Protection of the axonal cytoskeleton in anoxic optic nerve by decreased extracellular calcium

Nimodipine confers clinical improvement in two models of experimental autoimmune encephalomyelitis

Multiple sclerosis is characterised by inflammatory neurodegeneration, with axonal injury and neuronal cell death occurring in parallel to demyelination. Regarding the molecular mechanisms responsible for demyelination and axonopathy, energy failure, aberrant expression of ion channels and excitotoxicity have been suggested to lead to Ca2+ overload and subsequent activation of calcium-dependent damage pathways. Thus, the inhibition of Ca2+ influx by pharmacological modulation of Ca2+ channels may represent a novel neuroprotective strategy in the treatment of secondary axonopathy. We therefore investigated the effects of the L-type voltage-gated calcium channel blocker nimodipine in two different models of mouse experimental autoimmune encephalomyelitis (EAE), an established experimental paradigm for multiple sclerosis. We show that preventive application of nimodipine (10 mg/kg per day) starting on the day of induction had ameliorating effects on EAE in SJL/J mice immunised with encephalitic myelin peptide PLP139-151 , specifically in late-stage disease. Furthermore, supporting these data, administration of nimodipine to MOG35-55 -immunised C57BL/6 mice starting at the peak of pre-established disease, also led to a significant decrease in disease score, indicating a protective effect on secondary CNS damage. Histological analysis confirmed that nimodipine attenuated demyelination, axonal loss and pathological axonal β-amyloid precursor protein accumulation in the cerebellum and spinal cord in the chronic phase of disease. Of note, we observed no effects of nimodipine on the peripheral immune response in EAE mice with regard to distribution, antigen-specific proliferation or activation patterns of lymphocytes. Taken together, our data suggest a CNS-specific effect of L-type voltage-gated calcium channel blockade to inflammation-induced neurodegeneration.

Relationship of acute axonal damage, Wallerian degeneration, and clinical disability in multiple sclerosis

BACKGROUND

Axonal damage and loss substantially contribute to the incremental accumulation of clinical disability in progressive multiple sclerosis. Here, we assessed the amount of Wallerian degeneration in brain tissue of multiple sclerosis patients in relation to demyelinating lesion activity and asked whether a transient blockade of Wallerian degeneration decreases axonal loss and clinical disability in a mouse model of inflammatory demyelination.

METHODS

Wallerian degeneration and acute axonal damage were determined immunohistochemically in the periplaque white matter of multiple sclerosis patients with early actively demyelinating lesions, chronic active lesions, and inactive lesions. Furthermore, we studied the effects of Wallerian degeneration blockage on clinical severity, inflammatory pathology, acute axonal damage, and long-term axonal loss in experimental autoimmune encephalomyelitis using Wallerian degeneration slow (Wld ^S ) mutant mice.

RESULTS

The highest numbers of axons undergoing Wallerian degeneration were found in the perilesional white matter of multiple sclerosis patients early in the disease course and with actively demyelinating lesions. Furthermore, Wallerian degeneration was more abundant in patients harboring chronic active as compared to chronic inactive lesions. No co-localization of neuropeptide Y-Y1 receptor, a bona fide immunohistochemical marker of Wallerian degeneration, with amyloid precursor protein, frequently used as an indicator of acute axonal transport disturbance, was observed in human and mouse tissue, indicating distinct axon-degenerative processes. Experimentally, a delay of Wallerian degeneration, as observed in Wld ^S mice, did not result in a reduction of clinical disability or acute axonal damage in experimental autoimmune encephalomyelitis, further supporting that acute axonal damage as reflected by axonal transport disturbances does not share common molecular mechanisms with Wallerian degeneration. Furthermore, delaying Wallerian degeneration did not result in a net rescue of axons in late lesion stages of experimental autoimmune encephalomyelitis.

CONCLUSIONS

Our data indicate that in multiple sclerosis, ongoing demyelination in focal lesions is associated with axonal degeneration in the perilesional white matter, supporting a role for focal pathology in diffuse white matter damage. Also, our results suggest that interfering with Wallerian degeneration in inflammatory demyelination does not suffice to prevent acute axonal damage and finally axonal loss.

Acute anoxic changes in peripheral nerve: anatomic and physiologic correlations

INTRODUCTION

The response of the peripheral nerve to anoxia is modulated by many factors including glucose and temperature. The purposes of this article are to demonstrate the effects of these factors on the pathological changes induced by anoxia and to compare the electrophysiologic changes and pathological changes in the same nerves.

METHODS

Sciatic nerves were harvested from rats and placed in a perfusion apparatus where neurophysiologic responses could be recorded continuously during a 16 h experiment. After the experiment, light microscopy and electron microscopy were performed.

RESULTS

Light microscopic images showed mild changes from anoxia at normoglycemia. Hypoglycemic anoxia produced massive axonal swelling while hyperglycemic anoxia produced apparent changes in the myelin. Anoxic changes were not uniform in all axons. Electron microscopy showed only minor disruptions of the cytoskeleton with anoxia during normoglycemia. At the extremes of glucose concentration especially with hyperglycemia, there was a more severe disruption of intermediate filaments and loss of axonal structure with anoxia. Hypothermia protected axons from the effect of anoxia and produced peak axonal swelling in the 17-30°C range.

CONCLUSIONS

The combination of hyperglycemia or hypoglycemia and anoxia produces extremely severe axonal disruption. Changes in axonal diameter are complex and are influenced by many factors.

Axoplasmic reticulum Ca(2+) release causes secondary degeneration of spinal axons

OBJECTIVE

Transected axons of the central nervous system fail to regenerate and instead die back away from the lesion site, resulting in permanent disability. Although both intrinsic (eg, microtubule instability, calpain activation) and extrinsic (ie, macrophages) processes are implicated in axonal dieback, the underlying mechanisms remain uncertain. Furthermore, the precise mechanisms that cause delayed "bystander" loss of spinal axons, that is, ones that were not directly damaged by the initial insult, but succumbed to secondary degeneration, remain unclear. Our goal was to evaluate the role of intra-axonal Ca(2+) stores in secondary axonal degeneration following spinal cord injury.

METHODS

We developed a 2-photon laser-induced spinal cord injury model to follow morphological and Ca(2+) changes in live myelinated spinal axons acutely following injury.

RESULTS

Transected axons "died back" within swollen myelin or underwent synchronous pan-fragmentation associated with robust Ca(2+) increases. Spared fibers underwent delayed secondary bystander degeneration. Reducing Ca(2+) release from axonal stores mediated by ryanodine and inositol triphosphate receptors significantly decreased axonal dieback and bystander injury. Conversely, a gain-of-function ryanodine receptor 2 mutant or pharmacological treatments that promote axonal store Ca(2+) release worsened these events.

INTERPRETATION

Ca(2+) release from intra-axonal Ca(2+) stores, distributed along the length of the axon, contributes significantly to secondary degeneration of axons. This refocuses our approach to protecting spinal white matter tracts, where emphasis has been placed on limiting Ca(2+) entry from the extracellular space across cell membranes, and emphasizes that modulation of axonal Ca(2+) stores may be a key pharmacotherapeutic goal in spinal cord injury.

Central axons preparing to myelinate are highly sensitive [corrected] to ischemic injury

OBJECTIVE

Developing central white matter is subject to ischemic-type injury during the period that precedes myelination. At this stage in maturation, central axons initiate a program of radial expansion and ion channel redistribution. Here we test the hypothesis that during radial expansion axons display heightened ischemic sensitivity, when clusters of Ca(2+) channels decorate future node of Ranvier sites.

METHODS

Functionality and morphology of central axons and glia were examined during and after a period of modeled ischemia. Pathological changes in axons undergoing radial expansion were probed using electrophysiological, quantitative ultrastructural, and morphometric analysis in neonatal rodent optic nerve and periventricular white matter axons studied under modeled ischemia in vitro or after hypoxia-ischemia in vivo.

RESULTS

Acute ischemic injury of central axons undergoing initial radial expansion was mediated by Ca(2+) influx through Ca(2+) channels expressed in axolemma clusters. This form of injury operated only in this axon population, which was more sensitive to injury than neighboring myelinated axons, smaller axons yet to initiate radial expansion, astrocytes, or oligodendroglia. A pharmacological strategy designed to protect both small and large diameter premyelinated axons proved 100% protective against acute ischemia studied under modeled ischemia in vitro or after hypoxia-ischemia in vivo.

INTERPRETATION

Recent clinical data highlight the importance of axon pathology in developing white matter injury. The elevated susceptibility of early maturing axons to ischemic injury described here may significantly contribute to selective white matter pathology and places these axons alongside preoligodendrocytes as a potential primary target of both injury and therapeutics.

Haloperidol causes cytoskeletal collapse in N1E-115 cells through tau hyperphosphorylation induced by oxidative stress: Implications for neurodevelopment

Haloperidol a typical antipsychotic commonly used in the treatment of schizophrenia causes neuronal damage and extrapiramidal symptoms after several years of treatment. These symptoms have been associated with increased levels of oxidative stress. Reactive oxygen species produce cytoskeletal collapse and an excessive phosphorylation of tau, a microtubule-associated protein that plays a key role in microtubule stabilization, and in growth cone and neurite formation, which are cytoskeletal phenotypes that participate in neurodevelopment. Thus, we hypothesized that haloperidol produces neurocytoskeletal disorganization by increasing free radicals and tau hyperphosphorylation, and consequently, the loss of neurodevelopmental cytoskeletal phenotypes, neurites and growth cones. The purpose of this work was the characterization of neuronal cytoskeletal changes caused by haloperidol in neuroblastoma N1E-115 cells. We also studied the mechanisms by which haloperidol causes cytoskeletal changes. The results showed that haloperidol at 100microM caused a complete cytoskeleton collapse in the majority of the cells. Melatonin, a free radical scavenger, blocks tau hyperphosphorylation, and microtubule disorganization caused by haloperidol in a dose-response mode. Additionally, the indole blocks lipoperoxide formation in haloperidol treated cells. The results indicate that free radicals and tau hyperphosphorylation produced by haloperidol caused a cytoskeletal collapse and the lost of growth cones and neurites. These effects were blocked by melatonin. Data suggest that extrapiramidal symptoms in schizophrenic patients can be produced by cytoskeletal disorganization during adult brain neurodevelopment after prolonged haloperidol treatment that can be prevented by melatonin.

Hypoxia during pregnancy in rats leads to the changes of the cerebral white matter in adult offspring

The aim of the present study is to evaluate the effect of reduced fetal oxygen supply on cerebral white matter in the adult offspring and further assess its susceptibility to postnatal hypoxia and high-fat diet. Based on a 3 x 2 full factorial design consisting of three factors of maternal hypoxia, postnatal high-fat diet, and postnatal hypoxia, the ultrastructure of myelin, axon and capillaries were observed, and the expression of myelin basic protein (MBP), neurofilament-H+L(NF-H+L), and glial fibrillary acidic protein (GFAP) was analyzed in periventricular white matter of 16-month-old offspring. Demyelination, injured axon and damaged microvasculars were observed in maternal hypoxia offspring. The main effect of maternal hypoxia lead to decreased expression of MBP or NF-H+L, and increased expression of GFAP (all P<0.05). Moreover, there was positive three-way interaction among maternal hypoxia, high-fat diet and postnatal hypoxia on MBP, NF-H+L or GFAP expression (all P<0.05). In summary, our results indicated that maternal hypoxia during pregnancy in rats lead to changes of periventricular white matter in adult offspring, including demyelination, damaged axon and proliferated astroglia. This effect was amplified by high-fat diet and postnatal hypoxia.

Mechanisms of axonal injury: internodal nanocomplexes and calcium deregulation

Axonal degeneration causes morbidity in many neurological conditions including stroke, neurotrauma and multiple sclerosis. The limited ability of central nervous system (CNS) neurons to regenerate, combined with the observation that axonal damage causes clinical disability, has spurred efforts to investigate the mechanisms of axonal degeneration. Ca influx from outside the axon is a key mediator of injury. More recently, substantial pools of intra-axonal Ca sequestered in the 'axoplasmic reticulum' have been reported. These Ca stores are under the control of multimolecular 'nanocomplexes' located along the internodes under the myelin. The overactivation of these complexes during disease can lead to a lethal release of Ca from intra-axonal stores. Rich receptor pharmacology offers tantalizing therapeutic options targeting these nanocomplexes in the many diseases where axonal degeneration is prominent.

Proteomic changes in the crucian carp brain during exposure to anoxia

During exposure to anoxia, the crucian carp brain is able to maintain normal overall protein synthesis rates. However, it is not known if there are alterations in the synthesis or expression of specific proteins. This investigation addresses this issue by comparing the normoxic and anoxic brain proteome. Nine proteins were found to be reduced by anoxia. Reductions in the glycolytic pathway proteins creatine kinase, fructose biphosphate aldolase, glyceraldehyde-3-phosphate dehydrogenase, triosephosphate isomerase and lactate dehydrogenase reflect the reduced production and requirement for adenosine tri-phosphate during anoxia. In terms of neural protection, voltage-dependent anion channel, a protein associated with neuronal apoptosis, was reduced, along with gefiltin, a protein associated with the subsequent need for neuronal repair. Additionally the expression of proteins associated with neural degeneration and impaired cognitive function also declined; dihydropyrimidinase-like protein-3 and vesicle amine transport protein-1. One protein was found to be increased by anoxia; pre-proependymin, the precursor to ependymin. Ependymin fulfils multiple roles in neural plasticity, memory formation and learning, neuron growth and regeneration, and is able to reverse the possibility of apoptosis, thus further protecting the anoxic brain.

Mechanisms of amyloid plaque pathogenesis

The first ultrastructural investigations of Alzheimer's disease noted the prominence of degenerating mitochondria in the dystrophic neurites of amyloid plaques, and speculated that this degeneration might be a major contributor to plaque pathogenesis. However, the fate of these organelles has received scant consideration in the intervening decades. A number of hypotheses for the formation and progression of amyloid plaques have since been suggested, including glial secretion of amyloid, somal and synaptic secretion of amyloid-beta protein from neurons, and endosomal-lysosomal aggregation of amyloid-beta protein in the cell bodies of neurons, but none of these hypotheses fully account for the focal accumulation of amyloid in plaques. In addition to Alzheimer's disease, amyloid plaques occur in a variety of conditions, and these conditions are all accompanied by dystrophic neurites characteristic of disrupted axonal transport. The disruption of axonal transport results in the autophagocytosis of mitochondria without normal lysosomal degradation, and recent evidence from aging, traumatic injury, Alzheimer's disease and transgenic mice models of Alzheimer's disease, suggests that the degeneration of these autophagosomes may lead to amyloid production within dystrophic neurites. The theory of amyloid plaque pathogenesis has thus come full circle, back to the intuitions of the very first researchers in the field.

Hypoxic injury of isolated axons is independent of ionotropic glutamate receptors

Axonal injury in white matter is an important consequence of many acute neurological diseases including ischemia. A role for glutamate-mediated excitotoxicity is suggested by observations from in vitro and in situ models that AMPA/kainate blockers can reduce axonal injury. We assessed axonal vulnerability in primary murine neuronal cultures, with axons isolated from their cell bodies using a compartmented chamber design. Transient removal of oxygen and glucose in the axon compartment resulted in irreversible loss of axon length and neurofilament labeling. This injury was not prevented by addition of ionotropic glutamate receptor blockers and could not be reproduced by glutamate receptor agonists. However, hypoxic injury was prevented by blockade of voltage-gated sodium channels, inhibition of calpain and removal of extracellular calcium. These results suggest that isolated, unmyelinated axons are vulnerable to hypoxic injury which is mediated by influx of sodium and calcium but is independent of glutamate receptor activation.

General mechanisms of axonal damage and its prevention

Axonal degeneration is a prominent pathological feature in multiple sclerosis observed over a century ago. The gradual loss of axons is thought to underlie irreversible clinical deficits in this disease. The precise mechanisms of axonopathy are poorly understood, but likely involve excess accumulation of Ca ions. In healthy fibers, ATP-dependent pumps support homeostasis of ionic gradients. When energy supply is limited, either due to inadequate delivery (e.g., ischemia, mitochondrial dysfunction) and/or excessive utilization (e.g., conduction along demyelinated axons), ion gradients break down, unleashing a variety of aberrant cascades, ultimately leading to Ca overload. During Na pump dysfunction, Na can enter axons through non-inactivating Na channels, promoting axonal Na overload and depolarization by allowing K egress. This will gate voltage-sensitive Ca channels and stimulate reverse Na-Ca exchange, leading to further Ca entry. Energy failure will also promote Ca release from intracellular stores. Neurotransmitters such as glutamate can be released by reverse operation of Na-dependent transporters, in turn activating a variety of ionotropic and metabotropic receptors, further exacerbating overload of cellular Ca. Together, this Ca overload will inappropriately stimulate a variety of Ca-dependent enzyme systems (e.g., calpains, phospholipases), leading to structural and functional axonal injury. Pharmacological interruption at key points in these interrelated injury cascades (e.g., at voltage-gated Na channels or AMPA receptors) may confer significant neuroprotection to compromised central axons and supporting glia. Such agents may represent attractive adjuncts to currently available immunomodulatory therapies.

L-type calcium channel antagonist nifedipine reduces neurofilament restitution following traumatic optic nerve injury

BACKGROUND

The aim of the present study was to observe if the use of the L-type calcium channel antagonist nifedipine would offer advantages for the retinal ganglion cells and the restitution of the axonal cytoskeleton after optic nerve crush.

METHODS

Retinal ganglion cells were retrogradely labeled with a fluorescent calcium marker. With the in vivo confocal neuroimaging (ICON) method we observed the fluorescent cell metabolism marker Oregon Green BAPTA in the same retinal ganglion cells over 3 weeks after optic nerve crush. 2 micromol nifedipine were injected intraocularly 30 minutes following optic nerve crush.

FINDINGS

Investigation of the optic nerve immunostained for NF-H presented decreased restitution of the neurofilaments in the axonal cytoskeleton after 3 weeks in the optic nerve crush group treated with nifedipine as compared to the optic nerve crush only group.

INTERPRETATION

These results show that a single injection of the calcium L-type antagonist nifedipine shortly after optic nerve injury has long-lasting negative effects on the recovery of the retinal ganglion cells.

Calcium channel blockers ameliorate disease in a mouse model of multiple sclerosis

Multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE), an animal model of MS, are inflammatory demyelinating diseases of the central nervous system. The inflammatory attacks lead to glial dysfunction and death, axonal damage, and neurological deficits. Numerous studies in rat suggest that extracellular calcium influx, via voltage-gated calcium channels (VGCC), contributes to white matter damage in acute spinal cord injury and stroke. Our immunohistochemical finding that mouse spinal cord axons display subunits of L-type VGCC also supports this hypothesis. Furthermore, we hypothesized that VGCC also play a role in EAE, and possibly, MS. In our study, administration of the calcium channel blockers (CCB) bepridil and nitrendipine significantly ameliorated EAE in mice, compared with vehicle-treated controls. Spinal cord samples showed reduced inflammation and axonal pathology in bepridil-treated animals. Our data support the hypothesis that calcium influx via VGCC plays a significant role in the development of neurological disability and white matter damage in EAE and MS.

Proteomic analysis of neuronal hypoxia in vitro

We used high-throughput Western blotting to identify proteins that are up- or down-regulated by neuronal hypoxia in vitro. Exposure to hypoxia for 24 h produced > or = 1.5-fold increases in the expression of 10/700 proteins (1.4%) and decreases in the expression of 16/700 proteins (2.3%). Up-regulated proteins included Arc, doublecortin/calmodulin kinase-like 1, integrin alpha(v), and fibronectin; down-regulated proteins included nuclear autoantigenic sperm protein, protein kinase C-related kinase 2, and E2F transcription factor 1. The prominence of cytoskeleton-related proteins among those showing altered expression highlights the role of the cytoskeleton in neuronal responses to hypoxia.

Functional dichotomy: glutathione and vitamin E in homeostasis relevant to primary open-angle glaucoma

Primary open-angle glaucoma (POAG) is a complex chronic neurological disease that can result in blindness. The goal of understanding the aetiology of POAG is to be able to target effective treatment to individuals who will eventually go blind without it. Epidemiological studies of POAG have not specifically addressed the possibility that nutrition may play a role in the development of POAG. A handful of papers have considered that nutrition may have an impact on POAG patients. POAG is not believed to be a 'vitamin-deficiency disease'. The concept of 'vitamin-deficiency diseases' and the recommended daily allowances have not kept pace with the growing understanding of the cellular and molecular functions of vitamins and other micronutrients. The aetiology of POAG remains a mystery. Discoveries in cell physiology can be assimilated from the literature and applied to known homeostatic mechanisms of the eye. In this way the possible roles of nutritional components involved in the aetiology of POAG can be described. The mechanisms may be subject to many influences in ways that have yet to be defined. Two distinct changes in the trabecular meshwork can be identified: trabecular meshwork changes that cause intra-ocular pressure to increase and trabecular meshwork changes that are directly correlated to optic nerve atrophy. Compelling evidence suggests that collagen trabecular meshwork extracellular matrix (ECM) remodelling is correlated to increased intraocular pressure in POAG. Elastin trabecular meshwork ECM remodelling is correlated to POAG optic nerve atrophy. There appear to be two different pathways of ECM remodelling and apoptosis induction in POAG. The pathway for collagen remodelling and apoptosis induction seems to be exogenously influenced by water-soluble antioxidants, for example, glutathione. The pathway for elastin remodelling and apoptosis induction seems to be influenced by endogenous lipid-soluble antioxidants, for example, vitamin E. Roles can be defined for antioxidants in the two different pathways of ECM remodelling and apoptosis induction. This suggests that antioxidants are important in maintaining cellular homeostasis relevant to the aetiology of POAG.

Calpain inhibitor 2 prevents axonal degeneration of opossum optic nerve fibers

The ultrastructural change that characterizes the onset of Wallerian degeneration is the disintegration of axoplasmic microtubules and neurofilaments, which are converted into an amorphous and granular material, followed by myelin breakdown. The mechanism underlying such processes is an increase in the amount of intracellular calcium, leading to activation of proteases called calpains. The aim of this study was to evaluate by quantitative ultrastructural analysis whether nerve fibers can be preserved by the use of an exogenous inhibitor of these proteases (calpain inhibitor-2, Mu-F-hF-FMK), after optic nerve crush. For that, the left optic nerves of opossums, Didelphis aurita, were crushed with the aid of a fine forceps, and half of them received a calpain inhibitor mixed with Elvax resin. Ninety-six hours after the lesion, the animals were reanesthetized and transcardially perfused, and the optic nerves were removed, the right ones being used as normal nerves. Afterward, the optic nerves were dissected and processed for routine transmission electron microscopy and quantitative and statistical analysis. The results of this analysis showed that the group that received the calpain inhibitor presented a reduction of astrogliosis, maintaining the optic nerve structure in an organized state; a significant decrease in the number of degenerating fibers; and a significant increase in the number of fibers with preserved cytoskeleton and preservation of axonal and myelin area and integrity, reducing the enlargement and edema of the axon. In conclusion, our findings suggest that calpain inhibitor is able to provide neuroprotection of the central nervous system fibers after a crush lesion.

Depolarization-induced Ca2+ release in ischemic spinal cord white matter involves L-type Ca2+ channel activation of ryanodine receptors

The mechanisms of Ca(2+) release from intracellular stores in CNS white matter remain undefined. In rat dorsal columns, electrophysiological recordings showed that in vitro ischemia caused severe injury, which persisted after removal of extracellular Ca(2+); Ca(2+) imaging confirmed that an axoplasmic Ca(2+) rise persisted in Ca(2+)-free perfusate. However, depletion of Ca(2+) stores or reduction of ischemic depolarization (low Na(+), TTX) were protective, but only in Ca(2+)-free bath. Ryanodine or blockers of L-type Ca(2+) channel voltage sensors (nimodipine, diltiazem, but not Cd(2+)) were also protective in zero Ca(2+), but their effects were not additive with ryanodine. Immunoprecipitation revealed an association between L-type Ca(2+) channels and RyRs, and immunohistochemistry confirmed colocalization of Ca(2+) channels and RyR clusters on axons. Similar to "excitation-contraction coupling" in skeletal muscle, these results indicate a functional coupling whereby depolarization sensed by L-type Ca(2+) channels activates RyRs, thus releasing damaging amounts of Ca(2+) under pathological conditions in white matter.

Axonal injury in children after motor vehicle crashes: extent, distribution, and size of axonal swellings using beta-APP immunohistochemistry

The brains of 32 children (3 months to 16 years) who died as a result of motor vehicle collisions were examined for axonal injury using beta-APP immunohistochemistry. The extent and distribution of axonal injury was assessed and quantified throughout the forebrain, brainstem and cerebellum. The mean diameter of immunoreactive axons in the corpus callosum was measured for this pediatric group and, for comparison, a small adult sample. beta-APP immunoreactivity was seen in 14 pediatric cases (survival 35 mins to 87 h), most frequently in the parasagittal white matter (12/14), the corpus callosum (11/14), the brainstem (10/14) and cerebellum (9/14). In 2 cases, axon swelling was visualized in the internal capsule after only 35-45-min survival, earlier than has previously been reported. No immunoreactivity was seen in the remaining 18 cases who died within 1 h. The extent and distribution of axonal injury throughout the brain showed a rapid early increase with increasing survival time and then a slower progression. The diameter of individual callosal axons increased with increasing survival times, rapidly over the first 24 h and then more slowly. There was no statistical difference (p < 0.05) for callosal axon diameters at different survival times between the children and the adults sampled here. The extent and distribution of axonal injury throughout the brain appears to be similar in children to that previously reported in adults. The spatial and temporal spread of axonal damage suggests there may be therapeutic potential for the process to be arrested or slowed in its early stages.

Very early activation of m-calpain in peripheral nerve during Wallerian degeneration

Peripheral nerve injury results in a series of events culminating in degradation of the axonal cytoskeleton (Wallerian degeneration). In the time period between axotomy and cytoskeletal degradation (24-48 h in rodents), there is calcium entry and activation of calpains within the axon. The precise timing of these events during this period is unknown. In the present study, antibodies were generated to three distinct peptide epitopes of m-calpain, and a fusion protein antibody was generated to the intrinsic calpain inhibitor calpastatin. These antibodies were used to measure changes in these proteins in mouse sciatic nerves during Wallerian degeneration. In sciatic nerve homogenates and cultured dorsal root ganglion (DRG) neurites, m-calpain protein was significantly reduced in transected nerves very early after nerve injury, long before axonal degeneration occurred. Levels of m-calpain protein remained low as compared to control nerves for the remainder of the 72-h time course. No changes in calpastatin protein were evident. Systemic treatment of animals with the protease inhibitor leupeptin partially prevented the rapid loss of calpain protein. Removal of calcium in DRG cultures had the same effect. These data indicate that m-calpain protein is lost very early after axonal injury, and likely reflect activation and degradation of this protein long before the cytoskeleton is degraded. Calpain activation may be an early event in a proteolytic cascade that is initiated by axonal injury and culminates with axonal degeneration.

Glutamate, NMDA, and AMPA induced changes in extracellular space volume and tortuosity in the rat spinal cord

Glutamate release, particularly in pathologic conditions, may result in cellular swelling. The authors studied the effects of glutamate, N-methyl-D-aspartate (NMDA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) on extracellular pH (pH(e)), extracellular potassium concentration ([K(+)](e)), and changes in extracellular space (ECS) diffusion parameters (volume fraction alpha, tortuosity lambda) resulting from cellular swelling. In the isolated spinal cord of 4-to 12-day-old rats, the application of glutamate receptor agonists induced an increase in [K(+)](e), alkaline-acid shifts, a substantial decrease in alpha, and an increase in lambda. After washout of the glutamate receptor agonists, alpha either returned to or overshot normal values, whereas lambda remained elevated. Pretreatment with 20 mmol/L Mg(++), MK801, or CNQX blocked the changes in diffusion parameters, [K(+)](e) and pH(e) evoked by NMDA or AMPA. However, the changes in diffusion parameters also were blocked in Ca(2+)-free solution, which had no effect on the [K(+)](e) increase or acid shift. The authors conclude that increased glutamate release may produce a large, sustained and [Ca(2+)](e)-dependent decrease in alpha and increase in lambda. Repetitive stimulation and pathologic states resulting in glutamate release therefore may lead to changes in ECS volume and tortuosity, affecting volume transmission and enhancing glutamate neurotoxicity and neuronal damage.

Ampa/kainate receptor activation mediates hypoxic oligodendrocyte death and axonal injury in cerebral white matter

We developed an in situ model to investigate the hypothesis that AMPA/kainate (AMPA/KA) receptor activation contributes to hypoxic-ischemic white matter injury in the adult brain. Acute coronal brain slices, including corpus callosum, were prepared from adult mice. After exposure to transient oxygen and glucose deprivation (OGD), white matter injury was assessed by electrophysiology and immunofluorescence for oligodendrocytes and axonal neurofilaments. White matter cellular components and the stimulus-evoked compound action potential (CAP) remained stable for 12 hr after preparation. OGD for 30 min resulted in an irreversible loss of the CAP as well as structural disruption of axons and subsequent loss of neurofilament immunofluorescence. OGD also caused widespread oligodendrocyte death, demonstrated by the loss of APC labeling and the gain of pyknotic nuclear morphology and propidium iodide labeling. Blockade of AMPA/KA receptors with 30 microm NBQX or the AMPA-selective antagonist 30 microm GYKI 52466 prevented OGD-induced oligodendrocyte death. Oligodendrocytes also were preserved by the removal of Ca(2+), but not by a blockade of voltage-gated Na(+) channels. The protective action of NBQX was still present in isolated corpus callosum slices. CAP areas and axonal structure were preserved by Ca(2+) removal and partially protected by a blockade of voltage-gated Na(+) channels. NBQX prevented OGD-induced CAP loss and preserved axonal structure. These observations highlight convergent pathways leading to hypoxic-ischemic damage of cerebral white matter. In accordance with previous suggestions, the activation of voltage-gated Na(+) channels contributes to axonal damage. Overactivation of glial AMPA/KA receptors leads to oligodendrocyte death and also plays an important role in structural and functional disruption of axons.

Watery and dark axons in Wallerian degeneration of the opossum's optic nerve: different patterns of cytoskeletal breakdown?

In this paper we report a qualitative morphological analysis of Wallerian degeneration in a marsupial. Right optic nerves of opossums Didelphis marsupialis were crushed with a fine forceps and after 24, 48, 72, 96 and 168 hours the animals were anaesthetized and perfused with fixative. The optic nerves were immersed in fixative and processed for routine transmission electron microscopy. Among the early alterations typical of axonal degeneration, we observed nerve fibers with focal degeneration of the axoplasmic cytoskeleton, watery degeneration and dark degeneration, the latter being prevalent at 168 hours after crush. Our results point to a gradual disintegration of the axoplasmic cytoskeleton, opposed to the previous view of an "all-or-nothing" process (Griffin et al 1995). We also report that, due to an unknown mechanism, fibers show either a dark or watery pattern of axonal degeneration, as observed in axon profiles. We also observed fibers undergoing early myelin breakdown in the absence of axonal alterations.

Ionic mechanisms of aglycemic axon injury in mammalian central white matter

The authors investigated ionic mechanisms underlying aglycemic axon injury in adult rat optic nerve, a central white matter tract. Axon function was assessed using evoked compound action potentials (CAPs). Glucose withdrawal led to delayed CAP failure, an alkaline extracellular pH shift, and an increase in extracellular [K(+)]. Sixty minutes of glucose withdrawal led to irreversible axon injury. Aglycemic axon injury required extracellular calcium; the extent of injury progressively declined as bath [Ca(2+)] was decreased. To evaluate Ca(2+) movements during aglycemia, the authors recorded extracellular [Ca(2+)] ([Ca(2+)](o)) using Ca(2+)-sensitive microelectrodes. Under control conditions, [Ca(2+)](o) fell with a similar time course to CAP failure, indicating extracellular Ca(2+) moved to an intracellular position during aglycemia. The authors quantified the magnitude of [Ca(2+)]o decrease as the area below baseline [Ca(2+)]o during aglycemia and used this as a qualitative measure of Ca(2+) influx. The authors studied the mechanisms of Ca(2+) influx. Blockade of Na(+) influx reduced Ca(2+) influx and improved CAP recovery, suggesting Na(+)-Ca(2+) exchanger involvement. Consistent with this hypothesis, bepridil reduced axon injury. In addition, diltiazem or nifedipine decreased Ca(2+) influx and increased CAP recovery. The authors conclude aglycemic central white matter injury is caused by Ca(2+) influx into intracellular compartments through reverse Na(+)-Ca(2+) exchange and L-type Ca(2+) channels.

Axonal L-type Ca2+ channels and anoxic injury in rat CNS white matter

We studied the magnitude and route(s) of Ca2+ flux from extra- to intracellular compartments during anoxia in adult rat optic nerve (RON), a central white matter tract, using Ca2+ sensitive microelectrodes to monitor extracellular [Ca2+] ([Ca2+]o). One hour of anoxia caused a rapid loss of the stimulus-evoked compound action potential (CAP), which partially recovered following re-oxygenation, indicating that irreversible injury had occurred. After an initial increase caused by extracellular space shrinkage, anoxia produced a sustained decrease of 0.42 mM (29%) in [Ca2+]o. We quantified the [Ca2+]o decrease as the area below baseline [Ca2+]o during anoxia and used this as a qualitative index of suspected Ca2+ influx. The degree of RON injury was predicted by the amount of Ca2+ leaving the extracellular space. Bepridil, 0 Na+ artificial cerebrospinal fluid or tetrodotoxin reduced suspected Ca2+ influx during anoxia implicating reversal of the Na+/Ca2+ exchanger as a route of Ca2+ influx. Diltiazem reduced suspected Ca2+ influx during anoxia, suggesting that Ca2+ influx via L-type Ca2+ channels is a route of toxic Ca2+ influx into axons during anoxia. Immunocytochemical staining was used to demonstrate and localize high-threshold Ca2+ channels. Only alpha1(C) and alpha1(D) subunits were detected, indicating that only L-type Ca2+ channels were present. Double labeling with anti-neurofilament antibodies or anti-glial fibrillary acidic protein antibodies localized L-type Ca2+ channels to axons and astrocytes.

The gene for slow Wallerian degeneration (Wld(s)) is also protective against vincristine neuropathy

Neurological diseases are frequently associated with axonal degeneration, which leads to dysfunction though separation of neurons from their targets. The mechanisms of axonal degeneration are largely unknown and in many cases are independent of those occurring within cell bodies in neurodegenerative disorders. The Wld(s) mouse mutant demonstrates the unique phenotype of resistance to axonal degeneration after axotomy (slow Wallerian degeneration), making it a powerful tool for studying mechanisms of axonal degeneration. We asked whether the Wld(s) mutation also provides resistance to axonal degeneration in a slowly progressing neuropathy. Using cultured dorsal root ganglion neurons we compared the course of axonal degeneration in response to exposure to the neurotoxin vincristine and found that Wld(s) neurites were relatively resistant to vincristine neuropathy. These findings suggest common pathophysiologic mechanisms between axotomy-induced Wallerian degeneration and toxic neuropathy. The implications are wide-ranging and are relevant to the pathophysiology of axonal degeneration seen in a wide spectrum of neurological diseases ranging from stroke and head trauma to spinal cord injury and peripheral neuropathy.

Effect of nicardipine treatment on the expression of neurofilament 200 KDa immunoreactivity in the brain of spontaneously hypertensive rats

Neurofilaments (NFP) are components of neuronal cytoskeleton involved primarily in axonal transport and in the regulation of dynamic activities of nerve cells. NFP consist of three subunits denominated high- (200 kDa, NFP-H), intermediate- (160 kDa, NFP-I), and low-molecular weight (68 kDa, NFP-L) neurofilament proteins. Their function and polymerization depends on phosphorylation status, and is regulated by Ca2+ influx. Ca2+ overload enhances degradation of NFP and may compromise axonal transport. An increased susceptibility to ischemia occurs in hypertension, which is also a cause of brain damage. In this study, the expression of phosphorylated NFP (P-NFP) was investigated in the brain of spontaneously hypertensive rats (SHR) using immunohistochemical techniques with antibodies against the phosphorylated epitope of NFP RT-97. Microanatomical analysis included frontal cortex, occipital cortex, hippocampus and cerebellar cortex. The effect of long-term treatment with the dihydropyridine-type Ca2+ antagonist nicardipine on the expression of P-NFP was investigated as well. In hypertension a decreased P-NFP immunoreactivity was observed in frontal and occipital cortex, in the CA1 subfield of hippocampus and in the dentate gyrus, but not in the CA3 subfield of hippocampus or in the cerebellar cortex. Treatment with a daily dose of 3 mg/kg of nicardipine and 10 mg/kg of hydralazine significantly reduced systolic pressure in SHR. The above dose of nicardipine and to a lesser extent a non-hypotensive dose of the compound (0.1 mg/kg/day), but not hydralazine, increased P-NFP immunoreactivity in the cerebral cortex and hippocampus, except the CA3 subfield. The possibility that rescued P-NFP immunoreactivity by treatment with nicardipine depends on improved brain perfusion caused by the compound and/or by countering neuronal Ca2+ overload is discussed.

Early myelin breakdown following sural nerve crush: a freeze-fracture study

In this study we describe the early changes of the myelin sheath following surgical nerve crush. We used the freeze-fracture technique to better evaluate myelin alterations during an early stage of Wallerian degeneration. Rat sural nerves were experimentally crushed and animals were sacrificed by transcardiac perfusion 30 h after surgery. Segments of the nerves were processed for routine transmission electron microscopy and freeze-fracture techniques. Our results show that 30 h after the lesion there was asynchrony in the pattern of Wallerian degeneration, with different nerve fibers exhibiting variable degrees of axon disruption. This was observed by both techniques. Careful examination of several replicas revealed early changes in myelin membranes represented by vacuolization and splitting of consecutive lamellae, rearrangement of intramembranous particles and disappearance of paranodal transverse bands associated or not with retraction of paranodal myelin terminal loops from the axolemma. These alterations are compatible with a direct injury to the myelin sheath following nerve crush. The results are discussed in terms of a similar mechanism underlying both axon and myelin breakdown.

Modeling axonal injury in vitro: injury and regeneration following acute neuritic trauma

Traumatic injury to axons was modeled in vitro using sympathetic principal neurons from the rat superior cervical ganglion. Neurons were grown as a pure culture on collagen in parallel tracks, with cell somata confined to the center, and neurites occupying the periphery of the culture dish. Growing as fascicles on tracks, the neurites demonstrated periodic varicosities. Neuritic transection was reliably and reproducibly achieved with a motor driven rubber impactor injury device. During a period lasting at least 1 h, dieback involving the proximal neurites averaged 105 +/- 10 microm. This was followed by neurite regeneration, with the injured segment being traversed within 36 h at an average rate of regeneration of 595 +/- 15 microm/day. The distal neurite segments showed degenerative changes within 1 h following transection, with initial receding of neurites progressing to vacuolation, beading, blebbing, and eventual detachment from the underlying matrix. This in vitro model of axonal injury allows neuritic injury to be studied at the cellular and molecular levels, and also provides a unique opportunity to test potential neuromodulatory and neuroprotective strategies.

Lidocaine disrupts axonal membrane of rat sciatic nerve in vitro

Highly concentrated lidocaine has been reported to induce irreversible loss of membrane potential in crayfish nerve, which implies membrane disruption as one of the direct mechanisms of lidocaine-induced neurotoxicity. To confirm lidocaine-induced membrane disruption in mammalian nerve, a lactate dehydrogenase (LDH) leakage from rat sciatic nerve was measured in vitro. Before applying lidocaine, the desheathed nerve was incubated for 60 min in Krebs-Ringer solution at 37 degrees C to examine basal LDH activity. It was then incubated in 80 mM lidocaine solution at pH 7.3 for 15, 30, 60, or 120 min. Other nerves were immersed in 800 mM choline solution for 120 min. Total LDH activity per wet weight of nerve tissue was assayed using spectrophotometry. It was also determined using nerves cut into 10 segments and incubated in distilled water for 60 min. The LDH activity in the lidocaine group showed a time-dependent increase. After the 60- and 120-min incubation with lidocaine, the amount of LDH activity was significantly increased compared with the choline group and was similar to that of the group incubated in distilled water. We conclude that 80 mM lidocaine may be sufficient to cause membrane damage and facilitate the leakage of enzymes from cytoplasm.

IMPLICATIONS

This study demonstrates that exposing the rat myelinated nerve to lidocaine at a clinically used concentration for more than 30 min causes enough membrane damage to allow enzyme leakage. In clinical practice, the smallest effective dose should be used.

Effects of aging on myelinated nerve fibers in monkey primary visual cortex

In monkeys, myelin sheaths of the axons in the vertical bundles of nerve fibers passing through the deeper layers of primary visual cortex show age-related alterations in their structure. These alterations have been examined by comparing the myelin sheaths in young monkeys, 5-10 years old, with those in old monkeys, between 25 and 33 years of age. The age-related alterations are of four basic types. In some sheaths, there is local splitting of the major dense line to accommodate dense cytoplasm derived from the oligodendrocytes. Other sheaths balloon out, and in these locations, the intraperiod line in that part of the sheath opens up to surround a fluid-filled space. Other alterations are the formation of redundant myelin so that a sheath is too large for the enclosed axon and the formation of double sheaths in which one layer of compact myelin is surrounded by another one. These alterations in myelin increase in frequency with the ages of the monkeys, and there is a significant correlation between the breakdown of the myelin and the impairments in cognition exhibited by individual monkeys. This correlation also holds even when the old monkeys, 25 to 33 years of age, are considered as a group. It is suggested that the correlation between the breakdown of myelin in the old monkeys and their impairments in cognition has not to do specifically with visual function but to the role of myelin in axonal conduction throughout the brain. The breakdown of myelin could impair cognition by leading to a change in the conduction rates along axons, resulting in a loss of synchrony in cortical neuronal circuits.

White matter ischaemia

Brain and spinal cord white matter are vulnerable to the effects of ischaemia. Reduction of the energy supply leads to a cascade of events including depolarization, influx of Na(+) and the subsequent reverse operation of the membrane protein the Na(+)/Ca(2+) exchanger which ultimately terminates in intracellular Ca(2+) overload and irreversible axonal injury. Various points along the white matter damage cascade could be specifically targeted as a potential means of inhibiting the development of axonal irreversible injury.

Drug development for stroke: importance of protecting cerebral white matter

Multiple pharmacological mechanisms have been identified over the last decade which can protect grey matter from ischaemic damage in experimental models. A large number of drugs targeted at neurotransmitter receptors and related mechanisms involved in ischaemic damage have advanced to clinical trials in stroke and head injury based on their proven ability to reduce grey matter damage in animal models. The outcome to date of the clinical trials of neuroprotective drugs has been disappointing. Although the failure to translate preclinical pharmacological insight into therapy is multifactorial, we propose that the failure to ameliorate ischaemic damage to white matter has been a major factor. The recent development of quantitative techniques to assess ischaemic damage to cellular elements in white matter, both axons and oligodendrocytes, allows rigorous evaluation of pharmacologic mechanisms which may protect white matter in ischaemia. Such pharmacological approaches provide therapeutic opportunities which are both additional or alternatives to those currently being evaluated in man.

Axonal cytoskeleton changes in experimental optic neuritis

Axonal loss and degeneration in multiple sclerosis (MS) and experimental allergic encephalomyelitis (EAE) have been suggested by brain imaging, pathological and axonal transport studies. Further elucidation of the processes and mechanisms of axonal degeneration in demyelinating diseases is therefore of potential importance in order to alleviate the permanent disabilities of MS patients. However, detailed studies in this area are impeded by the small number of reliable models in which the onset and location of demyelination can be well-controlled. In this study, microinjection of polyclonal rabbit anti-galactocerebroside (anti-Gal C) antibody and guinea pig complement was used to induce local demyelination in the rat optic nerve. We found that treatment with appropriate volumes of the antibody and complement could induce local demyelination with minimal pressure- or trauma-induced damage. Local changes in neurofilaments (NFs) and microtubules (MTs) were examined with both immunohistochemistry (IHC) and electron microscopy (EM). On day 1 after microinjection, we observed moderate NF and MT disassembly in the local demyelinated area, although in most cases, no apparent inflammatory cell infiltration was seen. The NF and MT changes became more apparent on days 3, 5, 7 after microinjection, along with gradually increased inflammatory cell infiltration. These results suggested that acute demyelination itself may induce local cytoskeleton changes in the demyelinated axons, and that the ensuing local inflammation may further enhance the axonal damage. When the lesions were stained with specific antibodies for T lymphocytes, macrophages, and astrocytes, we found that most of the cells were macrophages, suggesting that macrophages may play a greater role in inflammation-related axonal degeneration and axonal loss. These results were confirmed and further characterized on the ultrastructural level.

The role of voltage-gated Ca2+ channels in anoxic injury of spinal cord white matter

Dorsal column axons of the rat spinal cord are partially protected from anoxic injury following blockade of voltage-sensitive Na+ channels and the Na+/--Ca2+ exchanger. To examine the potential contribution of voltage-gated Ca2+ channels to anoxic injury of spinal cord axons, we studied axonal conduction in rat dorsal columns in vitro following a 60-min period of anoxia. Glass microelectrodes were used to record field potentials from the dorsal columns following distal local surface stimulation. Perfusion solutions containing blockers of voltage-gated Ca2+ channels were introduced 60 min prior to onset of anoxia and continued until 10 min after reoxygenation. Pharmacological blocking agents which are relatively selective for L- (verapamil, diltiazem, nifedipine) and N- (omega-conotoxin GVIA) type calcium channels were significantly protective against anoxia-induced loss of conduction, as was non-specific block using divalent cations. Other Ca2+ channel blockers (neomycin and omega-conotoxin MVIIC) that affect multiple Ca2+ channel types were also neuroprotective. Ni2+, which preferentially blocks R-type Ca2+ channels more than T-type channels, was also protective in a dose-dependent manner. These data suggest that the influx of Ca2+, through L-, N- and possibly R-type voltage-gated Ca2+ channels, participates in the pathophysiology of the Ca2+-mediated injury of spinal cord axons that is triggered by anoxia.

Axonal injury caused by focal cerebral ischemia in the rat

The susceptibility of axons to blunt head injury is well established. However, axonal injury following cerebral ischemia has attracted less attention than damage in gray matter. We have employed immunocytochemical methods to assess the vulnerability of axons to cerebral ischemia in vivo. Immunocytochemistry was performed using antibodies to a synaptosomal-associated protein of 25 kDa (SNAP25), which is transported by fast anterograde transport; the 68-kDa neurofilament subunit (NF68kD); and microtubule-associated protein 5 (MAP5) on sections from rats subjected to 30 min and 1, 2, and 4 h of ischemia induced by permanent middle cerebral artery (MCA) occlusion. After 4 h of occlusion, there was increased SNAP25 immunoreactivity, which was bulbous in appearance, reminiscent of the axonal swellings that occur following blunt head injury. Increased SNAP25 immunoreactivity was present in circumscribed zones in the subcortical white matter and in the axonal tracts at the border of infarction, a pattern similar to that previously described for amyloid precursor protein. Although less marked, similar changes in immunoreactivity in axons were evident following 2 h of ischemia. MAP5 and NF68kD had striking changes in immunoreactivity in axonal tracts permeating the caudate nucleus within the MCA territory at 4 h. The appearance was roughened and disorganized compared with the smooth regular staining in axons within the nonischemic areas. Profiles reminiscent of axonal bulbs were evident in MAP5-stained sections. The changes seen with NF68kD and MAP5 were also evident at 2 h but were more subtle at 1 h. There were no changes in axonal immunoreactivity with SNAP25 or NF68kD at 30 min after MCA occlusion. Altered immunoreactivity following ischemia using SNAP25, MAP5, and NF68kD provides further evidence for the progressive breakdown of the axonal cytoskeleton following an ischemic insult. NF68kD and MAP5 appear to be sensitive markers of the structural disruption of the cytoskeleton, which precedes the subsequent accumulation of SNAP25 within the damaged axons. Axonal cytoskeletal breakdown and disruption of fast axonal transport, which are well-recognized features of traumatic brain injury, are also sequalae of an ischemic insult.

White matter changes in the gerbil brain under chronic cerebral hypoperfusion

BACKGROUND AND PURPOSE

An animal model of chronic cerebral hypoperfusion was developed with coiled clips applied to both carotid arteries of adult Mongolian gerbils for between 1 week and 2 months. In the brain of this animal model, rarefaction of white matter with dilatation of the ventricles was frequently observed. To better understand the mechanism of white matter alteration under cerebral hypoperfusion, the chronological sequence of molecular changes in the cerebral white matter of the animal model was determined.

METHODS

Specially designed coiled clips were placed around both carotid arteries of Mongolian gerbils to create stenosis without occlusion. Changes in levels of myelin basic protein (MBP) as a marker of myelin, neurofilament H (NFH) as a marker of axonal proteins, and glial fibrillary acidic protein (GFAP) in astroglia after 2 months of cerebral hypoperfusion were analyzed with Western blotting and enzyme-linked immunosorbent assay.

RESULTS

Western blotting of the white matter after 2 months of hypoperfusion showed that the levels of MBP and NFH decreased, whereas that of GFAP increased. The time course of MBP and NFH changes determined with enzyme-linked immunosorbent assay revealed that the change of MBP preceded that of NFH.

CONCLUSIONS

In the present study it was shown that the damage to myelin precedes that to the axon in the white matter in a chronic cerebral hypoperfusion animal model, suggesting that the change in myelin is the primary pathological event in the cerebral white matter under chronic hypoperfusion. The present study may help in understanding the mechanisms of white matter pathology in leukoaraiosis.

Mechanisms of calcium and sodium fluxes in anoxic myelinated central nervous system axons

Electron probe X-ray microanalysis was used to measure water content and concentrations of elements (i.e. Na, K, Cl and Ca) in selected morphological compartments of rat optic nerve myelinated axons. Transaxolemmal movements of Na+ and Ca2+ were modified experimentally and corresponding effects on axon element and water compositions were determined under control conditions and following in vitro anoxic challenge. Also characterized were effects of modified ion transport on axon responses to postanoxia reoxygenation. Blockade of Na+ entry by tetrodotoxin (1 microM) or zero Na+/Li(+)-substituted perfusion reduced anoxic increases in axonal Na and Ca concentrations. Incubation with zero-Ca2+/EGTA perfusate prevented axoplasmic and mitochondrial Ca accumulation during anoxia but did not affect Na increases or K losses in these compartments. Inhibition of Na(+)-Ca2+ exchange with bepridil (30 microM) selectively prevented increases in intra-axonal Ca, whereas neither nifedipine (5 microM) nor nimodipine (5 microM) influenced the effects of anoxia on axonal Na, K or Ca. X-ray microanalysis also showed that prevention of Na and Ca influx during anoxia obtunded severe elemental deregulation normally associated with reoxygenation. Results of the present study suggest that during anoxia, Na+ enters axons mainly through voltage-gated Na+ channels and that subsequent increases in axoplasmic Na+ are functionally coupled to extra-axonal Ca2+ import. Na+i-dependent, Ca2+o entry is consistent with reverse operation of the axolemmal Na(+)-Ca2+ exchanger and we suggest this route represents a primary mechanism of Ca2+ influx. Our findings also implicate a minor route of Ca2+ entry directly through Na+ channels.

Anoxic and ischemic injury of myelinated axons in CNS white matter: from mechanistic concepts to therapeutics

White matter of the brain and spinal cord is susceptible to anoxia and ischemia. Irreversible injury to this tissue can have serious consequences for the overall function of the CNS through disruption of signal transmission. Myelinated axons of the CNS are critically dependent on a continuous supply of energy largely generated through oxidative phosphorylation. Anoxia and ischemia cause rapid energy depletion, failure of the Na(+)-K(+)-ATPase, and accumulation of axoplasmic Na+ through noninactivating Na+ channels, with concentrations approaching 100 mmol/L after 60 minutes of anoxia. Coupled with severe K+ depletion that results in large membrane depolarization, high [Na+]i stimulates reverse Na(+)-Ca2+ exchange and axonal Ca2+ overload. A component of Ca2+ entry occurs directly through Na+ channels. The excessive accumulation of Ca2+ in turn activates various Ca(2+)-dependent enzymes, such as calpain, phospholipases, and protein kinase C, resulting in irreversible injury. The latter enzyme may be involved in "autoprotection," triggered by release of endogenous gamma-aminobutyric acid and adenosine, by modulation of certain elements responsible for deregulation of ion homeostasis. Glycolytic block, in contrast to anoxia alone, appears to preferentially mobilize internal Ca2+ stores; as control of internal Ca2+ pools is lost, excessive release from this compartment may itself contribute to axonal damage. Reoxygenation paradoxically accelerates injury in many axons, possibly as a result of severe mitochondrial Ca2+ overload leading to a secondary failure of respiration. Although glia are relatively resistant to anoxia, oligodendrocytes and the myelin sheath may be damaged by glutamate released by reverse Na(+)-glutamate transport. Use-dependent Na+ channel blockers, particularly charged compounds such as QX-314, are highly neuroprotective in vitro, but only agents that exist partially in a neutral form, such as mexiletine and tocainide, are effective after systemic administration, because charged species cannot penetrate the blood-brain barrier easily. These concepts may also apply to other white matter disorders, such as spinal cord injury or diffuse axonal injury in brain trauma. Moreover, whereas many events are unique to white matter injury, a number of steps are common to both gray and white matter anoxia and ischemia. Optimal protection of the CNS as a whole will therefore require combination therapy aimed at unique steps in gray and white matter regions, or intervention at common points in the injury cascades.

Resistance to anoxic injury in the dorsal columns of adult rat spinal cord following demyelination

In order to examine the relationship between myelination and sensitivity to anoxia in adult white matter, we studied action potential conduction in the spinal cord dorsal column of adult rats in which focal demyelinating lesions had been produced using ethidium bromide/X-irradiation. Acutely isolated spinal cords from control rats and following demyelination were maintained in vitro at 36 degrees C and compound action potentials were studied following supramaximal stimulation. The compound action potential was totally abolished within 12 min of the onset of anoxia in normal dorsal columns, but was not abolished until 50 min following the onset of anoxia in demyelinated dorsal columns. Compound action potentials showed significantly greater recovery (to 58.1 +/- 12.2% of control amplitude) in demyelinated dorsal columns compared to controls (30.8 +/- 5.3%) following 120 min of reoxygenation. These results show that focal demyelination is associated with reduced sensitivity to anoxia within white matter of the adult spinal cord.

Loss of axonal microtubules and neurofilaments after stretch-injury to guinea pig optic nerve fibers

Axonal swellings, characterized by focal accumulations of membranous organelles at presumed sites of interrupted axonal transport, occur in diffuse axonal injury (DAI) in human, blunt head injury and in animal models of nondisruptive axonal injury. Membranous organelles are transported by fast axonal transport in association with microtubules. Although loss of microtubules has been documented at levels of injury severe enough to result in permeabilization of the axolemma to tracers such as horseradish peroxidase, there has been no detailed analysis of responses by microtubules in less severe or milder forms of nondisruptive axonal injury. To test the hypothesis that in less severe forms of axonal injury there is a rapid response by axonal microtubules that might provide an explanation for loss of fast axonal transport, we have carried out a morphometric analysis of microtubules in CNS axons after stretch-injury. There is loss of microtubules at nodes of Ranvier with nodal blebs within 15 min of injury, and in internodal axonal swellings between 2 and 4 h. There is a return to control values at nodes of Ranvier by 4 h, and at the internode by 24 h. There is no loss of microtubules at paranodes, although there is a reduction in their density in the first 2 h after injury. The greatest loss of microtubules occurs at sites of axolemma infolding. Hypothetical mechanisms that might lead to this loss resulting in focal disruption of fast axonal transport and the formation of axonal swellings are discussed.

Removal of retrogradely transported material from rat lumbosacral alpha-motor axons by paranodal axon-Schwann cell networks

The aim of this study was to investigate the potential ability of Schwann cells to sequester axonally transported material via so called axon-Schwann cell networks (ASNs). These are entities consisting of sheets of Schwann cell adaxonal plasma membrane that invade the axon and segregate portions of axoplasm in paranodes of large myelinated mammalian nerve fibres. Rat hindlimb alpha-motor axons were examined in the L4-S1 ventral roots using light/fluorescence, confocal laser, and electron microscopy for detection of retrogradely transported red-fluorescent latex nanospheres taken up at a sciatic nerve crush, and intramuscularly injected horseradish peroxidase endocytosed by intact synaptic terminals. Survival times after tracer administration ranged from 27 hours to 4 weeks. During their retrograde transport toward the motor neuron perikarya, organelles carrying nanospheres/peroxidase accumulated at nodes of Ranvier, where they often appeared in close association with the paranodal myelin sheath. Serial section electron microscopy showed that many of the tracer-containing bodies were situated within ASN complexes, thereby being segregated from the main axon. Four weeks after nanosphere administration, several node-paranode regions still showed ASN-associated aggregations of spheres, some of which were situated in the adaxonal Schwann cell cytoplasm. The data establish the ability of Schwann cells to segregate material from motor axons with intact myelin sheaths, using the ASN as mediator. Taken together with our earlier observations that ASNs in alpha-motor axons are also rich in lysosomes, this process would allow a local elimination and secluded degradation of retrogradely transported foreign substances and degenerate organelles before reaching the motor neuron perikarya. In addition, ASNs may serve as sites for disposal of indigestable material.