Ionic channel distribution and heterogeneity of the axon membrane in myelinated fibers

The palmitoyl acyltransferase ZDHHC14 controls Kv1-family potassium channel clustering at the axon initial segment

The palmitoyl acyltransferase (PAT) ZDHHC14 is highly expressed in the hippocampus and is the only PAT predicted to bind Type-I PDZ domain-containing proteins. However, ZDHHC14’s neuronal roles are unknown. Here, we identify the PDZ domain-containing Membrane-associated Guanylate Kinase (MaGUK) PSD93 as a direct ZDHHC14 interactor and substrate. PSD93, but not other MaGUKs, localizes to the axon initial segment (AIS). Using lentiviral-mediated shRNA knockdown in rat hippocampal neurons, we find that ZDHHC14 controls palmitoylation and AIS clustering of PSD93 and also of Kv1 potassium channels, which directly bind PSD93. Neurodevelopmental expression of ZDHHC14 mirrors that of PSD93 and Kv1 channels and, consistent with ZDHHC14’s importance for Kv1 channel clustering, loss of ZDHHC14 decreases outward currents and increases action potential firing in hippocampal neurons. To our knowledge, these findings identify the first neuronal roles and substrates for ZDHHC14 and reveal a previously unappreciated role for palmitoylation in control of neuronal excitability.

Regional node-like membrane specializations in non-myelinated axons of rat retinal nerve fiber layer

The axons in the nerve fiber layer (NFL) of the adult rat retina were examined by transmission electron microscopy. NFL axons range in size from 0.12 to about 2.0 microm, with a peak at 0.3-0.4 microm. In addition to conventional small mitochondria in the NFL axons contain some large ones, which are similar to astrocytic gliosomes. Two types of regional axon membrane specialization are found in the NFL. One of these represents portions of the initial axon segments of retinal ganglion cells. Apart from features typical for initial axon segments in general, a corona of lamelliform, villous or blunt glial processes is always present. The glial processes originate from MUller cells. The other regional axon membrane specialization consists of patches of an electron-dense subaxolemmal undercoating with associated tufts of Miller cell processes. These patches cover a varying but always limited proportion of the axon circumference and their longitudinal extent varies between 0.5 and 5.0 microm. They are clearly distinct from the initial axon segment and from the initial heminode in the optic nerve. Similar undercoated patches in the optic disc axons are apposed by astrocytic processes. It is concluded that rat NFL axons represent an example of central non-myelinated axons with distinct regional membrane specializations, which have some structural characteristics in common with nodes of Ranvier.

Lambert-Eaton myasthenic syndrome with ophthalmoparesis and pseudoblepharospasm

We report a patient initially diagnosed as having ocular myasthenia gravis who showed progressive ophthalmoparesis and pseudoblepharospasm together with positive acetylcholine receptor antibodies. Repeated evaluation with high-frequency repetitive stimulation revealed an incremental response and elevated titers of antibodies against presynaptic calcium channels, confirming Lambert-Eaton myasthenic syndrome. Systemic evaluation revealed no malignant neoplasm but revealed euthyroid Hashimoto's disease. Immunomodulative therapy including plasma exchange and administration of an immunosuppressent (azathioprine) combined with a potassium-channel blocker (3,4-diaminopyridine) reduced the ocular abnormalities. We conclude that the ocular manifestations in this patient were probably caused by Lambert-Eaton myasthenic syndrome.

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.

Conduction properties of central demyelinated and remyelinated axons, and their relation to symptom production in demyelinating disorders

The conduction properties of central demyelinated and remyelinated axons are discussed, and related to the expression of symptoms in central demyelinating disease. The mechanisms underlying the block and restoration of conduction in segmentally demyelinated axons are described, together with the range of deficits expressed by the conducting axons. These abnormalities are related to clinical relapses and remissions, and to the phenomena of weakness, fatigue, the temperature sensitivity of symptoms, and the generation of 'positive' symptoms (e.g. Uhthoff's and Lhermitte's symptoms). The potential role of circulating 'blocking factors' in the symptomatology of central demyelinating disease is examined, and some approaches are advanced for the symptomatic therapy of such diseases.

AAEM Minimonograph #10: volume conduction

A volume conductor is any medium with the capability of passively conducting a current between regions of potential difference. The monophasic positive intracellular action potential produces a monophasic negative extracellular waveform and a triphasic extracellular waveform in a poor and good volume conductor, respectively. The observed waveform characteristics are dependent upon both the recording electrode montage and the type of volume conductor surrounding the excitable tissue. The extracellular current flow associated with an action potential can be divided into two current sources flanking a central current sink. If a recording electrode is located over the negative current sink, a negative potential is observed. When the two current sources approach a recording electrode, a positive potential is recorded. If a positive deflection of the baseline is observed, one may conclude that the wave of depolarization under investigation did not originate under, but traveled toward, the recording location. Electric currents from external sources are free to propagate extraneurally as the body is a good volume conductor. Care must be taken to not activate nearby nerves and, subsequently, obtain a waveform contaminated with potentials from undesired sources. Additionally, electrical activity from neighboring muscles and nerves can summate in the volume conductor and yield responses capable of masking pathology. An understanding of the principles of volume conduction theory can help the electrodiagnostician avoid artifactual errors and erroneous conclusions.

Spatial and morphological differentiation of trigger zones in afferent fibers to the teleost utricle

A morphological correlate of the trigger site (the locus of action potential initiation) was identified in afferent axons of the utricle in the ear of two species of teleost fish. These sites were identified by the ferric-ferrocyanide (Prussian blue) cytochemical procedure and they were correlated with the geometries of afferent intraepithelial arbors as visualized by means of a silver stain. The intraepithelial arbors of afferent fibers show regional distributions that correlate with axon diameter and Prussian blue staining. Afferent axons with diameters greater than 4-5 microns only innervate the striola regions of the epithelium and terminate as one of two distinct types of intraepithelial arbors. Afferent axons with diameters smaller than 4 microns are ubiquitously distributed throughout the epithelium. Arbors that stained by Prussian blue within the utricular epithelium are restricted to the striolar regions. These arbors possess nodal-like membrane in different branches as postsynaptic membrane. Afferents that innervate hair cells in the extrastriolar epithelial regions stained with Prussian blue only at the extraepithelial terminal heminode. The postsynaptic membrane of these afferents is passive or dendritic-like.

[Models of pain development in radicular compression.]

The problem of nociception and pain development in radicular pain syndromes is not clarified. In the pathophysiology of pain of radicular compression caused by stenosis or disc prolapse, morphological complex nerve root/ganglion is the key structure. Chronic compression forces on the nerve structure cause structural changes. Structural deterioration is linked with a change in the electrical membrane properties of the affected nerve root. The membrane threshold shift in nociceptive fibers is an important prerequisite for pain perception in nerve root compression. New biochemical aspects in the pathophysiology of radicular syndromes are presented, which could explain the discrepancy between pain and objective clinical findings. The article concludes that a better understanding of the nerve root pathophysiology will bring a more differentiated pain-management strategy.

A quantitative analysis of axon outgrowth, axon loss, and myelination in the rat pyramidal tract

A quantitative analysis of the development of the pyramidal tract (PT) was carried out at the level of the caudal medulla oblongata and at the sixth cervical spinal segment (C6), in rats ranging in age from embryonic day 20 (E20) to the adult of 90 days postnatally (P90). The axon number in the right medullary PT rises from 27,000 axons at E20 to 391,000 axons at P4. Growth cones are abundant during this period, but can still be observed occasionally at P7. After P4, the axon number is reduced by 62%, to 150,000 in the adult. A rapid axon loss until P14 is followed by a gradual axon loss, continuing beyond the third postnatal week. A similar biphasic axon loss was observed in the cervical PT. At P2 and at P7, concentrations of electron-dense material were observed in 0.5-0.7% of the axon profiles in the medullary PT. Since at P21 this feature was only observed in 0.2% of the axons, it might represent an early sign of axon loss. Myelination starts in the medullary PT at P7. Especially during the third postnatal week, the number of myelinated axons increases rapidly. In the adult rat PT, both at medullary and cervical levels, about one third of the axons are still unmyelinated. The results indicate that the development of the rat PT is characterized by a gradual outgrowth of its fibers and by a protracted, biphasic axon loss. Furthermore, comparing the PT at the medulla, at C3, and at C6, a rostrocaudal decrease in axon number was observed during development as well as at the adult stage. Therefore, no evidence was found for increased axon branching in the tract in the cervical intumescence.

An isoform of ankyrin is localized at nodes of Ranvier in myelinated axons of central and peripheral nerves

Two variants of ankyrin have been distinguished in rat brain tissue using antibodies: a broadly distributed isoform (ankyrinB) that represents the major form of ankyrin in brain and another isoform with a restricted distribution (ankyrinR) that shares epitopes with erythrocyte ankyrin. The ankyrinR isoform was localized by immunofluorescence in cryosections of rat spinal cord gray matter and myelinated central and peripheral nerves to: (a) perikarya and initial axonal segments of neuron cells, (b) nodes of Ranvier of myelinated nerve with no detectable labeling in other areas of the myelinated axons, and (c) the axolemma of unmyelinated axons. Immunogold EM on ultrathin cryosections of myelinated nerve showed that ankyrinR was localized on the cytoplasmic face of the axolemma and was restricted to the nodal and, in some cases, paranodal area. The major isoform of ankyrin in brain (ankyrinB) displayed a broad distribution on glial and neuronal cells of the gray matter and a mainly glial distribution in central myelinated axons with no significant labeling on the axolemma. These results show that (a) ankyrin isoforms display a differential distribution on glial and neuronal cells of the nervous tissue; (b) an isoform of ankyrin codistributes with the voltage-dependent sodium channel in both myelinated and unmyelinated nerve fibers. Ankyrin interacts in vitro with the voltage-dependent sodium channel (Srinivasan, Y., L. Elmer, J. Davis, V. Bennett, and K. Angelides. 1988. Nature (Lond.). 333:177-180). A specific interaction of an isoform of ankyrin with the sodium channel thus may play an important role in the morphogenesis and/or maintenance of the node of Ranvier.

Morphological and physiological properties of neurons after long-term axonal regeneration: observations on chronic and delayed sequelae of peripheral nerve injury

Axonal regeneration has been the focus of extensive investigation of mechanisms which mediate structural and functional recovery after injury to mammalian peripheral nerves and has proven to be a valuable model for development and plasticity in the nervous system. Although details of the acute morphological and physiological responses to nerve injury are well-described, less information is available to nerve injury are well-described, less information is available about long-term alterations which persist or develop after regenerated axons have established connections with their targets. The present paper briefly discusses the mammalian neuron's initial response to peripheral nerve injury and subsequent events which occur during regeneration. Morphological and physiological alterations observed in neurons after long-term axonal regeneration are described and are considered in the context of their potential implications for clinical recovery after nerve injury, as well as their potential contribution to the appearance of delayed neurological dysfunction. Selective responses to neuronal injury during development and in different fiber populations are discussed.

Demyelination in spinal cord injury

Morphological and physiological studies demonstrate that demyelination constitutes a significant component of the pathology in compressive spinal cord injury. In many cases of spinal cord injury, a rim of demyelinated axons surrounds a central core of hemorrhagic necrosis. This provides a pathophysiological basis for "discomplete" spinal cord injuries, characterized by apparently complete transection as judged by clinical criteria, but with neurophysiological evidence of conduction through the level of damage. Recovery of conduction in demyelinated axons may permit recovery of function, and can be mediated by several mechanisms, including remyelination by oligodendrocytes or Schwann cells. Alternatively, conduction of action potentials can occur in the absence of remyelination, but this requires plasticity of the demyelinated axon. The biophysics of conduction favors recovery of electrogenesis after demyelination of small diameter axons. This may account, in part, for the observation that functional recovery is more common after demyelination of visual, compared to spinal, axons. Restoration or modification of conduction in demyelinated fibers represents an important strategy for promoting functional recovery in spinal cord injury.

A theoretical analysis of extracellular punctate stimulation around dendrites

The polarization of neuronal trees by external point stimulation is modelled. In one form of model, an almost spheroidal field encloses the dendritic tree. Radially projecting, electrically linear dendrites, along with extracellular medium, are considered to occupy the entire field. The spheroid is modified by a penetrating cone that can surround the stimulating microelectrode; here, and in the rest of the infinite volume outside the field, there is only extracellular medium. A second form of linear electrical model, representing sections of membrane and cytoplasm by means of lumped electrical components commonly known as compartments, is used to validate the field construct. A similar spatial distribution of induced steady-state membrane potential emerges from the two forms of model, for a given morphology and electrophysiology. Compartmental models are also used to demonstrate time-courses of membrane charging. At the soma, if the point source is nearby, charging proves to be essentially complete in less than one time-constant. The soma thresholds under steady-state polarization from different electrode distances are plotted for field models of various electrical space-constant, size and shape of spheroid, and eccentricity of the soma. Characteristic cathodal or anodal thresholds, depending on the particular cell parameters, are revealed for specific electrode trajectories. The range of threshold-distance relations obtained in previously published experiments match those given by the models, when the time-course of charging is taken into account.

Membrane heterogeneity in sensory receptors of rat vibrissae: Merkel receptors differ from other nerve endings

The cupric/ferrocyanide reaction was used to analyse cation-binding sites in rat vibrissae. The heminode of the last myelinated segment had moderate staining. No staining of any complex receptor was found except for Merkel receptors. Both the Merkel cell and associated nerve ending were stained except for their apposed cell membranes, suggesting either reciprocal membrane specialization at that site, or prevention of cupric/ferrocyanide penetration into the space between the two cells.

Unmyelinated and myelinated axon membrane from rat corpus callosum: differences in macromolecular structure

The macromolecular structure of unmyelinated and myelinated internodal axon membrane was examined with freeze-fracture electron microscopy. Unmyelinated axons exhibited a gradient of axonal diameters, generally ranging from 0.1 to 0.5 micron, with some unmyelinated axons of up to 0.7 micron diameter. Myelinated fibers also displayed a range of axonal diameters, with axons generally 0.3-1.0 micron. The overlap in diameters, between unmyelinated and myelinated fibers, permitted a comparison of membrane structure in myelinated and unmyelinated axons of the same diameter. Small (less than 0.5 micron) diameter unmyelinated axons exhibited a moderate density (approximately 700/micron2) of P-face intramembranous particles (IMPs), while large (greater than or equal to 0.5 micron) caliber unmyelinated axons displayed a significantly greater P-face IMP density (approximately 1100/micron2). Internodal membrane of both small (less than 0.5 micron) and large (greater than or equal to 0.5 micron) diameter myelinated fibers exhibited densities of P-face particles (approximately 1400/micron2) that were similar to each other, but significantly different from unmyelinated fibers. These results demonstrate that there are differences in membrane structure between unmyelinated and myelinated axons of similar diameter. These findings also demonstrate that membrane structure of unmyelinated axons is not invariant for all unmyelinated fibers within a given CNS tract but, on the contrary, is related to diameter.

Sensitivity to 4-aminopyridine observed in mammalian sciatic nerves during acute pyridoxine-induced sensory neuropathy and recovery

In vitro physiologic properties of rat sciatic nerve were examined during pyridoxine-induced sensory neuropathy and following recovery. Compound action potential waveform characteristics, recovery cycle properties during paired stimulation, and ability to follow stimuli presented at high frequencies were examined. All parameters were assessed before and after potassium channel blockade with 4-aminopyridine in nerves from acutely toxic, recovered, and age-matched control rats. Compound action potential waveforms recorded from acutely toxic nerves were less affected by application of 4-aminopyridine than were those recorded from control nerves. Recovery cycles were less prolonged and frequency-following abilities less compromised a 3-month recovery period, experimental nerves demonstrated a more pronounced sensitivity to 4-aminopyridine than did age-matched control nerves. It is proposed that these differences in the physiological effects of 4-aminopyridine reflect the selective loss of large-caliber sensory fibers during the acute toxic phase and the increased sensitivity to the drug of regenerated sensory fibers following recovery.

The role of ultrastructural investigations in neurotoxicology

The ways in which ultrastructural approaches have been applied to the investigation of xenobiotic-induced toxicity of the nervous system have been briefly reviewed. These approaches have been grouped in 3 broad areas, viz. morphology, function and composition. Firstly, morphological approaches permit the visualisation of changes in intercellular relationships, the identification of the subcellular target(s) of a xenobiotic substance and the discrimination between what may appear ostensibly to be identical cellular responses to one or more chemically distinct toxins. Secondly, functional approaches using, e.g. cytochemistry, ion precipitation, immunocytochemistry and autoradiography provide indications of metabolic state, the identity or the intra- or extracellular location of the "reactive species". Thirdly, those approaches, viz. electronprobe X-ray microanalysis and electron energy loss spectroscopy which provide information of the elemental composition of cells and tissues permit an assessment of the subcellular distribution and compartmentalisation of endogenous substances and toxic or therapeutic xenobiotics. In concert, ultrastructural approaches possess the ability to contribute unique information on the effects of exposure of cells of the nervous system to toxic substances and so direct further investigation towards an understanding of the mechanism of action.

Cation-binding sites in trigeminal ganglia and maxillary nerve: unusual reactivity of perikarya, stem axons and satellite cells

We have used the cupric/ferrocyanide reaction to study cation-binding in trigeminal ganglia and maxillary nerve of adult rats. Unmyelinated axons did not react, whereas myelinated axons were stained at nodal, paranodal or cleft sites. At 'nodal' sites, metallic deposits were found in the axoplasm, along the axolemma, and at the extracellular interfaces of the paranodal myelin. At 'paranodal' sites, particles were concentrated in the paranodal axoplasm and in the intracellular spaces of the myelin loops. Most maxillary axons examined at successive sites had all nodal or all paranodal staining, but 13 of 51 had a mixture. In trigeminal ganglia there was no staining of perineurial sheath, endoneurial cells or mast cells. Satellite cells and their basal laminae were prominently stained, with those around small neurons more reactive than those of large neurons. Patches of neuronal membrane on cell bodies were stained, more often for small than large neurons. The axon hillock and proximal stem axon were not stained in some cases, but approximately half the neurons had staining of perikaryal cytoplasm at the axon hillock or a dense asymmetric band in the proximal stem axon. Strong intraaxonal staining was found at the junction between unmyelinated proximal and myelinated distal stem axon. In distal stem axons, staining was found at the first myelin segment and at each successively thicker myelin segment; staining was mostly weak and paranodal, with intensity proportional to myelin thickness. The T-junction between stem and main myelinated axon had nodal or paranodal patterns; unmyelinated T-junctions were not stained. The varied cation-binding patterns in trigeminal ganglia show unusual properties of satellite cells and important differences between stem and main axons. The results that the cell membrane of axon hillock and proximal stem regions of many sensory large and small neurons may have numerous sodium channels and could affect signal propagation.

Freeze-fracture studies on unmyelinated axolemma of rat cervical sympathetic trunk: correlation with saxitoxin binding

The density and diameter distributions of intramembranous particles (IMPs) within unmyelinated axolemma from rat cervical sympathetic trunk were examined with freeze-fracture electron microscopy. The axolemma displays a highly asymmetrical partitioning of IMPs with ca. 1200 IMPs microns-2 on P-faces and ca. 100 IMPs microns-2 on E-faces. Particle sizes (diameters) are unimodally distributed on both fracture faces, with a range from 2.4 nm to 15.6 nm. Approximately 16% of the particles on P-faces and 28% of particles on E-faces are of a large (greater than 9.6 nm) diameter. On both fracture faces, the IMPs appear to be randomly distributed; no aggregations of particles were observed. The results indicate that there are ca. 230 large IMPs microns-2 of unmyelinated axolemma from rat cervical sympathetic trunk. The density of these IMPs is similar to the density of saxitoxin binding sites on unmyelinated axolemma from rat cervical sympathetic trunk (Pellegrino et al. 1984 (Brain Res. 305, 357-360)), which suggests that many of the large diameter particles may be the morphological correlate of voltage-sensitive Na+ channels.

Ultrastructural cytochemical localization of 5'-nucleotidase activity in axon-myelin-Schwann cell complex

5'-Nucleotidase activity has been localized at the ultrastructural level in the axon-myelin-Schwann cell complex. Sciatic nerves of rabbits of pre- and postnatal development were used. Positive reaction was found on the plasma membrane, basal lamina, cytoplasm, and finger-like processes of the Schwann cells; on the intraperiod lines of the compact myelin, on the surface of myelin sheath, in the split myelin lamellae in the paranodal regions and Schmidt-Lanterman clefts, in segments of outermost and innermost lamellae, split off from the interparanodal myelin, in the mesaxons (outer and inner), in the loose myelin lamellae in the earlier stages of myelinization; on the axolemma (especially in the nodal and paranodal segments), in the periaxonal space, axoplasm. The alterations of 5'-nucleotidase distribution were associated with the developing myelin sheath.

Cytoplasmic membrane elaborations in oligodendrocytes during myelination of spinal motoneuron axons

The ultrastructure of paranodal oligodendroglial cytoplasm, which is located in proximity to the forming myelin sheath, was studied during maturation of spinal motoneuron axons in rat. At 8 days postnatal, the paranodal oligodendroglial loops contain a network of membrane-bound tubulovesicular elements. These membrane elaborations are most common in oligodendroglial loops attached to the outermost layers of the myelin sheath, i.e., paranodal loops closest to the nodal gap. The number of oligodendroglial cytoplasmic profiles per paranodal loop falls over the course of five to ten sequential paranodal loops, and these profiles are nearly absent in paranodal oligodendroglial cytoplasm located distant from the nodal gap. Oligodendrocytes in spinal cords of 14- and 20-day-old rats and of adult rats did not exhibit networks of tubulovesicular profiles. The appearance of these membrane organelles within oligodendroglial cytoplasm during myelin maturation suggests increased membrane turnover within paranodal cytoplasm located adjacent to the axon that is being myelinated. Membrane turnover within oligodendrocytes may reflect axonal modulation of glial function during myelination.

Physiological properties of regenerated rat sciatic nerve following lesions at different postnatal ages

Electrophysiological properties of regenerated sciatic nerves were examined in vitro following sciatic crush lesions performed on rats at 1 week to 4 months of age. Pharmacological blockade of potassium channels with 4-aminopyridine (4-AP) in control nerves resulted in only minimal changes in the waveform of whole nerve responses, but was associated with slight prolongation of relative refractory periods and compromised frequency-following abilities at all ages examined. A more marked sensitivity to application of 4-AP was observed in all regenerated nerves compared to control nerves. This sensitivity was characterized by the development of a prominent delayed negativity of the compound action potential. Specific features of the waveform alterations differed for nerves crushed before age 3 weeks compared to those injured at the older postnatal ages. Alteration of refractory properties and frequency-following abilities of regenerated nerves following 4-AP were also more pronounced than in control nerves with the most marked disruption being observed in nerves from animals lesioned at the older ages. These data suggest that the process of regeneration is modified by ongoing maturation at the time of crush.

Voltage-clamp analysis of nodes of Ranvier in regenerated rat sciatic nerve

Crush-lesioned adult rat sciatic nerves were allowed to regenerate during 70-181 days. Single large regenerated nerve fibres were isolated from levels distal to the crush site, and mounted in a feedback system for potential recordings and voltage clamp. After the experiment, each isolated fibre was fixed and embedded for morphological evaluation. Internodal lengths and fibre diameters were obtained form teased preparations of regenerated and normal sciatic nerves. The isolated regenerated fibres were excitable and had action potentials of large amplitude. TEA and 4-AP had negligible effects on the action potential and excitability properties. The time constant of the nodal segment was larger than in normal fibres. The nodal Na and K permeabilities (PNa and PK) were calculated both in absolute values and relative to the leak conductance (gL). Regenerated fibres had normal PNa/gL and PK/gL ratios. The potential dependence of PNa activation and inactivation was also normal. The isolated physiologically examined fibres had internodal lengths (L) of about 300 micron and diameters (D) of about 7-11 micron. In the teased fibre preparations L was about 300 micron (range 150-600 micron) and D was 3-11 micron. In addition, all teased regenerated nerve preparations exhibited a few scattered unusually short internodes (L less than 150 micron). In control nerves L ranged from 150 micron in the lower fibre size range (D = 2-3 micron) to 1.6 mm in the upper size range (D = 15 micron).

Cytochemical characteristics of the axon membrane at nodes of Ranvier in resting, tetrodotoxin-blocked, and electrically stimulated peripheral nerves of the rat

To test whether intraaxonal ferric ion-ferrocyanide staining at nodes of Ranvier is influenced by functional state of the nodal membrane, the tibial nerves of Sprague-Dawley rats were subjected to one of the following experimental procedures prior to fixation and staining: General anesthesia to induce nerves at rest, tetrodotoxin blocking of nerve activity, and high-frequency (100 Hz) electrical stimulation. In all cases most but not all nodes were stained. No statistically significant differences were found either in the percentage of total stained nodes or in the percentage of stained nodes from large- and small-diameter fibers among the three conditions tested. An explanation is offered to account for the apparent discrepancy between these results and those from other studies involving related cytochemical markers of the nodal apparatus.

Astrocytes play a role in regulation of synaptic density

Exposure of neonatal cerebellar explants to cytosine arabinoside destroys granule cells and arrests surviving glia in an early stage of maturation. Purkinje cells lack astroglial ensheathment and are hyperinnervated by sprouted Purkinje cell recurrent axon collateral terminals. Such granuloprival cultures were transplanted with optic nerve in order to supply mature glial cells. It was observed that not only were Purkinje cells almost completely ensheathed by astroglia, but there was a greater than 60% reduction in the number of somatic synapses compared to the non-transplanted granuloprival cultures. This astroglial ensheathment, which may be neuronally directed, could be the physical element provoking the reduction in the number of synapses.

Specific modulation of sodium channels in mammalian nerve by monoclonal antibodies

Monoclonal antibodies (mAbs) were generated against the sodium channels in the intact membrane of the eel electroplax. These antibodies bind to nodes of Ranvier, as indicated by immunofluorescence. When externally applied to rat nerve fibers one of these mAbs blocks impulse conduction. In voltage-clamp experiments, this mAb was found to attenuate sodium current amplitude without affecting the time course. The dose-response curve was very steep and had an ED50 of 133 nM. About half of the mAb effect was shown to be due to a shift, in the hyperpolarizing direction, of the steady-state sodium inactivation versus membrane potential curve. The remaining effect was voltage- and time-independent. This mAb had no effect on the potassium or leakage currents. The results indicate that on the external surface of the sodium channel, there are a number of antigenically similar determinants, which are functionally linked to specific elements of the sodium conductance system. These functionally related determinants were preserved through the course of evolution.

Mammalian optic nerve fibers display two pharmacologically distinct potassium channels

A suction electrode recording technique was used to study action potential characteristics of rat optic nerve fibers. Two pharmacologically distinct potassium channels are described. One is sensitive to 4-aminopyridine (4-AP) and the other to tetraethylammonium (TEA). 4-AP application leads to a substantial broadening of the optic nerve action potential, but TEA does not. 4-AP application also elicits a TEA-sensitive post-spike positivity, i.e. an intracellular hyperpolarization. From these results we suggest that the 4-AP-sensitive channel, not the TEA-sensitive channel, is primarily responsible for action potential repolarization of mammalian optic nerve fibers.

Neuronal cell adhesion molecules and cytotactin are colocalized at the node of Ranvier

Immunocytochemical methods were used to show that Ng-CAM (the neuron-glia cell adhesion molecule), N-CAM (the neural cell adhesion molecule), and the extracellular matrix protein cytotactin are highly concentrated at nodes of Ranvier of the adult chicken and mouse. In contrast, unmyelinated axonal fibers were uniformly stained by specific antibodies to both CAMs but not by antibodies to cytotactin. Ultrastructural immunogold techniques indicated that both N-CAM and Ng-CAM were enriched in the nodal axoplasm and axolemma of myelinated fibers as well as within the nodal regions of the myelinating Schwann cell. At embryonic day 14, before myelination had occurred, small-caliber fibers of chick embryos showed periodic coincident accumulations of the two CAMs but not of cytotactin, with faint labeling in the axonal regions between accumulations. Cytotactin was found on Schwann cells and in connective tissue. By embryonic day 18, nodal accumulations of CAMs were first observed in a few medium- and large-caliber fibers. Immunoblot analyses indicated that embryonic to adult conversion of N-CAM and a progressive decrease in the amount of Ng-CAM and N-CAM occurred while nodes were forming. Sciatic nerves of mouse mutants with defects in cell interactions showed abnormalities in the distribution patterns and amount of Ng-CAM, N-CAM, and cytotactin that were consistent with the known morphological nodal disorders. In trembler (+/Tr), intense staining for both CAMs appeared all along the fibers and the amounts of N-CAM in the sciatic nerve were found to be increased. In mice with motor endplate disease (med/med), Ng-CAM and N-CAM, but not cytotactin, were localized in the widened nodes. Both trembler and med/med Schwann cells stained intensely for cytotactin, in contrast to normal Schwann cells which stained only slightly. All of these findings are consistent with the hypothesis that surface modulation of neuronal CAMs mediated by signals shared between neurons and glia may be necessary for establishing and maintaining the nodes of Ranvier.

Relationship between ethanol-induced alterations in fluorescence anisotropy and adenylate cyclase activity

The effects of butanol, ethanol, and ketamine on adenylate cyclase activity and fluorescence anisotropy were determined in membranes prepared from L6 cells. The experiments were designed to test the hypothesis that the effects of ethanol on adenylate cyclase activity are a consequence of ethanol-induced changes in bulk membrane order. Butanol and ethanol elicited concentration-dependent increases in adenylate cyclase activity and caused decreases in the fluorescence anisotropy of diphenylhexatriene. Butanol was more potent than ethanol in reducing fluorescence anisotropy, and it elicited a greater reduction in fluorescence anisotropy than did ethanol. Butanol was also more potent than ethanol in activating adenylate cyclase, but the highest concentration of butanol used caused a smaller increase in enzyme activity than did the highest concentration of ethanol. When the percent change in adenylate cyclase activity was plotted against the percent change in fluorescence anisotropy at each concentration of alcohol, the increase in isoproterenol-stimulated adenylate cyclase activity per unit change in fluorescence polarization was greater with ethanol than with butanol. Ketamine decreased fluorescence anisotropy but, unlike the alcohols, ketamine caused a decrease in adenylate cyclase activity. A reduction in assay temperature attenuated both the ethanol-induced activation of adenylate cyclase activity and the ethanol-induced reduction in fluorescence anisotropy. Although the data are consistent with the theory that ethanol acts upon a hydrophobic region of the membrane to enhance adenylate cyclase activity, activation of the enzyme does not appear to be a consequence of a decrease in bulk membrane order.

Lithium distribution in the brain of normal mice and of "quaking" dysmyelinating mutants

Using nuclear reaction 6Li(n, alpha)3H and dielectric detectors, we have studied the distribution of Li in the brain of adult mice, following Li treatment of the animals. Two strains of animals were used in parallel: "quaking" dysmyelinating mutants and normally myelinated controls. The distribution appeared to be sharply regionalized in the brain of the normal mice (higher Li concentration in the gray rather than in the white matter, with the area postrema being particularly Li rich). In contrast, the Li distribution was practically homogeneous in the brain of the quaking dysmyelinating mutants, with a mean Li concentration comparable to that in the gray matter of the controls. The present method of Li detection has made it possible to estimate the Li equilibrium potentials (nerve cells with regard to plasma) in the different brain substructures. The results are consistent with (a) Li being actively extruded from nerve cells in all the cases and (b) myelination decreasing the relative importance of the passive component of Li transport in the nerve cells, as compared with the active component.

Molecular specialization of astrocyte processes at nodes of Ranvier in rat optic nerve

The HNK-1 and L2 monoclonal antibodies are thought to recognize identical or closely associated carbohydrate epitopes on a family of neural plasma membrane glycoproteins, including myelin-associated glycoprotein, the neural cell adhesion molecule, and the L1 and J1 glycoproteins, all of which have been postulated to play a part in mediating cell-cell interactions in the nervous system. We have used these two antibodies in immunofluorescence and immunogold-electron microscopic studies of semithin and ultrathin frozen sections of adult rat optic nerve, respectively, and we show that they bind mainly to astrocyte processes around nodes of Ranvier. Most other elements of the nerve, including astrocyte cell bodies and large astrocytic processes, are not labeled by the antibodies. To our knowledge, this is the first demonstration that perinodal astrocyte processes are biochemically specialized. We provide evidence that one of the HNK-1+/L2+ molecules concentrated around perinodal astrocyte processes is the J1 glycoprotein; our findings, taken together with previously reported observations, suggest that the other known HNK-1+/L2+ molecules are not concentrated on these processes. Since anti-J1 antibodies previously have been shown to inhibit neuron to astrocyte adhesion in vitro, we hypothesize that J1 may play an important part in the axon-glial interactions that presumably are involved in the assembly and/or maintenance of nodes of Ranvier.

Detection of sodium channel distribution in rat sciatic nerve following lysophosphatidylcholine-induced demyelination

In vivo application of lysophosphatidylcholine (LPC) to rat sciatic nerve induces impaired hind leg movement within 2 days which is recovered by 6 days. Segmental demyelination was seen at 2 days after LPC application, and remyelination had barely started in a few axons by 6 days. Using sodium channel-specific monoclonal antibodies and immunofluorescence microscopy, we observed altered distribution of sodium channels in demyelinated axons. Bright fluorescent labeling was found along the segmentally demyelinated axolemma at 6 days in contrast to the dim staining of the demyelinated nerve found at 2 days. In addition, radioimmunoassays detected an elevated number of antibody binding sites on sciatic nerve trunk from the sixth day. Our data provide the immunocytochemical evidence for the assumption that recruitment of sodium channels into demyelinated axolemma contributes to the recovery of function following axon demyelination by LPC.

Localizations of ruthenium red positive material in rabbit peripheral nerves

The penetration and distribution of ruthenium red in the axon-myelin-Schwann cell complex of developing rabbit peripheral nerve fibers are investigated. Ruthenium red positive material is established in the axoplasm, axolemma, periaxonal space, major dense lines and intraperiod lines of the compact myelin, mesaxons, split peripheral myelin lamellae, Schmidt-Lanterman and longitudinal incisures, paranodal loops and axo-glial contacts, Schwann cell cytoplasm and basal lamina, nodal extracellular matrix, desmosome-like structures, endoneural collagen. Some features of the distribution of the contrast material in the developing myelin sheath are described. Regional differences of the axolemma and of the Schwann cell cytoplasm and plasmalemma are established. The prevalence of glycoproteins or glycolipids in the ruthenium red stained material in its different localizations is discussed on the basis of trypsin and hyaluronidase digestion performed.

Sodium channel localization in rat sciatic nerve following lead-induced demyelination

Daily intraperitoneal injections of lead acetate for several weeks were followed by a peripheral neuropathy. The conduction of impulses in the sciatic nerve became slower, but their amplitude, duration and threshold remained normal. Sodium channel labeling with specific monoclonal antibodies revealed staining at demyelinated regions, while normal axons were stained exclusively at nodes of Ranvier. These results support the view that remodelling of sodium channel distribution may contribute to impulse conduction in demyelinated fibers.

Electrical activity of mouse motor endings during muscle reinnervation

An in vitro study of electrical activity of regenerating motor endings was performed 11-15 days after the crushing of one motor nerve supplying the triangularis sterni muscle in the adult mouse. For this purpose, presynaptic membrane currents elicited by electrical stimulation of the regenerating nerve were recorded by external electrodes. Ionic channel distribution along the length of the endings was deduced from wave form configuration in normal perfusing fluid together with changes produced by application of specific channel blocking agents. The sharp negative deflection which was shown to correspond to inward Na+ current by its sensitivity to tetrodotoxin application could be recorded along most of the length of the endings indicating a widespread distribution of Na channels. Frequent absence of the late wave form component which signals K+ current was taken to indicate an even K+ current density in the few last nodes, the heminode and the distal part of the endings. Therefore, it appears that regenerating motor endings are characterized by an overlap of Na and K conductances all along their length. In the course of regeneration, the heminode loses the sensitivity to K channel blocking agents and the remainder of the terminal becomes insensitive to tetrodotoxin, the former change occurring usually earlier than the latter.

Plasma membrane structure at the axon hillock, initial segment and cell body of frog dorsal root ganglion cells

Analysis of the plasmalemma of frog dorsal root ganglion cells by freeze-fracture demonstrates regional differences in the distribution of intramembranous particles. Although P-face particles are distributed rather uniformly, the E-face particle concentration at the cell body (approximately 300 micron -2) is much lower than that at the axon hillock (approximately 900 micron -2), proximal initial segment (approximately 1000 micron -2), or intermediate portion of the initial segment (approximately 800 micron -2). The particle concentrations in the latter regions approach that at the node of Ranvier and, moreover, particle size analysis reveals that the E-face particles, like those at the node, include a large number that are 10 nm or more in diameter. Thin sections reveal patches of a dense undercoating on the cytoplasmic surface of the axolemma in some regions of the initial segment but not the axon hillock. It is concluded from these results that the axon hillock and the initial segment of dorsal root ganglion cells have some of the structural characteristics of the node of Ranvier.

Membrane structure of vesiculotubular complexes in developing axons in rat optic nerve: freeze-fracture evidence for sequential membrane assembly

Intra-axonal vesiculotubular complexes, located within developing axons in the optic nerve of eight-day-old rats, were examined by freeze-fracture electron microscopy. The clusters usually fill most of the cross section of the axon and extend for approximately 1 micron along the fibre axis. As seen in freeze-fracture, the E- and P-faces of the membranes comprising these clusters exhibit a paucity of intramembranous particles (i.m.ps). This i.m.p.-poor membrane structure is different from that of the axolemma per se, which contains i.m.p. densities of ca. 120 micron-2 on the E-face and ca. 400 micron-2 on the P-face. Since earlier studies indicate that the vesiculotubular complexes fuse with the axon membrane so as to contribute to membrane growth, it is suggested that axonal differentiation involves a sequential mode of membrane development, in which an initial growth of a relatively undifferentiated membrane bilayer is followed by in situ insertion of specialized proteins within specific membrane domains.

Demyelination-induced plasticity in the axon membrane: an ultrastructural cytochemical study of lead neuropathy in the rat

We examined the distribution of ferric ion-ferrocyanide stain (a marker for excitable regions of myelinated fibers) in the lead-induced demyelinating neuropathy of the rat. By electron microscopy, we found that paranodal degeneration resulted in spreading of the reaction product from nodal to internodal axolemma. During repair, nodal-like stained areas formed at the contact zones between preremyelinating Schwann cells. These data suggest that the location and extent of excitable axonal regions are influenced by axoglial relationships. Additionally, some fibers displayed staining at paranodal axolemma adjacent to demyelinated segments, suggesting it might be an alternative site for impulse generation in demyelinated fibers.

Organization of ion channels in the myelinated nerve fiber

The functional organization of the mammalian myelinated nerve fiber is complex and elegant. In contrast to nonmyelinated axons, whose membranes have a relatively uniform structure, the mammalian myelinated axon exhibits a high degree of regional specialization that extends to the location of voltage-dependent ion channels within the axon membrane. Sodium and potassium channels are segregated into complementary membrane domains, with a distribution reflecting that of the overlying Schwann or glial cells. This complexity of organization has important implications for physiology and pathophysiology, particularly with respect to the development of myelinated fibers.

Cytochemical evidence for an inhomogeneous distribution of sodium channels in the squid giant axon

In a light microscopic study the ferric ion-ferrocyanide staining of the squid giant axon was investigated. In other neurons this stain is known to indicate excitable membranes with high sodium channel density, such as the node of Ranvier or the initial segment. In the squid the stain reacted in small patches of the axolemmal, suggesting an aggregation of sodium channels. These axolemma patches were always adjacent to the nuclei of special Schwann cells, suggesting a glial-axonal interaction.

Differences between mammalian ventral and dorsal spinal roots in response to blockade of potassium channels during maturation

Differences in potassium channel organization between motor and sensory fibres have been described in amphibians but have not previously been examined in mammals. In the present investigation, we studied whole nerve and single axon responses following pharmacological blockade of potassium conductance in rat ventral and dorsal spinal roots during maturation. Our results indicate a differential sensitivity in maturing mammalian motor and sensory fibres which is most apparent in younger roots. Specifically, application of 4-aminopyridine (4-AP) results in a broadening of the compound action potential in ventral roots which is associated with a delayed repolarization of the individual action potential of single fibres. In contrast, blockade of potassium channels in young dorsal roots results in a late negativity in the compound response which is correlated with multispike bursting activity recorded from single sensory fibres. The effects of 4-AP on ventral root fibres diminish earlier in the course of maturation than do the effects of 4-AP in dorsal root fibres. These results demonstrate developmental differences in the functional organization of potassium channels in mammalian motor and sensory axons which may have implications for differences in coding properties between these two classes of axons.

Computer simulation of action potentials and afterpotentials in mammalian myelinated axons: the case for a lower resistance myelin sheath

Depolarizing afterpotentials, recorded in peripheral nerves [Barrett and Barrett (1982) J. Physiol., Lond. 323, 117-144] and spinal axons [Blight and Someya (1985) Neuroscience 15, 1-12], have been interpreted as representing passive discharge of axolemmal capacitance. This interpretation requires a lower resistance pathway through the myelin sheath than previous measurements have suggested. A computer model was used to examine the contribution of the electrical characteristics of nerve fibers to action potential conduction and afterpotential generation. The model consisted of a resistance-capacitance network representing a chain of 20 internodes. The resistances of node, internode and myelin sheath, deduced from observations in the accompanying paper, [Blight and Someya (1985) Neuroscience, 15] were found to produce suitable length and time constants, and prolonged afterpotentials, when inserted into the model. Similar length and time constants were found using a conventional model of the axon, based on measurements from isolated peripheral fibers, but this did not reproduce the afterpotentials. Action-potential conduction velocity is enhanced by reducing the time constant and increasing the length constant. The problem of minimizing the internodal time constant was met in the conventional model through the low parallel resistance of the node, while in the new model it was met by reducing the resistance of the myelin sheath. The latter strategy required the nodal leakage resistance to be higher than values from single fiber measurements (ca 250 M omega rather than ca 50 M omega) in order to maintain the length constant similar to the conventional model. Simulation of the recorded potentials required the resistance of the myelin lamellae to be approx. 100 omega cm2. The model quantitatively reproduced the voltage response of the axon to injected current pulses and to propagated action potentials, using Frankenhaeuser-Huxley kinetics. [Frankenhaeuser and Huxley (1964) J. Physiol., Lond. 171, 302-315; Frankenhaeuser and Moore (1963) J. Physiol., Lond. 169, 431-437]. The short duration components of the afterpotential, observed in mammalian recordings were reproduced by assuming a leakage pathway in the myelin sheath, at the impalement site. The calculated lower resistance of the myelin sheath was such that it minimized the effective internodal time constant for a given nodal resistance. This appears to free the myelinated fiber from the alternative requirement for a high nodal leakage conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane ultrastructure of developing axons in glial cell deficient rat spinal cord

In order to investigate axolemmal development in a glial cell deficient environment, normal and irradiated dorsal funiculus in rat lumbosacral spinal cord was examined by freeze-fracture electron microscopy. At 3 days of age, normal fibres are all unmyelinated and of small (less than 0.5 micron) diameter. The unmyelinated axons have a moderate density (approximately 850 microns-2) of intramembranous particles (IMPs) on P-fracture faces and a low IMP density (approximately 300 microns-2) on E-faces. IMPs are homogeneously distributed along both fracture faces. By 19 days of age, the normal dorsal funiculus is well populated with myelinated axons and glial cells, as well as a sizable population of unmyelinated fibres. Nearly all of the myelinated fibres have a large (greater than 1.0 micron) diameter; whereas, most unmyelinated axons are of small (less than 0.5 micron) calibre. The axolemma of unmyelinated axons is relatively undifferentiated, with an asymmetrical distribution of IMPs (P-face: approximately 1100 microns-2; E-face: approximately 450 microns-2). Myelinated fibres show nodal and paranodal regions with P-face and E-face ultrastructure similar to previous descriptions. Internodal axolemma appears relatively homogeneous, with P-faces being highly particulate (approximately 2100 microns-2) and a low IMP density (approximately 200 microns-2) on E-faces. Following irradiation of the lumbosacral spinal cord at 3 days of age, there is a severe reduction in the number of glial cells and myelinated fibres in this region when the tissue is examined at 19 days of age. Despite the deficiency of glial cells in this tissue, axonal and axolemmal development continue. Numerous large (greater than 1.0 micron) diameter axons are present in this irradiated tissue. Large diameter axons show a high (approximately 2000 microns-2) density of IMPs on P-faces; E-face IMP density remains at approximately 440 micron-2. Small calibre axons also have an asymmetrical distribution of particles (P-face: approximately 1100 microns-2; E-face: 280 microns-2). The axolemmal E-faces of some glial cell deprived fibres exhibit regions with greater than normal (approximately 750 microns-2) density of IMPs. These results demonstrate that some aspects of axonal and axolemmal development continue in a glial cell deficient environment, and it is suggested that axolemmal ultrastructure is, at least in part, independent of glial cell association.

Ligature-induced injury in peripheral nerve: electrophysiological observations on changes in action potential characteristics following blockade of potassium conductance

The effects of the potassium channel blocking agent 4-aminopyridine (4-AP) on action potential properties were studied in chronically injured rat sciatic nerves. In normal, mature myelinated fibers, application of 4-AP does not lead to any significant change in action potential waveform or firing pattern in response to single stimuli. In contrast, application of 4-AP to nerves injured by the placement of loose ligatures results in the appearance of late rippled components in the compound action potential. This alteration in waveform is present at the injury site, but not at nerve segments proximal or distal to this region. Paired stimulation experiments demonstrate that this oscillation of the whole nerve response reflects repetitive firing in response to single stimuli following application of 4-AP. Intra-axonal recording following 4-AP application demonstrates bursts of action potentials, with several spikes of reduced amplitude arising from a depolarizing potential following the initial spike. Refractory period for the late spike is greater than that of the primary action potential. These results demonstrate that potassium channels are present and functional in chronically injured nerves, where blockage of these channels results in repetitive firing in response to single stimuli.

Myelinated non-axonal neuronal elements in the feline olfactory bulb lack sites with a nodal structural differentiation

Myelinated dendrites in the external plexiform layer (EPL) of the feline olfactory bulb and myelinated axons in the lateral olfactory tract (LOT), were examined by transmission electron microscopy. The results show that the non-axonal myelin sheaths are extremely thin and short and that the number of myelin lamellae does not increase with increasing dendritic diameter. In myelinated LOT axons the sheaths tend to be thicker and the myelin lamellar number increases with axon diameter. Domains with node-like structural characteristics are not encountered along myelinated dendrites, neither between successive myelin sheaths nor where single sheaths terminate. The partly myelinated neuronal perikarya, which occur in the EPL, also lack node-like domains. In contrast, typical nodes are easily found in myelinated LOT axons. In the periglomerular region dendrites and neuronal perikarya are surrounded by non-compacted glial sheets. It is concluded that myelination and node formation are relatively independent events and that morphogenetic glial-neuronal interactions may give different results in different parts of the same neuron.