Nguyen P V, Marin L, Atwood H L
Department of Physiology, Faculty of Medicine, University of Toronto, Ontario, Canada.
J Neurophysiol. 1997 Jul;78(1):281-94. doi: 10.1152/jn.1997.78.1.281.
Phasic and tonic motor neurons of crustaceans differ strikingly in their junctional synaptic physiology. Tonic neurons generally produce small excitatory postsynaptic potentials (EPSPs) that facilitate strongly as stimulation frequency is increased, and normally show no synaptic depression. In contrast, phasic neurons produce relatively large EPSPs with weak frequency facilitation and pronounced depression. We addressed the hypothesis that mitochondrial function is an important determinant of the features of synaptic transmission in these neurons. Mitochondrial fluorescence was measured with confocal microscopy in phasic and tonic axons and terminals of abdominal and leg muscles after exposure to supravital mitochondrial fluorochromes, rhodamine-123 (Rh123) and 4-diethylaminostyryl-N-methylpyridinium iodide (4-Di-2-Asp). Mitochondria of tonic axons and neuromuscular junctions had significantly higher mean Rh123 and 4-Di-2-Asp fluorescence than in phasic neurons, indicating more accumulation of the fluorochromes. Mitochondrial membrane potential, which is responsible for Rh123 uptake and is related to mitochondrial oxidative activity (the production of ATP by oxidation of metabolic substrates), is likely higher in tonic axons. Electron microscopy showed that tonic axons contain approximately fivefold more mitochondria per microm2 cross-sectional area than phasic axons. Neuromuscular junctions of tonic axons also have a much higher mitochondrial content than those of phasic axons. We tested the hypothesis that synaptic fatigue resistance is dependent on mitochondrial function in crayfish motor axons. Impairment of mitochondrial function by uncouplers of oxidative phosphorylation, dinitrophenol or carbonyl cyanide m-chlorophenylhydrazone, or by the electron transport inhibitor sodium azide, led to marked synaptic depression of a tonic axon and accelerated depression of a phasic axon during maintained stimulation. Iodoacetate, an inhibitor of glycolysis, and chloramphenicol, a mitochondrial protein synthesis inhibitor, had no significant effects on either mitochondrial fluorescence or synaptic depression in tonic or phasic axons. Collectively, the results provide evidence that mitochondrial oxidative metabolism is important for sustaining synaptic transmission during maintained stimulation of tonic and phasic motor neurons. Tonic neurons have a higher mitochondrial content and greater oxidative activity; these features are correlated with their greater resistance to synaptic depression. Conversely, phasic neurons have a lower mitochondrial content, less oxidative activity, and greater synaptic fatigability.
甲壳类动物的相位性和紧张性运动神经元在其连接性突触生理学方面存在显著差异。紧张性神经元通常产生小的兴奋性突触后电位（EPSP），随着刺激频率增加，这些电位会强烈增强，并且通常不会出现突触抑制。相比之下，相位性神经元产生相对较大的EPSP，频率增强较弱且有明显的抑制。我们探讨了线粒体功能是这些神经元突触传递特征的重要决定因素这一假说。在暴露于超活线粒体荧光染料罗丹明-123（Rh123）和4-二乙氨基苯乙烯基-N-甲基碘化吡啶鎓（4-Di-2-Asp）后，用共聚焦显微镜测量腹部和腿部肌肉的相位性和紧张性轴突及终末中的线粒体荧光。紧张性轴突和神经肌肉接头的线粒体平均Rh123和4-Di-2-Asp荧光显著高于相位性神经元，表明荧光染料积累更多。负责Rh123摄取且与线粒体氧化活性（通过代谢底物氧化产生ATP）相关的线粒体膜电位，在紧张性轴突中可能更高。电子显微镜显示，紧张性轴突每平方微米横截面积所含线粒体数量比相位性轴突多约五倍。紧张性轴突的神经肌肉接头线粒体含量也比相位性轴突高得多。我们检验了小龙虾运动轴突中突触抗疲劳性取决于线粒体功能这一假说。通过氧化磷酸化解偶联剂二硝基苯酚或羰基氰化物间氯苯腙，或电子传递抑制剂叠氮化钠损害线粒体功能，会导致在持续刺激期间紧张性轴突出现明显的突触抑制，相位性轴突的抑制加速。糖酵解抑制剂碘乙酸盐和线粒体蛋白质合成抑制剂氯霉素对紧张性或相位性轴突中的线粒体荧光或突触抑制均无显著影响。总体而言，这些结果提供了证据，表明线粒体氧化代谢对于在持续刺激紧张性和相位性运动神经元期间维持突触传递很重要。紧张性神经元线粒体含量更高且氧化活性更强；这些特征与其对突触抑制的更强抗性相关。相反，相位性神经元线粒体含量较低，氧化活性较低，且突触易疲劳性更强。
