Excitability properties of motor axons in the maturing mouse

Non-invasive excitability tests have been developed to appraise axonal membrane properties in peripheral nerves and are contributing to our understanding of neuropathies and neuronopathies. These techniques have been adapted to in vivo and in vitro rat models, but little data are available on mice, although mice provide more transgenic models of neurological disorders. This study was therefore undertaken to assess the suitability of mice to model human nerve excitability measurements and to document changes during maturation. Female mice, aged 4-19 weeks, were recorded under isoflurane anesthesia. Electrical stimuli were applied via surface electrodes to the caudal motor nerve and compound muscle action potentials (CMAPs) recorded from the tail with needle electrodes. Then, the sciatic nerve was stimulated above the ankle and CMAPs recorded from plantar muscles. The method was only minimally invasive, enabling the same animal to be tested up to eight times at weekly intervals. As in human studies, the multiple excitability program recorded stimulus-response, strength-duration, and current-threshold relationships; threshold electrotonus; and recovery cycle. The response waveforms were qualitatively similar to those from human axons. This resemblance was closer for the caudal nerve, which also showed more marked changes with age. Early hyperpolarizing electrotonus fell sharply from weeks 4 to 13 (p < 0.0001), while a progressive increase in superexcitability occurred throughout the period studied (p < 0.001). We conclude that multiple measures of nerve excitability can be performed reliably in mice in vivo, preferentially on the tail, and are suitable for longitudinal studies, but age matching is critical for younger animals.