Distortions of the nodes of Ranvier from axonal distension by filamentous masses in hexacarbon intoxication

A mechanism for neurofilament transport acceleration through nodes of Ranvier

Neurofilaments are abundant space-filling cytoskeletal polymers in axons that are transported along microtubule tracks. Neurofilament transport is accelerated at nodes of Ranvier, where axons are locally constricted. Strikingly, these constrictions are accompanied by sharp decreases in neurofilament number, no decreases in microtubule number, and increases in the packing density of these polymers, which collectively bring nodal neurofilaments closer to their microtubule tracks. We hypothesize that this leads to an increase in the proportion of time that the filaments spend moving and that this can explain the local acceleration. To test this, we developed a stochastic model of neurofilament transport that tracks their number, kinetic state, and proximity to nearby microtubules in space and time. The model assumes that the probability of a neurofilament moving is dependent on its distance from the nearest available microtubule track. Taking into account experimentally reported numbers and densities for neurofilaments and microtubules in nodes and internodes, we show that the model is sufficient to explain the local acceleration of neurofilaments within nodes of Ranvier. This suggests that proximity to microtubule tracks may be a key regulator of neurofilament transport in axons, which has implications for the mechanism of neurofilament accumulation in development and disease.

Local Acceleration of Neurofilament Transport at Nodes of Ranvier

Myelinated axons are constricted at nodes of Ranvier. These constrictions are important physiologically because they increase the speed of saltatory nerve conduction, but they also represent potential bottlenecks for the movement of axonally transported cargoes. One type of cargo are neurofilaments, which are abundant space-filling cytoskeletal polymers that function to increase axon caliber. Neurofilaments move bidirectionally along axons, alternating between rapid movements and prolonged pauses. Strikingly, axon constriction at nodes is accompanied by a reduction in neurofilament number that can be as much as 10-fold in the largest axons. To investigate how neurofilaments navigate these constrictions, we developed a transgenic mouse strain that expresses a photoactivatable fluorescent neurofilament protein in neurons. We used the pulse-escape fluorescence photoactivation technique to analyze neurofilament transport in mature myelinated axons of tibial nerves from male and female mice of this strain ex vivo Fluorescent neurofilaments departed the activated region more rapidly in nodes than in flanking internodes, indicating that neurofilament transport is faster in nodes. By computational modeling, we showed that this nodal acceleration can be explained largely by a local increase in the duty cycle of neurofilament transport (i.e., the proportion of the time that the neurofilaments spend moving). We propose that this transient acceleration functions to maintain a constant neurofilament flux across nodal constrictions, much as the current increases where a river narrows its banks. In this way, neurofilaments are prevented from piling up in the flanking internodes, ensuring a stable neurofilament distribution and uniform axonal morphology across these physiologically important axonal domains.SIGNIFICANCE STATEMENT Myelinated axons are constricted at nodes of Ranvier, resulting in a marked local decrease in neurofilament number. These constrictions are important physiologically because they increase the efficiency of saltatory nerve conduction, but they also represent potential bottlenecks for the axonal transport of neurofilaments, which move along axons in a rapid intermittent manner. Imaging of neurofilament transport in mature myelinated axons ex vivo reveals that neurofilament polymers navigate these nodal axonal constrictions by accelerating transiently, much as the current increases where a river narrows its banks. This local acceleration is necessary to ensure a stable axonal morphology across nodal constrictions, which may explain the vulnerability of nodes of Ranvier to neurofilament accumulations in animal models of neurotoxic neuropathies and neurodegenerative diseases.

Pathological classification of equine recurrent laryngeal neuropathy

Recurrent Laryngeal Neuropathy (RLN) is a highly prevalent and predominantly left-sided, degenerative disorder of the recurrent laryngeal nerves (RLn) of tall horses, that causes inspiratory stridor at exercise because of intrinsic laryngeal muscle paresis. The associated laryngeal dysfunction and exercise intolerance in athletic horses commonly leads to surgical intervention, retirement or euthanasia with associated financial and welfare implications. Despite speculation, there is a lack of consensus and conflicting evidence supporting the primary classification of RLN, as either a distal ("dying back") axonopathy or as a primary myelinopathy and as either a (bilateral) mononeuropathy or a polyneuropathy; this uncertainty hinders etiological and pathophysiological research. In this review, we discuss the neuropathological changes and electrophysiological deficits reported in the RLn of affected horses, and the evidence for correct classification of the disorder. In so doing, we summarize and reveal the limitations of much historical research on RLN and propose future directions that might best help identify the etiology and pathophysiology of this enigmatic disorder.

Acute demyelination disrupts the molecular organization of peripheral nervous system nodes

Intraneurally injected lysolecithin causes both segmental and paranodal demyelination. In demyelinated internodes, axonal components of nodes fragment and disappear, glial and axonal paranodal and juxtaparanodal proteins no longer cluster, and axonal Kv1.1/Kv1.2 K+ channels move from the juxtaparanodal region to appose the remaining heminodes. In paranodal demyelination, a gap separates two distinct heminodes, each of which contains the molecular components of normal nodes; paranodal and juxtaparanodal proteins are properly localized. As in normal nodes, widened nodal regions contain little or no band 4.1B. Lysolecithin also causes "unwinding" of paranodes: The spiral of Schwann cell membrane moves away from the paranodes, but the glial and axonal components of septate-like junctions remain colocalized. Thus, acute demyelination has distinct effects on the molecular organization of the nodal, paranodal, and juxtaparanodal region, reflecting altered axon-Schwann cell interactions.

The perineurium modifies the effects of phenol and glycerol in rat sciatic nerve

Endoneurial cell response and type of nerve fibre damage were studied after perineural injections of 7% phenol-aqua and pure glycerol. Our previous studies have shown that phenol and glycerol induce different types of nerve fibre degeneration after intraneural injections: phenol dissolves axons and Schwann cells inside the basal lamina tubes but glycerol breaks them down into cellular flakes. The current study investigated whether the difference in type of endoneurial damage also appears after perineural application and how the perineurium affects the effect of these neurolytic agents. Rat sciatic nerves were treated with perineural injections of 7% phenol-aqua or pure glycerol and were followed up to 6 months. The results support the previous findings that perineural phenol injection induces damage that covers almost the whole endoneurium, but glycerol injection results in minor subperineurial damage. An ultrastructural study showed that the endoneurial effects are much milder after perineural injection than after intraneural injections. Phenol-induced nerve fibre dissolving was only rarely seen and the nerve fibre damage appeared similar to that after regular Wallerian degeneration in both groups. Axonal regeneration began within 2 weeks of the injections. Endoneurial macrophages were numerous in the damaged area in many individual nerves even at 3-6 months in both groups, which may indicate impaired phagocytotic activity. Regenerating axonal sprouts were seen first at 1 week post injection and Schwann cells proliferated within 2 weeks in both groups. However, the number of axonal sprouts was higher (P=0.002) and the size of the sprouts appeared larger after glycerol injection at 4 weeks post injection. The present study shows that the effects of extraneurally applied neurolytic agents phenol and glycerol are modified by the perineurium. Phenol readily penetrates the perineurium, but glycerol causes only subperineurial damage. The type of damage is rather similar to regular Wallerian degeneration in both groups and the endoneurial effects differ from those seen after intraneural injections.

γ-Diketone neuropathy: axon atrophy and the role of cytoskeletal protein adduction

Multifocal giant neurofilamentous axonal swellings and secondary distal degeneration have been historically considered the hallmark features of gamma-diketone neuropathy. Accordingly, research conducted over the past 25 years has been directed toward discerning mechanisms of axonal swelling. However, this neuropathological convention has been challenged by recent observations that swollen axons were an exclusive product of long-term 2.5-hexanedione (HD) intoxication at lower daily dose-rates (e.g., 175 mg/kg/day); that is, higher HD dose-rates (e.g., 400 mg/kg/day) produced neurological deficits in the absence of axonal swellings. The observation that neurological toxicity can be expressed without axonal swelling suggests that this lesion is not an important pathophysiological event. Instead, several research groups have now shown that axon atrophy is prevalent in nervous tissues of laboratory animals intoxicated over a wide range of HD dose-rates. The well-documented nerve conduction defects associated with axon atrophy, in conjunction with the temporal correspondence between this lesion and the onset of neurological deficits, strongly suggest that atrophy has pathophysiological significance. In this commentary, we present evidence that supports a pathognomonic role for axon atrophy in gamma-diketone neuropathy and suggests that the functional consequences of this lesion mediate the corresponding neurological toxicity. Previous research has demonstrated that HD interacts with proteins via formation of pyrrole adducts. We therefore discuss the possibility that this chemical process is essential to the mechanism of atrophy. Evidence presented in this review suggests that "distal axonopathy" is an inaccurate classification and future nosological schemes should be based on the apparent primacy of axon atrophy.

gamma-diketone central neuropathy: quantitative morphometric analysis of axons in rat spinal cord white matter regions and nerve roots

A quantitative analytical method was used to measure myelinated axon morphometric parameters (e.g., axon area, ratio of axon area/fiber area, and index of circularity) in rat nervous tissue during intoxication with 2,5-hexanedione (HD). Parameters were assessed in nerve roots (dorsal and ventral) and in ascending (gracile fasciculus and spinocerebellar tract) and descending (corticospinal and rubrospinal tracts) spinal cord white matter tracts (L4-L5) of rats intoxicated with HD at two different daily dose-rates (175 or 400 mg HD/kg/day, gavage). For each dose-rate, tissue was sampled at four neurological endpoints: unaffected, slight, moderate, and severe toxicity, as determined by gait analysis and measurements of grip strength. Results indicate that, regardless of the HD dose-rate, axon atrophy (reduced axon area) was a widespread, abundant effect that developed in concert with neurological deficits. The atrophy response occurred contemporaneously in both ascending and descending spinal tracts, which suggests that loss of caliber developed simultaneously along the proximodistal axon axis. In contrast, swollen axons were a numerically small component and were present in nerve roots and spinal tracts only during subchronic intoxication at the lower HD dose-rate (i.e., 175 mg/kg/day). Intoxication at the higher dose-rate (400 mg/kg/day) produced neurological deficits in the absence of axonal swellings. These observations in conjunction with our previous studies of HD-induced peripheral neuropathy (Toxicol. Appl. Pharmacol. 135 (1995) 58; and Toxicol. Appl. Pharmacol. 165 (2000) 127) indicate that axon atrophy, and not axonal swelling, is a primary neuropathic phenomenon.

Imbalances in N-CAM, SAM and polysialic acid may underlie the paranodal ion channel barrier defect in diabetic neuropathy

Breakdown of protective tissue barrier systems characterizes the chronic diabetic complications affecting the retina, and peripheral and central nerve tracts. The progressive damages to the blood-retina-, blood-nerve-, and paranodal ion channel barriers have pathophysiological consequences for the relentless progression of these complications. The continuing damage to the paranodal ion channel barrier in the spontaneously diabetic BB/W rat is associated with an increasingly irreversible nerve conduction defect, due to impaired nodal Na+ currents associated with displacement of nodal Na+ channels across the damaged paranodal barrier. The structural substrate for the mechanical barrier of the paranode is provided by electron-dense junctional complexes made up by a moiety of neural cell adhesive-(N-CAM), neural-glial adhesive (Ng-CAM), substrate adhesive molecules (SAMs) and polysialic acid (PSA). To further explore the mechanism underlying the protective barrier defect in diabetic neuropathy we examined the expression and immunolocalization of these molecules in peripheral nerve. In 6-month diabetic BB/W rats, direct and indirect ELISAs revealed significantly up-regulated N-CAM (P < 0.05), tenascin (Ng-CAM), (P < 0.001) and N-cadherin (A-CAM) (P < 0.03). On the other hand, SAMs showed little change, except for PSA which showed a significantly (P < 0.03) decreased concentration in the diabetic nerve. Immunocytochemical identification of these molecules revealed no visually detectable differences between diabetic and control rats. In conclusion, these data suggest that imbalances between highly interactive molecules responsible for the adhesiveness between terminal Schwann cell loops and the paranodal axolemma may underlie the critical paranodal barrier defect in diabetic neuropathy.

An orderly development of paranodal axoglial junctions and bracelets of Nageotte in the rat sural nerve

The present study was designed to assess the normal development of the paranodal apparatus with particular emphasis on axoglial junctions (AGJs) which constitute the paranodal barrier system. The sural nerve was examined in 10- and 31-day-old rats. During the early phase of myelination AGJ attachment of terminal myelin loops to the axolemma proceeded from the node to the internode. The frequency of terminal loops with AGJ attachment increased with fiber growth. As myelination advanced internodal-most loops became almost 100% attached to the axolemma by AGJs, whereas at the same time an increasing number of nodal-most loops were unattached, suggesting a lack of AGJ formation at this site. The formation of bracelets of Nageotte increased with the progressive addition of myelin loops. They formed most frequently at the juxtanodal interface between unattached and attached loops, probably reflecting crowding of terminal loops along the unchanged length of the paranodal axolemma. The findings suggest a complex but orderly age- and fiber size-dependent maturation process of the paranode and its structural barrier system. The present data will serve as a basis for the evaluation of this anatomical region in regenerating and remyelinating fibers in various neuropathies.

Developmental and pathological changes at the node and paranode in human sural nerves

The nodes and paranodes of peripheral nerve fibers are complex structures that are especially prone to artificial and pathological changes which have to be distinguished from normal developmental changes. Alterations during normal development are mainly caused by an increase in axonal diameter and myelin sheath thickness. The nodal, and paranodal axon diameters in human sural nerves reach their adult values at 3-5 years of age, simultaneously to the internodal diameter. The ratio between internodal and paranodal axon diameters remain relatively constant, with an average value of 1.8 to 2.0 (range: 1.6 to 2.5). Despite a considerable increase of the number of myelin lamellae, the length of the paranodal myelin sheath attachment zone at the axon does not increase correspondingly, because of (1) attenuation of the terminal myelin loops, (2) separation of some of these from the axolemma, and their piling up in the paranode. Separation of variable numbers of terminal myelin loops from the underlying axolemma results in the formation of the spines on the "double bracelet épineux" of Nageotte, while the transverse bands of these loops disappear. The adaptation of the paranodal myelin sheath to axonal expansion during development probably occurs by uneven gliding of the paranodal myelin loops simultaneously with internodal slippage of myelin lamellae. Artificial changes are caused by insufficient fixation or mechanical stress during excision and further handling (cutting, dedydrating, embedding) of nerves whereas pathological changes may be induced by a multitude of causes. An attempt to classify these changes is presented in Table 2.

Early nodal changes in the acute motor axonal neuropathy pattern of the Guillain-Barré syndrome

The axonal patterns of Guillain-Barré syndrome, associated in many cases with antecedent Campylobacter jejuni infection, are now recognized as frequent causes of acute flaccid paralysis in some regions of the world. This study examined ultrastructurally the PNS of seven cases of the acute motor axonal neuropathy form of Guillain-Barré syndrome. In this disorder previous studies of advanced cases have found Wallerian-like degeneration of motor fibres in the spinal roots and peripheral nerves, with little lymphocytic inflammation or demyelination. The present study was focused on identifying early changes and establishing the sequence of changes. By electron microscopy the earliest and mildest changes consisted of lengthening of the node of Ranvier with distortion of the paranodal myelin, and in some instances with breakdown of the outermost myelin terminal loops. At this stage many nodes had overlying macrophages which extended their processes through the Schwann cell basal lamina covering the node and apposed the axolemma. Macrophage processes then extended beneath the myelin terminal loops, and the whole macrophage entered the periaxonal space at the paranode. Macrophage processes dissected the axon from the adaxonal Schwann cell plasmalemma and the macrophages advanced into the internodal periaxonal space, where they typically surrounded a condensed-appearing axon. At this stage the adaxonal Schwann cell cytoplasm regularly degenerated and disappeared, so that the periaxonal space was bounded by the innermost myelin lamella, and the axolemma of many fibres could not be seen. The internodal myelin sheath and the abaxonal Schwann cell cytoplasm remained normal. This arrangement appeared to be stable for some time, but in many fibres the axon subsequently underwent Wallerian-like degeneration. By interfering with impulse conduction, these nodal and periaxonal changes may explain paralysis in some pathologically mild cases. In addition, at early stages, these changes may be reversible, thus explaining the rapid recovery of some patients who become paralysed with acute motor axonal neuropathy. These observations, taken together with previous studies, suggest that acute motor axonal neuropathy is an antibody- and complement-mediated disorder in which the relevant epitopes are present on the nodal and internodal axolemma.

A morphological study of experimental mononeuropathy in the rat: early ischemic changes

This paper describes early morphological changes following the placement of four loose ligatures round the rat sciatic nerve, a method used to produce altered pain-related behaviour. The ligatures were tied so as not to constrict the nerve, but there was compression of some epineurial blood vessels. This mild lesion caused behavioural changes and nerve fiber abnormalities within 8.5 h. The distribution of nerve fiber changes was patchy, and initially due to stagnant ischemia, with necrosis of Schwann cells and evidence of axonal stasis. By 48 h, endoneurial oedema increased leading to a compressive lesion and eventually to severe nerve fiber degeneration.

Developmental changes at the node and paranode in human sural nerves: morphometric and fine-structural evaluation

Developmental alterations of paranodal fiber segments have not been investigated systematically in human nerve fibers at the light- and electron-microscopic level. We have therefore analyzed developmental changes in the fine structure of the paranode in 43 human sural nerves during the axonal growth period up to 5 years of age, and during the subsequent myelin development up to 20 years and thereafter. The nodal, internodal, and paranodal axon diameters reach their adult values at 4-5 years of age. The ratio between internodal and paranodal axon diameters remains constant at 1.8-2.0. Despite a considerable increase in myelin sheath thickness, the length of the paranodal myelin sheath attachment zone at the axon does not increase correspondingly, because of attenuation, separation from the axolemma, and piling up of myelin loops in the paranode. Separation of variable numbers of terminal myelin loops from the underlying axolemma results in the formation of bracelets of Nageotte, whereas the transverse bands of these loops disappear. The adaptation of the paranodal myelin sheath to axonal expansion during development probably occurs by uneven gliding of the paranodal myelin loops simultaneously with internodal slippage of myelin lamellae. Since mechanically stabilizing structures (tight junctions and desmosomes between adjacent paranodal myelin processes; transverse bands between myelin loops and paranodal axolemma) are unevenly arranged, especially during rapid axonal growth, paranodal axonal growth with simultaneous adaptation of the myelin sheath is probably discontinuous with time.

Axonal swellings in human intramuscular nerves

The presence, morphology, distribution, and abundance of axonal swellings in intramuscular nerves were evaluated. Axonal swellings were present in intramuscular nerves in 42% of 127 muscle biopsies from patients with a variety of conditions. The incidence was highest in muscle from patients with peripheral neuropathy, but swellings were present in muscle from patients with motor neuron disease, primary muscle diseases, and some individuals without clinical or histological evidence of neuromuscular disease. The greatest number of swellings in intramuscular nerves was in muscle from patients with chronic inflammatory demyelinating neuropathy. Swellings were spherical or elliptical, 4-20 microns in diameter, 5-30 microns in length, and composed of neurofilaments. Swellings were present only in myelinated axons of intramuscular nerves, proximal to nodes of Ranvier or in internodal regions. Swellings were not associated with axonal degeneration. They were probably not transported. The formation or accumulation of swellings may reflect altered axonal dynamics common to a number of disease processes.

Neurotoxic acrylamide and neurotrophic melanocortin peptides--can contrasting actions provide clues about modes of action?

Experimental acrylamide neuropathy has been studied as a model of degenerative neurological disorders of the 'dying-back' type for over 30 years. Functional, histological, ultrastructural, electrophysiological and biochemical aspects of acrylamide neuropathy have been described and several hypotheses concerning the mode of action proposed. However, the mechanism whereby acrylamide causes axonal degeneration and inhibits nerve sprouting remains unknown. By analogy with agonist/antagonist comparisons used by the pharmacologist, we have reconsidered the acrylamide problem in the light of the opposite effects summarized in Table 1, of neurotrophic peptides related to ACTH/MSH (collectively termed melanocortins). The contrasting effects on sprouting and the eventual quality of repair of mechanically lesioned nerves have suggested a mechanism whereby sprouting may regulate perikaryal adjustments to injury. We have also posed the question as to whether a common biochemical mechanism, namely selective proteolysis of neurofilament protein may underlie the opposing effects of acrylamide and melanocortins on nerve sprouting. This possibility implies a hitherto unknown role for neurofilament protein turnover in neuronal maintenance and repair, a suggestion that may provoke further research and discussion.

Taxol-induced neuropathy after nerve crush: long-term effects on Schwann and endoneurial cells

The present investigation is a continuation of previous studies showing taxol-induced changes up to 4 weeks after a nerve crush. To evaluate the long-term cellular response to taxol, we have extended our morphological analysis of these changes in the taxol-treated nerve crush for up to 40 weeks after a single injection of taxol (PI). The results showed that Schwann cells exhibited a long-lasting and marked response when taxol was injected into the crushed peripheral nerve. During the first 2 months PI, taxol-induced giant axonal bulbs showed the formation of primitive nodes of Ranvier as a result of Schwann cell invaginations. The Schwann cell invaginations developed into nodes of Ranvier after 3-4 months PI together with the recovery of axonal bulbs. Ultrastructurally, cytoplasmic microtubule-related abnormalities were numerous up to 3 months PI and microtubules were seen to enclose degenerative myelin. Taxol-induced abnormalities in Schwann cells did not prevent their ability to produce myelin sheaths, although the accumulation of microtubules between myelin lamellae caused swellings of Schmidt-Lanterman incisures and paranodal myelin loops. Abnormal, extracellular collagen-like 5-nm-thin fibrils were noted closely associated with Schwann cells up to 10 weeks PI. Endoneurial cells, present as long rows without interconnections were noted in areas devoid of axonal sprouts up to 6-8 weeks PI. These cells showed marked cytoplasmic elongations and were covered by thickened basal lamina and contained several microtubule-related cytoplasmic structures, some of which have not been described previously. Taxol, when injected into crushed sciatic nerve induced a long-lasting response upon the Schwann cells with several ultrastructural abnormalities which correlate with changes in myelination and the development of nodes of Ranvier. These findings suggest that normal microtubule turnover is necessary for Schwann cells during nerve fiber regeneration.

Myelin sheath remodelling in remyelinated rat sciatic nerve

In order to elicit de- and remyelination adult rat sciatic nerves were injected with diphtheria toxin dissolved in phosphate buffered saline (PBS). Control nerves were injected with PBS alone. After survival times of 1-10 weeks, the animals were perfused with glutaraldehyde. Specimens from the injected nerves were processed for light microscopic (LM) examination of teased fibres or for electron microscopic (EM) examination of longitudinal thin sections. LM examination of teased fibres after survival times of 6-10 weeks, showed that most remyelinated internodes are 150-300 microns long. In addition, some exceptionally short Schwann cell sheaths, with lengths of 15-150 microns, occur intercalated between conventional remyelinated internodes. EM analysis of thin sections showed that axonal evaginations penetrate in between the terminating myelin lamellae in fibres with nodal widening and/or paranodal demyelination, at early stages of demyelination. Such alterations are not present in relation to myelin sheaths formed during remyelination, which commences about 3 weeks after injection. In addition, some scattered contracted Schwann cell sheaths, which may be as short as 5-10 microns, occur at all stages. These are more frequent shortly after onset of remyelination than at later stages, and they are either composed of a cytoplasmic investment bordered by heminodes, or a more or less distorted myelin sheath bordered by nodes of Ranvier. This picture is very similar to the myelin sheath remodelling observed in regenerated rat sciatic nerves, and in some developing nerves with a mismatch between nerve growth and internodal elongation. It is concluded that a myelin sheath remodelling occurs in de- and remyelinated rat sciatic nerve, presumably as a result of the lack of longitudinal growth.

Ultrastructural and morphometric analysis of long-term peripheral nerve regeneration through silicone tubes

Light and electron microscopy were used to investigate long-term regeneration in peripheral nerves regenerating across a 10 mm gap through silicone tubes. Schwann cells and axons co-migrated behind an advancing front of fibroblasts, bridging the 10 mm gap between 28 and 35 days following nerve transection. Myelination of regenerated fibres started between 14 and 21 days after transection and occurred in a manner similar to that reported during development. Although these early events were successful in producing morphologically normal-appearing regenerated fibres, complete maturation of many of these fibres was never achieved. Axonal distortion by neurofilaments, axonal degeneration and secondary demyelination were seen at 56 days following nerve transection. These changes progressed in severity with time as more axons advanced through the distal stump towards their peripheral target. Since regeneration occurs in the absence of endoneurial tubes, and because constrictive forces act on the nerve during regeneration, we suggest that these extrinsic factors limit the successful advancement of axons through the distal stump to their target organ.

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.

Nodes of Ranvier in acrylamide neuropathy: voltage clamp and electron microscopic analysis of rat sciatic nerve fibres at proximal levels

Adult male rats were injected with acrylamide monomer (50 mg/kg i.p., 3 times/week). The animals developed hind limb paresis and distal motor nerve conduction velocity decreased. Three of 14 examined isolated myelinated sciatic nerve fibres showed a reduced excitability. In the remaining fibres the action potentials were normal. Potential clamp analysis of nodes of Ranvier in the single fibres revealed large delayed nodal K currents in 6 cases. Four of these 6 fibres exhibited a markedly increased membrane capacitance and in 2 fibres an increased Na permeability was found. Electron microscopic examination of sciatic nerves revealed comparatively subtle internodal and nodal-paranodal alterations in large myelinated fibres. Internodally, focal aggregates of tubulovesicular profiles could be found and some Schwann cells were hypertrophic. Paranodally, axonal evaginations penetrated in between the terminating myelin lamellae. Some paranodes had a very thin myelin covering and/or exhibited varying degrees of myelin sheath retraction. In the nodal axon domains lacking an axolemmal undercoating and partly non-undercoated axolemmal protrusions could be found. Similar physiological and morphological alterations occur in the rat sciatic nerve above a neuroma. Therefore, the presently observed proximal changes may, to some extent, represent non-specific alterations, secondary to a target deprivation caused by the distal axon degeneration typical for acrylamide neuropathy.

Ultrastructural observations of organelle accumulation in the equine recurrent laryngeal nerve

The left recurrent laryngeal nerves from five horses with sub-clinical neuropathy were examined by light and electron microscopy in a study designed to examine accumulation of axonal organelles at paranodal and internodal locations. Transverse sections of the nerve showed scattered fibres with split myelin sheaths and axonal accumulation of organelles. On longitudinal sections these collections were seen to result from an axonal outpouching in which dense lamellar bodies and mitochondria had accumulated. These paranodal collections, which could be found on both sides of the node, were often associated with infoldings of the terminal loops of myelin and with occasional paranodal demyelination. The fact that many of the organelles in the outpouches were lysosomal in nature was confirmed by their positive staining for cathepsin D activity. Longitudinal sections demonstrated a number of axons which were swollen over a long distance and which contained focal accumulations of similar organelles. In places, however, there was a clear separation between these organelles and the cytoskeletal proteins. In each case these swollen axons were surrounded by Schwann cell nuclei and their processes, forming well-ordered onion bulbs. The possibility that these two types of changes, i.e. the paranodal accumulations and the axonal swellings could result from a disturbance in axonal transport in this distal axonopathy is discussed.

Progressive axonopathy: an inherited neuropathy of boxer dogs. 4. Myelin sheath and Schwann cell changes in the nerve roots

Changes in the myelin sheath have been studied in the nerve roots of dogs with Progressive axonopathy, an autosomal recessive inherited neuropathy. The earliest changes were attenuation of the sheath at the proximal paranode and adjacent internode, probably in response to the axonal swelling which occurs in this area. Myelin bubbles were frequently observed along internodes. As the disease developed, progressively more fibres demonstrated short internodes of irregular length and thin myelin sheaths suggesting extensive remyelination and remodelling of the sheath. Short lengths of axons devoid of myelin, and occasional macrophages were also encountered. Sheaths of both original and newly formed internodes were highly irregular in outline. Occasional intra-axonal projections of adaxonal Schwann cell cytoplasm were observed, but complex interdigitations were unusual. A moderately electron-dense, granular material accumulated within the myelin sheath, becoming more obvious in the advanced disease. This material of unknown origin and composition was located predominantly at the intraperiod line principally between the adaxonal cytoplasm and the inner major dense line, but also at Schmidt-Lanterman incisures and between paranodal loops. Xenografts of the canine nerves into athymic mice failed to demonstrate any of the myelin sheath changes. The temporal and spatial relationship of the myelin sheath and axonal changes and the failure to reproduce the natural lesion in grafts suggest that Schwann cell alterations probably occur in response to the axonal changes.

Uncoupling of cerebral glucose supply and utilization after hexane-2,5-dione intoxication in the rat

Chronic administration of hexane-2,5-dione (2,5-HD) to rats causes an accumulation of neurofilaments within axons that may lead to their degeneration. This occurs in both the CNS and PNS. It has been suggested that one of the effects of 2,5-HD is an impairment of glucose utilization arising from an inhibition of specific glycolytic enzymes. This hypothesis is based principally on evidence obtained in vitro. In the present study, glucose utilization, glucose transport across the blood-brain barrier, and blood flow have been measured in vivo in brain regions of control rats and in three groups of rats treated with 2,5-HD as (a) a single intragastric dose (500 mg/kg of body weight), (b) high chronic doses of 500 mg/kg of body weight for 15 days, or (c) low chronic doses of 250 mg/kg of body weight for 21 days. Group b showed overt signs of neuropathy, whereas groups a and c did not. The results indicate two independent effects of 2,5-HD in the CNS: a dose-dependent inhibition of glucose utilization and an effect on glucose supply and transport across the blood-brain barrier, which is apparent only after chronic treatment.

The retrograde fast component of axonal transport in motor and sensory nerves of the rat during administration of 2,5-hexanedione

The retrograde and anterograde fast component of axonal transport (rFC and aFC) were examined in rats following intoxication with 2,5-hexanedione (1 g/kg/week) for 2, 4, 6 and 8 weeks. rFC and aFC were measured as accumulations distal and proximal to a double ligature after labeling of protein by injection of [35S]methionine into L5 spinal cord or L5 dorsal root ganglion. Clinical symptoms appeared after an accumulated dose of 6 g/kg of the toxin and was dominated by motor deficiencies. Abnormalities in retrograde build-up appeared earliest in motor nerves after an accumulated dose of 4 g/kg, while the sensory nerves showed a decreased retrograde build-up only after 6 g/kg. In motor nerves retrograde build-up for the short and the long accumulation interval (11.5-17.5 h and 11.5-25.5 h, respectively) was equally depressed, while in sensory nerves the retrograde build-up for the early accumulation interval (10-16 h) was less severely depressed as compared to the late collection interval (10-24 h). These findings point to a decreased amount of rFC, rather than a delay in turn-around. Furthermore, build-up was inversely correlated with the dose of 2,5-hexanedione and was most pronounced in motor nerves (82% vs 52% in motor and sensory nerves, respectively) at the final dose of 8 g/kg. When intoxication was stopped, rats improved in muscle-strength, but still showed atrophy of leg muscles. Coincidently, retrograde transport in motor nerves was normalized for the short accumulation interval, and improved for the longer collection interval, though not completely normalized.

Comparison of central and peripheral nervous system lesions caused by high-dose short-term and low-dose subchronic acrylamide treatment in rats

The effects of high-dose subacute acrylamide treatment of up to 50 mg/kg/day for 4 or 10 d were compared to those of subchronic exposure, up to 12 mg/kg/day for 90 d. In the subacute study, Purkinje cells, long ascending tracts of the spinal cord, optic tract terminal or preterminal regions in superior colliculus, sensory ganglion cells, and distal large-caliber peripheral axons were severely affected. Purkinje cells and fasciculus gracilis changes were the earliest lesions. In the subchronic study, the dominant lesion was confined to the distal peripheral axon, with only minor changes occurring in spinal cord and medulla. Paranodal swellings with the characteristic appearance of neurofilament aggregations were not seen. This morphological study suggests a significant difference between high- and low-dose acrylamide-induced lesions. If there is a reduced tendency for long-term low-dose acrylamide exposure to produce CNS lesions, the risk of irreversible nervous system damage would be less than that predicted from subacute studies.

Progressive axonopathy: an inherited neuropathy of boxer dogs. 3. The peripheral axon lesion with special reference to the nerve roots

Progressive axonopathy is an autosomal recessive inherited neuropathy of Boxer dogs with lesions in the CNS and PNS. This paper describes the axonal changes in the lumbar and cervical nerve roots and tibial nerve. By 2 months of age the proximal paranodal areas of many larger diameter fibres show small axonal swellings, sometimes with attenuation or loss of the associated myelin sheath. Axoplasmic changes within swollen and non-swollen fibres include disorganization of the peripheral neurofilaments and small accumulations of vesicles and vesiculo-tubular profiles, particularly in the sub-axolemmal area. Occasional fibres, more often in the cervical roots, are massively distended with disorganized neurofilaments. The frequency of the membranous accumulations decreases with progression of the disease. Many axons show a markedly irregular or corrugated outline and are surrounded by an attenuated sheath. The peripheral axonal cytoskeleton is disorganized and misaligned, whereas the central structures maintain a more normal arrangement. Regenerating axonal clusters are common in the cervical ventral roots but occur infrequently in the lumbar roots. Similar axonal changes occur in the peripheral nerves but at a much lower frequency. Any membranous accumulations or cytoskeletal disorganization are more probable in the proximal tibial nerves, while the frequency of axonal degeneration and regeneration increases distally. The morphological appearances indicate gross disturbances in axon-sheath cell relationships and suggest that abnormalities in the transport of various axoplasmic organelles may be involved in the pathogenesis of the axonal lesion.

Acute swelling of nodes of Ranvier caused by venoms which slow inactivation of sodium channels

The venoms of the spider Phoneutria nigriventer and the scorpions Leiurus quinquestriatus and Centruroides sculpturatus cause acute, transient swelling of axons at nodes of Ranvier. The changes in the morphology of the node and paranode were studied in the mouse. Venom was injected into the sciatic nerve by means of a glass micropipette. After survival times ranging from 15 min to 3 weeks the nerves were examined by light and electron microscopy. The increase in nodal axoplasmic volume led within an hour to disruption of neurofilaments and microtubules, swelling of the paranodes and displacement of the terminal loops of myelin away from the node. Axonal calibre recovered within a few hours, but restoration of nodal width took several days and seemed to be accomplished by elongation and remodelling of the paranodal myelin. Occasional internodes were interrupted by node-like discontinuities in the myelin sheath. These developed within a few hours and persisted for the duration of the study.

Progressive axonopathy: an inherited neuropathy of boxer dogs. 2. The nature and distribution of the pathological changes

This report describes the neuropathology of progressive axonopathy (PA), an autosomal recessive inherited neuropathy of Boxer dogs, which affects CNS and PNS. The nerve roots contain numerous myelin bubbles and proximal paranodal axonal swellings containing vesicles, vesiculo-tubular profiles and disorganized neurofilaments. The myelin sheath overlying such swellings is often attenuated. As the disease develops there are progressive changes in the myelin sheath with thinning at paranodal and internodal locations, loss of myelin from lengths of axon and the formation of short internodes with disproportionately thin sheaths. The abnormalities show a very definite selectivity for nerve roots and proximal nerves. Conversely, the frequency of degeneration and regeneration is greater distally except in the cervical ventral roots which contain numerous regenerating clusters. In the CNS numerous axonal spheroids are found in the lateral and ventral columns of the spinal cord and in various brain stem nuclei, particularly the superior olives, accessory cuneate nuclei and lateral lemniscus and its nucleus. Axonal degeneration which occurs mainly in the cord shows no obvious tract or proximal/distal selectivity. The optic pathways are also involved, predominantly adjacent to the chiasma. The autonomic nervous system is affected and distal limb muscles show varying, but usually minor, degrees of neurogenic atrophy. The condition, which has no obvious direct parallel in human or veterinary medicine, shows gross disturbances of axon-glial inter-relationships in both CNS and PNS.

Effects of IDPN-induced axonal swellings on conduction in motor nerve fibers

Paranodal demyelination produces a reduction of conduction velocity and conduction block. The relative proportions of these changes appear to vary among different demyelinating disorders. In this study we have examined the effects on conduction of paranodal demyelination produced by giant axonal swellings. The axonal swellings were induced in rats by administration of beta, beta'-iminodipropionitrile (IDPN). In this experimental model synchronous axonal swellings occur in the proximal region of virtually every alpha-motorneuron without evidence of segmental demyelination or fiber loss. Conduction across the motor neuron was evaluated by two methods: a monosynaptic reflex pathway and intracellular recording from single motor neurons. Increases in the delay across the central region of the monosynaptic reflex pathway began between 2 and 4 days after toxin administration. Intracellular studies confirmed that the slowing occurred across the proximal regions of the motor axons; more distal regions of the motor axons were unaffected. The substantial reduction in conduction velocity over the swollen segment occurs with only moderate evidence of conduction block, as assayed by a reduction in the H-reflex/M-response amplitude ratio. Parallel morphological studies showed that in the enlarged fibers the myelin terminal loops maintained contact with the axon but were displaced from the paranodal region into the internode. The appearance of this "passive" paranodal demyelination correlated closely with the increase in conduction delay. We suggest that the contact maintained by the displaced myelin terminal loops with the axolemma allows saltatory conduction to continue, and explains the paucity of conduction block in this model despite the prominent conduction slowing.

Reversible diabetic nerve dysfunction: structural correlates to electrophysiological abnormalities

Structural alterations of the nodal and paranodal areas were examined in the posterior tibial nerve in insulin-depleted and insulin-treated diabetic BB rats. The early metabolic phase of the distal symmetrical polyneuropathy was characterized by paranodal axonal swellings and nodal bulgings of the axon. These alterations correlate with intraaxonal sodium accumulation and decreased sodium equilibrium potentials which account for the early nerve conduction defect. Both the structural and electrophysiological abnormalities were completely normalized after vigorous insulin therapy. In the chronic diabetic polyneuropathy the paranodal area showed loss of paranodal axoglial junctions and paranodal myelin retraction. These changes may be partially responsible for the impaired electrical activity at the node as exemplified by irreversibly impaired sodium permeability and nerve conduction.

In vitro evidence that covalent crosslinking of neurofilaments occurs in gamma-diketone neuropathy

We have postulated that the toxic neuropathies associated with neurofilament-filled axonal swellings have a common pathogenesis, the covalent crosslinking of neurofilaments during anterograde transport. The newly described gamma-diketone, 3,4-dimethyl-2,5-hexanedione (DMHD), is a more potent analogue of the toxic metabolite of n-hexane, 2,5-hexanedione. The axonal swellings observed in DMHD toxicity are in the proximal axon, as seen in intoxication with beta, beta'-iminodipropionitrile, rather than in the distal axon, where neurofilamentous swellings are observed in n-hexane, carbon disulfide, and acrylamide neurotoxicity. In these studies, 14C-labeled DMHD and 2-butanone were synthesized and allowed to react with peripheral nerve. Only 14C-labeled DMHD resulted in stable radiolabeled protein polymers, which were retained by nitrocellulose filters with pore sizes as large as 12 microns. More specific evidence for covalent crosslinking of neurofilaments was obtained when DMHD was allowed to react with peripheral nerve in which the neurofilaments had been pulse-labeled with L-[35S]methionine.

The problems of neurons with long axons

Neurons with long axons are unique among cells in having to maintain a very large area of membrane. In this respect they have problems in common with red cells: the latter are separated from the source of their metabolites in time, the former by distance. In equilibrium, maintenance mechanisms are adequate; but in conditions of energy deprivation or deprivation of antioxidant substances such as glutathione and alpha-tocopherol, or when the transport of materials within the neuron is physically obstructed, the system may break down and the longest fibres will always suffer first. The problems are logistical, just as are those of red cells. The association between red-cell disease and neuropathy is not entirely fortuitous.

Cytochemical staining characteristics of peripheral nodes of Ranvier in hexacarbon intoxication

Changes in the distribution of stainable gap substance and subaxolemmal density at peripheral nodes of Ranvier in hexacarbon-intoxicated rats have been studied by light and electron microscopy. Cupric ion binding to the nodal gap substance was seen in normal nodes as a discrete annulus by the formation with ferrocyanide of Hatchett's Brown reaction product. Staining of osmicated fibres with ferrocyanide ions alone gave a deposit of black reaction product at the subaxolemmal region at nodes of Ranvier. Paranodal distension by neurofilamentous masses and separation of the terminal myelin loops in the early phase of paranodal dilatation produced no change in the distribution of the two kinds of stainable material. Paranodal myelin retraction with increases both in nodal gap width and nodal axon diameter resulted in displacement and attenuation of both stained regions. Axonal protrusion at the nodal region tended only to displace the stained gap substance, but sometimes it resulted in its attenuation. Occasionally loss of subaxolemmal staining was found. The possible functional relevance of these abnormal findings is discussed in relation to changes in conduction in affected nerves.

The effects of 2,5-hexanedione on axonal regeneration after nerve crush in the rat

The pattern of recovery of myelinated axons in the posterior tibial nerve after crushing was studied in rats chronically intoxicated with 2,5-hexanedione. It was given for 2 weeks before crushing (200 mg/kg i.p. 5 times a week) or additionally for two further weeks after the nerve crush. Two animals were examined from each group at approximately 1,2,3,4 and 8 weeks later. Return of function in poisoned animals was slower than in the controls. The numbers of regenerating myelinated fibres was severely reduced in poisoned animals up to 4 weeks later, but by 8 weeks the numbers equalled those in the control nerves. Marked impairment of initiation of neurite outgrowth was found, but once begun, axonal growth was comparable to controls and myelination occurred normally. Above the crush for 10 mm, filament-filled axonal swellings were found in poisoned animals accompanied by varying amounts of retrograde axonal degeneration. These findings are discussed in relation to the role of normal neurofilaments in axonal growth and the effects of probably cross-linking of these by 2,5-hexanedione on regenerating neurites.