Pharmacological modification of glutamate neurotoxicity in vivo

Evidence for a role of proline and hypothalamic astrocytes in the regulation of glucose metabolism in rats

The metabolism of lactate to pyruvate in the mediobasal hypothalamus (MBH) regulates hepatic glucose production. Because astrocytes and neurons are functionally linked by metabolic coupling through lactate transfer via the astrocyte-neuron lactate shuttle (ANLS), we reasoned that astrocytes might be involved in the hypothalamic regulation of glucose metabolism. To examine this possibility, we used the gluconeogenic amino acid proline, which is metabolized to pyruvate in astrocytes. Our results showed that increasing the availability of proline in rats either centrally (MBH) or systemically acutely lowered blood glucose. Pancreatic clamp studies revealed that this hypoglycemic effect was due to a decrease of hepatic glucose production secondary to an inhibition of glycogenolysis, gluconeogenesis, and glucose-6-phosphatase flux. The effect of proline was mimicked by glutamate, an intermediary of proline metabolism. Interestingly, proline's action was markedly blunted by pharmacological inhibition of hypothalamic lactate dehydrogenase (LDH) suggesting that metabolic flux through LDH was required. Furthermore, short hairpin RNA-mediated knockdown of hypothalamic LDH-A, an astrocytic component of the ANLS, also blunted the glucoregulatory action of proline. Thus our studies suggest not only a new role for proline in the regulation of hepatic glucose production but also indicate that hypothalamic astrocytes are involved in the regulatory mechanism as well.

Opioid system and Alzheimer's disease

The opioid system may be involved in the pathogenesis of AD, including cognitive impairment, hyperphosphorylated tau, Aβ production, and neuroinflammation. Opioid receptors influence the regulation of neurotransmitters such as acetylcholine, norepinephrine, GABA, glutamate, and serotonin which have been implicated in the pathogenesis of AD. Opioid system has a close relation with Aβ generation since dysfunction of opioid receptors retards the endocytosis and degradation of BACE1 and γ-secretase and upregulates BACE1 and γ-secretase, and subsequently, the production of Aβ. Conversely, activation of opioid receptors increases the endocytosis of BACE1 and γ-secretase and downregulates BACE1 and γ-secretase, limiting the production of Aβ. The dysfunction of opioid system (opioid receptors and opioid peptides) may contribute to hyperphosphorylation of tau and neuroinflammation, and accounts for the degeneration of cholinergic neurons and cognitive impairment. Thus, the opioid system is potentially related to AD pathology and may be a very attractive drug target for novel pharmacotherapies of AD.

Neuroprotection by lamotrigine in a rat model of neonatal hypoxic-ischaemic encephalopathy

Hypoxic-ischaemic (HI) encephalopathy is a severe complication of perinatal asphyxia and remains a frequent cause of a variety of brain disorders with long-term effects on the patients' life. The associated brain damage is strongly related to the toxic action of excitatory amino acids, especially glutamate and aspartate. Lamotrigine is an anti-epileptic drug that blocks the voltage-gated sodium channels of the presynaptic neuron and inhibits the release of glutamate. In the present study a well-established model of perinatal asphyxia in 7-d-old rats was used to investigate the effect of lamotrigine on HI-induced damage to different hippocampal brain structures, since disruption of this brain area is thought to play a key role in schizophrenia and epilepsy. Therefore, a combination of ischaemia, induced by unilateral occlusion of the left common carotid artery, followed by exposure to a 1-h period of hypoxia, was carried out in neonatal 7-d-old rats. Immediately after the insult, lamotrigine was given i.p. The histological outcome in the hippocampus was conducted and the tissue levels of glutamate, aspartate, GABA, and glutamine in the same area were determined. A remarkable reduction of HI-evoked damaged neurons in most of the investigated hippocampal regions was noted after lamotrigine administration. Furthermore, lamotrigine decreased the asphyxia-induced hippocampal tissue levels of glutamate and aspartate. Immediately after perinatal asphyxia GABA levels were enhanced, while levels of glutamine were decreased. Lamotrigine administration did not affect either GABA or glutamine levels. These results suggest a neuroprotective effect of lamotrigine in this particular animal model of neonatal HI encephalopathy.

Hippocampal neurons in organotypic slice culture are highly resistant to damage by endogenous and exogenous nitric oxide

Nitric oxide (NO) has been proposed to mediate neurodegeneration arising from NMDA receptor activity, but the issue remains controversial. The hypothesis was re-examined using organotypic slice cultures of rat hippocampus, with steps being taken to avoid known artefacts. The NO-cGMP signalling pathway was well preserved in such cultures. Brief exposure to NMDA resulted in a concentration-dependent delayed neuronal death that could be nullified by administration of the NMDA antagonist MK801 (10 microm) given postexposure. Two inhibitors of NO synthesis failed to protect the slices, despite fully blocking NMDA-induced cGMP accumulation. By comparing NMDA-induced cGMP accumulation with that produced by an NO donor, toxic NMDA concentrations were estimated to produce only physiological NO concentrations (2 nm). In studies of the vulnerability of the slices to exogenous NO, it was found that continuous exposure to up to 4.5 microm NO failed to affect ATP levels (measured after 6 h) or cause damage during 24 h, whereas treatment with the respiratory inhibitors myxothiazol or cyanide caused ATP depletion and complete cell death within 24 h. An NO concentration of 10 microm was required for ATP depletion and cell death, presumably through respiratory inhibition. It is concluded that sustained activity of neuronal NO synthase in intact hippocampal tissue can generate only low nanomolar NO concentrations, which are unlikely to be toxic. At the same time, the tissue is remarkably resistant to exogenous NO at up to 1000-fold higher concentrations. Together, the results seriously question the proposed role of NO in NMDA receptor-mediated excitotoxicity.

Mild Hypothermia Inhibits Exogenous Glutamate-Induced Increases in Nitric Oxide Synthesis

The purpose of this study was to investigate changes in nitric oxide (NO) synthesis induced by exogenous glutamate perfusion into the cerebral cortex, and the effects of mild hypothermia on this glutamate-induced NO synthesis. Glutamate-induced cortical lesions were produced by perfusion of 0.5 M glutamate solution via a microdialysis probe, and the extracellular concentrations of NO end-products (nitrite and nitrate) were measured by microdialysis in normothermic (37 degrees C) and hypothermic (32 degrees C) rats. The levels of NO end-products in the normothermia group were elevated markedly by glutamate perfusion, and this change was completely attenuated by the induction of hypothermia. The glutamate-induced increases were also attenuated markedly by both Nomega-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI). These results suggest that the perfusion of exogenous glutamate into the cortex induces NO synthesis, that is derived primarily from the activity of neuronal NO synthase. These results also demonstrate that hypothermia prevents this glutamate-induced increase in NO, suggesting that the protection afforded by the hypothermic condition is most likely linked to its inhibition of the glutamate-induced NO synthesis.

Treatment with 7-nitroindazole enhances kainic acid induced cholinergic neurotoxicity in the rat striatum: a neuroprotective role for neuronal nitric oxide

1. In this study we investigated the effect of 7-nitroindazole (7-NI), a preferential inhibitor of neuronal nitric oxide synthase (nNOS), on kainic acid (KA) induced neurotoxicity in rats. Choline acetyltransferase activity (CAT), a cholinergic marker, and histological changes were employed to assess neurotoxicity. 2. In control rats, the local intrastriatal injection of 0.5 microg of KA reduced CAT from 22.9 +/- 2.2 to 14.7 +/- 2.0 nmol/h/mg tissue ((38 +/- 6)% reduction) (P < 0.001). Greater reductions in CAT were observed with 1 and 2 microg of KA ((70 +/- 6)% and (80 +/- 3)%, respectively). 7-NI aggravated KA-induced cholinergic and histological damage. KA reduced CAT by (68.2 +/- 4)% in 7-NI-treated rats, by (38 +/- 6)% in saline-treated controls, and by (41 +/- 4)% in peanut-oil- (7-NI-vehicle-) treated rats (P = 0.0047). 3. After KA, CAT activity averaged 14.3 +/- 2.0 in peanut-oil-treated rats and 7.9 +/- 1.0 nmol/h/mg tissue in 7-NI- (peanut-oil-) treated rats (P = 0.015). Similarly to changes in CAT, 7-NI treatment aggravated KA-induced histological changes indicative of neuronal damage (acute ischemic neuronal changes, disorganization of myelinated fibers bundle, and vacuolation changes of the neuropil). Treatment with 7-NI was not associated with increased mortality. 4. Our findings suggest that neuronal NO plays a neuroprotective action on excitotoxicity.

Severe cerebral blood flow reduction inhibits nitric oxide synthesis

The purpose of this study was to investigate the relationship between cerebral blood flow (CBF) and nitric oxide (NO) synthesis using a rat model of transient forebrain ischemia of varying severity. Forebrain ischemia was induced for 30 min by occlusion of the bilateral common carotid arteries without hemorrhagic hypotension. The production of NO end-products (nitrite and nitrate) was measured by in vivo microdialysis, and CBF by the hydrogen clearance technique. Ischemia induced NO synthesis, although the increase in the quantity of NO end-products was not remarkable during the ischemic period but became prominent after reperfusion. Such increases were abolished by Nomega-nitro-L-arginine methyl ester (L-NAME), although 7-nitroindazole (7-NI) appeared to have only slight effects. The production of NO end-products during ischemia increased when the CBF during ischemia was less than 60 mL/100 g/min. In animals in which the CBF during ischemia was higher than 22.7 mL/100 g/min, the production of NO end-products increased gradually after the induction of ischemia and reached a peak during the reperfusion period, whereas in other animals in which the CBF during ischemia fell below 22.7 mL/100 g/min, the NO end-products decreased during ischemia and increased transiently after reperfusion. These results suggest that the increase in NO end-products is NO synthase (NOS)-dependent and that most of the increase is derived from endothelial NOS. It is also suggested that NO synthesis during ischemia is closely related to CBF, and that severe CBF reduction may inhibit NO synthesis.

Varied actions of proinflammatory cytokines on excitotoxic cell death in the rat central nervous system

Interleukin (IL)-1beta mediates diverse forms of neurodegeneration and exacerbates cell death induced by striatal injection of the excitotoxin alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) in the rat brain. The objective of this study was to determine whether this effect was specific to IL-1beta. Injection of IL-1alpha and AMPA in the striatum had effects identical to those of IL-1beta, whereas coinjection of IL-6 or tumor necrosis factor (TNF)-alpha with AMPA failed to induce significant cortical cell death. In contrast to IL-1alpha, IL-1beta, and IL-6, TNFalpha significantly reduced (by 38%) the local striatal damage. These findings suggest that the effect of IL-1 on AMPA receptor-mediated cell death in the rat striatum is not mimicked by other proinflammatory cytokines. Furthermore, TNFalpha shows neuroprotective effects against acute excitotoxic injury.

Site-specific actions of interleukin-1 on excitotoxic cell death in the rat striatum

The pro-inflammatory cytokine interleukin-1 (IL-1) contributes to and exacerbates many forms of neurodegeneration. When co-administered with the potent glutamatergic agonist S-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (S-AMPA) in the rat striatum, IL-1 induces marked and widespread cell death throughout the ipsilateral cortex. The mechanisms underlying this action of IL-1 are not known but may involve activation of polysynaptic neuronal pathways leading from the striatum to the cortex via other brain areas such as the hypothalamus. The aims of the present study were to identify specific sites of action of IL-1 in the rat striatum, in order to further understand these pathways. Ventral regions of the caudate-putamen and the lateral shell of the nucleus accumbens (NAcc) were particularly sensitive to the effects of IL-1 on excitotoxic damage. A high percentage of co-injections in these sites induced distant cortical damage, whereas injections in more dorsal areas of the caudate-putamen or core regions of the NAcc were less likely to result in cortical cell death. The 'positive' injection sites differ from the unresponsive areas in that they have extensive connections with the limbic system and it may be that IL-1 displays specific actions on limbic pathways that, in conjunction with AMPA/kainate receptor activation, contribute to the remote cell death in the cortex. These findings enhance our understanding of the actions of IL-1, and the mechanisms by which it participates in neurodegeneration through both local and long-range effects.

Profiles of glutamate and GABA efflux in core versus peripheral zones of focal cerebral ischemia in mice

Efflux of glutamate during cerebral ischemia is known to contribute to brain cell death via processes of excitotoxicity. However, gamma-aminobutyric acid (GABA) is also released during ischemia, and may be protective. In this study, we used in vivo microdialysis to map the efflux of glutamate and GABA from central core and peripheral zones of focal ischemia in mouse brain. We show that the temporal profiles of glutamate and GABA efflux are significantly different in core versus peripheral zones. Calculation of glutamate/GABA ratios demonstrate that, in the core, there is a significant increase above baseline ratios during the first 30 mm of ischemia, which then rapidly renormalizes. In contrast, no significant changes in glutamate/GABA ratios were seen in the ischemic periphery. These data suggest that imbalances in glutamate versus GABA efflux may be an initial trigger of excitotoxic brain damage in the core but not the peripheral zones of focal cerebral ischemia.

A model of acute subdural hematoma in the mouse

The availability of genetically modified mice has allowed the study of genetic influences on acute brain injury. An animal model of acute subdural hematoma (ASDH) has been previously described in the rat but not the mouse. We describe a method for producing ASDH in the mouse. Subdural injections of 50 and 30 microL of nonheparinized autologous blood were associated with excessive mortality. Injections of 10 and 20 microL were associated with mean percentage volumes of damage of 1.804% and 4.019%, respectively. Sham subdural injections of saline were associated with minimal hemisphere damage (0.152%). This mouse model provides a means of investigating the effects of genotype on the brain's response to ASDH.

Sustained, long-lasting inhibition of nitric oxide synthase aggravates the neural damage in some models of excitotoxic brain injury

Brain nitric oxide (NO) can be a mediator of physiological and neuroprotective actions and an effector of neural damage. The effectiveness of acute or chronic inhibition of NO production in in vivo experiments of neurotoxicity/neuroprotection is controversial. We report here on the effects of a chronic, sustained inhibition of nitric oxide synthase (NOS) on the neurodegenerative damage caused by three different excitotoxic lesions. The damage caused by intrastriatal injection of ibotenic or kainic acid was aggravated in rats subjected to chronic NOS inhibition. On the contrary, the drop of cortical cholinergic input consequent to ibotenic acid-mediated degeneration of basal forebrain neurons was not altered by chronic NOS inhibition. The worsening of the damage was not related to any overt differential sensitivity to excitotoxicity of NOS-containing striatal neurons under conditions of NOS inhibition. These results suggest that, contrary to what has been often reported for short-term, mild inhibition of NO production, chronic and sustained NOS inhibition may exacerbate neuropathology. Thus, long-lasting shortage of NO may be detrimental when neuroprotective mechanisms related to the physiological action of this free radical are severely impaired. Although we cannot exclude that inhibition of the endothelial NOS isoform could have contributed to the worsening of neuropathology, differences among the paradigms of neurotoxicity used in the present study suggest a primary involvement of the neuronal NOS isoform. In view of the potential therapeutic use of NOS inhibitors, the effects of a too drastic alteration of the balance between neuroprotective and neurodegenerative actions of NO should be carefully considered.

The combination of lamotrigine and mild hypothermia prevents ischemia-induced increase in hippocampal glutamate

The excessive release of glutamate during cerebral ischemia may play an important role in subsequent neuronal injury. Both lamotrigine and hypothermia have independently been shown to attenuate the release of glutamate. In this study, the authors sought to determine whether these effects were additive. Thirty-five New Zealand White rabbits were randomized to one of six groups: a normothermic control group; a lamotrigine-treated group; two hypothermic groups at 33 degreesC or 34.5 degreesC; or two groups treated with both hypothermia at 33 degreesC or 34.5 degreesC plus lamotrigine. Animals were anesthetized before implanting microdialysis probes in the hippocampus. Esophageal temperature was maintained at 38 degreesC in the control and lamotrigine groups, while the temperatures of animals in the hypothermia and hypothermia-plus-lamotrigine groups were cooled to 33 degreesC or 34.5 degreesC. Two 10 minute periods of global cerebral ischemia were produced by inflating a neck tourniquet. Levels of glutamate in the microdialysate were then determined using high-performance liquid chromatography. Extracellular glutamate concentrations increased only slightly from baseline during the first ischemic period. Glutamate levels during the second ischemic episode in the hypothermia-plus-lamotrigine group (34.5 degreesC) were significantly lower than those in the hypothermia group alone (34.5 degreesC), lamotrigine, or control groups (P < .01). The fact that mild hypothermia (34.5 degreesC) plus lamotrigine (20 mg/kg) together were more effective in inhibiting extracellular glutamate accumulation than hypothermia (34.5 degreesC) or lamotrigine (20 mg/kg) alone, suggests the potential for increased neuroprotection by the addition of lamotrigine to mild hypothermia.

Phase II clinical trial of sipatrigine (619C89) by continuous infusion in acute stroke

BACKGROUND

The sodium channel blocker sipatrigine (619C89) prevents ischemia-induced glutamate release and is neuroprotective in animal models of stroke. Previous clinical experience found neuropsychiatric effects attributed to sipatrigine that may have been related to peak plasma concentrations of the drug.

METHODS

Patients within 12 h of clinically diagnosed stroke were randomized to placebo or sipatrigine at total doses of 10, 18, 27, or 36 mg/kg by continuous intravenous infusion over 65 h. Pharmacokinetic and routine laboratory analyses were undertaken. The outcome at 30 days or 3 months was assessed by Barthel and Rankin scores.

RESULTS

Twenty-seven patients were recruited: 7 of 21 patients stopped the sipatrigine infusion early due to adverse events as compared with none of 6 placebo patients. Neuropsychiatric effects (reduced consciousness, agitation, confusion, visual perceptual disturbance, or frank hallucinations) occurred in 16 of 21 patients receiving sipatrigine and in no placebo patients. Nausea, vomiting, infusion site reactions, and hyponatremia were also commoner in sipatrigine patients. Pharmacokinetic parameters were similar to those observed previously. No effects on outcome measures were demonstrated.

CONCLUSIONS

Continuous infusion of sipatrigine is associated with neuropsychiatric effects. No difference in the nature of these events was evident between this regimen and repeated short infusions, but it was possible to administer higher doses.

Neurotoxicity of glutamate at the concentration released upon spinal cord injury

Damage caused by administering glutamate into the spinal cord was characterized histologically. Glutamate destroyed neurons for several hundred micrometers around the administering microdialysis fiber. At 24 h after treatment, significant (P = 0.036) loss of neurons was observed (75%) relative to control (47%) near the fiber when glutamate was administered for 1 h at a concentration outside the fiber approximating the maximum glutamate released upon spinal cord injury. Significant loss of neurons (P = 0.006, 0.022) was also caused by administering a combination of glutamate at about its average concentration released upon injury over the 1 h period of administration in combination with the maximum aspartate concentration released upon injury. This work provides a direct demonstration that the concentrations of excitatory amino acids released upon spinal cord injury are neurotoxic. The destruction of neurons by exposure to excitatory amino acids when there is also substantial loss of neurons simply from the presence of the microdialysis fiber may reflect sensitization of neurons to excitotoxicity by stress.

Dissociation between the effects of interleukin-1 on excitotoxic brain damage and body temperature in the rat

The cytokine interleukin-1beta (IL-1beta) mediates and exacerbates excitotoxic brain damage in the rat striatum. Co-injection of the selective glutamate receptor agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (S-AMPA) with human recombinant IL-1beta (hrIL-1beta) in the striatum of rats results in both local (striatal) damage and extensive, distant neuronal death in the cortex. The objective of the present study was to investigate the mechanisms underlying IL-1beta actions on excitotoxic damage, and to determine whether this effect of IL-1beta, a potent pyrogen, is due to modification of body temperature. Striatal infusion of S-AMPA (7.5 nmol) in anaesthetised rats produced localised striatal damage. Intrastriatal co-infusion of hrIL-1beta (10 ng) with S-AMPA caused similar striatal damage, but also produced extensive cortical damage, together with a modest increase in body temperature (0.9 degrees C) compared to rats infused with S-AMPA alone. Infusion of S-AMPA into the striatum, together with intracerebroventricular (i.c.v.) injection of hrIL-1beta, produced a similar rise in temperature to striatal co-infusion of S-AMPA and hrIL-1beta, but resulted in only local (striatal) neuronal damage, that was similar to that caused by striatal infusion of S-AMPA alone. These data suggest that the effects of IL-1beta on AMPA-receptor mediated neuronal damage in the striatum can be dissociated from its pyrogenic effects on body temperature.

Brain temperature modifies glutamate neurotoxicity in vivo

The purpose of this study was to examine the effects of mild hypothermia and hyperthermia on glutamate excitotoxicity. Glutamate-induced cortical lesions were produced in hypothermic (32 degrees C), normothermic (37 degrees C), and hyperthermic (40 degrees C) rats by perfusion of a 0.5 M glutamate solution via a microdialysis probe. The volume of the lesion 7 days after glutamate perfusion was quantified histologically by image analysis. This histological assessment was performed in two experiments; in one, each of the target temperatures was induced before glutamate perfusion, and in the other, each of the target temperatures was induced after stopping the glutamate perfusion. We also examined the effect of temperature on the diffusion of exogenously delivered material in the extracellular space using autoradiography of the perfused glutamate solution containing 14C-labeled sucrose. In the two experiments in which each of the target temperatures was induced before or after glutamate perfusion, the volume of damage was reduced by mild hypothermia and enlarged by mild hyperthermia. The volume of 14C diffusion also increased as brain temperature increased. These results provide evidence that small variations of brain temperature modify glutamate excitotoxicity. The results also suggest that the change in glutamate diffusion in the extracellular space is one mechanism by which mild hypothermia and hyperthermia exert their protective and harmful effects respectively.

A microdialysis study investigating the mechanisms of hydroxyl radical formation in rat striatum exposed to glutamate

Considerable evidence has linked hydroxyl radicals (.OH) to excitotoxicity. Glutamate infused through a microdialysis probe into rat striatum induced a massive .OH production, which was completely blocked by PBN and attenuated by dizocilpine, 2-amino-5-phosphonopentanoic acid (AP-5), NG-nitro-L-arginine methyl ester (L-NAME) and mepacrine. Thus, we suggest that the neurotoxic effects of glutamate in vivo may derive from an increased formation of .OH resulting from excessive activation of NMDA receptors and downstream enzymes such as NOS and PLA2.

Kappa-opioid potentiation of tumor necrosis factor-alpha-induced anti-HIV-1 activity in acutely infected human brain cell cultures

Opioids have been postulated to play an immunomodulatory role in the pathogenesis of HIV-1. Synthetic kappa-opioid receptor (KOR) ligands have been found to inhibit HIV-1 expression in acutely infected microglial cell cultures. We recently found that interleukin(IL)-1beta and tumor necrosis factor(TNF)-alpha have antiviral effects in acutely infected mixed glial/neuronal cell cultures. In the present study, we investigated whether selective KOR ligands would exert antiviral effects in acutely infected brain cell cultures. While the KOR ligand trans-3,4-dichloro-N-methyl-N[2-(1-pyrolidinyl)cyclohexyl]benze neaceamide methanesulfonate (U50,488) alone had little anti-HIV-1 activity, this opioid potentiated in a concentration-dependent manner the antiviral activity of TNF-alpha, but not of IL-1beta. The potentiating effect of U50,488 was detected after a 6-hr pretreatment and peaked at 24 hr. The KOR antagonist nor-binaltorphimine completely blocked the potentiating effect of U50,488, suggesting the involvement of a KOR-mediated mechanism. Antibodies to TNF-alpha completely blocked the potentiating effect of U50,488, suggesting a critical role for TNF-alpha. Antibodies to IL-1beta blocked the potentiating effect of U50,488, suggesting that IL-1beta was released following U50,488 treatment, which might contribute to the potentiating effect of U50,488. These in vitro findings support the notion that synthetic kappa-opioids could be considered as potential adjunctive therapeutic agents in HIV-1-related brain disease.

Neurodegeneration induced by reversed microdialysis of NMDA; a quantitative model for excitotoxicity in vivo

This study characterizes a quantifiable in vivo model of excitotoxicity. In halothane anesthetized rats, microdialysis probe was implanted into somatosensory cortex/striatum and perfused by various concentrations (1, 10, 50 and 100 mmol/l) of N-methyl-d-aspartate (NMDA) for 20 min. After 24 h, histological quantification confirmed that NMDA produced a concentration-dependent excitotoxic lesion. With 10 mmol/l NMDA, coadministration of magnesium reduced significantly, and 2-amino-5-phosphonovalerate blocked completely the development of excitotoxic injury.

Interleukin-1beta and the interleukin-1 receptor antagonist act in the striatum to modify excitotoxic brain damage in the rat

The cytokine interleukin-1 (IL-1) has been implicated in ischaemic, traumatic and excitotoxic brain damage. The results presented here reveal novel actions of IL-1 in the striatum which markedly exacerbate cortical neuronal damage elicited by local excitotoxins in the striatum or cortex. Intrastriatal infusion of IL-1 receptor antagonist, IL-1ra, markedly inhibited striatal neuronal damage caused by N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor activation in the rat. In contrast, intracortical infusion of IL-1ra failed to inhibit NMDA or AMPA receptor-induced damage in the cortex. Intrastriatal co-infusion of IL-1 with the NMDA or AMPA receptor agonist did not affect local striatal damage induced by activation of either glutamate receptor subtype, but caused extensive cortical damage when administered into the striatum with AMPA. This secondary damage was significantly reduced by pretreatment with the NMDA receptor antagonist (MK-801), which did not affect local (striatal) damage caused by AMPA. Infusion of IL-1beta into the striatum (but not the cortex) markedly enhanced cortical damage caused by infusion of an NMDA or AMPA receptor agonist into the cortex. These data reveal selective actions of IL-1 and IL-1ra in the striatum, which influence cortical neuronal loss and suggest that IL-1 selectively enhances damage caused by AMPA receptor activation.

Glutamate induces hydroxyl radical formation in vivo via activation of nitric oxide synthase in Sprague-Dawley rats

It was recently reported that neuronal nitric oxide synthase (NOS) generates oxygen-derived free radicals in vitro at low concentrations of L-arginine. Using the microdialysis technique, we monitored both hydroxyl radical (.OH) and nitric oxide (.NO) formation in rat striatum perfused with glutamate (500 mM). .OH and .NO were quantitated in microdialysates by measuring the amounts of the non-enzymatic hydroxylation product of salicylate (2,3-dihydroxybenzoic acid) and the metabolites of .NO (nitrite + nitrate), respectively. .OH levels were dramatically increased during glutamate perfusion, while .NO generation was virtually abolished. .OH production was inhibited by the specific NOS blocker, NG-nitro-L-arginine methyl ester. This effect was not reversed but potentiated by L-arginine. Thus, it is likely that NOS generates oxygen-derived free radicals instead of .NO in brain subjected to highly excitotoxic conditions.

Safety and tolerability of 619C89 after acute stroke

BACKGROUND

619C89 is a use-dependent sodium channel blocker which reduces hemispheric infarction volume by up to 60% after permanent middle cerebral artery occlusion in rats. Intravenous doses of up to 1 mg/kg have been well tolerated by healthy young and elderly volunteers. This study sought to assess safety and tolerability of 619C89 in the treatment of acute stroke.

METHODS

Patients were randomised within 12 h of onset of stroke to receive 619C89 or placebo as an intravenous loading dose, followed by maintenance doses given 8 hourly for 64 h in a double-blind, ascending-dose tolerance study. Dosing commenced at 0.5 mg/kg loading plus 0.25 mg/kg/8 h maintenance for the first group and increased in increments of 0.5 mg/kg loading +0.25 mg/kg/8 h maintenance thereafter. Safety evaluation was continued for 3 months.

RESULTS

48 patients were recruited. 12 received placebo and 36 received 619C89 in doses up to 2.5 mg/kg loading plus 1.25 mg/kg/8 h. Dose escalation was stopped after the occurrence of hallucinations in 5 of 18 patients who received 2 mg/kg/8 h or more. Gastro-intestinal upset and confusion were also possibly drug related. No drug-related effects on cardiovascular function were found.

CONCLUSIONS

619C89 was associated with significant central nervous system side-effects at doses of 2 mg/kg + 1 mg/kg/8 h or greater as discrete intravenous infusions within 12 h of stroke onset. It may also cause gastro-intestinal side-effects. Doses below this are well tolerated in patients. No adverse cardiovascular effects were seen.

Lamotrigine protects hippocampal CA1 neurons from ischemic damage after cardiac arrest

BACKGROUND AND PURPOSE

Lamotrigine (LTG) is an anticonvulsant drug whose mechanism of action may involve the inhibition of glutamate release by blocking voltage-dependent sodium channels. Glutamate neurotoxicity may contribute to cerebral ischemic damage after recovery from cardiac arrest. Thus, LTG may prevent the brain damage associated with global cerebral ischemia by reducing the release of glutamate from presynaptic vesicles during the ischemic insult or the early recovery period.

METHODS

LTG was studied in cardiac arrest-induced global cerebral ischemia with reperfusion in rats. In the first set of experiments, LTG (100 mg/kg, p.o.) was administered before induction of ischemia; and in the second experiment, LTG (10 mg/kg, i.v.) was given 15 minutes after ischemia and a second dose (10 mg/kg,i.v.) was given 5 hours later.

RESULTS

In both experiments LTG reduced the damage to the hippocampal CA1 cell population by greater than 50%. Neuroprotection was not associated with changes in brain temperature or plasma glucose concentration. Plasma concentrations of LTG ranged between 8 and 13 micrograms/mL. Patients taking LTG as a monotherapy for epilepsy typically have plasma levels of LTG in the 10 to 15 micrograms/mL range.

CONCLUSIONS

These data suggest that LTG may be effective in preventing brain damage after recovery from cardiac arrest. Patients on LTG monotherapy for epilepsy have plasma concentrations very similar to those found to be neuroprotective in this study. Although difficult to extrapolate, our data suggest that LTG at neuroprotective doses may be well tolerated by humans.

Intracortical glutamate perfusion in vivo induces cellular alterations in specific protein kinase C isoforms and amyloid precursor protein

This study investigated the immunostaining of protein kinase C (PKC) isoforms and amyloid precursor protein (APP) in rat brain cortex and determined alterations following an excitotoxic challenge in vivo. Cellular alterations in APP and PKC isoforms (alpha, beta, gamma, delta, epsilon, and zeta) following glutamate perfusion in the rat parietal cortex were compared with NaCl perfusion. In all animals, two histological zones could be defined consistently, a necrotic core and a boundary zone immediately adjacent to the core. Following glutamate and NaCl perfusion, cellular immunoreactivity to PKC isoforms and amino-terminal APP was significantly reduced within the necrotic core. Striking carboxy-terminal APP immunoreactivity was observed in some neurons remaining within the necrotic core. In the boundary of the glutamate lesion, the perikarya of most neurons were intensely immunoreactive to PKC alpha and beta. Furthermore, within the boundary zone, enhanced immunoreactivity within neuronal perikarya was observed to amino-terminal APP and, to a lesser extent, carboxy-terminal APP. Increased immunostaining of PKC alpha and beta and APP at the boundary zone was a consistent feature of intracortical glutamate perfusion and was not observed following NaCl perfusion. There were minimal alterations in PKC isoforms gamma, delta, epsilon, and zeta, in the boundary region following intracortical glutamate or NaCl perfusion. There was no astrocytic response, as detected by GFAP immunoreactivity, at the boundary zone. These findings indicate that there is a topographical relationship between cellular alterations of specific PKC isoforms and APP following an excitotoxic challenge in vivo.

alpha-Phenyl-N-tert-butylnitrone attenuates excitotoxicity in rat striatum by preventing hydroxyl radical accumulation

Various in vitro experiments have indicated that oxygen-derived free radicals may contribute to excitotoxic neuronal death. In the present study we induced excitotoxicity in rat striatum by perfusing glutamate at a high concentration through a microdialysis probe. We observed an increased formation of hydroxyl radicals (.OH) during the perfusion of the excitotoxin and an extensive striatal lesion 24 h after the insult. The spin trap, alpha-phenyl-N-tert-butylnitrone (PBN), attenuated both hydroxyl radical levels and the volume of the lesion. This result suggests that the neuroprotection may be due to a free radical scavenging mechanism. It also implies that PBN may be used in pathological situations involving excitotoxicity such as stroke, brain trauma, and chronic neurologic diseases.

Altered glutamatergic transmission in neurological disorders: from high extracellular glutamate to excessive synaptic efficacy

This review is a critical appraisal of the widespread assumption that high extracellular glutamate, resulting from enhanced pre-synaptic release superimposed on deficient uptake and/or cytosolic efflux, is the key to excessive glutamate-mediated excitation in neurological disorders. Indeed, high extracellular glutamate levels do not consistently correlate with, nor necessarily produce, neuronal dysfunction and death in vivo. Furthermore, we exemplify with spreading depression that the sensitivity of an experimental or pathological event to glutamate receptor antagonists does not imply involvement of high extracellular glutamate levels in the genesis of this event. We propose an extension to the current, oversimplified concept of excitotoxicity associated with neurological disorders, to include alternative abnormalities of glutamatergic transmission which may contribute to the pathology, and lead to excitotoxic injury. These may include the following: (i) increased density of glutamate receptors; (ii) altered ionic selectivity of ionotropic glutamate receptors; (iii) abnormalities in their sensitivity and modulation; (iv) enhancement of glutamate-mediated synaptic efficacy (i.e. a pathological form of long-term potentiation); (v) phenomena such as spreading depression which require activation of glutamate receptors and can be detrimental to the survival of neurons. Such an extension would take into account the diversity of glutamate-receptor-mediated processes, match the complexity of neurological disorders pathogenesis and pathophysiology, and ultimately provide a more elaborate scientific basis for the development of innovative treatments.

Failure of glycine site NMDA receptor antagonists to protect against L-2-chloropropionic acid-induced neurotoxicity highlights the uniqueness of cerebellar NMDA receptors

Cultured cerebellar granule cells and cerebellar slices from neonatal rats have been widely used to examine the biochemistry of excitatory amino acid-induced cell death mediated in part by the activation of NMDA receptors. However, the NMDA subunit stoichiometry, producing functional NMDA receptors is different in cultured granule cells, neonatal and adult rat cerebellum as compared to the NMDA receptors in forebrain regions. We have used the L-2-chloropropionic acid (L-CPA) (750 mg/kg) model of NMDA-mediated selective cerebellar granule cell necrosis in vivo to examine the role of the glycine binding site and possible effect of the NR2C subunit (which is largely expressed only in the cerebellum) on granule cell necrosis. The abilities of various NMDA receptor antagonists were examined in vivo to determine the relative contribution of both glutamate and glycine sites involved in the L-CPA-induced neurotoxicity. The potent neuroprotective, non-competitive NMDA receptor antagonist dizocilpine (MK-801) was compared with glutamate and glycine site NMDA antagonists. We have examined a number of markers for the L-CPA-induced granule cell necrosis. The L-CPA-induced reduction in cerebellar aspartate and glutamate concentrations were used as markers of granule cell necrosis. We also measured the cerebellar water content and sodium concentrations as measures of the L-CPA-induced cerebellar edema that accompanies the granule cell necrosis. Finally the ability of the NMDA antagonists to attenuate the L-CPA-induced reductions in body weight gain and the prevention of the loss in hindlimb function using a behavioral measure of hindlimb retraction were examined. The potent glutamate antagonists, CPP and CGP40116 and dizocilpine prevented the L-CPA-induced locomotor dysfunction and granule cell necrosis as measured by their ability to prevent L-CPA-induced reduction in aspartate and glutamate concentrations. CPP, CGP40116 and dizocilpine also prevented the appearance of cerebellar edema following L-CPA administration. In addition, dizocilpine, CPP and CGP40116 were able to partially prevent the L-CPA-induced loss in body weight over the 48 h experimental period. In contrast, none of the glycine partial agonists or antagonists, namely (+/-)HA-966, D-cycloserine, MDL-29,951, DPCQ, MNQX or L-701 252 were able to prevent the L-CPA-induced loss in body weight, L-CPA-induced granule cell necrosis and behavioral disturbances when administered to rats. None of the NMDA antagonists had any effect on the cerebellar neurochemistry when injected alone or had any effect on animal behavior except for dizocilpine, CPP, CGP40116 and (+/-)HA-966 which resulted in a transient sedation for between three and five hours immediately following their administration. In conclusion, we demonstrated that NMDA open channel blockade and glutamate antagonists can provide full neuroprotection against the L-CPA-induced granule cell necrosis. The failure of the glycine partial agonist and antagonists to provide any neuroprotection against L-CPA-induced neurotoxicity in the cerebellum contrast with their neuroprotective efficacy in other animal models of excitatory amino acid-induced cell death in forebrain regions in vivo. We therefore suggest that the glycine site plays a lesser role in modulating NMDA receptor function in the cerebellum and may explain why cells expressing NMDA receptors composed of NR1/NR2C subunits are particularly resistant to excitatory amino acid-induced neurotoxicity.

Patterns of increased glucose use following extracellular infusion of glutamate: an autoradiographic study

An apparent transient increase in local glucose utilization has been demonstrated in certain brain areas after global and focal ischemia in several models. A coincident transient increase in extracellular glutamate has been shown in the same brain regions in many of these models. To test the hypothesis that an increase in metabolism is an important component of the excitotoxic effect of glutamate, we perfused glutamate at different concentrations (0.01, 0.1, 0.5, 1 M) into the extracellular space, and performed 2-deoxyglucose autoradiography after 90 min of infusion. Furthermore, we infused 14C-labeled glutamate to investigate its diffusion characteristics within the brain using autoradiographic methods. Glutamate at 0.5 and 1 M concentration caused large consistent areas of brain damage with all the histological features of acute infarction, although ischemia does not occur in this model. Glucose utilization was significantly increased (115 +/- 20 vs. 56 +/- 13 mumol/100 g/min in controls p < 0.01) in a sharply demarcated concentric zone, at the boundary between histologically damaged and normal brain, suggesting that viable cells not yet destroyed by glutamate respond by increased glucose metabolism. [14C] Glutamate diffused into the brain in a dose-dependent manner, and the pattern of its diffusion corresponded closely to that of the histological lesion and the zone of increased glucose uptake. We speculate that the increase in glucose use, which is not caused by ischemia in this model, is due to a metabolic response to glutamate and may be due to attempts to restore ionic homeostasis or repair cell damage.

The relationship between glutamate release and cerebral blood flow after focal cerebral ischaemia in the cat: effect of pretreatment with enadoline (a kappa receptor agonist)

The effect of the kappa-opioid agonist enadoline (CI-977) upon the relationship between cerebral blood flow and glutamate release was simultaneously assessed (using microdialysis and hydrogen clearance techniques respectively) at the same anatomical locus in the cerebral cortex (suprasylvian gyrus) after permanent middle cerebral artery (MCA) occlusion in halothane-anaesthetised cats. During controlled graded ischaemia, pretreatment with enadoline (0.3 mg/kg i.v. followed by continuous infusion at 0.15 mg/kg/h), initiated 30 min prior to MCA occlusion, significantly attenuated the marked increases in extracellular glutamate, aspartate and GABA observed in the focal ischaemic penumbra. The present data are consistent with the hypothesis that the neuroprotective efficacy of enadoline in focal cerebral ischaemia is due to inhibition of glutamate release in the ischaemic penumbra.

Application of glutamate in the cortex of rats: a microdialysis study

Glutamate, a major neurotransmitter in the brain, is also involved in pathophysiological processes resulting in secondary lesions following ischaemia or trauma. In the present study we investigated the relationship between glutamate excitotoxicity free radical induction (indicated by ascorbic acid level) and glucose-lactate metabolism. Monosodium glutamate was applied through microdialysis probes (500 mM in perfusate) into the cortex of rats for 30 minutes and ascorbic acid (ASC), glucose (GLUC) and lactate (LAC) were measured in dialysates. Glutamate produced a cortical lesion with an average volume of 12.7 +/- 1.4 mm3. Analysis of dialysates revealed a significant increase of ASC (325 +/- 52% of baseline) and LAC (677 +/- 86%) in the core lesion. In the lesion periphery a non-significant and short-lasting elevation was measured for both parameters with a second microdialysis probe (about 1.3 mm frontally to the first probe). A concomitant decrease of GLUC was found in both probes, reaching 29 +/- 8% and 60 +/- 7% of basal levels in the core and periphery of the lesion, respectively. In addition, we studied the delivery characteristics of several glutamate concentrations (10, 100 or 1000 mM in perfusate) during a 90-minute application into the cortex. The delivery of glutamate from the perfusate to the brain was about 33-38% in the first 30 min and afterwards 11 25% of the total in the perfusate. The results show that cortical application of glutamate changes the composition of the extracellular fluid, which could contribute to the development of the lesion.

The role of nitric oxide in facial motoneuronal death

The present study aimed to determine whether nitric oxide synthase (NOS)/nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) activity would be induced in facial motoneurons after facial nerve avulsion and if so, whether such activity was related to neuronal death commonly observed after such injury. The left facial nerve in each of 28 Wistar albino rats was avulsed from the facial canal. Ten of them received either daily injections of N omega-nitro-L-arginine methyl ether (L-NAME) or the vehicle. After survival times ranging from 2-50 days, serial brainstem sections were processed for NOS immunocytochemistry and NADPH-d histochemistry respectively. The number of surviving, NOS and NADPH-d positive and NOS negative neurons were compared statistically. Two days after facial nerve avulsion, increased NADPH-d activity was noticed in the facial motoneurons and in the endothelial lining of many dilated blood vessels in the facial motor nucleus (FMN). NOS-positive neurons were not detectable until five days after operation. Both the number and staining intensity of NADPH-d and NOS-positive neurons increased steadily with increasing survival time while the number of surviving neurons decreased after nerve avulsion. Daily administration of L-NAME protected 17% the neurons from death in the affected FMN when examined at 30 days after nerve avulsion, suggesting a neurodestructive property of NO. It was also noticed that some of the surviving neurons were first NOS positive but became NOS negative later.

Nitric oxide does not mediate acute glutamate neurotoxicity, nor is it neuroprotective, in rat brain slices

Nitric oxide (NO), generated upon glutamate receptor activation, elicits cyclic GMP accumulation through stimulation of guanylyl cyclase. NO is also a potential cytotoxin that has been suggested, on the basis of tissue culture experiments, to mediate neuronal damage associated with excessive activity of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. We have investigated the involvement of NO in the toxicity of glutamate receptor agonists in brain slice preparations. Slices of cerebellum and hippocampus from the developing rat exhibited neuronal necrosis following exposure (5-30 min) to NMDA (100 microM or 1 mM). When the exposures were carried out in the presence of NO synthase inhibitors, at concentrations suppressing NMDA-induced NO formation (as judged by measurements of cyclic GMP accumulation), the extent of injury was unaffected. To determine if exogenous NO is able to replicate NMDA toxicity, the slices were exposed to high concentrations of NO donating compounds for up to 2 hr. No damage was detectable. NO donors, moreover, neither reduced NMDA toxicity, nor potentiated the degeneration caused by just suprathreshold NMDA concentrations. The toxicities of non-NMDA agonists, or of glutamate itself, were also unaltered by NO synthase inhibitors or NO donors. Similar results were obtained using hippocampal slices from more mature animals. We conclude that the acute neurodegeneration mediated by NMDA or non-NMDA receptors in the slice preparations is not mediated by NO, nor is NO neuroprotective under these conditions.

Anti-ischaemic efficacy of a nitric oxide synthase inhibitor and a N-methyl-D-aspartate receptor antagonist in models of transient and permanent focal cerebral ischaemia

1. We have recently developed a new model of transient focal ischaemia in the rat utilising topical application of endothelin-1 to the left middle cerebral artery (MCA). In order to validate this approach the present study assessed the neuroprotective efficacy of the NMDA receptor antagonist dizocilpine (MK-801) in the endothelin-1 model. The anti-ischaemic efficacy of the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) was subsequently evaluated, and contrasted with its efficacy against permanent focal ischaemia, to determine the utility of the endothelin-1 model for identification of novel pharmacoprotective agents. 2. MK-801 (0.12 mg kg-1 bolus, 108 micrograms kg-1 h-1 infusion i.v., either 1 or 2.5 h pre-transient MCA occlusion (MCAO)) induced hypotension that persisted for approximately 1.5 h so that mean arterial blood pressure (MABP) at the time of MCAO was significantly lower in the 1 h group compared with control (MABP: 86 +/- 11, 68 +/- 6 and 84 +/- 4 mmHg (mean +/- s.d.) for saline, 1 h MK-801 and 2.5 h MK-801 groups respectively). The 2.5 h pretreatment schedule resulted in significant reduction (71%) in the volume of hemispheric damage (assessed 4 h post onset of ischaemia) while the 1 h pretreatment schedule did not (volumes of hemispheric damage: 59 +/- 38, 51 +/- 51 and 17 +/- 28 mm3 for saline, 1 h and 2.5 h MK-801 groups). 3. Thus the considerable neuroprotective effect of MK-801 in the endothelin-1 model of transient focal cerebral ischaemia was highly sensitive to drug-induced hypotension. This result is in contrast to previous studies of permanent MCAO where MK-801-induced hypotension did not compromise its neuroprotective action.4. L-NAME (3 mg kg-1, i.v. 30 min pre-MCAO) moderately, but significantly, reduced (16%) the volume of ischaemic damage 4 h post-permanent MCA occlusion, whereas the 29% reduction in volume of damage achieved in the model of transient focal ischaemia did not attain significance due to the greater variability associated with this model. L-NAME did not significantly alter MABP in either model.5. The modest neuroprotection achieved with NO synthase inhibition suggests NO is of relatively minor importance as a mediator of neurotoxicity following permanent focal cerebral ischaemia. In addition the comparable efficacy of L-NAME against transient focal ischaemia suggests the presence of reperfusion does not enhance the contribution of NO to neuronal injury in the acute (4 h) phase following a focal ischaemic insult.

Resistance to nitroprusside neurotoxicity in perinatal rat brain

Nitric oxide (NO) may mediate some of the toxic effects of the excitatory amino acid (EAA) glutamate when there is overactivation of the N-methyl-D-aspartate (NMDA) receptor. In the developing rodent nervous system, NMDA neurotoxicity peaks at postnatal day 7. To assess whether NO toxicity exhibits a similar developmental profile, we injected the NO generator sodium nitroprusside into the immature and adult rodent hippocampal formation and striatum, using a dose known to damage the adult nervous system. Contrary to our expectations, we found the immature brain highly resistant to the toxic effects of sodium nitroprusside.