Pathophysiology of oligodendroglial excitotoxicity

Diffusion-weighted imaging of the brain in preterm infants with focal and diffuse white matter abnormality

OBJECTIVE

The most common finding on magnetic resonance imaging (MRI) of the brain in preterm infants at term-equivalent age is diffuse excessive high signal intensity (DEHSI) in the white matter. It is unclear whether DEHSI represents a biological abnormality. This study used diffusion-weighted imaging (DWI) to compare apparent diffusion coefficient (ADC) values in DEHSI with infants with normal imaging and those with overt brain damage to determine whether DEHSI shows the diffusion characteristics of normal or abnormal tissue.

METHODS

MRI, using conventional and diffusion-weighted imaging (DWI), was performed in 50 preterm infants at term-equivalent age using a 1.5 Tesla MR scanner. The infants were divided into 3 groups on the basis of their MRI results: 1) normal white matter, 2) DEHSI, or 3) overt white matter pathology. ADC values were measured in the frontal, central, and posterior white matter at the level of the centrum semiovale. ADC values in the 3 groups of preterm infants were compared using a 1-way analysis of variance with a Bonferroni test for multiple comparisons.

RESULTS

ADC values were significantly higher in infants with DEHSI and infants with overt white matter pathology than in infants with normal white matter. There was no significant difference between ADC values in infants with DEHSI and those with overt white matter pathology.

CONCLUSIONS

This study provides objective evidence that DEHSI represents diffuse white matter abnormality.

Calcium-permeable AMPA/kainate receptors mediate toxicity and preconditioning by oxygen-glucose deprivation in oligodendrocyte precursors

Hypoxic-ischemic brain injury in premature infants results in cerebral white matter lesions with prominent oligodendroglial injury and loss, a disorder termed periventricular leukomalacia (PVL). We have previously shown that glutamate receptors mediate hypoxic-ischemic injury to oligodendroglial precursor cells (OPCs) in a model of PVL in the developing rodent brain. We used primary OPC cultures to examine the mechanism of cellular toxicity induced by oxygen-glucose deprivation (OGD) to simulate brain ischemia. OPCs were more sensitive to OGD-induced toxicity than mature oligodendrocytes, and OPC toxicity was attenuated by nonselective [2,3-dihydroxy-6-nitro-7-sulfamoylbenzo[f]quinoxaline (NBQX), 6-cyano-7-nitroquinoxaline-2,3-dione], alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-preferring (GYKI 52466), kainate-preferring (gamma-d-glutamylaminomethanesulfonic acid), or Ca2+-permeable AMPA/kainate receptor antagonists (joro spider toxin, JSTx) administered either during or after OGD. Furthermore, NBQX or JSTx blocked OGD-induced Ca2+ influx. Relevant to recurrent hypoxic-ischemic insults in developing white matter, we examined the effects of sublethal OGD preconditioning. A prior exposure of OPCs to sublethal OGD resulted in enhanced vulnerability to subsequent excitotoxic or OGD-induced injury associated with an increased Ca2+ influx. AMPA/kainate receptor blockade with NBQX or JSTx either during or after sublethal OGD prevented its priming effect. Furthermore, OGD preconditioning resulted in a down-regulation of the AMPA receptor subunit GluR2 on cell surface that increased Ca2+ permeability of the receptors. Overall, these data suggest that aberrantly enhanced activation of Ca2+-permeable AMPA/kainate receptors may be a major mechanism in acute and repeated hypoxic-ischemic injury to OPCs in disorders of developing cerebral white matter, such as PVL.

Multiple sclerosis and glutamate

Experimental autoimmune encephalomyelitis reproduces in rodents the features of multiple sclerosis, an immune-mediated, disabling disorder of the human nervous system. No adequate therapy is available for multiple sclerosis, despite anti-inflammatory, immunosuppressive, and immunomodulatory measures. Increasingly glutamate is implicated in the pathogenesis of neurodegenerative diseases. Here we (1) review changes in the glutamatergic system in multiple sclerosis and (2) reveal the effects of glutamate AMPA antagonists in acute and chronic rodent models of multiple sclerosis. Administration of structurally diverse competitive and non-competitive AMPA antagonists reduces neurologic disability in rodents subjected to acute experimental autoimmune encephalomyelitis. In addition, AMPA antagonists are active in both the adoptive transfer and in chronic models of experimental autoimmune encephalomyelitis in rats and mice and affect both the acute and chronic relapsing phases. Moreover, short-term therapy with AMPA antagonists leads to sustained benefit well into the progressive phases. These results imply that therapeutic strategies for multiple sclerosis should be complemented by glutamate AMPA antagonists to reduce neurologic disability.

Protection of mature oligodendrocytes by inhibitors of caspases and calpains

Mature mouse oligodendrocytes (OLs) are susceptible to death in demyelinating diseases such as multiple sclerosis and in brain injury following neurotrauma, ischemia, or stroke. To understand mechanisms leading to death of mature OLs and develop strategies for protection, we utilized cultures of mature mouse OLs to investigate the role of caspases and calpains in OL cell death mediated by different mechanisms. The agents used were (i) staurosporine, which induces apoptotic death via inhibition of protein kinases; (ii) kainate, which activates non-NMDA glutamate receptors; (iii) thapsigargin, which releases intracellular calcium stores; and (iv) SNAP, which releases active NO species and causes necrotic cell death. Inhibitors blocking primary effector caspases (including caspase 3), the FAS (death receptor)-mediated initiator caspases (including caspase 8), and stress-induced caspases (including caspase 9), were tested for their protective effects. Inhibition of caspases 3, 8, and 9 each robustly protected OLs following insult with staurosporine, thapsigargin, or kainate when added at optimal times. The time of addition of the inhibitors for maximal protection varied with the agent, from 1 h of preincubation before addition of staurosporine to 6 h after addition of kainate. Much less protection was seen for the NO generator SNAP under any condition. The role of calcium in OL death in each model was investigated by chelating extracellular Ca++ with EGTA, and by inhibiting the Ca++-activated calpain proteases. Calcium chelation did not protect against staurosporine, but decreased OL death initiated by kainate, thapsigargin, or NO. The calpain inhibitors PD150606 and calpain inhibitor I protected from cell death initiated by staurosporine, kainate, and thapsigargin, but not from cell death initiated by the NO donor SNAP.

Type-2 astrocyte-like cells are more resistant than oligodendrocyte-like cells against non-N-methyl-D-aspartate glutamate receptor-mediated excitotoxicity

Glutamate causes excitotoxicity via non-N-methyl-D-aspartate (NMDA) glutamate receptors (GluR) in oligodendrocytes. Because both oligodendrocytes and type 2 astrocytes are differentiated from oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells, we investigated whether astrocytes are also vulnerable to non-NMDA GluR-mediated excitotoxicity. For these studies, oligodendrocyte-like cells (OLC) and type 2 astrocyte-like cells (2ALC) were derived from CG-4 cells, an immortalized rat O-2A progenitor cell line. About 50% of 2ALC were positive for glial fibrillary acidic protein and 90% were positive for A2B5, verifying that these cells have an type 2 astrocytic phenotype. A 24-hr exposure of OLC to 2 mM kainate, an activator of non-NMDA GluR, caused cell damage as shown by the release of lactate dehydrogenase. The extent of kainate-induced OLC damage was increased by cyclothiazide. In contrast, exposure of 2ALC to 2 mM kainate alone did not induce injury, though mild 2ALC injury was elicited by exposure to 2 mM kainate plus 100 microM cyclothiazide. Furthermore, we found that the kainate induced Ca(2+) uptake by 2ALC was 27.5% of that induced by kainate in OLC. Finally, both OLC and 2ALC expressed non-NMDA GluR subunit mRNAs, including GluR2, GluR3, GluR4, GluR6, GluR7, KA1, and KA2, but quantitative Western blot analysis revealed higher immunodetectable GluR2 and lower immunodetectable GluR3 and GluR4 in 2ALC than in OLC. Together, these results suggest that astrocytes are relatively resistant to non-NMDA GluR-mediated excitotoxicity because they have a higher expression of GluR2 and lower expression of GluR3 and GluR4.

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.

The role of glutamate receptor maturation in perinatal seizures and brain injury

The perinatal age window is characterized by vulnerability to age-specific patterns of injury. Hypoxia/ischemia occurs in a number of settings both in term and preterm neonates, yet the patterns of response appear dependent upon the age of the infant. In the preterm neonate, hypoxic/ischemic insults result in selective white matter injury, termed periventricular leukomalacia (PVL), with little or no cortical pathology. However, in term babies, hypoxic encephalopathy is the most common cause of seizures, and also can result in cortical infarction. Extracellular glutamate accumulates in the setting of hypoxia/ischemia, and excess activation of glutamate receptors has been implicated in hypoxic/ischemic cellular death. Glutamate receptors are developmentally regulated in both neuronal and glial cells within the brain. Using rodent models, we have shown that hypoxia/ischemia results in selective white matter injury in postnatal day (P) seven rat pups, while hypoxia causes seizures in P10-12 rats, but not at younger or older ages. We have further demonstrated that antagonists of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptor subtype block white matter injury at P7 and seizures at P10. We have shown that AMPA receptors are relatively overexpressed in oligodendrocytes (OLs) within white matter at P7 and in neurons in cortex and hippocampus at P10. Hence maturational patterns of glutamate receptor expression correlate with age-specific regional susceptibility to injury to hypoxia/ischemia. While glutamate receptor blockade represents a rational strategy in the treatment of perinatal hypoxic/ischemic brain injury, it is unclear what role variations in their expression play in normal development and plasticity. Further investigation of patterns of glutamate receptor subunit expression in human brain and in experimental animal models is necessary to determine potential age specific strategies as well as adverse effects.

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.

Ca(2+) influx through AMPA or kainate receptors alone is sufficient to initiate excitotoxicity in cultured oligodendrocytes

Oligodendrocytes are vulnerable to excitotoxic insults mediated by AMPA receptors and by low and high affinity kainate receptors, a feature that is dependent on Ca(2+) influx. In the current study, we have analyzed the intracellular concentration of calcium [Ca(2+)](i) as well as the entry routes of this cation, upon activation of these receptors. Selective activation of either receptor type resulted in a substantial increase (up to fivefold) of [Ca(2+)](i), an effect which was totally abolished by the non-NMDA receptor antagonist CNQX or by removing Ca(2+) from the culture medium. Blockade of voltage-gated Ca(2+) channels with La(3+) or nifedipine, reduced the amplitude of the Ca(2+) current triggered by AMPA receptor activation by approximately 65%, but not that initiated by low and high affinity kainate receptors. In contrast, KB-R7943, an inhibitor of the plasma membrane Na(+)-Ca(2+) exchanger, solely attenuated the rise in [Ca(2+)](i) by approximately 25% due to activation of low affinity kainate receptors. However, oligodendroglial death by glutamate receptor overactivation was largely unaffected in the presence of La(3+) or KB-R7943. These findings indicate that Ca(2+) influx via AMPA and kainate receptors alone is sufficient to initiate cell death in oligodendrocytes, which does not require the entry of calcium via other routes such as voltage-activated calcium channels or the plasma membrane Na(+)-Ca(2+) exchanger.

Neurobiology of periventricular leukomalacia in the premature infant

Brain injury in the premature infant is a problem of enormous importance. Periventricular leukomalacia (PVL) is the major neuropathologic form of this brain injury and underlies most of the neurologic morbidity encountered in survivors of premature birth. Prevention of PVL now seems ultimately achievable because of recent neurobiologic insights into pathogenesis. The pathogenesis of this lesion relates to three major interacting factors. The first two of these, an incomplete state of development of the vascular supply to the cerebral white matter, and a maturation-dependent impairment in regulation of cerebral blood flow underlie a propensity for ischemic injury to cerebral white matter. The third major pathogenetic factor is the maturation-dependent vulnerability of the oligodendroglial (OL) precursor cell that represents the major cellular target in PVL. Recent neurobiologic studies show that these cells are exquisitely vulnerable to attack by free radicals, known to be generated in abundance with ischemia-reperfusion. This vulnerability of OLs is maturation-dependent, with the OL precursor cell highly vulnerable and the mature OL resistant, and appears to relate to a developmental window characterized by a combination of deficient antioxidant defenses and active acquisition of iron during OL differentiation. The result is generation of deadly reactive oxygen species and apoptotic OL death. Important contributory factors in pathogenesis interact with this central theme of vulnerability to free radical attack. Thus, the increased likelihood of PVL in the presence of intraventricular hemorrhage could relate to increases in local iron concentrations derived from the hemorrhage. The important contributory role of maternal/fetal infection or inflammation and cytokines in the pathogenesis of PVL could be related to effects on the cerebral vasculature and cerebral hemodynamics, to generation of reactive oxygen species, or to direct toxic effects on vulnerable OL precursors. A key role for elevations in extracellular glutamate, caused by ischemia-reperfusion, is suggested by demonstrations that glutamate causes toxicity to OL precursors by both nonreceptor- and receptor-mediated mechanisms. The former involves an exacerbation of the impairment in antioxidant defenses, and the latter, an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptor-mediated cell death. Most importantly, these new insights into the pathogenesis of PVL suggest potential preventive interventions. These include avoidance of cerebral ischemia by detection of infants with impaired cerebrovascular autoregulation, e.g. through the use of in vivo near-infrared spectroscopy, the use of free radical scavengers to prevent toxicity by reactive oxygen species, the administration of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptor antagonists to prevent glutamate-mediated injury, or the use of maternal antibiotics or anticytokine agents to prevent toxicity from maternal/fetal infection or inflammation and cytokines.

Endogenous opioids and oligodendroglial function: possible autocrine/paracrine effects on cell survival and development

Previous work has shown that oligodendrocytes (OLs) express both micro- and kappa-opioid receptors. In developing OLs, micro receptor activation increases OL proliferation, while the kappa-antagonist nor-binaltorphimine (NorBNI) affects OL differentiation. Because exogenous opioids were not present in our defined culture medium, we hypothesized that NorBNI blocked endogenous opioids produced by the OLs themselves. To test this, intact and partially processed proenkephalin and prodynorphin-derived peptides were assessed in OLs using immunocytochemistry or Western blot analysis, or both. Immature OLs possessed large amounts of intact and partially processed proenkephalin precursors, as well as posttranslational products of prodynorphin including dynorphin A (1-17). With maturation, however, intact or partially processed proenkephalin was expressed by only about 50% of OLs, while dynorphin A (1-17) was undetectable. To assess the function of OL-derived opioids, the effect of kappa-agonists/antagonists on OL differentiation and death was explored. kappa-Agonists alone had no effect. In contrast, NorBNI significantly increased OL death. Additive OL losses were evident when NorBNI was paired with toxic levels of glutamate, suggesting that kappa-receptor blockade alone is sufficient to induce OL death. Thus, the results indicate that OLs express proenkephalin and prodynorphin peptides in a developmentally regulated manner, and further suggest that opioids produced by OLs modulate OL maturation and survival through local (i.e., autocrine and/or paracrine) mechanisms.

Multiple sclerosis: altered glutamate homeostasis in lesions correlates with oligodendrocyte and axonal damage

Glutamate excitotoxicity, recently demonstrated in an animal model of multiple sclerosis (MS), is evoked by altered glutamate homeostasis. In the present study, we investigated the major regulating factors in glutamate excitotoxicity by immunohistochemistry in MS and control white matter with markers for glutamate production (glutaminase), glutamate transport (GLAST, GLT-1 and EAAT-1), glutamate metabolism (glutamate dehydrogenase [GDH] and glutamine synthetase [GS]), axonal damage (SMI 32) and CNS cell types. Active MS lesions showed high-level glutaminase expression in macrophages and microglia in close proximity to dystrophic axons. Correlation between glutaminase expression and axonal damage was confirmed experimentally in animals. White matter from other inflammatory neurologic diseases displayed glutaminase reactivity, whereas normals and noninflammatory conditions showed none. All three glutamate transporters were expressed robustly, mainly on oligodendrocytes, in normal, control and MS white matter, except for GLT-1, which showed low-level expression around active MS lesions. GS and GDH were present in oligodendrocytes in normal and non-MS white matter but were absent from both active and chronic silent MS lesions, suggesting lasting metabolic impediments. Thus, imbalanced glutamate homeostasis contributes to axonal and oligodendroglial pathology in MS. Manipulation of this imbalance may have therapeutic import.

Ampa/kainate receptor activation mediates hypoxic oligodendrocyte death and axonal injury in cerebral white matter

We developed an in situ model to investigate the hypothesis that AMPA/kainate (AMPA/KA) receptor activation contributes to hypoxic-ischemic white matter injury in the adult brain. Acute coronal brain slices, including corpus callosum, were prepared from adult mice. After exposure to transient oxygen and glucose deprivation (OGD), white matter injury was assessed by electrophysiology and immunofluorescence for oligodendrocytes and axonal neurofilaments. White matter cellular components and the stimulus-evoked compound action potential (CAP) remained stable for 12 hr after preparation. OGD for 30 min resulted in an irreversible loss of the CAP as well as structural disruption of axons and subsequent loss of neurofilament immunofluorescence. OGD also caused widespread oligodendrocyte death, demonstrated by the loss of APC labeling and the gain of pyknotic nuclear morphology and propidium iodide labeling. Blockade of AMPA/KA receptors with 30 microm NBQX or the AMPA-selective antagonist 30 microm GYKI 52466 prevented OGD-induced oligodendrocyte death. Oligodendrocytes also were preserved by the removal of Ca(2+), but not by a blockade of voltage-gated Na(+) channels. The protective action of NBQX was still present in isolated corpus callosum slices. CAP areas and axonal structure were preserved by Ca(2+) removal and partially protected by a blockade of voltage-gated Na(+) channels. NBQX prevented OGD-induced CAP loss and preserved axonal structure. These observations highlight convergent pathways leading to hypoxic-ischemic damage of cerebral white matter. In accordance with previous suggestions, the activation of voltage-gated Na(+) channels contributes to axonal damage. Overactivation of glial AMPA/KA receptors leads to oligodendrocyte death and also plays an important role in structural and functional disruption of axons.

Perinatal brain injury: from pathogenesis to neuroprotection

Brain injury secondary to hypoxic-ischemic disease is the predominant form of all brain injury encountered in the perinatal period. The focus of this article is the most recent research developments in this field and especially those developments that should lead to the most profound effects on interventions in the first years of the new millennium. Neuronal injury is the predominant form of cellular injury in the term infant. The principal mechanisms leading to neuronal death after hypoxia-ischemia/reperfusion are initiated by energy depletion, accumulation of extracellular glutamate, and activation of glutamate receptors. The cascade of events that follows involves accumulation of cytosolic calcium and activation of a variety of calcium-mediated deleterious events. Notably this deleterious cascade, which evolves over many hours, may be interrupted even if interventions are instituted after termination of the insult, an important clinical point. Of the potential interventions, the leading candidates for application to the human infant in the relative short-term are mild hypothermia, inhibitors of free radical production, and free radical scavengers. Promising clinical data are available for the use of mild hypothermia.

Synaptic and non-synaptic AMPA receptors permeable to calcium

For a long time, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors permeable to calcium have been considered to be either non-existent or as "atypical". There is now ample evidence that these receptors exist in numerous regions of the nervous system and in many neuronal as well as non-neuronal cell populations. This evidence has been accumulated by several methods, including electrophysiological recording, calcium imaging and cobalt-loading. Functional AMPA receptors permeable to calcium are already expressed at very early stages of embryonic development, well before the onset of synaptogenesis. They are probably involved in the paracrine signaling necessary for construction of the nervous system before becoming involved in synaptic transmission. In immature cells, cyclothiazide strongly increases the steady-state level of responses not only to AMPA, but also to kainate. Ingestion, during pregnancy, of food or drug substances that can cross the placental barrier and act upon the embryonic receptors may constitute a risk for normal development. In the adult nervous system, synaptic as well as non-synaptic (paracrine) AMPA receptors permeable to calcium are probably widely expressed in both glial and neuronal cells. They may also participate in controlling some aspects related to adult neurogenesis, in particular the migration of newly formed neurons.

Glutamate excitotoxicity--a mechanism for axonal damage and oligodendrocyte death in Multiple Sclerosis?

Glutamate excitotoxicity mediated by the AMPA/kainate-type of glutamate receptors is known not only to damage neurons but also the myelin-producing cell of the central nervous system (CNS), the oligodendrocyte. In Multiple Sclerosis (MS), myelin, oligodendrocytes and axons are lost or damaged as a result of an inflammatory attack on the CNS. Activated immune cells produce glutamate in large quantities by deamidating glutamine via glutaminase. Thus, we hypothesized that during inflammation in MS, glutamate excitotoxicity may contribute to the lesion. This was addressed by treating mice sensitized to develop acute experimental autoimmune encephalomyelitis (EAE) with an AMPA/kainate antagonist, NBQX. Treatment resulted in substantial amelioration of disease, increased oligodendrocyte survival and reduced axonal damage, as indicated by the levels of dephosphorylated neurofilament-H. Despite the clinical differences, NBQX-treatment had no effect on lesion size and did not reduce the degree of CNS inflammation. In addition, NBQX did not alter the proliferative activity of antigen-primed T cells in vitro, further indicating a lack of effect at the level of the immune system. In separate studies, infiltrating immune cells present in perivascular cuffs, commonly the site of entry for invading immune cells, were found to express glutaminase in abundance, supporting the production of glutamate in inflammatory lesions. Thus, glutamate excitotoxicity appears to be an important mechanism in autoimmune demyelination and its prevention with AMPA/kainate antagonists may prove to be an effective therapy for MS.

The link between excitotoxic oligodendroglial death and demyelinating diseases

Oligodendrocytes, the myelinating cells of CNS axons, are highly vulnerable to excitotoxic signals mediated by glutamate receptors of the AMPA and kainate classes. Receptors in these cells are commonly activated by glutamate that is released from axons and glial cells. In addition, oligodendrocytes contribute to the control of extracellular glutamate levels by means of their own transporters. However, acute and chronic alterations in glutamate homeostasis can result in overactivation of AMPA and kainate receptors and subsequent excitotoxic oligodendroglial death. Furthermore, demyelinating lesions caused by excitotoxins can be similar to those observed in multiple sclerosis. This, together with the effect of AMPA and kainate receptor antagonists in ameliorating the neurological score of animals with experimental autoimmune encephalomyelitis (an animal model of multiple sclerosis), indicates that oligodendrocyte excitotoxicity could be involved in the pathogenesis of demyelinating disorders.

NBQX attenuates excitotoxic injury in developing white matter

The excitatory neurotransmitter glutamate is released from axons and glia under hypoxic/ischemic conditions. In vitro, oligodendrocytes (OLs) express non-NMDA glutamate receptors (GluRs) and are susceptible to GluR-mediated excitotoxicity. We evaluated the role of GluR-mediated OL excitotoxicity in hypoxic/ischemic white matter injury in the developing brain. Hypoxic/ischemic white matter injury is thought to mediate periventricular leukomalacia, an age-dependent white matter lesion seen in preterm infants and a common antecedent to cerebral palsy. Hypoxia/ischemia in rat pups at postnatal day 7 (P7) produced selective white matter lesions and OL death. Furthermore, OLs in pericallosal white matter express non-NMDA GluRs at P7. Unilateral carotid ligation in combination with hypoxia (6% O(2) for 1 hr) resulted in selective, subcortical white matter injury with a marked ipsilateral decrease in immature and myelin basic protein-expressing OLs that was also significantly attenuated by 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-dione (NBQX). Intracerebral AMPA demonstrated greater susceptibility to OL injury at P7 than in younger or older pups, and this was attenuated by systemic pretreatment with the AMPA antagonist NBQX. These results indicate a parallel, maturation-dependent susceptibility of immature OLs to AMPA and hypoxia/ischemia. The protective efficacy of NBQX suggests a role for glutamate receptor-mediated excitotoxic OL injury in immature white matter in vivo.

Neurotrophin-3 (NT-3) diminishes susceptibility of the oligodendroglial lineage to AMPA glutamate receptor-mediated excitotoxicity

Prior reports demonstrated that cells of the oligodendroglial lineage are susceptible to excitotoxic necrosis mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid glutamate receptors (AMPA-GluR), and also showed that these cells express the high affinity neurotrophin receptors, TrkC and TrkA. We now report that: a) oligodendroglial progenitors (OP) and immature oligodendroglia are more vulnerable to AMPA-GluR-mediated excitotoxicity than are mature oligodendroglia; b) TrkC expression falls substantially during differentiation of cultured OP to mature oligodendroglia, whereas TrkA expression increases markedly; and c) neurotrophin-3, and to a lesser extent, nerve growth factor, protect the oligodendroglial lineage against AMPA-GluR-mediated excitotoxicity.

Ion channels in glial cells

Functional and molecular analysis of glial voltage- and ligand-gated ion channels underwent tremendous boost over the last 15 years. The traditional image of the glial cell as a passive, structural element of the nervous system was transformed into the concept of a plastic cell, capable of expressing a large variety of ion channels and neurotransmitter receptors. These molecules might enable glial cells to sense neuronal activity and to integrate it within glial networks, e.g., by means of spreading calcium waves. In this review we shall give a comprehensive summary of the main functional properties of ion channels and ionotropic receptors expressed by macroglial cells, i.e., by astrocytes, oligodendrocytes and Schwann cells. In particular we will discuss in detail glial sodium, potassium and anion channels, as well as glutamate, GABA and ATP activated ionotropic receptors. A majority of available data was obtained from primary cell culture, these results have been compared with corresponding studies that used acute tissue slices or freshly isolated cells. In view of these data, an active glial participation in information processing seems increasingly likely and a physiological role for some of the glial channels and receptors is gradually emerging.

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

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

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

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

Cyclic GMP/cyclic GMP-dependent protein kinase system prevents excitotoxicity in an immortalized oligodendroglial cell line

Previously, we have demonstrated that excitotoxicity of oligodendrocyte-like cells (OLC), differentiated from immortalized rat O-2A progenitor cells (CG-4 cells), is prevented by cyclic AMP-elevating agents. We now report that some agents that elevate cyclic GMP prevent OLC excitotoxicity. Kainate-induced injury was prevented by cyclic GMP analogues (8-bromo-cyclic GMP and dibutyryl cyclic GMP), a guanylate cyclase activator [atrial natriuretic peptide (ANP)], and phosphodiesterase inhibitors [3-isobutyl-1-methylxanthine (IBMX), ibudilast, propentofylline, and rolipram]. When both forskolin and 8-bromo-cyclic GMP were added, kainate-induced injury was additively prevented. There was a strong positive correlation between suppression of kainate-induced Ca2+ influx and prevention of injury by these chemicals. The measurement of intracellular cyclic AMP and cyclic GMP by radioimmunoassay demonstrated the following: an increase of cyclic GMP with treatment with 8-bromo-cyclic GMP, dibutyryl cyclic GMP, and ANP; an increase of cyclic AMP with treatment with ibudilast and rolipram; and an increase of both cyclic AMP and cyclic GMP with treatment with IBMX and propentofylline. Kainate-induced Ca2+ influx was decreased by 8-(4-chlorophenylthiol)-guanosine-3',5'-monophosphate, an activator of cyclic GMP-dependent protein kinase (PKG), or okadaic acid, an inhibitor of protein phosphatases 1 and 2A. RT-PCR and westem blotting of OLC demonstrated transcription of PKG II gene and translation of PKG Ibeta mRNA, but no translation of PKG Ialpha mRNA. Therefore, we concluded that the cyclic GMP/PKG system prevents OLC excitotoxicity.

Mechanisms of perinatal brain injury

This article is focused on the mechanisms underlying primarily ischaemic/reperfusion brain injury in both the term and premature infant. Although the mechanisms involved include similar initiating events, principally ischaemia-reperfusion, and similar final common pathways to cell death, particularly free radical-mediated events, there are certain unique maturational factors influencing the type and pattern of cellular injury. We will therefore initially describe the physiological and cellular/molecular mechanisms of brain injury in the term infant, followed by the mechanisms in the premature infant.

Non-N-methyl-D-aspartate glutamate receptors mediate oxygen--glucose deprivation-induced oligodendroglial injury

Cells of oligodendroglial lineage are susceptible to oxygen and glucose deprivation. When oligodendrocyte-like cells differentiated from CG-4-immortalized rat O-2A progenitor cells were exposed to hypoxia alone or glucose deprivation alone for 48 h, release of lactate dehydrogenase (LDH) into the culture medium did not increase. However, when cells were deprived of both oxygen and glucose for 6 or 12 h preceding reoxygenation for 2 h, LDH release increased. Adding glucose to the medium protected against cell death and increased lactate production in a concentration-dependent manner. Cell damage induced by deprivation of oxygen and glucose was prevented by calcium-free medium or by non-N-methyl-D-aspartate glutamate receptor (GluR) antagonists, such as 6-cyano-7-nitroquinoxaline-2,3-dione or LY293558, but not by the voltage-dependent calcium channel blocker, nimodipine, or by the N-methyl-D-aspartate GluR antagonist, MK-801. The glutamate concentration in the medium from cells exposed to oxygen-glucose deprivation for 12 h was 49.70+/-3.04 microM/l, which is sufficient to activate GluRs during deprivation of oxygen and glucose. Apoptotic cells detected by terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end-labeling (TUNEL) or Hoechst 33258 staining did not increase in cells exposed to oxygen-glucose deprivation for 12 h and subsequent reoxygenation for 2 h. No DNA laddering was detected by agarose gel electrophoresis from cells exposed to deprivation of oxygen and glucose. Neither acetyl-YVAD-CHO, an inhibitor of caspase-1-like proteases, nor acetyl-DEVD-CHO, an inhibitor of caspase-3-like proteases, prevented oxygen-glucose deprivation-induced injury. Thus, oxygen and glucose deprivation causes calcium-influx-induced necrotic cell damage in cells of oligodendroglial lineage via non-N-methyl-D-aspartate GluR channels.

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

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

Cell death in the oligodendrocyte lineage: a molecular perspective of life/death decisions in development and disease

Cell death in the oligodendrocyte lineage occurs during development and in pathological conditions as the result of a balance between opposing molecular signals. This review focuses on the molecular mechanisms of activation of signal transduction pathways affecting life/death decisions in progenitor cells and in mature oligodendrocytes. Loss of trophic support, cytokine receptor activation, and oxidative stress may differentially contribute to the induction of cell death at specific stages of development and to the pathogenesis of demyelinating disorders. The execution of the death program leading to the morphological changes of apoptosis and/or necrosis is then determined by the generation of reactive oxygen species and the level of impairment of mitochondrial function. The final decision of a cell to die or survive is determined by a competition between survival and death signals. Depending on ligand availability, type, and levels of receptor expression and downstream cross-talks between distinct signaling pathways, the cell may activate a death execution program that will be affected by its stage of differentiation and its energetic metabolism.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

Glutamate excitotoxicity in a model of multiple sclerosis

Glutamate excitotoxicity mediated by the AMPA/kainate type of glutamate receptors damages not only neurons but also the myelin-producing cell of the central nervous system, the oligodendrocyte. In multiple sclerosis, myelin, oligodendrocytes and some axons are lost as a result of an inflammatory attack on the central nervous system. Because glutamate is released in large quantities by activated immune cells, we expected that during inflammation in MS, glutamate excitotoxicity might contribute to the lesion. We addressed this by using the AMPA/kainate antagonist NBQX to treat mice sensitized for experimental autoimmune encephalomyelitis, a demyelinating model that mimics many of the clinical and pathologic features of multiple sclerosis. Treatment resulted in substantial amelioration of disease, increased oligodendrocyte survival and reduced dephosphorylation of neurofilament H, an indicator of axonal damage. Despite the clinical differences, treatment with NBQX had no effect on lesion size and did not reduce the degree of central nervous system inflammation. In addition, NBQX did not alter the proliferative activity of antigen-primed T cells in vitro, further indicating a lack of effect on the immune system. Thus, glutamate excitotoxicity seems to be an important mechanism in autoimmune demyelination, and its prevention with AMPA/kainate antagonists may prove to be an effective therapy for multiple sclerosis.

AMPA and kainate receptors each mediate excitotoxicity in oligodendroglial cultures

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

A light and electron microscopic study of NG2 chondroitin sulfate proteoglycan-positive oligodendrocyte precursor cells in the normal and kainate-lesioned rat hippocampus

The adult brain contains a large population of oligodendrocyte precursor cells that can be identified using antibodies against the NG2 chondroitin sulfate proteoglycan. The functions of this newly recognized class of glial cells in the normal or pathological brain are not well understood. To begin to elucidate these functions, we have examined the morphology and distribution of oligodendrocyte precursor cells in the hippocampus and neocortex of normal and kainate-lesioned rats by anti-NG2 immunocytochemistry using light and electron microscopy. Large numbers of oligodendrocyte precursor cells were present in all layers of the neocortex and hippocampus. These cells differed in their morphology from astrocytes, oligodendrocytes and microglia. The processes of these cells often surrounded unlabeled areas of clear cytoplasm. At the electron microscopic level, some of the profiles that were enclosed by oligodendrocyte precursor cell processes contained synaptic vesicles. Other enclosed profiles were dendrites or dendritic spines. NG2-immunopositive processes were also observed to interpose between axon terminals containing round vesicles and dendrites with thick postsynaptic densities. After kainate injection, the NG2-positive oligodendrocyte precursor cells in the hippocampus displayed reactive changes characterized by swollen cell bodies, an increased number of small, filopodial-like processes, and higher levels of immunodetectable NG2. Both viable and degenerating oligodendrocyte precursor cells were observed with electron microscopy. These observations emphasize the dynamic nature of the oligodendrocyte precursor cell and suggest that, in addition to participating in the glial reactions to excitotoxic damage, oligodendrocyte precursor cells may regulate the stability, structure and function of synapses in the normal central nervous system.

Molecular pathways mediating activation by kainate of mitogen-activated protein kinase in oligodendrocyte progenitors

Oligodendroglial cells express ionotropic glutamate receptors of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid hydrobromide (AMPA) and kainate (KA) subtypes. Recently, we reported that AMPA receptor agonists increased 45Ca2+ uptake and phospholipase C (PLC) activity. To further elucidate the intracellular signaling mechanisms, we examined the effects of AMPA and KA on mitogen-activated protein kinase (MAPK). KA caused a time- and concentration-dependent increase in MAPK activity (predominantly the p42mapk or ERK2) and the effect was blocked by 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX), a competitive AMPA/KA receptor antagonist. Furthermore, the noncompetitive antagonists of AMPA receptor GYKI 52466 and LY 303070 prevented the actions of the agonists, indicating that the effect of KA on MAPK activation is mediated through AMPA receptors in oligodendrocyte progenitors. Chelation of extracellular Ca2+ by EDTA or inhibition of PLC with U73122 abolished MAPK activation by KA. In addition, KA-stimulated MAPK activation was reduced by the protein kinase C (PKC) inhibitors, H7 and bisindolylmaleimide, as well as downregulation of PKC by prolonged exposure to phorbol esters. The involvement of PKC in the signal transduction pathways was further supported by the ability of KA to induce translocation of PKC measured by [3H]PDBu binding. Interestingly, a wortmannin-sensitive phosphatidylinositol 3-kinase and a pertussis toxin (PTX)-sensitive G protein form part of the molecular pathways mediating MAPK activation by AMPA receptor. A specific inhibitor of MAPK kinase, PD 098059, blocked MAPK activation and reduced KA-induced c-fos gene expression. All together, these results indicate that MAPK is implicated in the transmission of AMPA signaling to the nucleus and requires extracellular Ca2+, and PLC/PKC activation.

Posthemorrhagic hydrocephalus and brain injury in the preterm infant: dilemmas in diagnosis and management

Advances in neonatal critical care have reduced the incidence of intraventricular hemorrhage (IVH) in the newborn. Paradoxically, however, the prevalence of the complications of IVH including posthemorrhagic hydrocephalus (PHHC) has increased. By virtue of its association with long-term neurodevelopmental disability, posthemorrhagic hydrocephalus is an ominous diagnosis in the premature infant. Animal models have demonstrated that ventricular distention may cause direct cerebral parenchymal injury. Evidence for secondary parenchymal injury in the premature infant with PHHC is by necessity indirect. The precise impact of secondary parenchymal injury on the overall neurological outcome of premature infants with PHHC remains unclear in large part because of the vulnerability of the immature brain to other forms of injury (e.g., periventricular leukomalacia) that may be difficult to distinguish from injury due to distention. Furthermore, parenchymal injury due to PVL may cause ventricular enlargement that does not benefit from CSF diversion. Because these primary and secondary mechanisms of injury may operate concurrently, the precise or dominant cause of ventricular enlargement is often difficult to establish with certainty in the neonatal period. These diagnostic dilemmas have in turn impeded the development and evaluation of therapies specifically aimed at reversing ventricular distention and preventing secondary parenchymal injury. This article focuses on the current dilemmas in diagnosis and management of this potentially reversible form of injury as well as on potential future strategies for its prevention.

Brain injury in the premature infant: overview of clinical aspects, neuropathology, and pathogenesis

Brain injury in the premature infant is an extremely important problem, in part because of the large absolute number of infants affected yearly. The two principal brain lesions that underlie the neurological manifestations subsequently observed in premature infants are periventricular hemorrhagic infarction and periventricular leukomalacia. The emphases of this article are the neurology, neuropathology, and pathogenesis of these two lesions. Recent work suggests that the ultimate goal, prevention of the lesions, is potentially achievable. Periventricular hemorrhagic infarction may be preventable by prevention of germinal matrix/intraventricular hemorrhage, and periventricular leukomalacia, by detection of impaired cerebrovascular autoregulation, prevention of impaired cerebral blood flow, and interruption of the cascade to oligodendroglial cell death by such agents as free-radical scavengers.

Characteristics of acute and chronic kainate excitotoxic damage to the optic nerve

Macroglial cells express ionotropic glutamate receptors. In the adult optic nerve, reverse transcription-PCR showed that the native receptors are formed by subunits belonging to the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate classes. Because activation of AMPA and kainate receptors can be toxic to oligodendrocytes in vitro, I examined the nature of the damage caused by kainate, an agonist of both receptor classes, applied directly onto the optic nerve. Acute infusion or chronic slow delivery of the agonist caused massive nerve damage that was more extensive in the latter. Interestingly, chronic delivery also produced inflammation and demyelination in well circumscribed areas of the nerve, together with other pathological features that closely resemble those observed in multiple sclerosis patients. Acute and chronic kainate lesions were both prevented by the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. However, GYKI53655, a specific AMPA receptor antagonist, did not significantly reduce the size of the lesion, suggesting that the kainate toxicity was mainly mediated through activation of kainate-preferring glutamate receptors. These observations suggest that alterations in glutamate signaling may be detrimental to oligodendrocytes and may be involved in the pathogenesis of multiple sclerosis and other demyelinating diseases.

Neuronal degeneration in the gerbil brainstem is associated with spongiform lesions

Spongiform lesions arise in dendrites and glia in the brainstem of domestic Mongolian gerbils. Most pronounced within the cochlear nucleus (CN), this disorder is dynamic and progressive; the lesions increase in number, size, and extent with age. It has not been clear whether these spongioid lesions either cause or are associated with significant neural degeneration. In contrast, feral Mongolian gerbils (wild-trapped in Tuva) and their offspring show few spongiform lesions. The Tuvan gerbils provide an appropriate within-species control. We compared degeneration in the brainstem of domestic and Tuvan gerbils using the amino-cupric-silver (ACS) stain of de Olmos et al. [(1994) Neurotoxicol. Teratol., 16:545-561]. Positive histologic controls were provided by cerebellar stab wounds in domestic gerbils and by unilateral kainic acid injections into the CN of Tuvan gerbils. The ACS stain revealed extensive degeneration of axons, terminals, dendrites, and neurons in the brainstem of domestic gerbils. Neurodegeneration was most pronounced in the CN and was coextensive with spongiform lesions. Neurodegeneration was also seen in the trapezoid body, lateral lemniscus, and inferior colliculus, but was less pronounced than in the CN. The cerebellar stab wounds resulted in silver-stained Purkinje cells restricted to the stab wound local region. Kainic acid produced extensive neuronal and spongiform degeneration of the injected CN that was very similar to that spontaneously occurring in domestic gerbils. In contrast, the non-injected CN of Tuvan gerbils showed no neuronal or spongiform degeneration with the ACS stain. We conclude that, in domestic gerbils, the naturally occurring spongiform lesions of the CN and the accompanying neurodegeneration are both results of a common mechanism, most probably excitotoxic.

Oligodendrocytes from forebrain are highly vulnerable to AMPA/kainate receptor-mediated excitotoxicity

Little is known of the molecular mechanisms that trigger oligodendrocyte death and demyelination in many acute central nervous system insults. Since oligodendrocytes express functional alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate-type glutamate receptors, we examined the possibility that oligodendrocyte death can be mediated by glutamate receptor overactivation. Oligodendrocytes in primary cultures from mouse forebrain were selectively killed by low concentrations of AMPA, kainate or glutamate, or by deprivation of oxygen and glucose. This toxicity could be blocked by the AMPA/kainate receptor antagonist 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-dione (NBQX). In vivo, differentiated oligodendrocytes in subcortical white matter expressed AMPA receptors and were selectively injured by microstereotaxic injection of AMPA but not NMDA. These data suggest that oligodendrocytes share with neurons a high vulnerability to AMPA/kainate receptor-mediated death, a mechanism that may contribute to white matter injury in CNS disease.

Brain injury in the premature infant--from pathogenesis to prevention

Brain injury in the premature infant is an extremely important problem, in part because of the large absolute number of infants affected yearly. The two principal brain lesions that underlie the neurological manifestations subsequently observed in premature infants are periventricular hemorrhagic infarction and periventricular leukomalacia. The emphases of this article are the neurology, neuropathology and pathogenesis of these two lesions. Recent work suggests that the ultimate goal, prevention of the lesions, is potentially achievable. Periventricular hemorrhagic infarction may be preventable by prevention of germinal matrix-intraventricular hemorrhage, and periventricular leukomalacia, by detection of impaired cerebrovascular autoregulation, prevention of impaired cerebral blood flow and interruption of the cascade to oligodendroglial cell death by such agents as free radical scavengers.