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

Anoxia-evoked intracellular pH and Ca2+ concentration changes in cultured postnatal rat hippocampal neurons

The ratiometric indicators 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein and Fura-2 were employed to examine, respectively, intracellular pH (pHi) and calcium ([Ca2+]i) changes evoked by anoxia in cultured postnatal rat hippocampal neurons at 37 degrees C. Under both HCO3-/CO2- and HEPES-buffered conditions, 3-, 5- or 10-min anoxia induced a triphasic change in pHi consisting of an initial fall in pHi, a subsequent rise in pHi in the continued absence of O2 and, finally, a further rise in pHi upon the return to normoxia, which recovered towards preanoxic steady-state pHi values if the duration of the anoxic insult was < or = 5 min. In parallel experiments performed on sister cultures, anoxia of 3, 5 or 10 min duration evoked rises in [Ca2+]i which, in all cases, commenced after the start of the fall in pHi, reached a peak at or just following the return to normoxia and then declined towards preanoxic resting levels. Removal of external Ca2+ markedly attenuated increases in [Ca2+]i, but failed to affect the pHi changes evoked by 5 min anoxia. The latency from the start of anoxia to the start of the increase in pHi observed during anoxia was increased by perfusion with media containing either 2 mM Na+, 20 mM glucose or 1 microM tetrodotoxin. Because each of these manoeuvres is known to delay the onset and/or attenuate the magnitude of anoxic depolarization, the results suggest that the rise in pHi observed during anoxia may be consequent upon membrane depolarization. This possibility was also suggested by the findings that Zn2+ and Cd2+, known blockers of voltage-dependent proton conductances, reduced the magnitude of the rise in pHi observed during anoxia. Under HCO3-/CO2-free conditions, reduction of external Na+ by substitution with N-methyl-D-glucamine (but not Li+) attenuated the magnitude of the postanoxic alkalinization, suggesting that increased Na+/H+ exchange activity contributes to the postanoxic rise in pHi. In support, rates of pHi recovery from internal acid loads imposed following anoxia were increased compared to control values established prior to anoxia in the same neurons. In contrast, rates of pHi recovery from acid loads imposed during anoxia were reduced, suggesting the possibility that Na+/H+ exchange is inhibited during anoxia. We conclude that the steady-state pHi response of cultured rat hippocampal neurons to transient anoxia is independent of changes in [Ca2+]i and is characterized by three phases which are determined, at least in part, by alterations in Na+/H- exchange activity and, possibly, by a proton conductance which is activated during membrane depolarization.

Block of rapid depolarization induced by in vitro energy depletion of rat dorsal vagal motoneurones

1. The ionic mechanisms contributing to the rapid depolarization (RD) induced by in vitro ischaemia have been studied in dorsal vagal motoneurones (DVMs) of brainstem slices. Compared with CA1 hippocampal neurones, RD of DVMs was slower, generally occurred from a more depolarized membrane potential and was accompanied by smaller increases in [K+]o. 2. RD was not induced by elevation of [K+]o to values measured around DVMs during in vitro ischaemia or by a combination of raised [K+]o and 2-5 microM ouabain. 3. Neither TTX (5-10 microM) nor TTX combined with bepridil (10-30 microM), a Na+-Ca2+ exchange inhibitor, slowed RD. Block of voltage-dependent Ca2+ channels with Cd2+ (0.2 mM) and Ni2+ (0.3 mM) led to an earlier onset of RD, possibly because [K+]o was higher than that measured during in vitro ischaemia in the absence of divalent ions. 4. When [Na+]o was reduced to 11.25-25 mM, RD did not occur, although a slow depolarization was observed. RD was slowed (i) by 10 mM Mg2+ and 0.5 mM Ca2+, (ii) by a combination of TTX (1.5-5 microM), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and D-2-amino-5-phosphonovalerate (AP5, 50 microM) and (iii) by TTX (1.5-5 microM) and AP5 (50 microM). 5. Ni2+ at concentrations of 0.6 or 1.33 mM blocked RD whereas 0.6 mM Cd2+ did not. A combination of Cd2+ (0.2 mM), Ni2+ (0.3 mM), AP5 (50 microM) and bepridil (10 microM) was largely able to mimic the effects of high concentrations of Ni2+. 6. It is concluded that RD is due to Na+ entry, predominantly through N-methyl-D-aspartate receptor ionophores, and to Ca2+ entry through voltage-dependent Ca2+ channels. These results are consistent with known changes in the concentrations of extracellular ions when ischaemia-induced rapid depolarization occurs.

Spinal cord injury in small animals 2. Current and future options for therapy

Although there can be substantial spontaneous improvements in functional status after a spinal cord injury, therapeutic intervention is desirable in many patients to improve the degree of recovery. At present only decompressive surgery and the neuroprotective drug methylprednisolone sodium succinate are effective and in widespread clinical use. There are limitations to the efficacy of these therapies in some clinical cases and they cannot restore satisfactory functional status to all patients. Many drugs have been investigated experimentally to assess their potential to preserve injured tissue and promote functional recovery in clinically relevant settings, and several of them would be suitable for assessment in future veterinary clinical trials. In addition, experimental techniques designed to mould the response of the CNS to injury, by the promotion of axonal regeneration across the lesion and the encouragement of local sprouting of undamaged axons, have recently been successful, suggesting that effective therapy for even very severe spinal cord injury may soon be available.

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.

Proton magnetic resonance spectroscopy of late delayed radiation-induced injury of the brain

We prospectively evaluated metabolite changes in late delayed radiation-induced injury to the temporal lobes on proton ((1)H) magnetic resonance spectroscopy (MRS) in 34 patients. Morphologically more severe injury on imaging tended to have lower N-acetyl aspartate (NAA)/creatine (Cr) and NAA/choline (Cho) ratios. A significantly higher Cho/Cr ratio was found in the most severe grade of cerebral necrosis, in which lactate might be present. The progressive decrease in NAA with increasing severity reflected neuronal loss at different stages of late delayed radiation-induced brain injury. The absence of Cho elevation in mild and moderate lesions did not suggest demyelination or glial hyperplasia as an etiologic mechanism of late delayed radiation-induced brain injury. The association of severe morphologic lesions with elevated lactate suggests ischemia as the underlying mechanism for severe lesions. (1)H MRS may provide metabolite information conducive to the understanding of the pathophysiology of late radiation-induced brain injury. J. Magn. Reson. Imaging 1999;10:130-137.

Pharmacological inhibition of the Na(+)/Ca(2+) exchanger enhances depolarizations induced by oxygen/glucose deprivation but not responses to excitatory amino acids in rat striatal neurons

BACKGROUND AND PURPOSE

Neuronal Na(+)/Ca(2+) exchanger plays a relevant role in maintaining intracellular Ca(2+) and Na(+) levels under physiological and pathological conditions. However, the role of this exchanger in excitotoxicity and ischemia-induced neuronal injury is still controversial and has never been studied in the same neuronal subtypes.

METHODS

We investigated the effects of bepridil and 3',4'-dichlorobenzamil (DCB), 2 blockers of the Na(+)/Ca(2+) exchanger, in rat striatal spiny neurons by utilizing intracellular recordings in brain slice preparations to compare the action of these drugs on the membrane potential changes induced either by oxygen and glucose deprivation (OGD) or by excitatory amino acids (EAAs).

RESULTS

Bepridil (3 to 100 micromol/L) and DCB (3 to 100 micromol/L) caused a dose-dependent enhancement of the OGD-induced depolarization measured in striatal neurons. The EC(50) values for these effects were 31 micromol/L and 29 micromol/L, respectively. At these concentrations neither bepridil nor DCB altered the resting membrane properties of the recorded cells (membrane potential, input resistance, and current-voltage relationship). The effects of bepridil and DCB on the OGD-induced membrane depolarization persisted in the presence of D-2-amino-5-phosphonovalerate (50 micromol/L) plus 6-cyano-7-nitroquinoxaline-2,3-dione (20 micromol/L), which suggests that they were not mediated by an enhanced release of EAAs. Neither tetrodotoxin (1 micromol/L) nor nifedipine (10 micromol/L) affect the actions of these 2 blockers of the Na(+)/Ca(2+) exchanger, which indicates that voltage-dependent Na(+) channels and L-type Ca(2+) channels were not involved in the enhancement of the OGD-induced depolarization. Conversely, the OGD-induced membrane depolarization was not altered by 5-(N, N-hexamethylene) amiloride (1 to 3 micromol/L), an inhibitor of the Na(+)/H(+) exchanger, which suggests that this antiporter did not play a prominent role in the OGD-induced membrane depolarization recorded from striatal neurons. Bepridil (3 to 100 micromol/L) and DCB (3 to 100 micromol/L) did not modify the amplitude of the excitatory postsynaptic potentials evoked by cortical stimulation. Moreover, these blockers did not affect membrane depolarizations caused by brief applications of glutamate (0.3 to 1 mmol/L), AMPA (0. 3 to 1 micromol/L), and NMDA (10 to 30 micromol/L).

CONCLUSIONS

These results provide pharmacological evidence that the activation of the Na(+)/Ca(2+) exchanger exerts a protective role during the early phase of OGD in striatal neurons, although it does not shape the amplitude and the duration of the electrophysiological responses of these cells to EAA.

Immunohistochemical detection of oxidative DNA damage induced by ischemia-reperfusion insults in gerbil hippocampus in vivo

There is much evidence to suggest that ischemic injury occurs during the reperfusion phase of ischemia-reperfusion insults, and that the injury may be due to reactive-oxygen-species (ROS)-mediated oxidative events, including lipid peroxidation and DNA damage. However, oxidative DNA damage has until now not been examined in situ. In the present study, we report for the first time observation of cell type- and region-specific oxidative DNA damages in 5 min transient ischemic model by immunohistochemical methods, using monoclonal antibody against 8-hydroxy-2'-deoxyguanosine (8-OHdG), an oxidative DNA product. The cell types containing 8-OHdG immunoreactivity were neurons, glia and endothelial cells in the hippocampus. The 8-OHdG immunoreactivity was present in the nucleus but not the cytoplasm of these cells. The level of 8-OHdG in CA1 increased significantly (P<0.05) at the end of 30 min after ischemia, but there was no increase within CA2 and CA3 areas. The 8-OHdG levels in the hippocampus increased significantly (about fourfold) after 3 h of reperfusion and remained significantly (P<0.01) elevated for at least 12 h. At 4 days after ischemia, 8-OHdG levels in the CA2 and CA3 areas decreased to levels of the sham without neuronal loss, while disappearance of 8-OHdG immunoreactivity in the CA1 coincided with neuronal death in this area. These findings strongly suggest that ischemia-induced DNA damage evolves temporally and spatially, and that oxidative DNA damage may be involved in delayed neuronal death in the CA1 region.

Breakdown of calcium homeostasis in relation to tissue depolarization: comparison between gray and white matter ischemia

In vitro studies suggest that ischemic injury of cerebral white matter is mediated by nonsynaptic cellular mechanisms, such as Ca2+ entry into axons through reversal of the Na+ -Ca2+ exchanger. The authors investigated extracellular Ca2+ concentration in relation to tissue depolarization (direct current potential) in vivo using ion-selective electrodes in cortical gray and subcortical white matter of alpha-chloralose-anesthetized cats during 120 minutes of global cerebral ischemia. On induction of ischemia, regional CBF, as measured by hydrogen clearance, ceased. The direct current potential decreased rapidly within minutes in gray matter and with little time delay in white matter. Extracellular Ca2+ concentration decreased just as quickly in gray matter. In white matter, in contrast, extracellular Ca2+ increased in the first 20 to 30 minutes, and a delayed and much slower decline, compared with gray matter, was observed thereafter, reaching a minimal level only about 60 minutes after occlusion. Our results suggest that smaller and delayed transmembrane shifts of Ca2+ are correlates of delayed ischemic membrane dysfunction in central white matter tracts, which may be explained by a lack of synaptic mechanisms.

Cerebral blood flow and glucose metabolism in long-term survivors of childhood acute lymphoblastic leukaemia

Central nervous system treatment for childhood acute lymphoblastic leukaemia (ALL) has been reported to cause changes in cerebral blood flow and glucose metabolism. Little is known about the association of these functional changes with neuropsychological defects and structural changes. The aim of the present study was to assess the relationship between changes in regional cerebral blood flow and glucose utilisation in long-term survivors of ALL, and the association of these functional abnormalities with neurocognitive and structural defects. 8 survivors of childhood ALL were studied with single photon emission tomography (SPECT) using Tc99m-ethyl cysteinate dimer (ECD) as tracer and with positron emission tomography (PET) using 18F-fluorodeoxyglucose (FDG) as tracer. 8 healthy controls also underwent FDG-PET. All subjects also underwent magnetic resonance imaging and neuropsychological assessment 5 years after cessation of the therapy. Focal cerebral blood flow abnormalities were found in ECD-SPECT in 5 of the 8 survivors. Glucose utilisation appeared normal in the corresponding regions. However, glucose utilisation was decreased in thalamus and cerebellum in the survivors of ALL as compared with healthy controls. 3 patients had severe and 5 patients mild neurocognitive difficulties. The changes in cerebral blood flow and FDG uptake did not correspond neuroanatomically with the neurocognitive defects. Focal defects in cerebral blood flow in long-term survivors of ALL are not associated with changes in local cerebral glucose utilisation. Neurocognitive difficulties are not consistently associated with either changes in cerebral blood flow or with decreased glucose utilisation. Therefore, based on the present set of studies FDG-PET and ECD-SPECT cannot yet be recommended for the evaluation of long-term neurocognitive defects associated with treatment of ALL.

Drug development for stroke: importance of protecting cerebral white matter

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

Applications of diffusion-perfusion magnetic resonance imaging in acute ischemic stroke

Diffusion-weighted imaging (DWI) and perfusion imaging (PI) are two new magnetic resonance technologies that are becoming increasingly available for evaluation of acute ischemic stroke patients. DWI provides information about the location of acute focal ischemic brain injury at early time points and PI can document the presence of disturbances in microcirculatory perfusion. DWI and PI are now being used in clinical practice and in clinical trials of potential acute stroke therapies to assess their utility. In the future, DWI and PI may aid in the development of effective acute stroke therapies and help identify which stroke patients are most likely to benefit from specific agents.

Role of sodium ion influx in depolarization-induced neuronal cell death by high KCI or veratridine

Intracellular Na+ concentration plays an important role in the regulation of cellular energy metabolism; i.e., increased intracellular Na+ concentration stimulates glucose utilization both in cultured neurons and astrocytes. Both high KCI and veratridine, which have been known to cause neuronal damage, elicit increased glucose utilization, presumably via increased intracellular Na+ concentration. In the present study, we examined the role of intracellular Na+ influx in the mechanisms of neuronal cell damage induced by high KCl or veratridine assayed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric method. Rat primary cultures of striatal neurons were incubated with high KCl (final concentrations: 25, 50 mM) or veratridine (0.1-100 microM) with or without various inhibitors. High KCl depolarizes cell membrane, thus, leading to Na+ influx through an activation of voltage-sensitive Na+ channels, while veratridine elicits Na+ influx by directly opening these channels. After 24-h incubation with elevated [K+]o or veratridine, glucose contents in the medium decreased significantly (approximately by 7 mM), but remained higher than 18 mM. High [K+]o reduced percent cell viability significantly (approximately 50% at 25 mM, approximately 40% at 50 mM [K+]o, P<0.01), but tetrodotoxin (100 nM) had no protective effect, indicating that Na+ influx was not essential to high K+ -induced cell death. DL-2-Amino-5-phosponovaleric acid (APV) (1 mM) completely blocked cell death induced by elevated [K+]o, while 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 microM) did not. In contrast, veratridine (>10 microM) caused cell damage in a dose-dependent and tetrodotoxin-sensitive manner, but none of APV, CNQX, or bepridil (Na+ -Ca2+ exchanger blocker) had any protective effect. Nifedipine (50 approximately 100 microM), however, reduced percent cell damage induced by veratridine.

Selective protection of gray and white matter during spinal cord ischemic injury

BACKGROUND

Ischemic injury in the gray matter is associated with excitatory amino acid neurotransmitters (EAA) release, and in the white matter is associated with intracellular sodium accumulation. We investigated the protective effect during spinal ischemia of the EAA antagonist, 2-carboxypiperazinyl-propylphosphonic acid (CPP), and the sodium channel blocker (2,6-dimethylphenylcarbamoylmethyl) triethylammonium bromide (QX).

METHODS

Sprague-Dawley rats were randomized in four groups, received intrathecally 10 microL of saline, CPP, QX, or QX/CPP, and underwent balloon occlusion of the aorta. Proximal pressure was lowered by exsanguination. In the acute protocol, 28 rats were used to calculate the length of occlusion, resulting in paraplegia in 50% of animals (P50). In the chronic study, 60 rats underwent 11' occlusion. The chronic animals were scored daily for 28 days and submitted to cord histology.

RESULTS

The P50 of QX (11'22") and QX/CPP (11'54") were longer than saline (10'39"), suggesting a beneficial effect. Neurologic scores of all treatment groups (p = 0.0001) and histologic scores of CPP (p = 0.003) and QX/CPP (p = 0.002) were better than saline.

CONCLUSIONS

Protection of spinal cord during ischemia can be achieved with intrathecal administration of selective agents directed to the gray and white matter.

A review of the pathophysiology of cervical spondylotic myelopathy with insights for potential novel mechanisms drawn from traumatic spinal cord injury

Cervical spondylotic myelopathy (CSM) is the most common cause of spinal cord dysfunction. Despite advances in diagnosis and surgical treatment, many patients still have severe permanent neurologic deficits caused by this condition. An improved understanding of the pathophysiology of cervical spondylotic myelopathy, particularly at a cellular and molecular level, may allow improved treatments in the future. A detailed review of articles in the literature pertaining to cervical spondylotic myelopathy was supplemented by an analysis of relevant mechanisms of spinal cord injury. The pathologic course of cervical spondylotic myelopathy is characterized by early involvement of the corticospinal tracts and later destruction of anterior horn cells, demyelination of lateral and dorsolateral tracts, and relative preservation of anterior columns. Static and mechanical factors and ischemia are critical to the development of cervical spondylotic myelopathy. Free radical-and cation-mediated cell injury, glutamatergic toxicity, and apoptosis may be of relevance to the pathophysiology of cervical spondylotic myelopathy. To date, research in cervical spondylotic myelopathy has focused exclusively on the role of mechanical factors and ischemia. Fundamental research at a cellular and molecular level, particularly in the areas of glutamatergic toxicity and apoptosis may result in clinically relevant treatments for this condition.