Hyde G E, Durham D
Department of Otolaryngology/Head and Neck Surgery, University of Washington, Seattle 98195.
J Neurosci. 1994 Jan;14(1):291-300. doi: 10.1523/JNEUROSCI.14-01-00291.1994.
Second-order auditory neurons in nucleus magnocellularis (NM) of the chick brainstem undergo a series of rapid metabolic changes following unilateral cochlea removal, culminating in the death of 25% of NM neurons. Within hours of cochlea removal, ipsilateral NM neurons show marked increases in histochemical staining for the mitochondrial enzymes succinate dehydrogenase and cytochrome oxidase (CO). We have shown previously in an ultrastructural study that these increases in oxidative capacity are mediated in part by a rapid increase in mitochondrial volume within deafferented neurons. In neurons that are destined to die as a result of deafferentation, mitochondria are smaller, stain poorly for CO, and often contain vacuoles associated with oxidative dysfunction. Our present set of experiments is designed to test the hypothesis that increases in oxidative metabolism are necessary for NM neuronal survival following removal of afferent input. We used chloramphenicol (CAP), a mitochondrial protein synthesis inhibitor, to block the characteristic increase in CO activity and mitochondrial proliferation following cochlea removal. We then studied the effects of CAP on NM neuronal survival following deafferentation. When CAP was administered continuously at 600 mg/kg/d for 5 d following cochlea removal, deafferentation-induced neuronal death in NM was significantly increased from 22% to 36%. Higher-dose (1200 mg/kg/d) pulses of CAP were administered for the first 6, 12, or 24 hr following cochlea removal. After 5 d survival, greater increases in neuronal cell death were found in animals treated for the first 12 or 24 hr (65% neuronal death). CAP administration for the first 6 hr had no significant effect on neuronal survival. The effects of CAP on neuronal cell death in NM are not likely to be due to systemic effects of the drug, but instead to a specific change in mitochondrial function. Our results suggest that enhanced oxidative enzyme function plays an important role in the survival of NM neurons during the first 24 hr after deafferentation. The nature of the signal(s) eliciting mitochondrial enhancement and the means by which it influences NM survival are not known.
雏鸡脑干大细胞神经核（NM）中的二级听觉神经元在单侧耳蜗切除后会经历一系列快速的代谢变化，最终导致25%的NM神经元死亡。在耳蜗切除后的数小时内，同侧NM神经元的线粒体酶琥珀酸脱氢酶和细胞色素氧化酶（CO）的组织化学染色显著增加。我们之前在一项超微结构研究中表明，这些氧化能力的增加部分是由去传入神经的神经元中线粒体体积的快速增加介导的。在因去传入神经而注定死亡的神经元中，线粒体较小，对CO染色不佳，且常含有与氧化功能障碍相关的液泡。我们目前的这组实验旨在检验以下假设：氧化代谢的增加是NM神经元在传入输入被去除后存活所必需的。我们使用氯霉素（CAP），一种线粒体蛋白质合成抑制剂，来阻断耳蜗切除后CO活性和线粒体增殖的特征性增加。然后我们研究了CAP对去传入神经后NM神经元存活的影响。在耳蜗切除后连续5天以600mg/kg/d的剂量持续给予CAP时，NM中去传入神经诱导的神经元死亡从22%显著增加到36%。在耳蜗切除后的前6、12或24小时给予更高剂量（1200mg/kg/d）的CAP脉冲。在存活5天后，发现在最初12或24小时接受治疗的动物中神经元细胞死亡的增加更大（65%的神经元死亡）。在最初6小时给予CAP对神经元存活没有显著影响。CAP对NM中神经元细胞死亡的影响不太可能是由于药物的全身作用，而是由于线粒体功能的特定变化。我们的结果表明，增强的氧化酶功能在去传入神经后的最初24小时内对NM神经元的存活起着重要作用。引发线粒体增强的信号的性质及其影响NM存活的方式尚不清楚。
