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.
