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

Role of RyRs and IP3 receptors after traumatic injury to spinal cord white matter

Calcium influx and elevation of intracellular free calcium (Ca2+i), with subsequent activation of degenerative enzymes is hypothesized to cause cell injury and death after trauma. We examined the effects of traumatic compressive injury on (Ca2+)i dynamics in spinal cord white matter. We conducted electrophysiological studies with ryanodine and inositol (1,4,5)-triphosphate (IP3) receptor agonists and antagonists in an in vitro model of spinal cord injury (SCI). A 25-30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber (37 degrees C) and injured with a modified clip (2-g closing force) for 15 sec. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential (CAP) with glass microelectrodes. The CAP decreased to 55.2+/-6.8% of control (p < 0.05) after spinal cord injury (SCI). Chelation of Ca2+i with BAPTA-AM (a high-affinity calcium chelator) promoted significantly greater recovery of CAP amplitude (83.2+/-4.2% of control; p < 0.05) after injury. Infusion of caffeine (1 and 10 mM) exacerbated CAP amplitude decline (45.1+/-5.9% of control; p < 0.05; 44.6+/-3.1% of control; p < 0.05) postinjury. Blockade of Ca2+i release through ryanodine-sensitive receptors (RyRs) with dantrolene (10 microM) and ryanodine (50 microM), conferred significant (p < 0.05) improvement in CAP amplitude after injury. On the other hand, blockade of Ca2+i with inositol (1,4,5)-triphosphate receptor (IP3Rs) blocker 2APB (10 microM) also conferred significant improvement in CAP amplitude after injury (82.9+/-7.9%; p < 0.05). In conclusion, the injurious effects of Ca2+i in traumatic central nervous system (CNS) white matter injury appear to be mediated both by RyRs and through IP3Rs calcium-induced calcium release receptors (CICRs).

Inhibition of cyclin-dependent kinases improves CA1 neuronal survival and behavioral performance after global ischemia in the rat

Increasing evidence suggests that cyclin-dependent kinases participate in neuronal death induced by multiple stresses in vitro. However, their role in cell death paradigms in vivo is not well characterized. Accordingly, the authors examined whether cyclin-dependent kinase inhibition resulted in functionally relevant and sustained neuroprotection in a model of global ischemia. Intracerebroventricular administration of the cyclin-dependent kinase inhibitor flavopiridol, immediately or at 4 hours postreperfusion after a global insult, reduced injury in the CA1 of the hippocampus when examined 7 days after reperfusion. No significant protection was observed when flavopiridol was administered 8 hours after reperfusion. The tumor-suppressor retinoblastoma protein, a substrate of cyclin-dependent kinase, was phosphorylated on a cyclin-dependent kinase consensus site after the global insult; this phosphorylation was inhibited by flavopiridol administration. Importantly, flavopiridol had no effect on core body temperature, suggesting that the mechanism of neuroprotection was through cyclin-dependent kinase inhibition but not through hypothermia. Furthermore, inhibition of cyclin-dependent kinases improved spatial learning behavior as assessed by the Morris water maze 7 to 9 days after reperfusion. However, the histologic protection observed at day 7 was absent 28 days after reperfusion. These results indicate that cyclin-dependent kinase inhibition provides an extended period of morphologic and functional neuroprotection that may allow time for other neuroprotective modalities to be introduced.

Role of Na(+)-Ca(2+) exchanger after traumatic or hypoxic/ischemic injury to spinal cord white matter

BACKGROUND CONTEXT

Spinal cord injury is a devastating condition in which clinical disability results from demyelination of white matter tracts. Changes in glial-axonal signaling, and enhanced Ca(2+) channel activity with excessive accumulation of intracellular Ca(2+), is a common phenomenon after hypoxia/ischemia or mechanical trauma to spinal cord dorsal column white matter tracts leading to irreversible injury.

PURPOSE

In the present study we examined the role of Na(+)-Ca(2+) exchanger (NCX) at physiological temperatures after hypoxia/ischemia and compressive injury to spinal cord dorsal column white matter in vitro.

STUDY DESIGN

A 30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber (maintained at 37 degrees C) and injured by exposure to a hypoxic atmosphere for 60 minutes or compressed with a modified aneurysm clip (2-gm closing force) for 15 seconds. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential (CAP) with glass microelectrodes.

RESULTS

The mean CAP decreased to 49.5 +/- 5.7% and 49.4 +/- 2.6% of control (p<.05) after hypoxia/ischemia and compressive injury, respectively. KB-R7943, a potent, selective NCX reverse mode inhibitor, significantly promoted greater recovery of CAP amplitude to 82.0 +/- 10.0% and 70.8 +/- 10.7% of control (p<.05) after hypoxic/ischemic or compressive injury to dorsal column white matter, respectively, when applied at 10 microM concentration. Bepridil (Research Biochemical Inc., Natick, MA, USA) (a less selective NCX inhibitor), when applied at 10 microM and 50 microM concentration promoted CAP amplitude recovery only to 46.8 +/- 7.8% and 29.9 +/- 3.3% of control, respectively, after hypoxic/ischemic injury to dorsal column white matter. Western blot analysis identified NCX presence with positive immunolabeling of 160 kD and 120 kD NCX proteins in the spinal cord white matter.

CONCLUSION

In conclusion, at physiological temperature NCX activation plays an important role in intracellular calcium overload after hypoxic/ischemic and compressive injury to spinal cord dorsal column white matter in vitro.

Effects of global ischemia duration on neuronal, astroglial, oligodendroglial, and microglial reactions in the vulnerable hippocampal CA1 subregion in rats

The hippocampal CA1 neurons are selectively vulnerable to global ischemia, and neuronal death occurs in a delayed manner. The threshold of global ischemia duration that induces neuronal death has been studied, but the relationship between ischemia duration and glial death in the hippocampal CA1 area has not been fully studied. We examined neuronal/glial viability and morphological changes in the CA1 subregion after different durations of global ischemia. Global ischemia was induced in Sprague-Dawley rats by 10, 5, and 3 min of bilateral common carotid artery occlusion and hypotension. At 1-56 days after ischemia, the morphological reactions of neurons, astrocytes, oligodendrocytes, and microglia were immunohistochemically evaluated. Most of the hippocampal CA1 pyramidal neurons underwent delayed death at 3 days after 10/5 min of ischemia, but not after 3 min of ischemia. The number of astrocytes gradually declined after 10/5 min of ischemia, and viable astrocytes showed characteristic staged morphological reactions. Oligodendrocytes also showed morphological changes in their processes after 10/5 min of ischemia. Microglia transformed into a reactive form at 5 days only after 10/5 min of ischemia. These data suggest that some morphological changes in glial cells were not dependent on neuronal cell death, but their own reactions to the different severity of ischemia.

Salvaging the ischaemic penumbra: more than just reperfusion?

1. The ischaemic penumbra is defined as a moderately hypoperfused region that retains structural integrity but has lost function. In animal models of ischaemic stroke, this region is prone to recurrent anoxic depolarization and will become infarcted if reperfusion does not occur. In the macaque model, an ischaemic penumbra has been identified for up to 3 h after ischaemic stroke onset, whereas in selected human patients it may exist for up to 48 h. 2. Although most definitions of the ischaemic penumbra stress a time-brain volume concept, few incorporate the idea that selective and delayed neuronal injury plays an important role. Thus, in addition to necrotic cell death caused by acute injury, it is important to also consider delayed death mediated by caspase-dependent and -independent apoptotic pathways. 3. Salvage of penumbral tissue is possible if reperfusion (e.g. after thrombolysis) occurs. However, neurons within this salvaged region may be still at risk of further delayed neuronal injury. 4. In the present review, we aim to revisit the concept of the ischaemic penumbra and explore the role of selective and delayed neuronal injury in enlargement of the volume of infarction, as well as pathogenic mechanisms of white matter ischaemia. Both animal and human models of cerebral ischaemia imaged using magnetic resonance and positron emission tomography techniques will be discussed.

Soluble guanylyl cyclase activator YC-1 protects white matter axons from nitric oxide toxicity and metabolic stress, probably through Na(+) channel inhibition

In the rat isolated optic nerve, nitric oxide (NO) activates soluble guanylyl cyclase (sGC), resulting in a selective accumulation of cGMP in the axons. The axons are also selectively vulnerable to NO toxicity. The experiments initially aimed to determine any causative link between these two effects. It was shown, using a NONOate donor, that NO-induced axonal damage occurred independently of cGMP. Unexpectedly, however, the compound YC-1, which is an allosteric activator of sGC, potently inhibited NO-induced axonopathy (IC(50) = 3 microM). This effect was not attributable to increased cGMP accumulation. YC-1 (30 microM) also protected the axons against damage by simulated ischemia, which (like NO toxicity) is sensitive to Na(+) channel inhibition. Although chemically unrelated to any known Na(+) channel inhibitor, YC-1 was effective in two biochemical assays for activity on Na(+) channels in synaptosomes. Electrophysiological recording from hippocampal neurons showed that YC-1 inhibited Na(+) currents in a voltage-dependent manner. At a concentration giving maximal protection of optic nerve axons from NO toxicity (30 microM), YC-1 did not affect normal axon conduction. It is concluded that the powerful axonoprotective action of YC-1 is unrelated to its activity on sGC but is explained by a novel action on voltage-dependent Na(+) channels. The unusual ability of YC-1 to protect axons so effectively without interfering with their normal function suggests that the molecule could serve as a prototype for the development of more selective Na(+) channel inhibitors with potential utility in neurological and neurodegenerative disorders.

Glial cells as targets for cytotoxic immune mediators

Oligodendrocytes and Schwann cells are the glia principally responsible for the synthesis and maintenance of myelin. Damage may occur to these cells in a number of conditions, but perhaps the most studied are the idiopathic inflammatory demyelinating diseases, multiple sclerosis in the CNS, and Guillain-Barré syndrome and its variants in the peripheral nervous system (PNS). This article explores the effects on these cells of cytotoxic immunological and inflammatory mediators: similarities are revealed, of which perhaps the most important is the sensitivity of both Schwann cells and oligodendrocytes to many such agents. This area of research is, however, characterised and complicated by numerous and often very substantial inter-observer discrepancies. Marked variability in cell culture techniques, and in assays of cell damage and death, provide artifactual explanations for some of this variability; true inter-species differences also contribute. Not the least important conclusion centres on the limited capacity of in vitro studies to reveal disease mechanisms: cell culture findings merely illustrate possibilities which must then be tested ex vivo using human tissue samples affected by the relevant disease.

Effect of spinal cord compression on cyclic 3',5'-guanosine monophosphate in the white matter columns of rabbit

Changes in the level of cyclic 3',5'-guanosine monophosphate (cGMP) were studied one day after a surgically induced spinal cord constriction performed at the Th7 segment level in the dorsal, lateral and ventral white matter columns and in the non-compartmentalized white matter of Th5-Th6 segments, i.e., above the site of the spinal cord constriction and in Th8-Th9 segments, located below the spinal cord constriction. The midthoracic spinal cord constriction caused a significant decrease in the level of cGMP in the ventral column of Th5-Th6 segments and a significant increase in the lateral column of Th8-Th9 segments. The level of cGMP in the dorsal column, located either rostrally or caudally to the site of the spinal cord injury, remained unchanged. In addition, no significant changes in the level of cGMP were found in the non-compartmentalized white matter of Th5-Th6 and Th8-Th9 segments in response to constriction of the Th7 segment.

Cyclin-dependent kinases and stroke

Cyclin-dependent kinases (CDKs) are a group of enzymes predominately known for their role in cell cycle regulation in proliferating cell types. Increasing evidence, however, suggests that CDKs also promote death in neurones. These observations have lead to the notion that CDKs may serve as a therapeutic target for neuropathological conditions such as stroke. Accordingly, in this review, we will examine the evidence which indicates a role for CDKs in neuronal death and evaluate the potential of CDK inhibitors as a therapeutic target for stroke.

Marked anemic hypoxia deteriorates cerebral hemodynamics and brain metabolism during massive intracerebral hemorrhage

The present study was undertaken to investigate the influence of imposed anemic hypoxia on cerebral hemodynamics and metabolism in a condition of massive ICH. Two groups of eight dogs, with a target hemoglobin concentration of 12 g/dl in nonanemic and 6 g/dl in anemic group, were included. Before the onset of the insult, anemic group had a significant reduction (p<0.05) in cerebral arteriovenous oxygen content difference (AVDO2), accompanied with a significant rise (p<0.05) in flow velocity (FV) of the basilar artery and cerebral extraction fraction of oxygen (CEO2) and a lower brain-tissue lactate clearance than did nonanemic group. Shortly after ICH, both groups displayed significant reductions (p<0.05) in FV, CEO2 and AVDO2, and simultaneous rises in arteriovenous lactate concentrations. In nonanemic group, the CEO2 and AVDO2 gradually returned after an initial decrease, and then the arteriovenous lactate concentrations slowly decreased. In contrast, anemic group showed progressive reductions in CEO2 and AVDO2 associated with persistent rises in arteriovenous lactate concentrations. Consequently, anemic group exhibited significantly greater brain-tissue lactate clearances (p<0.05), occurring at 10 min and 5 h postinjury, than did nonanemic group, although the former had relatively higher levels of CEO2 up to 3 h postinjury. We conclude that anemic hypoxia modulates a favorable change in cerebral hemodynamics and oxygenation, while it progressively deteriorates after an initial reduction during massive ICH, thus facilitating cerebral anaerobic glycolysis in biphasic periods. These results point to a complex interaction between cerebral hemodynamics, oxygen supply and glycolysis homeostasis upon the addition of anemic hypoxia in severe stress conditions of the brain.

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.

Recommendations for clinical trial evaluation of acute stroke therapies

The development of therapies for acute ischemic stroke has achieved a few notable successes and, unfortunately, many unsuccessful efforts. Many valuable lessons for the future assessment of new acute stroke therapies can be gleaned from the positive and negative prior trials. Phase I and II trials must be carefully designed and implemented to derive relevant, valuable information needed to proceed to phase III trials with promising interventions. The phase III trial should evaluate drug efficacy in an appropriately targeted stroke population evaluated by a meaningful and reliable outcome measure. Combinations of various types of stroke therapies will likely be increasingly assessed in future trials that are designed and implemented by cooperative efforts between the pharmaceutical industry, government agencies, academic advisors and clinical investigators. The chances for future success in demonstrating efficacy with acute stroke therapies will be enhanced by carefully conceived, scientifically based clinical trials. The recommendations contained in this document may help to focus attention on how to achieve the goal of developing an expanding number of a effective and safe acute stroke therapies.

Protective effect of nimodipine on behavior and white matter of rats with hydrocephalus

OBJECT

Hydrocephalus, a pathological dilation of the ventricles of the brain, causes damage to periventricular white matter, at least in part, through chronic ischemia. The authors tested the hypothesis that treatment with nimodipine, an L-type calcium channel-blocking agent with demonstrated efficacy in a range of cerebral ischemic disorders, would ameliorate the adverse effects of experimental hydrocephalus.

METHODS

Hydrocephalus was induced in 3-week-old rats by injection of kaolin into the cisterna magna. The rats were treated by continuous administration of nimodipine or control vehicle for 2 weeks, beginning 2 weeks after induction of hydrocephalus. During the treatment period, the animals underwent repeated tests of motor and cognitive behavior. At the end of the treatment period, the rat brains were analyzed by histopathological and biochemical means. Nimodipine treatment prevented the declines in motor and cognitive behavior that were observed in untreated control rats. During the treatment period, ventricular enlargement, determined by magnetic resonance imaging, was equal in the two groups, although the corpus callosum was thicker in the treated rats. Myelin content in white matter and synaptophysin content in gray matter, an indicator of synapses, did not differ.

CONCLUSIONS

The protective effect of nimodipine is most likely based on improved blood flow, although prevention of calcium influx-mediated proteolytic processes in axons cannot be excluded. Adjunctive pharmacological therapy may be beneficial to patients with hydrocephalus.

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.

Adenosine in relation to calcium homeostasis: comparison between gray and white matter ischemia

In vitro studies suggest that adenosine may attenuate anoxic white matter damage as an intrinsic protective substance. The authors investigated ischemic alterations of purines in relation to tissue depolarization and extracellular calcium and amino acid concentrations in vivo using microdialysis and ion-selective electrodes in cortical gray and subcortical white matter of 10 cats during 120 minutes of global brain ischemia. Immediately on induction of ischemia, regional cerebral blood flow ceased in all cats in both gray and white matter. The direct current potential rapidly decreased, the decline being slower and shallower in white matter. Extracellular calcium levels decreased in gray matter. In contrast, they first increased in white matter and started to decrease below control levels only after approximately 30 minutes. Adenosine levels transiently increased in both tissue compartments; the peak was delayed by 30 minutes in white matter. Thereafter, levels declined faster in gray than in white matter and remained elevated in the latter tissue compartment. Inosine and hypoxanthine elevations were progressive in both regions but smaller in white matter. Levels of gamma-aminobutyric acid, another putatively protective agent, steadily increased, starting immediately in gray matter and delayed by almost 1 hour in white matter. The delayed and prolonged accumulation of adenosine correlates with a slower adenosine triphosphate breakdown in white matter ischemia and may result in protection of white matter by suspending cellular calcium influx.

Delivery of genes encoding cardiac K(ATP) channel subunits in conjunction with pinacidil prevents membrane depolarization in cells exposed to chemical hypoxia-reoxygenation

Metabolic injury is a complex process affecting various: tissues with membrane depolarisation recognised as a common trigger event leading to cell death. To examine whether, under metabolic challenge, membrane potential homeostasis can be maintained by an activator of channel proteins, we here delivered Kir6.2 and SUR2A genes, which encode cardiac K(ATP) channel subunits, into a somatic cell line lacking native K(ATP) channels (COS-7 cells). Chemical hypoxia-reoxygenation was simulated in COS-7 cells by addition and removal of the mitochondrial poison 2,4 dinitrophenol (DNP). The membrane potential of COS-7 cells at rest was -31 +/- 3 mV. This value did not change following 3 min-long exposure to DNP (-32 +/- 4 mV). In contrast, washout of DNP induced significant membrane depolarisation (-17 +/- 2 mV). Delivery of Kir6.2/SUR2A genes did not change cellular response to hypoxia-reoxygenation. Similarly, pinacidil, potassium channel opener, did not have effect on hypoxia-reoxygenation-induced membrane depolarisation in cells lacking recombinant K(ATP) channel subunits. However, gene delivery combined with pinacidil prevented membrane depolarisation induced by hypoxia-reoxygenation. This effect of pinacidil, in cells expressing Kir6.2/SUR2A, was observed regardless of whether pinacidil was added only during hypoxia or reoxygenation. The present study demonstrates that combined use of K(ATP) channel subunits gene delivery and pharmacological targeting of recombinant proteins can be used to efficiently control membrane potential under hypoxia-reoxygenation.

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.

Subcellular distribution of calcium and ultrastructural changes after cerebral hypoxia-ischemia in immature rats

Recent data imply that mitochondrial regulation of calcium is critical in the process leading to hypoxic-ischemic brain injury. The aim was to study the subcellular distribution of calcium in correlation with ultrastructural changes after hypoxia-ischemia in neonatal rats. Seven-day-old rats were subjected to permanent unilateral carotid artery ligation and exposure to hypoxia (7.7% oxygen in nitrogen) for 90 min. Animals were perfusion-fixed after 30 min, 3 h or 24 h of reperfusion. Sections were sampled for light microscopy and electron microscopy combined with the oxalate-pyroantimonate technique. At 30 min and 3 h of reflow, a progressive accumulation of calcium was detected in the endoplasmic reticulum, cytoplasm, nucleus and, most markedly, in the mitochondrial matrix of neurons in the gray matter in the core area of injury. Some mitochondria developed a considerable degree of swelling reaching a diameter of several microm at 3 h of reflow whereas the majority of mitochondria appeared moderately affected. Chromatin condensation was observed in nuclei of many cells with severely swollen mitochondria with calcium deposits. A whole spectrum of morphological features ranging from necrosis to apoptosis was seen in degenerating cells. After 24 h, there was extensive injury in the cerebral cortex as judged by breaks of mitochondrial and plasma membranes, and a general decrease of cellular electron density. In the white matter of the core area of injury, the axonal elements exhibited varicosity-like swellings filled with calcium-pyroantimonate deposits. Furthermore, the thin myelin sheaths were loaded with calcium. Numerous oligodendroglia-like cells displayed apoptotic morphology with shrunken cytoplasm and chromatin condensation, whereas astroglial necrosis was not seen. In conclusion, markedly swollen 'giant' mitochondria with large amounts of calcium were found at 3 h of reperfusion often in neuronal cells with condensation of the nuclear chromatin. The results are discussed in relation to mitochondrial permeability transition and activation of apoptotic processes.

Astroglial cell death induced by excessive influx of sodium ions

Na(+) influx has been implicated to play an important role in the mechanisms of neuronal cell damage under ischemia as well as in neurodegenerative disorders. Thus far, however, the effects of Na(+) influx on astrocytic damage have not been studied extensively. In the present study, we have examined the effects of Na(+) influx induced by veratridine (Na(+) channel opener), monensin (Na(+) ionophore), and glutamate (co-transportation with Na(+)) on rat cultured astroglial damage. Cells were incubated with bicarbonate buffer with 25 mM glucose containing either 100 microM veratridine, 10 microM monensin, or 1 mM glutamate with or without 1 mM ouabain for 20 h. Cellular damage was evaluated quantitatively by lactate dehydrogenase (LDH) release or 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) reduction. Veratridine, monensin, or glutamate alone did not induce significant astroglial damage. Veratridine and monensin co-incubated with ouabain, which inhibits active extrusion of Na(+) by Na(+),K(+)-ATPase, thereby enhances intracellular Na(+) accumulation, caused significant cell death (P<0. 001, approximately 50% cell damage), whereas glutamate did not. Na(+)-free solution substituted by choline (impermeable cation) attenuated cell damage induced by veratridine and monensin markedly, while Li(+) substitution (permeable cation) rather exacerbated. Nifedipine (100 microM), a blocker of L-type Ca(2+) channel, reduced veratridine-induced glial damage by 50%. Neither bepridil nor benzamil, a blocker of Na(+)-Ca(2+) exchanger, had any protection. Cyclosporin A (1 or 10 microM), an inhibitor of mitochondrial permeability transition or 10 microM N-benzyloxycarbonyl-Val-Ala-Asp-(O-methyl)fluoromethyl ketone (zVAD-fmk), which inhibits a broad range of caspases, did not show protective effects.

The lipid peroxidation by-product 4-hydroxynonenal is toxic to axons and oligodendrocytes

Lipid peroxidation and the cytotoxic by-product 4-hydroxynonenal (4-HNE) have been implicated in neuronal perikaryal damage. This study sought to determine whether 4-HNE was involved in white matter damage in vivo and in vitro. Immunohistochemical studies detected an increase in cellular and axonal 4-HNE within the ischemic region in the rat after a 24-hour period of permanent middle cerebral artery occlusion. Exogenous 4-HNE (3.2 nmol) was stereotaxically injected into the subcortical white matter of rats that were killed 24 hours later. Damaged axons detected by accumulation of beta-amyloid precursor protein (beta-APP) were observed transversing medially and laterally away from the injection site after intracerebral injection of 4-HNE. In contrast, in the vehicle-treated animals, axonal damage was restricted to an area immediately surrounding the injection site. Exogenous 4-HNE produced oligodendrocyte cell death in culture in a time-dependent and a concentration-dependent manner. After 4 hours, the highest concentration of 4-HNE (50 micromol/L) produced 100% oligodendrocyte cell death. Data indicate that lipid peroxidation and production of 4-HNE occurs in white matter after cerebral ischemia and the lipid peroxidation by-product 4-HNE is toxic to axons and oligodendrocytes.

Calcium-mediated proteolytic damage in white matter of hydrocephalic rats?

Hydrocephalus is a pathological dilatation of the cerebrospinal fluid (CSF)-containing ventricles of the brain. Damage to periventricular white matter is multifactorial with contributions by chronic ischemia and gradual physical distortion. Acute ischemic and traumatic brain injuries are associated with calcium-dependent activation of proteolytic enzymes. We hypothesized that hydrocephalus is associated with calcium ion accumulation and proteolytic enzyme activation in cerebral white matter. Hydrocephalus was induced in immature and adult rats by injection of kaolin into the cisterna magna and several different experimental approaches were used. Using the glyoxal bis (2-hydroxyanil) method, free calcium ion was detected in periventricular white matter at sites of histological injury. Western blot determinations showed accumulation of calpain I (mu-calpain) and immunoreactivity for calpain I was increased in periventricular axons of young hydrocephalic rats. Proteolytic cleavage of a fluorogenic calpain substrate was demonstrated in white matter. Immunoreactivity for spectrin breakdown products was detected in scattered callosal axons of young hydrocephalic rats. The findings support the hypothesis that periventricular white matter damage associated with experimental hydrocephalus is due, at least in part, to calcium-activated proteolytic processes. This may have implications for supplemental drug treatments of this disorder.

Calcium imaging in live rat optic nerve myelinated axons in vitro using confocal laser microscopy

Intracellular Ca(2+) plays a major role in the physiological responses of excitable cells, and excessive accumulation of internal Ca(2+) is a key determinant of cell injury and death. Many studies have been carried out on the internal Ca(2+) dynamics in neurons. In constrast, there is virtually no such information for mammalian central myelinated axons, due in large part to technical difficulty with dye loading and imaging such fine myelinated structures. We developed a technique to allow imaging of ionized Ca(2+) in live rat optic nerve axons with simultaneous electrophysiological recording in vitro at 37 degrees C using confocal microscopy. The K(+) salt of the Ca(2+)-sensitive indicator Oregon Green 488 BAPTA-2 and the Ca(2+)-insensitive reference dye Sulforhodamine 101 were loaded together into rat optic nerves using a low-Ca(2+)/low-Na(+) solution. Axonal profiles, confirmed immunohistochemically by double staining with neurofilament-160 antibodies, were clearly visualized by S101 fluorescence up to 800 microm from the cut ends. The Ca(2+) signal was very low at rest, just above the background fluorescence intensity, indicating healthy tissue, and increased significantly after caffeine (20 mM) exposure designed to release internal Ca(2+) stores. The health of imaged regions was further confirmed by a virtual absence of spectrin breakdown, which is induced by calpain activation in damaged CNS tissue. Red and green fluorescence decayed to no less than 70% of control after 60 min of recording at 37 degrees C, with the green:red fluorescence ratio increasing slightly by 21% after 60 min. Electrophysiological responses recorded simultaneously with confocal images remained largely stable as well.

Role of L- and N-type calcium channels in the pathophysiology of traumatic spinal cord white matter injury

Recent work has suggested a potential role for voltage-gated Ca(2+) channels in the pathophysiology of anoxic central nervous system white matter injury. To examine the relevance of these findings to neurotrauma, we conducted electrophysiological studies with inorganic Ca(2+) channels blockers and L- and N-subtype-specific calcium channel antagonists in an in vitro model of spinal cord injury. Confocal immunohistochemistry was used to examine for localization of L- and N-type calcium channels in spinal cord white matter tracts. A 30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber and injured with a modified clip (2g closing force) for 15s. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential at two points with glass microelectrodes. The compound action potential decreased to 71.4+/-2.0% of control (P<0. 05) after spinal cord injury. Removal of extracellular Ca(2+) promoted significantly greater recovery of compound action potential amplitude (86.3+/-7.6% of control; P< 0.05) after injury. Partial blockade of voltage-gated Ca(2+) channels with cobalt (20 microM) or cadmium (200 microM) conferred improvement in compound action potential amplitude. Application of the L-type Ca(2+) channel blockers diltiazem (50 microM) or verapamil (90 microM), and the N-type antagonist omega-conotoxin GVIA (1 microM), significantly enhanced the recovery of compound action potential amplitude postinjury. Co-application of the L-type antagonist diltiazem with the N-type blocker omega-conotoxin GVIA showed significantly greater (P<0.05) improvement in compound action potential amplitude than application of either drug alone. Confocal immunohistochemistry with double labelling for glial fibrillary acidic protein, GalC and NF200 demonstrated L- and N-type Ca(2+) channels on astrocytes and oligodendrocytes, but not axons, in spinal cord white matter. In conclusion, the injurious effects of Ca(2+) in traumatic central nervous system white matter injury appear to be partially mediated by voltage-gated Ca(2+) channels. The presence of L- and N-type Ca(2+) channels on periaxonal astrocytes and oligodendrocytes suggests a role for these cells in post-traumatic axonal conduction failure.

Rate of neurotoxicant exposure determines morphologic manifestations of distal axonopathy

Exposure to a variety of agricultural, industrial, and pharmaceutical chemicals produces nerve damage classified as a central-peripheral distal axonopathy. Morphologically, this axonopathy is characterized by distal axon swellings and secondary degeneration. Over the past 25 years substantial research efforts have been devoted toward deciphering the molecular mechanisms of these presumed hallmark neuropathic features. However, recent studies suggest that axon swelling and degeneration are related to subchronic low-dose neurotoxicant exposure rates (i.e., mg toxicant/kg/day) and not to the development of neurophysiological deficits or behavioral toxicity. This suggests these phenomena are nonspecific and of uncertain pathophysiologic relevance. This possibility has significant implications for research investigating mechanisms of neurotoxicity, development of exposure biomarkers, design of risk assessment models, neurotoxicant classification schemes, and clinical diagnosis and treatment of toxic neuropathies. In this commentary we will review the evidence for the dose-related dependency of distal axonopathies and discuss how this concept might influence our current understanding of chemical-induced neurotoxicities.

Important role of reverse Na(+)-Ca(2+) exchange in spinal cord white matter injury at physiological temperature

Spinal cord injury is a devastating condition in which most of the clinical disability results from dysfunction of white matter tracts. Excessive cellular Ca(2+) accumulation is a common phenomenon after anoxia/ischemia or mechanical trauma to white matter, leading to irreversible injury because of overactivation of multiple Ca(2+)-dependent biochemical pathways. In the present study, we examined the role of Na(+)-Ca(2+) exchange, a ubiquitous Ca(2+) transport mechanism, in anoxic and traumatic injury to rat spinal dorsal columns in vitro. Excised tissue was maintained in a recording chamber at 37 degrees C and injured by exposure to an anoxic atmosphere for 60 min or locally compressed with a force of 2 g for 15 s. Mean compound action potential amplitude recovered to approximately 25% of control after anoxia and to approximately 30% after trauma. Inhibitors of Na(+)-Ca(2+) exchange (50 microM bepridil or 10 microM KB-R7943) improved functional recovery to approximately 60% after anoxia and approximately 70% after traumatic compression. These inhibitors also prevented the increase in calpain-mediated spectrin breakdown products induced by anoxia. We conclude that, at physiological temperature, reverse Na(+)-Ca(2+) exchange plays an important role in cellular Ca(2+) overload and irreversible damage after anoxic and traumatic injury to dorsal column white matter tracts.

Effect of midthoracic spinal cord constriction on catalytic nitric oxide synthase activity in the white matter columns of rabbit

The distribution and changes of catalytic nitric oxid synthase (cNOS) activity in the dorsal, lateral and ventral white matter columns at midthoracic level of the rabbit's spinal cord were studied in a model of surgically-induced spinal cord constriction performed at Th7 segment level and compared with the occurrence of nicotinamide adenine dinucleotide phosphate diaphorase expressing and neuronal nitric oxide synthase immunoreactive axons in the white matter of the control thoracic segments. Segmental and white-column dependent differences of cNOS activity were found in the dorsal (141.5 +/- 4.2 dpm/microm protein), lateral (87.3 +/- 11.5 dpm/microm protein) and ventral (117.1 +/- 7.6 dpm/microm protein) white matter columns in the Th5-Th6 segments and in the dorsal (103.3 +/- 15.5 dpm/microm protein), lateral (54.9 +/- 4.9 dpm/microm protein), and ventral (86.1 +/- 6.8 dpm/microm protein) white matter columns in the Th8-Th9 segments. A surgically-induced constriction of Th7 segment caused a disproportionate response of cNOS activity in the rostrally (Th5-Th6) and caudally (Th8-Th9) located segments in both lateral and ventral white matter columns. While a statistically significant decrease of cNOS activity was detected above the constriction site in the ventral columns, a considerable, statistically significant increase of cNOS activity was noted in the white lateral columns below the site of constriction. It is reasoned that the changes of cNOS activity may have adverse effects on nitric oxide (NO) production in the white matter close to the site of constriction injury, thus broadening the scope of the secondary mechanisms that play a role in neuronal trauma.

White matter injury in spinal cord ischemia: protection by AMPA/kainate glutamate receptor antagonism

BACKGROUND AND PURPOSE

Spinal cord ischemia is a serious complication of surgery of the aorta. NMDA receptor activation secondary to ischemia-induced release of glutamate is a major mechanism of neuronal death in gray matter. White matter injury after ischemia results in long-tract dysfunction and disability. The AMPA/kainate receptor mechanism has recently been implicated in white matter injury.

METHODS

We studied the effects of AMPA/kainate receptor blockade on ischemic white matter injury in a rat model of spinal cord ischemia.

RESULTS

Intrathecal administration of an AMPA/kainate antagonist, 6-nitro-7-sulfamoyl-(f)-quinoxaline-2, 3-dione (NBQX), 1 hour before ischemia reduced locomotor deficit, based on the Basso-Beattie-Bresnahan scale (0=total paralysis; 21=normal) (sham: 21+/-0, n=3; saline: 3.7+/-4.5, n=7; NBQX: 12. 7+/-7.0, n=7, P<0.05) 6 weeks after ischemia. Gray matter damage and neuronal loss in the ventral horn were evident after ischemia, but no difference was noted between the saline and NBQX groups. The extent of white matter injury was quantitatively assessed, based on axonal counts, and was significantly less in the NBQX as compared with the saline group in the ventral (sham: 1063+/-44/200x200 microm, n=3; saline: 556+/-104, n=7; NBQX: 883+/-103, n=7), ventrolateral (sham: 1060+/-135, n=3; saline: 411+/-66, n=7; NBQX: 676+/-122, n=7), and corticospinal tract (sham: 3391+/-219, n=3; saline: 318+/-23, n=7; NBQX: 588+/-103, n=7) in the white matter on day 42.

CONCLUSIONS

Results indicate severe white matter injury in the spinal cord after transient ischemia. NBQX, an AMPA/kainate receptor antagonist, reduced ischemia-induced white matter injury and improved locomotor function.

Advances in secondary spinal cord injury: role of apoptosis

The outcome of spinal cord injury depends on the extent of secondary damage produced by a series of cellular and molecular events initiated by the primary trauma. This article reviews the evidence that secondary spinal cord injury involves the apoptotic as well as necrotic death of neurons and glial cells. Also discussed are the major factors that can contribute to cell death, such as glutamatergic excitotoxicity, free radical damage, cytokines, and inflammation. The development of innovative therapeutic strategies to reduce secondary spinal cord injury depends on an increased understanding of secondary injury mechanisms at the molecular and biochemical level. Such therapeutic interventions may include the use of antiapoptotic drugs, free radical scavengers, and anti-inflammatory agents. These could be targeted to block key reactions on cellular and molecular injury cascades, thus reducing secondary tissue damage, minimizing side effects, and improving functional recovery.

Biochemical, cellular, and molecular mechanisms in the evolution of secondary damage after severe traumatic brain injury in infants and children: Lessons learned from the bedside

OBJECTIVE: To present a state-of-the-art review of mechanisms of secondary injury in the evolution of damage after severe traumatic brain injury in infants and children. DATA SOURCES: We reviewed 152 peer-reviewed publications, 15 abstracts and proceedings, and other material relevant to the study of biochemical, cellular, and molecular mechanisms of damage in traumatic brain injury. Clinical studies of severe traumatic brain injury in infants and children were the focus, but reports in experimental models in immature animals were also considered. Results from both clinical studies in adults and models of traumatic brain injury in adult animals were presented for comparison. DATA SYNTHESIS: Categories of mechanisms defined were those associated with ischemia, excitotoxicity, energy failure, and resultant cell death cascades; secondary cerebral swelling; axonal injury; and inflammation and regeneration. CONCLUSIONS: A constellation of mediators of secondary damage, endogenous neuroprotection, repair, and regeneration are set into motion in the brain after severe traumatic injury. The quantitative contribution of each mediator to outcome, the interplay between these mediators, and the integration of these mechanistic findings with novel imaging methods, bedside physiology, outcome assessment, and therapeutic intervention remain an important target for future research.

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.

Quantitative assessment of ischemic pathology in axons, oligodendrocytes, and neurons: attenuation of damage after transient ischemia

Axons and oligodendrocytes are vulnerable to cerebral ischemia. The absence of quantitative methods for assessment of white matter pathology in ischemia has precluded in vivo evaluation of therapeutic interventions directed at axons and oligodendrocytes. The authors demonstrate here that the quantitative extent of white matter pathology was reduced by restoration of cerebral blood flow after 2 hours of middle cerebral artery occlusion. Focal ischemia was induced in anesthetized rats by intraluminal thread placement, either transiently (for 2 hours) or permanently. At 24 hours after induction of ischemia, axonal damage was determined by amyloid precursor protein (APP) immunohistochemistry, and the ischemic insult to oligodendrocytes was assessed by Tau-1 immunostaining in the same sections. In adjacent sections, ischemic damage to neuronal perikarya was defined histologically. The hemispheric extent of axonal damage was reduced by 70% in the transiently occluded animals from that in permanently occluded animals. The volumes of oligodendrocyte pathology and of neuronal perikaryal damage were reduced by 62% and 58%, respectively, in the transiently occluded animals. These results demonstrate that this methodologic approach for assessing ischemic damage in axons and oligodendrocytes can detect relative alterations in gray and white matter pathology with intervention strategies.

NMDA receptor blockade fails to alter axonal injury in focal cerebral ischemia

The ability of the NMDA receptor antagonist, MK-801, to protect myelinated axons after focal cerebral ischemia has been examined. Amyloid precursor protein (APP) immunocytochemistry was used to assess the anatomic extent of axonal injury, and conventional histopathology was used to assess the volume of ischemic damage to neuronal perikarya. The middle cerebral artery was permanently occluded in 16 cats. The cats were treated with either vehicle or MK-801 as a 0.5-mg/kg bolus at 15 minutes before middle cerebral artery occlusion, followed by an infusion of 0.14 mg/kg per hour. After 6 hours, the animals were killed and the brains processed for histology and immunocytochemistry. The volume of neuronal necrosis was determined from 16 preselected coronal levels of the brain. The circumscribed zones of APP accumulation in axons were mapped onto images at the same 16 coronal levels, and quantitative analysis was performed using a transparent counting grid, randomly placed over each image. The histologic appearance and anatomic location of axons with increased APP immunoreactivity was similar in animals treated with vehicle and MK-801. MK-801 failed to reduce the hemispheric APP score significantly. In vehicle-treated animals, there was a significant association between the volume of neuronal necrosis and the amount of APP immunoreactivity. MK-801 significantly reduced the slope of the association between the volume of neuronal necrosis and the amount of APP immunoreactivity compared with that observed in vehicle-treated animals. As a result, the ratio of hemispheric APP score and volume of neuronal necrosis was significantly increased with MK-801 treatment. The inability of NMDA receptor antagonists to protect axons may limit their functional efficacy in improving functional outcome after stroke.

Traumatic axonal injury: practical issues for diagnosis in medicolegal cases

In the 25 years or so after the first clinicopathological descriptions of diffuse axonal injury (DAI), the criterion for diagnosing recent traumatic white matter damage was the identification of swollen axons ('bulbs') on routine or silver stains, in the appropriate clinical setting. In the last decade, however, experimental work has given us greater understanding of the cellular events initiated by trauma to axons, and this in turn has led to the adoption of immunocytochemical methods to detect markers of axonal damage in both routine and experimental work. These methods have shown that traumatic axonal injury (TAI) is much more common than previously realized, and that what was originally described as DAI occupies only the most severe end of a spectrum of diffuse trauma-induced brain injury. They have also revealed a whole field of previously unrecognized white matter pathology, in which axons are diffusely damaged by processes other than head injury; this in turn has led to some terminological confusion in the literature. Neuropathologists are often asked to assess head injuries in a forensic setting: the diagnostic challenge is to sort out whether the axonal damage detected in a brain is indeed traumatic, and if so, to decide what - if anything - can be inferred from it. The lack of correlation between well-documented histories and neuropathological findings means that in the interpretation of assault cases at least, a diagnosis of 'TAI' or 'DAI' is likely to be of limited use for medicolegal purposes.

Altered expression of the glutamate transporter EAAC1 in neurons and immature oligodendrocytes after transient forebrain ischemia

Glutamate uptake is reduced during ischemia because of perturbations of ionic gradients across neuronal and glial membranes. Using immunohistochemical and Western blot analyses, the authors examined the expression of the glutamate transporters EAAC1, GLAST, and GLT-1 in the rat hippocampus and cerebral cortex 8 hours and 1 to 28 days after transient forebrain ischemia. Densitometric analysis of immunoblots of CA1 homogenates showed a moderate increase in EAAC1 protein levels early after the insult. Consistently, it was observed that EAAC1 immunostaining in CA1 pyramidal neurons was more intense after 8 hours and 1 day of reperfusion and reduced at later postischemia stages. A similar transient increase of EAAC1 immunolabeling was detected in layer V pyramidal neurons of the cerebral cortex. In addition, the authors observed that EAAC1 also was located in oligodendroglial progenitor cells in subcortical white matter. The number of EAAC1-labeled cells in this region was increased after 3 and 28 days of reperfusion. Finally, changes in GLAST and GLT-1 expression were not observed in the CA1 region after ischemia using immunohistochemical study or immunoblotting. Enhanced expression of EAAC1 may be an adaptive response to increased levels of extracellular glutamate during ischemia.

Oligodendroglial cell behaviour in traumatic oedematous human cerebral cortex: a light and electron microscopic study

Cortical biopsies of 12 patients with traumatic brain injuries have been used in the present study to examine oligodendroglial cell changes and reactivity. The samples were processed for light and transmission electron microscopy. Four main types of oligodendrocyte populations have been found: resting or unreactive oligodendrocytes, reactive oligodendrocytes, anoxic-ischaemic oligodendroglial cells and hyperthrophic phagocytic oligodendrocytes. The unreactive or resting oligodendrocyte type exhibited a fusiform or elongated shape, a clear or dense band of scarce perikaryal cytoplasm and a nucleus with peripheral heterororomatin masses. Clear or dense reactive oligodendrocytes showed increased amount of perikaryal citoplasm, dilated endoplasmic reticulum and nuclear envelope, numerous clear, oedematous mitochondria and dense bodies. These oligodendrocytes appeared associated with degenerated myelinated axons. Anoxic-ischaenmic oligodendrocytes showed lacunar enlargement of endoplasmic reticulum, dilated Golgi complex and enlargement and disassembly of nuclear envelope. They appeared also in contact with degenerated myelinated axons. Hypertrophic phagocytic oligodendrocytes were observed engulfing the associated degenerated myelinated axons, invading the myelin sheath, separating the myelin lamellae and exerting myelinolitic effects. Oligodendroglialpseudopodic expansions were observed phagocyting the axoplasmic matrix and leaving a huge vacuolar axoplasmic space. The vasogenic and cytotoxic components of traumatic brain oedema are discussed in relation with the oligodendroglial cell changes and reactivity.

Redefining toxic distal axonopathies

Nerve damage classified as a central-peripheral distal axonopathy is produced by a variety of chemicals (e.g. acrylamide, n-hexane). Historically, axon swelling and secondary degeneration have been considered the morphologic hallmarks of toxic axonopathies and substantial research has been devoted toward deciphering corresponding molecular mechanisms. However, recent studies from the author's laboratory investigating rate (mg toxicant/kg/day) and route (i.p. vs gavage) of intoxication have shown that swelling and degeneration were related to neurotoxicant dosing conditions (i.e. low-dose, subchronic exposure) and not to development of neurophysiological deficits or classic behavioral toxicity. This suggests the presumed hallmarks of distal axonopathy are epiphenomena of uncertain pathophysiologic significance. Therefore, the current definition of and chemical classification scheme for toxic distal axonopathies requires re-evaluation.

Mechanisms of ischaemic damage to central white matter axons: a quantitative histological analysis using rat optic nerve

The mechanism of ischaemic injury to white matter axons was studied by transiently depriving rat optic nerves in vitro of oxygen and glucose. Light and electron microscopic analysis showed that increasing periods of oxygen/glucose deprivation (up to 1 h) caused, after a 90-min recovery period, the appearance of increasing numbers of swollen axons whose ultrastructure indicated that they were irreversibly damaged. This conclusion was supported by experiments showing that the damage persisted after a longer recovery period (3 h). To quantify the axonal pathology, an automated morphometric method, based on measurement of the density of swollen axons, was developed. Omission of Ca2+ from the incubation solution during 1 h of oxygen/glucose deprivation (and for 15 min either side) completely prevented the axonopathy (assessed following 90 min recovery). Omission of Na+ was also effective, though less so (70% protection). The classical Na+ channel blocker, tetrodotoxin (1 microM), provided 92% protection. In view of this evidence implicating Na+ channels in the pathogenesis of the axonal damage, the effects of three different Na+ channel inhibitors, with known neuroprotective properties towards gray matter in in vivo models of cerebral ischaemia, were tested. The compounds used were lamotrigine and the structurally-related molecules, BW619C89 and BW1003C87. All three compounds protected the axons to varying degrees, the maximal efficacies (observed at 30 to 100 microM) being in the order: BW619C89 (>95% protection) > BW1003C87 (70%) > lamotrigine (50%). At a concentration affording near complete protection (100 microM), BW619C89 had no significant effect on the optic nerve compound action potential. Experiments in which BW619C89 was added at different times indicated that its effects were exerted during two distinct phases, one (accounting for about 50% protection) was during the early stage of oxygen/glucose deprivation itself and the other (also about 50%) during the first 15 min of recovery in normal incubation solution. The results are consistent with a pathophysiological mechanism in which Na+ entry through tetrodotoxin-sensitive Na+ channels contributes to Na+ loading of the axoplasm which then results in a lethal Ca2+ overload through reversed Na(+)-Ca2+ exchange. The identification of BW619C89 as a compound able to prevent oxygen/glucose deprivation-induced injury to white matter axons without affecting normal nerve function opens the way to testing the importance of this pathway in white matter injury in vivo.

Mechanisms of ionotropic glutamate receptor-mediated excitotoxicity in isolated spinal cord white matter

Spinal cord injury involves a component of glutamate-mediated white matter damage, but the cellular targets, receptors, and ions involved are poorly understood. Mechanisms of excitotoxicity were examined in an in vitro model of isolated spinal dorsal columns. Compound action potentials (CAPs) were irreversibly reduced to 43% of control after 3 hr of 1 mM glutamate exposure at 37 degrees C. AMPA (100 microM) and kainate (500 microM) had similar effects. Antagonists (1 mM kynurenic acid, 10 microM NBQX, 30 microM GYKI52466) were each equally protective against a glutamate challenge, improving mean CAP amplitude to approximately 80% versus approximately 40% without antagonist. Joro spider toxin (0.75 microM), a selective blocker of Ca(2+)-permeable AMPA receptors, was also protective to a similar degree. Ca(2+)-free perfusate virtually abolished glutamate-induced injury ( approximately 90% vs approximately 40%). MK-801 (10 microM) had no effect. Glutamate caused damage (assayed immunohistochemically by spectrin breakdown products) to astrocytes and oligodendrocytes consistent with the presence of GluR2/3 and GluR4 in these cells. Myelin was also damaged by glutamate likely mediated by GluR4 receptors detected in this region; however, axon cylinders were unaffected by glutamate, showing no increase in the level of spectrin breakdown. These data may guide the development of more effective treatment for acute spinal cord injury by addressing the additional excitotoxic component of spinal white matter damage.

Na+/Ca2+ exchanger activity induces a slow DC potential after in vitro ischemia in rat hippocampal CA1 region

In rat hippocampal CA1 neurons recorded intracellularly from tissue slices, a rapid depolarization occurred approximately 5 min after application of ischemia-simulating medium. In extracellular recordings obtained from CA1 region, a rapid negative-going DC potential (rapid DC potential) was recorded, corresponding to a rapid depolarization. When oxygen and glucose were reintroduced after generating the rapid depolarization, the membrane further depolarized and the potential became 0 mV after 5 min. Contrary, the DC potential began to repolarize slowly and subsequently a slow negative-going DC potential (slow DC potential) occurred within 1 min. A prolonged application of ischemia-simulating medium suppressed the slow DC potential. Addition of a high concentration of ouabain in normoxic medium reproduced a rapid but not a slow DC potential. The slow DC potential was reduced in low Na+- or Co2+-containing medium, but was not affected in low Cl-, high K+ or K+-free medium, suggesting that the slow DC potential is Na+-and Ca2+-dependent. Ni2+ (Ca2+ channel blocker as well as the Na+/Ca2+ exchanger blocker) and benzamil hydrochloride (Na+/Ca2+ exchanger blocker) reduced the slow DC potential dose-dependently. These results suggest that the slow DC potential is mediated by forward mode operation of Na+/Ca2+ exchangers in non-neuronal cells, and that reactivation of Na+, K+-ATPase is necessary to the Na+/Ca2 +exchanger activity.

Expression of induced nitric oxide synthase in amoeboid microglia in postnatal rats following an exposure to hypoxia

The present study showed the expression of induced nitric oxide synthase (iNOS) immunoreactivity in amoeboid microglia following an exposure to transient hypoxia in postnatal rats. iNOS immunoreactivity was expressed mainly in the amoeboid microglia in corpus callosum and subependymal regions of the ventricles within 3 h after hypoxia. The expression declined after 5 h, and became undetectable after 15 h and in longer surviving rats. The immunoreactivity of these cells with OX-42, which is a marker for microglia cells and detects complement type three receptors (CR3), was comparable in the rats exposed to hypoxia and the control rats. Immunoglobulin G (IgG) immunoreactivity was observed in the amoeboid microglia up to 3 h after hypoxia but it was undetectable in longer surviving rats and in the control rats. The iNOS expression in the amoeboid mircoglial cells may be related to the host defense and maintenance of structural integrity of the highly vulnerable periventricular white matter after hypoxia. The immunostaining of amoeboid microglial cells with IgG following hypoxia indicates leakage of plasma immunoglobulin from the blood vessels and its removal by the amoeboid microglial cells.

Rapid ischemic cell death in immature oligodendrocytes: a fatal glutamate release feedback loop

Ischemic injury of immature oligodendrocytes is a major component of the brain injury associated with cerebral palsy, the most common human birth disorder. We now report that cultured immature oligodendrocytes [O4(+)/galactoceramide (GC)(-)] are exquisitely sensitive to ischemic injury (80% of cells were dead after 25.5 min of oxygen and glucose withdrawal). This rapid ischemic cell death was mediated by Ca(2+) influx via non-NMDA glutamate receptors. The receptors were gated by the release of glutamate from the immature oligodendrocytes themselves via reverse glutamate transport and included a significant element of autologous feedback of glutamate from cells onto their own receptors. High (> or = 100 microM) extracellular glutamate was protective against ischemic injury as a result of non-NMDA glutamate receptor desensitization. Other potential pathways of Ca(2+) influx, such as voltage-gated Ca(2+) channels, NMDA receptors, or the Na(+)-Ca(2+) exchanger, did not significantly contribute to ischemic Ca(2+) influx or cell injury. Release of Ca(2+) from intracellular stores was also not an important factor. In agreement with previous studies, more mature oligodendrocytes (O4(-)/GC(+)) were found to be less sensitive to ischemic injury than were the immature cells studied here.

Mechanisms of damage to myelin and oligodendrocytes and their relevance to disease

Oligodendrocytes synthesize and maintain myelin in the central nervous system (CNS). Damage may occur to these cells in a number of conditions, including infections, exposure to toxins, injury, degeneration, or autoimmune disease, arising both in the course of human disease and in experimental animal models of demyelination and dysmyelination; multiple sclerosis is the commonest human demyelinating disorder. Conventional classical accounts of the pathology of this and other myelin diseases have given great insights into their core features, but there remain considerable uncertainties concerning the timing, means and cause(s) of oligodendrocyte and myelin damage. At present, therapeutic efforts largely concentrate on immune manipulation and damage limitation, an approach that has produced only modest effects in multiple sclerosis. One reason for this must be the limited understanding of the mechanisms underlying cell damage - clearly, successful therapeutic strategies for preserving the oligodendrocyte-myelin unit must depend on knowledge of how oligodendrocyte damage and death occurs. In this review, mechanisms of oligodendrocyte and myelin damage are considered, and attempts made to relate them to disease processes, clinical and experimental. The hallmarks of different cell death processes are described, and oligodendrocyte-myelin injury by cellular and soluble mediators is discussed, both in vitro and invivo. Recent developments concerning the pathological involvement of oligodendrocytes in neurodegenerative disease are summarized. Finally, these neuropathological and applied neurobiological observations are drawn together in the context of multiple sclerosis.

AMPA and kainate receptors each mediate excitotoxicity in oligodendroglial cultures

Recent studies indicate that oligodendrocytes are vulnerable to excitotoxic insults mediated by glutamate receptors. The present study was carried out to characterize the type of glutamate receptors triggering cell death in optic nerve oligodendrocyte cultures. Acute activation of either AMPA or kainate receptors was toxic to oligodendrocytes, an effect that was prevented by CNQX. However, exposure to agonists of the NMDA and metabotropic glutamate receptors did not impair cell viability. Dose-response curves showed that toxicity was mediated by three distinct populations of receptors: an AMPA-type receptor and high- and low-affinity kainate-type receptors. Expression and immunocytochemical studies suggested that the glutamate receptor subunits give rise to the native receptors in each population. In all instances, Ca(2+) entry was a major determinant of glutamate receptor excitotoxicity. However, its influence varied for each receptor subtype. These results indicate that aberrantly enhanced activation of AMPA and/or kainate receptors may be involved in demyelinating diseases.

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.

Nitric oxide stimulates cGMP formation in rat optic nerve axons, providing a specific marker of axon viability

A major transduction pathway for nitric oxide (NO) is stimulation of soluble guanylyl cyclase and the generation of cyclic GMP (cGMP). In the central nervous system, the NO-cGMP pathway has previously been associated primarily with synapses, particularly glutamatergic synapses. We report here that NO caused a large increase in the levels of cGMP in a central white matter tract devoid of synapses, namely in the rat isolated optic nerve. Cyclic GMP immunohistochemistry indicated that this response was confined to the axons. Accordingly, nerves previously subjected to 1 h of oxygen/glucose deprivation, which leads to irreversible axonal damage, displayed an 80% reduction in their subsequent capacity to generate cGMP in response to NO and a corresponding reduction in the numbers of cGMP-immunostained axons. Protection of the axon cGMP response against this insult was achieved by omission of Ca2 + or Na + from the incubation medium, and by the pharmacological agents tetrodotoxin, lamotrigine, BW619C89 and BW1003C87, all of which protect axonal structure from oxygen/glucose deprivation-induced damage. The results suggest that the NO-cGMP pathway has a hitherto unsuspected function in the optic nerve. Additionally, the expression of NO-stimulated guanylyl cyclase in optic nerve axons provides a simple, sensitive and specific marker of their functional integrity that is likely to be valuable in investigating the mechanisms responsible for axon degeneration in ischaemia and other conditions.

Experimental spinal cord injury: spatiotemporal characterization of elemental concentrations and water contents in axons and neuroglia

To examine the role of axonal ion deregulation in acute spinal cord injury (SCI), white matter strips from guinea pig spinal cord were incubated in vitro and were subjected to graded focal compression injury. At several postinjury times, spinal segments were removed from incubation and rapidly frozen. X-ray microanalysis was used to measure percent water and dry weight elemental concentrations (mmol/kg) of Na, P, Cl, K, Ca, and Mg in selected morphological compartments of myelinated axons and neuroglia from spinal cord cryosections. As an index of axon function, compound action potentials (CAP) were measured before compression and at several times thereafter. Axons and mitochondria in epicenter of severely compressed spinal segments exhibited early (5 min) increases in mean Na and decreases in K and Mg concentrations. These elemental changes were correlated to a significant reduction in CAP amplitude. At later postcompression times (15 and 60 min), elemental changes progressed and were accompanied by alterations in compartmental water content and increases in mean Ca. Swollen axons were evident at all postinjury times and were characterized by marked element and water deregulation. Neuroglia and myelin in severely injured epicenter also exhibited significant disruptions. In shoulder areas (adjacent to epicenter) of severely injured spinal strips, axons and mitochondria exhibited modest increases in mean Na in conjunction with decreases in K, Mg, and water content. Following moderate compression injury to spinal strips, epicenter axons exhibited early (10 min postinjury) element and water deregulation that eventually recovered to near control values (60 min postinjury). Na(+) channel blockade by tetrodotoxin (TTX, 1 microM) perfusion initiated 5 min after severe crush diminished both K loss and the accumulation of Na, Cl, and Ca in epicenter axons and neuroglia, whereas in shoulder regions TTX perfusion completely prevented subcellular elemental deregulation. TTX perfusion also reduced Na entry in swollen axons but did not affect K loss or Ca gain. Thus graded compression injury of spinal cord produced subcellular elemental deregulation in axons and neuroglia that correlated with the onset of impaired electrophysiological function and neuropathological alterations. This suggests that the mechanism of acute SCI-induced structural and functional deficits are mediated by disruption of subcellular ion distribution. The ability of TTX to reduce elemental deregulation in compression-injured axons and neuroglia implicates a significant pathophysiological role for Na(+) influx in SCI and suggests Na(+) channel blockade as a pharmacotherapeutic strategy.

Neuroprotection of Acute Ischemic Stroke: Where are We?

Neuroprotective treatments for acute ischemic stroke are targeted at the large array of cellular biochemical and metabolic disturbances that occur after focal brain ischemia to prevent the evolution of injury toward irreversibility. Enhanced comprehension about the pathophysiology of focal brain ischemia has expanded the number of neuroprotective modalities under development and identification of the most likely target for these therapies. Many of the neuroprotective interventions are targeted at reducing calcium influx into ischemic cells and the downstream consequences of excessive intracellular calcium. Other neuroprotective strategies include: free radical scavengers, hyperpolarization of resting transmembrane potentials, and inhibition of the inflammatory response and growth factors. Some interventions potentially may enhance recovery and have neuroprotective effects (i.e., basic fibroblast growth factor [bFGF] and citicoline). Despite the lack of proven clinical efficacy with any neuroprotective intervention, the future will hopefully yield convincing evidence that neuroprotection can be effective and then be ultimately combined with thrombolysis to maximize improvement after ischemic stroke.