Ionic basis of neuronal hyperexcitability

在本财政年度，该项目已扩大到包括各种超氧化物歧化酶(SOD)突变形式产生的轴突兴奋性结果。超氧化物歧化酶是一种天然存在的蛋白质，存在于所有动物的神经系统中。在成年人类中，它偶尔会经历未知机制的突变。一些突变形式的超氧化物歧化酶在运动神经元中产生过度兴奋，这是肌萎缩侧索硬化症(ALS)的最早迹象之一。利用细胞内电压和电流钳技术，在鱿鱼巨型轴突中测试了实验室产生的重组形式的SOD。被称为A4V的超氧化物歧化酶在小鼠中产生最具攻击性的肌萎缩侧索硬化症，导致鱿鱼轴突准备中的过度兴奋。鱿鱼轴突通常是静止的，其静息电位为-60 mV。用含有A4V的标准磷酸盐缓冲液在细胞内灌流轴突，其浓度为5微摩尔(体内的超氧化物歧化酶的浓度)，可引起膜电位的亚稳态振荡，偶尔导致动作电位的自发放电。在初步的电压钳实验中确定的这种活动的机制似乎是持续钠离子电流的增加，这种电流也存在于哺乳动物神经元中。目前正在对这些结果进行详细的定量研究。

During the current fiscal year this project has been expanded to include results on axonal excitability produced by various mutant forms of superoxide dismutase (SOD). SOD is a naturally occuring protein found throughout the nervous system of all animals. In adult humans it occasionally undergoes mutations by unknown mechanisms. Some of the mutant forms of SOD produce hyperexcitability in motor neurons, one of the earliest signs of amyotrophic lateral sclerosis (ALS). Recombinant forms of SOD generated in the lab were tested in squid giant axons using the intracellular-voltage and current clamp technique. The SOD known as A4V which produces the most aggressive form of ALS in mice causes hyperexcitability in the squid axon preparation. Squid axons normally are quiescent having a rest potential of -60 mV. Intracellular perfusion of the axon with standard phosphate buffer containing A4V at a concentration of 5 microMolar - the concentration of SOD in vivo - produced subthrehsold oscillations of membrane potential occasionally leading to spontaneous firing of action potentials. The mechanism of this activity determined in preliminary voltage clamp experiments appears to be an augmentation of the persistent sodium ion current, a current also found in mammalian neurons. A detailed quantitative study of these results is currently in progress.