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

Sodium-mediated axonal degeneration in inflammatory demyelinating disease

Axonal degeneration is a major cause of permanent neurological deficit in multiple sclerosis (MS). The mechanisms responsible for the degeneration remain unclear, but evidence suggests that a failure to maintain axonal sodium ion homeostasis may be a key step that underlies at least some of the degeneration. Sodium ions can accumulate within axons due to a series of events, including impulse activity and exposure to inflammatory factors such as nitric oxide. Recent findings have demonstrated that partial blockade of sodium channels can protect axons from nitric oxide-mediated degeneration in vitro, and from the effects of neuroinflammatory disease in vivo. This review describes some of the reasons why sodium ions might be expected to accumulate within axons in MS, and recent observations suggesting that it is possible to protect axons from degeneration in neuroinflammatory disease by partial sodium channel blockade.

Na-K-Cl cotransporter-mediated intracellular Na+ accumulation affects Ca2+ signaling in astrocytes in an in vitro ischemic model

Na-K-Cl cotransporter isoform 1 (NKCC1) plays an important role in maintenance of intracellular Na+, K+, and Cl- levels in astrocytes. We propose that NKCC1 may contribute to perturbations of ionic homeostasis in astrocytes under ischemic conditions. After 3-8 hr of oxygen and glucose deprivation (OGD), NKCC1-mediated 86Rb influx was significantly increased in astrocytes from NKCC1 wild-type (NKCC1+/+) and heterozygous mutant (NKCC1+/-) mice. Phosphorylated NKCC1 protein was increased in NKCC1+/+ astrocytes at 2 hr of OGD. Two hours of OGD and 1 hr of reoxygenation (OGD/REOX) triggered an 3.6-fold increase in intracellular Na+ concentration ([Na+]i) in NKCC1+/+ astrocytes. Inhibition of NKCC1 activity by bumetanide or ablation of the NKCC1 gene significantly attenuated the rise in [Na+]i. Moreover, NKCC1+/+ astrocytes swelled by 10-30% during 20-60 min of OGD. Either genetic ablation of NKCC1 or inhibition of NKCC1 by bumetanide-attenuated OGD-mediated swelling. An NKCC1-mediated increase in [Na+]i may subsequently affect Ca2+ signaling through the Na+/Ca2+ exchanger (NCX). A rise in [Ca2+]i was detected after OGD/REOX in the presence of a sarcoplasmic-endoplasmic reticulum (ER) Ca2+-ATPase inhibitor thapsigargin. Moreover, OGD/REOX led to a significant increase in Ca2+ release from ER Ca2+ stores. Furthermore, KB-R7943 (2-[2-[4(4-nitrobenzyloxy)phenyl]ethyl]isothiourea mesylate), an inhibitor of reverse-mode operation of NCX, abolished the OGD/REOX-induced enhancement in filling of ER Ca2+ stores. OGD/REOX-mediated Ca2+ accumulation in ER Ca2+ stores was absent when NKCC1 activity was ablated or pharmacologically inhibited. These findings imply that stimulation of NKCC1 activity leads to Na+ accumulation after OGD/REOX and that subsequent reverse-mode operation of NCX contributes to increased Ca2+ accumulation by intracellular Ca2+ stores.

Coherent anti-stokes Raman scattering imaging of axonal myelin in live spinal tissues

We present a vibrational imaging study of axonal myelin under physiological conditions by laser-scanning coherent anti-Stokes Raman scattering (CARS) microscopy. We use spinal cord white matter strips that are isolated from guinea pigs and kept alive in oxygen bubbled Krebs' solution. Both forward- and epi-detected CARS are used to probe the parallel axons in the spinal tissue with a high vibrational contrast. With the CARS signal from CH2 vibration, we have measured the ordering degree and the spectral profile of myelin lipids. Via comparison with the ordering degrees of lipids in myelin figures formed of controlled lipid composition, we show that the majority of the myelin membrane is in the liquid ordered phase. By measuring the myelin thickness and axon diameter, the value of g ratio is determined to be 0.68 with forward- and 0.63 with epi-detected CARS. Detailed structures of the node of Ranvier and Schmidt-Lanterman incisure are resolved. We have also visualized the ordering of water molecules between adjacent bilayers inside the myelin. Our observations provide new insights into myelin organization, complementary to the knowledge from light and electron microscopy studies of fixed and dehydrated tissues. In addition, we have demonstrated simultaneous CARS imaging of myelin and two-photon excitation fluorescence imaging of intra- and extraaxonal Ca2+. The current work opens up a new approach to the study of spinal cord injury and demyelinating diseases.

Mitochondrial damage and histotoxic hypoxia: a pathway of tissue injury in inflammatory brain disease?

The immunological mechanisms leading to tissue damage in inflammatory brain diseases are heterogeneous and complex. They may involve direct cytotoxicity of T lymphocytes, specific antibodies and activated effector cells, such as macrophages and microglia. Here we describe that in certain inflammatory brain lesions a pattern of tissue injury is present, which closely reflects that found in hypoxic conditions of the central nervous system. Certain inflammatory mediators, in particular reactive oxygen and nitrogen species, are able to mediate mitochondrial dysfunction, and we suggest that these inflammatory mediators, when excessively liberated, can result in a state of histotoxic hypoxia. This mechanism may play a major role in multiple sclerosis, not only explaining the lesions formed in a subtype of patients with acute and relapsing course, but also being involved in the formation of diffuse "neurodegenerative" lesions in chronic progressive forms of the disease.

Chronically implanted electrodes for repeated stimulation and recording of spinal cord potentials

We have recorded and characterized the spinal cord evoked potentials (SCEPs) from the epidural space in the halothane-anesthetized rats. A group of 11 adult Wistar male rats was chronically implanted with two pairs of epidural electrodes. SCEPs were repeatedly elicited by applying electrical stimuli via bipolar U-shaped electrodes to the dorsal aspect of the spinal cord at C3-4 or Th11-12 levels, respectively. Responses were registered with the other pair of implanted electrodes, thus allowing us to monitor the descending (stimulation cervical/recording thoracic) and ascending SCEPs (stimulation thoracic/recording cervical). We studied the time-dependent changes of several SCEP parameters, among them the latency and amplitude of two major negative waves N1 and N2. During 4-weeks' survival, all major components of recordings remained stable and only minor changes in some parameters of the SCEPs were detected. We concluded that this technique enables repeated quantitative analysis of the conductivity of the spinal cord white matter in the rat. Our results indicate that SCEPs could be used in long-term experiments for monitoring progressive changes (degeneration/regeneration) in long projection tracts, primarily those occupying the dorsolateral quadrants of the spinal cord. These include projections that are of interest in spinal cord injury studies, i.e. ascending primary afferents, and important descending pathways including corticospinal, rubrospinal, reticulospinal, raphespinal and vestibulospinal tracts.

Na(+)-dependent chloride transporter (NKCC1)-null mice exhibit less gray and white matter damage after focal cerebral ischemia

We previously demonstrated that pharmacological inhibition of Na(+)-K(+)-Cl- cotransporter isoform 1 (NKCC1) is neuroprotective in in vivo and in vitro ischemic models. In this study, we investigated whether genetic ablation of NKCC1 provides neuroprotection after ischemia. Focal ischemia was induced by 2 hours occlusion of the left middle cerebral artery (MCAO) followed by 10 or 24 hours reperfusion. Two hours MCAO and ten or twenty-four hours reperfusion caused infarction (approximately 85 mm3) in NKCC1 wild-type (NKCC1(+/+)) mice. Infarction volume in NKCC1(-/-) mice was reduced by approximately 30% to 46%. Heterozygous mutant (NKCC1(+/-)) mice showed approximately 28% reduction in infarction (P>0.05). Two hours MCAO and twenty-four hours reperfusion led to a significant increase in brain edema in NKCC1(+/+) mice. In contrast, NKCC1(+/-) and NKCC1(-/-) mice exhibited approximately 50% less edema (P<0.05). Moreover, white matter damage was assessed by immunostaining of amyloid precursor protein (APP). An increase in APP was detected in NKCC1(+/+) mice after 2 hours MCAO and 10 hours reperfusion. However, NKCC1(-/-) mice exhibited significantly less APP accumulation (P<0.05). Oxygen-glucose deprivation (OGD) induced approximately 67% cell death and a fourfold increase in Na+ accumulation in cultured NKCC1(+/+) cortical neurons. OGD-mediated cell death and Na+ influx were significantly reduced in NKCC1(-/-) neurons (P<0.05). In addition, inhibition of NKCC1 by bumetanide resulted in similar protection in NKCC1(+/+) neurons and astrocytes (P<0.05). These results imply that stimulation of NKCC1 activity is important in ischemic neuronal damage.

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

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

Relationship between flow-metabolism uncoupling and evolving axonal injury after experimental traumatic brain injury

Blood flow-metabolism uncoupling is a well-documented phenomenon after traumatic brain injury, but little is known about the direct consequences for white matter. The aim of this study was to quantitatively assess the topographic interrelationship between local cerebral blood flow (LCBF) and glucose metabolism (LCMRglc) after controlled cortical impact injury and to determine the degree of correspondence with the evolving axonal injury. LCMRglc and LCBF measurements were obtained at 3 hours in the same rat from 18F-fluorodeoxyglucose and 14C-iodoantipyrine coregistered autoradiographic images, and compared to the density of damaged axonal profiles in adjacent sections and in an additional group at 24 hours using beta-amyloid precursor protein (beta-APP) immunohistochemistry. LCBF was significantly reduced over the ipsilateral hemisphere by 48 +/- 15% compared with sham-controls, whereas LCMRglc was unaffected, apart from foci of elevated LCMRglc in the contusion margin. Flow-metabolism was uncoupled, indicated by a significant 2-fold elevation in the LCMRglc/LCBF ratio within most ipsilateral structures. There was a significant increase in beta-APP-stained axons from 3 to 24 hours, which was negatively correlated with LCBF and positively correlated with the LCMRglc/LCBF ratio at 3 hours in the cingulum and corpus callosum. Our study indicates a possible dependence of axonal outcome on flow-metabolism in the acute injury stage.

Aglycemia and ischemia depress spinal synaptic transmission via inhibitory systems involving NMDA receptors

The effects of in vitro aglycemia (glucose-free) and ischemia (glucose-free and O(2)-free) were examined on the dorsal root-evoked ventral root spinal monosynaptic and polysynaptic reflexes in neonatal rat spinal cords. Aglycemia and ischemia depressed the reflexes in a time-dependent manner and abolished them by 35 min. The depression by ischemia began immediately while that by aglycemia began after 15 min. The NMDA receptor antagonist, DL-2-amino-5-phosphonovaleric acid (APV), blocked the depression induced by aglycemia completely and that by ischemia partially. Strychnine (glycine(A) receptor antagonist) or bicuculline (GABA(A) receptor antagonist) blocked the aglycemia-induced depression of the reflexes. In the case of ischemia, strychnine but not bicuculline, blocked the depression partially. The results indicate that aglycemia and ischemia depress the synaptic transmission involving NMDA receptors. Aglycemia involves both gamma-aminobutyric acid-ergic and glycinergic inhibitory transmission while ischemia involves other additional mechanisms.

Effects of the hyperbaric oxygen treatment on the Na+,K+ -ATPase and superoxide dismutase activities in the optic nerves of global cerebral ischemia-exposed rats

The effects of hyperbaric oxygen (HBO) treatment on the Na+,K+ -ATPase and superoxide dismutase (SOD) activities were examined in the optic nerves of the rats exposed to global cerebral ischemia. Animals were exposed to global cerebral ischemia of 20-min duration and were either sacrificed or exposed to the first HBO treatment immediately, 0.5, 1, 2, 6, 24, 48, 72 or 168 h after ischemic procedure (for Na+,K+ -ATPase activities measurement) or 2, 24, 48 or 168 h after ischemia (for SOD activities measurement). HBO procedure was repeated for 7 consecutive days. It was found that global cerebral ischemia induced a statistically significant decrease in the Na+,K+ -ATPase activity of the optic nerves, starting from 0.5 to 168 h of reperfusion. Maximal enzymatic inhibition was registered 24 h after the ischemic damage. The decline in the Na+,K+ -ATPase activity was prevented in the animals exposed to HBO treatment within the first 6 h of reperfusion. The results of the presented experiments demonstrated also a statistically significant increase in the SOD activity after 24, 48 and 168 h of reperfusion in the optic nerves of non-HBO-treated ischemic animals as well as in the ischemic animals treated with HBO. Our results indicate that global cerebral ischemia induced a significant alterations in the Na+,K+ -ATPase and SOD activities in the optic nerves during different periods of reperfusion. HBO treatment, started within the first 6 h of reperfusion, prevented ischemia-induced changes in the Na+,K+ -ATPase activity, while the level of the SOD activity in the ischemic animals was not changed after HBO administration.

Differential blockade of nerve injury–induced thermal and tactile hypersensitivity by systemically administered brain-penetrating and peripherally restricted local anesthetics

Systemic administration of local anesthetics has been shown to transiently reverse thermal and tactile hypersensitivity induced by peripheral nerve injury, effects that have been taken as suggesting direct actions on the peripheral nerves. The present study sought to determine whether a central site of action could contribute to, or account for, the effects of lidocaine on nerve injury-induced thermal and tactile hypersensitivity. Systemic lidocaine and its peripherally restricted analogues, QX-314 or QX-222, effectively reversed thermal hypersensitivity after spinal nerve ligation injury. Nerve injury-induced tactile hypersensitivity, however, was reversed by systemic lidocaine but not QX-314 or QX-222. Microinjection of either lidocaine or QX-314 into the rostral ventromedial medulla fully reversed spinal nerve ligation-induced thermal and tactile hypersensitivity. The data strongly suggest that nerve injury-induced thermal and tactile hypersensitivity are mediated through different mechanisms. In addition, the present study supports a prominent contribution of the central nervous system in the activity of systemically given lidocaine against nerve injury-induced tactile and thermal hypersensitivity. Thus, lidocaine might reverse tactile hypersensitivity mainly through its actions within the central nervous system, whereas its reversal of thermal hypersensitivity might occur through either central or peripheral sites.

PERSPECTIVE

Nerve injury-induced neuropathic pain has proved remarkably difficult to treat. Systemic administration of ion channel blockers such as lidocaine has been explored for the management of chronic pain. This work indicates that systemic rather than local administration of lidocaine would be more effective in treating tactile allodynia.

The Role of Excitotoxicity in Secondary Mechanisms of Spinal Cord Injury: A Review with an Emphasis on the Implications for White Matter Degeneration

Following an initial impact after spinal cord injury (SCI), there is a cascade of downstream events termed 'secondary injury', which culminate in progressive degenerative events in the spinal cord. These secondary injury mechanisms include, but are not limited to, ischemia, inflammation, free radical-induced cell death, glutamate excitotoxicity, cytoskeletal degradation and induction of extrinsic and intrinsic apoptotic pathways. There is emerging evidence that glutamate excitotoxicity plays a key role not only in neuronal cell death but also in delayed posttraumatic spinal cord white matter degeneration. Importantly however, the differences in cellular composition and expression of specific types of glutamate receptors in grey versus white matter require a compartmentalized approach to understand the mechanisms of secondary injury after SCI. This review examines mechanisms of secondary white matter injury with particular emphasis on glutamate excitotoxicity and the potential link of this mechanism to apoptosis. Recent studies have provided new insights into the mechanisms of glutamate release and its potential targets, as well as the downstream pathways associated with glutamate receptor activation in specific types of cells. Evidence from molecular and functional expression of glutamatergic AMPA receptors in white matter glia (and possibly axons), the protective effects of AMPA/kainate antagonists in posttraumatic white matter axonal function, and the vulnerability of oligodendrocytes to excitotoxic cell death suggest that glutamate excitotoxicity is associated with oligodendrocyte apoptosis. The latter mechanism appears key to glutamatergic white matter degeneration after SCI and may represent an attractive therapeutic target.

Implication of gamma-secretase in neuregulin-induced maturation of oligodendrocytes

Increasing evidences suggest that, after neuregulin (NRG) stimulation, ErbB4 undergoes a series of proteolysis, including gamma-secretase cleavage. The released ErbB4 intracellular domain (EICD) is translocated into nucleus and has a transcriptional function. Although NRG-ErbB4 signaling mediates maturation of oligodendrocytes (OLs), the role of EICD and gamma-secretase in this process remains elusive. Here, we showed that NRG-ErbB4 interaction accumulated EICD in the nucleus and promoted the expression of myelin basic protein expression in OLs. Conversely, inhibitor of ErbB4 or gamma-secretase blocked the capacity of NRG. Nuclear accumulation of EICD did not influence maturation of neurons and astrocytes and early development of OLs. We also found that EICD translocation accorded a temporal pattern, consistent with the developmental gradient of hippocampus. Our data suggest that gamma-secretase activation and EICD nuclear translocation are required for OL maturation induced by NRG, and ErbB4 acts as a functional receptor depending on a new signaling cascade.

Confocal imaging of changes in glial calcium dynamics and homeostasis after mechanical injury in rat spinal cord white matter

Periaxonal glia play an important role in maintaining axonal function in white matter. However, little is known about the changes that occur in glial cells in situ immediately after traumatic injury. We used fluo-3 and confocal microscopy to examine the effects of localized (<0.5 mm) mechanical trauma on intracellular calcium (Ca(i)(2+)) levels in glial cells in a mature rat spinal cord white matter preparation in vitro. At the injury site, the glial Ca(i)(2+) signal increased by 300-400% within 5 min and then irreversibly declined indicating cell lysis and death. In glial cells at sites adjacent to the injury (1.5-2 mm from epicenter), Ca(i)(2+) levels peaked at 10-15 min, and thereafter declined but remained significantly above rest levels. At distal sites (6-9 mm), Ca(i)(2+) levels rose and declined even slower, peaking at 80-90 min. Injury in zero calcium dampened Ca(i)(2+) responses, indicating a role for calcium influx in the generation and propagation of the injury-induced Ca(i)(2+) signal. By 50-80 min post-injury, surviving glial cells demonstrated an enhanced ability to withstand supraphysiological Ca(i)(2+) loads induced by the calcium ionophore A-23187. Glial fibrillary acidic protein (GFAP) and CNPase immunolabeling determined that the glial cells imaged with fluo-3 included both astrocytes and oligodendrocytes. These data provide the first direct evidence that the effects of localized mechanical trauma include a glial calcium signal that can spread along white matter tracts for up to 9 mm within less than 3 h. The results further show that trauma can enhance calcium regulation in surviving glial cells in the acute post-injury period.

Axonal protection using flecainide in experimental autoimmune encephalomyelitis

Axonal degeneration is a major cause of permanent neurological deficit in multiple sclerosis (MS), but no current therapies for the disease are known to be effective at axonal protection. Here, we examine the ability of a sodium channel-blocking agent, flecainide, to reduce axonal degeneration in an experimental model of MS, chronic relapsing experimental autoimmune encephalomyelitis (CR-EAE). Rats with CR-EAE were treated with flecainide or vehicle from either 3 days before or 7 days after inoculation (dpi) until termination of the experiment at 28 to 30 dpi. Morphometric examination of neurofilament-labeled axons in the spinal cord of CR-EAE animals showed that both flecainide treatment regimens resulted in significantly higher numbers of axons surviving the disease (83 and 98% of normal) compared with controls (62% of normal). These findings indicate that flecainide and similar agents may provide a novel therapy aimed at axonal protection in MS and other neuroinflammatory disorders.

Ischemic preconditioning does not improve neurological recovery after spinal cord compression injury in the rat

Ischemic preconditioning (IPC) has been defined as the endogenous cellular protective mechanism evoked by brief ischemic periods. IPC renders the tissue of the central nervous system more resistant to subsequent lethal ischemic insults, and similar protective effect of IPC has been observed after experimental traumatic brain injury. Spinal cord trauma differs from cerebral trauma in that the secondary processes are damaging mostly the white matter. In the present study, we have tested the hypothesis that a transient non-lethal ischemic insult would improve outcomes after subsequent traumatic spinal cord injury (SCI). In the IPC group, 5-min spinal cord ischemia has been induced by aortic occlusion combined with hypotension. Forty-eight hours after IPC, moderate spinal cord injury has been induced by epidural balloon inflation at T8 level. Control group underwent identical surgical procedures without ischemia followed by SCI after 48 h. During the 4-week survival, locomotor performance of all rats was repeatedly tested and evaluated according to BBB scale. After 4 weeks, the animals were perfusion-fixed for histopathology, and morphometric analyses were performed in order to quantify the extent of the spinal cord lesion. All animals were completely paraplegic after SCI, and showed partial neurological recovery during their survival period. No significant differences were detected either in neurological scores or in morphometric measurements after 4 weeks' survival. These results indicate that in contrary to cerebral trauma, IPC does not improve the outcome after SCI.

Focal Lesions in the Rat Central Nervous System Induced by Endothelin-1

Axon injury following cerebral ischemia has received little scientific attention compared to the abundance of information dealing with the pathophysiology of grey matter ischemia. There are differences in the initial response of grey and white matter to ischemia in vitro. In this study we investigate whether the vasoactive peptide, endothelin-1, can generate a focal ischemic lesion in the white matter and compare the findings with endothelin-1-induced lesions in the grey matter. Using a minimally invasive technique to microinject endothelin-1 into selected brain regions, we observed an acute reduction in local MRI perfusion in the injected hemisphere after 1 hour. Twenty-four hours after microinjection of 10 pmoles of endothelin-1, we observed a loss of neurons in the grey matter. At 72 hours, neutrophils were absent and a macrophage/microglia response and astrocyte gliosis were detected. No breakdown in the blood-brain barrier was detected. After injection of 10 pmoles endothelin-1 into the cortical white matter, we observed prolific amyloid precursor protein-positive immunostaining (indicative of axonal disruption) and an increase in tau-1 immunostaining in oligodendrocytes at 6 hours. Similar to the grey matter lesions, no neutrophils were present, a macrophage/microglia response did not occur until 72 hours and there was no disruption in the blood-brain barrier. Focal injections of endothelin-1 into specific areas of the rat CNS represent a model to investigate therapeutic approaches to white matter ischemia.

Deprenyl blocks the aglycemia-induced depression of the synaptic transmission but not the ischemia-induced depression in neonatal rat spinal cord in vitro

The protective action of R-(-)-deprenyl against the aglycemia (glucose-free) and the ischemia (glucose-free and O2-free)-induced changes in the synaptic transmission was investigated. The in vitro "glucose-free and O2-free" condition mimics in vivo ischemia where there is a deficiency of O2 and energy substrate, hence the term ischemia was used. The monosynaptic reflex (MSR) and polysynaptic reflex (PSR) potentials were elicited in the ventral root by stimulating the corresponding dorsal root in an isolated spinal cord from the neonatal rat. Aglycemia and ischemia depressed the spinal reflexes in a time-dependent manner and abolished them within 30 min. The 50% depression of the reflexes (T-50) occurred around 25 min for aglycemia and 15 min for ischemia. Creatine phosphate, an energy supplement, attenuated the aglycemia- and ischemia-induced depression of the reflexes. The T-50 values for both the reflexes were around 40 and 25 min for aglycemia and ischemia, respectively. Deprenyl (10 microM) blocked the aglycemia-induced depression completely but failed to block the ischemia-induced depression. The present results indicate that aglycemia and ischemia abolished the synaptic transmission simultaneously and energy supplementation partially attenuated the depression. The protective effects of deprenyl against aglycemia may not be due to its MAO-B action and suggest for the involvement of non-MAO-B mechanisms.

Upregulation of neurogenesis and reduction in functional deficits following administration of DEtA/NONOate, a nitric oxide donor, after traumatic brain injury in rats

OBJECT

Neurogenesis, which is upregulated by neural injury in the adult mammalian brain, may be involved in the repair of the injured brain and functional recovery. Therefore, the authors sought to identify agents that can enhance neurogenesis after brain injury, and they report that (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA/NONOate), a nitric oxide donor, upregulates neurogenesis and reduces functional deficits after traumatic brain injury (TBI) in rats.

METHODS

The agent DETA/NONOate (0.4 mg/kg) was injected intraperitoneally into 16 rats daily for 7 days, starting 1 day after TBI induced by controlled cortical impact. Bromodeoxyuridine (100 mg/kg) was also injected intraperitoneally daily for 14 days after TBI to label the newly generated cells in the brain. A neurological functional evaluation was performed in all rats and the animals were killed at 14 or 42 days postinjury. Immunohistochemical staining was used to identify proliferating cells.

CONCLUSIONS

Compared with control rats, the proliferation, survival, migration and differentiation of neural progenitor cells were all significantly enhanced in the hippocampus, subventricular zone, striatum, corpus callosum, and the boundary zone of the injured cortex, as well as in the contralateral hemisphere in rats with TBI that received DETA/ NONOate treatment. Neurological functional outcomes in the DETA/NONOate-treated group were also significantly improved compared with the untreated group. These data indicate that DETA/NONOate may be useful in the treatment of TBI.

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.

A modeling study predicts the presence of voltage gated Ca2+ channels on myelinated central axons

The objective of this current study was to investigate whether voltage gated Ca(2+) channels are present on axons of the adult rat optic nerve (RON). Simulations of axonal excitability using a Hodgkin-Huxley based one-compartment model incorporating I(Na), I(K) and leak currents were used to predict conditions under which the potential contribution of a Ca(2+) current to an evoked action potential could be measured. Under control conditions the inclusion of a high threshold Ca(2+) current (I(Ca)) in the model had a negligible effect on the action potential. Reducing I(K), by decreasing the value of g(K), elongated the repolarizing phase of the action potential, increasing its duration. Subsequent incorporation of I(Ca) in the model revealed a significant I(Ca) contribution to the repolarizing phase of the action potential. The simulation thus suggests that Ca(2+) channels may be present on RON axons, but that pharmacological intervention is required to unmask their presence. Experiments based on the simulations revealed that there was no significant contribution of I(Ca) to the control action potential. However, as predicted by the simulation, reducing the repolarizing effect of I(K) by adding the K(+) channel blocker 4-AP revealed a Ca(2+) component on the repolarizing phase of the action potential that was blocked by the Ca(2+) channel inhibitor nifedipine.

Axonal damage: a key predictor of outcome in human CNS diseases

Axonal damage has recently been recognized to be a key predictor of outcome in a number of diverse human CNS diseases, including head and spinal cord trauma, metabolic encephalopathies, multiple sclerosis and other white-matter diseases (acute haemorrhagic leucoencephalitis, leucodystrophies and central pontine myelinolysis), infections [malaria, acquired immunodeficiency syndrome (AIDS) and infection with human lymphotropic virus type 1 (HTLV-I) causing HTLV-I-associated myelopathy (HAM)/tropical spastic paraparesis (TSP)] and subcortical ischaemic damage. The evidence for axonal damage and, where available, its correlation with neurological outcome in each of these conditions is reviewed. We consider the possible pathogenetic mechanisms involved and how increasing understanding of these may lead to more effective therapeutic or preventive interventions.

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 sodium channel blocker RS100642 reverses down-regulation of the sodium channel alpha-subunit Na(v) 1.1 expression caused by transient ischemic brain injury in rats

In this study we evaluated the expression of five sodium channel (NaCh) Alpha-subunit genes after transient middle cerebral artery occlusion (MCAo) in the rat and the effects of treatment with the NaCh blocker and experimental neuroprotective agent RS100642 as compared to the prototype NaCh blocker mexiletine. The expression of Na(v) 1.1, Na(v) 1.2, Na(v) 1.3, Na(v) 1.7, Na(v) 1.8 and the housekeeping gene beta-actin were studied in vehicle or drug-treated rats at 6, 24 and 48 h post-MCAo using real-time quantitative RT-PCR. RS100642 (1 mg/kg), mexiletine (10 mg/kg), or vehicle (1 ml/kg) was injected (i.v.) at 30 min, 2, 4, and 6 h post-injury. Following MCAo only the Na(v) 1.1 and Na(v) 1.2 genes were significantly down-regulated in the ipsilateral hemisphere of the injured brains. RS100642 treatment significantly reversed the down-regulation of Na(v) 1.1 (but not Na(v) 1.2) at 24-48 h post-injury. Mexiletine treatment, on the other hand, had no significant effect on the down-regulation of either gene. These findings demonstrate that treatment with a neuroprotective dose of RS100642 significantly reverses the down-regulation of Na(v) 1.1 caused by ischemic brain injury and suggests that RS100642 selectively targets the Na(v) 1.1 Alpha-subunit of the NaCh. Furthermore, our findings strengthen the hypothesis that ischemic injury may produce selective depletion of voltage-gated NaChs, and suggest that the Na(v) 1.1 NaCh Alpha-subunit may play a key role in the neuronal injury/recovery process.

Restoring function after spinal cord injury

BACKGROUND

By affecting young people during the most productive period of their lives, spinal cord injury is a devastating problem for modern society. A decade ago, treating SCI seemed frustrating and hopeless because of the tremendous morbidity and mortality, life-shattering impact, and limited therapeutic options associated with the condition. Today, however, an understanding of the underlying pathophysiological mechanisms, the development of neuroprotective interventions, and progress toward regenerative interventions are increasing hope for functional restoration.

REVIEW SUMMARY

This study addresses the present understanding of SCI, including the etiology, pathophysiology, treatment, and scientific advances. The discussion of treatment options includes a critical review of high-dose methylprednisolone and GM-1 ganglioside therapy. The concept that limited rebuilding can provide a disproportionate improvement in quality of life is emphasized throughout.

CONCLUSIONS

New surgical procedures, pharmacologic treatments, and functional neuromuscular stimulation methods have evolved over the last decades that can improve functional outcomes after spinal cord injury, but limiting secondary injury remains the primary goal. Tissue replacement strategies, including the use of embryonic stem cells, become an important tool and can restore function in animal models. Controlled clinical trials are now required to confirm these observations. The ultimate goal is to harness the body's own potential to replace lost central nervous system cells by activation of endogenous progenitor cell repair mechanisms.

Role of calcineurin in calcium-mediated hypoxic injury to white matter

BACKGROUND CONTEXT

Calcium influx into cells is responsible for initiating the "final pathway" to cell death in neuronal tissue after traumatic or hypoxic injury. The specific pathways in this cascade are myriad and the importance each one plays is controversial. It is clear, though, that blocking individual pathways confers protection to these tissues.

PURPOSE

In the present study we examined the role of Cyclosporin A (CsA), FK-506 and rapamycin in modulating the effects of Ca(2+) influx through their interactions with immunophilins and specifically the end result of calcineurin modulation.

METHODS

Dorsal columns were isolated from the spinal cord of adult rats and injured by exposure to hypoxic conditions for 60 minutes. The samples were monitored electrophysiologically in an in vitro recording chamber (maintained at 37 C degrees ) during injury, and the compound action potential (CAP) was monitored with glass microelectrodes. The dorsal column was exposed to hypoxic Ringers solution alone or with the different immunosuppressants and compared with baseline readings. Functional recovery of the dorsal column was then assessed by recovery of the CAP.

RESULTS

The mean CAP decreased to about 20% of baseline control levels during hypoxia and returned 53.8+/-7.6% of baseline (p<.05) after reoxygenation. CsA, an immunosuppressant known to inhibit calcineurin, promoted a significantly greater recovery of CAP amplitude to 76.8+/-5.2% and 72.1+/-13.2% of control (p<.05) after hypoxic injury and reoxygenation of dorsal column white matter when applied at concentrations of 1 microM and 10 microM, respectively. FK-506, which also inhibits calcineurin, was applied at a concentration of 0.1 microM, and promoted CAP amplitude recovery to 82.6+/-5.0% of control after hypoxic injury and reoxygenation of dorsal column white matter. The addition of rapamycin (1 microM), which binds to the same immunophilin as FK-506, to the FK-506 (0.1 microM) solution during hypoxic injury showed recovery of CAP amplitudes to only 56.9+/-6.7% of control. Electron microscopy revealed remarkable protection of axons and prevention of organelle disruption in segments treated with CsA and FK-506 during hypoxia when compared with hypoxic controls.

CONCLUSION

In conclusion, both CsA and FK-506 confer in vitro protection to dorsal columns during hypoxic injury at physiological temperatures, and rapamycin blocks the protective effect of FK-506. Thus, calcineurin may play an important role in the physiology of neuronal injury.

New perspectives on developing acute stroke therapy

The development of additional acute stroke therapies to complement and supplement intravenous recombinant tissue-type plasminogen activator within the first 3 hours after stroke onset remains an important and pressing need. Much has been learned about the presumed target of acute stroke therapy, the ischemic penumbra, and clinically available imaging modalities such as magnetic resonance imaging and computed tomography hold great promise for at least partially identifying this region of potentially salvageable ischemic tissue. Understanding the biology of ischemia-related cell injury has also evolved rapidly. New treatment approaches to improve outcome after focal brain ischemia will likely be derived by looking at naturally occurring adaptive mechanisms such as those related to ischemic preconditioning and hibernation. Many clinical trials previously performed with a variety of neuroprotective and thrombolytic drugs provide many lessons that will help to guide future acute stroke therapy trials and enhance the likelihood of success in future trials. Combining knowledge from these three areas provides optimism that additional acute stroke therapies can be developed to maximize beneficial functional outcome in the greatest proportion of acute stroke patients possible.

Moderate hypoglycemia aggravates effects of hypoxia in hippocampal slices from diabetic rats

We recorded the effects of hypoxia combined with relative hypoglycemia on pre- and post-synaptic potentials in the CA1 area of slices from 4-month-old control and diabetic (streptozotocin-treated) Wistar rats. In experiments on slices kept in 10 or 4 mM glucose (at 33 degrees C), hypoxia was applied until the pre-synaptic afferent volley disappeared--after 12-13 min in most slices, but much earlier (5+/-0.8 min) in diabetic slices kept in 4 mM glucose. When oxygenation was resumed, the afferent volley returned in all slices, for an overall mean recovery of 86.5% (+/-8.8%). Field post-synaptic potentials were fully blocked within 2-3 min of the onset of hypoxia. After the end of hypoxia, they failed to reappear in some slices: overall, their recovery varied between 62 and 68% in control slices, as well as in diabetic slices kept in 10 mM glucose; but recovery was very poor in diabetic slices kept in 4 mM glucose (only 15+/-0.94%). In the latter, hypoxic injury discharges occurred earlier (4.2+/-0.68 min vs. 6.5-8 min for other groups). We conclude that diabetes appears to make hippocampal slices more prone to irreversible loss of synaptic function and early block of axonal conduction when temporary hypoxia is combined with moderate hypoglycemia.

Axon pathology in neurological disease: a neglected therapeutic target

In the C57BL/Wld(S) mouse, a dominant mutation dramatically delays Wallerian degeneration in injury and disease, possibly by influencing multi-ubiquitination. Studies on this mouse show that axons and synapses degenerate by active and regulated mechanisms that are akin to apoptosis. Axon loss contributes to neurological symptoms in disorders as diverse as multiple sclerosis, stroke, traumatic brain and spinal cord injury, peripheral neuropathies and chronic neurodegenerative diseases, but it has been largely neglected in neuroprotective strategies. Defects in axonal transport, myelination or oxygenation could trigger such mechanisms of active axon degeneration. Understanding how these diverse insults might initiate an axon-degeneration process could lead to new therapeutic interventions.

Resistance of isolated mammalian spinal cord white matter to oxygen-glucose deprivation

We found that isolated guinea pig spinal cord white matter is resistant to acute oxygen-glucose deprivation. Sixty minutes of oxygen-glucose deprivation resulted in a 60% reduction of compound action potential (CAP) conductance, and there was a near complete recovery after 60 min reperfusion. Corresponding horseradish peroxidase-exclusion assay showed little axonal membrane damage. To further deprive the axons of metabolic substrate, we added 2 mM sodium cyanide or 2 mM sodium azide, both mitochondrial suppressors, to the ischemic medium, which completely abolished CAP and resulted in a 15 to approximately 30% recovery postreperfusion. Both compounds preferentially reduced the conductance of large diameter axons. We suggest the residual ATP in our ischemic model can protect anatomic integrity and physiological functioning of spinal axons following ischemic insult. This further suggests that oxygen-glucose deprivation alone cannot be solely responsible for short-term functional and anatomic damage. The damaging effects of ischemia in vivo may be mediated by factors originating from the gray matter of the cord or other systemic factors; both were largely eliminated in our in vitro white matter preparation.

New method for the quantitative assessment of axonal damage in focal cerebral ischemia

Quantification of damage in both grey and white matter is required for comprehensive assessment of neuroprotective drug efficacy. Although methods for quantification of neuronal perikaryal damage after ischemia are well established, assessment of axonal damage has been limited. This article describes a new method for quantitation of axonal injury after middle cerebral artery (MCA) occlusion in rats and its application to the study of the antioxidant ebselen. The methodology is based on immunohistochemical detection of amyloid precursor protein (APP) accumulation in deformed, swollen axons in zones of ischemia. Sixty-five axon-rich sites throughout the MCA territory are assessed for the presence (scored 1) or absence (scored 0) of accumulated APP in axonal swellings. Scores for individual sites are summated in predefined neuroanatomic regions (e.g. corpus callosum), stereotaxic levels, or for a total hemisphere APP score. Both intra-rater and inter-rater reproducibility were high (r = 0.87 and 0.80, respectively). Ebselen (1 mg kg(-1) hr(-1), intravenously) significantly reduced the volume of neuronal perikaryal damage (24%, P < 0.01) and axonal damage (total APP score reduced from 27 [23.9 to 35.1, 95% CI] to 21.5 [18.2 to 23.3], P = 0.002 with ebselen treatment). In conclusion, a robust and generally applicable method is described for assessing pathologic features in myelinated fiber tracts that is sensitive for detection of drug effects on axonal damage.

Calpain-dependent neurofilament breakdown in anoxic and ischemic rat central axons

Neurofilaments are key structural components of white matter axons. The effect of in vitro anoxia or oxygen-glucose deprivation (OGD) on the integrity of the 160 and 200 kDa neurofilament isoforms was studied by immunoblot, and correlated with physiological function. Adult rat optic nerves were exposed to 60 min of either anoxia or OGD. Compound action potential area recovered to 22+/-6% of control after 60 min of anoxia, and to 4+/-1% after 60 min of OGD. Ca(2+)-free (+EGTA) perfusate allowed complete recovery after OGD (108+/-42%). Tetrodotoxin (TTX, 1 microM) was less protective (45+/-6%). Both anoxia and OGD induced breakdown of neurofilament 160 (NF160) and NF200 revealed by the appearance of multiple lower molecular weight bands mainly in the 75-100 kDa range. Zero-Ca(2+)/EGTA completely prevented NF breakdown. TTX only partially reduced NF160 degradation. Non-phosphorylated NF200 appeared after reperfusion post-anoxia or OGD, and was also greatly reduced by zero-Ca(2+) or TTX. Calpain inhibitors (10 microM calpain inhibitor I or 50 microM MDL 28,170) significantly reduced NF160 and NF200 breakdown/dephosphorylation, but did not improve electrophysiological recovery. Significant calpain-mediated breakdown of NF160 and NF200 indicates structural damage to the axonal cytoskeleton, which was completely Ca(2+)-dependent. While pharmacological inhibition of calpain alone greatly reduced NF proteolysis, there was no concomitant improvement in function. These results imply that calpain inhibition is necessary but not sufficient for white matter protection, and emphasize the existence of multiple Ca(2+)-dependent degradative pathways activated in injured white matter.

Effects of AMPA-receptor and voltage-sensitive sodium channel blockade on high potassium-induced glutamate release and neuronal death in vivo

High extracellular potassium induces spreading depression-like depolarizations and elevations of extracellular glutamate. Both occur in the penumbra of a focal ischemic infarct, and may be responsible for the spread of cell death from the infarct core to the penumbra. We have modeled this situation with microdialysis of an isotonic high-potassium solution into the normal rat amygdala for 70 min. This elevates extracellular glutamate up to 8-fold or more and produces irreversibly damaged, acidophilic neurons. NMDA-receptor blockade protects neurons and reduces the elevation of extracellular glutamate. Here we investigated the effects of sodium channel blockade with the voltage-sensitive sodium channel blocker tetrodotoxin and the AMPA receptor antagonist 2,3-dihydroxy-6-nitro-1,2,3,4-tetrahydrobenzo(f)quinoxaline-7-sulfonamide disodium (NBQX disodium) on high potassium-induced neuronal death and extracellular glutamate elevations. The acidophilic neurons produced are necrotic by ultrastructural examination. Tetrodotoxin, at dialysate concentrations of 33, 330 and 3300 microM (only a small fraction is extracted by tissue), markedly reduced the elevations of glutamate in rat amygdala at nearly all time points during high-potassium perfusion, but it reduced tissue edema only at the highest concentration, and it was neuroprotective only if dialyzed prior to high-potassium microdialysis (at 330 microM concentration). Although both 250 microM (6.2% is extracted by tissue) and 500 microM NBQX reduced elevations of glutamate, neither was neuroprotective, and neuropil edema was not reduced by either concentration. Our results suggest that in vivo, sodium influx through voltage-sensitive sodium channels but not through ligand-gated AMPA receptor channels contributes to high potassium-induced neuronal necrosis.

Sodium channel-blocking agents are not of benefit to rats with kaolin-induced hydrocephalus

OBJECTIVE

Hydrocephalus causes damage to periventricular white matter at least in part through chronic ischemia. The sodium channel-blocking agents mexiletine and riluzole have been shown to be of some protective value in various models of neurological injury. We hypothesized that these agents would ameliorate the effects of experimental childhood-onset hydrocephalus.

METHODS

Hydrocephalus was induced in 4-week-old rats by injection of kaolin into the cisterna magna. Tests of cognitive and motor function were performed on a weekly basis. In a blinded and randomized manner, mexiletine (0.7, 7, or 42 mg/kg/d) or riluzole (1.4 or 13.6 mg/kg/d) was administered by osmotic minipump for 2 weeks, beginning 2 weeks after induction of hydrocephalus. The brains were then subjected to histopathological and biochemical analyses.

RESULTS

Compared with untreated hydrocephalic rats, neither mexiletine nor riluzole was associated with a protective effect on behavioral, structural, or biochemical abnormalities.

CONCLUSION

Protection of hydrocephalic brains through pharmacological sodium channel blockade is probably an approach not worth pursuing.

Toward wisdom from failure: lessons from neuroprotective stroke trials and new therapeutic directions

BACKGROUND

Neuroprotective drugs for acute stroke have appeared to work in animals, only to fail when tested in humans. With the failure of so many clinical trials, the future of neuroprotective drug development is in jeopardy. Current hypotheses and methodologies must continue to be reevaluated, and new strategies need to be explored. Summary of Review- In part 1, we review key challenges and complexities in translational stroke research by focusing on the "disconnect" in the way that neuroprotective agents have traditionally been assessed in clinical trials compared with animal models. In preclinical studies, determination of neuroprotection has relied heavily on assessment of infarct volume measurements (instead of functional outcomes), short-term (instead of long-term) end points, transient (instead of permanent) ischemia models, short (instead of extended) time windows for drug administration, and protection of cerebral gray matter (instead of both gray and white matter). Clinical trials have often been limited by inappropriately long time windows, insufficient statistical power, insensitive outcome measures, inclusion of protocol violators, failure to target specific stroke subtypes, and failure to target the ischemic penumbra. In part 2, we explore new concepts in ischemic pathophysiology that should encourage us also to think beyond the hyperacute phase of ischemia and consider the design of trials that use multiagent therapy and exploit the capacity of the brain for neuroplasticity and repair.

CONCLUSIONS

By recognizing the strengths and limitations of animal models of stroke and the shortcomings of previous clinical trials, we hope to move translational research forward for the development of new therapies for the acute and subacute stages after stroke.

Excitotoxicity in glial cells

Excitotoxicity results from prolonged activation of glutamate receptors expressed by cells in the central nervous system (CNS). This cell death mechanism was first discovered in retinal ganglion cells and subsequently in brain neurons. In addition, it has been recently observed that CNS glial cells can also undergo excitotoxicity. Among them, oligodendrocytes are highly vulnerable to glutamate signals and alterations in glutamate homeostasis may contribute to demyelinating disorders. We review here the available information on excitotoxity in CNS glial cells and its putative relevance to glio-pathologies.

AMPA receptor-mediated toxicity in oligodendrocyte progenitors involves free radical generation and activation of JNK, calpain and caspase 3

The molecular mechanisms underlying AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate) receptor-mediated excitotoxicity were characterized in rat oligodendrocyte progenitor cultures. Activation of AMPA receptors, in the presence of cyclothiazide to selectively block desensitization, produced a massive Ca(2+) influx and cytotoxicity which were blocked by the antagonists CNQX and GYKI 52466. A role for free radical generation in oligodendrocyte progenitor cell death was deduced from three observations: (i) treatment with AMPA agonists decreased intracellular glutathione; (ii) depletion of intracellular glutathione with buthionine sulfoximine potentiated cell death; and (iii) the antioxidant N -acetylcysteine replenished intracellular glutathione and protected cultures from AMPA receptor-mediated toxicity. Cell death displayed some characteristics of apoptosis, including DNA fragmentation, chromatin condensation and activation of caspase-3 and c-Jun N-terminal kinase (JNK). A substrate of calpain and caspase-3, alpha-spectrin, was cleaved into characteristic products following treatment with AMPA agonists. In contrast, inhibition of either caspase-3 by DEVD-CHO or calpain by PD 150606 protected cells from excitotoxicity. Our results indicate that overactivation of AMPA receptors causes apoptosis in oligodendrocyte progenitors through mechanisms involving Ca(2+) influx, depletion of glutathione, and activation of JNK, calpain, and caspase-3.

Nerve terminals as the primary site of acrylamide action: a hypothesis

Acrylamide (ACR) is considered to be prototypical among chemicals that cause a central-peripheral distal axonopathy. Multifocal neurofilamentous swellings and eventual degeneration of distal axon regions in the CNS and PNS have been traditionally considered the hallmark morphological features of this axonopathy. However, ACR has also been shown to produce early nerve terminal degeneration of somatosensory, somatomotor and autonomic nerve fibers under a variety of dosing conditions. Recent research from our laboratory has demonstrated that terminal degeneration precedes axonopathy during low-dose subchronic induction of neurotoxicity and occurs in the absence of axonopathy during higher-dose subacute intoxication. This relationship suggests that nerve terminal degeneration, and not axonopathy, is the primary or most important pathophysiologic lesion produced by ACR. In this hypothesis paper, we review evidence suggesting that nerve terminal degeneration is the hallmark lesion of ACR neurotoxicity, and we propose that this effect is mediated by the direct actions of ACR at nerve terminal sites. ACR is an electrophile and, therefore, sulfhydryl groups on presynaptic proteins represent rational molecular targets. Several presynaptic thiol-containing proteins (e.g. SNAP-25, NSF) are critically involved in formation of SNARE (soluble N-ethylmaleimide (NEM)-sensitive fusion protein receptor) complexes that mediate membrane fusion processes such as exocytosis and turnover of plasmalemmal proteins and other constituents. We hypothesize that ACR adduction of SNARE proteins disrupts assembly of fusion core complexes and thereby interferes with neurotransmission and presynaptic membrane turnover. General retardation of membrane turnover and accumulation of unincorporated materials could result in nerve terminal swelling and degeneration. A similar mechanism involving the long-term consequences of defective SNARE-based turnover of Na+/K(+)-ATPase and other axolemmal constituents might explain subchronic induction of axon degeneration. The ACR literature occupies a prominent position in neurotoxicology and has significantly influenced development of mechanistic hypotheses and classification schemes for neurotoxicants. Our proposal suggests a reevaluation of current classification schemes and mechanistic hypotheses that regard ACR axonopathy as a primary lesion.

Regulation of intracellular calcium in N1E-115 neuroblastoma cells: the role of Na(+)/Ca(2+) exchange

In fura 2-loaded N1E-115 cells, regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) following a Ca(2+) load induced by 1 microM thapsigargin and 10 microM carbonylcyanide p-trifluoromethyoxyphenylhydrazone (FCCP) was Na(+) dependent and inhibited by 5 mM Ni(2+). In cells with normal intracellular Na(+) concentration ([Na(+)](i)), removal of bath Na(+), which should result in reversal of Na(+)/Ca(2+) exchange, did not increase [Ca(2+)](i) unless cell Ca(2+) buffer capacity was reduced. When N1E-115 cells were Na(+) loaded using 100 microM veratridine and 4 microg/ml scorpion venom, the rate of the reverse mode of the Na(+)/Ca(2+) exchanger was apparently enhanced, since an approximately 4- to 6-fold increase in [Ca(2+)](i) occurred despite normal cell Ca(2+) buffering. In SBFI-loaded cells, we were able to demonstrate forward operation of the Na(+)/Ca(2+) exchanger (net efflux of Ca(2+)) by observing increases (approximately 6 mM) in [Na(+)](i). These Ni(2+) (5 mM)-inhibited increases in [Na(+)](i) could only be observed when a continuous ionomycin-induced influx of Ca(2+) occurred. The voltage-sensitive dye bis-(1,3-diethylthiobarbituric acid) trimethine oxonol was used to measure changes in membrane potential. Ionomycin (1 microM) depolarized N1E-115 cells (approximately 25 mV). This depolarization was Na(+) dependent and blocked by 5 mM Ni(2+) and 250-500 microM benzamil. These data provide evidence for the presence of an electrogenic Na(+)/Ca(2+) exchanger that is capable of regulating [Ca(2+)](i) after release of Ca(2+) from cell stores.

Nitric oxide toxicity in CNS white matter: an in vitro study using rat optic nerve

Excessive nitric oxide formation may contribute to the pathology occurring in diseases affecting central white matter, such as multiple sclerosis. The rat isolated optic nerve preparation was used to investigate the potential toxicity of the molecule towards such tissue. The nerves were exposed to a range of concentrations of different classes of nitric oxide donor for up to 23 h, with or without a subsequent period of recovery, and the damage assessed by quantitative histological methods. Degeneration of axons and macroglia occurred in a time- and concentration-dependent manner, the order of susceptibility being: axons>oligodendrocytes>astrocytes. Use of NONOate donors differing in half-life indicated that nitric oxide delivered in an enduring manner at relatively low concentration was more toxic than the same amount supplied rapidly at high concentration. The mechanism by which nitric oxide affects axons was studied using a donor [3-(n-propylamino)propylamine/NO adduct, PAPA/NO] with an intermediate half-life that produced selective axonopathy after a 2-h exposure (plus 2 h recovery). Axon damage was abolished if, during the exposure, Na(+) or Ca(2+) was removed from the bathing medium or the sodium channel inhibitors tetrodotoxin or BW619C89 (sipatrigine) were added. In electrophysiological experiments, the donor elicited a biphasic depolarisation. The second, larger component (occurring after 7-10 min) was associated with a block of nerve conduction and could be inhibited by tetrodotoxin. Coincident with the secondary depolarisation was a reduction in ATP levels by about 50%, an effect that was also inhibited by tetrodotoxin. It is concluded that nitric oxide, in submicromolar concentrations, can kill axons and macroglia in white matter. The findings lend support to the hypothesis that nitric oxide may be of importance to white matter pathologies, particularly those in which inducible nitric oxide synthase is expressed. The axonopathy, at least when elicited over relatively short time intervals, is likely to be caused by metabolic inhibition. As in anoxia and anoxia/aglycaemia, nitric oxide-induced destruction of axons is likely to be caused by the Ca(2+) overload that follows a reduction in ATP levels in the face of continued influx of Na(+) through voltage-dependent channels.

Retinal and optic nerve degeneration after chronic carotid ligation: time course and role of light exposure

BACKGROUND AND PURPOSE

Carotid artery disease can cause chronic retinal ischemia, resulting in transient or permanent blindness, pupillary reflex dysfunction, and retinal degeneration. This experiment investigated the effects of chronic retinal ischemia in an animal model involving permanent carotid occlusion. The time course of retinal pathology and the role of light in this pathology were examined.

METHODS

Sprague-Dawley rats underwent permanent bilateral occlusion of the common carotid arteries or sham surgery. Half of the animals were postsurgically housed in darkness, and half were housed in a 12-hour light/dark cycle. Animals were killed at 3, 15, and 90 days after surgery. Stereological techniques were used to count the cells of the retinal ganglion cell layer. Thy-1 immunoreactivity was assessed to specifically quantify loss of retinal ganglion cells. The thicknesses of the remaining retinal sublayers were measured. Optic nerve degeneration was quantified with the Gallyas silver staining technique.

RESULTS

Permanent bilateral occlusion of the common carotid arteries resulted in loss of the pupillary reflex to light in 58% of rats. Eyes that lost the reflex showed (1) optic nerve degeneration at 3, 15, and 90 days after surgery; (2) a reduction of retinal ganglion cell layer neurons and Thy-1 immunoreactivity by 15 and 90 days; and (3) a severe loss of photoreceptors by 90 days when postsurgically housed in the light condition only.

CONCLUSIONS

Ischemic damage to the optic nerve caused loss of pupillary reflex and death of retinal ganglion cells in a subset of rats. Subsequently, light toxicity induced death of the photoreceptors.

Na(+)-K(+)-ATPase inhibition and depolarization induce glutamate release via reverse Na(+)-dependent transport in spinal cord white matter

Excitotoxic mechanisms involving alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA)/kainate receptors play an important role in mediating cellular damage in spinal cord injury. However, the precise cellular mechanisms of glutamate release from non-synaptic white matter are not well understood. We examined how the collapse of transmembrane Na(+) and K(+) gradients induces reverse operation of Na(+)-dependent glutamate transporters, leading to glutamate efflux and injury to rat spinal dorsal columns in vitro. Compound action potentials were irreversibly reduced to 43% of control after ouabain/high K(+)/low Na(+) exposure (500 microM ouabain for 30 min to increase [Na(+)](i), followed by 1 h ouabain+high K(+) (129 mM)/low Na(+) (27 mM), to further reverse transmembrane ion gradients) followed by a 2 h wash. Ca(2+)-free perfusate was very protective (compound action potential amplitude recovered to 87% vs. 43%). The broad spectrum glutamate antagonist kynurenic acid (1 mM) or the selective AMPA antagonist GYKI52466 (30 microM) were partially protective (68% recovery). Inhibition of Na(+)-dependent glutamate transport with L-trans-pyrrolidine-2,4-dicarboxylic acid (1 mM) also provided significant protection (71% recovery), similar to that seen with glutamate receptor antagonists. Blocking reverse Na(+)-Ca(2+) exchange with KB-R7943 (10 microM) however, was ineffective in this paradigm (49% recovery). Semiquantitative glutamate immunohistochemistry revealed that levels of this amino acid were significantly depleted in axon cylinders and, to a lesser degree, in oligodendrocytes (but not in astrocytes) by ouabain/high K(+)/low Na(+), which was largely prevented by glutamate transport inhibition. Our data show that dorsal column white matter contains the necessary glutamate pools and release mechanisms to induce significant injury. When Na(+) and K(+) gradients are disrupted, even in the absence of reduced cellular energy reserves, reverse operation of Na(+)-dependent glutamate transport will release enough endogenous glutamate to activate AMPA receptors and cause substantial Ca(2+)-dependent injury. This mechanism likely plays an important role during ischemic and traumatic white matter injury, where collapse of transmembrane Na(+) and K(+) gradients occurs.

Delayed detection of motor pathway dysfunction after selective reduction of thoracic spinal cord blood flow in pigs

OBJECTIVE

Clinical monitoring of myogenic motor evoked potentials to transcranial stimulation provides rapid evaluation of motor-pathway function during surgical procedures in which spinal cord ischemia can occur. However, a severe reduction of spinal cord blood flow that remains confined to the thoracic spinal cord might render ischemic only the descending axons of the corticospinal pathway. In this situation lower-limb motor evoked potentials could respond relatively late compared with a similar spinal cord blood flow reduction of the lumbar spinal cord that renders predominantly motoneurons ischemic.

METHODS

Selective thoracic and lumbar spinal cord ischemia was induced by sequential clamping of segmental arteries during continuous assessment of laser-Doppler spinal cord blood flow at the thoracic and lumbar spinal cord. Myogenic motor evoked potentials were recorded from the upper and lower limbs. The time to loss of motor evoked potentials was compared (n = 11) during reduction of laser-Doppler spinal cord blood flow below 25% of baseline (ischemic segment), and flow was maintained at greater than 75% of baseline in the nonischemic segment, both during thoracic and lumbar spinal cord ischemia.

RESULTS

Average laser-Doppler spinal cord blood flow in the ischemic segment was similar during thoracic (26% +/- 15% [+/- SD]) and lumbar (26% +/- 16%) ischemia, whereas normal flow was maintained in the nonischemic segment. The time to motor evoked potentials loss was considerably longer after thoracic spinal cord ischemia (15 +/- 11 minutes) than after lumbar spinal cord ischemia (3 +/- 2 minutes, P <.005).

CONCLUSION

In this experimental model of selective spinal cord ischemia, a severe reduction of lumbar spinal cord blood flow results in rapid loss of myogenic motor evoked potentials, whereas a similar blood flow reduction in the thoracic spinal cord results in relatively slow loss of motor evoked potentials. The effectiveness of motor evoked potentials to rapidly assess spinal cord integrity might be limited when spinal cord ischemia is confined to the thoracic segments.