Investigations into the mechanism of excitant amino acid cytotoxicity using a well-characterized glutamatergic system

Modeling the glutamate-glutamine neurotransmitter cycle

Glutamate is the principal excitatory neurotransmitter in brain. Although it is rapidly synthesized from glucose in neural tissues the biochemical processes for replenishing the neurotransmitter glutamate after glutamate release involve the glutamate–glutamine cycle. Numerous in vivo¹³C magnetic resonance spectroscopy (MRS) experiments since 1994 by different laboratories have consistently concluded: (1) the glutamate–glutamine cycle is a major metabolic pathway with a flux rate substantially greater than those suggested by early studies of cell cultures and brain slices; (2) the glutamate–glutamine cycle is coupled to a large portion of the total energy demand of brain function. The dual roles of glutamate as the principal neurotransmitter in the CNS and as a key metabolite linking carbon and nitrogen metabolism make it possible to probe glutamate neurotransmitter cycling using MRS by measuring the labeling kinetics of glutamate and glutamine. At the same time, comparing to non-amino acid neurotransmitters, the added complexity makes it more challenging to quantitatively separate neurotransmission events from metabolism. Over the past few years our understanding of the neuronal-astroglial two-compartment metabolic model of the glutamate–glutamine cycle has been greatly advanced. In particular, the importance of isotopic dilution of glutamine in determining the glutamate–glutamine cycling rate using [1−¹³C] or [1,6-¹³C₂] glucose has been demonstrated and reproduced by different laboratories. In this article, recent developments in the two-compartment modeling of the glutamate–glutamine cycle are reviewed. In particular, the effects of isotopic dilution of glutamine on various labeling strategies for determining the glutamate–glutamine cycling rate are analyzed. Experimental strategies for measuring the glutamate–glutamine cycling flux that are insensitive to isotopic dilution of glutamine are also suggested.

AMPA Neurotoxicity in Rat Cerebellar and Hippocampal Slices: Histological Evidence for Three Mechanisms

Excitatory amino acid-induced death of central neurons may be mediated by at least two receptor types, the so-called NMDA (N-methyl-d-aspartate) and AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate) receptors. We have studied the neurodegenerative mechanisms set in motion by AMPA receptor activation using incubated slices of 8-day-old rat cerebellum and hippocampus. In both preparations, AMPA induced a pattern of degeneration that differed markedly from the one previously shown to be elicited by NMDA. In cerebellar slices, AMPA induced the degeneration of most Purkinje cells together with a population of Golgi cells; in hippocampal slices the neurons were affected in the order CA3 > CA1 > dentate granule cells. Three mechanisms could be discerned: an acute one in which neurons (e.g. cerebellar Golgi cells) underwent a rapid degeneration; a delayed one in which the neurons (Purkinje cells and hippocampal neurons) appeared to be only mildly affected immediately after a 30 min exposure but then underwent a protracted degeneration during the postincubation period (1.5 - 3 h); and finally a slow toxicity, which took place during long (2 h) exposures to AMPA (3 - 30 microM). Although Purkinje cells were vulnerable in both cases, the efficacy of AMPA was higher for the delayed mechanism than for the slow one. The pathology displayed by the acutely destroyed Golgi neurons was a classical oedematous necrosis, whereas most neurons vulnerable to the delayed and slow mechanisms displayed a 'dark cell degeneration', whose cytological features bore a close resemblance to those of neurons irreversibly damaged by ischaemia, hypoglycaemia or status epilepticus in vivo.

Polyamine amides are neuroprotective in cerebellar granule cell cultures challenged with excitatory amino acids

Primary cultures of rat cerebellar granule cells have been used to assess the potential neuroprotective effects of philanthotoxins and argiotoxin-636 (ArgTX-636). These polyamine amides are potent antagonists of ionotropic L-glutamate (L-Glu) receptors. In granule cells loaded with fluo-3, ArgTX-636 and philanthotoxin-343 (PhTX-343) antagonised increases of intracellular free calcium concentration ([Ca2+]i) that were stimulated by N-methyl-D-aspartate (NMDA). The antagonism was use-dependent. Antagonism by PhTX-343 was fully reversible, but recovery following antagonism by ArgTX-636 was slow and only partial during the time-course of an experiment. Neither compound inhibited K(+)-induced increases in [Ca2+]i. In excitotoxicity studies with cerebellar granule cells, the release of lactate dehydrogenase (LDH) and morphological observations were used to assess cell death. A 20-30 min exposure to 500 microM NMDA, 100 microM L-Glu or 500 microM kainate was sufficient to kill > 90% of the cells after 18-20 h. When added 5 min prior to, and during agonist exposure, PhTX-343 and ArgTX-636 provided total neuroprotection. ArgTX-636 was about 20-30 fold more potent than PhTX-343 against NMDA, but was approximately equipotent with PhTX-343 against a kainate challenge. Neither of the toxins showed any inherent toxicity even at 400 microM and 100 microM respectively. Some analogues of PhTX-343 are more potent, both in terms of antagonism of NMDA-stimulated increases of [Ca2+]i and neuroprotection, than PhTX-343 and ArgTX-636.

Cyclothiazide treatment unmasks AMPA excitotoxicity in rat primary hippocampal cultures

Mechanisms of non-NMDA receptor-mediated excitotoxicity were studied in embryonic rat hippocampal cultures using kainic acid (KA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) as agonists. Under basal culture conditions, overnight treatment with AMPA resulted in negligible excitotoxicity as assessed by phase-contrast microscopy and measurement of lactate dehydrogenase (LDH) release. In contrast, similar treatment with KA resulted in marked excitotoxic morphologic changes and release of LDH. Cotreatment of cultures with AMPA but not NMDA effectively blocked KA toxicity, suggesting that AMPA-induced rapid desensitization of the AMPA/KA receptor could account for the lack of prominent direct toxicity as well as AMPA's ability to block KA toxicity. To test this hypothesis, cultures were briefly pretreated with 10 microM cyclothiazide, a drug reported to block desensitization of the AMPA/KA receptor, and then exposed overnight to cyclothiazide plus AMPA and/or KA. Cyclothiazide-treated cultures were now vulnerable to AMPA as well as KA; moreover, AMPA was unable to block KA toxicity completely, suggesting that cyclothiazide impaired AMPA/KA receptor desensitization. These and related studies suggest that a regulatory site may exist on the AMPA/KA receptor that modulates non-NMDA receptor-mediated excitotoxicity.

Blockade of ionotropic quisqualate receptor desensitization in rat hippocampal neurons by wheatgerm agglutinin and other lectins

Previous experiments with wheatgerm agglutinin, an inhibitor of ionotropic quisqualate receptor desensitization, suggest that desensitization regulates quisqualate receptor-mediated synaptic transmission and excitotoxicity. Using whole-cell recordings from cultured postnatal rat hippocampal neurons, we have examined the wheatgerm agglutinin effect in further detail and compared it to other lectins. Wheatgerm agglutinin and other lectins belonging to the glucose/mannose, N-acetylglucosamine, and sialic acid classes inhibited desensitization. However, wheatgerm agglutinin was the most effective and had the most rapid onset of action. The inhibition was dose-dependent, and it was reduced and reversed by N-acetylglucosamine and N-acetylneuraminic acid. Treating neurons with neuraminidase, which cleaves sialic acid residues from the surface of cells, also diminished the effect. These results suggest that wheatgerm agglutinin reversibly inhibits ionotropic quisqualate receptor desensitization by interacting with carbohydrate residues on or near the quisqualate receptor complex. Future studies using the lectins may help to clarify the functional role of carbohydrate chains on the ionotropic quisqualate receptor.

Molecular analysis of Drosophila glutamate receptors

Insects and other invertebrates use L-glutamate as a neurotransmitter in the central nervous system and at the neuromuscular junction. In contrast to the well-studied effects of L-glutamate on invertebrate muscle cells, relatively little is known about the physiological role of glutamate receptors (GluRs) in the invertebrate central nervous system. We have applied a molecular cloning approach to elucidate the molecular structure of neuronal and muscle-specific Drosophila glutamate receptor subunits (DGluRs). Several domains conserved between rat GluR subunits and DGluRs indicate regions of high functional significance. Drosophila genetics may now be used as a valuable experimental tool to gain further insight into the role of DGluRs in development, synaptic plasticity and control of gene expression.

Properties of vertebrate glutamate receptors: calcium mobilization and desensitization

Glutamate is now recognized as a major excitatory neurotransmitter in the vertebrate CNS, participating in a number of physiological and pathological processes. The importance of glutamate in the mobilization of intracellular Ca2+ as well as the relationship between excitatory and toxic properties has made it important to understand factors that regulate the responsivity of glutamate receptors. In recent years considerable insight has been gained about regulatory sites on NMDA receptors, with the recognition that these receptors are modulated by multiple endogenous and exogenous agents. Less is known about the regulation of responses mediated by AMPA, kainate, ACPD or APB receptors. Desensitization represents a potentially powerful means by which glutamate responses may be regulated. Indeed, two agents closely linked to the physiology of NMDA receptors, glycine and Ca2+, appear to modulate different types of desensitization. In the case of glycine, alteration of a rapid form of desensitization may be important in the role of this amino acid as a necessary cofactor for NMDA receptor activation. Additionally, changes in the affinity of the receptor complex for glycine may underlie the use-dependent decline in NMDA responses under certain conditions. Likewise, Ca2+ is a crucial player in the synaptic and toxic effects mediated by NMDA receptors, and is involved in a slower form of desensitization, in effect helping to regulate its own influx into neurons. The site and mechanism of the Ca2+ regulatory effects remain uncertain with evidence supporting both intracellular and ion channel sites of action. A clear role for Ca(2+)-dependent desensitization in the function of NMDA receptors under physiological conditions has not yet been demonstrated. AMPA receptor desensitization has been an area of intense investigation in recent years. The rapidity and degree of this process, coupled with its apparent rapid recovery, has suggested that desensitization is a key mechanism for the short-term regulation of responses mediated by these receptors. Furthermore, rapid desensitization appears to be one factor determining the time course and efficacy of fast excitatory synaptic transmission mediated by AMPA receptors, highlighting the physiological relevance of the process. The molecular mechanisms underlying desensitization remain uncertain. Traditionally, desensitization, like inactivation of voltage-gated channels, has been thought to represent a conformational change in the ion channel complex (Ochoa et al., 1989). However, it is unknown to what extent desensitization, in particular rapid AMPA receptor desensitization, has mechanistic features in common with inactivation. In voltage-gated channels, conformational changes in the channel protein restrict ion flow through the channel (Stuhmer, 1991).(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular cloning of an invertebrate glutamate receptor subunit expressed in Drosophila muscle

Insects and other invertebrates use glutamate as a neurotransmitter in the central nervous system and at the neuromuscular junction. A complementary DNA from Drosophila melanogaster, designated DGluR-II, has been isolated that encodes a distant homolog of the cloned mammalian ionotropic glutamate receptor family and is expressed in somatic muscle tissue of Drosophila embryos. Electrophysiological recordings made in Xenopus oocytes that express DGluR-II revealed depolarizing responses to L-glutamate and L-aspartate but low sensitivity to quisqualate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), and kainate. The DGluR-II protein may represent a distinct glutamate receptor subtype, which shares its structural design with other members of the ionotropic glutamate receptor family.

Amino acid receptors from insect muscle: electrophysiological characterization in Xenopus oocytes following expression by injection of mRNA

Poly(A)+ Messenger ribonucleic acid (mRNA) was extracted from leg muscles of the locust Schistocerca gregaria and injected into oocytes of Xenopus laevis. After 5-10 days incubation, receptors for L-glutamate, L-quisqualate, DL-ibotenate and gamma-aminobutyric acid (GABA) were expressed. Agonist-induced currents were dose-dependent, and, in the concentration range 1 microM to 1 mM, generally had peak values of 50 nA. The responses to all agonists, apart from GABA, exhibited desensitization which could not be reversed even by prolonged washing with Ringer. Application of 100 microM GABA to oocytes voltage clamped at -60 mV produced a smooth inward current with a reversal potential of -22 +/- 1 mV, which is consistent with the involvement of chloride ions. At 100 microM, picrotoxin reversibly abolished this current, while 100 microM bicuculline had no effect. L-Glutamate elicited a smooth current with a reversal potential of -52 +/- 3 mV. L-Quisqualate elicited an inward current at -60 mV with a reversal potential of -9 +/- 2 mV; this current occasionally had an oscillatory component. The response to ibotenate comprised a smooth inward current with a reversal potential of -21 +/- 3 mV which was probably mediated by chloride ions.

Blockade of desensitization augments quisqualate excitotoxicity in hippocampal neurons

Glutamate neurotoxicity is thought to play a role in the pathogenesis of several neurodegenerative diseases. While prolonged activation of either NMDA or non-NMDA receptors causes neuronal damage, NMDA receptors appear to mediate most of the glutamate toxicity. The reasons why NMDA toxicity predominates are uncertain but may relate to more effective neuroprotective mechanisms acting at non-NMDA receptors. To determine whether desensitization is one such mechanism, we studied the effects of the lectin wheat germ agglutinin (WGA) on quisqualate currents and toxicity in cultured postnatal rat hippocampal neurons. After WGA treatment, quisqualate currents exhibit little desensitization and a 4- to 8-fold increase in steady-state amplitude. WGA also markedly augments the degree of acute, quisqualate-induced neuronal degeneration. These results suggest that non-NMDA desensitization serves a neuroprotective function in hippocampal neurons.

Glutamate in the mammalian CNS

The excitatory amino acid glutamate plays an important role in the mammalian CNS. Studies conducted from 1940 to 1950 suggested that oral administration of glutamate could have a beneficial effect on normal and retardate intelligence. The neurotoxic nature of glutamate resulting in excitotoxic lesions (neuronal death) is thought possibly to underlie several neurological diseases including Huntington's disease, status epilepticus. Alzheimer's dementia and olivopontocerebellar atrophy. This neurodegenerative effect of glutamate also appears to regulate the formation, modulation and degeneration of brain cytoarchitecture during normal development and adult plasticity, by altering neuronal outgrowth and synaptogenesis. In addition to its function as a neurotransmitter in several regions of the CNS, glutamate seems to be specifically implicated in the memory process. Long-term potentiation (LTP) and long-term depression (LTD), two forms of synaptic plasticity associated with learning and memory, both involve glutamate receptors. Studies with antagonists of glutamate receptors reveal a highly selective dependency of LTP and LTD on the N-methyl-D-aspartate and quisqualate receptors respectively. The therapeutic value of glutamate receptor antagonists is being actively investigated. The most promising results have been obtained in epilepsy and to some extent in ischaemia and stroke. The major drawback remains the inability of antagonists to permeate the blood-brain barrier when administered systemically. Efforts should be directed towards finding antagonists that are lipid soluble and able to cross the blood-brain barrier and to find precursors that would yield the antagonist intracerebrally.

Treatment with polyamines can prevent monosodium glutamate neurotoxicity in the rat retina

It has been previously shown that treatment of newborn rats with the polyamines putrescine, spermidine and spermine can rescue sympathetic neurons from naturally occurring cell death and from induced death after axotomy or immunosympathectomy. The present study demonstrates that polyamine treatment can also prevent the neurodegenerative effects in the retina and the loss of body weight caused by monosodium glutamate. The findings indicate that polyamine treatment may have a rather general beneficial effect on neuron survival.

The action of natural and synthetic isomers of quisqualic acid at a well-defined glutamatergic synapse

L-, D- and DL-quisqualic acid have been synthesized and their activities at the glutamatergic locust nerve-muscle junction have been compared with those of natural quisqualic acid and with glutamic acid. Two well-characterised locust nerve-muscle preparations were used in these studies, the retractor unguis nerve-muscle system and the extensor tibiae nerve-muscle system. The amino acids were tested on the whole nerve-muscle system in the former, when reduction in neurally evoked twitch contraction amplitude was the measured parameter, and by ionophoretic application to single excitatory junctional sites in the latter, when amplitude of junctional depolarization was the measured parameter. Synthetic L-quisqualic acid exhibited identical potency to its natural counterpart. However, D-quisqualic acid and DL-quisqualic acid were more active than expected from the known stereospecificity of this glutamatergic system towards D- and L-glutamic acid. The hydantoin analogue of quisqualic acid was inactive. X-ray crystallographic analysis of L-quisqualic acid and the hydantoin analogue showed that the ring junction in the former is pyramidal whereas in the latter it is planar. This may account for the high potency of L-quisqualic acid on a receptor system which identifies a partially folded conformation of L-glutamic acid. A pyramidal configuration of D-quisqualic acid would allow either rapid interconversion between active and inactive configurations at its ring junction or adoption of a trigonal configuration in solution. Either interpretation could explain the unexpected potency of D-quisqualic acid.

Reversible and irreversible neuronal damage caused by excitatory amino acid analogues in rat cerebellar slices

Slice preparations of the developing rat cerebellum were used to investigate the light and electron microscopic correlates of reversible and irreversible neuronal injury caused by the neurotoxic excitatory amino acid receptor agonists, kainate and N-methyl-D-aspartate. The slices were examined after various periods of exposure to the agonists (up to 30 min) with or without a 90 min recovery period in agonist-free medium. N-Methyl-D-aspartate (100 microM) caused necrosis of deep nuclear neurons and differentiating granule cells, the exposure times necessary to induce non-recoverable damage (leading to necrosis), being, respectively, 10 min and 20-30 min. Exposure periods of only 2-4 min with kainate (100 microM) were needed for Golgi cells to subsequently undergo necrosis. Other cell types (Purkinje, granule and deep nuclear neurons) were altered histologically by kainate but most recovered fully from 30 min exposures. Before the recovery period, the worst affected of these cells (deep nuclear neurons) displayed increased cytoplasmic and nuclear electron density and microvacuolation due to swelling of Golgi cisterns but little or no chromatin clumping or mitochondrial expansion. The neurons which were injured irreversibly by the agonists within 30 min displayed, near the time of lethal injury, increased cytoplasmic and nuclear electron lucency, marked focal aggregation of chromatin and swelling of Golgi apparatus. Mitochondrial swelling did not appear to precede lethal injury and even after exposure times sufficient, or more than sufficient, to lead to necrosis, large numbers of mitochondria remained in a condensed configuration. The significance of the histological changes is discussed and they are compared with those occurring in other pathological conditions. The time scales required for the receptor agonists to induce irreversible cellular lesions would be consistent with this being a process which is responsible for acute neuronal necrosis in the brain.

Ionic requirements for neurotoxic effects of excitatory amino acid analogues in rat cerebellar slices

The ionic requirements for the neurotoxic effects of N-methyl-D-aspartate and kainate in incubated slices of developing rat cerebellum were studied using light and electron microscopy. Under normal conditions, 30 min exposure to 100 microM N-methyl-D-aspartate followed by a 90 min recovery period in agonist-free medium resulted in the necrosis of differentiating granule cells and deep nuclear neurons, while the corresponding effect of 100 microM kainate was the death of Golgi cells. Substitution of 96% of the Cl- in the medium with isethionate did not prevent the toxicity of either agonist. However, all the ordinarily vulnerable cells survived and exhibited normal ultrastructure if the slices were exposed to the excitants in a Ca2+-free medium and were subsequently allowed to recover in a Ca2+-containing solution. Prior to this recovery period, granule, Golgi and deep nuclear neurons exposed to N-methyl-D-aspartate were markedly swollen but their mitochondria were hypercontracted and there was no clumping of chromatin or obvious swelling of the rough endoplasmic reticulum or Golgi apparatus, in contrast to observations made on slices exposed to this agonist in normal medium. Substitution of all the Na+ in the medium with a mixture of choline (118 mM) and Tris (25 mM) itself caused necrosis of granule cells and deep nuclear neurons and an intense microvacuolation of Purkinje cells, due, in large part, to high amplitude mitochondrial swelling. A low (25 mM) Na+ medium was well tolerated under control conditions. This medium protected granule cells but not deep nuclear neurons from the toxicity of N-methyl-D-aspartate and failed to prevent kainate-induced death of Golgi cells. It is concluded that the acute neurotoxic effects of the two excitatory amino acid receptor agonists in the slices are dependent on extracellular Ca2+ and are independent of extracellular Cl-. Where apparent, the protective effect of reducing extracellular Na+ on the toxicity of N-methyl-D-aspartate is likely to reflect the involvement of this ion in the primary depolarizing mechanism.

Amino acid neurotoxicity: relationship to neuronal depolarization in rat cerebellar slices

It has long been proposed that the excitatory and toxic properties of acidic amino acid receptor agonists are linked. To test this hypothesis, the depolarizing effects of quisqualate, kainate and N-methyl-D-aspartate in adult and immature rat cerebellar slices have been studied in relation to their neurotoxic effects in the same tissues (reported separately). A "grease-gap" method was used to measure the depolarizing responses of Purkinje cells and granule cells in lobule VI to the agonists. The depolarizing potencies of kainate and quisqualate were apparently similar on both cell types and at both ages studied although maximal responses to kainate were always larger. N-Methyl-D-aspartate was a very weak agonist in the adult slices but was much more effective in the immature tissues, apparently on both Purkinje cells and granule cells. Comparison of the depolarizing effects of the agonists with their neurotoxic effects on Purkinje cells and granule cells suggested that: (a) the ability to depolarize is a required condition for an agonist to be neurotoxic, (b) the magnitude of depolarization, rather than depolarizing potency, is the more pertinent determinant of neurotoxic potency and (c) resistance to the neurotoxicity of an agonist is not necessarily associated with resistance to its depolarizing actions. Histological studies indicated that the neurotoxicity of N-methyl-D-aspartate and kainate in immature cerebellar slices could largely not be replicated by veratridine (50 microM) or high extracellular K+ (124 mM) indicating that receptor-mediated ionic fluxes may be needed in addition to those caused by depolarization. Exposure of the slices to anoxia in the absence of glucose partially reproduced the toxicity of the receptor agonists. Application of ouabain for 30 min caused necrosis of all the cells which are vulnerable to the agonists but spared the cells which are not vulnerable. Profound ionic imbalance thus appears to be a sufficient explanation for amino acid neurotoxicity.

In vitro neurotoxicity of excitatory acid analogues during cerebellar development

The neurotoxic effects of the selective excitatory amino acid receptor agonists, quisqualate, kainate and N-methyl-D-aspartate, were studied in slice preparations of cerebellum from rats at different stages of postnatal development. With increasing age, (i) Purkinje cells became more vulnerable to kainate and quisqualate but remained insensitive to N-methyl-D-aspartate (up to 300 microM); (ii) Golgi cells became more sensitive to kainate, quisqualate and N-methyl-D-aspartate; (iii) granule cells became more sensitive to kainate, less sensitive to N-methyl-D-aspartate and remained unaffected by quisqualate (up to 100 microM), and (iv) basket and stellate cells and, up to 14 days of age, neurones of the deep cerebellar nuclei, became more vulnerable to kainate and quisqualate, but their sensitivity to N-methyl-D-aspartate stayed the same. The neurotoxicity of N-methyl-D-aspartate, but not that of kainate in 8-day-old cerebellar slices was prevented by 2-amino-5-phosphonovaleric acid; tetrodotoxin did not affect the toxicity of the agonists in 8-day-old or adult slices. The results with kainate are consistent with other studies indicating an insensitivity of the immature brain to its neurotoxic effects, but suggest that this property is not a peculiarity of kainate. Alterations in excitatory potency can explain some of the observed developmental changes. However, other observations cannot readily be accounted for on the basis of either changes in excitatory potency, the functional maturation of cerebellar circuits, changes in synaptic density, or the developmental appearance of Ca2+ channels in susceptible cells, suggesting that additional factors play an important role in the neurotoxic effects of the excitants.

The binding of acidic amino acids to snail, Helix aspersa, periesophagic ring membranes reveals a single high-affinity glutamate/kainate site

The characterization of specific acidic amino acid binding sites to snail, Helix aspersa, ganglia membranes has been assayed using tritiated glutamate (L-[3H]Glu), aspartate (L-[3H]Asp), cysteine sulfinate (L-[3H]CSA) and kainate. At 2 degrees C, only L-[3H]Glu and [3H]kainate specific binding could be measured using a filtration procedure to separate bound from free ligand. The analysis of L-[3H]Glu specific binding reveals the presence of one class of high-affinity binding sites with Kd = 0.12 microM and Bmax = 30 pmol/mg protein. This L-[3H]Glu binding was specific, reversible and saturable. The order of potency of different substances, agonists or antagonists of the rat brain excitatory amino acid receptors, has been determined. Kainate was the best displacing agent, followed by ibotenate = L-Glu greater than L-alpha-aminoadipate (L-alpha-AA) greater than homocysteate (HCA). Using 10 nM [3H]kainate, a single class of binding site was detected. Its pharmacological properties indicate that it is likely identical to the L-[3H]Glu binding site. This L-Glu-kainate site possesses most of the properties expected for a specific receptor. However, whereas L-[3H]Glu binding could be detected on purified neuronal membranes, the major component of specifically bound L-[3H]Glu appeared to be located on the sheaths surrounding neuronal cell bodies. These findings suggest that Glu or another endogenous acidic amino acid may function as a transmitter at neuromuscular junctions in Helix periesophagic ring, acting at a receptor distinct from those on nerve cells.

Glutamate neurotoxicity in cortical cell culture is calcium dependent

Brief exposure to glutamate produced widespread neuronal death in mature, but not young, cortical cell cultures. Extracellular sodium replacement or addition of tetrodotoxin produced only minor reduction in this toxic neuronal loss. However, removal of extracellular calcium markedly reduced neuronal loss, and elevation of extracellular calcium accentuated neuronal loss. These observations suggest that the toxicity of glutamate on cortical neurons may depend primarily on the presence of extracellular Ca, probably through a mechanism which is distinct from simple 'excitotoxicity'.

Comparison of the ultrastructural myopathy induced by anticholinesterase agents at the end plates of rat soleus and extensor muscles

Rats were treated with single subcutaneous injections of the irreversible AChE inhibitors, sarin (90 to 100 micrograms/kg) or soman (55 micrograms/kg), and with chronic doses of the reversible carbamate inhibitor, pyridostigmine. In surviving animals with severe behavioral symptoms, we examined the end-plate regions of the slow-twitch soleus and the fast-twitch extensor digitorum longus muscles, using the electron microscope. Within 30 min, sarin administration caused a recognizable subjunctional myopathy. The progress of morphologic damage was followed for 7 days, during which time the occurrence of damage diminished. The initial swelling of subjunctional organelles and vacuole generation progressed to the point where nerve terminals and attached postjunctional folds were lifted away from the muscle surface. This appeared to be caused by a combination of enlarging vacuoles and insertion of Schwann and macrophage cells into the lesions, and was followed by degeneration of the postjunctional folds. A new component of anti-AChE myopathy was recognized: progressive swelling of chromatin in subjunctional muscle nuclei. The soleus muscle was considerably more sensitive to these effects than the extensor muscle. Soman had a much less prominent ultrastructural effect on the muscle end plates. Chronic pyridostigmine treatment had effects similar to those of a single sarin injection on the soleus as well as a pronounced effect on the extensor muscle.

Maturation of kainic acid seizure-brain damage syndrome in the rat. II. Histopathological sequelae

The histopathological sequelae of parenteral administration of kainic acid were investigated in immature rats (3-35 days of age). The brains were fixed 1-14 days after the administration of kainate and the damage evaluated by means of argyrophylic (Fink-Heimer, Gallyas or Nauta-Gygax) and Nissl stains. In animals of less than 18 days of age there was no sign of damage even after 1-2 h of severe tonico-clonic convulsions. Between 18 and 35 days after birth, there was a progressive increase in the severity of the damage, the adult pattern being reached at the latter age. As in adult animals, brain damage was most severe in structures which are part of the limbic system, i.e. the hippocampal formation, lateral septum, amygdaloid complex, claustrum, piriform cortex, etc. In addition to neuronal abnormalities, the following reactions were observed: hypertrophy and swelling of satellite oligodendroglia, proliferation of hypertrophic microglia, proliferation of astroglia and hypertrophy of endothelial cells in the capillary wall. The latter type of change, together with local coagulative necrosis, was almost exclusively restricted to the granular and molecular layers of the fascia dentata. In the hippocampal formation we found a temporal gradient of vulnerability. The earliest and most consistent neuronal alterations were largely restricted to interneurons of the hilar region and to a lesser extent to non-pyramidal neurons of strata oriens and radiatum. The severe necrotic destruction of the pyramidal layer of CA3 is conspicuous at a later age (postnatal day 30-35) and with longer survival times. Our results suggest that: (1) the neurotoxin only induces brain damage once it also causes limbic motor seizures and its associated metabolic activations, notably in the amygdala; (2) the earliest pathological sequelae occur in interneurons of the hilar region and (3) sclerosis of the vulnerable region of the Ammon's horn--the CA3 region--is only obtained once the dentate granules and their mossy fibres are fully operational, thereby reflecting the crucial role of this axonal connection in eliciting hippocampal damage.

The nature of the chick's magnesium-sensitive retinal spreading depression

Spreading depression (SD) in the chick retina is completely suppressed by 10 mM MgCl2 in the bathing solution (Mg-sensitive SD). However, after increasing the KCl concentration in the Mg solution to values between 10 and 20 mM the retina can again exhibit SDs (Mg-insensitive SD). It has been postulated that the Mg-sensitive SD is a glutamatergic phenomenon. This is supported by the effect of four gl(utamate)-antagonists--L-proline, glutamic acid diethyl ester (GDEE), D-alpha-aminoadipate (D-AA), and 2-amino-4-phosphonobutyrate (APB)--which all suppressed this type of SD. It was suggested that this effect is due to competitive binding of glutamate involved in the Mg-sensitive SD and the gl-antagonist to glutamate receptors. The suppression of SD could be reversed by washing the preparation in a physiologic salt solution. The gl-antagonists in relatively high concentrations had a cytotoxic effect which, when severe, suppressed SD and prevented the recovery of this phenomenon by washing the compound out of the tissue. The compounds examined had, in addition to their gl-antagonistic properties, a gl-agonistic effect, which was postulated to enhance the Na+ permeability of neural membranes resulting in a release of K+ into the extracellular space. In preparations bathed in 10 mM MgCl2 (which suppresses Mg-sensitive SDs) the four compounds investigated promoted Mg-insensitive SDs supposedly when the extracellular K+ concentration reached values between 10 and 20 mequiv.

Calcium accumulation precedes the degenerative effects of L-glutamate on locust muscle fibres

Agonist-induced degeneration of locust muscle occurs only when desensitization of the excitatory glutamate receptors present on this tissue is inhibited. It has been suggested that an increase in intracellular Ca2+ is responsible for this degeneration. To test this proposal the accumulation of 45Ca by locusts muscle has been studied under various conditions, including those under which receptor desensitization was inhibited. Retractor unguis muscles from the metathoracic leg of locusts (Schistocerca gregaria) were used in these studies. All muscles exposed to L-glutamate exhibited an early increase in intracellular radioactivity but this was 2-3 times greater in muscle pretreated with concanavalin A (Con A) to block receptor desensitization. In the desensitizing system the increase in muscle radioactivity was not maintained, intracellular Ca2+-levels declining to control values after 30 min in 45Ca-saline-containing glutamate. In Con A-treated muscles intracellular Ca2+-levels plateaued well above control levels within 5 min of exposure to glutamate and were maintained at these levels throughout the period of glutamate treatment. These results support the contention that agonist-induced degeneration of locust muscle is triggered by entry of Ca2+ and a rise in intracellular concentration of this cation to a toxic level.