Mechanisms of 1S,3R-ACPD-induced neuroprotection in rat hippocampal slices subjected to oxygen and glucose deprivation

Neuroprotection Against Diisopropylfluorophosphate in Acute Hippocampal Slices

Diisopropylfluorophosphate (DFP) is an irreversible inhibitor of acetylcholine esterase and a surrogate of the organophosphorus (OP) nerve agent sarin. The neurotoxicity of DFP was assessed as a reduction of population spike (PS) area elicited by synaptic stimulation in acute hippocampal slices. Two classical antidotes, atropine, and pralidoxime, and two novel antidotes, 4R-cembranotriene-diol (4R) and a caspase nine inhibitor, were tested. Atropine, pralidoxime, and 4R significantly protected when applied 30 min after DFP. The caspase inhibitor was neuroprotective when applied 5-10 min before or after DFP, suggesting that early synaptic apoptosis is responsible for the loss of PSs. It is likely that apoptosis starts at the synapses and, if antidotes are not applied, descends to the cell bodies, causing death. The acute slice is a reliable tool for mechanistic studies, and the assessment of neurotoxicity and neuroprotection with PS areas is, in general, pharmacologically congruent with in vivo results and predicts the effect of drugs in vivo. 4R was first found to be neuroprotective in slices and later we demonstrated that 4R is neuroprotective in vivo. The mechanism of neurotoxicity of OPs is not well understood, and there is a need for novel antidotes that could be discovered using acute slices.

γ-Aminobutyric acid type A receptor inhibition triggers a nicotinic neuroprotective mechanism

Nicotinic acetylcholine receptor (nAChR)-mediated neuroprotection has been implicated in the treatment of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases and hypoxic ischemic events as well as other diseases hallmarked by excitotoxic and apoptotic neuronal death. Several modalities of nicotinic neuroprotection have been reported. However, although this process generally involves α4β2 and α7 subtypes, the underlying mechanisms are largely unknown. Interestingly, both activation and inhibition of α7 nAChRs have been reported to be neuroprotective. We have shown that inhibition of α7 nAChRs protects the function of acute hippocampal slices against excitotoxicity in an α4β2-dependent manner. Neuroprotection was assessed as the prevention of the N-methyl-D-aspartate-dependent loss of the area of population spikes (PSs) in the CA1 area of acute hippocampal slices. Our results support a model in which α7 AChRs control the release of γ-aminobutyric acid (GABA). Blocking either α7 or GABA(A) receptors reduces the inhibitory tone on cholinergic terminals, thereby promoting α4β2 activation, which in turn mediates neuroprotection. These results shed light on how α7 nAChR inhibition can be neuroprotective through a mechanism mediated by activation of α4β2 nAChRs.

Excitotoxic mechanisms of ischemic injury in myelinated white matter

Axonal injury and dysfunction in white matter (WM) are caused by many neurologic diseases including ischemia. We characterized ischemic injury and the role of glutamate-mediated excitotoxicity in a purely myelinated WM tract, the mouse optic nerve (MON). For the first time, excitotoxic WM injury was directly correlated with glutamate release. Oxygen and glucose deprivation (OGD) caused duration-dependent loss of axon function in optic nerves from young adult mice. Protection of axon function required blockade of both alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate receptors, or removal of extracellular Ca(2+). Blockade of N-methyl-D-aspartate receptors did not preserve axon function. Curiously, even extended periods of direct exposure to glutamate or kainate or AMPA failed to induce axon dysfunction. Brief periods of OGD, however, caused glutamate receptor agonist exposure to become toxic, suggesting that ionic disruption enabled excitotoxic injury. Glutamate release, directly measured using quantitative high-performance liquid chromatography, occurred late during a 60-mins period of OGD and was due to reversal of the glutamate transporter. Brief periods of OGD (i.e., 15 mins) did not cause glutamate release and produced minimal injury. These results suggested that toxic glutamate accumulation during OGD followed the initial ionic changes mediating early loss of excitability. The onset of glutamate release was an important threshold event for irreversible ischemic injury. Regional differences appear to exist in the specific glutamate receptors that mediate WM ischemic injury. Therapy for ischemic WM injury must be designed accordingly.

Cell signaling pathways in the neuroprotective actions of the green tea polyphenol (-)-epigallocatechin-3-gallate: implications for neurodegenerative diseases

Accumulating evidence supports the hypothesis that brain iron misregulation and oxidative stress (OS), resulting in reactive oxygen species (ROS) generation from H2O2 and inflammatory processes, trigger a cascade of events leading to apoptotic/necrotic cell death in neurodegenerative disorders, such as Parkinson's (PD), Alzheimer's (AD) and Huntington's diseases, and amyotrophic lateral sclerosis (ALS). Thus, novel therapeutic approaches aimed at neutralization of OS-induced neurotoxicity, support the application of ROS scavengers, transition metals (e.g. iron and copper) chelators and non-vitamin natural antioxidant polyphenols, in monotherapy, or as part of antioxidant cocktail formulation for these diseases. Both experimental and epidemiological evidence demonstrate that flavonoid polyphenols, particularly from green tea and blueberries, improve age-related cognitive decline and are neuroprotective in models of PD, AD and cerebral ischemia/reperfusion injuries. However, recent studies indicate that the radical scavenger property of green tea polyphenols is unlikely to be the sole explanation for their neuroprotective capacity and in fact, a wide spectrum of cellular signaling events may well account for their biological actions. In this article, the currently established mechanisms involved in the beneficial health action and emerging studies concerning the putative novel molecular neuroprotective activity of green tea and its major polyphenol (-)-epigallocatechin-3-gallate (EGCG), will be reviewed and discussed.

The Neurotoxicity Induced by Ethanol Withdrawal in Mature Organotypic Hippocampal Slices Might Involve Cross-Talk Between Metabotropic Glutamate Type 5 Receptors and N-Methyl-d-Aspartate Receptors

BACKGROUND

We recently reported that the sodium salt of acamprosate (Na-acamprosate) demonstrates the characteristics of an antagonist at metabotropic glutamate type 5 receptors (mGluR5s) rather than at N-methyl-d-aspartate receptors (NMDARs). Because mGluR5s are able to enhance the function of NMDARs, this interplay may be involved in the dysregulation of glutamatergic transmission during ethanol withdrawal. The following studies use organotypic hippocampal slice cultures at a mature age to investigate the potential for this interplay in the neurotoxicity associated with withdrawal from long-term ethanol exposure.

METHODS

At 25 days in vitro, organotypic hippocampal slice cultures prepared from male and female 8-day-old rats were exposed to an initial concentration of 100 mM ethanol for 10 days before undergoing a 24-hr period of withdrawal. The effects of Na-acamprosate; 2-methyl-6-(2-phenylethenyl)pyridine (SIB-1893), a noncompetitive antagonist at mGluR5s; 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester, a noncompetitive antagonist at mGluR1s; dizocilpine (MK-801), a noncompetitive NMDAR antagonist; and staurosporine on the neurotoxicity induced by ethanol withdrawal were assessed by determining differences in propidium iodide uptake. Polypeptide levels of mGluR5s and the NR1 and NR2B subunits of NMDARs were also determined via Western blot analyses after 10 days of ethanol exposure.

RESULTS

Significant neurotoxicity was always evident in the CA1 hippocampal region after a 24-hr withdrawal period. This spontaneous neurotoxicity resulted from intrinsic changes induced by the long-term presence of ethanol. Na-acamprosate (200-1000 microM), SIB-1893 (200-500 microM), MK-801 (20 microM), and staurosporine (200 nM) were all neuroprotective. The polypeptide levels of mGluR5s and NR1 and NR2B subunits of NMDARs were all increased after ethanol exposure; however, the increase in mGluR5s did not achieve statistical significance.

CONCLUSIONS

From this model of long-term ethanol exposure and withdrawal, the functional interplay between mGluR5s and NMDARs might represent a novel target for the prevention of neurotoxicity associated with ethanol withdrawal.

Regulation of Cl--HCO3- exchangers by cAMP-dependent protein kinase in adult rat hippocampal CA1 neurons

The contributions of HCO(3)(-)-dependent, DIDS-sensitive mechanisms to the maintenance of steady-state pH(i), and the regulation of their activities by cAMP-dependent protein kinase (PKA), were investigated in CA1 neurons with the H(+)-sensitive fluorophore, BCECF. The addition of HCO(3)(-)/CO(2) to neurons with "low" (pH(i) < or = 7.20) and "high" (pH(i) > 7.20) initial pH(i) values under Hepes-buffered conditions, increased and decreased steady-state pH(i), respectively. Conversely, under HCO(3)(-)/CO(2)-buffered conditions, DIDS caused pH(i) to decrease and increase in neurons with low and high initial pH(i) values, respectively. In the presence, but not the absence, of HCO(3)(-), the PKA inhibitor Rp-adenosine-3',5'-cyclic monophosphorothioate (Rp-cAMPS; 50 microM) evoked DIDS-sensitive increases and decreases in pH(i) in neurons with low and high initial pH(i) values, respectively. In contrast, in neurons with low initial pH(i) values, activation of PKA with the Sp isomer of cAMPS (Sp-cAMPS; 25 microM) elicited increases in pH(i) that were smaller in the presence than in the absence of HCO(3)(-), whereas in neurons with high initial pH(i) values, Sp-cAMPS-evoked rises in pH(i) were larger in the presence than in the absence of HCO(3)(-); the differences between the effects of Sp-cAMPS on pH(i) under the different buffering conditions were attenuated by DIDS. Consistent with the possibility that changes in the activities of HCO(3)(-)-dependent, DIDS-sensitive mechanisms contribute to the steady-state pH(i) changes evoked by the PKA modulators, in neurons with initial pH(i) values < or = 7.20, Rp-cAMPS concurrently inhibited Na(+)-independent Cl(-)-HCO(3)(-) exchange and stimulated Na(+)-dependent Cl(-)-HCO(3)(-) exchange; in contrast, Sp-cAMPS concurrently stimulated Na(+)-independent Cl(-)-HCO(3)(-) exchange and inhibited Na(+)-dependent Cl(-)-HCO(3)(-) exchange. Data from a limited number of neurons with initial pH(i) values > 7.20 suggested that the directions of the reciprocal changes in anion exchange activities (inhibition or stimulation) evoked by Rp- and Sp-cAMPS may be opposite in cells with low vs. high resting pH(i) values. Taken together, the results indicate that the effects of modulating PKA activity on steady-state pH(i) in rat CA1 neurons under HCO(3)(-)/CO(2)-buffered conditions reflect not only changes in Na(+)-H(+) exchange activity but also changes in Na(+)-dependent and Na(+)-independent Cl(-)-HCO(3)(-) exchange activity that, in turn, may be dependent upon the initial pH(i).

DCG-IV but not other group-II metabotropic receptor agonists induces microglial BDNF mRNA expression in the rat striatum. Correlation with neuronal injury

We have previously described a neuroprotective action of (2S,2'R,3'R)-2-(2'3'-dicarboxycyclopropyl)glycine (DCG-IV), an agonist for group-II metabotropic receptors, on dopaminergic nerve terminals against the degeneration induced by 1-methyl-4-phenylpyridinium (MPP+). This effect was accompanied by an up-regulation of brain-derived neurotrophic factor (BDNF) mRNA expression in the rat striatum. We have now analyzed the phenotypic nature of the BDNF mRNA-expressing cells in response to intrastriatal injection of DCG-IV. Dual in situ hybridization and immunohistochemistry revealed that microglial cells but not astrocytes were responsible for this induction. Subsequent analysis demonstrated that this effect was accompanied by striking loss of striatal glutamic acid decarboxylase (GAD) mRNA and massive appearance of internucleosomal DNA fragmentation, a hallmark of apoptosis. A dose-response study demonstrated that doses of DCG-IV as low as 5 nmol was very toxic in terms GAD mRNA and apoptosis. 0.5 nmol of DCG-IV did not induce toxicity at all in terms of GAD mRNA and apoptosis. Activation of group-II metabotropic receptors in striatum with N-Acetyl-Asp-Glu (NAAG; a mGlu3 agonist) and (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (a mGlu2 and mGlu3 agonist) did not induce neither loss of GAD mRNA nor appearance of apoptosis (doses up to 20 nmol). In additional experiments, NAAG, in contrast to DCG-IV, failed to protect the striatal dopaminergic system against the degeneration induced by MPP+ as studied by microdialysis. Finally, we studied the mechanism by which DCG-IV is highly toxic. For that, selective antagonists of either metabotropic--(R,S)-alpha-methyl-4-carboxyphenylglycine and LY 341495--or ionotropic (N-methyl-D-aspartate, NMDA)--DL-2-amino-5-phosphonovaleric acid (AP-5) glutamate receptors --were co-administered with DCG-IV. Only AP-5 highly protected the striatum against the degeneration induced by DCG-IV. Since DCG-IV also activates the NMDA receptor at concentrations higher than 3 microM, it is conceivable that a intrastriatal concentration equal or higher than 3 microM after a single striatal injection of 5-20 nmol of DCG-IV. Our findings suggest that much caution must be exerted when testing the numerous neuroprotective effects ascribed to group-II metabotropic receptor activation, in particular when using DCG-IV. We conclude that the neuroprotectant capability of a given compound on a specific system does not exclude the possibility of inducing toxicity on a different one.

Involvement of protein kinase C activation and cell survival/ cell cycle genes in green tea polyphenol (-)-epigallocatechin 3-gallate neuroprotective action

Studies from our laboratory have demonstrated that the major green tea polyphenol, (-)-epigallocatechin 3-gallate (EGCG), exerts potent neuroprotective actions in the mice model of Parkinson's disease. These studies were extended to neuronal cell culture employing the parkinsonism-inducing neurotoxin, 6-hydroxydopamine (6-OHDA). Pretreatment with EGCG (0.1-10 microm) attenuated human neuroblastoma (NB) SH-SY5Y cell death, induced by a 24-h exposure to 6-OHDA (50 microm). Potential cell signaling candidates involved in this neuroprotective effect were further examined. EGCG restored the reduced protein kinase C (PKC) and extracellular signal-regulated kinases (ERK1/2) activities caused by 6-OHDA toxicity. However, the neuroprotective effect of EGCG on cell survival was abolished by pretreatment with PKC inhibitor GF 109203X (1 microm). Because EGCG increased phosphorylated PKC, we suggest that PKC isoenzymes are involved in the neuroprotective action of EGCG against 6-OHDA. In addition, gene expression analysis revealed that EGCG prevented both the 6-OHDA-induced expression of several mRNAs, such as Bax, Bad, and Mdm2, and the decrease in Bcl-2, Bcl-w, and Bcl-x(L). These results suggest that the neuroprotective mechanism of EGCG against oxidative stress-induced cell death includes stimulation of PKC and modulation of cell survival/cell cycle genes.

Intracellular pH response to anoxia in acutely dissociated adult rat hippocampal CA1 neurons

The effects of anoxia on intracellular pH (pH(i)) were examined in acutely isolated adult rat hippocampal CA1 neurons loaded with the H(+)-sensitive fluorophore, 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein. During perfusion with HCO/CO(2)- or HEPES-buffered media (pH 7.35) at 37 degrees C, 5- or 10-min anoxic insults were typified by an intracellular acidification on the induction of anoxia, a subsequent rise in pH(i) in the continued absence of O(2), and a further internal alkalinization on the return to normoxia. The steady-state pH(i) changes were not consequent on changes in [Ca(2+)](i) and, examined in the presence of HCO, were not significantly affected by (DIDS). In the absence of HCO, the magnitude of the postanoxic alkalinization was attenuated when external Na(+) was reduced by substitution with N-methyl-D-glucamine (NMDG(+)), but not Li(+), suggesting that increased Na(+)/H(+) exchange activity contributes to this phase of the pH(i) response. In contrast, 100-500 microM Zn(2+), a known blocker of H(+)-conductive pathways, reduced the magnitudes of the internal alkalinizations that occurred both during and following anoxia. The effects of NMDG(+)-substituted medium and Zn(2+) to reduce the increase in pH(i) that occurred after anoxia were additive. Consistent with the steady-state pH(i) changes, rates of pH(i) recovery from internal acid loads imposed immediately after anoxia were increased, and the application of Zn(2+) and/or perfusion with NMDG(+)-substituted medium slowed pH(i) recovery. Reducing extracellular pH from 7.35 to 6.60, or reducing ambient temperature from 37 degrees C to room temperature, also attenuated the increases in steady-state pH(i) observed during and after anoxia and reduced rates of pH(i) recovery from acid loads imposed in the immediate postanoxic period. Finally, inhibition of the cAMP/protein kinase A second-messenger system reduced the magnitude of the rise in pH(i) after anoxia in a manner that was dependent on external Na(+); conversely, activation of the system with isoproterenol increased the postanoxic alkalinization, an effect that was attenuated by pretreatment with propranolol, Rp-cAMPS, or when NMDG(+) (but not Li(+)) was employed as an external Na(+) substitute. The results suggest that a Zn(2+)-sensitive acid efflux mechanism, possibly a H(+)-conductive pathway activated by membrane depolarization, contributes to the internal alkalinization observed during anoxia in adult rat CA1 neurons. The rise in pH(i) after anoxia reflects acid extrusion via the H(+)-conductive pathway and also Na(+)/H(+) exchange, activation of the latter being mediated, at least in part, through a cAMP-dependent signaling pathway.

Hypothermia inhibits translocation of CaM kinase II and PKC-alpha, beta, gamma isoforms and fodrin proteolysis in rat brain synaptosome during ischemia-reperfusion

To clarify the involvement of intracellular signaling pathway and calpain in the brain injury and its protection by mild hypothermia, immunoblotting analyses were performed in the rat brain after global forebrain ischemia and reperfusion. After 30 min of ischemia followed by 60 min of reperfusion, Ca2+/calmodulin-dependent kinase II (CaM kinase II) and protein kinase C (PKC)-alpha, beta, gamma isoforms translocated to the synaptosomal fraction, while mild hypothermia (32 degrees C) inhibited the translocation. The hypothermia also inhibited fodrin proteolysis caused by ischemia-reperfusion, indicating the inhibition of calpain. These effects of hypothermia may explain the mechanism of the protection against brain ischemia-reperfusion injury through modulating synaptosomal function.

Group II mGlu receptor agonists fail to protect against various neurotoxic insults induced in murine cortical, striatal and cerebellar granular pure neuronal cultures

Since group II metabotropic glutamate (mGlu) receptors are a potential target for the amelioration of neuronal injury, we evaluated the ability of group II mGlu receptor agonists to attenuate toxicity induced by various insults in cortical, striatal and cerebellar granular (CGCs) pure neuronal cultures. The three cultures, when maintained under serum-free, anti-oxidant rich conditions for up to 13 days in vitro (div) were shown by immunocytochemistry to contain a maximum of 2-7% glia. At 6, 9 and 13 div a graded pattern of injury to cortical and striatal cultures was achieved with either hydrogen peroxide (60-110 microM), staurosporine (1 microM), N-methyl-D-aspartate (NMDA, 70 microM), alpha-amino-3-hydroxy-methylisoxazole-4-propionate (AMPA, 100 microM) or kainate (100 microM) over either 4, 24 or 48 h. CGCs were similarly exposed to low K(+) (5.4 mM KCl). Cell viability was examined via phase-contrast microscopy and assessed by a 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide assay. Treatment with group II mGlu receptor agonists (1-300 microM), 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate ((2R,4R)-APDC), (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I), (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) and N-acetylaspartylglutamate (NAAG) failed to attenuate the toxicity. Pretreatment of cultures with the agonists and treatment following acute insult also failed to attenuate toxicity. Further investigations demonstrated the presence of second messenger activation whereby (2R,4R)-APDC reduced forskolin-stimulated production of cAMP in each culture. Thus, despite receptor coupling to intracellular signaling cascades, and regardless of culture development, agonist concentration, extent and mode of injury, group II mGlu receptor agonists were unable to protect against injury induced in cortical, striatal and cerebellar granular pure neuronal cultures. This result is in contrast to mixed cultures of neurones and glia and implies an important role for glia in the neuroprotective effects of group II mGlu receptor agonists.

The metabotropic glutamate receptor agonist 1S,3R-ACPD stimulates and modulates NMDA receptor mediated excitotoxicity in organotypic hippocampal slice cultures

The potential toxic effects of the metabotropic glutamate receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) and its interactions with the N-methyl-D-aspartate (NMDA) receptor were studied in hippocampal brain slice cultures, using densitometric measurements of the cellular uptake of propidium iodide (PI) to quantify neuronal degeneration. Cultures exposed to ACPD, showed a concentration (2-5 mM) and time (1-4 days) dependent increase in PI uptake in CA1, CA3 and dentate subfields after 24 h and 48 h of exposure, with CA1 pyramidal cells being most sensitive. The neurodegeneration induced by 2 mM ACPD was completely abolished by addition of 10 microM of the NMDA receptor antagonist (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), while 20 microM of the 2-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainic acid receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) had no effect. Co-exposing cultures to a subtoxic dose of 300 microM ACPD together with 10 microM NMDA, which at this dose is known to induce a fairly selective degeneration of CA1 pyramidal cells, significantly increased the PI uptake in both CA1 and CA3, compared to cultures exposed to 10 microM NMDA only. Adding the 300 microM ACPD as pretreatment for 30 min followed by a 30 min wash in normal medium before the ACPD/NMDA co-exposure, eliminated the potentiation of NMDA toxicity. The potentiation was also blocked by addition of 10 or 100 microM 2-methyl-6-(phenylethynyl)pyridine (MPEP) (mGluR5 antagonist) during the co-exposure, while a corresponding addition of 10 or 100 microM 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) (mGluR1 antagonist) had no effect. We conclude that, stimulation of metabotropic glutamate receptors with ACPD at concentrations of 2 mM or higher induces a distinct subfield-related and time and concentration dependent pattern of hippocampal degeneration, and that ACPD at subtoxic concentrations modulates NMDA-induced excitotoxicity through the mGluR5 receptor in a time dependent way.

Neuroprotection against hypoxic/hypoglycaemic injury after the insult by the group III metabotropic glutamate receptor agonist (R, S)-4-phosphonophenylglycine

The role of group III metabotropic glutamate receptors (mGluR) in ischaemic neurodegeneration is still unsettled. In order to examine a possible modulatory effect of these receptors on ischaemia-induced damage we tested the novel selective agonist (R, S)-4-phosphonophenylglycine [(R,S)-PPG] after an hypoxic/hypoglycaemic insult in rat hippocampal slices. The recovery of population spike amplitudes in the CA1-region was used as parameter for neuronal viability. (R,S)-PPG significantly improved the recovery of synaptic transmission in the CA1-region even when applied only during the recovery period. The results imply that presynaptic glutamate release after an insult contributes to neurodegeneration. Since agonists of group III mGluR reduce neurotransmitter release - probably via presynaptic autoreceptors - we interpret the results obtained in our in vitro model of hypoxia/hypoglycaemia as support of the hypothesis that group III mGluR agonists might be beneficial drugs against diseases where excitotoxicity is one of the dominant pathological mechanisms.

Involvement of metabotropic glutamate receptors in ischemia-induced taurine release in the developing and adult hippocampus

Metabotropic glutamate receptors have recently been envisaged as involved in both potentiation and prevention of ischemic and excitotoxic neuronal damage. The release of the inhibitory amino acid taurine is markedly enhanced in ischemia in both the immature and mature mouse hippocampus. The modulation of [3H]taurine release by metabotropic receptor agonists and antagonists was studied in hippocampal slices from developing (7-day-old) and adult (3-month-old) mice using a superfusion system. Agonists of group I, II and III metabotropic glutamate receptors generally reduced the ischemia-induced release in adult animals. In the immature hippocampus the group I agonists (S)-3,5-dihydroxyphenylglycine and (1+/-)-1-aminocyclopentane-trans-1,3-dicarboxylate, which mainly enhance neuronal excitation, potentiated initial taurine release in ischemia. Ionotropic glutamate receptor agonists also enhance the ischemia-induced taurine release in developing mice. This glutamate-activated taurine release may thus constitute an important protective mechanism against excitotoxicity in the immature hippocampus.

Neuroprotective action of group I metabotropic glutamate receptor agonists against oxygen-glucose deprivation-induced neuronal death

The metabotropic glutamate receptor (mGluR) non-selective agonist (1S,3R)-1-aminocycloheptane-trans-1,3-dicarboxylic acid [(1S, 3R)ACPD] and group I selective receptor agonist 3, 5-dihydrophenylglycine (DHPG) effectively attenuated oxygen-glucose deprivation (OGD)-induced death of the cultured cerebellar granule cells. Furthermore, (1S,3R)ACPD (100 microM) reduced the number of apoptotic cells. Antiapoptotic action of (1S,3R)ACPD was prevented by the group I selective antagonist (RS)-1-aminoindan-1, 5-dicarboxylic acid (AIDA, 100 microM) and protein kinase C (PKC) inhibitor bisindolylmaleimide (BMI, 1 microM).

Metabotropic glutamate receptor agonists protect from oxygen-glucose deprivation- and colchicine-induced apoptosis in primary cultures of cerebellar granule cells

The effects of the metabotropic glutamate receptor agonists against apoptosis induced by oxygen-glucose deprivation or colchicine were studied in the primary cultures of mature cerebellar granule cells. Oxygen-glucose deprivation (90 min) or addition of colchicine (1 microM) resulted in neuronal damage as revealed by Trypan Blue assay 12 h later. Further analysis demonstrated that the cells exposed to oxygen-glucose deprivation or colchicine exhibit typical features of apoptosis: internucleosomal DNA fragmentation, condensation and fragmentation of chromatin and typical DNA ladder on agarose gel electrophoresis. Metabotropic glutamate receptor agonist, (1S,3R)-1-aminocycloheptane-trans-1,3-dicarboxylic acid, acting at group I and II receptors, and selective agonist, (2S,1'R,2R',3R')-2(2,3-dicarboxycyclopropyl)glycine, acting at group II receptors, added to cells recovering from oxygen-glucose deprivation exerted neuroprotective action against oxygen-glucose deprivation-induced apoptosis. Similar neuroprotective effects of metabotropic glutamate receptor agonists were observed against colchicine-induced apoptosis. The results thereby provide evidence that metabotropic glutamate receptor agonists have therapeutic potential in the treatment of pathologies associated with increased neuronal apoptosis. The selective protein kinase C inhibitor bisindolylmaleimide (100 nM) abolished the neuroprotective action of (1S,3R)-1-aminocycloheptane-trans-1,3-dicarboxylic acid, whereas the activator of adenylyl cyclase forskolin (10 microM) abolished the neuroprotective action of (2S,1'R,2R',3R')-2(2,3-dicarboxycyclopropyl)glycine (30 microM) against colchicine-induced apoptosis. It is concluded that both phosphoinositide hydrolysis with consequent activation of protein kinase C and inhibition of adenylyl cyclase seem to contribute to the neuroprotective action of metabotropic glutamate receptor agonists against neuronal apoptosis in the primary culture of cerebellar granule cells.

Protective effect of group I metabotropic glutamate receptor activation against hypoxic/hypoglycemic injury in rat hippocampal slices: timing and involvement of protein kinase C

Excessive release of glutamate during ischemia leads to sustained neuronal damage. In this study we investigated the influence of metabotropic glutamate receptor (mGluR) activation on neuronal recovery from a hypoxic/hypoglycemic event in hippocampal slices from rats. The slices were transiently exposed to an oxygen- and glucose-free environment in an interface chamber and the synaptically evoked population spike in the CA1 region was taken as a measure of neuronal viability. Under control conditions the population spike amplitude recovered to 41.4% of baseline value within 1 h after hypoxia/hypoglycemia. The specific mGluR group I agonist 3,5-dihydroxyphenylglycine (DHPG, 10 microM) increased the recovery rate to 88.3% of baseline value when applied from 20 min before until 10 min after the event. Similar recovery rates were obtained when DHPG was present only 10 or 20 min before hypoxia/hypoglycemia (89.3% and 79.3% of baseline value, respectively). However, when applied later, DHPG had no protective effect. Co-application of the protein kinase C (PKC) inhibitors staurosporine (100 nM) or chelerythrine (30 microM) prevented the protective effect of DHPG. Our data suggest that group I mGluR agonists are only protective when present prior to the onset of the hypoxic/hypoglycemic event and that the activation of PKC is a critical step of the protective mechanism.

(1S,3R)-ACPD, a metabotropic glutamate receptor agonist, enhances damage after global ischaemia

There are opposing results in the literature concerning the influence of (1S,3R)-ACPD [(1S,3R)-1-aminocyclopentane-1,3-dicarboxylate: group I/II metabotropic glutamate receptor agonist) on neurodegeneration, showing both enhancement and reduction of damage. We examined the effects of (1S,3R)-ACPD, given in various application schedules, on global ischaemia in gerbils. The most pronounced effect was a significant increase of hippocampal damage when the drug was applied at 20 mg/kg i.p. pre ischaemia. All other experiments with lower concentrations (0.02-2 mg/kg), other time schedules (post-ischaemic application) or co-application of a selective group I metabotropic glutamate receptor antagonist (4-CPG: (S)-4-carboxyphenylglycine), had no influence on neuronal density.

Evidence that brain-derived neurotrophic factor neuroprotection is linked to its ability to reverse the NMDA-induced inactivation of protein kinase C in cortical neurons

Several lines of evidence indicate that a rapid loss of neuronal protein kinase C (PKC) activity is a characteristic feature of cerebral ischemia and is a necessary step in the NMDA-induced death of cultured neurons. Exposing embryonic day 18 primary rat cortical neurons to 50 microM NMDA or 50 microM glutamate for 10 min caused approximately 80% cell death over the next 24 h, but excitotoxic death was largely averted, i.e., by 70-80%, in cells pretreated with brain-derived neurotrophic factor (BDNF). An 8-h preexposure to BDNF (50-100 ng/ml) maximally protected cortical cells from the effects of NMDA and glutamate, although the transient application of BDNF between 8 and 4 h before NMDA was equally protective. These effects of BDNF were abolished at supralethal, i.e., >100 microM, NMDA concentrations. It is significant that BDNF pretreatment prevented the inactivation of PKC in cortical cells normally seen 30 min to 2 h following lethal NMDA or glutamate exposure. This BDNF effect did not arise from changes in NMDA channel activity because neither whole-cell NMDA current amplitudes nor increases in intracellular free Ca2+ concentration were altered by the 8-h BDNF pretreatment. Furthermore, BDNF offered no neuroprotection to cells treated with the PKC inhibitors staurosporine (10-20 nM), calphostin C (1-2.5 microM), or GF-109203X (100 nM) at the time of NMDA addition. These results underscore the importance of PKC inactivation in glutamate-induced neuronal death. They also suggest that BDNF neuroprotection arises, at least in part, via its ability to block the mechanism by which pathophysiological Ca2+ influx through the NMDA receptor causes membrane PKC inactivation.

(S)-4C3HPG reduces infarct size after focal cerebral ischemia

This study investigated whether the metabotropic glutamate receptor ligand (S)-4C3HPG can reduce brain damage after focal ischemia in rats. Application of 1 micromol of (S)-4C3HPG (intracerebroventricularly) 5 min after occlusion of the middle cerebral artery significantly reduced the infarct size by 72.3% of the saline control.

Antagonists for group I mGluRs attenuate excitotoxic neuronal death in cortical cultures

Activation of ion channel-linked glutamate receptors, especially N-methyl-D-aspartate (NMDA) receptors, mediates the excitotoxic effects of glutamate upon central neurons. We examined the hypothesis that activation of group I metabotropic glutamate receptors (mGluRs) would increase NMDA receptor-mediated cortical neuronal death. Addition of the selective group I mGluR agonists, dihydroxyphenylglycine (DHPG) or trans-azetidine-2,4-dicarboxylic acid (t-ADA) potentiated NMDA-induced neuronal death, and application of the group I mGluR-selective antagonist, aminoindan-1,5-dicarboxylic acid (AIDA), as well as the non-selective antagonists methyl-4-carboxyphenylglycine (MCPG) or 4-carboxyphenylglycine (4CPG) reduced NMDA- and kainate-induced neuronal death in murine cortical cultures. The pro-excitotoxic effect of group I mGluR activation may be mediated largely by enhancement of glutamate release, as DHPG potentiated high potassium-stimulated glutamate release, and the protective effects of both AIDA and MCPG were abolished when NMDA and alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) receptors were blocked immediately after toxic NMDA receptor overstimulation. The present data support the possibility that antagonizing group I mGluRs may be a useful strategy for attenuating excitotoxic neuronal death in certain disease states.

A fluorescence confocal assay to assess neuronal viability in brain slices

Hippocampal slice models are used to study the mechanisms of ischemia-induced neurotoxicity and to assess the neuroprotective potential of novel therapeutic agents. A number of morphological and functional endpoints are available to assess neuronal viability. The slice model also allows the study of selectively vulnerable neuronal populations within the same preparation. The fluorescence procedure described here provides a method of assessing the viability of neurons in rat hippocampal slices exposed to hypoxic-hypoglycemic conditions. Control and/or treated slices that had been subjected to a 10 min oxygen-glucose deprivation insult are double stained with calcein-AM (4 microM), which stains live cells green, and ethidium homodimer (6 microM), which stains the nucleus of dead cells red. The stained slices are then imaged using confocal microscopy. Vulnerable neurons in the CA1 region of slices deprived of oxygen and glucose became increasingly permeant to ethidium homodimer over the 4 h reperfusion period. Exposure to low Ca2+ concentration (0.3 mM) or the N-, P- and Q-type Ca2+ channel antagonist MVIIC (100 nM), which have been shown to be neuroprotective in this model of ischemia using field evoked post-synaptic potential (EPSP) measures as an endpoint, were also shown to be protective using the fluorescence assay.