Optical measurement of conduction in single demyelinated axons

Restore axonal conductance in a locally demyelinated axon with electromagnetic stimulation

Objective. Axonal demyelination leads to failure of axonal conduction. Current research on demyelination focuses on the promotion of remyelination. Electromagnetic stimulation is widely used to promote neural activity. We hypothesized that electromagnetic stimulation of the demyelinated area, by providing excitation to the nodes of Ranvier, could rescue locally demyelinated axons from conductance failure.Approach. We built a multi-compartment NEURON model of a myelinated axon under electromagnetic stimulation. We simulated the action potential (AP) propagation and observed conductance failure when local demyelination occurred. Conductance failure was due to current leakage and a lack of activation of the nodes in the demyelinated region. To investigate the effects of electromagnetic stimulation on locally demyelinated axons, we positioned a miniature coil next to the affected area to activate nodes in the demyelinated region.Main results. Subthreshold microcoil stimulation caused depolarization of node membranes. This depolarization, in combination with membrane depolarization induced by the invading AP, resulted in sufficient activation of nodes in the demyelinated region and restoration of axonal conductance. Efficacy of restoration was dependent on the amplitude and frequency of the stimuli, and the location of the microcoil relative to the targeted nodes. The restored axonal conductance was due to the enhanced Na+current and reduced K+current in the nodes, rather than a reduction in leakage current in the demyelinated region. Finally, we found that microcoil stimulation had no effect on axonal conductance in healthy, myelinated axons.Significance. Activation of nodes in the demyelinated region using electromagnetic stimulation provides an alternative treatment strategy to restore axonal function under local demyelination conditions. Results provide insights to the development of microcoil technology for the treatment of focal segmental demyelination cases, such as neuropraxia, spinal cord injury, and auditory nerve demyelination.

Myelin loss and axonal ion channel adaptations associated with gray matter neuronal hyperexcitability

Myelination and voltage-gated ion channel clustering at the nodes of Ranvier are essential for the rapid saltatory conduction of action potentials. Whether myelination influences the structural organization of the axon initial segment (AIS) and action potential initiation is poorly understood. Using the cuprizone mouse model, we combined electrophysiological recordings with immunofluorescence of the voltage-gated Nav1.6 and Kv7.3 subunits and anchoring proteins to analyze the functional and structural properties of single demyelinated neocortical L5 axons. Whole-cell recordings demonstrated that neurons with demyelinated axons were intrinsically more excitable, characterized by increased spontaneous suprathreshold depolarizations as well as antidromically propagating action potentials ectopically generated in distal parts of the axon. Immunofluorescence examination of demyelinated axons showed that βIV-spectrin, Nav1.6, and the Kv7.3 channels in nodes of Ranvier either dissolved or extended into the paranodal domains. In contrast, while the AIS in demyelinated axons started more closely to the soma, ankyrin G, βIV-spectrin, and the ion channel expression were maintained. Structure-function analysis and computational modeling, constrained by the AIS location and realistic dendritic and axonal morphologies, confirmed that a more proximal onset of the AIS slightly reduced the efficacy of action potential generation, suggesting a compensatory role. These results suggest that oligodendroglial myelination is not only important for maximizing conduction velocity, but also for limiting hyperexcitability of pyramidal neurons.

Demyelination in multiple sclerosis

This review, focused on demyelination in multiple sclerosis, is divided in two parts. The first part addresses the many and not exclusive mechanisms leading to demyelination in the central nervous system. Although the hypothesis that a primary oligodendrocyte or myelin injury induces a secondary immune response in the central nervous system is still a matter of debate, most recent advances underline the influence of a primary immune response against myelin antigen(s), with a diversity of potential targets. Whereas multiple sclerosis was long considered as a T cell-mediated disease, the role of B lymphocytes is now increasingly recognized, and the influence of antibodies on tissue damage actively investigated. The second part of the review describes the axonal consequences of demyelination. Segmental demyelination results in conduction block or slowing of conduction through adaptative responses, notably related to modifications in the distribution of voltage gated sodium channels along the denuded axon. If demyelination persists, these changes, as well as the loss of trophic and metabolic support, will lead to irreversible axonal damage and loss. In this respect, favouring early myelin repair, during a window of time when axonal damage is still reversible, might pave the way for neuroprotection.

Neuronal activity in the hub of extrasynaptic Schwann cell-axon interactions

The integrity and function of neurons depend on their continuous interactions with glial cells. In the peripheral nervous system glial functions are exerted by Schwann cells (SCs). SCs sense synaptic and extrasynaptic manifestations of action potential propagation and adapt their physiology to support neuronal activity. We review here existing literature data on extrasynaptic bidirectional axon-SC communication, focusing particularly on neuronal activity implications. To shed light on underlying mechanisms, we conduct a thorough analysis of microarray data from SC-rich mouse sciatic nerve at different developmental stages and in neuropathic models. We identify molecules that are potentially involved in SC detection of neuronal activity signals inducing subsequent glial responses. We further suggest that alterations in the activity-dependent axon-SC crosstalk impact on peripheral neuropathies. Together with previously reported data, these observations open new perspectives for deciphering glial mechanisms of neuronal function support.

Congenital CNS hypomyelination in the Fig4 null mouse is rescued by neuronal expression of the PI(3,5)P(2) phosphatase Fig4

The plt (pale tremor) mouse carries a null mutation in the Fig4(Sac3) gene that results in tremor, hypopigmentation, spongiform degeneration of the brain, and juvenile lethality. FIG4 is a ubiquitously expressed phosphatidylinositol 3,5-bisphosphate phosphatase that regulates intracellular vesicle trafficking along the endosomal-lysosomal pathway. In humans, the missense mutation FIG4(I41T) combined with a FIG4 null allele causes Charcot-Marie-Tooth 4J disease, a severe form of peripheral neuropathy. Here we show that Fig4 null mice exhibit a dramatic reduction of myelin in the brain and spinal cord. In the optic nerve, smaller-caliber axons lack myelin sheaths entirely, whereas many large- and intermediate-caliber axons are myelinated but show structural defects at nodes of Ranvier, leading to delayed propagation of action potentials. In the Fig4 null brain and optic nerve, oligodendrocyte (OL) progenitor cells are present at normal abundance and distribution, but the number of myelinating OLs is greatly compromised. The total number of axons in the Fig4 null optic nerve is not reduced. Developmental studies reveal incomplete myelination rather than elevated cell death in the OL linage. Strikingly, there is rescue of CNS myelination and tremor in transgenic mice with neuron-specific expression of Fig4, demonstrating a non-cell-autonomous function of Fig4 in OL maturation and myelin development. In transgenic mice with global overexpression of the human pathogenic FIG4 variant I41T, there is rescue of the myelination defect, suggesting that the CNS of CMT4J patients may be protected from myelin deficiency by expression of the FIG4(I41T) mutant protein.

Transient demyelination increases the efficiency of retrograde AAV transduction

Adeno-associated virus (AAV) is capable of mediating retrograde viral transduction of central and peripheral neurons. This occurs at a relatively low efficiency, which we previously found to be dependent upon capsid serotype. We sought to augment retrograde transduction by providing increased axonal access to peripherally delivered AAV. Others have described utilizing full transection of peripheral nerves to mediate retrograde viral transduction of motor neurons. Here, we examined the ability of a transient demyelinating event to modulate levels of retrograde AAV transduction. Transient demyelination does not cause lasting functional deficits. Ethidium bromide (EtBr)-induced transient demyelination of the sciatic nerve resulted in significant elevation of retrograde transduction of both motor and sensory neurons. Retrograde transduction levels of motor neurons and heavily myelinated, large-diameter sensory neurons increased at least sixfold following peripheral delivery of self-complementary AAV serotype 1 (scAAV1) and serotype 2 (scAAV2), when preceded by demyelination. These findings identify a means of significantly enhancing retrograde vector transport for use in experimental paradigms requiring either retrograde neuronal identification and gene expression, or translational treatment paradigms.

THE EFFECT OF BUPIVACAINE ON COMPOUND ACTION POTENTIAL PARAMETERS OF SCIATIC NERVE FIBERS

The aim of this study was to document the effects of the local anesthetic agent bupivacaine on individual fibers of peripheral nerve. To accomplish this objective, compound action potentials (CAPs) were recorded from isolated frog sciatic nerves treated with bupivacaine for seven individual concentration levels. Numerical and fast Fourier transform (FFT) analysis were performed on these recordings. The areas, latency periods, maximum and minimum derivatives, and power spectrums of the CAPs were computed. The results show that the area and the absolute values of maximum and minimum derivatives decrease linearly as bupivacaine concentration increases. The power spectrum of the CAPs, which resides in the 0-1000 Hz interval, initially shifts to higher frequencies then returns to lower frequency region again with increasing bupivacaine concentration. Due to this result, it is thought that bupivacaine inhibits nerve fibers in a dose-dependent manner. It primarily affects the fibers having the least myelin sheets (motor fibers), then it begins to depress the fast conducting (neurosensorial) fibers as the bupivacaine concentration increases, and finally blocks the unmyelinated C-fibers.

Assesment criteria for experimental demyelination induced in frog peripheral nerve

In ideal conditions the area under compound action potential may be used as an index for the number of activated fibers in a nerve trunk whereas peak amplitude, maximum time derivative, and duration may be used as an indicator for the rate of contribution to compound action potential and the degree of velocity dispersion. In this study, the time domain effect of demyelination on compound action potential has been investigated in experimentally demyelinated frog sciatic nerve. The results were analyzed in order to suggest criteria for demyelination. The results suggest that the changes in peak amplitude and maximum time derivative of compound action potential that is made up by the contribution of the active fibers may be more useful in the assessment of early phase of demyelination. Therefore, it may be concluded that these two parameters, intrinsically, carry augmented information on the velocity dispersion originated from larger-diameter fibers.

Synthesis, spectra, delivery and potentiometric responses of new styryl dyes with extended spectral ranges

Styryl dyes have been among the most widely used probes for mapping membrane potential changes in excitable cells. However, their utility has been somewhat limited because their excitation wavelengths have been restricted to the 450-550 nm range. Longer wavelength probes can minimize interference from endogenous chromophores and, because of decreased light scattering, improve recording from deep within tissue. In this paper we report on our efforts to develop new potentiometric styryl dyes that have excitation wavelengths ranging above 700 nm and emission spectra out to 900 nm. We have prepared and characterized dyes based on 47 variants of the styryl chromophores. Voltage-dependent spectral changes have been recorded for these dyes in a model lipid bilayer and from lobster nerves. The voltage sensitivities of the fluorescence of many of these new potentiometric indicators are as good as those of the widely used ANEP series of probes. In addition, because some of the dyes are often poorly water soluble, we have developed cyclodextrin complexes of the dyes to serve as efficient delivery vehicles. These dyes promise to enable new experimental paradigms for in vivo imaging of membrane potential.

The role of the ankyrin-binding protein NrCAM in node of Ranvier formation

Molecular events involved in Na+ channel clustering at the node of Ranvier have been investigated during early development. NrCAM, an ankyrinG-binding protein, precedes Na+ channels at cluster sites adjacent to the tips of Schwann cell processes. Both Na+ channel and ankyrinG sequestration at developing nodes are delayed in NrCAM null mutants. The action of NrCAM is manifest locally at individual nodes, rather than affecting overall neuronal expression, and is linked to glial interactions. During remyelination, Na+ channel clusters at new nodes are initially labile, and anchoring to the cytoskeleton appears to grow progressively with time. The distance between Na+ channel clusters across remyelinating Schwann cells (nascent internodes) increases markedly from 83 to 274 microm during node formation, arguing against schemes in which the loci of nodes are fixed in advance by the axon. A hypothesis for node formation in which axonal Na+ channels move by lateral diffusion from regions of Schwann cell contact, with clustering dependent on linkage to the cytoskeleton by ankyrinG, is proposed and tested in a computational model. To match experimental measurements, this latter reaction needs fast kinetics, and the early arrival of NrCAM is postulated to contribute to this requirement.

The role of the ankyrin-binding protein NrCAM in node of Ranvier formation

Molecular events involved in Na+ channel clustering at the node of Ranvier have been investigated during early development. NrCAM, an ankyrinG-binding protein, precedes Na+ channels at cluster sites adjacent to the tips of Schwann cell processes. Both Na+ channel and ankyrinG sequestration at developing nodes are delayed in NrCAM null mutants. The action of NrCAM is manifest locally at individual nodes, rather than affecting overall neuronal expression, and is linked to glial interactions. During remyelination, Na+ channel clusters at new nodes are initially labile, and anchoring to the cytoskeleton appears to grow progressively with time. The distance between Na+ channel clusters across remyelinating Schwann cells (nascent internodes) increases markedly from 83 to 274 microm during node formation, arguing against schemes in which the loci of nodes are fixed in advance by the axon. A hypothesis for node formation in which axonal Na+ channels move by lateral diffusion from regions of Schwann cell contact, with clustering dependent on linkage to the cytoskeleton by ankyrinG, is proposed and tested in a computational model. To match experimental measurements, this latter reaction needs fast kinetics, and the early arrival of NrCAM is postulated to contribute to this requirement.

Molecular constituents of the node of Ranvier

The interaction between neurons and glial cells that results in myelin formation represents one of the most remarkable intercellular events in development. This is especially evident at the primary functional site within this structure, the node of Ranvier. Recent experiments have revealed a surprising level of complexity within this zone, with several components, including ion channels, sequestered with a very high degree of precision and sharply demarcated borders. We discuss the current state of knowledge of the cellular and molecular mechanisms responsible for the formation and maintenance of the node. In normal axons, Na+ channels are present at high density within the nodal gap, and voltage-dependent K+ channels are sequestered on the internodal side of the paranode--a region known as the juxtaparanode. Modifying the expression of certain surface adhesion molecules that have been recently identified, markedly alters this pattern. There is a special emphasis on contactin, a protein with multiple roles in the nervous system. In central nervous system (CNS) myelinated fibers, contactin is localized within both the nodal gap and paranodes, and appears to have unique functions in each zone. New experiments on contactin-null mutant mice help to define these mechanisms.

Molecular constituents of the node of Ranvier

The interaction between neurons and glial cells that results in myelin formation represents one of the most remarkable intercellular events in development. This is especially evident at the primary functional site within this structure, the node of Ranvier. Recent experiments have revealed a surprising level of complexity within this zone, with several components, including ion channels, sequestered with a very high degree of precision and sharply demarcated borders. We discuss the current state of knowledge of the cellular and molecular mechanisms responsible for the formation and maintenance of the node. In normal axons, Na+ channels are present at high density within the nodal gap, and voltage-dependent K+ channels are sequestered on the internodal side of the paranode--a region known as the juxtaparanode. Modifying the expression of certain surface adhesion molecules that have been recently identified, markedly alters this pattern. There is a special emphasis on contactin, a protein with multiple roles in the nervous system. In central nervous system (CNS) myelinated fibers, contactin is localized within both the nodal gap and paranodes, and appears to have unique functions in each zone. New experiments on contactin-null mutant mice help to define these mechanisms.

Ion channel sequestration in central nervous system axons

Na+ and K+ channel localization and clustering are essential for proper electrical signal generation and transmission in CNS myelinated nerve fibres. In particular, Na+ channels are clustered at high density at nodes of Ranvier, and Shaker-type K+ channels are sequestered in juxtaparanodal zones, just beyond the paranodal axoglial junctions. The mechanisms of channel localization at nodes of Ranvier in the CNS during development in both normal and hypomyelinating mutant animals are discussed and reviewed. As myelination proceeds, Na+ channels are initially found in broad zones within gaps between neighbouring oligodendroglial processes, and then are condensed into focal clusters. This process appears to depend on the formation of axoglial junctions. K+ channels are first detected in juxtaparanodal zones, and in mutant mice lacking normal axoglial junctions, these channels fail to cluster. In these mice, despite the presence of numerous oligodendrocytes, Na+ channel clusters are rare, and when present, are highly irregular. A number of molecules have recently been described that are candidates for a role in the neuron-glial interactions driving ion channel clustering. This paper reviews the cellular and molecular events responsible for formation of the mature node of Ranvier in the CNS.

The pathophysiology of multiple sclerosis: the mechanisms underlying the production of symptoms and the natural history of the disease

The pathophysiology of multiple sclerosis is reviewed, with emphasis on the axonal conduction properties underlying the production of symptoms, and the course of the disease. The major cause of the negative symptoms during relapses (e.g. paralysis, blindness and numbness) is conduction block, caused largely by demyelination and inflammation, and possibly by defects in synaptic transmission and putative circulating blocking factors. Recovery from symptoms during remissions is due mainly to the restoration of axonal function, either by remyelination, the resolution of inflammation, or the restoration of conduction to axons which persist in the demyelinated state. Conduction in the latter axons shows a number of deficits, particularly with regard to the conduction of trains of impulses and these contribute to weakness and sensory problems. The mechanisms underlying the sensitivity of symptoms to changes in body temperature (Uhthoff's phenomenon) are discussed. The origin of 'positive' symptoms, such as tingling sensations, are described, including the generation of ectopic trains and bursts of impulses, ephaptic interactions between axons and/or neurons, the triggering of additional, spurious impulses by the transmission of normal impulses, the mechanosensitivity of axons underlying movement-induced sensations (e.g. Lhermitte's phenomenon) and pain. The clinical course of the disease is discussed, together with its relationship to the evolution of lesions as revealed by magnetic resonance imaging and spectroscopy. The earliest detectable event in the development of most new lesions is a breakdown of the blood-brain barrier in association with inflammation. Inflammation resolves after approximately one month, at which time there is an improvement in the symptoms. Demyelination occurs during the inflammatory phase of the lesion. An important mechanism determining persistent neurological deficit is axonal degeneration, although persistent conduction block arising from the failure of repair mechanisms probably also contributes.

Formation of compact myelin is required for maturation of the axonal cytoskeleton

Although traditional roles ascribed to myelinating glial cells are structural and supportive, the importance of compact myelin for proper functioning of the nervous system can be inferred from mutations in myelin proteins and neuropathologies associated with loss of myelin. Myelinating Schwann cells are known to affect local properties of peripheral axons (de Waegh et al., 1992), but little is known about effects of oligodendrocytes on CNS axons. The shiverer mutant mouse has a deletion in the myelin basic protein gene that eliminates compact myelin in the CNS. In shiverer mice, both local axonal features like phosphorylation of cytoskeletal proteins and neuronal perikaryon functions like cytoskeletal gene expression are altered. This leads to changes in the organization and composition of the axonal cytoskeleton in shiverer unmyelinated axons relative to age-matched wild-type myelinated fibers, although connectivity and patterns of neuronal activity are comparable. Remarkably, transgenic shiverer mice with thin myelin sheaths display an intermediate phenotype indicating that CNS neurons are sensitive to myelin sheath thickness. These results indicate that formation of a normal compact myelin sheath is required for normal maturation of the neuronal cytoskeleton in large CNS neurons.

Formation of compact myelin is required for maturation of the axonal cytoskeleton

Although traditional roles ascribed to myelinating glial cells are structural and supportive, the importance of compact myelin for proper functioning of the nervous system can be inferred from mutations in myelin proteins and neuropathologies associated with loss of myelin. Myelinating Schwann cells are known to affect local properties of peripheral axons (de Waegh et al., 1992), but little is known about effects of oligodendrocytes on CNS axons. The shiverer mutant mouse has a deletion in the myelin basic protein gene that eliminates compact myelin in the CNS. In shiverer mice, both local axonal features like phosphorylation of cytoskeletal proteins and neuronal perikaryon functions like cytoskeletal gene expression are altered. This leads to changes in the organization and composition of the axonal cytoskeleton in shiverer unmyelinated axons relative to age-matched wild-type myelinated fibers, although connectivity and patterns of neuronal activity are comparable. Remarkably, transgenic shiverer mice with thin myelin sheaths display an intermediate phenotype indicating that CNS neurons are sensitive to myelin sheath thickness. These results indicate that formation of a normal compact myelin sheath is required for normal maturation of the neuronal cytoskeleton in large CNS neurons.

Disruption and reorganization of sodium channels in experimental allergic neuritis

The axonal distribution of voltage-dependent Na+ channels was determined during inflammatory demyelinating disease of the peripheral nervous system. Experimental allergic neuritis was induced in Lewis rats by active immunization. In diseased spinal roots Na+ channel immunofluorescence at many nodes of Ranvier changed from a highly focal ring to a more diffuse pattern and, as the disease progressed, eventually became undetectable. The loss of nodal channels corresponded closely with the development of clinical signs. Electrophysiological measurements and computations showed that a lateral spread of nodal Na+ channels could contribute significantly to temperature sensitivity and conduction block. During recovery new clusters of Na+ channels were seen. In fibers with large-scale demyelination, the new aggregates formed at the edges of adhering Schwann cells and appeared to fuse to form new nodes. At nodes with demyelination limited to paranodal retraction, Na+ channels were often found divided into two symmetric highly focal clusters. These results suggest that reorganization of Na+ channels plays an important role in the pathogenesis of demyelinating neuropathies.

Conduction in segmentally demyelinated mammalian central axons

The prominent symptoms associated with central demyelinating diseases such as multiple sclerosis (MS) are primarily caused by conduction deficits in affected axons. The symptoms may go into remission, but the mechanisms underlying remissions are uncertain. One factor that could be important is the restoration of conduction to affected axons, but it is not known whether demyelinated central axons resemble their peripheral counterparts in being able to conduct in the absence of repair by remyelination. In the present study we have made intra-axonal recordings from central axons affected by a demyelinating lesion, and then the axons have been labeled ionophoretically to permit their subsequent identification. Ultrastructural examination of 23 labeled preparations has established that some segmentally demyelinated central axons can conduct, and that they can do so over continuous lengths of demyelination exceeding several internodes (2500 micron). Such segmentally demyelinated central axons were found to conduct with the anticipated reduction in velocity and a refractory period of transmission (RPT) as much as 34 times the value obtained from the nondemyelinated portion of the same axon; the RPT was typically prolonged to 2-5 times the normal value. We conclude that some segmentally demyelinated central axons can conduct, and we propose that the restoration of conduction to such axons is likely to contribute to the remissions commonly observed in diseases such as MS.

Sodium channel distribution in axons of hypomyelinated and MAG null mutant mice

Na+ channel organization was studied with immunofluorescence in the peripheral nervous system of mice genetically altered to produce abnormal myelin. In two of these strains, transcription of inserted transgenes was targeted to myelinating Schwann cells through linkage to a promoter for the myelin protein P0. Adults of both of these strains had hindlimb paralysis and a tremor on lifting by the tail. In one case, Schwann cells were eliminated via expression of the diphtheria toxin A chain (DT-A). During postnatal days 3-7, Na+ channel clustering at forming nodes was dramatically reduced compared with that of normal animals. At 1-3 months of age, Na+ channel immunofluorescence was often found spread over long stretches of the axolemma, instead of being confined to nodal gaps. In the second P0-linked transgenic model, Schwann cell expression of the large T antigen tsA-1609 resulted in cell cycle dysfunction. Adult axons had regions of diffuse Na+ channel labeling. Focal clusters were rare within these zones, which were characterized by a series of cells of myelinating phenotype tightly apposed to the axon. Previous studies suggested that Schwann cells had to reach the stage of ensheathment characterized by periaxonal myelin associated glycoprotein (MAG) expression in order to induce Na+ channel clustering. However, in MAG-deficient mice, Na+ channel labeling patterns within sciatic nerves were normal.

Conduction in segmentally demyelinated mammalian central axons

The prominent symptoms associated with central demyelinating diseases such as multiple sclerosis (MS) are primarily caused by conduction deficits in affected axons. The symptoms may go into remission, but the mechanisms underlying remissions are uncertain. One factor that could be important is the restoration of conduction to affected axons, but it is not known whether demyelinated central axons resemble their peripheral counterparts in being able to conduct in the absence of repair by remyelination. In the present study we have made intra-axonal recordings from central axons affected by a demyelinating lesion, and then the axons have been labeled ionophoretically to permit their subsequent identification. Ultrastructural examination of 23 labeled preparations has established that some segmentally demyelinated central axons can conduct, and that they can do so over continuous lengths of demyelination exceeding several internodes (2500 micron). Such segmentally demyelinated central axons were found to conduct with the anticipated reduction in velocity and a refractory period of transmission (RPT) as much as 34 times the value obtained from the nondemyelinated portion of the same axon; the RPT was typically prolonged to 2-5 times the normal value. We conclude that some segmentally demyelinated central axons can conduct, and we propose that the restoration of conduction to such axons is likely to contribute to the remissions commonly observed in diseases such as MS.

Clusters of axonal Na+ channels adjacent to remyelinating Schwann cells

Rat sciatic nerve fibres were demyelinated by injection of lysolecithin and examined at several stages as Schwann cells proliferated, adhered, and initiated remyelination. Immunoperoxidase EM has been used to follow the clustering of Na+ channels that represents an early step in the formation of new nodes of Ranvier. At the peak of demyelination, 1 week post-injection, only isolated sites, suggestive of the original nodes, were labelled. As Schwann cells adhered and extended processes along the axons, regions of axonal Na+ channel immunoreactivity were often found just beyond their leading edges. These channel aggregates were associated only with the axolemma and Na+ channels were not detected on glial membranes. Sites with more than one cluster in close proximity and broadly labelled aggregates between Schwann cells suggested that new nodes of Ranvier formed as neighbouring Na+ channel groups merged. Schwann cells thus seem to play a major role in ion channel distributions in the axolemma. In all of these stages Na+ channel label was found primarily just outside the region of close contact between axon and Schwann cell. This suggests that Schwann cell adherence acts in part to exclude Na+ channels, or that diffusible substances are involved and can act some distance from regions of direct contact.

Control of myelination, axonal growth, and synapse formation in spinal cord explants by ion channels and electrical activity

The involvement of axonal electrical activity and ion channels as mediators of neuron-glial communication during myelin formation has been tested in explant culture. Transverse slices of embryonic mouse spinal cord were maintained under conditions normally leading to extensive myelination. Axonal conduction was measured optically through the use of a voltage-sensitive dye. Glial development was at a very early stage at the time of plating, and oligodendrocyte precursor cells had not yet appeared. Spontaneous electrical activity was blocked either by tetrodotoxin or by elevation of external K+ concentrations. Myelin development was unaffected by tetrodotoxin and was also present, though quantitatively reduced, in elevated K+. Tetraethylammonium ion (TEA+), a blocker of many K+ channels, almost entirely eliminated myelination at a concentration of 1 mM, but axonal growth and conduction were unaffected. Synapse formation was followed both morphologically and functionally, and was altered neither by conduction block nor by 1 mM TEA+. It is concluded that in the spinal cord oligodendrocyte development and myelination can proceed in the absence of axonal action potentials, but ion channels, possibly in glial membranes, play an important role in these events.

Survival, development, and electrical activity of central nervous system myelinated axons exposed to tumor necrosis factor in vitro

Spinal cord explants from CD-1 mouse embryos were cultured in Maximow slide assemblies to promote myelin development. At about 20 days in vitro, recombinant human or mouse tumor necrosis factor alpha (TNF alpha) was added. Observed 3-8 days later, myelin was largely intact. The myelin blistering and oligodendrocyte damage seen in other strains were generally absent. Axonal conduction was measured optically through the use of a voltage-sensitive dye. Action potential shape, conduction velocity, and refractory period were all unchanged by exposure to TNF alpha. Two series of explants were grown with TNF alpha present continuously throughout the culture period. Observed with light and electron microscopy, myelin developed in at least 50% of the explants treated with recombinant mouse TNF alpha and 80% of those exposed to recombinant human TNF alpha. Optically recorded action potentials were of normal shape and refractory period. Conduction velocities were slightly lower than controls. CD-1 mouse central nervous system contains TNF alpha receptors and yet was resistant to myelin damage. The apparent strain specificity of TNF alpha disruption of myelin may result from more indirect modes of action, including interaction with other cytokines produced by glial cells. Survival of axonal conduction suggests that Na+ channel function remains intact following TNF alpha exposure.

Resolving three types of chloride channels in demyelinated Xenopus axons

Axons from Xenopus sciatic nerve were demyelinated by intraneural injection of lysolecithin rendering the entire internodal axolema accessible to a patch electrode. In this region, three types of anion selective pores were found and characterized at the single-channel level. These included outwardly rectifying, inwardly rectifying, and maxi Cl- channels. The outwardly rectifying Cl- channels (24 pS) are activated by depolarization with a weak voltage dependence of 42 mV per e-fold change in open probability. The inwardly rectifying Cl- channels (27 pS) are insensitive to voltage, but can be blocked by internal application of 100 microM SITS or DIDS. The I-V curves of rectifying channels are S-shaped and can be fitted by a kinetic model with a single free energy barrier. The rectification may be related to the location of this barrier. The maxi Cl- channel (335 pS) is often open at the resting potential, but is inactivated by a large depolarization. The rectification, voltage dependence, and inactivation of these channels may contribute to the regulation of axonal Cl- balance and resting potential.

Axonal coding of action potentials in demyelinated nerve fibers

Conduction in individual axons of Xenopus has been measured optically in response to short trains of stimuli, following demyelination of the sciatic nerve. In many cases the initial action potential in a burst is absent. Failure may also occur later in the train, resulting in a profound alteration of signal coding by the axon. Integration leading to delayed transmission occurred at the heminode forming the proximal border of the demyelinated zone, as well as at new nodes of Ranvier forming in remyelinating axons. This process appeared to involve a depolarizing afterpotential and seemed to be analogous to the threshold changes involved in superexcitability. Axonal coding was very sensitive to small changes in the stimulus pattern. Neither 1 mM tetraethylammonium ion, which blocks nodal and Ca2+ activated K+ channels, nor 1 mM 4-aminopyridine, which blocks fast internodal K+ channels, prevented loss of the initial spike in a burst. Similarly, neither block of Ca2+ channels by Cd2+ nor lowering of Cl- had a notable effect. Ouabain, on the other hand, had small but possibly significant effects on responses to repetitive stimuli. A computational model was used to test mechanisms involving passive cable properties. Lowering the myelin resistance and the nodal leakage conductance, in accord with recent evidence from intracellular recordings, reproduced many of the results and was accurate with respect to stimulus frequency, temperature and sensitivity to average potential. The coding of action potentials observed here may have clinical consequences in demyelinating diseases such as multiple sclerosis.

Optical determination of impulse conduction velocity during development of embryonic chick cervical vagus nerve bundles

1. Employing an optical method for multiple-site simultaneous recording of electrical activity, we have determined the conduction velocity in cervical vagus nerve bundles isolated from 5- to 21-day-old chick embryos, and investigated its developmental changes. 2. The preparations were stained with a voltage-sensitive merocyanine-rhodanine dye (NK2761), and action potential- (impulse-) related optical signals were elicited by brief stimuli applied to the end of the vagus nerve bundle with a suction electrode. Optical signals were recorded simultaneously from many contiguous regions using a 12 x 12-element photodiode array. 3. The optical signals spread with small delay from the site of stimulation. From the relationship between the delay and distance from the current-applying electrode, conduction velocities were estimated in each tested preparation: the conduction velocity was very small and increased monotonically from about 0.1 m s-1 at 5 days embryonic age to about 0.4 m s-1 by hatching. The increase in the conduction velocity was closely related to a developmental increase in the diameter of the vagus nerve bundle. 4. In addition, we have examined the spread of electrotonic potentials. The space constant was very small (200-450 microns) and increased as development proceeded. 5. Compound optical action signals having two distinct components were also recorded. They often appeared to be concentrated in the preparations from 8- to 12-day-old embryos. The conduction velocity of the second component was slower than that of the first. We suggest that appearance of the second component reflects degeneration of a subset of axons resulting from 'neural cell death' during the development of the vagus nerve.

Remyelination of nerve fibers in the transected frog sciatic nerve

This project tests an important aspect of the cellular events controlling the processes of recovery of function and remyelination that follow demyelination in the peripheral nervous system. Frog sciatic nerves have been shown to survive and remain functional for up to 10 days following transection. We have utilized this property in order to dissociate the recovery process from possible control by the neuronal soma. Xenopus sciatic nerves were demyelinated in one branch by an intraneural injection of lysolecithin. The nerve was cut proximally to the injection site either immediately before, or several days after the lysolecithin injection. Recovery of function and remyelination were then followed by electrophysiological, optical, and ultrastructural techniques applied both to whole branches and single fibers. Controls included the cut but uninjected branch, and injected but uncut nerves. The progression of events during both demyelination and recovery in cut axons was indistinguishable from that in uncut fibers. This suggests that this process may be under local control and can be initiated and carried out in the absence of constant communication with the nerve cell body.