Nerve excitability studies: past, present, future?

Blood and CSF Biomarker Dynamics in Multiple Sclerosis: Implications for Data Interpretation

Background. Disability in multiple sclerosis (MS) is related to neuroaxonal degeneration. A reliable blood biomarker for neuroaxonal degeneration is needed. Objectives. To explore the relationship between cerebrospinal fluid (CSF) and serum concentrations of a protein biomarker for neuroaxonal degeneration, the neurofilaments heavy chain (NfH). Methods. An exploratory cross-sectional (n = 51) and longitudinal (n = 34) study on cerebrospinal fluid (CSF) and serum NfH phosphoform levels in patients with MS. The expanded disability status scale (EDSS), CSF, and serum levels of NfH-SMI34 and NfH-SMI35 were quantified at baseline. Disability progression was assessed at 3-year followup. Results. At baseline, patients with primary progressive MS (PPMS, EDSS 6) and secondary progressive MS (SPMS, EDSS 6) were more disabled compared to patients with relapsing remitting MS (RRMS, EDSS 2, P < .0001). Serum and CSF NfH phosphoform levels were not correlated. Baseline serum levels of the NfH-SMI34 were significantly (P < .05) higher in patients with PPMS (2.05 ng/mL) compared to SPMS (0.03 ng/mL) and RRMS (1.56 ng/mL). In SPMS higher serum than CSF NfH-SMI34 levels predicted disability progression from baseline (ΔEDSS 2, P < .05). In RRMS higher CSF than serum NfH-SMI35 levels predicted disability progression (ΔEDSS 2, P < .05). Conclusion. Serum and CSF NfH-SMI34 and NfH-SMI35 levels did not correlate with each other in MS. The quantitative relationship of CSF and serum NfH levels suggests that neuroaxonal degeneration of the central nervous system is the likely cause for disability progression in RRMS. In more severely disabled patients with PP/SPMS, subtle pathology of the peripheral nervous system cannot be excluded as an alternative source for blood NfH levels. Therefore, the interpretation of blood protein biomarker data in diseases of the central nervous system (CNS) should consider the possibility that pathology of the peripheral nervous system (PNS) may influence the results.

An ENU-induced mutation in mouse glycyl-tRNA synthetase (GARS) causes peripheral sensory and motor phenotypes creating a model of Charcot-Marie-Tooth type 2D peripheral neuropathy

Mutations in the enzyme glycyl-tRNA synthetase (GARS) cause motor and sensory axon loss in the peripheral nervous system in humans, described clinically as Charcot-Marie-Tooth type 2D or distal spinal muscular atrophy type V. Here, we characterise a new mouse mutant, Gars(C201R), with a point mutation that leads to a non-conservative substitution within GARS. Heterozygous mice with a C3H genetic background have loss of grip strength, decreased motor flexibility and disruption of fine motor control; this relatively mild phenotype is more severe on a C57BL/6 background. Homozygous mutants have a highly deleterious set of features, including movement difficulties and death before weaning. Heterozygous animals have a reduction in axon diameter in peripheral nerves, slowing of nerve conduction and an alteration in the recovery cycle of myelinated axons, as well as innervation defects. An assessment of GARS levels showed increased protein in 15-day-old mice compared with controls; however, this increase was not observed in 3-month-old animals, indicating that GARS function may be more crucial in younger animals. We found that enzyme activity was not reduced detectably in heterozygotes at any age, but was diminished greatly in homozygous mice compared with controls; thus, homozygous animals may suffer from a partial loss of function. The Gars(C201R) mutation described here is a contribution to our understanding of the mechanism by which mutations in tRNA synthetases, which are fundamentally important, ubiquitously expressed enzymes, cause axonopathy in specific sets of neurons.

A rat in vitro model for the measurement of multiple excitability properties of cutaneous axons

OBJECTIVE

To establish an in vitro model for measurement of the excitability properties of cutaneous sensory axons.

METHODS

We used a saphenous skin-nerve preparation from adult rat in combination with computerized threshold tracking. We measured strength-duration time constant, the recovery of excitability after a supramaximal stimulus and the accommodation to conditioning subthreshold polarizing stimuli (threshold electrotonus, current-threshold relationship) and compared these with previously published recordings from sensory axons in human median nerve.

RESULTS

Threshold electrotonus and the amplitude of superexcitability were indistinguishable between human median nerve in vivo and rat saphenous nerve in vitro, but several excitability parameters were significantly different in the rat: strength-duration time constant was significantly shorter (0.19+/-0.01 vs. 0.53+/-0.02 ms); the refractory period was shorter (1.9+/-1.1 ms vs. 3.5+/-1.0 ms) and late subexcitability was smaller (6.3+/-0.3% vs. 11.3+/-0.5%); thirdly, during recording of current-threshold relationship, rat nerves displayed more inward rectification to strong hyperpolarizing currents. Parameters were stable over more than 3h.

CONCLUSIONS

Excitability changes of sensory Abeta-fibres can be reliably studied in the rat in vitro and are qualitatively similar to humans.

SIGNIFICANCE

This rat model will facilitate pharmacological studies of nerve excitability and work on models of neuropathy.