Axonal Membrane Properties Research Articles

Increase of HCN current in SOD1-associated amyotrophic lateral sclerosis.

The clinical manifestations of sporadic amyotrophic lateral sclerosis (ALS) vary widely. However, the current classification of ALS is mainly based on clinical presentations, while the roles of electrophysiological and biomedical biomarkers remain limited. Herein, we investigated a group of patients with sporadic ALS and an ALS mouse model with superoxide dismutase 1 (SOD1)/G93A transgenes using nerve excitability tests (NET) to investigate axonal membrane properties and chemical precipitation, followed by enzyme-linked immunosorbent assay analysis to measure plasma misfolded protein levels. Six of 19 patients (31.6%) with sporadic ALS had elevated plasma misfolded SOD1 protein levels. In sporadic ALS patients, only those with elevated misfolded SOD1 protein levels showed an increased inward rectification in the current-threshold (I/V) curve and an increased threshold reduction in the hyperpolarizing threshold electrotonus (TE) in the NET study. Two familial ALS patients with SOD1 mutations also exhibited similar electrophysiological patterns of NET. For patients with sporadic ALS showing significantly increased inward rectification in the I/V curve, we noted an elevation in plasma misfolded SOD1 level, but not in total SOD1, misfolded C9orf72, or misfolded phosphorylated TDP43 levels. Computer simulations demonstrated that the aforementioned axonal excitability changes are likely associated with an increase in hyperpolarization-activated cyclic nucleotide-gated (HCN) current. In SOD1/G93A mice, NET also showed an increased inward rectification in the I/V curve, which could be reversed by a single injection of the HCN channel blocker, ZD7288. Daily treatment of SOD1/G93A mice with ZD7288 partially prevented the early motor function decline and spinal motor neuron death. In summary, sporadic ALS patients with elevated plasma misfolded SOD1 exhibited similar patterns of motor axonal excitability changes as familial ALS patients and ALS mice with mutant SOD1 genes, suggesting the existence of SOD1-associated sporadic ALS. The observed NET pattern of increased inward rectification in the I/V curve was attributable to an elevation in the HCN current in SOD1-associated ALS.

MScanFit motor unit number estimation and muscle velocity recovery cycle recordings in diabetic polyneuropathy

ObjectiveMotor Unit Number Estimation (MUNE) methods may be valuable in tracking motor unit loss in diabetic polyneuropathy (DPN). Muscle Velocity Recovery Cycles (MVRCs) provide information about muscle membrane properties. This study aimed to examine the utility of the MScanFit MUNE in detecting motor unit loss and to test whether the MVRCs could improve understanding of DPN pathophysiology. MethodsSeventy-nine type-2 diabetic patients were compared to 32 control subjects. All participants were examined with MScanFit MUNE and MVRCs in anterior tibial muscle. Lower limb nerve conduction studies (NCS) in peroneal, tibial and sural nerves were applied to diagnose large fiber neuropathy. ResultsNCS confirmed DPN for 47 patients (DPN + ), with 32 not showing DPN (DPN−). MScanFit showed significantly decreased MUNE values and increased motor unit sizes, when comparing DPN + patients with controls (MUNE = 71.3 ± 4.7 vs 122.7 ± 3.8), and also when comparing DPN− patients (MUNE = 103.2 ± 5.1) with controls. MVRCs did not differ between groups. ConclusionsMScanFit is more sensitive in showing motor unit loss than NCS in type-2 diabetic patients, whereas MVRCs do not provide additional information. SignificanceThe MScanFit results suggest that motor changes are seen as early as sensory, and the role of axonal membrane properties in DPN pathophysiology should be revisited.

A repeated molecular architecture across thalamic pathways.

Thalamus is the central communication hub of the forebrain, providing cerebral cortex with inputs from sensory organs, subcortical systems, and cortex itself. Multiple thalamic regions send convergent information to each cortical region, but the organizational logic of thalamic projections has remained elusive. Through comprehensive transcriptional analyses of retrogradely labeled thalamic neurons in adult mice, we identify three major profiles of thalamic pathway. These profiles exist along a continuum that is repeated across all major projection systems, such as those for vision, motor control, and cognition. The largest component of gene expression variation in mouse thalamus is topographically organized with features conserved in humans. Transcriptional differences between these thalamic neuronal identities are tied to cellular features critical for function, such as axonal morphology and membrane properties. Molecular profiling therefore reveals covariation in properties of thalamic pathways serving all major input modalities and output targets, establishing a molecular framework for understanding thalamus.

P50-T Changes in peripheral nerve excitability by transcutaneous direct currents in healthy subjects – pilot study

Background Anodal stimulation results in subthreshold depolarisation, while cathodal stimulation hyperpolarises neuronal membranes. There have been few studies on the possible modulatory effect of long lasting transcutaneous direct current stimulation on the axonal membrane properties of peripheral nerves, especially on the possible modulatory effect of such currents. We aimed to assess the applicability, safety and effects on axonal membrane excitability of transcutaneous peripheral nerve stimulation with direct currents (pDCS). Material and methods We included 15 healthy subjects. pDCS was performed with the active electrode placed over the forearm and the reference electrode on the back of the hand using 5 × 5 cm rubber electrodes. The current applied was 2.5 mA during 15 min. Following a sham stimulation, two different settings were randomly used, cathodal and anodal current. Excitability measurements were performed using the TRONDNF nerve excitability protocol of the QTRAC program. Results The protocol was completed and well tolerated in all subjects. The TEd (90–100 ms) was significantly lower for anodal stimulation (p = 0.04). There were no other significant differences. These results were not replicated in a repeated trial, and in addition multiTRAC assessment failed to demonstrate significant after-effects on excitability. Conclusions TEd 90–100 ms reflects function of slow K+ channels, mainly located at the nodes. Initial findings suggested that slow K+-channel expression or conductance may be reduced after the application of anodal DC current, however repeated studies and real-time assessment of excitability measures argue against a consistent effect. Further research should be performed, using different intensities and stimulation time, as well as other excitability measurements.

Excitability of sensory axons in amyotrophic lateral sclerosis

ObjectiveTo evaluate the excitability of sensory axons in patients with amyotrophic lateral sclerosis (ALS). MethodsComprehensive sensory nerve excitability studies were prospectively performed on 28 sporadic ALS patients, compared to age-matched controls. Sensory nerve action potentials were recorded from digit 2 following median nerve stimulation at the wrist. Disease severity was measured using motor unit number estimation (MUNE), the revised ALS Functional Rating Scale (ALSFRS-R) and the MRC scale. ResultsThere were no significant differences in standard and extended measures of nerve excitability between ALS patients and controls. These unchanged excitability measures included accommodation to long-lasting hyperpolarization and the threshold changes after two supramaximal stimuli during the recovery cycle. Excitability parameters did not correlate with MUNE, ALSFRS-R, APB MRC scale or disease duration. ConclusionsThis cross-sectional study has identified normal axonal membrane properties in myelinated sensory axons of ALS patients. Previously described sensory abnormalities could be the result of axonal fallout, possibly due to a ganglionopathy, or to involvement of central sensory pathways rostral to gracile and cuneate nuclei. SignificanceThese results demonstrate the absence of generalized dysfunction of the membrane properties of sensory axons in ALS in the face of substantial deficits in motor function.

Furosemide depresses the presynaptic fiber volley and modifies frequency-dependent axonal excitability in rat hippocampus.

The loop diuretic furosemide is known to have anticonvulsant effects, believed to be exerted through blockade of glial Na+-K+-2Cl- cotransport causing altered volume regulation in brain tissue. The possibility that direct effects of furosemide on neuronal properties could also be involved is supported by previous observations, but such effects have not been thoroughly investigated. In the present study we show that furosemide has two opposing effects on stimulus-induced postsynaptic excitation in the nonepileptic rat hippocampal slice: 1) an enhancement of e-s coupling, which depended on intact GABAA transmission and was partially mimicked by selective blockade of K+-2Cl- cotransport, and 2) a decrement of field excitatory postsynaptic potentials. The balance between these effects varied, depending on the amount of synaptic drive. In addition, the compound action potential (fiber volley) recorded from the stimulated Schaffer collateral axons in stratum radiatum showed a progressive decrease during perfusion of furosemide. This effect was activity-independent, was mimicked by the stilbene derivative DIDS, and could be reproduced on fiber volleys in the alveus. Furosemide also reduced the initial enhancement of the fiber volley observed during trains of high-frequency stimulation (HFS). Results of hyperosmotic expansion of the extracellular volume, with 30 mM sucrose, indicated that both the induction and antagonism of the HFS-induced enhancement were independent of signaling via the extracellular space. Furosemide caused an increased decay of paired-pulse-induced supranormal axonal excitability, which was antagonized by ZD7288. We conclude that furosemide decreases axonal excitability and prevents HFS-induced hyperexcitability via mechanisms downstream of blockage of anion transport, which could include hyperpolarization of axonal membranes.NEW & NOTEWORTHY This study shows that the anion transporter antagonists furosemide and DIDS cause a marked decrease of axonal excitability in rat hippocampal CA1 region and prevent the induction of activity-dependent hyperexcitability in Schaffer collateral axons. The data are consistent with direct effects on axonal membrane properties. We also find that activity-dependent enhancement and depression of axonal excitability can be modified independently, suggesting that these events are governed by different underlying processes.

Nerve excitability changes after intravenous immunoglobulin infusions in multifocal motor neuropathy and chronic inflammatory demyelinating neuropathy

Intravenous immunoglobulin (IVIg) infusions may provide clinical benefits in multifocal motor neuropathy (MMN) and chronic inflammatory demyelinating polyneuropathy (CIDP). The short delay in the clinical response to IVIg therapy is not consistent with a process of remyelination or axonal regeneration. We assessed whether or not the efficacy of IVIg infusions in MMN and CIDP could reflect changes in axonal membrane properties and nerve excitability. Ulnar motor nerve excitability was studied before and after three to five consecutive days of IVIg infusions (0.4 g/kg/day) in 10 patients with MMN, 10 patients with CIDP, and 10 neurological controls (CTRLs). Excitability recovery cycle, stimulus–response and strength–duration properties were investigated. The recovery cycle parameters (absolute and relative refractory period durations, refractoriness and supernormality) were similar in all groups and did not change after IVIg infusions. At baseline, patients with CIDP, but not with MMN, showed a reduced strength–duration time constant (chronaxie) and increased rheobase when compared to CTRLs. After IVIg infusions, strength–duration time constant remained stable in CTRLs, but decreased in patients with MMN or CIDP. Rheobase increased in the three groups after treatment. The decreased strength–duration time constant after IVIg infusions in patients with MMN or CIDP could reflect a reduction of persistent Na + current, able to limit intraaxonal Na + accumulation and then to produce neuroprotective effects. However, this could also reflect compensatory mechanisms that did not directly underlie the therapeutic effect. Whatever the underlying process, this result revealed that IVIgs were able to produce early nerve excitability changes.

Myosin-dependent targeting of transmembrane proteins to neuronal dendrites

The distinct electrical properties of axonal and dendritic membranes are largely a result of specific transport of vesicle-bound membrane proteins to each compartment. How this specificity arises is unclear because kinesin motors that transport vesicles cannot autonomously distinguish dendritically projecting microtubules from those projecting axonally. We hypothesized that interaction with a second motor might enable vesicles containing dendritic proteins to preferentially associate with dendritically projecting microtubules and avoid those that project to the axon. Here we show that in rat cortical neurons, localization of several distinct transmembrane proteins to dendrites is dependent on specific myosin motors and an intact actin network. Moreover, fusion with a myosin-binding domain from Melanophilin targeted Channelrhodopsin-2 specifically to the somatodendritic compartment of neurons in mice in vivo. Together, our results suggest that dendritic transmembrane proteins direct the vesicles in which they are transported to avoid the axonal compartment through interaction with myosin motors.

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.

Effects of age on excitability properties in human motor axons

Objective The threshold tracking technique is a new approach to non-invasively assess biophysical properties of axonal membrane in human subjects. The aim of this study was to evaluate the effects of age and gender on excitability properties of human motor axons. Methods Computerized threshold tracking was used to measure multiple excitability indices in median motor axons of 93 normal subjects (50 men; age, 20–86 years). Results Regression analyses showed that the higher age was associated with longer strength-duration time constant ( p = 0.03), smaller threshold changes in depolarizing threshold electrotonus ( p = 0.02), smaller supernormality ( p = 0.01), and steeper slope of the current–threshold relationship for hyperpolarizing currents ( p < 0.001). There were slight sex differences in rheobase, threshold electrotonus, supernormality, late subnormality, and current–threshold slope, though they were significant only in the subgroup with age <50 years. Conclusions Aging may increase persistent sodium currents, inward rectification, and possibly, outward potassium currents. The combination of changes raises the possibility of slight membrane depolarization in elderly people. For the sex-related differences, further studies will be required with the evaluation of sex hormonal effects. Significance Age-related effects on excitability properties are subtle, but should be taken into consideration in the clinical application of nerve excitability testing, particularly in elderly subjects.

Axonal function in a family with episodic ataxia type 2 due to a novel mutation

Episodic ataxia type 2(EA-2) is a rare, autosomal dominant disorder characterised by recurrent episodes of ataxia and dysarthria,due to mutations in the CACNA1A gene on chromosome 19 encoding voltage-dependent Ca2+ channels. The aim of the present study was to explore whether axonal membrane properties, assessed using nerve excitability techniques, were abnormal in patients with EA-2 . Nerve excitability techniques were applied to the median nerve of three individuals from three generations of a single family, all of whom had typical features of EA-2. This family was found to have a novel mutation at codon 1451 of the Ca2+ channel alpha 1A subunit. Nerve excitability testing demonstrated significant abnormalities,with all patients outside the normal 95 % confidence limits in having a high rheobase and reduced early hyperpolarizing threshold electrotonus. On average there were also significant reductions in refractoriness,late sub excitability and early depolarizing threshold electrotonus. Mathematical modelling indicated that a similar pattern of abnormalities may result from a reduced voltage dependence of slow K+ channels (KCNQ channels). There are significant and distinctive changes in peripheral nerve excitability in EA-2 patients,which are presumably induced indirectly. These findings raise the possibility that excitability testing may prove a convenient screening test for patients with this suspected channelopathy.

Velocity recovery cycles of single C fibres innervating rat skin

To improve knowledge about axonal membrane properties in nociceptive and non-nociceptive C fibres, we studied impulse-dependent velocity changes by in vivo microneurography in the rat sciatic nerve. Cutaneous C fibres were classified, based primarily on their activity-dependent slowing profile, as Type 1A (mechano-responsive nociceptors; CMR; n = 23), Type 1B (mechano-insensitive nociceptors; CMI; n = 24), Type 2 (cold units; n = 2), Type 3 units (unknown function; n = 4) or Type 4 (presumed sympathetics; n = 23) units. They were excited by single, double and triple electrical stimuli to the skin at mean rates of 0.25, 0.5, 1 and 2 Hz and with interstimulus intervals ranging from 2 to 1000 ms. All CMRs exhibited only postspike subnormality at 0.25 and 0.5 Hz. They gradually developed supernormality with higher stimulation rates, and 12/19 CMRs were supernormal at 1 Hz. The CMIs showed a greater tendency towards supernormality, with 10/21 already supernormal at 0.25 Hz, 17/24 at 0.5 Hz and all were supernormal at 1 Hz. In some CMIs but in none of the CMRs, the supernormal period was directly followed by a peak in late subnormality. Among non-nociceptive fibres, all Type 4 units exhibited long-lasting supernormality independent of the stimulation rate, whereas the cold units showed short-lived supernormality. In both, supernormality increased with higher stimulation rates. Regardless of fibre function or stimulation rate, a second conditioning stimulus always induced additional slowing, providing evidence for a passive origin of supernormality in all rat C fibre subtypes. However, the degree and time-course of extra slowing due to a preconditioning stimulus was highly dependent on fibre function and stimulation rate. These data indicate axonal membrane differences between different functional classes of C fibres, which resemble those previously described in human C fibres.

Capacitance fluctuations causing channel noise reduction in stochastic Hodgkin–Huxley systems

Voltage-dependent ion channels determine the electric properties of axonal cell membranes. They not only allow the passage of ions through the cell membrane, but also contribute to an additional charging of the cell membrane resulting in the so-called capacitance loading. The switching of the channel gates between an open and a closed configuration is intrinsically related to the movement of gating charge within the cell membrane. At the beginning of an action potential, the transient gating current is opposite to the direction of the current of sodium ions through the membrane. Therefore, the excitability is expected to become reduced due to the influence of a gating current. Our stochastic Hodgkin–Huxley-like modeling takes into account both the channel noise—i.e. the fluctuations of the number of open ion channels—and the capacitance fluctuations that result from the dynamics of the gating charge. We investigate the spiking dynamics of membrane patches of a variable size and analyze the statistics of the spontaneous spiking. As a main result, we find that the gating currents yield a drastic reduction of the spontaneous spiking rate for sufficiently large ion channel clusters. Consequently, this demonstrates a prominent mechanism for channel noise reduction.

Nerve excitability changes in critical illness polyneuropathy

Patients in intensive care units frequently suffer muscle weakness and atrophy due to critical illness polyneuropathy (CIP), an axonal neuropathy associated with systemic inflammatory response syndrome and multiple organ failure. CIP is a frequent and serious complication of intensive care that delays weaning from mechanical ventilation and increases mortality. The pathogenesis of CIP is not well understood and no specific therapy is available. The aim of this project was to use nerve excitability testing to investigate the changes in axonal membrane properties occurring in CIP. Ten patients (aged 37-76 years; 7 males, 3 females) were studied with electrophysiologically proven CIP. The median nerve was stimulated at the wrist and compound action potentials were recorded from abductor pollicis brevis muscle. Strength-duration time constant, threshold electrotonus, current-threshold relationship and recovery cycle (refractoriness, superexcitability and late subexcitability) were recorded using a recently described protocol. In eight patients a follow-up investigation was performed. All patients underwent clinical examination and laboratory investigations. Compared with age-matched normal controls (20 subjects; aged 38-79 years; 7 males, 13 females), CIP patients exhibited reduced superexcitability at 7 ms, from -22.3 +/- 1.6% to -7.6 +/- 3.1% (mean +/- SE, P approximately 0.0001) and increased accommodation to depolarizing (P < 0.01) and hyperpolarizing currents (P < 0.01), indicating membrane depolarization. Superexcitability was reduced both in patients with renal failure and without renal failure. In the former, superexcitability correlated with serum potassium (R = 0.88), and late subexcitability was also reduced (as also occurs owing to hyperkalaemia in patients with chronic renal failure). In patients without renal failure, late subexcitability was normal, and the signs of membrane depolarization correlated with raised serum bicarbonate and base excess, indicating compensated respiratory acidosis. It is inferred that motor axons in these CIP patients are depolarized, in part because of raised extracellular potassium, and in part because of hypoperfusion. The chronic membrane depolarization may contribute to the development of neuropathy.

Axonal potassium conductance and glycemic control in human diabetic nerves

Objective To investigate the effects of hyperglycemia on axonal excitability and potassium conductance in human diabetic nerves. Methods Threshold tracking was used to measure excitability indices, which depend on potassium channels (supernormality, late subnormality, threshold electrotonus, and a current/threshold relationship) in median motor axons of 96 diabetic patients. The effects of hyperglycemia on these indices were analyzed. Results Among diabetic patients, higher serum hemoglobin A1c (HbA1c) levels were significantly associated with greater supernormality ( P=0.04) and smaller late subnormality ( P=0.02), suggestive of reduced nodal/paranodal potassium currents under hyperglycemia. Threshold electrotonus and current/threshold relationships did not correlate with HbA1c levels, but partly related with nerve conduction slowing. Conclusions Hyperglycemia could reduce nodal potassium conductances, possibly due to reduced membranous potassium gradient or suppression of potassium channels. In contrast, internodal potassium conductances may be determined by both metabolic factors and structural changes such as exposure of internodal channels by demyelination. Significance Measurements of the excitability indices could provide new insights into nodal and internodal axonal membrane properties in human diabetic neuropathy, whereas multiple factors can affect especially internodal properties.

Velocity recovery cycles of C fibres innervating human skin.

Velocity changes following single and double conditioning impulses were studied by microneurography in single human C fibres to provide information about axonal membrane properties. C units were identified as mechano-responsive (n = 19) or mechano-insensitive (12) nociceptors, cold-sensitive (8) or sympathetic fibres (9), and excited by single, double and triple electrical stimuli to the skin at mean rates of 0.25-2 Hz. The interval between single or paired (20 ms apart) conditioning stimuli and test stimulus was then varied between 500 and 2 ms, and recovery curves of velocity change against inter-spike interval constructed, allowing for changes in these variables with distance. All fibres exhibited an initial (4-24 ms) relative refractory phase, and a long-lasting (>500 ms) 'H2' phase of reduced velocity, attributed to activation of Na+/K+-ATPase. Mechano-responsive nociceptors exhibited an intermediate phase of either supernormality or subnormality, depending on stimulation rate. Mechano-insensitive nociceptors behaved similarly, but all were supernormal at 1 Hz. Sympathetic units exhibited only a long-lasting supernormality, while cold fibres exhibited a briefer supernormal and a late subnormal phase (H1), similar to A fibres. A pre-conditioning impulse doubled H2 and increased H1, but did not augment supernormality or the subnormality of similar time course. Like A fibre supernormality, these phenomena were explained by a passive cable model, so that they provide an estimate of membrane time constant. Nociceptor membrane time constants (median 110 ms, n = 17) were rather insensitive to membrane potential, indicating few active voltage-dependent potassium channels, whereas sympathetic time constants were longer and reduced by activity-dependent hyperpolarisation.

Nerve excitability properties in Charcot-Marie-Tooth disease type 1A.

Charcot-Marie-Tooth disease type 1A (CMT1A) is commonly considered a prototype of a hereditary demyelinating polyneuropathy. Apart from the myelin involvement, there has been little information on axonal membrane properties in this condition. Taking advantage of the uniform nature of the disease process, we undertook the in vivo assessment of multiple axonal excitability properties at the median nerve in nine CMT1A patients with PMP22 (peripheral myelin protein 22) gene duplication and 53 controls. The thresholds of CMT1A patients were much higher than normal, and threshold electrotonus (TE) exhibited a consistent pattern of abnormalities: early steep changes (fanning out) of both hyperpolarizing and depolarizing responses were followed by increased inward rectification to hyperpolarizing currents and unusually fast accommodation to depolarizing currents. Strength-duration time constants and the shapes of recovery cycles were normal, although refractoriness and superexcitability were reduced relative to controls. The high thresholds and early fanning out of electrotonus indicated altered cable properties, such that a greater proportion than normal of applied currents reached internodal rather than nodal axolemma. The rapid accommodation to depolarizing currents suggested activation of fast K+ channels, which are normally sequestered from the nodal membrane. The excitability abnormalities are therefore consistent with a demyelinating pathology and exposure or spread of K+ channels from under the myelin. It remains to be seen whether the TE abnormalities in CMT1A, which resemble previous recordings from normal immature rats, can be distinguished from those in acquired demyelinating neuropathies.

Nerve excitability changes in chronic renal failure indicate membrane depolarization due to hyperkalaemia.

Multiple nerve excitability measurements were used to investigate axonal membrane properties in patients with chronic renal failure (CRF). Nine patients were studied during routine haemodialysis therapy. The median nerve was stimulated at the wrist and compound muscle action potentials recorded from abductor pollicis brevis. Stimulus-response behaviour, strength-duration time constant, threshold electrotonus, current-threshold relationship and recovery cycle (refractoriness, superexcitability and late subexcitability) were recorded using a recently described protocol. In six patients, sequential studies were performed before, during and after haemodialysis. All patients underwent standard electrolyte and renal function tests before and after haemodialysis. Before dialysis, there were significant abnormalities in axonal excitability: reduced superexcitability; increased accommodation to depolarizing and hyperpolarizing currents; and a steeper current-threshold relationship compared with normal controls. These excitability parameters are the most sensitive to membrane potential and the abnormalities, which were all reduced by haemodialysis, closely resembled those in normal axons depolarized by ischaemia. Before dialysis, the excitability parameters correlated significantly with serum potassium (range 4.3-6.1 mM), but not with other markers of renal dysfunction: patients with normal axonal resting potentials had normal serum potassium, although urea and creatinine were elevated. We conclude that nerves are depolarized in many CRF patients and that the depolarization is primarily due to hyperkalaemia.

Evidence for axonal membrane hyperpolarization in multifocal motor neuropathy with conduction block

Multiple nerve excitability measurements were used to investigate axonal membrane properties of patients diagnosed with multifocal motor neuropathy (MMN). Six patients were selected, all with evidence of distal focal motor conduction block involving the median nerve in the forearm. In all patients, the median nerve was stimulated at the wrist, just distal to the site of block, and the resulting compound muscle action potentials were recorded from abductor pollicis brevis. Stimulus-response behaviour, the strength--duration time constant, threshold electrotonus to 100 ms polarizing currents, a current-threshold relationship and the recovery of excitability following supramaximal activation were recorded using a protocol described recently. When compared with control values, patients demonstrated significantly greater superexcitability, a 'fanning out' of threshold electrotonus recordings, and a significant change in the slope of the current--threshold relationship. These abnormalities in axonal membrane excitability parameters closely resembled those in normal axons hyperpolarized following release from ischaemia. To test for axonal hyperpolarization, DC depolarizing currents were applied to the nerves of three patients, and all the excitability parameters were normalized by depolarization. Attempts to trace excitability measures proximally towards the site of block were unsuccessful, as the nerve became inexcitable in all cases. It is suggested that the distal hyperpolarization is probably linked to focal depolarization and that the clinical features of MMN are consistent with a depolarizing/hyperpolarizing lesion.

ALTERED TONIC CONDUCTANCES IN AXONS OF TRANSGENIC MOUSE EXPRESSING THE HUMAN NEUROFILAMENT HEAVY GENE: A MOUSE MODEL OF AMYOTROPHIC LATERAL SCLEROSIS

Neurofilaments (NFs; made by copolymerization of three intermediate filament proteins NF‐L, NF‐M, and NF‐H, for light, medium, and heavy) constitute the most abundant cytoskeletal structure in large myelinated axons. The presence of aberrant NF accumulation has been associated with neurodegenerative diseases (such as ALS). The possible causal role of NF in neurodegeneration has been supported by studies on recently available transgenic mice in which expression of human NF‐H (hNF‐H +/+) leads to overt neuropathy. We have examined electrophysiological properties of myelinated axons in hNF‐H +/+ mice using intraaxonal microelectrode recording from isolated sciatic and tibial nerves. Transgenic mice showed several deficits in physiological properties of low threshold myelinated fibers: conduction velocity and resting membrane potential were significantly decreased (20 +/− 1.6 vs 40 +/− 2 m/s; −71.3 +/− 0.9 vs −75.5 +/− 0.5 m/s; mean +/− SE; n = 25; 22 degrees C). While the amplitude of action potentials was of comparable size (82 +/− 5 vs 86 +/− 3 mV) duration of action potential (at half‐amplitude, AP/2) in hNF‐H +/+ was significantly prolonged (0.82 +/− 0.02 vs 0.65 +/− 0.02 ms). Voltage‐current properties of axonal membrane indicate a significant decrease in inward and outward rectification. Occasionally, impaled axons of hNF‐H +/+ showed membrane oscillations and repetitive activity (reminiscent of fasciculations) never observed in normal animals. These results are compatible with an imbalance between ion conductances in axons from transgenic animals (an increase in Na+ and a decrease in K+ conductances), in agreement with recent suggestion based on clinical studies on ALS patients (H. Bostock et al., 1995, Brain 118, 217–225). One may hypothesize that these changes could contribute to neurodegenerative processes (i.e., via an increase in [Na+](i)), as well as clinical symptoms (fasciculations) observed in patients with degenerative motor neuron diseases.