Anoxic injury of rat optic nerve: ultrastructural evidence for coupling between Na+ influx and Ca(2+)-mediated injury in myelinated CNS axons

Preservation of thalamocortical circuitry is essential for good recovery after cardiac arrest

Continuous electroencephalographam (EEG) monitoring contributes to prediction of neurological outcome in comatose cardiac arrest survivors. While the phenomenology of EEG abnormalities in postanoxic encephalopathy is well known, the pathophysiology, especially the presumed role of selective synaptic failure, is less understood. To further this understanding, we estimate biophysical model parameters from the EEG power spectra from individual patients with a good or poor recovery from a postanoxic encephalopathy. This biophysical model includes intracortical, intrathalamic, and corticothalamic synaptic strengths, as well as synaptic time constants and axonal conduction delays. We used continuous EEG measurements from hundred comatose patients recorded during the first 48 h postcardiac arrest, 50 with a poor neurological outcome [cerebral performance category ( CPC = 5 ) ] and 50 with a good neurological outcome ( CPC = 1 ). We only included patients that developed (dis-)continuous EEG activity within 48 h postcardiac arrest. For patients with a good outcome, we observed an initial relative excitation in the corticothalamic loop and corticothalamic propagation that subsequently evolved towards values observed in healthy controls. For patients with a poor outcome, we observed an initial increase in the cortical excitation-inhibition ratio, increased relative inhibition in the corticothalamic loop, delayed corticothalamic propagation of neuronal activity, and severely prolonged synaptic time constants that did not return to physiological values. We conclude that the abnormal EEG evolution in patients with a poor neurological recovery after cardiac arrest may result from persistent and selective synaptic failure that includes corticothalamic circuitry and also delayed corticothalamic propagation.

Persistent sodium currents in SCN1A developmental and degenerative epileptic dyskinetic encephalopathy

Pathogenic variants in the voltage-gated sodium channel gene (SCN1A) are amongst the most common genetic causes of childhood epilepsies. There is considerable heterogeneity in both the types of causative variants and associated phenotypes; a recent expansion of the phenotypic spectrum of SCN1A associated epilepsies now includes an early onset severe developmental and epileptic encephalopathy with regression and a hyperkinetic movement disorder. Herein, we report a female with a developmental and degenerative epileptic-dyskinetic encephalopathy, distinct and more severe than classic Dravet syndrome. Clinical diagnostics indicated a paternally inherited c.5053G>T; p. A1685S variant of uncertain significance in SCN1A. Whole-exome sequencing detected a second de novo mosaic (18%) c.2345G>A; p. T782I likely pathogenic variant in SCN1A (maternal allele). Biophysical characterization of both mutant channels in a heterologous expression system identified gain-of-function effects in both, with a milder shift in fast inactivation of the p. A1685S channels; and a more severe persistent sodium current in the p. T782I. Using computational models, we show that large persistent sodium currents induce hyper-excitability in individual cortical neurons, thus relating the severe phenotype to the empirically quantified sodium channel dysfunction. These findings further broaden the phenotypic spectrum of SCN1A associated epilepsies and highlight the importance of testing for mosaicism in epileptic encephalopathies. Detailed biophysical evaluation and computational modelling further highlight the role of gain-of-function variants in the pathophysiology of the most severe phenotypes associated with SCN1A.

Prophylactic Riluzole Attenuates Oxidative Stress Damage in Spinal Cord Distraction

Spinal cord injury (SCI) without radiographical abnormalities (SCIWORA) presents a significant challenge because of the loss of function despite an apparent normal anatomy. The cause of dysfunction is not understood, and specific treatment options are lacking. Some scoliosis corrective surgeries result in SCIWORA, where stretching of the spinal cord can lead to vascular compromise and hypoxia. The iatrogenic nature of this injury allows for the implantation of neuroprotective strategies that are designed to prevent damage. We utilized a model of atraumatic SCI to evaluate the efficacy of the sodium-channel blocker, riluzole, as a prophylactic neuroprotectant. As expected, the stretch injury caused a significant reduction in intraparenchymal oxygen in distraction (-53.09 ± 22.23%) and riluzole pre-treated distraction animals (-43.04 ± 22.86%). However, in contrast to the oxidative stress and metabolic impairments observed in vehicle-treated distraction animals, in which protein carbonylation increased significantly (5.88 ± 1.3 nmol/mL), riluzole kept these levels within the normal range (1.8 ± 1.0 nmol/mL). This neurprotection also prevented ventral motor neuron hypoplasia and pyknosis, characteristic features of this atraumatic SCI model, and maintained normal gait function (e.g., stride length and stance time). This study provides evidence for the use of prophylactic neuroprotective strategies in which thoracic or spine surgeries present the risk of causing atraumatic SCI.

Axonal Damage in Traumatic Brain Injury

Axonal damage is one of the most common and important pathologic features of traumatic brain injury. Severe diffuse axonal injury, resulting from inertial forces applied to the head, is associated with prolonged unconsciousness and poor outcome. The susceptibility of axons to mechanical injury appears to be due to both their viscoelastic properties and their highly organized structure in white matter tracts. Although axons are supple under normal conditions, they become brittle when exposed to rapid deformations associated with brain trauma. Accordingly, rapid stretch of axons can damage the axonal cytoskeleton, resulting in a loss of elasticity and impairment of axoplasmic transport. Subsequent swelling of the axon occurs in discrete bulb formations or in elongated varicosities that accumulate organelles. Calcium entry into damaged axons is thought to initiate further damage by the activation of proteases and the induction of mitochondrial swelling and dysfunction. Ultimately, swollen axons may become disconnected and contribute to additional neuropathologic changes in brain tissue. However, promising new therapies that reduce proteolytic activity or maintain mitochondrial integrity may attenuate progressive damage of injured axons following experimental brain trauma. Future advancements in the prevention and treatment of traumatic axonal injury will depend on our collective understanding of the relationship between the biomechanics and pathophysiology of various phases of axonal trauma.

REVIEW ■ : Physical Injury of Neurons: Important Roles for Sodium and Chloride Ions

There is growing evidence that ions other than Ca2+ play important roles in the deterioration of neuronal elements in both gray and white matter after physical injury. This review features information gathered with a tissue culture model of dendrite transection regarding the contributions of Na+ and CI- to ultrastructural damage and neuronal death. This information and the results of other in vitro investigations of physical and ischemic/excitotoxic injuries indicate that elevation of internal Na+ is an early event that may contribute significantly to neuronal injury through effects on Na+-driven transport mechanisms. Proposed deleterious consequences include cytoplasmic acidification, reduced mitochondrial energy production, and elevation of intracellular Ca2+ and extracellular excitatory amino acids to toxic levels. Prevention of Na+ entry into neurons after injury has been found to limit ultrastructural damage, prevent death, and preserve electrophysiological function. Although the role of CI- in neuronal injury is less well defined, there is also evidence that elevation of intracellular CI- contributes to structural damage, particularly to the smooth endoplasmic reticulum. In terventions that limit Na+- and CI--mediated damage to injured neurons may have utility in neurosurgery and as acute phase treatments for nervous system trauma and other pathological states. NEURO SCIENTIST 3:89-101, 1997

Ischemic Preconditioning in White Matter: Magnitude and Mechanism

Ischemic preconditioning (IPC) is a robust neuroprotective phenomenon whereby brief ischemic exposure confers tolerance to a subsequent ischemic challenge. IPC has not been studied selectively in CNS white matter (WM), although stroke frequently involves WM. We determined whether IPC is present in WM and, if so, its mechanism. We delivered a brief in vivo preconditioning ischemic insult (unilateral common carotid artery ligation) to 12- to 14-week-old mice and determined WM ischemic vulnerability [oxygen-glucose deprivation (OGD)] 72 h later, using acutely isolated optic nerves (CNS WM tracts) from the preconditioned (ipsilateral) and control (contralateral) hemispheres. Functional and structural recovery was assessed by quantitative measurement of compound action potentials (CAPs) and immunofluorescent microscopy. Preconditioned mouse optic nerves (MONs) showed better functional recovery after OGD than the non-preconditioned MONs (31 ± 3 vs 17 ± 3% normalized CAP area, p < 0.01). Preconditioned MONs also showed improved axon integrity and reduced oligodendrocyte injury compared with non-preconditioned MONs. Toll-like receptor-4 (TLR4) and type 1 interferon receptor (IFNAR1), key receptors in innate immune response, are implicated in gray matter preconditioning. Strikingly, IPC-mediated WM protection was abolished in both TLR4(-/-) and IFNAR1(-/-) mice. In addition, IPC-mediated protection in WM was also abolished in IFNAR1(fl/fl) LysM(cre), but not in IFNAR1(fl/fl) control, mice. These findings demonstrated for the first time that IPC was robust in WM, the phenomenon being intrinsic to WM itself. Furthermore, WM IPC was dependent on innate immune cell signaling pathways. Finally, these data demonstrated that microglial-specific expression of IFNAR1 plays an indispensable role in WM IPC.

SIGNIFICANCE STATEMENT

Ischemic preconditioning (IPC) has been studied predominantly in gray matter, but stroke in humans frequently involves white matter (WM) as well. Here we describe a novel, combined in vivo/ex vivo mouse model to determine whether IPC occurs in WM. It does. Using genetically altered mice, we identified two innate immune cell receptors, Toll-like receptor 4 and type 1 interferon receptor (IFNAR1), that are required for IPC-mediated protection in WM. Furthermore, using microglia-targeted IFNAR1 knockdown, we demonstrate that interferon signaling specifically in microglia is essential for this protection. The discovery of IPC as an intrinsic capability of WM is novel and important. This is also the first in vivo demonstration that cell-type-specific expression of an individual gene plays an indispensable role in IPC-mediated protection.

Nodopathies of the peripheral nerve: an emerging concept

Peripheral nerve diseases are traditionally classified as demyelinating or axonal. It has been recently proposed that microstructural changes restricted to the nodal/paranodal region may be the key to understanding the pathophysiology of antiganglioside antibody mediated neuropathies. We reviewed neuropathies with different aetiologies (dysimmune, inflammatory, ischaemic, nutritional, toxic) in which evidence from nerve conductions, excitability studies, pathology and animal models, indicate the involvement of the nodal region in the pathogenesis. For these neuropathies, the classification in demyelinating and axonal is inadequate or even misleading, we therefore propose a new category of nodopathy that has the following features: (1) it is characterised by a pathophysiological continuum from transitory nerve conduction block to axonal degeneration; (2) the conduction block may be due to paranodal myelin detachment, node lengthening, dysfunction or disruption of Na(+) channels, altered homeostasis of water and ions, or abnormal polarisation of the axolemma; (3) the conduction block may be promptly reversible without development of excessive temporal dispersion; (4) axonal degeneration, depending on the specific disorder and its severity, eventually follows the conduction block. The term nodopathy focuses to the site of primary nerve injury, avoids confusion with segmental demyelinating neuropathies and circumvents the apparent paradox that something axonal may be reversible and have a good prognosis.

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.

Pathophysiology of immune-mediated demyelinating neuropathies--Part II: Neurology

In the second part of this review we deal with the clinical aspects of immune-mediated demyelinating neuropathies. We describe the relationship between pathophysiology and symptoms and discuss the pathophysiology of specific disease entities, including Guillain-Barré syndrome, chronic inflammatory demyelinating polyneuropathy, multifocal motor neuropathy, anti-myelin-associated glycoprotein neuropathy, and POEMS syndrome.

Ionic transporter activity in astrocytes, microglia, and oligodendrocytes during brain ischemia

Glial cells constitute a large percentage of cells in the nervous system. During recent years, a large number of studies have critically attributed to glia a new role which no longer reflects the long-held view that glia constitute solely a silent and passive supportive scaffolding for brain cells. Indeed, it has been hypothesized that glia, partnering neurons, have a much more actively participating role in brain function. Alteration of intraglial ionic homeostasis in response to ischemic injury has a crucial role in inducing and maintaining glial responses in the ischemic brain. Therefore, glial transporters as potential candidates in stroke intervention are becoming promising targets to enhance an effective and additional therapy for brain ischemia. In this review, we will describe in detail the role played by ionic transporters in influencing astrocyte, microglia, and oligodendrocyte activity and the implications that these transporters have in the progression of ischemic lesion.

Neurological perspectives on voltage-gated sodium channels

The activity of voltage-gated sodium channels has long been linked to disorders of neuronal excitability such as epilepsy and chronic pain. Recent genetic studies have now expanded the role of sodium channels in health and disease, to include autism, migraine, multiple sclerosis, cancer as well as muscle and immune system disorders. Transgenic mouse models have proved useful in understanding the physiological role of individual sodium channels, and there has been significant progress in the development of subtype selective inhibitors of sodium channels. This review will outline the functions and roles of specific sodium channels in electrical signalling and disease, focusing on neurological aspects. We also discuss recent advances in the development of selective sodium channel inhibitors.

Disrupted axo-glial junctions result in accumulation of abnormal mitochondria at nodes of ranvier

Mitochondria and other membranous organelles are frequently enriched in the nodes and paranodes of peripheral myelinated axons, particularly those of large caliber. The physiologic role(s) of this organelle enrichment and the rheologic factors that regulate it are not well understood. Previous studies suggest that axonal transport of organelles across the nodal/paranodal region is locally regulated. In this study, we have examined the ultrastructure of myelinated axons in the sciatic nerves of mice deficient in the contactin-associated protein (Caspr), an integral junctional component. These mice, which lack the normal septate-like junctions that promote attachment of the glial (paranodal) loops to the axon, contain aberrant mitochondria in their nodal/paranodal regions. These mitochondria are typically large and swollen and occupy prominent varicosities of the nodal axolemma. In contrast, mitochondria located outside the nodal/paranodal regions of the myelinated axons appear normal. These findings suggest that paranodal junctions regulate mitochondrial transport and function in the axoplasm of the nodal/paranodal region of myelinated axons of peripheral nerves. They further implicate the paranodal junctions in playing a role, either directly or indirectly, in the local regulation of energy metabolism in the nodal region.

Axonal protection achieved by blockade of sodium/calcium exchange in a new model of ischemia in vivo

Ischemic white matter injury has been relatively little studied despite its importance to the outcome of stroke. To aid such research a new rat model has been developed in vivo and used to assess whether blockade of the sodium/calcium exchanger is effective in protecting central axons from ischemic injury. Vasoconstrictive agent endothelin-1 was injected into the rat spinal cord to induce ischemia. KB-R7943 or SEA0400 was administered systemically to block the operation of the sodium/calcium exchanger. Endothelin-1 caused profound reduction of local blood perfusion and resulted in a prompt loss of axonal conduction. Whereas recovery of conduction following vehicle administration was only to 10.5 ± 9% of baseline (n = 8) 4.5 h after endothelin-1 injection, recovery following KB-R7943 (30 mg/kg, i.a.) administration was increased to 35 ± 9% of baseline (n = 6; P < 0.001). SEA0400 (30 mg/kg, i.a.) was also protective (33.2 ± 6% of baseline, n = 4; P < 0.001). Neither drug improved conduction by diminishing the severity of the ischemia. The protective effect of KB-R7943 persisted for at least 3 days after ischemia, as it improved axonal conduction (76.3 ± 11% for KB-R7943 vs. 51.0 ± 19% for vehicle; P < 0.01) and reduced lesion area (55.6 ± 15% for KB-R7943 vs. 77.9 ± 9% for vehicle; P < 0.01) at this time. In conclusion, a new model of white matter ischemia has been introduced suitable for both structural and functional studies in vivo. Blocking the sodium/calcium exchanger protects central axons from ischemic injury in vivo.

Proinflammatory and proapoptotic markers in relation to mono and di-cations in plasma of autistic patients from Saudi Arabia

Objectives

Autism is a developmental disorder characterized by social and emotional deficits, language impairments and stereotyped behaviors that manifest in early postnatal life. This study aims to clarify the relationship amongst absolute and relative concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺ and/or proinflammatory and proapoptotic biomarkers.

Materials and methods

Na⁺, K⁺, Ca²⁺, Mg²⁺, Na⁺/K⁺, Ca²⁺/Mg²⁺ together with IL6, TNFα as proinflammatory cytokines and caspase3 as proapoptotic biomarker were determined in plasma of 25 Saudi autistic male patients and compared to 16 age and gender matching control samples.

Results

The obtained data recorded that Saudi autistic patients have a remarkable lower plasma caspase3, IL6, TNFα, Ca²⁺ and a significantly higher K⁺ compared to age and gender matching controls. On the other hand both Mg²⁺ and Na⁺ were non-significantly altered in autistic patients. Pearson correlations revealed that plasma concentrations of the measured cytokines and caspase-3 were positively correlated with Ca²⁺ and Ca²⁺/K⁺ ratio. Reciever Operating Characteristics (ROC) analysis proved that the measured parameters recorded satisfactory levels of specificity and sensitivity.

Conclusion

Alteration of the selected measured ions confirms that oxidative stress and defective mitochondrial energy production could be contributed in the pathogenesis of autism. Moreover, it highlights the relationship between the measured ions, IL6, TNFα and caspase3 as a set of signalling pathways that might have a role in generating this increasingly prevalent disorder. The role of ions in the possible proinflammation and proapoptic mechanisms of autistics' brains were hypothesized and explained.

Anaerobic function of CNS white matter declines with age

The mammalian central nervous system (CNS) is generally believed to be completely dependent on the presence of oxygen (O(2)) to maintain energy levels necessary for excitability. However, previous studies on CNS white matter (WM) have shown that a large subset of CNS-myelinated axons of mice aged 4 to 6 weeks remains excitable in the absence of O(2). We investigated whether this surprising WM tolerance to anoxia varied with age. Acutely isolated mouse optic nerve (MON), a purely myelinated WM tract, was studied electrophysiologically. Excitability in the MONs from 1-month-, 4-month-, and 8-month-old mice was assessed quantitatively as the area under the supramaximal compound action potential (CAP). Anoxia-resistant WM function declined with age. After 60  minutes of anoxia, ∼23% of the CAP remained in 1-month-old mice, 8% in 4-month-old mice, and ∼0 in the 8-month-old group. Our results indicated that although some CNS axons function anaerobically in young adult animals, they lose this ability in later adulthood. This finding may help explain the clinical impression that favorable outcome after stroke and other brain injuries declines with age.

Wallerian-like axonal degeneration in the optic nerve after excitotoxic retinal insult: an ultrastructural study

Background

Excitotoxicity is involved in the pathogenesis of a number neurodegenerative diseases, and axonopathy is an early feature in several of these disorders. In models of excitotoxicity-associated neurological disease, an excitotoxin delivered to the central nervous system (CNS), could trigger neuronal death not only in the somatodendritic region, but also in the axonal region, via oligodendrocyte N-methyl-D-aspartate (NMDA) receptors. The retina and optic nerve, as approachable regions of the brain, provide a unique anatomical substrate to investigate the "downstream" effect of isolated excitotoxic perikaryal injury on central nervous system (CNS) axons, potentially providing information about the pathogenesis of the axonopathy in clinical neurological disorders.

Herein, we provide ultrastructural information about the retinal ganglion cell (RGC) somata and their axons, both unmyelinated and myelinated, after NMDA-induced retinal injury. Male Sprague-Dawley rats were killed at 0 h, 24 h, 72 h and 7 days after injecting 20 nM NMDA into the vitreous chamber of the left eye (n = 8 in each group). Saline-injected right eyes served as controls. After perfusion fixation, dissection, resin-embedding and staining, ultrathin sections of eyes and proximal (intraorbital) and distal (intracranial) optic nerve segments were evaluated by transmission electron tomography (TEM).

Results

TEM demonstrated features of necrosis in RGCs: mitochondrial and endoplasmic reticulum swelling, disintegration of polyribosomes, rupture of membranous organelle and formation of myelin bodies. Ultrastructural damage in the optic nerve mimicked the changes of Wallerian degeneration; early nodal/paranodal disturbances were followed by the appearance of three major morphological variants: dark degeneration, watery degeneration and demyelination.

Conclusion

NMDA-induced excitotoxic retinal injury causes mainly necrotic RGC somal death with Wallerian-like degeneration of the optic nerve. Since axonal degeneration associated with perikaryal excitotoxic injury is an active, regulated process, it may be amenable to therapeutic intervention.

Glutamate excitotoxicity inflicts paranodal myelin splitting and retraction

Paranodal myelin damage is observed in white matter injury. However the culprit for such damage remains unknown. By coherent anti-Stokes Raman scattering imaging of myelin sheath in fresh tissues with sub-micron resolution, we observed significant paranodal myelin splitting and retraction following glutamate application both ex vivo and in vivo. Multimodal multiphoton imaging further showed that glutamate application broke axo-glial junctions and exposed juxtaparanodal K⁺ channels, resulting in axonal conduction deficit that was demonstrated by compound action potential measurements. The use of 4-aminopyridine, a broad-spectrum K⁺ channel blocker, effectively recovered both the amplitude and width of compound action potentials. Using CARS imaging as a quantitative readout of nodal length to diameter ratio, the same kind of paranodal myelin retraction was observed with applications of Ca²⁺ ionophore A23187. Moreover, exclusion of Ca²⁺ from the medium or application of calpain inhibitor abolished paranodal myelin retraction during glutamate exposure. Examinations of glutamate receptor agonists and antagonists further showed that the paranodal myelin damage was mediated by NMDA and kainate receptors. These results suggest that an increased level of glutamate in diseased white matter could impair paranodal myelin through receptor-mediated Ca²⁺ overloading and subsequent calpain activation.

Bioenergetics of cerebral ischemia: a cellular perspective

In cerebral ischemia survival of neurons, astrocytes, oligodendrocytes and endothelial cells is threatened during energy deprivation and/or following re-supply of oxygen and glucose. After a brief summary of characteristics of different cells types, emphasizing the dependence of all on oxidative metabolism, the bioenergetics of focal and global ischemia is discussed, distinguishing between events during energy deprivation and subsequent recovery attempt after re-circulation. Gray and white matter ischemia are described separately, and distinctions are made between mature and immature brains. Next comes a description of bioenergetics in individual cell types in culture during oxygen/glucose deprivation or exposure to metabolic inhibitors and following re-establishment of normal aerated conditions. Due to their expression of NMDA and non-NMDA receptors neurons and oligodendrocytes are exquisitely sensitive to excitotoxicity by glutamate, which reaches high extracellular concentrations in ischemic brain for several reasons, including failing astrocytic uptake. Excitotoxicity kills brain cells by energetic exhaustion (due to Na(+) extrusion after channel-mediated entry) combined with mitochondrial Ca(2+)-mediated injury and formation of reactive oxygen species. Many (but not all) astrocytes survive energy deprivation for extended periods, but after return to aerated conditions they are vulnerable to mitochondrial damage by cytoplasmic/mitochondrial Ca(2+) overload and to NAD(+) deficiency. Ca(2+) overload is established by reversal of Na(+)/Ca(2+) exchangers following Na(+) accumulation during Na(+)-K(+)-Cl(-) cotransporter stimulation or pH regulation, compensating for excessive acid production. NAD(+) deficiency inhibits glycolysis and eventually oxidative metabolism, secondary to poly(ADP-ribose)polymerase (PARP) activity following DNA damage. Hyperglycemia can be beneficial for neurons but increases astrocytic death due to enhanced acidosis.

Acute energy restriction triggers Wallerian degeneration in mouse

Acute exposure of peripheral axons to the free radical Nitric Oxide (NO) may trigger conduction block and, if prolonged, Wallerian degeneration. It was hypothesized that this neurotoxic effect of NO may be due primarily to energy restriction by inhibition of mitochondrial respiration. We compared the neurotoxic effect of NO with the effect of the mitochondrial uncoupler 2,4-dinitrophenol (DNP) on electrically active axons of mouse sciatic nerve. The right tibial nerve was stimulated at the ankle. Muscle responses were recorded from plantar muscles and ascending nerve action potentials were recorded form the exposed sciatic nerve by means of a hook electrode. The sciatic nerve was focally immersed over a length of 1 cm in either phosphate buffered saline (PBS), a solution of approximately 4 microM NO obtained from 10 mM of the NO-donor DETA NONOate, or a solution of up to 1 mM DNP. Following 3 hours of 200 Hz stimulation, the nerves were washed in PBS for 1 hour, the surgical wounds were closed and the mice were left to recover. Following repetitive stimulation in PBS, the nerve responses recovered within 1 hour and the muscle responses within 1 day. The effects of focal acute exposure to NO or DNP were similar: (i) a transient conduction failure that rapidly normalized within one hour of washout and (ii) subsequent Wallerian degeneration of some axons confirmed at morphological studies. Taken together, these data support the hypothesis that neurotoxicity may be caused by energy restriction. Since the pharmacologic effect of NO and DNP was only transient, our data suggest that even a brief period of focal energy restriction can trigger Wallerian degeneration.

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.

Interferon-alpha2b may impair myelinization of rat optic nerve

This story investigated the effects of interferon-alpha-2b (IFN-alpha2b) on the optic nerves of 17 adult male Wistar albino rats. Animals were divided into 3 groups: 6 rats (group 1) received 7.5 units (5 mIU/m2) IFN-alpha2b-a normal treatment dose, and 6 (groups 2) received 30.0 units (20 mIU/m2)-a high dose; 5 rats (control group) received 0.5 mL saline. Test substances were delivered by intraperitoneal injection 3 times a week for 3 weeks with animals under inhalation anesthesia. After the rats were sacrificed, their optic nerves were dissected, sectioned, and examined under an electron microscope. The mean thicknesses of the basal membranes of blood vessels were 86.354 nm in the control group, 104.297 nm in group 1, and 140.181 nm in group 2. Basal membrane changes in IFN groups were dose dependent. Mitochondrial swelling, degeneration, increased diameter of vacuoles, and vacuolization in the cytoplasm of oligodendrocytes and astrocytes were also observed. IFN-alpha2b has histopathologic effects on blood vessels and cells of the optic nerve.

Protective effects of betaxolol in eyes with kainic acid-induced neuronal death

In the present study, we investigated whether betaxolol, a selective beta1-adrenoceptor antagonist, has neuroprotective effect on kainic acid (KA)-induced retinal damage. Neurotoxicities were induced in adult male rats by intravitreal injection of KA (total amount, 6 nmol). To examine the neuroprotective effects of betaxolol, rats were pretreated with betaxolol topically 60 min before KA injection to the rat eyes and twice daily for 1, 3, and 7 days after KA injection. The neuroprotective effects of betaxolol were estimated by measuring the thickness of the various retinal layers, and by counting the number of choline acetyltransferase (ChAT)- and tyrosine hydroxylase (TH)-positive cells in each retinal layer. The retina is highly vulnerable to KA-induced neuronal damage. Morphometric analysis of retinal damage in KA injected eyes, the thickness of the retinal layers decreased markedly after KA injection period of both 3 and 7 days. Furthermore, the numbers of ChAT- and TH-positive cells were significantly reduced by intravitreal injection of KA. However, when two drops of betaxolol, once before KA injection and twice daily for 7 days after KA injection, were continuously administered, the reductions in the retinal thickness and the retinal ChAT- and TH-positive cells were significantly attenuated. The present study suggests that topically applied betaxolol has neuroprotective effect on the retinal cell damage due to KA-induced neurotoxicity.

Cytosolic Ca2+ under high glucose with suppressed Na+/K+ pump activity in rat sensory neurons

Cytosolic Ca2+ concentration ([Ca2+]i) was measured in isolated rat dorsal root ganglion (DRG) neurons using the fluorescent Ca2+ indicator fura-2. Exposure to high (50 mM) extracellular K+ evoked a robust increase in [Ca2+]i, which was almost totally abolished by concomitant presence of nisoldipine (10 microM) and omega-conotoxin GVIA (10 microM). Whereas either high (30 mM) D-glucose alone or ouabain (100 microM) alone did not appreciably affect the high K+-induced [Ca2+]i elevation, neurons pretreated with high D-glucose together with ouabain exhibited a significantly larger [Ca2+]i response to high K+ stimulation, which was almost completely inhibited by nisoldipine and omega-conotoxin GVIA. These results suggest that a combination of high glucose and suppressed Na+/K+ pump activity potentiates the [Ca2+]i elevation stimulated by activation of the voltage-gated Ca2+ channels in rat DRG neurons.

Activation of voltage-sensitive sodium channels during oxygen deprivation leads to apoptotic neuronal death

Sodium (Na(+)) entry into neurons during hypoxia is known to be associated with cell death. However, it is not clear whether Na(+) entry causes cell death and by what mechanisms this increased Na(+) entry induces death. In this study we used cultures of rat neocortical neurons to show that an increase in intracellular sodium (Na(i)(+)) through voltage-sensitive sodium channels (VSSCs), during hypoxia contributes to apoptosis. Hypoxia increased Na(i)(+) and induced neuronal apoptosis, as assessed by electron microscopy, annexin V staining, and terminal UDP nick end labeling staining. Reducing Na(+) entry with the VSSC blocker, tetrodotoxin (TTX), attenuated apoptotic neuronal death via a reduction in caspase-3 activation. Since the attenuation of apoptosis by TTX during hypoxia suggested that the activation of VSSCs and Na(+) entry are crucial events in hypoxia-induced cell death, we also determined whether the activation of VSSCs per se could lead to apoptosis under resting conditions. Increasing Na(+) entry with the VSSC activator veratridine also induced neuronal apoptosis and caspase-3 activation. These data indicate that a) Na(+) entry via VSSCs during hypoxia leads to apoptotic cell death which is mediated, in part, by caspase-3 and b) activation of VSSCs during oxygen deprivation is a major event by which hypoxia induces cell death.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

Beta-adrenergic receptor agonists and antagonists counteract LPS-induced neuronal death in retinal cultures by different mechanisms

Treatment with lipopolysaccharide (LPS) for 72 h was shown to dose-dependently increase nitric oxide production from 6-day-old retinal cultures. Cell death, as determined by lactate dehydrogenase (LDH) release and an increase in neuronal labelling for TUNEL, was elevated concurrently. During treatment there was an increase of both inducible nitric oxide synthase and glial fibrillary acidic protein labelling in glial cells and a reduction in the number of gamma-aminobutyric acid-positive neurones. The NOS inhibitors, N-nitro-L-arginine methyl ester, dexamethasone and indomethacin potently inhibited both nitric oxide stimulation and cell death caused by LPS. In this study, the beta(2)- (ICI-18551), beta(1)- (betaxolol) and mixed beta(1)/beta(2)- (timolol, metipranolol) adrenergic receptor antagonists were all shown to attenuate LPS-induced LDH release from these cultures, but to have no effect on LPS-stimulated nitric oxide production. This effect was mimicked by the calcium channel blocker, nifedipine. Interestingly, the beta-adrenergic receptor agonists, salbutamol, arterenol and isoproterenol were also able to attenuate cell death caused by LPS. Moreover, these compounds also inhibited LPS-stimulated nitric oxide release. These studies suggest that LPS stimulates nitric oxide release from cultured retinal glial cells and that this process leads to neurone death. beta-adrenergic receptor agonists prevent the effects of LPS by inhibiting the stimulation of nitric oxide production. The data also suggest that beta-adrenergic receptor antagonists can attenuate LPS-induced death of neurones, but that these compounds act in a manner that is neurone-dependent, is mimicked by blockade of calcium channels and is independent of the stimulation of nitric oxide release.

Aberrant chloride transport contributes to anoxic/ischemic white matter injury

Rundown of ionic gradients is a central feature of white matter anoxic injury; however, little is known about the contribution of anions such as Cl-. We used the in vitro rat optic nerve to study the role of aberrant Cl- transport in anoxia/ischemia. After 30 min of anoxia (NaN3, 2 mm), axonal membrane potential (V(m)) decreased to 42 +/- 11% of control and to 73 +/- 11% in the presence of tetrodotoxin (TTX) (1 microm). TTX + 4,4'-diisothiocyanatostilbene-2,2' disulfonic acid disodium salt (500 microm), a broad spectrum anion transport blocker, abolished anoxic depolarization (95 +/- 8%). Inhibition of the K-Cl cotransporter (KCC) (furosemide 100 microm) together with TTX was also more effective than TTX alone (84 +/- 14%). The compound action potential (CAP) area recovered to 26 +/- 6% of control after 1 hr anoxia. KCC blockade (10 microm furosemide) improved outcome (40 +/- 4%), and TTX (100 nm) was even more effective (74 +/- 12%). In contrast, the Cl- channel blocker niflumic acid (50 microm) worsened injury (6 +/- 1%). Coapplication of TTX (100 nm) + furosemide (10 microm) was more effective than either agent alone (91 +/- 9%). Furosemide was also very effective at normalizing the shape of the CAPs. The KCC3a isoform was localized to astrocytes. KCC3 and weaker KCC3a was detected in myelin of larger axons. KCC2 was seen in oligodendrocytes and within axon cylinders. Cl- gradients contribute to resting optic nerve membrane potential, and transporter and channel-mediated Cl- fluxes during anoxia contribute to injury, possibly because of cellular volume changes and disruption of axo-glial integrity, leading to propagation failure and distortion of fiber conduction velocities.

The beta-adrenoceptor antagonists metipranolol and timolol are retinal neuroprotectants: comparison with betaxolol

beta-adrenoceptor antagonists are used clinically to reduce elevated intraocular pressure in glaucoma which is characterised by a loss of retinal ganglion cells. Previous studies have shown that the beta(1)-selective adrenoceptor antagonist, betaxolol, is additionally able to protect retinal neurones in vitro and ganglion cells in vivo from the detrimental effects of either ischemia-reperfusion or from excitotoxicity, after topical application. The neuroprotective effect of betaxolol is thought not to be elicited through an interaction with beta-adrenoceptors, but by its ability to reduce influx of sodium and calcium through voltage-sensitive calcium and sodium channels. In the present study it is shown that the non-selective beta-adrenoceptor antagonists, metipranolol and timolol behave like betaxolol. When topically applied they all attenuate the detrimental effect of ischemia-reperfusion. Protection of the retina was determined by evaluating changes in the electroretinogram and by assessing the loss of mRNA for Thy-1, which is expressed in retinal ganglion cells. In addition, studies conducted on neurones in mixed retinal cultures demonstrated that metipranolol, betaxolol and timolol were all able to partially counteract anoxia-induced cell loss and viability reduction. The influence of timolol was, however, not significant. Within the confines of these investigations, an order of neuroprotective efficacy was delineated for the three beta-adrenoceptor antagonists: betaxolol>metipranolol>timolol. The ability of the beta-adrenoceptor antagonists to attenuate ligand-induced stimulation of calcium and sodium entry into neuronal preparations showed a similar order of effectiveness. In conclusion, the ability to confer neuroprotection to retinal neurones is a common feature of three ophthalmic beta-adrenoceptor antagonists (betaxolol, metipranolol and timolol). A comparison of the effectiveness of the individual compounds in protecting retinal cells in vivo was not possible in these studies. However, in vitro studies show that the capacity of the individual beta-adrenoceptor antagonists to act as neuroprotectants appears to relate to their capacity to attenuate neuronal calcium and sodium influx.

Blockers of sodium and calcium entry protect axons from nitric oxide-mediated degeneration

Axonal degeneration can be an important cause of permanent disability in neurological disorders in which inflammation is prominent, including multiple sclerosis and Guillain-Barré syndrome. The mechanisms responsible for the degeneration remain unclear, but it is likely that axons succumb to factors produced at the site of inflammation, such as nitric oxide (NO). We previously have shown that axons exposed to NO in vivo can undergo degeneration, especially if the axons are electrically active during NO exposure. The axons may degenerate because NO can inhibit mitochondrial respiration, leading to intraaxonal accumulation of Na(+) and Ca(2+) ions. Here, we show that axons can be protected from NO-mediated damage using low concentrations of Na(+) channel blockers, or an inhibitor of Na(+)/Ca(2+) exchange. Our findings suggest a new strategy for axonal protection in an inflammatory environment, which may be effective in preventing the accumulation of permanent disability in patients with neuroinflammatory disorders.

The role of nitric oxide in multiple sclerosis

Nitric oxide (NO) is a free radical found at higher than normal concentrations within inflammatory multiple sclerosis (MS) lesions. These high concentrations are due to the appearance of the inducible form of nitric oxide synthase (iNOS) in cells such as macrophages and astrocytes. Indeed, the concentrations of markers of NO production (eg, nitrate and nitrite) are raised in the CSF, blood, and urine of patients with MS. Circumstantial evidence suggests that NO has a role in several features of the disease, including disruption of the blood-brain barrier, oligodendrocyte injury and demyelination, axonal degeneration, and that it contributes to the loss of function by impairment of axonal conduction. However, despite these considerations, the net effect of NO production in MS is not necessarily deleterious because it also has several beneficial immunomodulatory effects. These dual effects may help to explain why iNOS inhibition has not provided reliable and encouraging results in animal models of MS, but alternative approaches based on the inhibition of superoxide production, partial sodium-channel blockade, or the replacement of lost immunomodulatory function, may prove beneficial.

Time course studies on the effectiveness of tetrodotoxin in reducing consequences of spinal cord contusion

Focal injection of the sodium channel blocker tetrodotoxin (TTX) into the injury site at either 5 or 15 min after a standardized thoracic contusion spinal cord injury (SCI) reduces white matter pathology and loss of axons in the first 24 hr after injury. Focal injection of TTX at 15 min after SCI also reduces chronic white matter loss and hindlimb functional deficits. We have now tested the hypothesis that the reduction in chronic deficits with TTX treatment is associated with long-term preservation of axons after SCI and compared both acute (24 hr) and chronic (6 weeks) effects of TTX administered at 15 min prior to and 5 min or 4 hr after SCI. Our results indicate a significant reduction of acute white matter pathology in rats treated with TTX at 15 min before and 5 min after injury but no effect when treatment was delayed until 4 hr after contusion. Compared with injury controls, groups treated with TTX at 5 min and 4 hr after injury did not show a significant deficit reduction, nor was there a significant sparing of white matter at 6 weeks compared with injury controls. In contrast, the group treated with TTX at 15 min before SCI demonstrated significantly reduced hindlimb functional deficits beginning at 1 week after injury and throughout the 6 weeks of the study. This was associated with a significantly higher axon density in the ventromedial white matter at 6 weeks. The results demonstrate that blockade of sodium channels preserves axons from loss after SCI and points to the importance of time of administration of such drugs for therapeutic effectiveness.

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.

Traumatic axonal injury induces calcium influx modulated by tetrodotoxin-sensitive sodium channels

Diffuse axonal injury (DAI) is one of the most common and important pathologies resulting from the mechanical deformation of the brain during trauma. It has been hypothesized that calcium influx into axons plays a major role in the pathophysiology of DAI. However, there is little direct evidence to support this hypothesis, and mechanisms of potential calcium entry have not been explored. In the present study, we used an in vitro model of axonal stretch injury to evaluate the extent and modulation of calcium entry after trauma. Using a calcium-sensitive dye, we observed a dramatic increase in intra-axonal calcium levels immediately after injury. Axonal injury in a calcium-free extracellular solution resulted in no change in calcium concentration, suggesting an extracellular source for the increased post-traumatic calcium levels. We also found that the post-traumatic change in intra-axonal calcium was completely abolished by the application of the sodium channel blocker tetrodotoxin or by replacement of sodium with N-methyl-d-glucamine. In addition, application of the voltage-gated calcium channel (VGCC) blocker omega-conotoxin MVIIC attenuated the post-traumatic increase in calcium. Furthermore, blockade of the Na(+)-Ca(2+) exchanger with bepridil modestly reduced the calcium influx after injury. In contrast to previously proposed mechanisms of calcium entry after axonal trauma, we found no evidence of calcium entry through mechanically produced pores (mechanoporation). Rather, our results suggest that traumatic deformation of axons induces abnormal sodium influx through mechanically sensitive Na(+) channels, which subsequently triggers an increase in intra-axonal calcium via the opening of VGCCs and reversal of the Na(+)-Ca(2+) exchanger.

Influence of posttraumatic hypoxia on behavioral recovery and histopathological outcome following moderate spinal cord injury in rats

Pulmonary dysfunction leading to secondary hypoxia is a common complication of spinal cord injury (SCI). The purpose of this study was to clarify the behavioral and histopathological consequences of posttraumatic hypoxia in an established model of traumatic SCI. Forty-five female Sprague-Dawley rats were randomly assigned to one of four groups, including (1) laminectomy and normoxia (n = 10), (2) laminectomy and hypoxia (n = 11), (3) NYU weight-drop and normoxia (n = 12), and (4) NYU weight-drop and hypoxia (n = 11). For these studies, a moderate injury was induced by adjusting the height of the weight drop (10 g) to 12.5 mm above the exposed spinal cord (T10). Immediately after injury, PaO2 in the hypoxic rats was kept between 30 and 35 mm Hg for 30 min. PaO2 in the normoxic group was maintained over 100 mm Hg, while PaCO2 in all rats was maintained at 35-40 mm Hg. The behavior of the rats was checked every 7 days using the Basso, Beattie, and Bresnahan (BBB) locomotor rating scale. Rats were sacrificed at 8 weeks for quantitative histopathological analysis of lesion areas. During the hypoxic insults, the mean arterial blood pressure dropped in both sham control and weight-drop rats (p < 0.01). At the end of the 8-week monitoring period, BBB scores were 12.5 +/- 3.1 (mean +/- SEM) and 14.2 +/- 3.4 in the normoxic and hypoxic traumatized rats, respectively. No significant difference between the traumatized groups was documented with BBB monitoring. In contrast, the percent of gray matter necrosis at the impact epicenter was significantly increased in hypoxic versus normoxic SCI rats (p < 0.01). These data demonstrate that posttraumatic hypoxia complicated by mild hypotension aggravates the histopathological consequences of SCI and further emphasize the need to control for secondary hypoxic insults after experimental and clinical SCI. Potential explanations for the lack of a correlation between the behavioral and histopathological findings are discussed.

Hypoglycemia enhances ionotropic but reduces metabotropic glutamate responses in substantia nigra dopaminergic neurons

It is widely accepted that energy deprivation causes a neuronal death that is mainly determined by an increase in the extracellular level of glutamate. Consequently an excessive membrane depolarization and a rise in the intracellular concentration of sodium and calcium are produced. In spite of this scenario, the function of excitatory and inhibitory amino acids during an episode of energy failure has not been studied yet at a cellular level. In a model of cerebral hypoglycemia in the rat substantia nigra pars compacta, we measured neuronal responses to excitatory amino acid agonists. Under single-electrode voltage-clamp mode at -60 mV, the application of the ionotropic glutamate receptor agonists N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, kainate, and the metabotropic group I agonist (S)-3,5-dihydroxyphenilglycine (DHPG) produced reversible inward currents in the dopaminergic cells. In addition, an outward current was caused by the superfusion of the metabotropic GABA(B) agonist baclofen. Glucose deprivation enhanced the inward responses caused by each ionotropic glutamate agonist. In contrast, hypoglycemia depressed the DHPG-induced inward current and the baclofen-induced outward current. These effects of hypoglycemia were reversible. To test whether a failure of the Na(+)/K(+) ATPase pump could account for the modification of the agonist-induced currents during hypoglycemia, we treated the midbrain slices with strophanthidin (1-3 microM). Strophanthidin enhanced the inward currents caused by glutamate agonists. However, it did not modify the GABA(B)-induced outward current. Our data suggest that glucose deprivation enhances the inward current caused by the stimulation of ionotropic glutamate receptors while it dampens the responses caused by the activation of metabotropic receptors. Thus a substantial component of the augmented neuronal response to glutamate, during energy deprivation, is very likely due to the failure of Na(+) and Ca(2+) extrusion and might ultimately favor excitotoxic processes in the dopaminergic cells.

On the applicability of two different stimulation techniques for intra-operative peroneal nerve conduction testing

Dysfunction of the peroneal nerve is an important complication of knee surgery. We compared two monitoring procedures of peroneal nerve function during a standardized operation, a closing wedge high tibial osteotomy. For two types of stimulation the evoked compound motor unit action potentials (CMAPs) were recorded on the tibialis anterior muscle. We used direct perineural electrical stimulation of the common peroneal nerve distal to the cuff (dCMAPs) after nerve identification in the surgical field. Additionally, magnetic stimulation of the sacral plexus proximal to the cuff (pCMAPs) was performed. It was found that dCMAPs were recorded during almost one hour of tourniquet time whereas the pCMAPs were blocked after 25-30 min in 9 out of 11 cases. On the other hand, the CMAP obtained after proximal stimulation exhibited a latency shift with tourniquet yielding an indicator of ischaemic changes present beneath and distal to the tourniquet cuff. In conclusion, different applicabilities of both stimulation techniques under tourniquet conditions were demonstrated.

Topically applied betaxolol attenuates ischaemia-induced effects to the rat retina and stimulates BDNF mRNA

It has previously been reported that the beta(1)-adrenoceptor antagonist, betaxolol, can protect retinal neurones from ischaemia when applied topically. It has further been shown that betaxolol can reduce influx of both sodium or calcium into neurones through interaction at neurotoxin site 2 of the sodium channel and the L-type calcium channel, respectively. The present study sought to further investigate the neuroprotective mode of action of betaxolol in the rat retina. Rats were treated topically with L-betaxolol for 10, 5 and 1 min before ischaemia, induced by raising the intraocular pressure above systolic blood pressure for 45 min. This was followed by reperfusion of 3 or 5 days where L-betaxolol was applied topically twice daily. Ischaemia plus reperfusion caused both a loss of immunoreactivity for choline acetyl transferase (ChAT) and a marked reduction of the b-wave of the electroretinogram (ERG). Treatment, as described, with topical L-betaxolol, completely blunted the effects upon ChAT immunoreactivity and caused a significant reversal of the ERG changes. Furthermore, other rats treated topically with commercially available racemic betaxolol (Betoptic Solution, 0.5%) for 6 hr had raised levels of mRNA for brain derived neurotrophic factor (BDNF) but not for basic fibroblast growth factor (bFGF) in their retinas. The combined data provide further evidence that betaxolol can blunt the effects of ischaemia to the rat retina when applied topically just before the insult. Furthermore, the finding that retinal levels of BDNF mRNA are raised following topical betaxolol treatment shows that not only can this drug reach the retina but that it can also induce changes in expression of factors which are known, themselves, to provide neuroprotection to retinal neurones.

Effects of sodium and chloride on neuronal survival after neurite transection

An in vitro investigation was undertaken to study the roles of Na+ and Cl- in mammalian spinal cord (SC) neuron deterioration and death after injury involving physical disruption of the plasma membrane. Individual SC neurons in monolayer cultures were subjected to UV laser microbeam transection of a primary dendrite. Neurons lesioned in modified ionic environments (MIEs) where 50%-75% of the NaCl was replaced with sucrose had higher survival (65%-75%) than neurons lesioned in medium with normal (125 mM) NaCl (28%; p < 0.001). Subsequent experiments found a comparable increase in lesioned neuron survival in MIEs in which only Na+ was replaced with specific ionic substitutes; however, replacement of Cl- was not protective. Electron microscope examinations of neurons fixed <16 min after lesioning showed a dramatic decrease in vesiculation of the smooth endoplasmic reticulum and Golgi apparatus in the low NaCl or low Na+ MIEs. It is hypothesized that Na+ entry after membrane disruption may stimulate elevation of [Ca+2]i leading to ultrastructural disruption and death of injured neurons. The results of these studies suggest that a low NaCl MIE may be useful as an irrigant to limit damage spread and cell death within CNS tissues during surgery or after trauma.

Quantitative ultrastructural analysis of a single spinal cord demyelinated lesion predicts total lesion load, axonal loss, and neurological dysfunction in a murine model of multiple sclerosis

Infection of susceptible mice with Theiler's murine encephalomyelitis virus results in neurological dysfunction from progressive central nervous system demyelination that is pathologically similar to the human disease, multiple sclerosis. We hypothesized that the development of neuropathology proceeds down a final common pathway that can be accurately quantified within a single spinal cord lesion. To test this hypothesis, we conducted quantitative ultrastructural analyses of individual demyelinated spinal cord lesions from chronically infected mice to determine whether pathological variables assessed within a single lesion accurately predicted global assessments of morphological and functional disease course. Within lesions we assessed by electron microscopy the frequencies of normally myelinated, remyelinated, and demyelinated axons, as well as degenerating axons and intra-axonal mitochondria. The frequency of medium and large remyelinated fibers within a single lesion served as a powerful indicator of axonal preservation and correlated with preserved neurological function. The number of degenerating axons and increased intra-axonal mitochondria also correlated strongly with global measures of disease course, such as total lesion load, spinal cord atrophy, and neurological function. This is the first study to demonstrate that functional severity of disease course is evident within a single demyelinated lesion analyzed morphometrically at the ultrastructural level.

The N-methyl-D-aspartate antagonist CNS 1102 protects cerebral gray and white matter from ischemic injury following temporary focal ischemia in rats

BACKGROUND AND PURPOSE

Cerebral white matter is as sensitive as gray matter to ischemic injury and is probably amenable to pharmacological intervention. In this study we investigated whether an N-methyl-D-aspartate (NMDA) antagonist, CNS 1102, protects not only cerebral gray matter but also white matter from ischemic injury.

METHODS

Ten rats underwent 15 minutes of temporary focal ischemia and were blindly assigned to CNS 1102 intravenous bolus injection (1. 13 mg/kg) followed by intravenous infusion (0.33 mg/kg per hour) for 3.75 hours or to vehicle (n=5 per group) immediately after reperfusion. Seventy-two hours after ischemia, the animals were perfusion fixed for histology. The severity of neuronal necrosis in the cortex and striatum was semiquantitatively analyzed. The Luxol fast blue-periodic acid Schiff stain and Bielschowsky's silver stain were used to measure optical densities (ODs) of myelin and axons, respectively, in the internal capsule of both hemispheres, and the OD ratio was calculated to reflect the severity of white matter damage.

RESULTS

Neuronal damage in both the cortex and the striatum was significantly better in the drug-treated group than in the placebo group (P<0.05). The OD ratio of both the axons (0.93+/-0.08 versus 0.61+/-0.18; P<0.01) and the myelin sheath (0.95+/-0.07 versus 0.67+/-0.19; P=0.01) was significantly higher in the CNS 1102 group than in the placebo group. The neurological score was significantly improved in the drug-treated group (P<0.05).

CONCLUSIONS

The NMDA receptor antagonist CNS 1102 protects not only cerebral gray matter but also white matter from ischemic injury, most probably by preventing degeneration of white matter structures such as myelin and axons.

Betaxolol, a beta(1)-adrenoceptor antagonist, reduces Na(+) influx into cortical synaptosomes by direct interaction with Na(+) channels: comparison with other beta-adrenoceptor antagonists

Betaxolol, a beta(1)-adrenoceptor antagonist used for the treatment of glaucoma, is known to be neuroprotective in paradigms of ischaemia/excitotoxicity. In this study, we examined whether betaxolol and other beta-adrenoceptor antagonists interact directly with neurotoxin binding to sites 1 and 2 of the voltage-sensitive sodium channel (Na(+) channel) in rat cerebrocortical synaptosomes. Betaxolol inhibited specific [(3)H]-batrachotoxinin-A 20-alpha-benzoate ([(3)H]-BTX-B) binding to neurotoxin site 2 in a concentration-dependent manner with an IC(50) value of 9.8 microM. Comparison of all the beta-adrenoceptor antagonists tested revealed a potency order of propranolol>betaxolol approximately levobetaxolol>levobunolol approximately carteolol>/=timolol>atenolol. None of the drugs caused a significant inhibition of [(3)H]-saxitoxin binding to neurotoxin receptor site 1, even at concentrations as high as 250 microM. Saturation experiments showed that betaxolol increased the K(D) of [(3)H]-BTX-B binding but had no effect on the B(max). The association kinetics of [(3)H]-BTX-B were unaffected by betaxolol, but the drug significantly accelerated the dissociation rate of the radioligand. These findings argue for a competitive, indirect, allosteric mode of inhibition of [(3)H]-BTX-B binding by betaxolol. Betaxolol inhibited veratridine-stimulated Na(+) influx in rat cortical synaptosomes with an IC(50) value of 28. 3 microM. Carteolol, levobunolol, timolol and atenolol were significantly less effective than betaxolol at reducing veratridine-evoked Na(+) influx. The ability of betaxolol to interact with neurotoxin site 2 of the Na(+) channel and inhibit Na(+) influx may have a role in its neuroprotective action in paradigms of excitotoxicity/ischaemia and in its therapeutic effect in glaucoma.

Exacerbated responses to oxidative stress by an Na(+) load in isolated nerve terminals: the role of ATP depletion and rise of [Ca(2+)](i)

We have explored the consequences of a [Na(+)](i) load and oxidative stress in isolated nerve terminals. The Na(+) load was achieved by veratridine (5-40 microM), which allows Na(+) entry via a voltage-operated Na(+) channel, and oxidative stress was induced by hydrogen peroxide (0.1-0.5 mM). Remarkably, neither the [Na(+)](i) load nor exposure to H(2)O(2) had any major effect on [Ca(2+)](i), mitochondrial membrane potential (Deltapsim), or ATP level. However, the combination of an Na(+) load and oxidative stress caused ATP depletion, a collapse of Deltapsim, and a progressive deregulation of [Ca(2+)](i) and [Na(+)](i) homeostasis. The decrease in the ATP level was unrelated to an increase in [Ca(2+)](i) and paralleled the rise in [Na(+)](i). The loss of Deltapsim was prevented in the absence of Ca(2+) but unaltered in the presence of cyclosporin A. We conclude that the increased ATP consumption by the Na,K-ATPase that results from a modest [Na(+)](i) load places an additional demand on mitochondria metabolically compromised by an oxidative stress, which are unable to produce a sufficient amount of ATP to fuel the ATP-driven ion pumps. This results in a deregulation of [Na(+)](i) and [Ca(2+)](i), and as a result of the latter, collapse of Deltapsim. The vicious cycle generated in the combined presence of Na(+) load and oxidative stress could be an important factor in the neuronal injury produced by ischemia or excitotoxicity, in which the oxidative insult is superimposed on a disturbed Na(+) homeostasis.