Spinal neuronal pathology associated with continuous intrathecal infusion of N-methyl-D-aspartate in the rat

NBQX treatment improves mitochondrial function and reduces oxidative events after spinal cord injury

The purpose of this study was to examine the effects of inhibiting ionotropic glutamate receptor subtypes on measures of oxidative stress events at acute times following traumatic spinal cord injury (SCI). Rats received a moderate contusion injury and 15 min later were treated with one of two doses of 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzol[f]quinoxaline-7-sulfonamide disodium (NBQX), MK-801, or the appropriate vehicle. At 4 h following injury, spinal cords were removed and a crude synaptosomal preparation obtained to examine mitochondrial function using the MTT assay, as well as measures of reactive oxygen species (ROS), lipid peroxidation, and glutamate and glucose uptake. We report here that intraspinal treatment with either 15 or 30 nmol of NBQX improves mitochondrial function and reduces the levels of ROS and lipid peroxidation products. In contrast, MK-801, given intravenously at doses of 1.0 or 5.0 mg/kg, was without effect on these same measures. Neither drug treatment had an effect on glutamate or glucose uptake, both of which are reduced at acute times following SCI. Previous studies have documented that drugs acting on non-N-methyl-D-aspartate (NMDA) receptors exhibit greater efficacy compared to NMDA receptor antagonists on recovery of function and tissue sparing following traumatic spinal cord injury. The results of this study provide a potential mechanism by which blockade of the non-NMDA ionotropic receptors exhibit positive effects following traumatic SCI.

Tranexamic acid, a widely used antifibrinolytic agent, causes convulsions by a gamma-aminobutyric acid(A) receptor antagonistic effect

Application of 4-(aminomethyl)cyclohexanecarboxylic acid (tranexamic acid; TAMCA) to the central nervous system (CNS) has been shown to result in hyperexcitability and convulsions. However, the mechanisms underlying this action are unknown. In the present study, we demonstrate that TAMCA binds to the gamma-aminobutyric acid (GABA) binding site of GABA(A) receptors in membranes from rat cerebral cortex and does not interfere with N-methyl-D-aspartate receptors. Patch-clamp studies using human embryonic kidney cells transiently transfected with recombinant GABA(A) receptors composed of alpha 1 beta 2 gamma 2 subunits showed that TAMCA did not activate these receptors but dose dependently blocked GABA-induced chloride ion flux with an IC(50) of 7.1 +/- 3.1 mM. Application of TAMCA to the lumbar spinal cord of rats resulted in dose-dependent hyperexcitability, which was completely blocked by coapplication of the GABA(A) receptor agonist muscimol. These results indicate that TAMCA may induce hyperexcitability by blocking GABA-driven inhibition of the CNS.

Differential expression of brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5 in the adult rat spinal cord: regulation by the glutamate receptor agonist kainic acid

Previous in vitro studies indicate that select members of the neurotrophin gene family, namely brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5), contribute to survival and differentiation of spinal cord motoneurons. To investigate the potential roles of these factors in the adult spinal cord, we examined their cellular localization and regulation after systemic exposure to an excitotoxic stimulus, kainic acid (KA). Of the neurotrophins examined, NT-4/5 mRNA was most robustly expressed in the lumbosacral spinal cord of the normal adult rat, including expression by neurons throughout the gray matter, and in a subpopulation of white and gray matter glia. Both BDNF and NT-3 mRNAs were also densely expressed by alpha motoneurons of lamina IX, but were detected at lower levels elsewhere in the gray matter. NT-3 mRNA was additionally expressed by spinal cord glia, but was less widespread compared to NT-4/5. In response to systemic administration of KA, NT-4/5 and BDNF mRNAs were dramatically upregulated in a spatially and temporally restricted fashion, whereas levels of NT-3 mRNA were unchanged. These results provide strong in vivo evidence to support the idea that BDNF, NT-3, and in particular, NT-4/5, play a role in the normal function of the adult spinal cord. Furthermore, our results indicate that the actions of BDNF and NT-4/5 participate in the response of the cord to excitotoxic stimuli, and that those of NT-4/5 and NT-3 include both neurons and glia.

Hypothermia prevents biphasic glutamate release and corresponding neuronal degeneration after transient spinal cord ischemia in the rat

1. Spinal cord ischemia evoked a biphasic increase in CSF-Glu during 20 min of ischemia (40%) and at 2 hr after reperfusion (70%) in the nontreated group that was attenuated by all treated groups. But MK-801 (15 micrograms i.t.) did not affect the increased Glu at 2 hr (80%). 2. The argyrophilia observed in laminae II-V at 8 hr after reperfusion was attenuated by hypothermia (33 degrees C) and combination with MK-801, but the attenuation was less with MK-801. 3. Mild hypothermia attenuated the biphasic increase in CSF-Glu and corresponding development of neuronal damage after spinal cord ischemia. 4. Mild hypothermia with NMDA antagonism did not yield any further effects, suggesting that hypothermia itself plays a pivotal role in the protection.

Excitotoxic death of a subset of embryonic rat motor neurons in vitro

We have used cultures of purified embryonic rat spinal cord motor neurons to study the neurotoxic effects of prolonged ionotropic glutamate receptor activation. NMDA and non-NMDA glutamate receptor agonists kill a maximum of 40% of the motor neurons in a concentration- and time-dependent manner, which can be blocked by receptor subtype-specific antagonists. Subunit-specific antibodies stain all of the motor neurons with approximately the same intensity and for the same repertoire of subunits, suggesting that the survival of the nonvulnerable population is unlikely to be due to the lack of glutamate receptor expression. Extracellular Ca2+ is required for excitotoxicity, and the route of entry initiated by activation of non-NMDA, but not NMDA, receptors is L-type Ca2+ channels. Ca2+ imaging of motor neurons after application of specific glutamate receptor agonists reveals a sustained rise in intracellular Ca2+ that is present to a similar degree in most motor neurons, and can be blocked by appropriate receptor/channel antagonists. Although the lethal effects of glutamate receptor agonists are seen in only a subset of cultured motor neurons, the basis of this selectivity is unlikely to be simply the glutamate receptor phenotype or the level/pattern of rise in agonist-evoked intracellular Ca2+.

AMPA receptor-mediated slow neuronal death in the rat spinal cord induced by long-term blockade of glutamate transporters with THA

Excitotoxicity secondary to the loss of glutamate transporters (GluT) has been proposed as a possible pathogenetic mechanism for neuronal degeneration in amyotrophic lateral sclerosis. We therefore investigated whether prolonged in vivo pharmacologic inhibition of GluT would result in neuronal damage in the rat. DL-Threo-beta-hydroxyaspartate (THA), a potent GluT inhibitor, and glutamate were continuously infused into the rat spinal subarachnoid space by using a mini-osmotic pump. Animals that received both THA and glutamate, but not those received either singly, displayed tail paralysis with or without hind-limb paralysis and urinary incontinence after the third postoperative day. Pathologically, symptomatic animals exhibited neuronal loss with a variable extent of gliosis preferentially involving the dorsal horn of the lumbosacral cord. In the rostral spinal segments adjacent to those regions of intense pathologic changes, small neurons in the dorsal horn were selectively destroyed, a pattern similar to the late-onset neuronal damage induced by continuous intrathecal administration of 1-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) [R. Nakamura et al., Brain Res. 654 (1994) 279-285]. These behavioral and pathologic changes were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), suggesting that pharmacologic blockade of GluT causes selective neuronal damage in vivo by AMPA receptor activation.

Concentration-dependent changes in motor behavior produced by continuous intrathecal infusion of excitatory amino acids in the rat spinal cord

A growing body of evidence has suggested that glutamate receptors mediate selective degeneration of neurons in the central nervous system during the development of neurodegenerative diseases. Glutamate receptors are divided into N-methyl-D-aspartate (NMDA)-type and non-NMDA-type. Neurotoxicity mediated by the latter has attracted much interest as a possible causative mechanism underlying amyotrophic lateral sclerosis (ALS). As the clinical course of ALS is chronic and progressive, investigation of chronic effects of non-NMDA receptor agonists on neuronal function would be useful for evaluating the role of glutamate receptor-mediated neurotoxicity in ALS. However, chronic non-NMDA receptor-mediated neurotoxicity has been investigated less thoroughly than acute neurotoxicity. We infused an aqueous solution of R,S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) intrathecally and continuously by an osmotic minipump in rats. This method of continuous infusion enabled us to keep the drug concentration relatively constant in the cerebrospinal fluid surrounding the spinal cord. The present method of AMPA administration is more suitable for investigating ALS pathogenesis than acute injections, in view of the gradual progression of the disease and the selectivity of lesions produced.

NMDA and non-NMDA receptor antagonists protect against excitotoxic injury in the rat spinal cord

The neuroprotective properties of the N-methyl-D-aspartate (NMDA) antagonist dizocilpine (MK-801) and the non-NMDA antagonists 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo[f]quinoxaline (NBQX) and alpha-methyl-4-carboxyphenylglycine (MCPG) were evaluated against neuronal injury produced by the intraspinal injection of NMDA and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA). Forty-nine animals were divided into eight groups in order to evaluate the effects of different drug combinations: (a) NMDA; (b) NMDA + MCPG; (c) NMDA + NBQX; (d) NMDA + MK-801; (e) AMPA; (f) AMPA + MCPG; (g) AMPA + MK-801; and (h) AMPA + NBQX. Drugs were microinjected into spinal segments T12-L3 through a micropipette attached to a Hamilton microliter syringe. Spinal cords were evaluated after a survival period of 48 h at which time NMDA and AMPA were found to produce morphological changes over the concentration ranges of 125-500 mM and 75-500 microM, respectively. Neuronal loss following injections of NMDA + MK-801 or AMPA + NBQX was significantly less than that following injections of NMDA or AMPA alone. By contrast, neuronal loss following co-injections of NMDA or AMPA with inappropriate antagonists, i.e., NMDA + NBQX/MCPG or AMPA + MCPG/MK-801, was not significantly different from that produced by NMDA or AMPA. The results suggest that elevations in spinal levels of glutamate followed by prolonged activation of NMDA and AMPA receptor subtypes initiate an excitotoxic cascade resulting in neuronal injury. Blockade of NMDA and AMPA effects by MK-801 and NBQX respectively confirms the well documented neuroprotective effects of these drugs and lends support to the potential importance of NMDA and especially AMPA receptor antagonists as therapeutic agents in the treatment of acute spinal cord injury.

Effects of NMDA and its antagonists on ventral horn cholinergic neurons in organotypic roller tube spinal cord cultures

Neurotoxic effects of excitatory amino acid (EAA) receptor agonist N-methyl-D-aspartic acid (NMDA) and its antagonists on ventral horn cholinergic neurons were studied in organotypic rollertube cultures of spinal cord (OTC-SCs) using biochemical assays of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity, and AChE histochemistry. NMDA exposure decreased ChAT and AChE activity by 83% and 66%, respectively. Cultures treated with NMDA also showed a marked loss of AChE staining in both dorsal and ventral horns and a significant, dose-dependent decrease in the number of ventral horn AChE-positive neurons (VHANs). NMDA treatment primarily resulted in the loss of small VHANs (< 300 microns2). VHANs with a size and distribution typical of alpha-motoneurons were relatively well preserved. The effects of NMDA on OTC-SCs appeared to be independent of the age of the cultures. The NMDA antagonist DL-AP5 completely prevented the NMDA-induced loss of ChAT activity, but only attenuated the effect of NMDA on ChE activity. The antagonists DL-AP5, D-AP5 and MK-801, used alone, caused significant loss and/or shrinkage of VHANs. These effects appeared to be distinct from the NMDA mediated toxicity. The results indicate that NMDA and its antagonists exert powerful toxic effects on ventral horn cholinergic neurons. The large cholinergic alpha-motoneurons, however, appear to be relatively immune to these toxic effects.

Striatal efferent involvement and its correlation to levodopa efficacy in patients with multiple system atrophy

We report an immunohistochemical investigation of the striatal efferents in the striatum, globus pallidus, and substantia nigra of five patients with multiple system atrophy (MSA): olivopontocerebellar atrophy (2), striatonigral degeneration (2), and Shy-Drager syndrome (1). All patients manifested parkinsonism during the clinical course of their illness. The administration of levodopa improved the symptoms of two patients, but not of the other three. Brain tissues from five age-matched neurologically normal subjects served as controls. Immunohistochemical assays were carried with antibodies against met-enkephalin, substance P, and calbindin-D28k. Irrespective of the clinical form of multiple system atrophy, the immunoreactivity with the antibodies was reduced at the dorsolateral portion of the striatum and the ventrolateral portions of the globus pallidus and of the substantia nigra. The woolly fiber arrangement of reaction product deposits seen in both segments of the globus pallidus of normal individuals was totally absent in the ventrolateral portions of the three patients who did not have a response to levodopa. By contrast, there were positively stained woolly fibers in globus pallidum segments of the two levodopa-responsive patients, even though their number and size were decreased in comparison with controls. These results indicate that the three clinical forms of multiple system atrophy share common topographic alterations of the striatal efferent system and that the severity of the involvement correlates with the clinically observed effect of levodopa on the parkinsonism.

Amyotrophic lateral sclerosis: the involvement of intracellular Ca2+ and protein kinase C

The neurodegenerative disease, amyotrophic lateral sclerosis (ALS), is characterized by the selective death of motoneurones and corticospinal tract neurones. Abnormalities in excitatory amino acids and their receptors, as well as disordered function of voltage-dependent Ca2+ channels and superoxide dismutase have been reported in ALS patients. Furthermore, the activity of protein kinase C (PKC), a Ca2+, phospholipid-dependent enzyme, is also substantially increased in tissue from ALS patients, suggesting that alterations in intracellular free Ca2+ may be central to many of the diverse pathogenic mechanisms potentially responsible for ALS as discussed here by Charles Krieger and colleagues. Increased PKC activity, in turn, may have direct or indirect effects on neuronal viability and influence the pathogenic process in ALS by modifying the phosphorylation of voltage-dependent Ca2+ channels, neurotransmitter receptors and structural proteins.

Polyamines and NMDA receptors modulate pericapillary astrocyte swelling following cerebral cryo-injury in the rat

Four hours following cryo-injury rat cerebral pericapillary astrocytes from the perilesional area were markedly swollen occupying 17% of the pericapillary space as compared to 11% in sham operated controls. Ornithine decarboxylase activity and polyamine levels were increased over sham controls. The astrocytic swelling, the percentage of the pericapillary space occupied by astrocytic processes, and polyamine levels were reduced to near control levels by the following: (1) alpha-difluoromethylornithine; (2) Ifenprodil; and (3) MK-801. alpha-Difluoromethylornithine is a specific inhibitor of ornithine decarboxylase, Ifenprodil is an inhibitor of the polyamine binding site on the n-methyl-d-aspartate receptor, and MK-801 is an antagonist to n-methyl-d-aspartate binding to the n-methyl-d-aspartate receptor. Addition of putrescine, the product of ornithine decarboxylase activity, reversed the effect of alpha-difluoromethylornithine and restored the pericapillary swelling. Putrescine did not affect the MK-801-induced reduction in pericapillary astrocytic swelling. Therefore, polyamines and the n-methyl-d-aspartate receptor modulate excitotoxic responses to cryo-injury in pericapillary cerebral astrocytes.

Acute and late neurotoxicity in the rat spinal cord in vivo induced by glutamate receptor agonists

We investigated glutamate receptor-mediated neurotoxicity in vivo by means of infusing three specific agonists for non-NMDA receptors (acromelic acid A (ACRO), kainic acid and 1-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)) into the adult rat spinal subarachnoid space. ACRO induced long-lasting pure motor, rigid-spastic paraparesis in a dose-dependent manner (EC50: 220 pmol/h) that was accompanied by selective degeneration of spinal interneurons; leaving large anterior horn cells intact. Kainate and AMPA induced paraplegia but with relatively non-selective neuronal damage when given in doses more than 40-fold larger than those required for ACRO. When AMPA (> 100 nmol/h) was infused continuously using a mini-osmotic pump for more than 2 days, rats displayed progressive changes in motor behavior due to extensive damage in the caudal spinal cord where small neurons in the dorsal horns were the most vulnerable. Co-administration of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) specifically prevented neurotoxicity, suggesting a non-NMDA receptor-mediated mechanism. These results indicate that the non-NMDA receptor is heterogeneous, mediating neuronal damage with different selectivity. It is also suggested that chronic activation of glutamate receptors is capable of inducing slowly progressive neuronal death, which suggests relevance to the pathogenesis of ALS.

An experimental model combining microdialysis with electrophysiology, histology, and neurochemistry for studying excitotoxicity in spinal cord injury. Effect of NMDA and kainate

We used an experimental model that we previously developed to characterize the damage caused by the agonists of glutamate receptors, N-methyl-D-aspartate (NMDA) and kainate, in the spinal cord in vivo, thereby testing further the utility of this model. Microdialysis was used to administer the toxins and to sample the release of other substances in response to these agents. The blockage of electrical conduction was monitored by recording the amplitudes of evoked potentials during administration of the damaging substances, and damage was assessed by postmortem histological examination. The released amino acids in microdialysates were measured by HPLC. Administration of 5 mM NMDA + 5 mM kainate into the gray matter blocked most postsynaptic responses and caused the release of amino acids. Administration of 10 mM NMDA and 10 mM kainate significantly destroyed cell bodies near the fiber. The advantage of this model is that histological, neurochemical, and electrophysiological parameters were obtained in the same experiment.

The role of intracellular free calcium in motor neuron disease

The intracellular calcium (Ca2+) concentrations of motoneurons can be altered by the influx of Ca2+ into the cell by the opening of voltage-dependent Ca2+ channels and ligand-gated channels linked to Ca2+ influx, especially by the N-methyl-D-aspartate (NMDA) type of excitatory amino acid receptor. Intracellular Ca2+ concentration is also affected by the release of Ca2+ buffered in mitochondria and endoplasmic reticulum. Evidence that motoneurons may be selectively vulnerable to Ca(2+)-induced cell death include the following observations: (i) the presence of excitatory amino acid receptors on the cell membranes of motoneurons, some of which would permit Ca2+ influx (e.g. NMDA receptors); (ii) the availability of the presynaptic terminal for antibody-mediated effects leading to changes in cell permeability and Ca2+ influx; and (iii) the limited amounts of intracellular Ca(2+)-binding proteins such as calbindin D28K and parvalbumin in motoneurons. Elevation of intracellular free Ca2+ may also be a common event in a number of independent mechanisms leading to motoneuron death in motor neuron disease.

A segmental chronic pain syndrome in rats associated with intrathecal infusion of NMDA: evidence for selective action in the dorsal horn

We explored the effects of chronic lumbar intrathecal NMDA infusion (mini-osmotic pumps) in Sprague-Dawley rats on motor and sensory axon integrity. Several different infusion protocols, each given over a 4 week period were examined: 0.15 M NMDA in phosphate buffered saline; phosphate buffered saline without NMDA; and 0.20 M magnesium sulfate plus 0.15 M NMDA; 0.35 M NMDA. In two additional protocols, 0.15 M NMDA or phosphate buffered saline were infused for a total of 8 weeks. Within 1-2 weeks of the onset of NMDA, but not phosphate buffered saline infusions, the rats exhibited irritability, circling, biting and excessive grooming resulting in loss of hair, and skin ulcerations from autotomy localized to lumbar and sacral innervated dermatomes. Co-infusion of NMDA with magnesium sulfate almost completely prevented these findings. The behavioural changes were not associated with abnormalities of sensory or motor conduction. Intrathecal infusion of NMDA induces a chronic "central" experimental pain disorder in rats, localized to the cord segment with the greatest exposure to the infusion, without involvement of peripheral sensory axons and sparing the axonal integrity of anterior horn cells.

Patterns of neuronal degeneration in the motor cortex of amyotrophic lateral sclerosis patients

We examined patterns of neuronal degeneration in the motor cortex of amyotrophic lateral sclerosis (ALS) patients using traditional cell stains and several histochemical markers including neurofilament, parvalbumin, NADPH-diaphorase, ubiquitin, Alz-50 and tau. Three grades of ALS (mild, moderate, severe) were defined based on the extent of Betz cell depletion. Non-phosphorylated neurofilament immunoreactive cortical pyramidal neurons and non-pyramidal parvalbumin local circuit neurons were significantly depleted in all grades of ALS. In contrast, NADPH-diaphorase neurons and Alz-50-positive neurons were quantitatively preserved despite reduced NADPH-diaphorase cellular staining and dendritic pruning. The density of ubiquitin-positive structures in the middle and deep layers of the motor cortex was increased in all cases. Axonal tau immunoreactivity was not altered. These histochemical results suggest that cortical degeneration in ALS is distinctive from other neurodegenerative diseases affecting cerebral cortex. Unlike Huntington's disease, both pyramidal and local cortical neurons are affected in ALS; unlike Alzheimer's disease, alteration of the neuronal cytoskeleton is not prominent. The unique pattern of neuronal degeneration found in ALS motor cortex is consistent with non-N-methyl-D-aspartate glutamate receptor-mediated cytotoxicity.

Neuronal degeneration and spinal cavitation following intraspinal injections of quisqualic acid in the rat

Microinjections of quisqualic acid were made in the spinal cord to evaluate the excitotoxic effects of this excitatory amino acid agonist on spinal neurons in the rat. Animals were divided into four groups based on post injection survival times of 7-49 days. Injections ranging from 0.3 to 2.0 microL of 8.3, 83, and 125 mM quisqualic acid or normal saline were made in the lower thoracic and upper lumbar spinal cord. At all survival times evaluated unilateral injections of quisqualic acid produced unilateral or bilateral cell death and a prominent inflammatory reaction. In 23/25 animals spinal cavities were also observed. Spinal cord segments at or near quisqualate injection sites contained darkly stained, hypertrophied neuronal profiles, and increased staining for glial fibrillary acidic factor. Immunostaining for glial fibrillary acidic factor was especially intense in areas of neuronal degeneration and in border areas of spinal cavities. The results of this study suggest that the intraspinal injection of quisqualic acid may be an effective method to study the mechanisms of excitatory amino acid neurotoxicity, and the pathogenesis of spinal cavitation following neuronal injury.

Ibotenic acid-induced calcium deposits in rat substantia nigra. Ultrastructure of their time-dependent formation

The excitotoxin ibotenic acid (IBO) induces local calcium deposits upon injection into rat substantia nigra. Their formation has been investigated at the ultrastructural level in a time course study from 2 days to 8 weeks survival. Potassium bichromate stain was used to visualize pathological calcium accumulation. Two days after IBO application, reaction product for calcium was observed in mitochondria of degenerating perikarya and dendrites, but not in axons, boutons or glia. Four days after the lesion, calcium stain was found, in addition, in a seemingly free form in a few dendrites, especially those still contacted by intact boutons and not sequestrated by invading glia. Two days later, most of these calcium-accumulating dendrites were separated by astroglia from their synaptic partners. At the border between glia and dendrite a fibrillar matrix was formed which further accumulated calcium. During the following weeks this matrix enlarged stepwise and was infiltrated with calcium, thus giving a picture resembling the annual growth rings of trees. The evolving bodies incorporated smaller deposits in their vicinity, finally representing the large concretions seen at the light microscopic level from the 4th postoperative week onward. Similarities and dissimilarities of these observations with the results from other ultrastructural studies on excitotoxin lesions are detailed. It is suggested that the different outfit of neuronal subpopulations and of glia with ligand-gated and metabotropic glutamate receptors in the single brain region, as well as the local response repertoire of glial cells towards the excitotoxic injury with the subsequent formation of a calcium-accumulating matrix provide the molecular basis for the formation of calcium deposits.

Intraventricular infusion of N-methyl-D-aspartate. 2. Acute neuronal consequences

This study documents the ultrastructural features of acute neuronal injury following N-methyl-D-aspartate (NMDA) receptor activation. NMDA (100 nmol/microliters) or vehicle was infused over a 15-min period into the lateral ventricle of adult rats. After perfusion fixation, specimens demonstrating normal and abnormal patterns of vascular permeability to horseradish peroxidase were sampled for ultrastructural analysis. In NMDA-infused rats, brain regions exhibiting protein extravasation contained swollen dendritic profiles and abnormal neuronal perikarya. Although periventricular regions were most severely affected, parenchymal abnormalities were also detected in the cerebral cortex, septum, striatum, thalamus, hypothalamus and cerebellum. Mildly affected dendrites contained dark compact mitochondria, while in severely swollen dendrites mitochondria were enlarged with ruptured cristae. Focal sites of plasma membrane disruption were also detected within swollen dendrites. Swollen neurons commonly displayed peripheral pallor and increased numbers of cytoplasmic vacuoles. Other neurons appeared dark and shrunken, some containing disrupted mitochondria and pyknotic nuclei. Pretreatment with the NMDA antagonist MK-801 (2 mg/kg) attenuated the neuronal and dendritic alterations. In conditions where cerebrospinal fluid levels of glutamate are abnormally elevated, excessive NMDA receptor activation may lead to early vascular and neuronal complications which could work in concert to promote brain injury.

Vascular changes in the spinal cord in N-methyl-D-aspartate-induced excitotoxicity: morphological and permeability studies

Our previous studies have demonstrated toxicity in spinal cord neuronal systems of middle-aged rats with continuous intrathecal infusion of N-methyl-D-aspartate (NMDA). The present study was undertaken to determine when during the course of excitotoxicity vascular changes occur. The model used was intrathecal infusion of NMDA in the region of the lumbar enlargement of the spinal cord. Horseradish peroxidase (HRP) was used as a marker of vascular permeability alterations occurring in this model. Pathological changes were observed in the cord gray matter of all rats infused with 30-60 micrograms/min NMDA for 30 or 60 min. The changes consisted of swelling of dendrites which gave the neuropil a vacuolated appearance. There was expansion of the extracellular spaces in these areas and neurons were shrunken with pyknotic nuclei. These changes were more frequently encountered in the posterior than anterior horns and were specific for NMDA since they did not occur in NMDA-infused rats pretreated with MK-801, a specific NMDA antagonist. Endothelial dysfunction manifested as increased permeability to HRP. This was a consistent finding in all rats infused with the higher dose of NMDA and was less frequent in those infused with 30 micrograms/min and no vascular changes were observed in rats infused with NMDA for 30 min despite the presence of tissue changes. Increased permeability affected all types of vessels but principally, capillaries and venules. There was no evidence of endothelial necrosis or vascular occlusion. This study demonstrates that in excitotoxin-mediated tissue damage, breakdown of the blood-brain barrier follows the development of nervous tissue damage. Thus, edema is not a significant feature of early lesions in excitotoxin-induced brain injury.

Excitotoxicity in the embryonic chick spinal cord

Recent evidence implicates excitatory amino acids (EAAs), acting as excitotoxic agents, in the pathogenesis of neurological disorders involving the spinal cord. In this study, we used the chick embryo spinal cord as an in vitro model for studying the sensitivity of spinal neurons to the excitotoxic effects of EAA agonists. Compounds tested include the prototypic receptor-specific agonists, N-methyl-D-aspartate (NMDA), quisqualic acid (Quis), and kainic acid (KA), and the plant-derived excitotoxic food poisons, beta-N-oxalylamino-L-alanine, beta-N-methylamino-L-alanine, and domoic acid. Each agonist induced concentration-dependent acute degeneration of neurons distributed throughout the spinal cord. These cytopathological changes consisted of acute edematous degeneration of dendrosomal structures in the dorsal horn and intermediate zone, and dark cell changes with intracytoplasmic vacuolization of motor neurons; this damage is identical to that induced by excitotoxin agonists in other regions of the central nervous system. The NMDA receptor-specific antagonist MK-801 completely blocked toxicity of NMDA, and the nonNMDA antagonist CNQX preferentially blocked the toxicity of Quis- and KA-type agonists in the spinal cord. Our findings suggest that (1) the majority of spinal neurons have all three subtypes of EAA receptors, making them acutely vulnerable to excitotoxin exposure; and (2) EAA antagonists are effective in preventing excitotoxin-induced damage of the spinal cord.