[Frontiers in Bioscience, d701-718, July 20, 1998]
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EXCITATORY AMINO ACID NEUROTRANSMISSION. PATHWAYS FOR METABOLISM, STORAGE AND REUPTAKE OF GLUTAMATE IN BRAIN

Monica Palmada and Josep Joan Centelles

Departament de Bioquímica i Biologia Molecular, Facultat de Química, Universitat de Barcelona, Martí i Franquès, 1, 08028-Barcelona, Spain

Received 5/7/98 Accepted 5/23/98

6. DYSFUNCTION OF EXCITATORY AMINOACID NEUROTRANSMISSION

It is important to maintain low levels (1-3m M) of extracellular glutamate as excessive receptor stimulation or excessive ammonium generated by the glutamate dehydrogenase can lead to neural injury and/or death ("excitotoxicity").

Glutamate appears to be remarkably potent and rapidly acting neurotoxin. Exposure to 100 m M glutamate for 5 min is enough to destroy large numbers of cultured cortical neurons (116). By the way, glutamate neurotoxicity may be blocked by antagonist compounds and attenuated by antagonists added after glutamate exposure (116).

Neurodegenerative diseases, such as Alzheimer's disease (AD), Huntington’s disease (HD), Parkinson’s disease (PD) and amyotrophic lateral sclerosis (ALS), particularly affect old people and result in: i) a modification of the individual personality; ii) the need for constant help from relatives; iii) a high economical costs for family and institutions. In Spain alone, over 500.000 people are affected with those diseases with this increasing as the aged population increases. Due to the increased lifespan in developed societies, neurodegenerative diseases have been increased in the European Union and represent one third of deaths (after cancer and cardiovascular problems). It is though that with Alzheimer's disease alone, at the beginning of next century, 8 million Europeans will be affected.

Many reports have been published on increased levels of glutamate in certain neurodegenerative diseases such as AD, HD, PD and ALS (117-120). Increased extracellular glutamate has also been implicated in the onset of neurodegeneration associated with hypoxic damage (stroke). The release in glutamate following hypoxia has been suggested to be due, at least in part, to a calcium-independent mechanism following the reversal of the neuronal glutamate uptake carrier (121). This increase in extracellular glutamate acts afterwards postsynaptically to increase cellular calcium levels with subsequent cell death.

The mechanisms by which glutamate is increased in neurodegenerative diseases is unknown. The cause of this increased extracellular glutamate has been ascribed to a decrease in the activity of glutamate dehydrogenase (PD) (122) or a decrease in the number of Na+-glutamate transporters (ALS, HD) (123). As it is a neurotoxic, it is likely that the high glutamate concentration observed in neurodegenerative diseases are the most likely cause of the neurodegeneration.

Marangos et al. (124) stated the glutamate hypothesis for early Alzheimer’s disease: "Cell death due to neuronal toxicity could result from excessive synthesis or release of glutamate or a glutamate-like substance, faulty glutamate reuptake, decreased glutamate degradation, or decreased inhibition of excitatory neurons. Any of these aberrant processes could, early in the disease, increase local levels of glutamate and so initiate a slow, progressive degeneration and eventual death of neurons".

Several post-mortem studies have compared brain glutamate levels in Alzheimer’s disease to those in control subjects. Some investigators (125-127) have found lower glutamate levels in the frontal cortex and the temporal cortex of Alzheimer’s patients than in control subjects. Cerebro spinal fluid (CSF) concentration of free glutamate was significantly higher in patients with Alzheimer’s disease than in comparison subjects (117). It should be noted that measurements of glutamate in CSF are likely to give better approximation of glutamate concentrations at synapses than are plasma glutamate concentrations.

In Huntington’s disease, glutamate and GABA concentrations decrease in striatum and caudate nucleus from brain (118,128). However, no reduction at all was observed in the frontal cortex of patients. A likely possibility is that the low glutamate content of the caudate and the putamen in HD results from chronic failure of the normal reuptake mechanism for glutamate released at synapses, with or without any excessive rate of release of this neurotransmitter. This possibility is supported by the finding of Cross et al. (129), who observed large reductions in high-affinity glutamate uptake sites in autopsy specimens of caudate and putamen from HD patients.

If either excessive release or decreased reuptake of glutamate occurred in the striatum in HD, concentrations of glutamate might become high at synapses, with resulting damage to neurons. Some of the excess glutamate accumulate in synaptic clefts in HD would be carried away in the extracellular fluid, thus eventually causing a lowered glutamate content in striatal tissue. This fact is supported by an increased glutamate concentration in CSF of living HD patients as it was observed for Alzheimer’s disease patients.

In Parkinson’s disease, Schapire et al. (130) demonstrated a reduced activity of complex I of the mitochondrial respiratory chain in the region of substantia nigra in brain. Deficiencies on complex II and IV have been also observed in muscle biopsia from PD patients (131). Cedarbaum et al. (122) observed a deficiency of GDH but not on pyruvate dehydrogenase complex. Since complex I is the point of entry for reducing equivalents (as NADH) to the respiratory chain, a decrease in complex I activity might result in feedback endproduct inhibition of GDH. Decreased levels of GDH might exert an excitotoxic glutamate effect via NMDA receptors on striatal dopamine nerve terminals (132) and contribute to cellular degeneration in PD.

Amyotrophic lateral sclerosis (ALS) is a disease resulting in degeneration of the motor cortex, the brain-stem and the spinal chord. While there are a number of hypotheses underlying this disease, an increase in glutamatergic neurotransmission has been proposed as a key event in the disease onset and recently, this has been ascribed, at least in part, to a defect in the function of the GLT-1 glutamate transporter which is localized on astrocytes (123).

Recent in vitro studies have demonstrated that a blockade of the GLT-1 transporter either by transport inhibitors or by an antisense approach resulted in a slow, selective loss of motor neurons, thus strengthening the case for a critical role for GLT-1 in the etiology of ALS (120,133). Furthermore, this decrease in uptake does not appear to be associated with a decrease in transporter expression levels (57) and there is no evidence of specific protein mutations associated with the disease. Because of the potential role of free radicals in ALS and the upset in function of the superoxide dismutase enzyme in the familial form of ALS, previous studies have suggested that an increase in free radicals may impair the function of GLT-1 (134).

The role of free radicals in the etiology of ALS has been strengthened by the selection of point mutations in the cytosolic Cu/Zn superoxide dismutase (SOD-1) associated with the familial form of ALS (FALS) (135). These changes were not detected in control individuals and do not represent normal allelic variants. SOD-1 catalyses the dismutation of the superoxide radical (O2-) to hydrogen peroxide (H2O2) and represents the first line of defense against oxygen toxicity. The mechanism responsible for tissue damage associated with reduced SOD activity remains to be defined. Direct toxicity due to the superoxide radical is probably minor in comparison to the generation of the hydroxyl radical (OH.) which is much more reactive. In addition, the superoxide may interact with endogenously formed nitric oxide to form peroxynitrite which can oxidize methionine residues in proteins and peptides as well as thiols and thioethers (136). It has therefore been suggested that an increase in free radicals in ALS may be, at least in part, responsible for the upset in functioning of the GLT-1 transporter and that the disease may have both a free radical and excitatory amino acid basis.

Glutamate, and particularly the glutamate transporter system, have also been implicated in the ischemic damage associated with anoxia/hypoxia (stroke). For the first few minutes of ischemia, there is a slow acid shift of the cellular pH with a slow rise of extracellular potassium concentration and a subsequent decrease in extracellular sodium and calcium. The rise in potassium depolarizes the cells to around -20mV (anoxic depolarization) and the release of glutamate (137). The mechanism by which glutamate is released in ischemia has been controversial. Some reports suggest that the release is calcium-dependent, suggesting conventional vesicular release, while others claim that the release is calcium-independent, implying a non-exocytotic mechanism such as the reversed operation of the glutamate uptake carrier (138).