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的结果。SOD是一种天然存在的蛋白质，存在于所有动物的神经系统中。在成年人中，它偶尔会发生未知机制的突变。SOD的一些突变形式在运动神经元中产生高兴奋性，这是肌萎缩性侧索硬化症（ALS）的早期症状之一。利用细胞内电压和电流箝位技术，在鱿鱼巨轴突中检测了实验室产生的SOD重组形式。被称为A4V的超氧化物歧化酶在小鼠中产生最具侵袭性的ALS形式，导致鱿鱼轴突准备过度兴奋。鱿鱼轴突通常处于静止状态，静息电位为-60毫伏。轴突细胞内灌注含有5微摩尔浓度A4V（体内SOD浓度）的标准磷酸盐缓冲液，会产生膜电位的亚阈值振荡，偶尔导致动作电位的自发放电。在初步的电压箝位实验中确定的这种活动的机制似乎是持续的钠离子电流的增强，这种电流也在哺乳动物神经元中发现。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.