Late-onset selective neuronal damage in the rat spinal cord induced by continuous intrathecal administration of AMPA

Exciting Complexity: The Role of Motor Circuit Elements in ALS Pathophysiology

Amyotrophic lateral sclerosis (ALS) is a fatal disease, characterized by the degeneration of both upper and lower motor neurons. Despite decades of research, we still to date lack a cure or disease modifying treatment, emphasizing the need for a much-improved insight into disease mechanisms and cell type vulnerability. Altered neuronal excitability is a common phenomenon reported in ALS patients, as well as in animal models of the disease, but the cellular and circuit processes involved, as well as the causal relevance of those observations to molecular alterations and final cell death, remain poorly understood. Here, we review evidence from clinical studies, cell type-specific electrophysiology, genetic manipulations and molecular characterizations in animal models and culture experiments, which argue for a causal involvement of complex alterations of structure, function and connectivity of different neuronal subtypes within the cortical and spinal cord motor circuitries. We also summarize the current knowledge regarding the detrimental role of astrocytes and reassess the frequently proposed hypothesis of glutamate-mediated excitotoxicity with respect to changes in neuronal excitability. Together, these findings suggest multifaceted cell type-, brain area- and disease stage- specific disturbances of the excitation/inhibition balance as a cardinal aspect of ALS pathophysiology.

Motor Neuron Susceptibility in ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the death of both upper and lower motor neurons (MNs) in the brain, brainstem and spinal cord. The neurodegenerative mechanisms leading to MN loss in ALS are not fully understood. Importantly, the reasons why MNs are specifically targeted in this disorder are unclear, when the proteins associated genetically or pathologically with ALS are expressed ubiquitously. Furthermore, MNs themselves are not affected equally; specific MNs subpopulations are more susceptible than others in both animal models and human patients. Corticospinal MNs and lower somatic MNs, which innervate voluntary muscles, degenerate more readily than specific subgroups of lower MNs, which remain resistant to degeneration, reflecting the clinical manifestations of ALS. In this review, we discuss the possible factors intrinsic to MNs that render them uniquely susceptible to neurodegeneration in ALS. We also speculate why some MN subpopulations are more vulnerable than others, focusing on both their molecular and physiological properties. Finally, we review the anatomical network and neuronal microenvironment as determinants of MN subtype vulnerability and hence the progression of ALS.

Driven to decay: Excitability and synaptic abnormalities in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron (MN) disease and is clinically characterised by the death of corticospinal motor neurons (CSMNs), spinal and brainstem MNs and the degeneration of the corticospinal tract. Degeneration of CSMNs and MNs leads inexorably to muscle wastage and weakness, progressing to eventual death within 3-5 years of diagnosis. The CSMNs, located within layer V of the primary motor cortex, project axons constituting the corticospinal tract, forming synaptic connections with brainstem and spinal cord interneurons and MNs. Clinical ALS may be divided into familial (∼10% of cases) or sporadic (∼90% of cases), based on apparent random incidence. The emergence of transgenic murine models, expressing different ALS-associated mutations has accelerated our understanding of ALS pathogenesis, although precise mechanisms remain elusive. Multiple avenues of investigation suggest that cortical electrical abnormalities have pre-eminence in the pathophysiology of ALS. In addition, glutamate-mediated functional and structural alterations in both CSMNs and MNs are present in both sporadic and familial forms of ALS. This review aims to promulgate debate in the field with regard to the common aetiology of sporadic and familial ALS. A specific focus on a nexus point in ALS pathogenesis, namely, the synaptic and intrinsic hyperexcitability of CSMNs and MNs and alterations to their structure are comprehensively detailed. The association of extramotor dysfunction with neuronal structural/functional alterations will be discussed. Finally, the implications of the latest research on the dying-forward and dying-back controversy are considered.

Time-dependent, bidirectional, anti- and pro-spinal hyper-reflexia and muscle spasticity effect after chronic spinal glycine transporter 2 (GlyT2) oligonucleotide-induced downregulation

The loss of local spinal glycine-ergic tone has been postulated as one of the mechanisms contributing to the development of spinal injury-induced spasticity. In our present study using a model of spinal transection-induced muscle spasticity, we characterize the effect of spinally-targeted GlyT2 downregulation once initiated at chronic stages after induction of spasticity in rats. In animals with identified hyper-reflexia, the anti-spasticity effect was studied after intrathecal treatment with: i) glycine, ii) GlyT2 inhibitor (ALX 1393), and iii) GlyT2 antisense oligonucleotide (GlyT2-ASO). Administration of glycine and GlyT2 inhibitor led to significant suppression of spasticity lasting for a minimum of 45-60 min. Treatment with GlyT2-ASO led to progressive suppression of muscle spasticity seen at 2-3 weeks after treatment. Over the subsequent 4-12 weeks, however, the gradual appearance of profound spinal hyper-reflexia was seen. This was presented as spontaneous or slight-tactile stimulus-evoked muscle oscillations in the hind limbs (but not in upper limbs) with individual hyper-reflexive episodes lasting between 3 and 5 min. Chronic hyper-reflexia induced by GlyT2-ASO treatment was effectively blocked by intrathecal glycine. Immunofluorescence staining and Q-PCR analysis of the lumbar spinal cord region showed a significant (>90%) decrease in GlyT2 mRNA and GlyT2 protein. These data demonstrate that spinal GlyT2 downregulation provides only a time-limited therapeutic benefit and that subsequent loss of glycine vesicular synthesis resulting from chronic GlyT2 downregulation near completely eliminates the tonic glycine-ergic activity and is functionally expressed as profound spinal hyper-reflexia. These characteristics also suggest that chronic spinal GlyT2 silencing may be associated with pro-nociceptive activity.

Inducing Chronic Excitotoxicity in the Mouse Spinal Cord to Investigate Lower Motor Neuron Degeneration

We report the methodology for the chronic delivery of an excitotoxin to the mouse spinal cord via surgically implanted osmotic mini-pumps. Previous studies have investigated the effect of chronic application of excitotoxins in the rat, however there has been little translation of this model to the mouse. Using mice that express yellow fluorescent protein (YFP), motor neuron and neuromuscular junction alterations can be investigate following targeted, long-term (28 days) exposure to the α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor excitotoxin, kainic acid. By targeting the L3-4 region of the lumbar spinal cord, with insertion of an intrathecal catheter into the subarachnoid space at L5, chronic application of the kainic acid results in slow excitotoxic death in the anterior ventral horn, with a significant (P < 0.05) reduction in the number of SMI-32 immunopositive neurons present after 28 days infusion. Use of the Thy1-YFP mice provides unrivaled visualization of the neuromuscular junction and enables the resultant distal degeneration in skeletal muscle to be observed. Both neuromuscular junction retraction at the gastrocnemius muscle and axonal fragmentation in the sciatic nerve were observed after chronic infusion of kainic acid for 28 days. Lower motor neuron, and distal neuromuscular junction, degeneration are pathological hallmarks of the devastating neurodegenerative disease Amyotrophic Lateral Sclerosis (ALS). This mouse model will be advantageous for increasing our understanding of how the pathophysiological phenomena associated with this disease can lead to lower motor neuron loss and distal pathology, as well as providing a robust in vivo platform to test therapeutic interventions directed at excitotoxic mechanisms.

Identifying the primary site of pathogenesis in amyotrophic lateral sclerosis - vulnerability of lower motor neurons to proximal excitotoxicity

There is a desperate need for targeted therapeutic interventions that slow the progression of amyotrophic lateral sclerosis (ALS). ALS is a disorder with heterogeneous onset, which then leads to common final pathways involving multiple neuronal compartments that span both the central and peripheral nervous system. It is believed that excitotoxic mechanisms might play an important role in motor neuron death in ALS. However, little is known about the mechanisms by which excitotoxicity might lead to the neuromuscular junction degeneration that characterizes ALS, or about the site at which this excitotoxic cascade is initiated. Using a novel compartmentalised model of site-specific excitotoxin exposure in lower motor neurons in vitro, we found that spinal motor neurons are vulnerable to somatodendritic, but not axonal, excitotoxin exposure. Thus, we developed a model of somatodendritic excitotoxicity in vivo using osmotic mini pumps in Thy-1-YFP mice. We demonstrated that in vivo cell body excitotoxin exposure leads to significant motor neuron death and neuromuscular junction (NMJ) retraction. Using confocal real-time live imaging of the gastrocnemius muscle, we found that NMJ remodelling preceded excitotoxin-induced NMJ degeneration. These findings suggest that excitotoxicity in the spinal cord of individuals with ALS might result in a die-forward mechanism of motor neuron death from the cell body outward, leading to initial distal plasticity, followed by subsequent pathology and degeneration.

The C-terminal domain of tetanus toxin protects motoneurons against acute excitotoxic damage on spinal cord organotypic cultures

The C-terminal domain of tetanus toxin (Hc-TeTx) has been suggested to act as a neuroprotective agent by activating signaling pathways related to neurotrophins and also to exert anti-apoptotic effects. Here, we show the beneficial properties of the recombinant protein Hc-TeTx to protect spinal motoneurons against excitotoxic damage. In vitro spinal cord organotypic cultures were used to assess acute glutamate excitotoxic damage. Our results indicate that Hc-TeTx treatment improves motoneuron survival within a short therapeutical window (the first 2 h post-injury). Within this interval, we found that p44/p42 MAP kinase (ERK1/2) and glycogen synthase kinase-3 (GSK3β) signaling pathways play a crucial role in the neuroprotective effect. Moreover, we demonstrated that Hc-TeTx treatment initiate autophagy which is ERK1/2- and GSK3β-dependent. These findings suggest a possible therapeutical tool to improve motoneuron survival immediately after excitotoxic insults or during the secondary injury phase that occurs after spinal cord trauma.

Lithium prevents excitotoxic cell death of motoneurons in organotypic slice cultures of spinal cord

Several studies have reported the neuroprotective effects of lithium (Li) suggesting its potential in the treatment of neurological disorders, among of them amyotrophic lateral sclerosis (ALS). Although the cause of motoneuron (MN) death in ALS remains unknown, there is evidence that glutamate-mediated excitotoxicity plays an important role. In the present study we used an organotypic culture system of chick embryo spinal cord to explore the presumptive neuroprotective effects of Li against kainate-induced excitotoxic MN death. We found that chronic treatment with Li prevented excitotoxic MN loss in a dose dependent manner and that this effect was mediated by the inhibition of glycogen synthase kinase-3beta (GSK-3beta) signaling pathway. This neuroprotective effect of Li was potentiated by a combined treatment with riluzole. Nevertheless, MNs rescued by Li displayed structural changes including accumulation of neurofilaments, disruption of the rough endoplasmic reticulum and free ribosome loss, and accumulation of large dense core vesicles and autophagic vacuoles. Accompanying these changes there was an increase in immunostaining for (a) phosphorylated neurofilaments, (b) calcitonin gene-related peptide (CGRP) and (c) the autophagic marker LC3. Chronic Li treatment also resulted in a reduction in the excitotoxin-induced rise in intracellular Ca(2+) in MNs. In contrast to the neuroprotection against excitotoxicity, Li was not able to prevent normal programmed (apoptotic) MN death in the chick embryo when chronically administered in ovo. In conclusion, these results show that although Li is able to prevent excitotoxic MN death by targeting GSK-3beta, this neuroprotective effect is associated with conspicuous cytopathological changes.

Excitotoxic motoneuron degeneration induced by glutamate receptor agonists and mitochondrial toxins in organotypic cultures of chick embryo spinal cord

Glutamate receptor-mediated excitotoxicity and mitochondrial dysfunction appear to play an important role in motoneuron (MN) degeneration in amyotrophic lateral sclerosis (ALS). In the present study we used an organotypic slice culture of chick embryo spinal cord to explore the responsiveness of mature MNs to different excitotoxic stimuli and mitrochondrial inhibition. We found that, in this system, MNs are highly vulnerable to excitotoxins such as glutamate, N-methyl-D-aspartate (NMDA), and kainate (KA), and that the neuroprotective drug riluzole rescues MNs from KA-mediated excitotoxic death. MNs are also sensitive to chronic mitochondrial inhibition induced by malonate and 3-nitropropionic acid (3-NP) in a dose-dependent manner. MN degeneration induced by treatment with mitochondrial toxins displays structural changes similar to those seen following excitotoxicity and can be prevented by applying either the antiexcitotoxic drug 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX) or riluzole. Excitotoxicity results in an increased frequency of normal spontaneous Ca2+ oscillations in MNs, which is followed by a sustained deregulation of intracellular Ca2+. Tolerance to excitotoxic MN death resulting from chronic exposure to excitotoxins correlates with a reduced excitotoxin-induced increase in intracellular Ca2+ and increased thapsigargin-sensitive Ca2+ stores.

Glutamate transporters and the excitotoxic path to motor neuron degeneration in amyotrophic lateral sclerosis

Responsible for the majority of excitatory activity in the central nervous system (CNS), glutamate interacts with a range of specific receptor and transporter systems to establish a functional synapse. Excessive stimulation of glutamate receptors causes excitotoxicity, a phenomenon implicated in both acute and chronic neurodegenerative diseases [e.g., ischemia, Huntington's disease, and amyotrophic lateral sclerosis (ALS)]. In physiology, excitotoxicity is prevented by rapid binding and clearance of synaptic released glutamate by high-affinity, Na(+)-dependent glutamate transporters and amplified by defects to the glutamate transporter and receptor systems. ALS pathogenetic mechanisms are not completely understood and characterized, but excitotoxicity has been regarded as one firm mechanism implicated in the disease because of data obtained from ALS patients and animal and cellular models as well as inferred by the documented efficacy of riluzole, a generic antiglutamatergic drug, has in patients. In this article, we critically review the several lines of evidence supporting a role for glutamate-mediated excitotoxicity in the death of motor neurons occurring in ALS, putting a particular emphasis on the impairment of the glutamate-transport system.

Sendai virus-mediated gene transfer of GDNF reduces AIF translocation and ameliorates ischemic cerebral injury

The therapeutic effect of a novel RNA viral vector, Sendai virus (SeV)-mediated glial cell line-derived neurotrophic factor (GDNF) gene (SeV/GDNF), on the infarct volume, was investigated after 90 minutes of transient middle cerebral artery occlusion (tMCAO) in rats with relation to nuclear translocation of apoptosis inducing factor (AIF). The topical administration of SeV/GDNF induced high level expression of GDNF protein, which effectively reduced the infarct volume when administrated 0 and 1 hours as well after the reperfusion. Twenty-four hours after ischemia, the obvious nuclear translocation of AIF was found in neurons of peri-infarct area, which significantly reduced with administration of SeV/GDNF 0 or 1 hour after reperfusion, as well as the number of TUNEL positive cells. These results demonstrate that SeV vector-mediated gene transfer of GDNF effectively reduced ischemic infarct volume after tMCAO and extended the therapeutic time window compared with previous viral vectors, and that promoting neuronal survival of GDNF might be related to the reduction of AIF nuclear translocation, indicating the high therapeutic potency of SeV/GDNF for cerebral ischemia.

Plasma glutamate and glycine levels in patients with amyotrophic lateral sclerosis: the effect of riluzole treatment

OBJECTIVES

Defective glutamate (glu) metabolism and excitotoxicity have been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). Moreover, glycine (gly) has been shown to potentiate excitatory transmission. The "antiglutamatergic" agent riluzole has been shown to prolong survival in ALS. The aim of the study was to investigate a possible effect of riluzole on plasma glu and gly levels, correlating with clinical response to treatment.

PATIENTS AND METHODS

Plasma concentrations of glu and gly were measured in 20 healthy volunteers and 22 ALS patients before treatment and after 6 months on riluzole.

RESULTS

At baseline, increased plasma glu correlated with spinal onset and male gender whereas gly levels did not differ between patients and controls. No significant change was observed for both amino acids post-treatment, despite a lower rate of disease progression.

CONCLUSION

These results suggest that riluzole may affect disease progression without a significant impact on plasma glu and gly levels, possibly indicating different mechanisms of drug action.

The neuroprotective potential of phase II enzyme inducer on motor neuron survival in traumatic spinal cord injury in vitro

(1) Phase II enzyme inducer is a kind of compound which can promote the expression of antioxidative enzymes through nuclear factor erythroid 2-related factor 2 (Nrf2) activation. Recently, it has been reported that these compounds show neuroprotective effect via combating oxidative stress. The purpose of this study is to determine whether phase II enzyme inducers have neuroprotective effects on traumatic spinal cord injury. (2) An organotypic spinal cord culture system was used, Phase II enzyme inducers were added to culture medium for 1 week, motor neurons were counted by SMI-32 staining, glutamate, Nrf2, and Heme oxygenase-1(HO-1) mRNA were tested. (3) This study showed motor neuron loss within 1 week in culture. After 1 week in culture, the system was stable. Moreover, Glutamate was increased when in culture 48 h and decreased after 1 week in culture. There was no significant change between 1 and 4 weeks in culture. Necrotic motor neuron and damaged mitochondrial were observed in culture 48 h. Furthermore, phase II enzyme inducers: tert-butyhydroquinone (t-BHQ), 3H-1,2-dithiole-3-thione (D3T), and 5,6-dihydrocyclopenta-1,2-dithiole-3-thione (CPDT) were shown to promote motor neuron survival after dissection, it was due to increasing Nrf2 and HO-1 mRNA expression and protecting mitochondrial not due to decreasing glutamate level. (4) The loss of motor neuron due to dissection can mimic severe traumatic spinal cord injury. These results demonstrate that glutamate excitotoxicity and the damage of mitochondrial is possibly involve in motor neuron death after traumatic spinal cord injury and phase II enzyme inducers show neuroprotective potential on motor neuron survival in traumatic spinal cord injury in vitro.

Excitotoxicity and amyotrophic lateral sclerosis

Since its description by Charcot more than 130 years ago, the pathogenesis of selective motor neuron degeneration in amyotrophic lateral sclerosis (ALS) remains unsolved. Over the years, many pathogenic mechanisms have been proposed. Amongst others these include: oxidative stress, excitotoxicity, aggregate formation, inflammation, growth factor deficiency and neurofilament disorganization. This multitude of contributing factors indicates that ALS is a complex disease and also suggests that ALS is a multifactorial disorder. Excitotoxicity is not the newest and most spectacular hypothesis in the ALS field, but it is undoubtedly one of the most robust pathogenic mechanisms supported by an impressive amount of evidence. Moreover, the therapeutic efficacy of riluzole, the only drug proven to slow disease progression in ALS, is most likely related to its anti-excitotoxic properties. In this review, we will give an overview of the arguments in favor of the involvement of excitotoxicity in ALS and of the possible mechanisms leading to motor neuron death. We will also summarize the intrinsic properties of motor neurons that render these cells particularly vulnerable to excitotoxicity and could explain the selective vulnerability of motor neurons in ALS. All this information could help to develop new and better therapeutic strategies that could protect motor neurons from excitotoxicity.

Slow and selective death of spinal motor neurons in vivo by intrathecal infusion of kainic acid: implications for AMPA receptor-mediated excitotoxicity in ALS

Excitotoxicity mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors has been proposed to play a major role in the selective death of motor neurons in sporadic amyotrophic lateral sclerosis (ALS), and motor neurons are more vulnerable to AMPA receptor-mediated excitotoxicity than are other neuronal subclasses. On the basis of the above evidence, we aimed to develop a rat model of ALS by the long-term activation of AMPA receptors through continuous infusion of kainic acid (KA), an AMPA receptor agonist, into the spinal subarachnoid space. These rats displayed a progressive motor-selective behavioral deficit with delayed loss of spinal motor neurons, mimicking the clinicopathological characteristics of ALS. These changes were significantly ameliorated by co-infusion with 6-nitro-7-sulfamobenso(f)quinoxaline-2,3-dione (NBQX), but not with d(-)-2-amino-5-phosphonovaleric acid (APV), and were exacerbated by co-infusion with cyclothiazide, indicative of an AMPA receptor-mediated mechanism. Among the four AMPA receptor subunits, expression of GluR3 mRNA was selectively up-regulated in motor neurons but not in dorsal horn neurons of the KA-infused rats. The up-regulation of GluR3 mRNA in this model may cause a molecular change that induces the selective vulnerability of motor neurons to KA by increasing the proportion of GluR2-lacking (i.e. calcium-permeable) AMPA receptors. This rat model may be useful in investigating ALS etiology.

The role of excitotoxicity in the pathogenesis of amyotrophic lateral sclerosis

Unfortunately and despite all efforts, amyotrophic lateral sclerosis (ALS) remains an incurable neurodegenerative disorder characterized by the progressive and selective death of motor neurons. The cause of this process is mostly unknown, but evidence is available that excitotoxicity plays an important role. In this review, we will give an overview of the arguments in favor of the involvement of excitotoxicity in ALS. The most important one is that the only drug proven to slow the disease process in humans, riluzole, has anti-excitotoxic properties. Moreover, consumption of excitotoxins can give rise to selective motor neuron death, indicating that motor neurons are extremely sensitive to excessive stimulation of glutamate receptors. We will summarize the intrinsic properties of motor neurons that could render these cells particularly sensitive to excitotoxicity. Most of these characteristics relate to the way motor neurons handle Ca(2+), as they combine two exceptional characteristics: a low Ca(2+)-buffering capacity and a high number of Ca(2+)-permeable AMPA receptors. These properties most likely are essential to perform their normal function, but under pathological conditions they could become responsible for the selective death of motor neurons. In order to achieve this worst-case scenario, additional factors/mechanisms could be required. In 1 to 2% of the ALS patients, mutations in the SOD1 gene could shift the balance from normal motor neuron excitation to excitotoxicity by decreasing glutamate uptake in the surrounding astrocytes and/or by interfering with mitochondrial function. We will discuss point by point these different pathogenic mechanisms that could give rise to classical and/or slow excitotoxicity leading to selective motor neuron death.

Excitotoxicity and ALS: what is unique about the AMPA receptors expressed on spinal motor neurons?

It has been repeatedly reported that spinal motor neurons are selectively vulnerable to AMPA receptor-mediated excitotoxicity. Therefore, identifying the uniqueness of AMPA receptors that are expressed on motor neurons, especially in individuals affected with sporadic amyotrophic lateral sclerosis (ALS) is essential for elucidating the etiology of this disorder. The mechanism that initiates motor neuronal death appears to be an exaggerated influx of Ca(2+) through AMPA receptors. The determinants that affect this Ca(2+) influx are Ca(2+) permeability, which is regulated by the presence of the GluR2 subunit and by RNA editing at the Q/R site of GluR2; channel desensitization, which is regulated by alternative splicing at the flip/flop site and by RNA editing at the R/G site of GluR subunits; and receptor density on the cell surface, which is controlled by many factors including regulatory proteins, direct phosphorylation and RNA editing at the Q/R site. This review focuses on recent progress on the molecular dynamics of AMPA receptors and discusses the pathophysiology of selective motor neuron death mediated by AMPA receptors in individuals affected with sporadic ALS.

Expression profile of AMPA receptor subunit mRNA in single adult rat brain and spinal cord neurons in situ

The differential expression of AMPA receptor subunit mRNA was analyzed in the adult rat brain and spinal cord using quantitative RT-PCR with laser capture microdissection. The expression of all four AMPA receptor subunits was demonstrable, with the mRNA expression level for GluR2 being the highest in all the brain areas and neuronal subsets examined. Both the absolute expression level of GluR2 mRNA and its expression relative to the other subunits were the lowest in motoneurons. The unique AMPA receptor expression profile of motoneurons may render them selectively vulnerable to AMPA receptor-mediated excitotoxicity.

Naloxone lowers cerebrospinal fluid levels of excitatory amino acids after thoracoabdominal aortic surgery

OBJECTIVE

Although naloxone has been used to prevent ischemic spinal cord injury (SCI), its effect on excitatory amino acids (EAAs) has not been understood. We investigated the clinical significance of naloxone by measuring EAAs in the cerebrospinal fluid (CSF) in patients undergoing thoracoabdominal aortic surgery.

METHODS AND SUBJECTS

Twenty-seven patients (15 men and 12 women; mean age, 66 +/- 12 years) undergoing prosthetic replacement of the thoracoabdominal aorta (n = 19) or the descending thoracic aorta (n = 8) from April 1997 to June 2003 under distal perfusion and mild hypothermia were enrolled in this cohort study with historical controls. Their etiology was 7 dissections and 20 nondissections. In 16 patients (naloxone group), intravenous infusion of naloxone (1 microg/kg/h) was continued until the patients became alert. In the remaining 11 patients (control group) naloxone was not given. CSF drainage was used in all patients. CSF levels of EAAs, glutamate, aspartate, and glycine were measured at 6 points in time until 72 hours postoperatively, using a high-performance liquid chromatography method.

RESULTS

In 5 patients with SCI (2 patients in control group, 3 in naloxone group), CSF levels of glutamate and glycine continued to increase even at 72 hours postoperatively, and were significantly more elevated than those in patients without SCI ( P < .0001, glutamate; P = .0006, glycine). Postoperative maximum levels of CSF glutamate and glycine were also significantly higher in patients with postoperative SCI than in patients without SCI (glutamate: 215.3% +/- 158.6% vs 32.9% +/- 37.3% increase from baseline, P < .0001; glycine: 309.1% +/- 218.2% vs 89.2% +/- 103.1% increase from baseline, P = .0036). CSF levels of glutamate and aspartate in naloxone group were significantly lower than those in control group ( P = .0161, glutamate; P < .0001, aspartate). Postoperative maximum level of CSF aspartate was also significantly lower in the naloxone group than in the control group (8.3% +/- 75.5% vs 119.7% +/- 120.6% increase from baseline, P = .0077). In multivariate logistic regression analysis, postoperative maximum CSF glutamate >100% from baseline ( P < .001) and postoperative maximum level of CSF glycine ( P = .005)were identified as the independent risk factors for SCI. Both SCI ( P < .001) and postoperative maximum level of CSF glycine ( P = .005) were the independent predictors for postoperative maximum level of CSF glutamate >100% from baseline.

CONCLUSIONS

CSF levels of EAAs are elevated in patients with SCI. CSF glutamate is the strongest independent predictor of SCI. Naloxone is effective in lowering CSF levels of EAAs.

AMPA receptor activation, but not the accumulation of endogenous extracellular glutamate, induces paralysis and motor neuron death in rat spinal cord in vivo

The mechanisms of motor neuron (MN) degeneration in amyotrophic lateral sclerosis (ALS) are unknown, but glutamate-mediated excitotoxicity may be involved. To examine directly this idea in vivo, we have used microdialysis in the rat lumbar spinal cord and showed that four- to fivefold increases in the concentration of endogenous extracellular glutamate during at least 1 h, by perfusion with the glutamate transport inhibitor L-2,4-trans-pyrrolidine-dicarboxylate, elicited no motor alterations or MN damage. Stimulation of glutamate release with 4-aminopyridine induced transitory ipsilateral hindlimb muscular twitches but no MN damage. In contrast, perfusion of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) did not modify glutamate levels but produced intense muscular spasms, followed by ipsilateral permanent hindlimb paralysis and a remarkable loss of MNs. These effects of AMPA were prevented by co-perfusion with the AMPA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline. Perfusion with NMDA or kainate produced no motor effects or MN damage. Thus, the elevation of endogenous extracellular glutamate in vivo due to blockade of its transport is innocuous for spinal MNs. Because this resistance is observed under the same experimental conditions in which MNs are highly vulnerable to AMPA, these results indicate that excitotoxicity due to this mechanism might not be an important factor in the pathogenesis of ALS.

Recent advances in the study of AMPA receptors

As glutamate is a dominant excitatory neurotransmitter in the central nervous system, glutamate receptors, and especially AMPA receptors, are located ubiquitously in all brain areas. In this paper, we reviewed recent advances of studies on AMPA receptor functions. AMPA receptors are cation-conducting complexes composed of various combinations of four subunits (GluR1 to GluR4). The glutamine residue located in the pore-forming segment of GluR2 subunit (Q/R site) is changed to arginine by RNA editing at the pre mRNA stage in normal adult mammalian animal. The edited GluR2 subunit is a major determination of Ca(2+) permeability of the AMPA receptor; only edited GluR2-lacking receptor shows high-Ca(2+) permeability. The assembly of glutamate AMPA receptor subunit is not completely according to the stochastic theory. The heteromeric subunits assembly is more rapid than the homomeric assembly is. The transfer of AMPA receptor subunit to the plasma membrane is conducted in multiple ways. Many molecules that interact with the intracellular domain of AMPA receptor subunits are reported as the modulators of AMPA receptor subunit transfer. In the motoneuron of sporadic amyotrophic lateral sclerosis (ALS) patients, the efficiency of RNA editing at the GluR2 Q/R site is significantly decreased. Relative low level of edited GluR2 subunit expression is likely responsible for motoneuronal death in ALS. Recently, AMPA receptors in glial cells have been studied. Bergmann glial cells in cerebellum express Ca(2+)-permeable AMPA receptors. Conversion of these AMPA receptors to Ca(2+)-impermeable type receptors induces morphological and functional changes. Glioblastoma cells also express Ca(2+)-permeable AMPA receptors, and their conversion to Ca(2+)-impermeable receptors inhibits cell locomotion and induces apoptosis.

Oxidative Stress: a common denominator in the pathogenesis of amyotrophic lateral sclerosis

PURPOSE OF REVIEW

Amyotrophic lateral sclerosis, or Lou Gehrig disease, is a progressive neurodegenerative disease of adult onset characterized by a loss of motor neurons in the spinal cord and motor cortex. In the last several years, substantial progress has been made in defining the pathogenesis of motor neuron injury and relationships between disease mechanisms and the selective vulnerability of the motor neuron in both familial and sporadic forms of amyotrophic lateral sclerosis.

RECENT FINDINGS

Current theories have shifted from a neuron-centered pathology to a focus on the interaction between motor neurons and glia, and their respective contributions to pathways implicated in amyotrophic lateral sclerosis. Although multiple mechanisms clearly can contribute to the pathogenesis of motor neuron injury, recent advances suggest that oxidative stress may play a significant role in the amplification, and possibly the initiation, of disease.

SUMMARY

This article reviews the clinical aspects of amyotrophic lateral sclerosis and potential mechanisms of disease pathogenesis in the context of recent data supporting a major role for oxidative stress throughout the disease course. Evidence suggesting an important role for intercellular signaling is emphasized.

Human spinal motoneurons express low relative abundance of GluR2 mRNA: an implication for excitotoxicity in ALS

AMPA receptor-mediated neurotoxicity is currently the most plausible hypothesis for the etiology of amyotrophic lateral sclerosis (ALS). The mechanism initiating this type of neuronal death is believed to be exaggerated Ca2+-influx through AMPA receptors, which is critically determined by the presence or absence of the glutamate receptor subunit 2 (GluR2) in the assembly. We have provided the first quantitative measurements of the expression profile of AMPA receptor subunits mRNAs in human single neurons by means of quantitative RT-PCR with a laser microdissector. Among the AMPA subunits, GluR2 shared the vast majority throughout the neuronal subsets and tissues examined. Furthermore, both the expression level and the proportion of GluR2 mRNA in motoneurons were the lowest among all neuronal subsets examined, whereas those in motoneurons of ALS did not differ from the control group, implying that selective reduction of the GluR2 subunit cannot be a mechanism of AMPA receptor-mediated neurotoxicity in ALS. However, the low relative abundance of GluR2 might provide spinal motoneurons with conditions that are easily affected by changes of AMPA receptor properties including deficient GluR2 mRNA editing in ALS.

Spinal amino acid release and repeated withdrawal in spinal morphine tolerant rats

1. We used spinal microdialysis in awake rats to investigate whether the repeated withdrawal with naloxone during continuous spinal infusion of morphine would lead to a progressively greater spinal glutamate release and a more pronounced intrathecal tolerance. 2. Rats received lumbar intrathecal (IT) infusion of morphine (IT-M: 20 nmol microl(-1) h(-1)) or saline (IT-S: 1 microl h(-1)) continuously for 3 days. Both groups were further subdivided to receive intraperitoneal (i.p.) injection of naloxone (IP-N: 0.6 mg kg(-1)) or saline (IP-S: 3 ml kg(-1)) every 24 h after the beginning of IT infusion. Daily thermal escape latencies, withdrawal signs, the resting basal release of spinal amino acids before IP injection and the release immediately after the injection (evoked) were measured. 3. Rats receiving IT morphine showed a maximum increase in thermal escape latency on day 1, after which this value declined, with the fastest decline observed in IT morphine + IP naloxone group. On day 1, no significant difference was observed among groups in the resting basal release of amino acids. Rats in IT morphine + i.p. naloxone group displayed a progressive increase in this value. The release was not significantly altered in other groups. 4. For the IT-M + IP-N group, basal resting dialysate concentrations of Glu, Asp and Tau rose steadily over the 3-day infusion interval. No change in basal resting release was noted for any other treatment. 5. Evoked release (after i.p. naloxone) in IT-M animals displayed a progressive increase over the three repeated exposures. Evoked release did not change significantly in other treatment groups. 6. The degree of precipitated withdrawal significantly correlated with the increase in glutamate acutely evoked by i.p. injection. 7. The present results show that periodic transient withdrawal of spinal opiate agonist activity leads to a progressive increase in glutamate outflow and withdrawal signs, in a manner consistent with an enhanced development of spinal tolerance.

Glutamate release and neuronal injury after intrathecal injection of local anesthetics

High concentrations of local anesthetics are neurotoxic, but the mechanism for this neurotoxicity is obscure. Here, we report increased concentrations of glutamate in the cerebrospinal fluid after intrathecal injections of high concentrations of tetracaine (a local anesthetic). The peak concentrations of glutamate after administration of 1%, 2%, and 4% tetracaine were 4-fold, 6-fold, and 10-fold higher than baseline values, respectively. Animals in the 1% group were all neurologically normal one week after tetracaine injection. In the group receiving 4%, no animal was able to hop and vacuolation of the white matter and/or central chromatolysis of the motor neurons were observed. Because high concentrations of glutamate are known to be neurotoxic, our results may provide some insight into the mechanisms for neurotoxicity of intrathecal local anesthetics.

Reduction of GluR2 RNA editing, a molecular change that increases calcium influx through AMPA receptors, selective in the spinal ventral gray of patients with amyotrophic lateral sclerosis

Enhancement of calcium influx through the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor is a plausible mechanism underlying selective neuronal death in amyotrophic lateral sclerosis (ALS). The calcium conductance of the AMPA receptor is regulated by the GluR2 subunit that is edited at the glutamine/arginine residue site in the subunit assembly. We investigated the molecular changes of GluR2 mRNA in the spinal cord of ALS cases, those of cases with other neurological diseases, and those of normal cases using reverse transcription-polymerase chain reaction combined with restriction enzyme cleavage. We found that the editing efficiency was significantly lower only in the ventral gray of ALS cases (virtually 0% in 2 cases) than in any spinal region of the disease controls and normal controls. In addition, expression of GluR2 mRNA is lower in the ventral gray of the ALS cases and disease controls than in that of the normal controls. The above molecular changes of GluR2 mRNA in the ventral gray of ALS cases may enhance calcium influx through AMPA receptors, thereby promoting neuronal vulnerability. The decrement of GluR2 mRNA editing efficiency is unique to the ventral gray of ALS cases and may be closely linked to the etiology of ALS.

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.

Receptor localization in the mammalian dorsal horn and primary afferent neurons

The dorsal horn of the spinal cord is a primary receiving area for somatosensory input and contains high concentrations of a large variety of receptors. These receptors tend to congregate in lamina II, which is a major receiving center for fine, presumably nociceptive, somatosensory input. There are rapid reorganizations of many of these receptors in response to various stimuli or pathological situations. These receptor localizations in the normal and their changes after various pertubations modify present concepts about the wiring diagram of the nervous system. Accordingly, the present work reviews the receptor localizations and relates them to classic organizational patterns in the mammalian dorsal horn.

An immunocytochemical study of the distribution of AMPA selective glutamate receptor subunits in the normal human motor system

Glutamate is the major mediator of fast excitatory neurotransmission in the mammalian central nervous system. Disturbances of this neurotransmitter system have been implicated in chronic degenerative neurological disease. Recently, major advances in our knowledge and understanding of the molecular biology of the glutamatergic receptor system have been made. It is now known that functional glutamate receptors consist of various combinations of some 20 identified subunits. A growing body of circumstantial evidence suggests that the non-N-methyl-D-aspartate subtype of glutamate receptors may mediate, at least in part, the selective motor neuron death seen in the human neurodegenerative disease amyotrophic lateral sclerosis. We have used subunit specific immunocytochemistry to study the distribution and potential subunit composition of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) selective glutamate receptors, (a subgroup of non-N-methyl-D-aspartate selective glutamate receptors formed by combinations of GluR1-4 subunits), in the human motor system. Motor neurons in the spinal cord, brainstem, and motor cortex were relatively strongly immunoreactive with the GluR2/3 subunit antibody, moderately so with the GluR4 subunit antibody, and showed relatively low levels of immunoreactivity with the GluR1 subunit antibody. This is the first detailed study of AMPA receptor subunit expression in the human motor system. Motor neurons express a distinct subunit profile when compared with other groups of neurons in the human nervous system. There were no significant differences in the pattern of relative AMPA subunit expression (GluR2/3 > or = GluR4 > GluR1) between groups of motor neurons typically affected (in the spinal cord and hypoglossal nucleus), or spared (oculomotor and Onufs nucleus) by the amyotrophic lateral sclerosis disease process. However, oculomotor motor neurons had higher levels of expression of all AMPA subunit proteins which may indicate greater AMPA mediated glutamatergic input in the normal function of this neuronal population. This study does not support a role for differential subunit composition of AMPA receptors in determining the selective vulnerability of motor neurons in amyotrophic lateral sclerosis. However, the overall density of receptors may be of importance.

Selective degeneration of inhibitory interneurons in the rat spinal cord induced by intrathecal infusion of acromelic acid

We investigated the characteristics of the neurotoxicity mediated by non-N-methyl-D-aspartic acid (NMDA)-type glutamate receptors in the spinal cord by infusing rats intrathecally with three specific agonists (acromelic acid A (ACRO), kainic acid and 1-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)). When ACRO was infused intrathecally continuously for 2 h, the rats developed dose-dependent flaccid paraplegia during the infusion, followed by pure motor, rigid-spastic, long-lasting paraparesis (ED50: 220 pmol/h). The paraparetic rats showed selective degeneration of interneurons in the spinal cord with about 50% loss of their contents of glycine and aspartic and glutamic acids. The alpha-motoneurons in the ventral horns were largely free from permanent damage. These changes were selectively ameliorated by concomitant administration of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an antagonist at non-NMDA receptors. Kainate induced long-lasting paraplegia with neuronal damage at doses about 40-times higher than those of ACRO that caused paraplegia. When examined 30 days after kainate infusion, the neurotransmitter markers had decreased markedly in the lumbar cords of the rats with long-lasting paraplegia. Intrathecal infusion of large doses of AMPA caused long-lasting flaccid paraplegia, which were accompanied by extensive necrosis in the caudal spinal cord. It seems that ACRO exerts its unique pharmacological actions by activating a subclass of non-NMDA receptors distinct from those activated by kainate and AMPA, and may become a useful tool for investigating the biological roles of glutamate receptors.

Amyotrophic lateral sclerosis is a multifactorial disease

Amyotrophic lateral sclerosis (ALS) is probably biphasic. An initial trigger(s) is followed by a terminal cascade coinciding with the onset of neurological deficits. The terminal cascade involves interactive multifactorial pathogenic mechanisms. Aging must play a crucial role leading to multiple defective or degraded gene products accumulating with progressing years. This in turn leads to failure of receptor integrity and resulting excitotoxicity, free radical accumulation, failure of neurotrophism, and possibly immunological disturbances. These events are predated by months or years by a trigger which is also likely to be multifactorial and cumulative. Evidence suggests that environmental factors may be important triggers. Failure of specific glutamate transporters and calcium binding proteins may account for selective vulnerability of the corticomotoneuronal system. It is postulated that in ALS the primary target cell is the corticomotoneuron or the local circuit interneurons which modulate its activity. Glia cells may play an important role in the demise of the corticomotoneuronal cell. The disordered corticomotoneuron induces excessive excitatory transmitter (glutamate?) release at the corticomotoneuronal-spinal-motoneuronal synapse resulting in the subsequent demise of this neuron.

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.