Abnormal excitatory amino acid metabolism in amyotrophic lateral sclerosis

Changes in N-acetyl-aspartate content during focal and global brain ischemia of the rat

N-Acetyl-aspartate (NAA) is almost exclusively localized in neurons in the mature brain and might be used as a neuronal marker. It has been reported that the NAA content in human brain is decreased in neurodegenerative diseases and in stroke. Since the NAA content can be determined by nuclear magnetic resonance techniques, it has potential as a diagnostic and prognostic marker. The objective of this study was to examine the change of NAA content and related substances following cerebral ischemia and compare the results to the damage of the tissue. We used rats to study the changes of NAA, N-acetyl-aspartyl-glutamate (NAAG), glutamate, and aspartate contents over a time course of 24 h in brain regions affected by either permanent middle cerebral artery occlusion (focal ischemia) or decapitation (global ischemia). The decreases of NAA and NAAG contents following global brain ischemia were linear over time but significant only after 4 and 2 h, respectively. After 24 h, the levels of NAA and NAAG were 24 and 44% of control values, respectively. The concentration of glutamate did not change, whereas the aspartate content increased at a rate comparable with the rate of decrease of NAA content. This is consistent with NAA being preferentially degraded by the enzyme amidohydrolase II in global ischemia. In focal ischemia, there was a rapid decline of NAA within the first 8 h of ischemia followed by a slower rate of reduction. The reductions of NAA and NAAG contents in focal ischemia were significant after 4 and 24 h, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective loss of glial glutamate transporter GLT-1 in amyotrophic lateral sclerosis

The pathogenesis of sporadic amyotrophic lateral sclerosis (ALS) is unknown, but defects in synaptosomal high-affinity glutamate transport have been observed. In experimental models, chronic loss of glutamate transport can produce a loss of motor neurons and, therefore, could contribute to the disease. With the recent cloning of three glutamate transporters, i.e., EAAC1, GLT-1, and GLAST, it has become possible to determine if the loss of glutamate transport in ALS is subtype specific. We developed C-terminal, antioligopeptide antibodies that were specific for each glutamate transporter. EAAC1 is selective for neurons, while GLT-1 and GLAST are selective for astroglia. Tissue from various brain regions of ALS patients and controls were examined by immunoblot or immunocytochemical methods for each transporter subtype. All tissue was matched for age and postmortem delay. GLT-1 immunoreactive protein was severely decreased in ALS, both in motor cortex (71% decrease compared with control) and in spinal cord. In approximately a quarter of the ALS motor cortex specimens, the loss of GLT-1 protein (90% decrease from control) was dramatic. By contrast, there was only a modest loss (20% decrease from control) of immunoreactive protein EAAC1 in ALS motor cortex, and there was no appreciable change in GLAST. The minor loss of EAAC1 could be secondary to loss of cortical motor neurons. As a comparison, glial fibrillary acidic protein, which is selectively localized to astroglia, was not changed in ALS motor cortex. Because there is no loss of astroglia in ALS, the dramatic abnormalities in GLT-1 could reflect a primary defect in GLT-1 protein, a secondary loss due to down regulation, or other toxic processes.

Inhibitory and excitatory amino acids in cerebrospinal fluid of neurolathyrism patients, a highly prevalent motorneurone disease

Neurolathyrism is caused by overconsumption of seeds containing 3-N-oxalyl-L-2,3-diaminopropanoic acid (beta-ODAP). Amino acids levels of cerebrospinal fluid (CSF) were studied in 50 patients with neurolathyrism and 12 healthy volunteers. The levels of excitatory amino acids glutamate and aspartate were 281% and 71% respectively of control values. The concentration of inhibitory amino acids glycine and taurine were 277% and 198% respectively of the levels in CSF from control individuals. There was a significant correlation between the level of glycine and the duration of the disease. We also found increased levels of threonine, serine and alanine. In contrast to reports on other motor neurone diseases where an increase of isoleucine was observed we found a significant decrease of isoleucine. The results suggest a disturbance of amino acid metabolism due to excitotoxic damages caused by beta-ODAP, a dietary excitatory amino acid.

N-Acetyl-aspartylglutamate (NAAG) in human cerebrospinal fluid: Determination by high performance liquid chromatography, and influence of biological variables

NAAG is one of the neuropeptides found in highest concentrations in the CNS. The presence of micromolar concentrations of NAAG in human CSF was demonstrated by using two different and complementary analytical approaches: 1) isocratic separation of endogenous NAAG by reverse-phase high performance liquid chromatography (HPLC) with dual wavelength detection and 2) derivatization of endogenous NAAG with acidic methanol and subsequent HPLC analysis of the derivative NAAG-trimethyl ester. The NAAG concentration was between 0.44µmol/l and 7.16µmol/l (mean of 2.19 ± 1.53µmol/l) in CSF samples from forty neuropsychiatric patients. Endogenous NAAG or [(3)H]NAAG added to CSF samples were not significantly degraded when the CSF was incubated at 37°C during one hour, suggesting that the peptide is a highly stable metabolite in the subarachnoid space. In addition, evidence is provided that NAAG does not present a concentration gradient along the lower subarachnoid space.

Brainstem motoneuron pools that are selectively resistant in amyotrophic lateral sclerosis are preferentially enriched in parvalbumin: evidence from monkey brainstem for a calcium-mediated mechanism in sporadic ALS

Some brainstem motoneuron groups appear more resistant to the process of neurodegeneration in ALS (for example, oculomotor, trochlear, and abducens nuclei) than others (for example, trigeminal, facial, ambiguus, and hypoglossal nuclei). The possibility that the differential presence of the calcium-chelating protein parvalbumin might underlie this difference in vulnerability was examined immunohistochemically as a way to determine whether a calcium-mediated mechanism might be involved in ALS. In normal monkey brainstem, we found that the abundance of parvalbumin-containing neurons in the oculomotor, trochlear, and abducens nuclei was approximately 90% of the abundance of choline acetyltransferase (CHAT)-containing motoneurons. In contrast, the abundance of parvalbumin-containing neurons in the other brainstem motor nuclei innervating skeletal muscle (trigeminal, facial, ambiguus, and hypoglossal) was only about 30-60% of the abundance of CHAT-containing motoneurons. Since some of these motoneuron pools contain nonmotoneuron internuclear neurons that might be parvalbumin-containing, we also carried out double-label studies to specifically determine the percentage of cholinergic motoneurons that contained parvalbumin in each of these motoneuron pools. We found that 85-100% of the oculomotor, trochlear, and abducens motoneurons were parvalbumin-containing. In contrast, only 20-30% of the trigeminal, facial, ambiguus, and hypoglossal motoneurons were parvalbumin-containing. These results raise the possibility that motoneuron death in sporadic ALS is related to some defect that promotes cytosolic calcium accumulation in motoneurons. This excess calcium entry may promote cell death via an excitotoxic pathway. Motoneurons rich in parvalbumin may resist the deleterious effects of this putative calcium gating defect because they are better able to sequester the excess calcium.

[3H]D-aspartate binding sites in the normal human spinal cord and changes in motor neuron disease: a quantitative autoradiographic study

The distribution and density of glutamate transporter sites was determined in human cervical and lumbar spinal cord, by quantitative autoradiography using [3H]D-aspartate. In the normal human spinal cord (n = 8) there was specific binding of [3H]D-aspartate throughout the spinal grey matter, with the highest levels observed in the substantia gelatinosa and central grey matter. In the ventral horns, particularly at the L5 level, focal hot spots of binding were observed in a distribution corresponding to that of lower motor neuron somata. Comparison of motor neuron disease (MND) cases (n = 12) with normal controls showed a reduction in the density of [3H]D-aspartate binding in the intermediate grey matter and the substantia gelatinosa of the lumbar cord. These changes were more marked in the amyotrophic lateral sclerosis (ALS) compared to the progressive muscular atrophy (PMA) subgroup, and may be due to loss of glutamatergic terminals of the corticospinal tract. The changes observed in the cervical cord were milder and did not reach statistical significance. No differences were found between [3H]D-aspartate binding in the spinal cords of the normal controls and a neurological disease control group (n = 6), suggesting that the changes observed in MND are disease specific. These findings provide further evidence in support of a disturbance of glutamatergic neurotransmission in MND.

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

Neurotoxicity mediated by 1-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) was investigated by infusing this agent continuously for 7 days intrathecally to adult rats using a mini-osmotic pump. Behavioral changes were apparent only after the second postoperative day, when the rats displayed hindlimb palsy or incontinence of urine. The behavioral deficits became progressively severe and the rats usually displayed both hindlimb paraplegia and incontinence of urine by the 7th postoperative day. These progressive behavioral deficits were induced in a dose-dependent manner in the rats that received AMPA at a dose of > 100 pmol/h (100 microM at 1 microliter/h, 17 nmol in total dose). The severity of behavioral deficits was in parallel with that of neuropathological changes in the lumbosacral cords. In spinal segments rostrally adjacent to those with severe pathological changes, only the neurons in the dorsal horns (Rexed's laminae II-IV) were destroyed with intense gliosis. These changes were not induced by infusing AMPA for 1 day. The concomitant administration of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an antagonist for non-N-methyl-D-aspartate (NMDA) receptors, with AMPA, but not that of 2-amino-5-phosphonovalerate (APV), an antagonist for NMDA receptor, prevented induction of the behavioral and neuropathological changes. The findings of the present study suggest that this late-onset, selective neurotoxicity is mediated by AMPA-type glutamate receptors.

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.

Excitotoxicity and motor neurone disease: a review of the evidence

Excitotoxic mechanisms have a well established role in the pathogenesis of neuronal injury following acute CNS insults such as ischaemia and trauma. Their role in the selective cell death which occurs in chronic neurodegenerative disorders such as motor neurone disease (MND) is more speculative. The traditional classification of glutamate receptor subtypes which mediate excitotoxicity requires modification in the light of new molecular data. There is much greater structural and functional diversity in this receptor family than previously envisaged and it is quite possible that specific populations of neurones will be characterised by a unique profile of glutamate receptor subtypes which may be a factor determining their selective vulnerability. The molecular mechanisms underlying excitotoxic neuronal injury are still being elucidated but it is clear that the cascade of events resulting from elevation of intracellular free calcium is likely to play a major role. As well as being a primary mechanism of neuronal injury, excitotoxicity can secondarily damage neurones whose energy metabolism is impaired from some primary pathological process. The 8 lines of evidence that primary or secondary excitotoxic mechanisms may be involved in the selective neuronal injury of MND are discussed. The evidence, while still circumstantial, is sufficient to warrant further research effort in this field, not least because the emergence of pharmacological agents which modify specific aspects of excitatory amino acid neurotransmission offer the possibility of therapeutic intervention in MND.

Extracellular N-acetylaspartate in the rat brain: in vivo determination of basal levels and changes evoked by high K+

The purpose of this study was to determine the extracellular concentrations of N-acetylaspartate (NAA) in the rat cerebral cortex, striatum, and hippocampus of halothane-anaesthetised rats by intracerebral microdialysis, and to examine the effects of high K(+)-induced local depolarisation, which provokes synchronous neurotransmitter release, cell swelling, and acid-base changes. Basal levels of NAA in the extracellular fluid (ECF) were determined by the zero net flux method. Tissue levels of NAA in the cortex, striatum, and hippocampus were 8.4, 5.7, and 7.2 mmol/kg, respectively. The corresponding extracellular concentrations of NAA were much lower (35.1, 83.7, and 23.0 microM). High tissue/ECF concentration ratios may suggest little release or leakage of NAA under basal conditions, and potent reuptake mechanisms for NAA in the cellular membrane of CNS cells. There was no change in ECF NAA during K(+)-induced local depolarising stimuli produced in the striatum, but NAA levels consistently increased after the K+ stimuli, irrespective of whether or not Ca2+ was present in the perfusion medium. These data confirm that NAA is not a neurotransmitter and suggest strongly that NAA is not directly involved in the release and reuptake or metabolism of neuroactive compounds. The increase of NAA in the ECF immediately after K+ stimulation may reflect an involvement in brain osmoregulation and/or acid-base homeostasis.

Excitatory amino acids, free radicals and the pathogenesis of motor neuron disease

The cause of motor neuron disease (MND) remains unknown, but the pathogenic involvement of excitatory amino acid (EAA) neurotransmitters and related exogenous compounds has been proposed. We discuss current concepts of the mechanisms of action of EAAs and the evidence for links between these neurotransmitters and free radical hypotheses of neuronal damage. These concepts are especially pertinent following reports of mutations in the gene encoding the free radical scavenging enzyme, copper-zinc superoxide dismutase, in familial MND. New approaches to treatment are suggested by advances in understanding of the disease.

Trial of immunosuppression in amyotrophic lateral sclerosis using total lymphoid irradiation

Although the cause of amyotrophic lateral sclerosis (ALS) remains unknown, recent studies have suggested an autoimmune mechanism of pathogenesis. Previous trials of immunosuppressive treatment have yielded inconclusive results. Our study was designed to determine whether more powerful and prolonged immunosuppression, produced by total lymphoid irradiation (TLI), would alter the course of ALS. In a double-blind, randomized, placebo-controlled study, 30 patients with classic ALS were treated with TLI, and 31 were given sham radiation. Quantitative measurements of muscle strength, functional motor activity, and humoral and cellular immune status were followed for 2 years, or until death or respirator dependence. Motor function in the TLI-treated and control groups showed no significant differences throughout the study. Overall survival was not significantly different in the TLI-treated and control groups. TLI effectively suppressed cellular and humoral immune function throughout the 2-year study period. Analysis of the relationship between immunosuppression and motor functions showed no consistent effect of treatment. We conclude that powerful and prolonged immunosuppression produced by TLI did not benefit patients with ALS. This fails to support the concept of an autoimmune mechanism of pathogenesis of ALS.

Oxygen radical-induced neurotoxicity in spinal cord neuron cultures

The neurotoxic effects of oxygen radicals on spinal cord neuron cultures derived from fetal mouse have been studied. Oxygen radicals, superoxide radical and hydrogen peroxide, were generated by adding xanthine oxidase and hypoxanthine in the culture medium. Exposure of neurons to this oxygen radical-generating system resulted in a significant cell death and decrease of choline acetyltransferase (ChAT) activity in a time-dependent manner in spinal cord neuron cultures. The decrease in cell viability and ChAT enzyme activity induced by the oxygen radicals was blocked by scavengers such as superoxide dismutase (SOD), catalase, and tetrakis (2-pyridylmethyl) ethylenediamine (TPEN), a metal chelator. Antagonists of the N-methyl-D-aspartate (NMDA) receptor, including MK801 (a noncompetitive NMDA antagonist), D-2-amino-5-phosphonovaleric acid (APV) (a competitive NMDA antagonist), and 7-chlorokynurenic acid (an antagonist at the glycine site associated with the NMDA receptor), similarly blocked oxygen radical-induced decrease in cell viability and ChAT activity in spinal cord neuron cultures. These results indicate that both oxygen radicals and excitotoxic amino acids were involved in the oxidant-initiated neurotoxicity of spinal cord neurons.

Abnormal acidic amino acids and N-acetylaspartylglutamate in hereditary canine motoneuron disease

Hereditary canine spinal muscular atrophy (HCSMA) is a lower motor neuron disease found in Brittany Spaniels that shares clinical and pathological features with human amyotrophic lateral sclerosis (ALS). Since acidic excitatory amino acids and the neuropeptide N-acetyl-aspartyl-glutamate (NAAG) are reduced in spinal cord and cerebral cortex in ALS, the levels of these substances were measured in nervous tissue in Brittany Spaniels heterozygous and homozygous for HCSMA. Significant reductions in the levels of endogenous aspartate, glutamate, N-acetylaspartate (NAA), and NAAG were found in the spinal cord in homozygous but not heterozygous HCSMA. In contrast, the activity of N-acetylated-alpha-linked-amino dipeptidase (NAALADase), an enzyme that cleaves NAAG into NAA and Glu, was significantly increased. None of these parameters was affected in the motor cortex or occipital cortex. Since NAA and NAAG are highly concentrated in motoneurons, they may play a role in the pathogenesis of motor neuron disease.

Neuronal damage induced by beta-N-oxalylamino-L-alanine, in the rat hippocampus, can be prevented by a non-NMDA antagonist, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline

The neurotoxin beta-N-oxalylamino-L-alanine (BOAA), found in Lathyrus sativus seeds, is thought to be the causative agent of neurolathyrism. We have investigated the in vivo mechanism of action of BOAA by focal injection (1 microliter) in the dorsal hippocampus of male Wistar rats and comparing the pathological outcome with the effects of injections (1 microliter) of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA), kainate (KA) or N-methyl-D-aspartate (NMDA). Cellular damage induced by the excitatory amino acids in the pyramidal (CA1-CA4) and dentate granule neurones (DG) was assessed histologically 24 h after the injection. The study shows that BOAA (50 nmol) induces hippocampal toxicity with a highly selective pattern of regional cellular damage. The CA1, CA4 and DG subfields show 70-90% neuronal injury whereas CA2 and CA3 show only minimal damage. This pattern of cellular damage is similar to that induced by AMPA (1 nmol) and NMDA (25 nmol) but not KA (0.5 nmol). BOAA-induced neurotoxicity is prevented in a dose-dependent manner by focal co-injection of the non-NMDA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX) (1-25 nmol) but not by a dose of MK-801 (3 mg/kg i.p.) which is neuroprotective against an injection of NMDA. Delayed focal injections of NBQX (25 nmol) up to 2 h after the BOAA injection result in a significant protection of all pyramidal and granular cell regions. These results indicate that the in vivo hippocampal toxicity of BOAA is mediated by AMPA receptors rather than by KA or NMDA receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxidative stress, glutamate, and neurodegenerative disorders

There is an increasing amount of experimental evidence that oxidative stress is a causal, or at least an ancillary, factor in the neuropathology of several adult neurodegenerative disorders, as well as in stroke, trauma, and seizures. At the same time, excessive or persistent activation of glutamate-gated ion channels may cause neuronal degeneration in these same conditions. Glutamate and related acidic amino acids are thought to be the major excitatory neurotransmitters in brain and may be utilized by 40 percent of the synapses. Thus, two broad mechanisms--oxidative stress and excessive activation of glutamate receptors--are converging and represent sequential as well as interacting processes that provide a final common pathway for cell vulnerability in the brain. The broad distribution in brain of the processes regulating oxidative stress and mediating glutamatergic neurotransmission may explain the wide range of disorders in which both have been implicated. Yet differential expression of components of the processes in particular neuronal systems may account for selective neurodegeneration in certain disorders.

Amyotrophic lateral sclerosis: quantitative autoradiography of [3H]MK-801/NMDA binding sites in spinal cord

We have characterized a high-affinity binding site for [3H]MK-801, an NMDA receptor ion channel antagonist, in cervical spinal cords from patients who have died with amyotrophic lateral sclerosis (ALS) and from control subjects. In cervical spinal cord [3H]MK-801 labelled at least two binding sites, the highest affinity site having a Kd of between 9-16 nM. No significant differences in affinity were observed between spinal cords from ALS patients or controls. In spinal cords from ALS patients, large reductions in [3H]MK-801 receptor binding (between 30-40% reductions) were detected in both the dorsal and ventral horns. These data may reflect the death of receptor-bearing cells, or a form of receptor regulation.

A case of Canavan disease: the first biochemically proven case in a Japanese girl

Canavan disease (CD) has only been diagnosed on autopsy or brain biopsy, however, specific biochemical markers, such as N-acetylaspartic acid (NAA) and aspartoacylase activity, have recently been described in CD. We report a case of CD having the above biochemical markers. High levels of NAA were found in her urine, serum and CSF. Fibroblasts did not exhibit aspartoacylase activity. Clinically, she presented progressive psychomotor retardation, cerebellar signs, pyramidal signs and relative megalencephaly. CT and MRI showed findings of leukodystrophy. The evoked potentials showed widespread involvement in the brainstem. Magnetic resonance spectra showed a high level of NAA in the white matter. In Japan, this case is the first of CD determined on the basis of biochemical markers.

Excitotoxic neuronal damage and neuropsychiatric disorders

Excitatory amino acids (EAA) serve important physiological functions in the vertebrate CNS, including participation in fast excitatory synaptic transmission, modulation of synaptic plasticity and regulation of neuronal morphology during development. However, paradoxically they also harbor neurotoxic (excitotoxic) potential, which, if unleashed, can cause widespread degeneration of CNS neurons. Accumulating evidence suggests a role for excitotoxins in a variety of human neuropsychiatric disorders. This paper reviews the classes of EAA receptors in the CNS, the mechanisms underlying EAA-mediated neuronal damage and the role of EAA in specific human disorders.

Chronic inhibition of glutamate uptake produces a model of slow neurotoxicity

Defects in neurotransmitter glutamate transport may be an important component of chronic neurotoxicity in diseases such as amyotrophic lateral sclerosis. There are no reliable models of slow glutamate neurotoxicity. Most previous in vitro systems have studied the rapid neurotoxic effects of direct-acting glutamate agonists. Therefore, we developed a model of slow toxicity in cultured organotypic spinal cord slices. The model was based on selective inhibition of glutamate transport, which continuously raised the concentration of glutamate in the culture medium. This resulted in the slow degeneration of motor neurons over several weeks. Motor neuron toxicity was selectively prevented by non-N-methyl-D-aspartate glutamate receptor antagonists and glutamate synthesis or release inhibitors but not by N-methyl-D-aspartate receptor antagonists. Thus, selective inhibition of glutamate transport produces a model of clinically relevant slow neurotoxicity and appears to be mediated by the action of non-N-methyl-D-aspartate receptors. This data supports the hypothesis that the slow loss of motor neurons in amyotrophic lateral sclerosis could be due, in part, to defective glutamate transport.

Fasting plasma and CSF amino acid levels in amyotrophic lateral sclerosis: a subtype analysis

Data from the literature about plasma and CSF amino acid (AA) levels in amyotrophic lateral sclerosis (ALS) remain controversial. To refine such analyses we used HPLC, and report a study of plasma and CSF AA concentrations in patients with ALS, the type of the disease (spinal and bulbar onset) being precisely determined. In ALS, there is a decrease in the plasma levels of the large neutral amino acids (LNAA) alanine, isoleucine, leucine, methionine and tyrosine which was particularly striking in the bulbar type (p < 0.05). Plasma glutamate levels do not differ between ALS and controls, but are significantly increased in ALS with spinal onset and decreased in the bulbar type (p < 0.05 vs controls, p < 0.001 bulbar vs spinal). In CSF, the analysis of the whole ALS group shows no difference from controls. However, there is an increase of CSF serine, glutamine and alanine in ALS with spinal onset (p < 0.05). Our results do not support an abnormal profile of excitatory AA concentrations in ALS. The heterogeneous changes we observed, mainly concerning LNAAs, may be explained by a blood-CSF barrier disturbance in the disease. As AA levels clearly differ between ALS types, with low concentrations in bulbar ALS, this dual profile probably explains some of the discrepancies between previous studies.

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.

Synaptosomal glutamate uptake declines progressively in the spinal cord of a mutant mouse with motor neuron disease

It has been suggested that the degeneration of motor neurons in amyotrophic lateral sclerosis is a consequence of excitotoxicity resulting from a loss of synaptosomal glutamate uptake. The role of synaptosomal glutamate uptake in the pathogenesis of motor neuron disease was studied in the Mnd mouse. Glutamate uptake in spinal-cord synaptosomes declined in parallel with the onset of behavioral deficits in Mnd mice but lagged considerably behind the appearance of pathology in motor neurons. Glutamate uptake did not decline significantly in corpus striatum, and GABA uptake did not change significantly in either spinal cord or striatum. The presence of pronounced histopathological changes before the loss of glutamate uptake suggests that the decline of glutamate uptake is a consequence rather than the primary cause of motor neuron disease in the Mnd mouse.

Assessment of reliability and biological significance of glutamate levels in cerebrospinal fluid

The published information on glutamate levels in cerebrospinal fluid (CSF) and modifications in neurological disorders is controversial. In the present study, we demonstrated a metabolic instability of glutamate in untreated CSF and a spurious elevation of its levels by the current methods of CSF acidification. These findings may explain the discrepancies observed with different methods of CSF processing and analysis. We suggest a method of inactivating CSF enzymes that yields stable glutamate levels under different storage conditions. Use of such a method may be necessary for clinical studies.

Cell culture evidence for neuronal degeneration in amyotrophic lateral sclerosis being linked to glutamate AMPA/kainate receptors

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder affecting motor neurons. Glutamate, a potent central-nervous-system toxin, has been proposed as one possible factor in this motoneuron disease. Serum from patients with ALS is known to be toxic when added to neurons in culture. We report on the toxicity to rat neurons in culture of cerebrospinal fluid (CSF) from patients with ALS. CSF were obtained from 10 ALS patients, 10 neurological controls, and 10 other controls. ALS CSF was added at dilutions of 50%, 20%, or 10% and neuron survival was assessed after 24 h. The neuroprotective effects of antagonists to two glutamate receptors were also assessed. ALS CSF was significantly neurotoxic, with a neuronal survival rate of only 47% compared with 80% or so for control CSF. This neurotoxicity was blocked by CNQX, an antagonist to the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptor but not by two N-methyl-D-aspartate (NMDA) antagonists. ALS CSF contains a specific neurotoxic factor which is AMPA/kainate-like which could have a role in the neuronal degeneration of this disease.

The gene encoding the glutamate receptor subunit GluR5 is located on human chromosome 21q21.1-22.1 in the vicinity of the gene for familial amyotrophic lateral sclerosis

Genomic clones of the human non-N-methyl-D-aspartate (non-NMDA) glutamate receptor subunit GluR5 were isolated by high-stringency screening of a cosmid library using the rat cDNA as a probe. The chromosomal localization of the human GluR5 gene has been established. Southern hybridization of DNA isolated from mapping panels of Chinese hamster-human hybrid cell lines and high-resolution in situ suppression hybridization localize the GluR5 gene to chromosome 21q21.1-22.1. This coincides with the localization of a mutant gene causing familial amyotrophic lateral sclerosis (ALS), as Siddique et al. established by linkage analyses [Siddique, T., Figlewicz, D. A., Pericak-Vance, M. A., Haines, J. L., Rouleau, G., Jeffers, A. J., Sapp, P., Hung, W. Y., Bebout, J., McKenna-Yasek, D., Deng, G., Horvitz, H. R., Gusella, J. F., Brown, R. H. & Roses, A. D. (1991) N. Engl. J. Med. 324, 1381-1384]. Convergent evidence from other investigators suggests that chronic pathologic activation of motor neurons via non-NMDA glutamate receptors might induce excitotoxic injury of motor neurons, culminating in ALS. Together with the demonstration that GluR5 transcripts are expressed in the ventral horn of the spinal cord, the region in which susceptible motor neurons reside, the chromosomal localization suggests that a mutated GluR5 gene may be responsible for the familial form of ALS.

Altered metabolism of excitatory amino acids, N-acetyl-aspartate and N-acetyl-aspartyl-glutamate in amyotrophic lateral sclerosis

Since recent studies provided evidence for abnormal glutamate metabolism in amyotrophic lateral sclerosis, we measured amino acid levels in the fasting plasma of 52 ALS patients and an equal number of controls of a similar age. In addition, the content of amino acids, N-acetyl-aspartate (NAA) and N-acetyl-aspartyl-glutamate (NAAG) were measured in spinal cord and brain tissue obtained at autopsy from patients dying of ALS. Results showed significant elevations (by about 70%) in the plasma levels of glutamate in the ALS patients as compared to controls. In contrast, glutamate levels were significantly decreased in all CNS regions studied of ALS patients (by 21-40%), with the greatest changes occurring in the spinal cord. The ratio of glutamine to glutamate was altered significantly in the spinal cord ALS tissue. In addition, reductions in the levels of aspartate (by 32-35%), NAA, and NAAG (by 40-48%) were found in the spinal cord of ALS patients. These results are consistent with a generalized defect in the metabolism of neuroexcitotoxic amino acids. An altered distribution of these compounds may occur between their intracellular and extracellular pools with resultant abnormal potentiation of excitatory transmission mediated by glutamate receptors and selective degeneration of motor neurons.

What do we really know about amyotrophic lateral sclerosis?

The cause of amyotrophic lateral sclerosis is unknown. In this review clinical and scientific data that are pertinent to understanding this disease are reviewed. There are currently several major controversies concerning the possible role of immunological factors, genetic factors, environmental toxins, and viral infection in pathogenesis. These concepts must be considered in relation to what is known about the disease in all its aspects, including epidemiological data, information on the classical and molecular pathology of the disease, and on associated involvement of other systems, e.g., the spinocerebellar pathways and frontal dementia. Only when all this information is assimilated can full understanding of the disease and, hopefully, a logical approach to treatment and prevention, be achieved.

Nerve conduction studies in amyotrophic lateral sclerosis

Nerve conduction studies (NCS) are an integral part of the evaluation of amyotrophic lateral sclerosis (ALS) patients and are useful in distinguishing ALS from disorders that mimic it. The question often arises whether in the presence of severe atrophy and reduction of the compound muscle action potential amplitude, abnormal conduction velocity (CV), distal latency (DL), or F-wave latency (F) exceeds what can be expected from ALS alone. To determine the limits of abnormality in classic ALS, we prospectively evaluated NCS data from 61 patients who met a strict clinical definition of ALS. We related CV, DL, and F to distal evoked amplitude (AMP) in peroneal (n = 63 observations), median (n = 50), and ulnar (n = 52) nerves. In nerves with reduced AMP, CV rarely fell to less than 80% of the lower limit of normal, and DL and F rarely exceeded 1.25 times the upper limit of normal. Utilizing the entire data set and regression analyses, 95% confidence limits for expected values for CV, F, and DL as a function of AMP were calculated. These limits thus derived suggest criteria for NCS abnormalities in ALS and may be useful in differentiating ALS from other illnesses.

Proton magnetic resonance spectroscopy of the brain in schizophrenic and affective patients

Water-suppressed 1H magnetic resonance spectra were recorded from two brain regions of psychiatric patients and normal volunteers. The two regions studied were (a) the basal ganglia structures surrounding the anterior horn of the lateral ventricle and (b) the occipital cortex. N-Acetylaspartate (NAA), phosphocreatine-creatine (PCr-Cr), choline and inositol resonances were seen in both regions. Ratios of metabolite peak integrals to PCr-Cr peak integral were calculated for each spectrum. To control for partial volume effects, comparisons between patients and controls were made only from identical regions i.e. basal ganglia vs basal ganglia, and likewise for occipital cortex. Metabolite ratios from the occipital region of patients were similar to those from the occipital region of normal subjects. Bipolar patients being treated with lithium had elevated NAA/PCr-Cr in the basal ganglia region when compared to normals. These patients also demonstrated elevated choline/PCr-Cr and inositol/PCr-Cr ratios in the basal ganglia region.

Glutamate and GABA levels in CSF from patients affected by dementia and olivo-ponto-cerebellar atrophy

The modifications in the CSF content of glutamate and GABA in patients afflicted with primary degenerative dementia (PDD) and olivo-ponto-cerebellar atrophy (OPCA) have been evaluated. Control subjects (with disk herniation) were also included in the study. The amino-acids assays were carried out utilizing enzymatic-bioluminescence technique. GABA levels in controls were 803 +/- 98 (n = 7) and in demented patients 702 +/- 98 (n = 7) pmol/ml. Glutamate levels were 2067 +/- 244 (n = 10) in controls, 1190 +/- 81 (n = 16) pmol/ml (vs controls p less than 0.01) in demented patients, and 1116 +/- 146 (vs controls p less than 0.01) in OPCA patients. These results suggest that CSF glutamate levels in severely demented patients might be a result of generalized neuronal loss in the brain with a reactive gliosis.

Decreased glutamate transport by the brain and spinal cord in amyotrophic lateral sclerosis

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a chronic degenerative neurologic disorder characterized by the death of motor neurons in the cerebral cortex and spinal cord. Recent studies have suggested that the metabolism of glutamate, a potentially neurotoxic amino acid, is abnormal in patients with ALS. We hypothesized that the high-affinity glutamate transporter is the site of the defect.

METHODS

We measured high-affinity, sodium-dependent glutamate transport in synaptosomes from neural tissue obtained from 13 patients with ALS, 17 patients with no neurologic disease, and 27 patients with other neuro-degenerative diseases (Alzheimer's disease in 15 patients and Huntington's disease in 12 patients). The groups were comparable with respect to age and the interval between death and autopsy. Synaptosomes were prepared from spinal cord, motor cortex, sensory cortex, visual cortex, striatum, and hippocampus. We also measured sodium-dependent transport of gamma-aminobutyric acid and phenylalanine in the synaptosomal preparations.

RESULTS

In patients with ALS, there was a marked decrease in the maximal velocity of transport for high-affinity glutamate uptake in synaptosomes from spinal cord (-59 percent, P less than 0.001), motor cortex (-70 percent, P less than 0.001), and somatosensory cortex (-39 percent, P less than 0.05), but not in those from visual cortex, striatum, or hippocampus. The affinity of the transporter for glutamate was not altered. No abnormalities in glutamate transport were found in synaptosomes from patients with other chronic neurodegenerative disorders. The transport of gamma-aminobutyric acid and phenylalanine was normal in patients with ALS.

CONCLUSIONS

ALS is associated with a defect in high-affinity glutamate transport that has disease, region, and chemical specificity. Defects in the clearance of extracellular glutamate because of a faulty transporter could lead to neurotoxic levels of extracellular glutamate and thus be pathogenic in ALS.

Decreased brain N-acetylaspartate in Huntington's disease

The concentration of N-acetylaspartic acid (NAA) was measured in perchloric acid extracts of postmortem brain tissue obtained from patients with Huntington's disease and from control subjects. The material in the desalted extracts was resolved on an ion exclusion column and the content of NAA was determined by subsequent fluorometric quantitation of aspartate in hydrolyzates of the resolved NAA. The concentration of NAA in the putamen from patients with Huntington's disease was less than half that of controls (2.74 vs. 6.06 mumol/g wet weight). A smaller but significant reduction was also evident in samples of cerebral cortex from Brodmann area 10 (3.99 vs. 5.29 mumol/g), while the difference in concentrations in the cerebellum was not statistically significant. Though NAA could play a direct role in Huntington's disease, it seems more likely that the changes observed reflect illness or death of neurons, and that it may be feasible to monitor the course of Huntington's disease from NAA determinations. The same tissue extracts were also examined for the presence of D-isomers of amino acids. Only traces were found in NAA, aspartate, or glutamate.

Alterations in spinal cord excitatory amino acid receptors in amyotrophic lateral sclerosis patients

Excitatory amino acids (EAA) have been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). We have analyzed the distribution of the N-methyl-D-aspartate (NMDA) 1-(1-(2-thienyl)-cyclohexyl) piperidine (TCP), kainate and alpha-amino-3-hydroxy-5-methyl-4 isoxazole propionic acid (AMPA) quisqualate subtypes of EAA receptors using quantitative receptor autoradiography in the cervical and thoracic spinal cords of patients who have died with ALS, and of controls. We observed that in control spinal cords [3H]TCP/NMDA binding sites were located both in the ventral and dorsal horns with the highest densities being situated in lamina II. [3H]AMPA and [3H]kainate binding sites were present almost exclusively in the substantia gelatinosa of the dorsal horn. In ALS, the distribution of these 3 types of receptors was unchanged, but [3H]TCP/NMDA binding was decreased both in the dorsal and ventral horns. [3H]kainate binding was possibly decreased in substantia gelatinosa, of ALS cords. However, the limited sample size available for [3H]kainate binding did not permit statistical analysis. [3H]AMPA binding sites were unaltered in ALS. These results indicate that there is a preferential reduction in NMDA receptors in ALS. We suggest that should an excitotoxic mechanism be involved in the pathogenesis of ALS, then NMDA receptors may be the target of this effect.

Chronic treatment with l-threonine in amyotrophic lateral sclerosis: a pilot study

Thirty patients suffering from amyotrophic lateral sclerosis were included in an open therapeutical trial. They were randomized to receive either L-threonine (Thr), a precursor of the inhibitory amino acid glycine, or vitamin B or carnitine. Thirteen patients (9 patients on Thr and 4 control subjects) completed the 1-year trial. No statistical differences were observed between the treated group and the control patients in the decline of the clinical assessment score. Nevertheless, Thr-treated patients complained less frequently of respiratory failure than the control group despite bulbar involvement being more common in the Thr group at entry.

Cholinergic markers in ALS spinal cord

We analyzed binding sites for quinuclidinyl benzilate (QNB) and hemicholinium-3 (HC-3) by quantitative slice autoradiography and the activities of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) in spinal cord of 5-7 patients with amyotrophic lateral sclerosis (ALS). In the ventral horn, QNB binding sites were markedly reduced (38% of controls; P less than 0.001), whereas HC-3 binding sites were only moderately affected (76%, P less than 0.01). Losses in cholinergic marker enzymes were inconsistent. The loss of muscarinic binding sites in the ventral horn was the most reliable cholinergic disease marker in ALS.