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Excitotoxicity refers to the ability of glutamate or related excitatory amino acids to mediate the death of central neurons under certain conditions, for example, after intense exposure. Such excitotoxic neuronal death may contribute to the pathogenesis of brain or spinal cord injury associated with several human disease states. Excitotoxicity has substantial cellular specificity and, in most cases, is mediated by glutamate receptors. On average, NMDA receptors activation may be able to trigger lethal injury more rapidly than AMPA or kainate receptor activation, perhaps reflecting a greater ability to induce calcium influx and subsequent cellular calcium overload. It is possible that excitotoxic death may share some mechanisms with other forms of neuronal death.

Excitotoxic cell death

Keywords: Cell Communication Reference LNDC-REVIEW-1996-002View record in PubMed Record created on 2010-01-08, modified on 2017-05-12

https://www.jneurosci.org/content/jneuro/16/3/877.full.pdf

Metabolic coupling between glia and neurons

Astrocytes play a critical role in the regulation of brain metabolic responses to activity. One detailed mechanism proposed to describe the role of astrocytes in some of these responses has come to be known as the astrocyte-neuron lactate shuttle hypothesis (ANLSH). Although controversial, the original concept of a coupling mechanism between neuronal activity and glucose utilization that involves an activation of aerobic glycolysis in astrocytes and lactate consumption by neurons provides a heuristically valid framework for experimental studies. In this context, it is necessary to provide a survey of recent developments and data pertaining to this model. Thus, here, we review very recent experimental evidence as well as theoretical arguments strongly supporting the original model and in some cases extending it. Aspects revisited include the existence of glutamate-induced glycolysis in astrocytes in vitro, ex vivo, and in vivo, lactate as a preferential oxidative substrate for neurons, and the notion of net lactate transfer between astrocytes and neurons in vivo. Inclusion of a role for glycogen in the ANLSH is discussed in the light of a possible extension of the astrocyte-neuron lactate shuttle (ANLS) concept rather than as a competing hypothesis. New perspectives offered by the application of this concept include a better understanding of the basis of signals used in functional brain imaging, a role for neuron-glia metabolic interactions in glucose sensing and diabetes, as well as novel strategies to develop therapies against neurodegenerative diseases based upon improving astrocyte-neuron coupled energetics.

Activity-dependent regulation of energy metabolism by astrocytes: an update.

Cyclo-oxygenase-2 (COX-2), a rate-limiting enzyme for prostanoid synthesis, is induced during inflammation and participates in inflammation-mediated cytotoxicity. Cerebral ischemia is followed by an inflammatory reaction that plays a role in the evolution of the tissue damage. We studied whether COX-2 is induced after cerebral ischemia and if so, whether such expression contributes to ischemic brain damage. The middle cerebral artery was occluded in rats, and the ischemic area was sampled for analysis 3–96 hr later. COX-2 mRNA was determined by the competitive reverse-transcription PCR. COX-2 mRNA was upregulated in the ischemic hemisphere, but not contralaterally, beginning 6 hr after ischemia. The upregulation reached a maximum at 12 hr, at which time a fivefold induction of the message occurred. Twenty-four hours after ischemia, the concentration of prostaglandin E2 was elevated in the injured brain by 292 ± 57% ( n = 6). COX-2 immunoreactivity was observed in neurons at the medial edge of the ischemic area. Administration of the COX-2 inhibitor NS-398 attenuated the elevation in prostaglandin E2 in the postischemic brain and reduced the volume of the infarct by 29 ± 6% ( p < 0.05). Thus, cerebral ischemia leads to upregulation of COX-2 message, protein, and reaction products in the injured hemisphere. The data implicate COX-2 in the mechanisms of delayed neuronal death at the infarct border and provide the rationale for neuroprotective strategies employing COX-2 inhibitors.

https://www.jneurosci.org/content/jneuro/17/8/2746.full.pdf

Cyclo-Oxygenase-2 Gene Expression in Neurons Contributes to Ischemic Brain Damage

The effects of severe hypoxia were studied in a primary culture of astrocytes prepared from newborn rat cerebral cortex. Hypoxia was created by placing cultures in an airtight chamber that was flushed with 95% N2/5% CO2 for 15 min before being sealed. The hypoxic environment was maintained constant for up to 24 h. During the first 12 h of hypoxia, astrocytes showed no morphological changes by phase-contrast microscopy. After 18 h of hypoxia, some astrocytes in culture became swollen and started to detach from the culture dish. All cells in the culture were destroyed after 24 h of hypoxia. The lactate dehydrogenase level in the culture medium increased more than tenfold between 12 and 24 h of hypoxia. Glutamate uptake was inhibited 80% by similar hypoxic conditions. The cell volume of astrocytes, as measured by 3-O-methyl-[14C]-D-glucose uptake, was increased. These observations suggested cell membrane dysfunction. The malondialdehyde level of hypoxic cultures increased two-fold after 24 h of hypoxia. Verapamil (0.5 mM), furosemide (1 mM), indomethacin (1 mM), MgCl2 (10 mM), and mannitol (10 mM) reduced but never completely abolished the release of lactate dehydrogenase from hypoxic astrocytes. These data suggest multifactorial causes for severe injury in hypoxic astrocytes.

https://journals.sagepub.com/doi/pdf/10.1038/jcbfm.1989.3

Hypoxia-Induced Dysfunctions and Injury of Astrocytes in Primary Cell Cultures:

The effects of arachidonic acid on glutamate and gamma-aminobutyric acid (GABA) uptake were studied in primary cultures of astrocytes and neurons prepared from rat cerebral cortex. The uptake rates of glutamate and GABA in astrocytic cultures were 10.4 nmol/mg protein/min and 0.125 nmol/mg protein/min, respectively. The uptake rates of glutamate and GABA in neuronal cultures were 3.37 nmol/mg protein/min and 1.53 nmol/mg protein/min. Arachidonic acid inhibited glutamate uptake in both astrocytes and neurons. The inhibitory effect was observed within 10 min of incubation with arachidonic acid and reached approximately 80% within 120 min in both types of culture. The arachidonic acid effect was not only time-dependent, but also dose-related. Arachidonic acid, at concentrations of 0.015 and 0.03 mumol/mg protein, significantly inhibited glutamate uptake in neurons, whereas 20 times higher concentrations were required for astrocytes. The effects of arachidonic acid were not as deleterious on GABA uptake as on glutamate uptake in both astrocytes and neurons. In astrocytes, GABA uptake was not affected by any of the doses of arachidonic acid studied (0.015-0.6 mumol/mg protein). In neuronal cultures, GABA uptake was inhibited, but not to the same degree observed with glutamate uptake. Lower doses of arachidonic acid (0.03 and 0.015 mumol/mg protein) did not affect neuronal GABA uptake. Other polyunsaturated fatty acids, such as docosahexaenoic acid, affected amino acid uptake in a manner similar to arachidonic acid in both astrocytes and neurons. However, saturated fatty acids, such as palmitic acid, exerted no such effect. The significance of the arachidonic acid-induced inhibition of neurotransmitter uptake in cultured brain cells in various pathological states is discussed.

Effects of arachidonic acid on glutamate and γ-aminobutyric acid uptake in primary cultures of rat cerebral cortical astrocytes and neurons

The glycogen content of primary cultured astrocytes was approximately doubled by incubation with 1 mM L-glutamate or L-aspartate. Other amino acids and excitatory neurotransmitters were without effect. The increase in glycogen level was not blocked by the glutamate receptor antagonist kynurenic acid but was completely blocked by the glutamate uptake inhibitor threo-3-hydroxy-D,L-aspartate and by removal of Na+ from the medium. Incubation with radiolabeled glucose and glutamate revealed that the increased glycogen content was derived almost entirely from glucose. Glutamate at 1 mM was also found to cause a 53 +/- 12% decrease in glucose utilization and a 112 +/- 69% increase in glucose-6-phosphate levels. These results suggest that the glycogen content of astrocytes is linked to the rate of glucose utilization and that glucose utilization can, in turn, be affected by the availability of alternative metabolic substrates. These relationships suggest a mechanism by which brain glycogen accumulation occurs during decreased neuronal activity.

Glutamate increases glycogen content and reduces glucose utilization in primary astrocyte culture.

Publisher Summary This chapter explains that free polyenoic fatty acids (PUFAs), especially arachidonic acid and docosahexaenoic acid are rapidly released following ischemia, electroconvulsive seizures, and various pathological insults. Free PUFAs and arachidonic acid in particular, have both physiological and pathological effects on cellular systems. It has been described that free arachidonic acid readily intercalates into the membrane and produces significant changes in the packing of the lipid molecules. PUFA-induced membrane fluidity has been associated with the stimulation of chloride transport in corneal epithelium; it enhanced activities of both membrane-associated adenylate cyclase, and guanylate cyclase. The molecular mechanisms of PUFA-induced cellular edema are studied further in the in vitro cortical slices system. The studies have suggested that PUFAs, especially arachidonic acid, play a key role in membrane damage and the development of edema following ischemia and injury.

Cellular and molecular effects of polyunsaturated fatty acids in brain ischemia and injury.

The effects of arachidonic acid and other polyunsaturated fatty acids (PUFAs) on both oxidative and metabolic perturbation were studied in primary cultures of rat cerebral cortical astrocytes. In the presence of 0.1 mM arachidonic acid, the rate of the reduction of nitroblue tetrazolium (NBT) to nitroblue formazan (NBF) was stimulated from 0.65 +/- 0.10 to 1.43 +/- 0.15 and from 0.092 +/- 0.006 to 0.162 +/- 0.009 nmol/min/mg protein in intact and broken cell preparations, respectively. The rate of superoxide radical formation, as measured by the superoxide dismutase (SOD)-inhibitable NBT reduction was 0.042 nmol/mg protein in broken cells and was negligible in intact cells. The latter is due to the impermeability of SOD into the intact cell preparation. NBF formation in intact astrocytes stimulated by arachidonic acid was both time- and dose-dependent. Other PUFAs, including linoleic acid, linolenic acid, and docosahexaenoic acid, were also effective in stimulating NBF formation in astrocytes, whereas saturated palmitic acid and monounsaturated oleic acid were ineffective. Similar effects of these PUFAs were observed in malondialdehyde formation in cells and lactic acid accumulation in incubation medium. These data indicate that both membrane integrity and cellular metabolism were perturbed by arachidonic acid and by other PUFAs. The sites of superoxide radical formation appeared to be intracellular and may be associated with membrane phospholipid domains, because liposome-entrapped SOD, which was taken up by intact astrocytes, reduced the level of superoxide radicals and lactic acid content, whereas free SOD was not effective.

Albert C. H. Yu

Papers

Hypoxia-Induced Dysfunctions and Injury of Astrocytes in Primary Cell Cultures:

Effects of arachidonic acid on glutamate and γ-aminobutyric acid uptake in primary cultures of rat cerebral cortical astrocytes and neurons

Glutamate increases glycogen content and reduces glucose utilization in primary astrocyte culture.

Cellular and molecular effects of polyunsaturated fatty acids in brain ischemia and injury.

Induction of intracellular superoxide radical formation by arachidonic acid and by polyunsaturated fatty acids in primary astrocytic cultures.