状态4超极化；随后线粒体去极化，这可能是DCD的结果而非原因。此外，我们的研究没有显示在DCD之前线粒体通透性转换的证据。在谷氨酸暴露期间，线粒体超氧阴离子的生成增强，一个可行的假说是DCD可能是由质膜上钙外排途径的氧化损伤引起的。
