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

The protective action of tetrodotoxin and (+/-)-kavain on anaerobic glycolysis, ATP content and intracellular Na+ and Ca2+ of anoxic brain vesicles

Because recent reports point to Na+ channel blockers as protective agents directed against anoxia-induced neuronal damage including protection of anaerobic glycolysis, the influences of tetrodotoxin (TTX) and (+/-)-kavain on anoxic rat brain vesicles were investigated with respect to lactate synthesis, vesicular ATP content and cytosolic free Na+ and Ca2+ ([Na+]i, [Ca2+]i), both of the latter determined fluorometrically employing SBFI and FURA-2, respectively. After anoxia, basal lactate production was increased from 2.9 to 9.8 nmol lactate/min/mg protein. Although lactate synthesis seemed to be stable for at least 45 min of anoxia, as deduced from the linearity of lactate production, the ATP content declined continuously with a half life (tau 1/2) of 14.5 min, indicating that anaerobic glycolysis was insufficient to cover the energy demand of anoxic vesicles. Correspondingly, [Na+]i and [Ca2+]i increased persistently after anoxia by 22.1 mmol/l Na+ and 274.9 nmol/l Ca2+, determined 6.3 min after onset. An additional stimulation of vesicles with veratridine accelerated the drop of ATP (tau 1/2 = 5.1 min) and provoked a massive Na+ overload, which levelled off to 119 mmol/l Na+ within a few minutes. Concomitantly, [Ca2+]i increased linearly with a rate of 355 nmol Ca2+/l/min. Despite the massive perturbation of ion homeostasis, lactate production was unaffected during the first 8 min of veratridine stimulation. However, complete inhibition of lactate synthesis took place 30 min after veratridine was added. The Na+ channel blockers TTX and (+/-)-kavain, if applied before anoxia, preserved vesicular ATP content, diminished anoxia-induced increases in [Na+]i and [Ca2+]i and prevented both the veratridine-induced increases of [Na+]i and [Ca2+]i and the inhibition of lactate production. The data indicate a considerable Na+ influx via voltage-dependent Na+ channels during anoxia, which speeds up the decline in ATP and provokes an increase in [Ca2+]i. A massive Na+ and Ca2+ overload induced by veratridine failed to influence lactate synthesis directly, but initiated its inhibition.

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.

Effects of the sodium channel blocker tetrodotoxin on acute white matter pathology after experimental contusive spinal cord injury

Focal microinjection of tetrodotoxin (TTX), a potent voltage-gated sodium channel blocker, reduces neurological deficits and tissue loss after spinal cord injury (SCI). Significant sparing of white matter (WM) is seen at 8 weeks after injury and is correlated to a reduction in functional deficits. To determine whether TTX exerts an acute effect on WM pathology, Sprague Dawley rats were subjected to a standardized weight-drop contusion at T8 (10 gm x 2.5 cm). TTX (0. 15 nmol) or vehicle solution was injected into the injury site 5 or 15 min later. At 4 and 24 hr, ventromedial WM from the injury epicenter was compared by light and electron microscopy and immunohistochemistry. By 4 hr after SCI, axonal counts revealed reduced numbers of axons and significant loss of large (>/=5 micrometer)-diameter axons. TTX treatment significantly reduced the loss of large-diameter axons. In addition, TTX significantly attenuated axoplasmic pathology at both 4 and 24 hr after injury. In particular, the development of extensive periaxonal spaces in the large-diameter axons was reduced with TTX treatment. In contrast, there was no significant effect of TTX on the loss of WM glia after SCI. Thus, the long-term effects of TTX in reducing WM loss after spinal cord injury appear to be caused by the reduction of acute axonal pathology. These results support the hypothesis that TTX-sensitive sodium channels at axonal nodes of Ranvier play a significant role in the secondary injury of WM after SCI.

The changing landscape of ischaemic brain injury mechanisms

Thrombolysis has become established as an acute treatment for human stroke. But despite multiple clinical trials, neuroprotective strategies have yet to be proved effective in humans. Here we discuss intrinsic tissue mechanisms of ischaemic brain injury, and present a perspective that broadening of therapeutic targeting beyond excitotoxicity and neuronal calcium overload will be desirable for developing the most effective neuroprotective therapies.

An intrathecal bolus of cyclosporin A before injury preserves mitochondrial integrity and attenuates axonal disruption in traumatic brain injury

Traumatic brain injury evokes multiple axonal pathologies that contribute to the ultimate disconnection of injured axons. In severe traumatic brain injury, the axolemma is perturbed focally, presumably allowing for the influx of Ca2+ and initiation of Ca2+ -sensitive, proaxotomy processes. Mitochondria in foci of axolemmal failure may act as Ca2+ sinks that sequester Ca2+ to preserve low cytoplasmic calcium concentrations. This Ca2+ load within mitochondria, however, may cause colloid osmotic swelling and loss of function by a Ca2+ -induced opening of the permeability transition pore. Local failure of mitochondria, in turn, can decrease production of high-energy phosphates necessary to maintain membrane pumps and restore ionic balance in foci of axolemmal permeability change. The authors evaluated the ability of the permeability transition pore inhibitor cyclosporin A (CsA) to prevent mitochondrial swelling in injured axonal segments demonstrating altered axolemmal permeability after impact acceleration injury in rat. At the electron microscopic level, statistically fewer abnormal mitochondria were seen in traumatically injured axons from CsA-pretreated injured animals. Further, this mitochondrial protection translated into axonal protection in a second group of injured rats, whose brains were reacted with antibodies against amyloid precursor protein, a known marker of injured axons. Pretreatment with CsA significantly reduced the number of axons undergoing delayed axotomy, as evidenced by a decrease in the density of amyloid precursor protein-immunoreactive axons. Collectively, these studies demonstrate that CsA protects both mitochondria and the related axonal shaft, suggesting that this agent may be of therapeutic use in traumatic brain injury.

Neuroprotection--rationale for pharmacological modulation of Na(+)-channels

The primary factor detrimental to neurons in neurological disorders associated with deficient oxygen supply or mitochondrial dysfunction is insufficient ATP production relative to their requirement. As a large part of the energy consumed by brain cells is used for maintenance of the Na+ gradient across the cellular membrane, reduction of energy demand by down-modulation of voltage-gated Na(+)-channels is a rational strategy for neuroprotection. In addition, preservation of the inward Na+ gradient may be beneficial because it is an essential driving force for vital ion exchanges and transport mechanisms such as Ca2+ homeostasis and neurotransmitter uptake.

Axonal injury caused by focal cerebral ischemia in the rat

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

Oligodendrocytes and microglia are selectively vulnerable to combined hypoxia and hypoglycemia injury in vitro

The major classes of glial cells, namely astrocytes, oligodendrocytes, and microglial cells were compared in parallel for their susceptibility to damage after combined hypoxia and hypoglycemia or hypoxia alone. The three glial cell types were isolated from neonatal rat brains, separated, and incubated in N2/CO2-gassed buffer-containing glucose or glucose substitutes, 2-deoxyglucose or mannitol (both nonmetabolizable sugars). The damage to the cells after 6 hours' exposure was determined at 0, 1, 3, 7 days based on release of lactate dehydrogenase and counting of ethidium bromide-stained dead cells, double-stained with cell-type specific markers. When 2-deoxyglucose replaced glucose during 6 hours of hypoxia, both oligodendrocytes and microglia rarely survived (18% and 12%, respectively). Astroglia initially increased the release of lactate dehydrogenase but maintained 98% to 99% viability. When mannitol, a radical scavenger and osmolarity stabilizer, replaced glucose during 6 hours of hypoxia, oligodendrocytes rarely survived (10%), astroglia survival remained at 99%, but microglia survival increased to 50%. After exposure to 6 and 42 hours, respectively, of hypoxic conditions alone, oligodendrocytes exhibited 10% survival whereas microglia and astroglia were only temporarily stressed and subsequently survived. In conclusion, oligodendrocytes, then microglia, are the most vulnerable glial cell types in response to hypoxia or hypoglycemia conditions, whereas astrocytes from the same preparations recover.

The role of calcium ion in anoxia/reoxygenation damage of cultured brain capillary endothelial cells

Capillary endothelial cells are critical targets in both ischemia and reperfusion of the brain. Arachidonic acids and oxygen free radicals have been shown to cause disruption of blood-brain barrier (BBB) by destruction of capillary endothelial cell membrane. However, the exact mechanism of BBB breakdown by cerebral ischemia/reperfusion remains undetermined. The aim of the present study is to clarify the mechanism of intracellular calcium ion ([Ca2+]i) change in brain capillary endothelial cells under anoxia/reoxygenation. Brains capillary endothelial cells were isolated from ten male Sprague-Dawley rats by a two step enzymatic process. [Ca2+]i was measured by means of a confocal laser scanning microscope using Indo 1-A/M as a calcium indicator. The endothelial cells were subjected to anoxia and reoxygenization under different conditions. [Ca2+]i increased gradually during anoxia and slightly decreased after reoxygenation. Indomethacin and SOD suppressed the elevation of [Ca2+]i during anoxia. NG-nitro-L-arginine methyl ester and catalase moderately suppressed the elevation, however nifedipine did not suppress it at all. In this model, rapid [Ca2+]i change was not observed during the reoxygenation phase. The results indicate that the anoxia induced elevation of [Ca2+]i in the brain capillary endothelial cells depends on superoxide and peroxynitrite generation.

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

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

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

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

Immunolocalization of the Na(+)-Ca2+ exchanger in mammalian myelinated axons

Previous studies on the pathophysiology of white matter anoxic injury have revealed that the Na(+)-Ca2+ exchanger is an important mediator of Ca2+ overload. To date, however, the localization of this key Ca2+ transporter in myelinated axons has not been demonstrated. The present study uses immunofluorescence labeling with a monoclonal antibody (R3F1) to the canine cardiac type I Na(+)-Ca2+ exchanger to localize exchanger protein to rat peripheral and central myelinated axons. The indirect immunofluorescence labeling technique was used to study paraformaldehyde fixed frozen cryostat sections of sciatic nerve, optic nerve and spinal cord. Examination of sciatic nerve sections with both conventional and confocal microscopy revealed a staining pattern which suggested both a glial and axonal localization of the exchanger. In the rat optic nerve, positive label was associated with cell bodies and their processes, likely glia, and with numerous finer processes arranged in parallel, running longitudinally. These finer processes likely represent axonal profiles. A similar staining pattern was observed in lateral and dorsal columns from spinal cord. Immunoelectron microscopy of dorsal root axons revealed gold particles associated with the paranodal and internodal myelin, in the axoplasm, and close to the nodal/paranodal axon membrane. The high density of Na(+)-Ca2+ exchanger demonstrated in central and peripheral myelinated mammalian axons supports the importance of this transporter in Ca2+ regulation in these tissues.

Axonal structure and function after axolemmal leakage in the squid giant axon

Membrane leakage is a common consequence of traumatic nerve injury. In order to measure the early secondary effects of different levels of membrane leakage on axonal structure and function we studied the squid giant axon after electroporation at field strengths of 0.5, 1.0, 1.6, or 3.3 kV/cm. Immediately after mild electroporation at 0.5 kV/cm, 40% of the axons had no action potentials, but by 1 h all of the mildly electroporated axons had recovered their action potentials. Many large organelles (mitochondria) were swollen, however, and their transport was reduced by 62% 1 h after this mild electroporation. One hour after moderate electroporation at 1.0 kV/cm, most of the axons had no action potentials, most large organelles were swollen, and their transport was reduced by 98%, whereas small organelle transport was reduced by 75%. Finally at severe electroporation levels of 1.65-3.0 kV/cm all conduction and transport was lost and the gel-like axoplasmic structure was clumped or liquefied. The structural damage and transport block seen after severe and moderate poration were early secondary injuries that could be prevented by placing the porated axons in an intracellular-type medium (low in Ca2+, Na+, and Cl-) immediately after poration. In moderately, but not severely, porated axons this protection of organelle transport and structure persisted, and action potential conduction returned when the axons were returned to the previously injurious extracellular-type medium. This suggests that the primary damage, the axolemmal leak, was repaired while the moderately porated axons were in the protective intracellular-type medium.

Quantitative analysis of acute axonal pathology in experimental spinal cord contusion

The major sensorimotor deficits that result from traumatic spinal cord injury (SCI) are due to loss of axons in ascending and descending pathways of the white matter (WM). Experimental treatments administered after a standardized SCI can reduce WM loss and long-term functional deficits. Thus, a significant proportion of WM loss occurs secondary to the mechanical injury and may be a target for therapeutic intervention. Presently, we know little of how and when secondary injury mechanisms operate in the WM after SCI. We therefore used a standardized rat model of clinically relevant contusion injury to examine axonal pathology over the first 24 h by light and electron microscopy. Based on qualitative evaluation of tissue at 15 min, 4 h, and 24 h after a "mild" SCI produced with a weight-drop device (10 g x 2.5 cm), we selected areas from the ventromedial WM at the lesion epicenter for quantitative analyses. We compared axon number and the proportion of axons with various axoplasmic and myelin abnormalities over time after SCI, as well as the effect of axon size on degree of pathology and loss. We found by 4 h postinjury (pi) axonal pathology was more severe than at 15 min and that a significant loss of large diameter axons had occurred; no significant additional loss of axons was seen by 24 h pi. When we compared axonal pathology after a more severe contusion (10 g x 17.5 cm), we found a greater loss of axons at 4 h. In addition, a higher proportion of the remaining axons demonstrated pathological alterations. We developed a semi-quantitative Axonal Injury Index (AII) as an overall measure of axonal pathology that was sensitive to the effects of injury severity at 4 h pi. The AII has greater statistical power than our individual measures of axonal pathology. Our results suggest that it may be possible to use the AII at 4 h pi to assess effects of potential therapeutic agents on acute axonal pathology after SCI.

Differential expression of sodium channel genes in retinal ganglion cells

Action potential electrogenesis in the axons of retinal ganglion cells is supported by voltage-gated sodium channels, and a tetrodotoxin (TTX)-inhibitable sodium conductance participates in anoxic injury of these axons within the optic nerve. However, the subtypes of sodium channels expressed in retinal ganglion cells have not been identified. In this study, we used reverse transcription-polymerase chain reaction (RT-PCR) and restriction enzyme mapping, together with in situ hybridization, to examine the expression of transcripts for sodium channel alpha-subunits I, II, III, NaG, Na6, hNE/PN1 and SNS, and beta-subunits 1 and 2, in the retina of the adult rat. RT-PCR yielded high levels of amplification of I, II, III, Na6, beta1 and beta2 transcripts. In situ hybridization demonstrated the presence of all these mRNAs in the cell bodies of retinal ganglion cells. Retinal ganglion cells thus express multiple sodium channel mRNAs, suggesting that they deploy several different types of sodium channels.

Anoxic injury in the rat spinal cord: pharmacological evidence for multiple steps in Ca(2+)-dependent injury of the dorsal columns

To examine anoxic injury in spinal cord white matter, we studied axonal conduction in the dorsal columns during and following a standard 60 min anoxic insult at 36 degrees C. Perfusion of the spinal cord in 0-Ca2+ Ringer solution resulted in significantly improved recovery of the compound action potential. Similarly, removal of Na+ from the perfusate resulted in significantly improved recovery of conduction in dorsal column axons. Exposure of the anoxic spinal cord to the Na+ channel blocker tetrodotoxin (TTX), the Na-Ca exchange blockers benzamil and bepridil, Na(+)-H+ exchange blockers amiloride and harmaline, and perfusion in Ringer solution with pH adjusted to 6.4, all resulted in improved recovery. The tertiary anesthetics procaine and lidocaine, as well as phenytoin and carbamazepine, also resulted in improved recovery of compound action potential amplitude after 60 min of anoxia. These results demonstrate that a significant component of irreversible loss of conduction, following anoxic injury of the dorsal columns, is Ca(2+)-dependent. Moreover, these results demonstrate that TTX-inhibitable Na+ channels participate in the pathophysiology of anoxic injury in spinal cord white matter, and indicate that reverse Na-Ca exchange provides a route for at least part of the damaging influx of Ca2+ into an intracellular compartment in anoxic spinal cord white matter. Our results also suggest that extracellular acidosis may have a protective effect on anoxic spinal cord white matter, and support the hypothesis that anoxic injury of spinal cord white matter may involve the Na(+)-H+ exchanger.

Mechanism of calcium entry during axon injury and degeneration

Axon degeneration is a hallmark consequence of chemical neurotoxicant exposure (e.g., acrylamide), mechanical trauma (e.g., nerve transection, spinal cord contusion), deficient perfusion (e.g., ischemia, hypoxia), and inherited neuropathies (e.g., infantile neuroaxonal dystrophy). Regardless of the initiating event, degeneration in the PNS and CNS progresses according to a characteristic sequence of morphological changes. These shared neuropathologic features suggest that subsequent degeneration, although induced by different injury modalities, might evolve via a common mechanism. Studies conducted over the past two decades indicate that Ca2+ accumulation in injured axons has significant neuropathic implications and is a potentially unifying mechanistic event. However, the route of Ca2+ entry and the involvement of other relevant ions (Na+, K+) have not been adequately defined. In this overview, we discuss evidence for reverse operation of the Na+-Ca2+ exchanger as a primary route of Ca2+ entry during axon injury. We propose that diverse injury processes (e.g., axotomy, ischemia, trauma) which culminate in axon degeneration cause an increase in intraaxonal Na+ in conjunction with a loss of K+ and axolemmal depolarization. These conditions favor reverse Na+-Ca2+ exchange operation which promotes damaging extraaxonal Ca2+ entry and subsequent Ca2+-mediated axon degeneration. Deciphering the route of axonal Ca2+ entry is a fundamental step in understanding the pathophysiologic processes induced by chemical neurotoxicants and other types of nerve damage. Moreover, the molecular mechanism of Ca2+ entry can be used as a target for the development of efficacious pharmacotherapies that might be useful in preventing or limiting irreversible axon injury.

Altered Na(+)-channel function as an in vitro model of the ischemic penumbra: action of lubeluzole and other neuroprotective drugs

Veratridine blocks Na(+)-channel inactivation and causes a persistant Na(+)-influx. Exposure of hippocampal slices to 10 microM veratridine led to a failure of synaptic transmission, repetitive spreading depression (SD)-like depolarizations of increasing duration, loss of Ca(+)-homeostasis, a large reduction of membrane potential, spongious edema and metabolic failure. Normalization of the amplitude of the negative DC shift evoked by high K+ ACSF 80 min after veratridine exposure was taken as the primary endpoint for neuroprotection. Compounds whose mechanisms of action includes Na(+)-channel modulation were neuroprotective (IC50-values in microM): tetrodotoxin 0.017, verapamil 1.18, riluzole 1.95, lamotrigine > or = 10, and diphenylhydantoin 16.1. Both NMDA (MK-801 and PH) and non-NMDA (NBQX) excitatory amino acid antagonists were inactive, as were NOS-synthesis inhibitor (nitro-L-arginine and L-NAME) Ca(2+)-channel blockers (cadmium, nimodipine) and a K(+)-channel blocker (TEA). Lubeluzole significantly delayed in time before the slices became epileptic, postponed the first SD-like depolarization, allowed the slices to better recover their membrane potential after a larger number of SD-like DC depolarizations, preserved Ca2+ and energy homeostasis, and prevented the neurotoxic effects of veratridine (IC50-value 0.54 microM). A concentration of lubeluzole, which was 40 x higher than its IC50-value for neuroprotection against veratridine, had no effect on repetitive Na(+)-dependent action potentials induced by depolarizing current in normal ACSF. The ability of lubeluzole to prevent the pathological consequences of excessive Na(+)-influx, without altering normal Na(+)- channel function may be of benefit in stroke.

Activation of voltage-dependent sodium channels in cultured cerebellar poffule cells induces neurotoxicity that is not mediated by glutamate release

Exposure of rat cerebellar granule cell cultures to neurotoxins that specifically enhance the open state probability of voltage-dependent Na+ channels, resulted in neuronal death as estimated by a cell viability assay based on fluorescent staining and 51Cr-uptake. Toxicity was detected within 1 h after addition of 100 microM veratridine and was complete within 10-18 h; it was dose-dependent and was found to be completely abolished by tetrodotoxin, an Na+ channel blocker. When veratridine was replaced by an alpha-scorpion toxin, similar observations were done. In contrast, when cultured neurons prepared ffom the cerebral hemisphere of fetal rat brain were exposed to either veratridine or alpha-scorpion toxin for 18 h or even for a longer time of incubation, no neuronal death was observed. DNA fragmentation analysis showed that the toxicity was not mediated by apoptosis. Neuronal death was neither prevented by glutamate receptor antagonists, nor by depletion of endogenous glutamate, nor by voltage sensitive calcium channel antagonists such as omega-Conotoxin-GVIA (N-type channels), omega-Agatoxin-IVA (P-type channels), nimodipine and nitrendipine (L-type channels). Our study indicates that prolonged opening of Na+ channels induced neuronal death of cerebellar granule cells which is not mediated by glutamate and reveals novel neurotoxic mechanism in addition to the well-established excitatory amino acid receptor pathway.

Cytochemical evidence for redistribution of membrane pump calcium-ATPase and ecto-Ca-ATPase activity, and calcium influx in myelinated nerve fibres of the optic nerve after stretch injury

There has been controversy for some time as to whether a posttraumatic influx of calcium ions occurs in stretch/nondisruptively injured axons within the central nervous system in both human diffuse axonal injury and a variety of models of such injury. We have used the oxalate/pyroantimonate technique to provide cytochemical evidence in support of such an ionic influx after focal axonal injury to normoxic guinea pig optic nerve axons, a model for human diffuse axonal injury. We present evidence for morphological changes within 15 min of injury where aggregates of pyroantimonate precipitate occur in nodal blebs at nodes of Ranvier, in focal swellings within axonal mitochondria, and at localized sites of separation of myelin lamellae. In parallel with these studies, we have used cytochemical techniques for localization of membrane pump Ca(2+)-ATPase and ecto-Ca-ATPase activity. There is loss of labelling for membrane pump Ca(2+)-ATPase activity on the nodal axolemma, together with loss of ecto-Ca-ATPase from the external aspect of the myelin sheath at sites of focal separation of myelin lamellae. Disruption of myelin lamellae and loss of ecto-Ca-ATPase activity becomes widespread between 1 and 4 h after injury. This is correlated with both infolding and retraction of the axolemma from the internal aspect of the myelin sheath to form periaxonal spaces which are characterized by aggregates of pyroantimonate precipitate, and the development of myelin intrusions into invaginations of the axolemma such that the regular profile of the axon is lost. There is novel labelling of membrane pump Ca(2+)-ATPase on the cytoplasmic aspect of the internodal axolemma between 1 and 4 h after injury. There is loss of an organized axonal cytoskeleton in a proportion of nerve fibres by 4-6 h after injury. We suggest that these changes demonstrate a progressive pathology linked to calcium ion influx after stretch (non-disruptive) axonal injury to optic nerve myelinated fibres. We posit that calcium influx, linked to or correlated with changes in Ca(2+)-ATPase activities, results in dissolution of the axonal cytoskeleton and axotomy between 4 and 6 h after the initial insult to axons.

Protective effects of antiarrhythmic agents against anoxic injury in CNS white matter

Irreversible anoxic injury is dependent on extracellular Ca2+ in mammalian CNS white matter, with a large portion of the pathologic Ca2+ influx occurring through reverse Na(+)-Ca2+ exchange, stimulated by increased intracellular [Na+]. This Na+ leak likely occurs via incompletely inactivated voltage-gated Na+ channels. This study reports that clinically used antiarrhythmic compounds, likely by virtue of their Na+ channel-blocking properties, significantly protect CNS white matter from anoxia at concentrations that cause little suppression of the preanoxic response. Rat optic nerves were pretreated with various agents for 60 min, then subjected to 60 min of anoxia in vitro. Functional recovery was measured electrophysiologically as the area under the compound action potential (CAP). Without drug, the CAP areas recovered to a mean of 32 +/- 12% of control after 1 h of reoxygenation. Recoveries using prajmaline 10 microM were 82 +/- 15% (p < 0.0001), and using tocainide 1 mM, 78 +/- 8% (p < 0.0001), with little suppression (< or = 10%) of the preanoxic response. Ajmaline (10-100 microM), disopyramide (10-300 microM) and bupivacaine (10-100 microM) were somewhat less effective, whereas verapamil produced 52 +/- 11% recovery before reduction of the preanoxic CAP was observed at 30 microM. Procainamide (100-300 microM) was ineffective. These results suggest that Na+ channel blockers, including commonly used antiarrhythmic agents, may be effective in protecting central white matter, which is a target for anoxic/ischemic injury in diseases such as stroke and spinal cord injury.