EXCITABILITY PROPERTIES OF MOUSE MOTOR AXONS IN THE MUTANT SOD1 G93A MODEL OF AMYOTROPHIC LATERAL SCLEROSIS DELPHINE BOE ¨ RIO, PhD, BERNADETT KALMAR, PhD, LINDAGREENSMITH, PhD, and HUGH BOSTOCK, PhD Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, Queen Square, London WC1N 3BG, UK Accepted 5 October 2009 ABSTRACT: Non-invasive excitability studies of motor axons in patients with amyotrophic lateral sclerosis (ALS) have revealed a changing pattern of abnormal membrane properties with disease progression, but the heterogeneity of the changes has made it difficult to relate them to pathophysiology. The SOD1 G93A mouse model of ALS displays more synchronous motoneuron pathology. Multiple excitability measures of caudal and sciatic nerves in mutant and wild-type mice were compared before onset of signs and during disease progression (4–19 weeks), and they were related to changes in muscle fiber histo- chemistry. Excitability differences indicated a modest membrane depolarization in SOD1 G93A axons at about the time of symp- tom onset (8 weeks), possibly due to deficient energy supply. Previously described excitability changes in ALS patients, sug- gesting altered sodium and potassium conductances, were not seen in the mice. This suggests that those changes relate to features of the human disease that are not well represented in the animal model. Muscle Nerve 41: 774–784, 2010 Amyotrophic lateral sclerosis (ALS) is a fatal neu- rodegenerative disorder characterized by a selec- tive loss of motoneurons in the spinal cord, brain- stem, and motor cortex. Although the majority of ALS cases are sporadic, approximately 10% of cases are familial (FALS), of which 10–20% are due to a mutation of Cu/Zn superoxide dismutase type 1 (SOD1). 1 Clinically, ALS is characterized by both lower motoneuron signs (e.g., muscle weak- ness, atrophy, and fasciculations) and upper moto- neuron signs (e.g., hyperreflexia). The fascicula- tions are caused by spontaneous discharges of single motor units 2 and reflect ectopic activity of hyperexcitable motor axons. 3 Beside conventional electrophysiological meth- ods, new threshold-tracking techniques have been developed to non-invasively investigate ion con- ductance and axonal membrane properties in peripheral nerves. 4 An automated sequence of multiple excitability tests 5 has been particularly useful for studying the pathophysiology of diseased axons. 6–8 Applied to ALS patients, axonal excitabil- ity testing has found evidence of abnormalities in depolarizing threshold electrotonus, supernomal- ity, and strength–duration time constant consistent with an increase in sodium conductance and a reduction in potassium conductance. 9–11 However, there is also evidence of a changing pattern with disease progression, so that a reduced compound muscle action potential (CMAP) amplitude is asso- ciated with a contrasting flattening of electroto- nus. 10 These abnormalities in axonal properties in ALS are difficult to resolve or to relate to other facets of the disease in patients, because axons of the same nerve may be affected differently. Transgenic mice that express mutant SOD1 dis- play a similar phenotype and pathology to human ALS, 12 although the mouse model is characterized by a relatively early onset and rapidity of disease progression. 13 Nevertheless, SOD1 mice present more predictable and synchronous motoneuron pathology and may therefore facilitate evaluation of the pathophysiological significance of the changes in axonal excitability. We recently devel- oped a method of recording multiple measures of nerve excitability in wild-type (WT) mice in vivo that is suitable for longitudinal studies. 14 That study documented the effects of maturation on axonal excitability properties and established nor- mative data from a juvenile (4 weeks) to an adult stage (19 weeks) on the mouse caudal and sciatic motor nerves. This study was undertaken to examine the changes in peripheral motor nerve excitability dur- ing disease progression in the SOD1 G93A mouse model using the multiple excitability measure- ments previously applied to ALS patients. The recordings were performed on two motor nerves: the caudal and the sciatic nerves. These two nerves supply muscles that are involved in differ- ent motor tasks. The tail muscles play a postural role, and the plantar muscles are more involved in locomotion. The purpose of this study was to char- acterize the changes in axonal excitability during the course of the disease in SOD1 mice and to appraise whether motor nerves that innervate muscles with different functional roles were affected in a different way. Experimental Procedures. Mice. In these experi- ments, 25 mutant SOD1 G93A female mice, 4–19 weeks of age, were examined, and the data were Abbreviations: ALS, amyotrophic lateral sclerosis; ATP, adenosine tri- phosphate; CMAP, compound muscle action potential; EDL, extensor dig- itorum longus; FALS, familial form of amyotrophic lateral sclerosis; HCN channels, hyperpolarization-activated and cyclic-nucleotide gated cation channels; IV slope, current–threshold slope; MEM, multiple excitability measures; SDH, succinate dehydrogenase; SOD, superoxide dismutase; TE, threshold electrotonus; WT, wild-type Correspondence to: H. Bostock; e-mail: h.bostock@ion.ucl.ac.uk V C 2010 Wiley Periodicals, Inc. Published online 21 January 2010 in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/mus.21579 Key words: ALS; animal model; axonal depolarization; motor neuron; nerve excitability; neurodegeneration; SOD1 G93A mice; threshold-tracking 774 Excitability in SOD1 Motor Axons MUSCLE & NERVE June 2010