Diuretic-sensitive electroneutral Na+ movement and temperature effects on central axons

Key points

Optic nerve axons get less excitable with warming.

F‐fibre latency does not shorten at temperatures above 30°C.

Action potential amplitude falls when the Na⁺‐pump is blocked, an effect speeded by warming.

Diuretics reduce the rate of action potential fall in the presence of ouabain.

Our data are consistent with electroneutral entry of Na⁺ occurring in axons and contributing to setting the resting potential.

Abstract

Raising the temperature of optic nerve from room temperature to near physiological has effects on the threshold, refractoriness and superexcitability of the shortest latency (fast, F) nerve fibres, consistent with hyperpolarization. The temperature dependence of peak impulse latency was weakened at temperatures above 30°C suggesting a temperature‐sensitive process that slows impulse propagation. The amplitude of the supramaximal compound action potential gets larger on warming, whereas in the presence of bumetanide and amiloride (blockers of electroneutral Na⁺ movement), the action potential amplitude consistently falls. This suggests a warming‐induced hyperpolarization that is reduced by blocking electroneutral Na⁺ movement. In the presence of ouabain, the action potential collapses. This collapse is speeded by warming, and exposure to bumetanide and amiloride slows the temperature‐dependent amplitude decline, consistent with a warming‐induced increase in electroneutral Na⁺ entry. Blocking electroneutral Na⁺ movement is predicted to be useful in the treatment of temperature‐dependent symptoms under conditions with reduced safety factor (Uhthoff's phenomenon) and provide a route to neuroprotection.

Optic nerve axons get less excitable with warming.

F‐fibre latency does not shorten at temperatures above 30°C.

Action potential amplitude falls when the Na⁺‐pump is blocked, an effect speeded by warming.

Diuretics reduce the rate of action potential fall in the presence of ouabain.

Our data are consistent with electroneutral entry of Na⁺ occurring in axons and contributing to setting the resting potential.

Differentiating lower motor neuron syndromes

Lower motor neuron (LMN) syndromes typically present with muscle wasting and weakness and may arise from pathology affecting the distal motor nerve up to the level of the anterior horn cell. A variety of hereditary causes are recognised, including spinal muscular atrophy, distal hereditary motor neuropathy and LMN variants of familial motor neuron disease. Recent genetic advances have resulted in the identification of a variety of disease-causing mutations. Immune-mediated disorders, including multifocal motor neuropathy and variants of chronic inflammatory demyelinating polyneuropathy, account for a proportion of LMN presentations and are important to recognise, as effective treatments are available. The present review will outline the spectrum of LMN syndromes that may develop in adulthood and provide a framework for the clinician assessing a patient presenting with predominantly LMN features.

In vivo assessment of neurological channelopathies: Application of peripheral nerve excitability studies

With the rapid evolution of understanding of neurological channelopathies comes a need for sensitive tools to evaluate patients in clinical practice. Neurological channelopathies with a single-gene basis can manifest as seizures, headache, ataxia, vertigo, confusion, weakness and neuropathic pain and it is likely that other genetic factors contribute to the phenotype of many of these disorders. Ion channel dysfunction can result in abnormal cell membrane excitability but utilisation of advanced neurophysiology techniques has lagged behind developments in clinical, genetic and imaging evaluation of channelopathies. However, momentum in the application of in vivo axonal excitability testing sees these tests emerging as valuable tools, with the capacity to provide sensitive and specific insights into the mechanism of disease. While single-channel function cannot be directly measured in vivo, evaluation of subjects with single-gene channelopathies has provided insights into the effects of mutation-related alterations of membrane excitability, as well as compensatory adaptive changes. By showing how ion channel dysfunction can affect axonal excitability in vivo, studies of the excitability of peripheral nerve axons complement in vitro analysis of single channel activity. The interpretation of results is enhanced by mathematical modelling of axonal function and insights provided by in vitro work. This article is part of the Special Issue entitled 'Channelopathies.'

The evolution of motor cortical dysfunction in amyotrophic lateral sclerosis

OBJECTIVE

The present study aimed to investigate alterations in cortical function in amyotrophic lateral sclerosis (ALS) related to disease progression.

METHODS

In total, clinical assessments were evaluated in 189 ALS patients, combined with assessment of cortical function utilising threshold tracking transcranial magnetic stimulation. Results were compared with disease stage. Disease stage was defined in three ways: (1) as a proportion of disease duration in deceased patients; (2) from the time of ALS onset; and (3) using the ALS rating scale-revised (ALSFRS-R).

RESULTS

Prospective studies in ALS patients demonstrated decreased neurophysiological index (p<0.0001) and decreased compound muscle action potential (CMAP) (p<0.0001), combined with abnormalities of central function including prolonged central motor conduction time (CMCT) (p<0.05), increased motor evoked potential/CMAP amplitude ratio (p<0.0001) and decreased short interval intracortical inhibition (SICI) (p<0.001). SICI at 3ms (p<0.05, β=-0.21) and averaged SICI (p<0.05, β=-0.21) decreased with disease progression, measured using proportion of disease duration. Alternatively, using time from disease onset, CMCT prolonged with disease progression (p<0.01, β=0.25), while ALSFRS-R decline correlated with decreased SICI at 3ms (p<0.01, β=0.20).

CONCLUSIONS

Clinical measures combined with assessment of cortical function established that SICI decreased with disease progression.

SIGNIFICANCE

These findings may suggest dysfunction of inhibitory interneurons with disease progression.

Peripheral nerve diffusion tensor imaging as a measure of disease progression in ALS

Clinical trial design in amyotrophic lateral sclerosis (ALS) remains hampered by a lack of reliable and sensitive biomarkers of disease progression. The present study evaluated peripheral nerve diffusion tensor imaging (DTI) as a surrogate marker of axonal degeneration in ALS. Longitudinal studies were undertaken in 21 ALS patients studied at 0 and 3 months, and 19 patients at 0, 3 and 6 months, with results compared to 13 age-matched controls. Imaging metrics were correlated across a range of functional assessments including amyotrophic lateral sclerosis functional rating scale revised (ALSFRS-R), lower limb muscle strength (Medical Research Council sum score, MRCSS-LL), compound muscle action potential amplitudes and motor unit number estimation (MUNE). Fractional anisotropy was reduced at baseline in ALS patients in the tibial (p < 0.05), and peroneal nerve (p < 0.05). Fractional anisotropy and axial diffusivity declined in the tibial nerve between baselines, 3- and 6-month scans (p < 0.01). From a functional perspective, ALSFRS-R correlated with fractional anisotropy values from tibial (R = 0.75, p < 0.001) and peroneal nerves (R = 0.52, p = 0.001). Similarly, peroneal nerve MUNE values correlated with fractional anisotropy values from the tibial (R = 0.48, p = 0.002) and peroneal nerve (R = 0.39, p = 0.01). There were correlations between the change in ALSFRS-R and tibial nerve axial diffusivity (R = 0.38, p = 0.02) and the change in MRCSS-LL and peroneal nerve fractional anisotropy (R = 0.44, p = 0.009). In conclusion, this study has demonstrated that some peripheral nerve DTI metrics are sensitive to axonal degeneration in ALS. Further, that DTI metrics correlated with measures of functional disability, strength and neurophysiological measures of lower motor neuron loss.

Laterality of motor cortical function measured by transcranial magnetic stimulation threshold tracking

INTRODUCTION

Threshold tracking paired-pulse transcranial magnetic stimulation (TTTMS) examines cortical function and is useful for diagnosis of motor neuron disorders. Differences in cortical function have been identified between dominant and non-dominant limbs using constant stimulus methods, but they remain unclear, potentially due to methodological differences. In this study we aimed to clarify differences in cortical function between dominant and non-dominant limbs using TTTMS.

METHODS

Single-pulse TMS, TTTMS, and nerve conduction studies were performed in 25 healthy, right-handed participants by recording from the abductor pollicis brevis muscle.

RESULTS

There were no side-to-side differences observed in resting motor threshold, motor evoked potential (MEP) amplitude, MEP latency, central motor conduction time, cortical silent period, short-interval intracortical inhibition and facilitation, compound muscle action potential (CMAP) amplitude, CMAP latency, F-wave latency, or neurophysiological index.

CONCLUSIONS

These findings suggest that, when using TTTMS, there are no differences in cortical function between dominant and non-dominant hemispheres. Muscle Nerve 55: 424-427, 2017.

Effect of fampridine on axonal excitability in multiple sclerosis

OBJECTIVE

To investigate the effects of fampridine on nerve excitability, the present study utilized peripheral axonal excitability techniques in 18 MS patients receiving treatment with fampridine.

METHODS

Studies were performed at baseline and repeated 3months after institution of fampridine at standard dosing.

RESULTS

Following treatment with fampridine there were significant changes in axonal excitability for those parameters associated with fast K(+) channels that shifted towards normal control values. Specifically, increases were noted in the peak superexcitability of recovery cycle (fampridine, -25.6±1.6%; baseline -22.8±1.7%; p<0.004), peak depolarizing threshold electrotonus (fampridine, 69.1±1.0%; baseline 67.0±1.4%; p<0.004), and depolarizing threshold electrotonus between 40 and 60ms after onset of depolarization (fampridine, 52.8±1.3%; baseline 49.9±1.4%; p=0.02).

CONCLUSION

The present study has established that fampridine at standard doses exerts effects on peripheral nerve function that may be mediated by reduction of fast K(+) conductances.

SIGNIFICANCE

Modulation of fast K(+) conductances by fampridine may contribute to the improvement observed in MS symptoms including motor fatigue.

Accommodation to hyperpolarization of human axons assessed in the frequency domain

Human axons in vivo were subjected to subthreshold currents with a threshold impedance amplitude profile to allow the use of frequency domain techniques to determine the propensity for resonant behavior and to clarify the relative contributions of different ion channels to their low-frequency responsiveness. Twenty-four studies were performed on the motor and sensory axons of the median nerve in six subjects. The response to oscillatory currents was tested between direct current (DC) and 16 Hz. A resonant peak at ∼2-2.5 Hz was found in the response of hyperpolarized axons, but there was only a small broad response in axons at resting membrane potential (RMP). A mathematical model of axonal excitability developed using DC pulses provided a good fit to the frequency response for human axons and indicated that the hyperpolarization-activated current Ih and the slow potassium current IKs are principally responsible for the resonance. However, the results indicate that if axons are hyperpolarized by more than -60% of resting threshold, the only conductances that are appreciably active are Ih and the leak conductance, i.e., that the activity of these conductances can be studied in vivo virtually in isolation at hyperpolarized membrane potentials. Given that the leak conductance dampens resonance, it is suggested that the -60% hyperpolarization used here is optimal for Ih As expected, differences between the frequency responses of motor and sensory axons were present and best explained by reduced slow potassium conductance GKs, up-modulation of Ih, and increased persistent Na(+) current INaP (due to depolarization of RMP) in sensory axons.

Cortical Function in Asymptomatic Carriers and Patients With C9orf72 Amyotrophic Lateral Sclerosis

IMPORTANCE

The identification of the chromosome 9 open reading frame 72 (c9orf72) gene hexanucleotide repeat expansion represents a major advance in the understanding of amyotrophic lateral sclerosis (ALS) pathogenesis. The pathophysiological mechanism by which the c9orf72 gene expansion leads to neurodegeneration is not yet elucidated. Cortical hyperexcitability is potentially an important pathophysiological process in sporadic ALS and familial ALS (FALS).

OBJECTIVE

To investigate whether cortical hyperexcitability forms the pathophysiological basis of c9orf72 FALS using the threshold-tracking transcranial magnetic stimulation technique.

DESIGN, SETTING, AND PARTICIPANTS

Prospective case-control single-center study that took place at hospitals and outpatient clinics from January 1, 2013, to January 1, 2015. Clinical and functional assessments along with transcranial magnetic stimulation studies were taken on 15 patients with c9orf72 FALS and 11 asymptomatic expansion carriers of c9orf72 who were longitudinally followed up for 3 years. Results were compared with 73 patients with sporadic ALS and 74 healthy control participants.

MAIN OUTCOMES AND MEASURES

Cortical excitability variables, including short-interval intracortical inhibition, were measured in patients with c9orf72 FALS and results were compared with asymptomatic c9orf72 carriers, patients with sporadic ALS, and healthy control participants.

RESULTS

Mean (SD) short-interval intracortical inhibition was significantly reduced in patients with c9orf72 FALS (1.2% [1.8%]) and sporadic ALS (1.6% [1.2%]) compared with asymptomatic c9orf72 expansion carriers (10.2% [1.8%]; F = 16.1; P < .001) and healthy control participants (11.8% [1.0%]; F = 16.1; P < .001). The reduction of short-interval intracortical inhibition was accompanied by an increase in intracortical facilitation (P < .01) and motor-evoked potential amplitude (P < .05) as well as a reduction in the resting motor threshold (P < .05) and cortical silent period duration (P < .001).

CONCLUSIONS AND RELEVANCE

This study establishes cortical hyperexcitability as an intrinsic feature of symptomatic c9orf72 expansion-related ALS but not asymptomatic expansion carriers.

In vivo evidence of reduced nodal and paranodal conductances in type 1 diabetes

OBJECTIVES

Diabetic neuropathy is a debilitating complication of diabetes. Animal models of type 1 diabetes (T1DM) suggest that functional and structural changes, specifically axo-glial dysjunction, may contribute to neuropathy development. The present study sought to examine and characterise early sensory axonal function in T1DM patients in the absence of clinical neuropathy.

METHODS

Thirty patients with T1DM (15M:15F) without neuropathy underwent median nerve sensory and motor axonal excitability studies to examine axonal function. A verified mathematical model of human motor and sensory axons was used to elucidate the underlying causes of observed alterations.

RESULTS

Compared to controls (NC), T1DM patients demonstrated significant axonal excitability abnormalities in sensory and motor axons. These included marked reductions in sensory and motor subexcitability during the recovery cycle (T1DM 7.9 ± 0.4:10.4 ± 0.6%, NC 10.4 ± 0.7:15.4 ± 1.2%, P<0.01) and during hyperpolarizing threshold electrotonus at 10-20 ms (T1DM -75.5 ± 0.8:-69.7 ± 0.8%, NC -78.4 ± 1:-72.7 ± 0.9%, P<0.01). Mathematical modelling demonstrated that these changes were due to reduced nodal Na(+) currents, nodal/paranodal K(+) conductances and Na(+)/K(+) pump dysfunction, consistent with axo-glial dysjunction as outlined in animal models of T1DM.

CONCLUSIONS

The study provided support for the occurrence of early changes in nodal and paranodal conductances in patients with T1DM.

SIGNIFICANCE

These data indicate that axonal excitability techniques may detect early changes in diabetic patients, providing a window of opportunity for prophylactic intervention in T1DM.

Sensitivity and specificity of threshold tracking transcranial magnetic stimulation for diagnosis of amyotrophic lateral sclerosis: a prospective study

BACKGROUND

Diagnosis of amyotrophic lateral sclerosis (ALS) remains problematic, with substantial diagnostic delays. We assessed the sensitivity and specificity of a threshold tracking transcranial magnetic stimulation (TMS) technique, which might allow early detection of upper motor neuron dysfunction, for the diagnosis of the disorder.

METHODS

We did a prospective study of patients referred to three neuromuscular centres in Sydney, Australia, in accordance with the Standards for Reporting of Diagnostic Accuracy. Participants had definite, probable, or possible ALS, as defined by the Awaji criteria; or pure motor disorder with clinical features of upper and lower motor neuron dysfunction in at least one body region, progressing over a 6 month follow-up period; or muscle wasting and weakness for at least 6 months. All patients underwent threshold tracking TMS at recruitment (index test), with application of the reference standard, the Awaji criteria, to differentiate patients with ALS from those with non-ALS disorders. The investigators who did the index test were masked to the results of the reference test and all other investigations. The primary outcome measures were the sensitivity and specificity of TMS in differentiating ALS from non-ALS disorders; these measures were derived from receiver operator curve analysis.

FINDINGS

Between Jan 1, 2010, and March 1, 2014, we screened 333 patients; 281 met our inclusion criteria. We eventually diagnosed 209 patients with ALS and 68 with non-ALS disorders; the diagnosis of four patients was inconclusive. The threshold tracking TMS technique differentiated ALS from non-ALS disorders with a sensitivity of 73·21% (95% CI 66·66-79·08) and specificity of 80·88% (69·53-89·40) at an early stage in the disease. All patients tolerated the study well, and we did not record any adverse events from performance of the index test.

INTERPRETATION

The threshold tracking TMS technique reliably distinguishes ALS from non-ALS disorders and, if these findings are replicated in larger studies, could represent a useful diagnostic investigation when combined with the Awaji criteria to prove upper motor neuron dysfunction at early stages of ALS.

FUNDING

Motor Neuron Disease Research Institute of Australia, National Health and Medical Research Council of Australia, and Pfizer.

Axonal ion channel dysfunction in c9orf72 familial amyotrophic lateral sclerosis

IMPORTANCE

Abnormalities of axonal excitability characterized by upregulation of persistent sodium (Na+) conductances and reduced potassium (K+) currents have been reported in sporadic amyotrophic lateral sclerosis (SALS) phenotypes and linked to the development of clinical features such as fasciculations and neurodegeneration.

OBJECTIVE

To investigate whether abnormalities of axonal ion channel function, particularly upregulation of persistent Na+ conductances and reduced K+ currents, form the pathophysiological basis of chromosome 9 open reading frame 72 (c9orf72) familial amyotrophic lateral sclerosis (FALS).

DESIGN, SETTING, AND PARTICIPANTS

This was a prospective study. Clinical and functional assessment, along with motor-nerve excitability studies, were undertaken in 10 clinically affected patients with c9orf72 FALS, 9 asymptomatic c9orf72 mutation carriers, and 21 patients with SALS from 3 hospitals and 2 outpatient clinics.

MAIN OUTCOMES AND MEASURES

Axonal excitability variables were measured in patients with c9orf72 ALS and results compared with matched patients with SALS and healthy control participants.

RESULTS

Strength-duration time constant (τSD) was significantly increased in the patients with c9orf72 FALS and those with SALS (mean [SD], c9orf72 FALS: 0.50 [0.02] milliseconds; SALS: 0.52 [0.02] milliseconds; P < .01) when compared with control participants (mean [SD], 0.44 [0.01] milliseconds). In contrast, there were no significant changes of τSD in asymptomatic c9orf72 mutation carriers (P = .42). An accompanying increase in depolarizing threshold electrotonus at 90 to 100 milliseconds (TEd 90-100 milliseconds) was also evident in the c9orf72 FALS (P < .05) and SALS (P < .01) cohorts. Mathematical modeling suggested that an increase in persistent Na+ conductances, along with reduced K+ currents, best explained the changes in axonal excitability. Importantly, these abnormalities in axonal excitability correlated with the motor amplitude (τSD: R = -0.38, P < .05 and TEd 90-100 milliseconds: R = -0.44, P < .01), muscle weakness (TEd 90-100 milliseconds: R = -0.32, P < .05), and the ALS Functional Rating Scale (TEd 90-100 milliseconds: R = -0.34, P < .05).

CONCLUSIONS AND RELEVANCE

Findings from the present study establish that upregulation of persistent Na+ conductances and reduced K+ currents were evident in both c9orf72 FALS and SALS cohorts, and these changes in axonal excitability were associated with motor neuron degeneration.

Axonal excitability in X-linked dominant Charcot Marie Tooth disease

OBJECTIVE

We investigated peripheral nerve function in X-linked Charcot-Marie-Tooth disease type 1 (CMTX1), and considered the functional consequences of mutant connexin-32.

METHODS

Twelve subjects (9 female, 3 male) were assessed clinically, by nerve conduction and excitability studies. A model of myelinated axon was used to clarify the contributing changes.

RESULTS

All subjects had abnormal nerve conduction. Excitability studies on median nerve axons showed greater threshold changes to hyperpolarising currents, with "fanning out" in threshold electrotonus, and modest changes in the recovery cycle. Modelling suggested shortening of internodal length, increase in nodal fast potassium currents, shift of the voltage activation hyperpolarisation-activated cyclic-nucleotide-gated channels, and axonal hyperpolarisation. Plotting threshold versus extent of hyperpolarising threshold change in threshold electrotonus distinguished the CMTX1 patients from other chronic demyelinating neuropathies reported in the literature except hereditary neuropathy with pressure palsies (HNPP).

CONCLUSIONS

Some measures of axonal excitability are similar in CMTX1 and HNPP (though not the recovery cycle), but they differ from those in other chronic demyelinating neuropathies. The findings in CMTX1 are consistent with known pathology, but are not correlated to neuropathy severity.

SIGNIFICANCE

The findings in CMTX1 could be largely the result of morphological alterations, rather than plasticity in channel expression or distribution.

Evidence for a causal relationship between hyperkalaemia and axonal dysfunction in end-stage kidney disease

OBJECTIVE

Potassium (K(+)) has been implicated as a factor in the development of uraemic neuropathy. This study was undertaken to investigate whether hyperkalaemia plays a causal role in axonal dysfunction in end-stage kidney disease (ESKD).

METHODS

Median motor nerve excitability studies were undertaken in four haemodialysis patients during a modified dialysis session. The serum K(+) level was "clamped" (fixed) for the first 3h of dialysis, whilst allowing all other solutes to be removed, this was followed by dialysis against low dialysate K(+) for a further 4 h. Blood chemistry and nerve excitability studies were undertaken prior to, during and following dialysis. Results were compared to results from the same patients during routine dialysis sessions.

RESULTS

All patients demonstrated significant nerve excitability abnormalities reflective of nerve membrane depolarization in pre-dialysis recordings (p<0.01). After the 3 h clamp period, serum K(+) remained elevated (5.0 mmol/L) and nerve excitability remained highly abnormal, despite the significant clearance of other uraemic toxins. In contrast, studies undertaken during routine dialysis sessions demonstrated significant improvement in both serum K(+) and nerve function after 3 h.

CONCLUSIONS

The current study has established a causal relationship between serum K(+) and axonal membrane depolarization in haemodialysis patients.

SIGNIFICANCE

From a clinical perspective, strict K(+) control may help improve nerve function in ESKD.

A structured, group-based diabetes self-management education (DSME) programme for people, families and whanau with type 2 diabetes (T2DM) in New Zealand: an observational study

BACKGROUND

Group-based diabetes self-management education (DSME) programmes have been shown to be effective. A programme tailored for the unique social and ethnic environment of New Zealand (NZ) was developed using concepts from internationally developed programmes.

AIM

To assess the effectiveness of a 6 week New Zealand specific DSME programme.

METHODS

In this observational study people with type 2 diabetes (aged 18-80 years) from diverse cultural backgrounds were recruited from primary care. Seventeen groups of six education sessions were run. Clinical data were collected from primary care at baseline, 3, 6 and 9 months. Participants also completed a self-administered questionnaire on diabetes knowledge, and self-management behaviours.

RESULTS

107 participants, mean age 56.7±11.3 years and mean duration of diabetes 7.5±7 years (NZ European (44%), Maori (24%), Pacific (16%) and Indian (16%)), were enrolled. Confidence in self-managing diabetes, regular examination of feet, physical activity levels and smoking rates all improved. Glycaemic control improved between baseline and 6 months (HbA1C 64.9±20.0 mmol/mol to 59.9±13.9 mmol/mol (p<0.05) (baseline 8.07%±1.80, 6 months 7.62%±1.25)), but was no different to baseline at 9 months. Systolic BP reduced from 131.9±16.4 to 127.4±18.2 mmHg (p<0.05) at 6 months, but increased to baseline levels by 9 months. Diastolic BP, triglycerides and urine microalbumin:creatinine ratio were significantly reduced at 3, 6 and 9 months.

CONCLUSION

A group-based DSME programme designed specifically for the NZ population was effective at improving aspects of diabetes care at 6 months. The attenuation of these improvements after 6 months suggests a refresher course at that time may be beneficial.