A metabotropic glutamate receptor agonist does not mediate neuronal degeneration in cortical culture

Phospholipase A(2) activity of beta-bungarotoxin is essential for induction of cytotoxicity on cerebellar granule neurons

The aim of this study was to investigate the mechanism of the cytotoxic effect of beta-bungarotoxin (beta-BuTX), a presynaptic neurotoxin, on rat cerebellar granule neurons (CGNs). The maturation of CGNs is characterized by the prominent dense neurite networks that became fragmented after treatment with beta-BuTX, and this cytotoxic effect of beta-BuTX on CGNs was in a dose- and time-dependant manner. The cytotoxic effect of beta-BuTX was found to be more potent than other toxins, such as alpha-BuTX, cardiotoxin, melittin, and Naja naja atra venom phospholipase A(2). Meanwhile, undifferentiated neuroblastoma neuronal cell lines, IMR-32 and SK-N-MC, and astrocytes were found to be resistant to beta-BuTX. These results indicated that only the mature CGNs were sensitive to beta-BuTX insults. None of the following chemicals: antioxidants, K(+)-channel activator, K(+)-channel antagonists, intracellular Ca(2+) chelator, Ca(2+)-channel blockers, NMDA receptor antagonists, and nitric oxide synthase inhibitor tested, were able to reduce beta-BuTX-induced cytotoxicity. However, secretory type phospholipase A(2) inhibitors (glycyrrhizin and aristolochic acid) and a free radical scavenger (5,5-dimethyl pyrroline N-oxide, DMPO) could attenuate not only beta-BuTX-induced cytotoxicity but also ROS production and caspase-3 activation. These data suggest that phospholipase A(2) activity of beta-BuTX may be responsible for free radical generation and caspase-3 activation that accounts for the observed cytotoxic effect. It is proposed that the CGNs can be a useful tool for studying interactions of the molecules on neuronal plasma membrane with beta-BuTX that mediates the specific cytotoxicity.

Efficacy of Choto-san on vascular dementia and the protective effect of the hooks and stems of Uncaria sinensis on glutamate-induced neuronal death

Two different multicenter studies on the efficacy of Choto-san on patients with vascular dementia, one a well-controlled but non-double blind (60 patients), the other a double-blind controlled study (139 patients), were performed. In the well-controlled study, Choto-san was superior in global improvement rating, utility rating and improvement of subjective symptoms, psychiatric symptoms and disturbance in daily living activities. In the double-blind study, with more objective criteria than the well-controlled study, Choto-san was also superior in global improvement rating, utility rating and improvement of subjective symptoms, psychiatric symptoms and disturbance in daily living activities. These results suggest that Choto-san is effective in the treatment of vascular dementia. Uncaria sinensis (OLIV.) HAVIL. (US) is the main medicinal plant composing Choto-san. Glutamate-induced cell death of cultured cerebellar granule cells was protected by the application of water extract of US in a dose-dependent manner, and concentrations of 10(-5) to 10(-4) g/ml had a significant effect compared to exposure to glutamate only. Further, the increase of 45Ca2+ influx into cells by glutamate was also blocked by the water extract in a dose-dependent manner. These results suggest that US has a protective effect on glutamate-induced neuronal death in cultured cerebellar granule cells through the inhibition of Ca2+ influx.

NMDA and non-NMDA receptor-mediated excitotoxicity are potentiated in cultured striatal neurons by prior chronic depolarization

The excitatory input from cortex and/or thalamus to striatum appears to promote the maturation of glutamate receptors on striatal neurons, but the mechanisms by which it does so have been uncertain. To explore the possibility that the excitatory input to striatum might influence glutamate receptor maturation on striatal neurons, at least in part, by its depolarizing effect on striatal neurons, we examined the influence of chronic KCl depolarization on the development of glutamate receptor-mediated excitotoxic vulnerability and glutamate receptors in cultured striatal neurons. Dissociated striatal neurons from E17 rat embryos were cultured for 2 weeks in Barrett's medium containing either low (3 mM) or high (25 mM) KCl. The vulnerability of these neurons to NMDA receptor agonists (NMDA and quinolinic acid), non-NMDA receptor agonists (AMPA and KA), and a metabotropic glutamate receptor agonist (trans-ACPD) was examined by monitoring cell loss 24 h after a 1-h agonist exposure. We found that high-KCl rearing potentiated the cell loss observed with 500 microM NMDA or 250 microM KA and yielded cell loss with 250 microM AMPA that was not evident under low KCl rearing. In contrast, neither QA up to 5 mM nor trans-ACPD had a significant toxic effect in either KCl group. ELISA revealed that chronic high KCl doubled the abundance of NMDA NR2A/B, AMPA GluR2/3, and KA GluR5-7 receptor subunits on cultured striatal neurons and more than doubled AMPA GluR1 and GluR4 subunits, but had no effect on NMDA NR1 subunit levels. These receptor changes may contribute to the potentiation of NMDA and non-NMDA receptor-mediated excitotoxicity shown by these neurons following chronic high-KCl rearing. Our studies suggest that membrane depolarization produced by corticostriatal and/or thalamostriatal innervation may be required for maturation of glutamate receptors on striatal neurons, and such maturation may be important for expression of NMDA and non-NMDA receptor-mediated excitotoxicity by striatal neurons. Striatal cultures raised under chronically depolarized conditions may, thus, provide a more appropriate culture model to study the role of NMDA or non-NMDA receptor subtypes in excitotoxicity in striatum.

The neural mechanisms underlying cholinergic cell death within the basal forebrain

The basal forebrain region includes a large group of cholinergic neurons within the medial septal area and nucleus basalis magnocellularis (NBM) that project to the hippocampus and throughout the neocortex, respectively. This chapter will consider the mechanisms that influence why cholinergic cells within the NBM die and discuss studies that have manipulated the features of these cells that could make them differentially vulnerable to degeneration with aging and Alzheimer's Disease (AD). This chapter will focus upon the NBM cholinergic system because this regions typically demonstrates a greater degree of cell loss with aging and AD.

Mechanisms of neuronal damage in brain hypoxia/ischemia: focus on the role of mitochondrial calcium accumulation

Following a hypoxic-ischemic insult, the collapse of ion gradients results in the inappropriate release of excitatory neurotransmitters. Although excitatory amino acids such as glutamate are the likely extracellular mediators of the ensuing neuronal cell death, the intracellular events occurring downstream of glutamate receptor activation are much less clear. The present review attempts to summarize how Ca2+ overload of neurons following a hypoxic-ischemic insult is neurotoxic. In particular, the interlocked relation between mitochondrial Ca2+ accumulation and subsequent neuronal cell death is examined.

Metabotropic glutamate receptor activation in vivo induces intraneuronal amyloid immunoreactivity in guinea pig hippocampus

Stimulation of metabotropic glutamate receptors in vitro has been shown to accelerate the breakdown of amyloid precursor protein (APP) to form increased production of non-amyloidogenic secreted APP (sAPP). The mechanism whereby this occurs is not entirely clear but it is presumed to be linked to generation of diacylglycerol and activation of protein kinase C because other neurotransmitter receptors such as m1 and m3 muscarinic receptors, known to be coupled to this second messenger cascade, likewise increase sAPP production. Although it is presumed that a reciprocal relationship exists between the formation of amyloid beta protein (Abeta) and the production of sAPP, recent evidence suggests alternative processing can occur. Given the fact that much of the observations on APP metabolism have been made in vitro we sought to investigate the effect of metabotropic receptor activation on Abeta in vivo in a species known to contain the same amino acid sequence of Abeta as found in humans. Intrahippocampal injection of the mGluR agonist 1S,3R-ACPD in guinea pigs produced neurodegeneration of CA1 hippocampal pyramidal neurons at 12 h postinjection. Immunocytochemistry of sections from ACPD injected animals using selective antibodies to Abeta revealed the presence of punctate intraneuronal granules in pyramidal neurons of the hippocampus. These structures appeared to be localized within the nucleus and were particularly prominent in neurons within the region of neurodegeneration. Immunoreactivity was not observed in vehicle injected controls nor in sections from ACPD injected animals stained with preadsorbed antiserum. Abeta immunodetection was correlated with the onset of neurodegeneration since animals evaluated at 1 h and 4 h postinjection lacked both Abeta immunoreactivity as well as neurodegeneration. Evaluation of animals injected with NMDA revealed neurodegeneration but no Abeta immunoreactivity suggesting Abeta formation did not appear to be due to non-selective excitotoxicity. Staining of sections with antibodies directed to various regions of APP demonstrated increased C-terminal APP immunoreactivity in pyramidal neurons in the vicinity of degeneration. These data support recent in vitro studies illustrating that Abeta can be found intracellularly within neurons.

Regulation of the nigrostriatal pathway by metabotropic glutamate receptors during development

Dopamine neurons in the substantia nigra heavily innervate the striatum, making it the nucleus with the highest levels of dopamine in the adult brain. The present study analyzes the time course and the density of striatal innervation by nigral dopamine neurons and characterizes the role of the neurotransmitter glutamate during the development of the nigrostriatal pathway. For this purpose, organotypic cultures containing the cortex, the striatum, and the substantia nigra (triple cultures) were prepared from rat brains at postnatal day (PND) 0-2 and were cultured for up to 60 d in vitro (DIV). Dopamine fibers and neurons were labeled using tyrosine hydroxylase (TH) immunohistochemistry. Striatal TH-ir fiber density was quantitatively analyzed using confocal laser scanning microscopy (CLSM). In long-term triple cultures (44 +/- 3 DIV), the striatal dopamine fiber density was high and was weakly correlated with the number of nigral dopamine neurons. The high striatal dopamine fiber density mainly resulted from an enhanced ingrowth and ramification of dopamine fibers from nigral neurons during 8-17 DIV. The metabotropic glutamate receptor (mGluR) antagonist L(+)-2-amino-3-phosphonopropionic acid (L-AP-3) selectively inhibited this dopaminergic innervation of the striatum, whereas ionotropic GluR antagonists had no effect. The L-AP-3-mediated inhibition was prevented by the mGluR agonist 1S, 3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). The inhibition of the striatal dopaminergic innervation by L-AP-3 was further confirmed by anterograde tracing of the nigrostriatal projection with Phaseolus vulgaris leucoagglutinin. These results indicate that glutamate, by acting on group I mGluRs, plays an important "trophic" role for the development of the nigrostriatal dopamine pathway.

Regulation of striatal dopamine release by metabotropic glutamate receptors

In vivo microdialysis in conscious rats was used to assess the effect of metabotropic glutamate receptor stimulation on striatal dopamine release. Local application of the metabotropic glutamate agonist (+/-)-trans-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), via a microdialysis probe, produced a concentration-dependent response: infusion of 50 microM ACPD did not produce a significant effect on extracellular dopamine levels, while application of 100 microM or 500 microM ACPD increased dopamine release by approximately 50% or 100%, respectively. To examine the contribution of impulse flow and multisynaptic mechanisms to the ACPD-induced increase in dopamine release, 500 microM ACPD were coapplied with 2 microM tetrodotoxin (TTX). An increase in extracellular dopamine levels was observed after the application of 500 microM ACPD, despite the presence of TTX. To further study the actions of metabotropic glutamate receptor-stimulation on terminal release characteristics of dopamine, the effect of ACPD on 40 mM K+-stimulated dopamine release was investigated. It was found that application ofACPD reduces dopamine release in response to K+ stimulation. These data suggest that during basal conditions, metabotropic glutamate receptor activation facilitates striatal dopamine release, possibly through presynaptic, impulse-independent mechanisms. However, during conditions of hyperstimulation, activation of metabotropic receptors, in contrast to ionotropic receptors, reduces excess dopamine release.

Factors that reverse the persistent depolarization produced by deprivation of oxygen and glucose in rat hippocampal CA1 neurons in vitro

In CA1 pyramidal neurons in rat hippocampal tissue slices, superfusion with ischemia-simulating medium produced a rapid depolarization after 6 min of exposure. The membrane potential eventually reached 0 after 5 min (a persistent depolarization), even when oxygen and glucose were reintroduced. The role of various ions in the reversal of this persistent depolarization after reintroduction of oxygen and glucose was investigated. The peak of the persistent depolarization was decreased in solutions containing reduced Na+ or Ca2+ and in solutions containing Co2+ or Ni2+. In contrast, the depolarization was not affected by reduction of external K+ or Cl- or by addition of tetrodotoxin (TTX), flunarizine, or nifedipine. These results suggest that sustained Na+ and Ca2+ influxes produce the persistent depolarization. The membrane potential recovered after reintroduction of oxygen and glucose in low Ca2+, low Cl-, or K+-rich medium and in TTX- or tetraethylammonium-containing medium, but not in low Na+ or low K+ medium and in flunarizine- or nifedipine-containing medium. Either reduction in extracellular Ca2+ or addition of Co2+ was the most effective in promoting recovery from the persistent depolarization, suggesting that Ca2+ influx has a key role in causing the membrane dysfunction. The peak of the persistent depolarization was reduced by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), DL-2-amino-5-phosphonopentanoic acid (AP5), DL-amino-3-phosphonopropionic acid (AP3), or DL-amino-4-phosphonobutyric acid, suggesting that activation of non-N-methyl-D-aspartate (non-NMDA), NMDA, and metabotropic glutamate (Glu) receptors is involved in the generation and maintenance of the persistent depolarization. Among these Glu receptor antagonists, only CNQX or AP5 was able to reduce dose dependently the level of depolarization, suggesting that Ca2+ influx via both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate type II receptors and NMDA receptors contributes to the membrane dysfunction. trans-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) did not affect the peak potential of the persistent depolarization, but it dose-dependently restored the membrane potential. AP3 antagonized the protective action of t-ACPD. The membrane potential also recovered after reintroduction when the slice was pretreated by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester, ryanodol 3-(1H-pyrrole-2-carboxylate), 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride, and procaine, suggesting that raised [Ca2+]i from Ca2+-induced Ca2+ release pool contributes to the membrane dysfunction. It, therefore, is concluded that raised [Ca2+]i has a dominant role in causing irreversible changes. The increase in [Ca2+]i during the persistent depolarization may be the result of Ca2+ entry via both a leaky membrane and Glu-activated receptor channels as well as Ca2+ released from internal stores.

Acute and chronic effects of estrogenic compounds on glutamate-stimulated phosphatidylinositol metabolism in primary neuronal cultures

Glutamate (Glu)-stimulated phosphatidylinositol (PI) metabolism in primary neuronal cultures was found to be modulated by acute and chronic treatment with two estrogenic compounds. 17 beta-Estradiol 3-benzoate (0.1 and 1 microM), when applied with Glu, significantly reduced Glu (40 microM)-stimulated PI metabolism by 20-36%, an effect not seen with 17 alpha-estradiol. The weak estrogen phenol red (20 microM), had no effect when added immediately before Glu stimulations. Two-week pretreatment with 17 beta-estradiol 3-benzoate (1 microM) resulted in a significant decrease in Glu-stimulated PI metabolism (10-100 microM). Chronic treatment with 20 microM phenol red, at a concentration commonly found in culture medium, resulted in parallel but not statistically significant effects to those observed with chronic estradiol treatment. Estrogenic compounds may modulate the excitatory responses of neurons by both genomic and non-genomic means.

DCG-IV selectively attenuates rapidly triggered NMDA-induced neurotoxicity in cortical neurons

Molecular cloning has revealed the existence of at least eight subtypes of metabotropic glutamate receptors (mGluRs). We examined the effect of (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), a selective agonist of the mGluR 2/3 subtype, on excitotoxicity in mouse cortical cell cultures. Addition of DCG-IV to the exposure medium partially attenuated the rapidly triggered excitotoxic death induced by a 5 min exposure to 200 microM NMDA. This neuroprotective effect was reversed by coapplication of alpha-methyl-4-carboxyphenylglycine (MCPG), an antagonist of mGluRs, by pertussis toxin pretreatment and also by preincubation with dibutyryl cAMP, a stable analogue of cAMP. These results suggest that the activation of mGluR 2/3 is neuroprotective in our system. However, DCG-IV did not attenuate the slowly triggered neuronal death induced by 24 h exposure to low concentrations of NMDA, alpha-amino-1,3-cyclopentanedicarboxylic acid (AMPA) or kainate. The failure of DCG-IV to block slowly triggered NMDA neurotoxicity is likely due to weak NMDA agonist activity, as demonstrated in whole-cell recording.

Neuroprotection and selective vulnerability of neurons within the nucleus basalis magnocellularis

Neurons within the nucleus basalis may die due to their selective vulnerability to endogenous excitatory amino acid neurotransmitters, nitric oxide and free radicals. The factors influencing the selective vulnerability of neurons within the nucleus basalis depend upon many different factors related to the presence of these agents and the neuron's ability to defend itself against the consequences of exposure. Many different mechanisms have been investigated to provide neuroprotection for neurons within the nucleus basalis and throughout the central nervous system. This review summarizes the results of studies that have investigated our current capability to either attenuate the neurotoxicity of endogenous excitatory amino acids, such as glutamate, or to provide effective neuroprotection during circumstances of neurotoxin exposure.

Nitric oxide-mediated death of cultured neonatal retinal ganglion cells: neuroprotective properties of glutamate and chondroitin sulfate proteoglycan

The release of nitric oxide and stimulation of glutamate receptors by excitatory amino acids has been linked to neuronal degeneration and toxicity. In the rat retina approximately 60% of retinal ganglion cells (RGCs) die during the first postnatal week. In this study we examined the effects of nitric oxide synthase blockers and glutamate on the survival of neonatal RGCs in vitro over a 16 h assay period. Less than 10% of P1 RGCs survived in serum free defined media alone (control), however survival was increased, in a dose-dependent manner, when L-glutamate (10 microM-10 mM) was added to the media; a maximum of 70% of RGCs could be maintained with the addition of 5 mM glutamate. This effect was blocked by the NMDA and non-NMDA receptor blockers APV and DNQX and was age dependent; the survival of RGCs from P5 but not P7 rats was enhanced by the addition of glutamate even in high calcium concentrations (10 mM). When the nitric oxide synthase blockers L-NAME (5 mM) or haemoglobin (25 microM) were added to the culture media, up to 61% of P1 RGCs survived. The addition of the 480 kDa chondroitin sulfate proteoglycan (SCCP) previously shown to enhance RGC survival in vivo and in vitro, potentiated the action of glutamate and L-NAME and increased RGC survival to over 90% with almost all RGCs expressing a profusion of processes. These results suggest that the release of nitric oxide and glutamate by cells within the retina may contribute to the regulation of RGC numbers in vivo during development.

Activation of the metabotropic glutamate receptor is neuroprotective during nitric oxide toxicity in primary hippocampal neurons of rats

Metabotropic glutamate receptors (mGluRs) can influence neuronal survival and have been shown to be neuroprotective during glutamate toxicity in retinal cells and in cortical neurons. The mechanisms that mediate protection by this group of receptors are not clear. Since nitric oxide (NO) production can lead to neuronal cell death during excessive glutamate release, we examined whether neuronal survival was directly linked to mGluR activity and the NO pathway. Treatment with the mGluR4 receptor subtype agonist, L-(+)-2-amino-4-phosphonobutyric acid, in hippocampal cell cultures protected neurons during NO exposure. Treatment with L-(+)-2-amino-3-phosphonopropionic acid, an antagonist of the mGluR1 receptor subtype and inhibitor of inositol trisphosphate formation, did not significantly alter neuronal survival during NO administration. We conclude that activation of the mGluR4 receptor protects hippocampal neurons from NO toxicity and that the mechanism of NO induced neurodegeneration does not appear to involve inhibition of the mGluR1 receptor subtype activity or the phosphoinositide system.

Roles of metabotropic glutamate receptors in brain plasticity and pathology

In summary, the mGluRs are a large family of receptor subtypes with diverse properties in terms of transduction coupling, pharmacology, and anatomical distribution. Many divergent studies have demonstrated that activation of these receptors can result in either neuroprotection or neuropathology. We hypothesized that the mGluRs of astrocytes may have a role in determining the response following administration of mGluR agonists in vivo, and we have defined a suitable in vitro model for the study of these receptors. The experimental plasticity demonstrated in the astrocyte culture model may represent a more general principle that conditions in the microenvironment may differentially alter mGluR subtype expression as part of development, functional specialization, or pathology. This astrocyte model of receptor regulation provides a system suitable for studying the effects of specific growth factors, neurotrophins, cytokines, and other substances released by neurons and glia that may act in both autocrine and paracrine fashions. Alteration in the ratios of receptors by such variables could then modify future signaling properties and neuroglial interactions, a form of conditioning of the astrocytic response that would alter the physiological output following glutamate release. One measure of the value of this model will be its usefulness in stimulating the generation of hypotheses that can be tested in vivo. For example, the morphology of the astrocytes when cultured in the defined medium has similarities to the morphology of astrocytes undergoing reactive gliosis in pathological states. It is also interesting to note that treatments that have been reported to increase excitatory amino acid-stimulated PI hydrolysis in ex vivo brain slices (lesions, ischemia, and kindling) are accompanied by reactive gliosis. Those findings combined with the present in vitro results lead us to speculate that mGluR5 expression may also be altered in vivo during reactive gliosis. If so, it will be important to examine the functional consequences of such a change with regard to the astrocytic response to injury and maintaining the balance between excitatory transmission and excitotoxicity.

The metabotropic glutamate receptors: structure and functions

Glutamate is the main excitatory neurotransmitter in the brain. For many years it has been considered to act only on ligand-gated receptor channels--termed NMDA, AMPA and kainate receptors--involved in the fast excitatory synaptic transmission. Recently, glutamate has been shown to regulate ion channels and enzymes producing second messengers via specific receptors coupled to G-proteins. The existence of these receptors, called metabotropic glutamate receptors, is changing our views on the functioning of fast excitatory synapses.

Excitotoxicity of glutamate and four analogs in primary spinal cord cell cultures

Continuous glutamate exposure produced widespread neuronal damage in mixed whole dissociated murine spinal cord cell cultures. Ethidium bromide and acridine orange staining revealed that a 24 h glutamate exposure produced nearly 98% neuronal cell death but the underlying glia were spared. Continuous exposure to glutamate, N-methyl-D-aspartate (NMDA), kainate and quisqualate produced time-dependent and dose-dependent cell death as measured by the assay of lactate dehydrogenase activity in the cell culture media. Glutamate (500 microM), NMDA (100 microM) and kainate (500 microM) were equally neurotoxic. In contrast, quisqualate (100 microM) was only partially neurotoxic compared to the other glutamate analogs. The neurotoxicity of glutamate was blocked by the NMDA antagonist, MK-801. The neurotoxicity of kainate and quisqualate was blocked with the non-NMDA antagonist CNQX. Continuous exposure to (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) was not neurotoxic, even at concentrations up to 1 mM.

Investigations of neurotoxicity and neuroprotection within the nucleus basalis of the rat

The present study investigated the specific ways by which cytotoxicity due to glutamate receptor stimulation could be attenuated by the administration of agonists and antagonists of the ionotropic and metabotropic glutamate receptors within the nucleus basalis magnocellularis (NBM) of rats as measured by cortical choline acetyltransferase activity. The results of these studies suggest that (1) the cytotoxicity of ibotenate to NBM cholinergic cells is not dependent upon stimulation of metabotropic glutamate receptors, but results from activation of N-methyl-D-aspartate (NMDA) receptors, (2) the cytotoxicity of quisqualate to cholinergic cells within the NBM is not dependent upon stimulation of NMDA or metabotropic receptors, and (3) the cytotoxicity of NMDA was prevented by administration (i.p.) of the un-competitive NMDA antagonist memantine (30 mg/kg), resulting in plasma levels of 2.5 micrograms/ml, a concentration known to block efficiently NMDA receptors in vitro. Finally, performance of a food-motivated, delayed-alternation task on a T-maze was impaired by injections of NMDA into the NBM, but was prevented by co-administration of NMDA with memantine.

Glutamate-induced neuronal death in cerebellar culture is mediated by two distinct components: a sodium-chloride component and a calcium component

The relative contribution of sodium, chloride and calcium ions in the neuronal death induced by glutamate is controversial. We have therefore reassessed the effects of extracellular ion substitution on glutamate-induced neuronal death in cerebellar granule cell culture. Sodium or chloride substitution by impermeant ions prevented the initial swelling observed after glutamate exposure (100 microM, 15 min) in balanced salt solution but did not prevent the progressive degeneration of cerebellar neurons over the next few hours. In low calcium medium, glutamate exposure also led to degeneration of granule neurons. In contrast, sodium or chloride substitution and calcium omission prevented both the initial swelling and the delayed neuronal death after glutamate exposure. These morphological observations were confirmed both by measurement of the intracellular water space with [3H]methylglucose and by quantification of cell viability by 3-(4,5-dimethylthiazol-2-yl-)-2,5-diphenyl tetrazolium bromide (MTT) staining. We conclude that glutamate-induced neuronal death is mediated by two distinct components: a calcium-independent sodium-chloride dependent component and a calcium-dependent component. Each one of these components leads to the death of cerebellar neurons after glutamate exposure.

Brain content of glycosphingolipids after oral administration of monosialogangliosides GM1 and LIGA20 to rats

Natural (GM1) and semisynthetic [113-Neu-5-AcGgOse4-2-D-erythro-1,3- dihydroxy-2-dichloroacetylamide-4-trans-octadecene (LIGA20)] glycosphingolipids, given parenterally, protect neurones against glutamate-induced death without producing the side effects typical of glutamate receptor antagonists. Chronic glutamate-related neurotoxicity (e.g., in recurring strokes in elderly hypertensive patients, and in Parkinson disease) could be prevented also by glycosphingolipids treatment, but this therapeutic intervention will require a protracted administration of orally active glycosphingolipids. Here we demonstrate that 3-6 h after oral administration of 68 mumol/kg of LIGA20 and GM1 to rats, the brain content of LIGA20 is 50-fold higher than that of GM1. The brain concentration of LIGA20 remains elevated for at least 12-24 h. Because the LIGA20 that reaches the brain is slowly metabolized, repeated oral administrations of this glycosphingolipid can yield to its accumulation in brain, and can yield various brain levels depending on the dose and frequency of drug administration. In contrast this is not possible with GM1, which given orally for 7 d, cannot accumulate in brain in pharmacologically significant concentrations.

(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid attenuates N-methyl-D-aspartate-induced neuronal cell death in cortical cultures via a reduction in delayed Ca2+ accumulation

The effects of (1S,3R)-ACPD, a selective metabotropic glutamate receptor agonist, on NMDA-induced 45Ca2+ accumulation and delayed neuronal cell death were determined using primary cerebrocortical cultures. Exposure to (1S,3R)-ACPD alone, although causing small increases in 45Ca2+ accumulation, was not neurotoxic. The presence of (1S,3R)-ACPD during exposure to NMDA attenuated the resulting sustained accumulation of 45Ca2+ and delayed neuronal cell death. Reductions in sustained Ca2+ accumulation were associated both with Ca2+ efflux, in the absence of cell death, and inhibition of delayed intracellular Ca2+ accumulation. The protective effects of (1S,3R)-ACPD on NMDA-induced cell death were inhibited by pretreatment of cultures with pertussis toxin. These results suggest that activation of metabotropic glutamate receptors may stimulate intracellular processes capable of limiting sustained elevations in intracellular calcium and the resulting excitotoxic neuronal damage.

Attenuation of excitatory amino acid toxicity by metabotropic glutamate receptor agonists and aniracetam in primary cultures of cerebellar granule cells

Activation of glutamate ionotropic receptors represents the primary event in the neurotoxicity process triggered by excitatory amino acids. We demonstrate here that the concentration-dependent stimulation of metabotropic glutamate receptor (mGluR) by the selective agonist trans-1-aminocyclopentane-1,3-dicarboxylate or by quisqualate counteracts both glutamate- and kainate-induced neurotoxicity in primary cultures of rat cerebellar granule cells. The mGluR-evoked responses are potentiated by aniracetam, which per se also elicits neuroprotection. Aniracetam concentration-dependently counteracted glutamate-, kainate-, or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced cell death and greatly facilitated neuroprotective response achieved by different concentrations of both quisqualate and trans-1-aminocyclopentane-1,3-dicarboxylate. In addition, aniracetam potentiated the mGluR-coupled stimulation of phospholipase C, as revealed by the measurement of 3H-inositol phosphate formation. Thus, mGluRs could be a suitable target for novel pharmacological strategies pointing to the treatment of neurodegenerative diseases.

Superoxide and nitric oxide cooperation in hypoxia/reoxygenation-induced neuron injury

Oxygen-derived free radicals are implicated in hypoxia- and reoxygenation-related brain injury. In addition, excitatory amino acid neurotransmitters seem to be involved in this neurotoxicity and could act through the L-arginine/nitric oxide (NO) synthase pathway. In the present study we have used rat forebrain neurons in culture submitted to hypoxia/reoxygenation to investigate the relative role of free radicals, glutamate, and nitric oxide in hypoxic neuronal injury. Hypoxia (5 h) followed by reoxygenation (0-24 h) induced cell damage assessed by lacticodehydrogenase release into culture medium. Superoxide dismutase (SOD, 500 U/mL), D-L-2-amino-5-phosphonovaleric acid (100 microM), a glutamate receptor antagonist, and NG-nitro-L-arginine (100 microM), an NO synthase inhibitor, protected the neurons. The effect of NG-nitro-L-arginine was reversed by adding L-arginine (10 mM) in the culture medium, and hemoglobin, which scavenges NO, also afforded protection. Hypoxia (5 h) provoked glutamate release from neurons, and this effect was inhibited by SOD. Exogenous glutamate (1-100 microM) induced lacticodehydrogenase release, and this effect was inhibited by glutamate antagonism, NO synthase inhibition, or superoxide radical scavenging. These data are consistent with the following sequence of events in hypoxia-related neurotoxicity: free radical formation, glutamate release, and activation of NO synthase leading to superoxide and NO cooperative toxicity.

In rats, the metabotropic glutamate receptor-triggered hippocampal neuronal damage is strain-dependent

The effect of intrahippocampal (i.h.) and intraocular (i.o.) administration of the selective metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) was studied in different rat strains. A massive hippocampal damage was observed in CD/SD and Fischer 344 but not in SD/Rij and Brown Norway rats 7 days following the i.h. injection of 1S,3R-ACPD, while no retinal damage was observed following its i.o. administration. Moreover, 1S,3R-ACPD reduced the N-methyl-D-aspartate (NMDA) toxicity in the retina of both CD/SD and SD/Rij rats. Regardless of its toxic action on hippocampal neurons the i.h. injection of 1S,3R-ACPD caused an acute stimulation of motor activity in both CD/SD and SD/Rij rats. This effect was blocked by the intracerebroventricular (i.c.v.) administration of the putative mGluR antagonist L-2-amino-3-phosphono-propionic acid (L-AP3). It is suggested that the differential expression of mGluR subtypes might determine their role in brain pathology.

Excitotoxicity, energy metabolism and neurodegeneration

There is increasing evidence that the neurotoxic effects of excitatory amino acids and their analogues are part of the pathogenesis of neuronal degeneration in acute and chronic neurological disease. Recent studies indicate that activation of excitatory amino acid receptors is also induced in the mechanism of neuronal damage induced by impairment of cellular energy metabolism. This article briefly summarizes the evidence for the presence of such a mechanism and discusses metabolic diseases in which excitatory amino acids alone or in combination with energy deficiency could play a pathogenetic role. In these and other metabolic diseases, antagonists to excitatory amino acid receptors may offer a therapeutic opportunity; however, there are potential limits that may prevent chronic use.

Cellular alterations produced by the experimental increase in intracellular calcium and the nature of protective effects from pretreatment with nimodipine

The immortalized septal cell line, SN56 B5 G4, generated by the fusion of mouse septal area cells and neuroblastoma cells, was used to determine if nimodipine, an antagonist of voltage sensitive calcium 'L' channels, might act in a neuroprotective fashion when intracellular calcium levels were raised by incubation in ouabain and monensin. Fluorescent indicator dyes and the automated spectrofluorometer, the CytoFluor 2300, were used to analyze specific cellular targets and functions affected by ouabain and monensin and possible protection by prior incubation with nimodipine. Ouabain and monensin were used together to create a time- and dose-dependent toxic episode. Increases in the emission intensity of Fluo3-AM demonstrated that the concentration of intracellular calcium was monotonically increased by increasing levels of ouabain-monensin. The calcein-AM fluorescent probe indicated that there were no changes in plasma membrane permeability during the toxic episode. Lysosomal integrity decreased as indicated by decreases in neutral red retention. The concentration of free radicals increased as shown by the increase in emission intensity of 2',7'-dichlorfluorescein. Nimodipine pretreatment of the cells incubated with ouabain and monensin resulted in apparent protection of lysosomes and a reduction in the level of free radicals. While nimodipine, by itself, produced a small decrease in intracellular calcium, it actually augmented the ouabain-monensin induced increase in intracellular calcium. The data suggest that in immortalized septal cells, (a) nimodipine offers protection to certain of the responses induced by ouabain-monensin, (b) the protection offered by nimodipine may be independent of antagonism of voltage sensitive calcium channels, and (c) that the protective changes can occur at the same time that intracellular calcium is increasing. These latter observations question the hypothesis that the protection against cell death and dysfunction offered by nimodipine is due solely to maintaining calcium homeostasis.

Neuroprotective synergism of 2-amino-3-phosphonoproprionate (d,l-AP3) and MK-801 against ibotenate induced brain injury

The neuroprotective characteristics of the functional antagonist of metabotropic stimulated phosphoinositide hydrolysis, 2-amino-3-phosphonoproprionate (D,L-AP3), were examined alone and in combination with the non-competitive N-methyl-D-aspartate (NMDA) antagonist, MK-801, against ibotenate induced brain injury. Postnatal day (PND) 7 rats received unilateral stereotaxic intrastriatal injections of 10 nmol ibotenate and treated with either D,L-AP3 (600 nmol i.c.), MK-801 (1 mg/kg i.p.) or both. The severity of brain injury was assessed on PND 12 by comparison of the weights of injected and contralateral cerebral hemispheres. Ibotenate induced injury was partially reduced by treatment with MK-801 (34.0 +/- 4.4% protection, P < 0.05 vs. PBS treated, independent t-test) but not D,L-AP3. However, combined treatment with both MK-801 and D,L-AP3 produced marked synergistic neuroprotection (83.5 +/- 7.6% protection, P < 0.001 vs. PBS treated, independent t-test). The data suggest that metabotropic stimulated phosphoinositide hydrolysis contributes to excitotoxic neuronal injury in the presence of concurrent ionotropic receptor activation.

Role of excitotoxicity in human neurological disease

An increasing body of evidence has implicated excitoxicity as a mechanism of neuronal death in both acute and chronic neurological diseases. A major recent advance has been the successful cloning and expression of the non-NMDA, NMDA, and metabotropic glutamate receptors. The cellular mechanisms responsible for cell death following activation of these receptors are still being clarified. A recent advance in conceptualizing excitotoxicity is the notion that a slow excitotoxic process may occur as a consequence of either a receptor abnormality or an impairment of energy metabolism. It is possible that such a mechanism may occur in neurodegenerative illnesses. Recent therapeutic studies have focused on glycine site antagonists and on the efficacy of non-NMDA antagonists in ischemia.

Activation of the glutamate metabotropic receptor protects retina against N-methyl-D-aspartate toxicity

Intraocular pretreatment with the specific metabotropic glutamate receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) in the adult rat reduced the excitotoxic effects induced in the retina by a single intraocular injection of N-methyl-D-aspartate (NMDA). Damage was estimated by assessing NMDA-induced loss of retinal choline acetyltransferase (ChAT) activity. The interaction between metabotropic and ionotropic glutamate receptors may, therefore, be considered an important target for in vivo pharmacological neuroprotection.

The metabotropic excitatory amino acid receptor agonist 1S,3R-ACPD selectively potentiates N-methyl-D-aspartate-induced brain injury

The role of metabotropic type excitatory amino acid receptors in brain injury was assessed using the selective metabotropic receptor agonist, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). Intrastriatal stereotaxic injection of 1S,3R-ACPD (250 nmol) in PND 7 rats produced little brain injury as assessed by hemispheric weight disparities. However, 1S,3R-ACPD markedly potentiated N-methyl-D-aspartate (NMDA)-, but not alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA)-, mediated brain injury. This effect was stereoselective since the inactive metabotropic agonist, 1R,3S-ACPD, did not potentiate NMDA toxicity.

Activation of hippocampal metabotropic excitatory amino acid receptors leads to seizures and neuronal damage

A role for ionotropic (NMDA, AMPA, and kainate) excitatory amino acid (EAA) receptors in seizure and seizure-related brain damage is well documented. To study the possible role of metabotropic (G-protein linked) EAA receptors in this regard, a highly selective metabotropic EAA agonist was injected into the hippocampus of halothane-anesthetized rats. This resulted in delayed-onset seizures and selective hippocampal neuronal damage that was indirectly mediated by NMDA receptors. This provides direct evidence for a novel role of metabotropic EAA receptors in the etiology of seizures and neuronal damage.