Clinical experience with excitatory amino acid antagonist drugs

Increased production of extracellular glutamate by the mitochondrial glutaminase following neuronal death

Elevated extracellular concentrations of the excitatory transmitter glutamate are an important cause of neuronal death in a variety of disorders of the nervous system. The concentrations and rates of clearance and production of extracellular glutamate were measured in the medium of primary cultures from mouse neocortex containing neurons, astrocytes, or both cell types. Measurements were performed in the presence and absence of 2 mM glutamine with or without neuronal injury caused by 5-h exposure to hypoxia or 500 microM N-methyl-D-aspartate or a freeze-thaw cycle. High rates of glutamate generation (0.5-0.8 microM/min in the 0.4-ml culture well) occurred if neurons were both damaged and exposed to glutamine. Intact neurons or glia exposed to glutamine generated only small amounts of glutamate (0.03 microM/min). Glutamate generation by damaged neurons was dependent on the presence of glutamine, activated by phosphate, and inhibited by 6-diazo-5-oxo-L-norleucine and p-chloromercuriphenylsulfonic acid (pCMPS), strongly implicating the mitochondrial glutaminase. Following 5-h exposure to 500 microM N-methyl-D-aspartate, the glutaminase was localized to fragments of damaged neurons and was accessible to inhibition by the membrane-impermeant pCMPS. The glutaminase activity from damaged neurons is sufficient to account for the neurotoxic concentrations of glutamate in hypoxic mixed neuronal-glial cultures exposed to 2 mM glutamine. Finally, pCMPS is neuroprotective and also prevents the increased rate of generation of glutamate observed in neuronal cultures after prolonged exposure to glutamine. The cumulative data indicate the following: 1) excitotoxic neuronal death activates the hydrolysis of extracellular glutamine by the mitochondrial glutaminase, and 2) the glutaminase in damaged neurons is sufficient to cause neuronal death in in vitro models of neuronal injury.

Characterization of NMDA- and AMPA-induced enhancement of AP-1 DNA binding activity in rat cerebellar granule cells

Effects of the glutamate receptor agonists, N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), on the activator protein-1 (AP-1) DNA binding activity were studied in primary cultures of rat cerebellar granule cells. Application of NMDA as well as of AMPA produced a concentration-dependent enhancement of AP-1 binding. Further examination revealed that only a brief exposure (10 min) to NMDA or AMPA was required for the initiation of a significant, four- to sixfold enhancement of AP-1 DNA binding activity. Blockade of the desensitization of AMPA receptors by cyclothiazide further reduced the exposure time needed to activate the AP-1 complex. The time needed to achieve a maximal increase of AP-1 binding activity varied depending on the glutamate receptor agonist used. NMDA gave maximal AP-1 stimulation after 60 min exposure, whereas stimulation with AMPA alone reached a maximum after 240 min exposure. When AMPA was applied together with cyclothiazide the maximal enhancement of AP-1 binding was reached much faster, within 120 min. Supershift analysis with specific antibodies against the members of Fos and Jun protein families (c-Fos, Fos B, c-Jun, Jun B, Jun D) revealed that the NMDA-induced AP-1 complex was composed predominantly of Jun D and c-Fos. The composition of the AP-1 complex activated by AMPA alone was similar to that produced by NMDA, but with an additional contribution of Fos B. In contrast, application of AMPA plus cyclothiazide induced an AP-1 transcription with contribution of Jun D, c-Fos, Fos B, c-Jun and Jun B proteins. These findings indicate that glutamate is able to enhance AP-1 DNA binding activity in cerebellar granule cells through both NMDA and AMPA glutamate receptors.

Pharmacological characterization of MDL 105,519, an NMDA receptor glycine site antagonist

MDL 105,519, (E)-3-(2-phenyl-2-carboxyethenyl)-4,6-dichloro-1 H-indole-2-carboxylic acid, is a potent and selective inhibitor of [3H]glycine binding to the NMDA receptor. MDL 105,519 inhibits NMDA (N-methyl-D-aspartate)-dependent responses including elevations of [3H]N-[1,(2-thienyl)cyclohexyl]-piperidine ([3H]TCP) binding in brain membranes, cyclic GMP accumulation in brain slices, and alterations in cytosolic CA2+ and NA(+)-CA2+ currents in cultured neurons. Inhibition was non-competitive with respect to NMDA and could be nullified with D-serine. Intravenously administered MDL 105,519 prevented harmaline-stimulated increases in cerebellar cyclic GMP content, providing biochemical evidence of NMDA receptor antagonism in vivo. This antagonism was associated with anticonvulsant activity in genetically based, chemically induced, and electrically mediated seizure models. Anxiolytic activity was observed in the rat separation-induced vocalization model, but muscle-relaxant activity was apparent at lower doses. Higher doses impair rotorod performance, but were without effect on mesolimbic dopamine turnover or prepulse inhibition of the startle reflex. This pattern of activities differentiates this compound from (5R,10S)-(+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) and indicates a lower psychotomimetic risk.

Huntington's disease: N-methyl-D-aspartate receptor coagonist glycine is increased in platelets

Experiments in vertebrates and striatal tissue cultures have provided evidence for a neuroexcitotoxic cause for the neurodegeneration in Huntington's disease (HD), via N-methyl-D-aspartate (NMDA) receptors. Glycine in vitro increases the response of NMDA receptors to its agonists via the NMDA receptor-associated glycine receptor, and the same effect has been observed in vivo. Significantly increased levels of glycine have previously been found in the cerebrospinal fluid of patients with HD. In this present study glycine was measured in platelets and plasma of patients with HD and in controls by high-pressure liquid chromatography. Mean glycine concentration was significantly increased (P < or = 0.01) in platelets in HD compared to controls, though plasma glycine was normal. A possible role for glycine in the pathogenesis of HD, based on the excitotoxicity hypothesis of HD, is discussed.

Isoflurane and propofol block neurotoxicity caused by MK-801 in the rat posterior cingulate/retrosplenial cortex

In acute brain injury syndromes, the potent N-methyl-D-aspartate (NMDA) antagonist, MK-801, can prevent neuronal degeneration, and the general anesthetics, isoflurane and propofol, may also provide neuroprotective benefits. An obstacle to the use of NMDA antagonists for neuroprotective purposes is that they can cause a neurotoxic vacuole reaction in cerebrocortical neurons. This study demonstrates the ability of isoflurane and propofol to prevent the neurotoxic vacuole reaction induced by MK-801. Low sedative doses of inhaled isoflurane (1%) or intravenous (i.v.) propofol (7.5 mg/kg/h) were as effective as higher general anesthetic doses. Thus, in the clinical management of acute brain injury conditions such as stroke and brain trauma, administration of one of these anesthetic agents together with an NMDA antagonist may be an excellent formula for obtaining optimal neuroprotection while eliminating serious side effects.

Enzyme-catalyzed production of the neuroprotective NMDA receptor antagonist 7-chlorokynurenic acid in the rat brain in vivo

NMDA receptors play a critical role in neurotransmission and are also involved in the occurrence of excitotoxic nerve cell death. Synthetic halogenated analogs of the endogenous broad spectrum excitatory amino acid receptor blocker kynurenic acid are among the most potent and selective antagonists of the glycine co-agonist site of the NMDA receptor complex. Pharmacological blockade of this site provides neuroprotection in animal models of cerebral ischemia, epilepsy and neurodegenerative disorders, and does not appear to be associated with some of the undesirable side effects linked to classic competitive and non-competitive NMDA receptor antagonists. Here we demonstrate the neuroprotective quantities of 7-chloro-kynurenic acid (7-Cl-KYNA), one of the most selective and well-studied glycine site antagonists, can be synthesized in the brain from its bioprecursor L-4-chlorokynurenine (4-Cl-KYN). Intracerebral infusion of 4-Cl-KYN dose-dependently reduced quinolinate neurotoxicity in the rat hippocampus after enzymatic conversion to 7-Cl-KYNA by kynurenine aminotransferase. In accordance with previous studies demonstrating that kynurenine aminotransferase is preferentially localized in astrocytes, both the enzymatic formation of 7-Cl-KYNA and the neuroprotective potency of 4-Cl-KYN were substantially reduced following an intrahippocampal injection of the gliotoxin fluorocitrate. In situ produced 7-Cl-KYNA offers a novel neuroprotective strategy for targeting the glycine/NMDA site while avoiding excessive receptor blockade and reducing the clinical risks associated with conventional NMDA receptor antagonism.

Nimodipine improves kainic acid induced neurotoxicity in cerebellar granular cell culture: a double-blind dose-response study

The neuroprotective role of nimodipine was tested in kainic acid (50 and 100 microM) induced neurotoxicity in cerebellar granular cell cultures of 4 to 7 day-old rat pups. Nimodipine was applied in 50, 100 and 200 microM concentrations. Kainate, in either dose, induced cerebellar granular cell death in respect to controls and the results were statistically significant (P = 0.000 for both doses). However, kainic acid in 100 microM concentration led to higher rates of cell death than 50 microM (P = 0.017). The neuroprotective role of nimodipine in kainate induced neurotoxicity was dose dependent. Kainate toxicity in 50 microM concentration was blocked by 50 and 100 microM nimodipine concentrations (P = 0.006 and P = 0.002, respectively) while 200 microM nimodipine was found ineffective. The most effective nimodipine dose for 100 microM kainic acid neurotoxicity was 200 microM (P = 0.000) while 50 and 100 microM concentrations of nimodipine were found ineffective. In this study, we have proven the dose-dependent neuroprotective role of nimodipine in kainate induced neurotoxicity in cerebellar granular cell cultures of rat pups.

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.

AMPA neurotoxicity in cultured cerebellar granule neurons: mode of cell death

Various forms of cell death induced by the glutamate receptor agonist, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), were analyzed by determining the capacity of cultured cerebellar granule cells to metabolize 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) into formazan, by measuring the leakage of lactate dehydrogenase (LDH), by using confocal microscopy to visualize propidium iodide staining of apoptotic nuclei, and by using field inversion gel electrophoresis (FIGE) for the detection of AMPA-produced cleavage of DNA into high molecular-weight fragments (50 kbp). All these measures indicated that stimulation of AMPA receptors may be involved in the neurotoxic effects of glutamate, and that AMPA-induced neurotoxicity in cerebellar granule cells display morphologically distinct features of both necrotic and apoptotic modes of cell death. In agreement with previous observations, a blockade of AMPA receptor desensitization was necessary to unmask AMPA-induced functional responses in cultured cerebellar granule neurons in vitro. Microfluorimetric measurements of free cytoplasmic calcium concentrations ([Ca2+]i) in single cerebellar neurons revealed that AMPA neurotoxicity was accompanied by a pronounced elevation of [Ca2+]i. Our current results add further evidence to the notion that glutamate-induced neurotoxicity in cerebellar granule cells is mediated not only through NMDA receptors but also through a direct activation of AMPA receptor-regulated cation channels.

The rationale for new therapies in acute ischaemic stroke

Although stroke is a major cause of morbidity and mortality, it is only relatively recently that a concerted effort has been made to develop acute treatments. Thrombolytics, such as recombinant tissue plasminogen activator (rt-PA), may benefit selected patients within 3 h of cerebral infarction. CUrrently, rt-PA is only licensed for use in the United States. Many potential strategies for neuroprotection exist and are currently under investigation. Because the mechanisms of neurotoxicity involve numerous interdependent processes, it may be that the interpretation of a single site in the cascade of events is insufficient to provide effective neuroprotection. Drugs acting at several sites in the neurotoxic cascade may be more effective, and the results of Phase III studies with the novel neoroprotectant lubeluzole are anticipated.

The influence of repeated doses, route and time of administration on the neuroprotective effects of BIII 277 CL in a rat model of focal cerebral ischemia

OBJECTIVE

We investigated the influence of dose, route and time of administration on the neuroprotective effects of the noncompetitive N-methyl-D-aspartic acid antagonist BIII 277 CL ([2R-[2alpha, 3(R*), 6alpha]]-1,2,3,4,5,6-hexahydro-3-(2-methoxy-propyl)-6,11,11-trimethyl-2,6-methao-3-benzazocin-9-ol hydrochloride).

METHODS

Focal cerebral ischemia was induced in isoflurane-anaesthetized Fischer rats by permanent occlusion of the left middle cerebral artery. Rats were treated with BIII 277 CL three times at doses of 1 and 3 mg/kg intraperitoneally (IP) (5 to 10 minutes and 4 and 24 hours after occlusion) or twice with 0.1, 0.3, and 1.0 mg/kg subcutaneously (SC) (5 to 10 minutes and 3 hours after occlusion) or twice with 1 mg/kg SC (30 minutes and 3 hours 30 minutes; 1 and 4 hours; 2 and 5 hours; or 4 and 7 hours after occlusion). Other rats received (+)MK-801 (dizocilpine) three times at doses of 0.3, 1.0, and 3.0 mg/kg IP (5 to 10 minutes and 4 and 24 hours after occlusion). Control rats received an equal volume of saline. Infarct volume was determined 48 hours after occlusion by standard histological techniques.

RESULTS

IP administration of BIII 277 CL caused a dose-dependent reduction of infarct volume (1 mg/kg, 13%; 3 mg/kg, 25%). (+)MK-801 had similar effects (0.3 mg/kg, 13%; 1.0 mg/kg, 21%; 3 mg/kg, 27%). BIII 277 CL also dose-dependently reduced the infarct volume after SC administration (0.1 mg/kg, 14%; 0.3 mg/kg, 30%; 1.0 mg/kg, 28%). Furthermore, significant neuroprotective effects of BIII 277 CL were observed even when initial treatment was delayed up to 1 hour after occlusion (30 minutes, 28%; 1 hour, 23%; 2 hours, 5%; 4 hours, 4%).

CONCLUSIONS

These results indicate that BIII 277 CL shows significant neuroprotective effects at doses as low as 0.1 mg/kg SC. The effects after IP administration are comparable with those of (+)MK-801, and significant effects were observed even when the BIII 277 CL was first administered up to 1 hour after the beginning of ischemia.

Binding of the radiolabeled glycine site antagonist [3H]MDL 105,519 to homomeric NMDA-NR1a receptors

We have characterized the binding of [3H]MDL 105,519 ((E)-3-(2-phenyl-2-carboxyethenyl)-4,6-dichloro-1 H-indole-2-carboxylic acid), a NMDA receptor glycine recognition site antagonist, to homomeric NMDA subunit 1a (NR 1a) receptors. Chinese hamster ovary cells (CHO-K1) were transfected with the rat NR 1a gene and cell lines stably expressing the receptor were isolated from amongst clones resistant to the neomycin analog G418. Saturation analysis indicated that the radioligand bound to the homomeric receptor with a similar high affinity (Kd = 1.8 nM) to that reported for the native receptor. The binding capacity (Bmax) was 370 fmol/mg protein reflecting approximately 110000 receptors per cell. The radioligand interacted with a single class of binding sites as indicated by linear Scatchard transformation of the saturation data and a unitary Hill slope in competition experiments. Thus, the MDL 105,519 recognition site is present on the NR 1a subunit and has similar radioligand binding properties to the native brain-derived receptor. However, pharmacologic characterization of [3H]MDL 105,519 binding indicated that agonists were weaker competitors at the homometric receptor relative to the native receptors. In contrast, representative of three distinct chemical classes of glycine site antagonists exhibited similar potencies at both types of binding sites.

The protective effect of MK-801 on infarct development over a period of 24 h as assessed by diffusion-weighted magnetic resonance imaging

Diffusion-weighted MRI has been used to investigate therapeutic intervention with MK-801 in an animal model of permanent focal cerebral ischaemia. The animals were imaged continuously for 4 h and again at 24 h following occlusion of the middle cerebral artery (MCA) allowing the development of the ischaemic lesion to be monitored continuously in the same animals. An increased DWI signal, seen as a region of hyperintensity, was detected 1 h after MCA-occlusion in the lateral cortex and caudate nucleus in both control and MK-801 (administered at a dose of 3 mg/kg i.p. 5 min post-ischaemia) treated animals. However, the volume of hemispheric and cortical hyperintensity was smaller in the MK-801-treated animals. The area of hyperintensity progressively increased in the control group over the 4 h imaging time and there was also an increase in the area of hyperintensity between 4 and 24 h. At these time points the area of hyperintensity encompassed the dorsolateral cortex and caudate nucleus. MK-801 treated animals also demonstrated some progressive increase in the area of hyperintensity between 1 and 3 h, but no significant increase in the area of hyperintensity was seen after this time. The hyperintense regions at 4 and 24 h were restricted to the so-called 'core areas' of the lesion in MK-801-treated animals. Thus, using DWI the tissue 'at risk' following ischaemia could be identified and the protective effect of therapeutic intervention demonstrated.

Endothelin receptor antagonist increases cerebral perfusion and reduces ischaemic damage in feline focal cerebral ischaemia

These investigations characterised the cerebrovascular effects of an endothelin ETA-receptor antagonist PD156707 in normal and ischaemic cat brain. A dose of PD156707 that inhibited the effects of exogenous endothelin-1 was established in nonischaemic cerebral resistance arterioles. Perivascular microapplication of the endothelin-receptor antagonist PD156707 (0.03-3 microM) had a minimal effect on nonischaemic pial resistance arterioles. The perivascular co-application of PD156707 and ET-1 (10 nM) effected a dose-dependent attenuation of the ET-1 vasoconstrictive response (IC50 = 0.1 microM). Intravenous administration of PD156707 (3 mumol/kg bolus + 5 mumol/kg/h infusion) attenuated the vasoconstriction elicited by perivascular ET-1 (10 nM) in normal pial arterioles (ET-1 vasoconstriction: -37 +/- 13% from preinjection baseline; after intravenous PD156707: 6 +/- 10% from preinjection baseline). In the focal ischaemia studies, cerebral perfusion was measured in the suprasylvian and ectosylvian gyri (by laser Doppler flowmetry). Occlusion of the middle cerebral artery reduced cerebral perfusion in the suprasylvian and ectosylvian gyri by approximately 50%. Intravenous administration of PD156707 (3 mumol/kg bolus + 5 mumol/kg/h infusion), initiated 30 min after middle cerebral artery occlusion, effected a progressive increase in cerebral perfusion up to preocclusion baseline levels, whereas cerebral perfusion in vehicle-treated animals did not vary from its postocclusion level. In these animals, the intravenous administration of PD156707 reduced the hemispheric volume of ischaemic damage by 45% (vehicle: 2,376 +/- 1,107 mm3; PD156707: 1,307 +/- 548 mm3; p < 0.05). Our investigations indicate that endothelin receptor antagonism may be a new therapeutic strategy for the amelioration of focal ischaemic damage.

Neuroprotective effects of human recombinant interleukin-1 receptor antagonist in focal cerebral ischaemia in the rat

Recombinant human interleukin-1 receptor antagonist (rhIL-1ra) markedly protects against focal cerebral ischaemia in the rat, implicating endogenous IL-1 in the events leading to cerebral infarction. The present experiments investigated the effect of intracerebroventricular (i.c.v.) administration of IL-1 beta or rhIL-1ra on ischaemia damage and physiological parameters after permanent middle cerebral artery occlusion in the rat. IL-1 beta (5 ng. i.c.v.) markedly (92%) enhanced infarct volume and caused a significant rise in body temperature, but rhIL-1ra (10 micrograms, i.c.v.) significantly reduced infarct volume and did not significantly affect heart rate, blood pressure, or body temperature, rhIL-1ra administered 30 min before, or at the time of ischaemia significantly reduced infarct volume in cortex (55 and 60%, respectively) and striatum (52 and 41%, respectively). rhIL-1ra administered 30 min after ischaemia significantly reduced total and cortical infarct volume (26 and 29%, respectively), but did not significantly protect striatal tissue. The effects of rhIL-1ra were still evident in both cortex and striatum 7 days after ischaemia. These results support the role of IL-1 in ischaemic brain damage, revealing potent, sustained, neuroprotective effects of rhIL-1ra in the cortex and striatum, which cannot be attributed directly to changes in physiological parameters.

NMDA-antagonists reverse increased hypoxic tolerance by preceding chemical hypoxia

Glutamate antagonists mitigate hypoxic damage upon acute inhibition of energy metabolism. The goal of this study was to investigate their effect on increased hypoxic tolerance induced by preceding chemical inhibition of energy metabolism. While recovery of population spike amplitude (psap) is 30% of onset in slices prepared from control animals (15 min hypoxia, 45 min recovery), recovery exceeds 90% in slices prepared from animals that underwent mild chemical hypoxia in vivo by treatment with 20 mg/kg 3-nitropropionic acid 1 h prior to slice preparation (p-slices). In p-slices perfused for 5 min with D(-)-2-amino-5-phosphonopentanoic acid (APV) (100 microM) 45 min prior to hypoxia, recovery declines to 42 +/- 13% (mean +/- SEM). In contrast, posthypoxic recovery after similar perfusion with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 microM) is 72 +/- 15% (P < 0.05). We conclude that increased hypoxic tolerance is abolished by N-methyl-D-aspartate (NMDA)-antagonists but not non-NMDA-antagonists.

State-dependent inhibition of Na+ currents by the neuroprotective agent 619C89 in rat hippocampal neurons and in a mammalian cell line expressing rat brain type IIA Na+ channels

The compound 619C89 [4-amino-2-(4-methyl-1-piperazinyl)-5-(2,3,5-tricholorophenyl)-pyr imidine] is an effective neuroprotective agent in in vivo models of cerebral ischaemia. It has been suggested to act by inhibiting voltage-gated Na+ channels. To test this hypothesis, the action of 619C89 on recombinant rat brain type IIA Na+ channels expressed in Chinese hamster ovary cells and on native Na+ channels in acutely dissociated rat hippocampal neurons has been studied using whole-cell voltage-clamp recording techniques. In the cell line expressing type IIA Na+ channels, 619C89 caused a reversible inhibition of Na+ currents in a concentration- and voltage-dependent manner. A half-maximal inhibitory concentration (IC50) of approximately 50 microM was obtained at a holding potential of -90 mV whereas, with a conditioning prepulse to -60 mV for 30 s, the IC50 was reduced to 8 microM. Furthermore, the inhibition was markedly enhanced by a use-dependent action, which was dependent not only on the frequency of stimulation, but also on the duration (3.5-40 ms) of the pulses. Trains (10-50 Hz) of up to 60 depolarizing pulses of 0.7 ms duration did not evoke any use-dependent inhibition in the presence of 619C89, suggesting that this compound is not an open channel blocker. The voltage- and use-dependent inhibition by 619C89 was also observed on native Na+ channels in hippocampal neurons. 619C89 (10 microM) produced a small hyperpolarizing shift in the fast inactivation curve and a substantial (13 mV) hyperpolarizing shift in slow inactivation. The compound dramatically delayed the recovery from inactivation without affecting the development of inactivation. Moreover, 619C89 has no effect on the shape of the current-voltage relationship or on the voltage activation curve. These data indicate that 619C89 interacts selectively with the inactivated state of the Na+ channel with an estimated affinity of 3 microM. This primary action of 619C89 may underlie its neuroprotective effects.

Neuroprotective effects of preischemia intraarterial magnesium sulfate in reversible focal cerebral ischemia

The known cytoprotective properties of MgSO4 led the authors to study its effects on infarct size in rats when administered intraarterially before reversible focal ischemia. Following an intracarotid infusion of MgSO4 in the amount of 30 mg/kg (24 animals), 90 mg/kg (18 animals), or an equal volume of vehicle (23 animals), middle cerebral artery occlusion was produced in rats by means of an intraluminal suture technique. Reperfusion occurred after 1.5 (42 animals) or 2 hours (23 animals) of ischemia. Automated, volumetric measurements of 2',3',5'-triphenyl-2H-tetrazolium chloride-stained coronal brain sections demonstrated a statistically significant decrease in infarct size for MgSO4 treatment groups compared to controls. Cytoprotection was greater in animals subjected to 1.5 hours of ischemia (28.4% reduction in infarct volume, p < 0.001, Student's t-test), than in those having 2 hours of ischemia (19.3% reduction, p < 0.05). Animals given 90 mg/kg MgSO4 prior to 1.5 hours of ischemia (12 animals) showed a 59.8% reduction in infarct volume compared to controls (11 animals, p < 0.001) and a 43.1% reduction compared to the 30 mg/kg group (11 animals, p < 0.001). Analysis of variance demonstrated the statistically significant effects of MgSO4 doses on infarct volume across all groups (F = 22.95, p < 0.0001). The neuroprotective effect of intraarterial MgSO4 in this model is robust, dose dependent, and related to the duration of ischemia. The compound may be valuable for limiting infarction if given intraarterially before induction of reversible ischemia during cerebrovascular surgery.

Prophylaxis of migraine with oral magnesium: results from a prospective, multi-center, placebo-controlled and double-blind randomized study

In order to evaluate the prophylactic effect of oral magnesium, 81 patients aged 18-65 years with migraine according to the International Headache Society (IHS) criteria (mean attack frequency 3.6 per month) were examined. After a prospective baseline period of 4 weeks they received oral 600 mg (24 mmol) magnesium (trimagnesium dicitrate) daily for 12 weeks or placebo. In weeks 9-12 the attack frequency was reduced by 41.6% in the magnesium group and by 15.8% in the placebo group compared to the baseline (p < 0.05). The number of days with migraine and the drug consumption for symptomatic treatment per patient also decreased significantly in the magnesium group. Duration and intensity of the attacks and the drug consumption per attack also tended to decrease compared to placebo but failed to be significant. Adverse events were diarrhea (18.6%) and gastric irritation (4.7%). High-dose oral magnesium appears to be effective in migraine prophylaxis.

Dose escalation safety and tolerance study of the N-methyl-D-aspartate antagonist dextromethorphan in neurosurgery patients

Experimental studies have shown that dextromethorphan, a noncompetitive N-methyl-D-aspartate antagonist is neuroprotective in experimental models of ischemic cerebral injury. The authors studied the safety and tolerability of oral dextromethorphan (DM) in humans, and correlated serum levels of this drug with cerebrospinal fluid (CSF) and brain levels. Neurosurgical patients undergoing intracranial surgery or endovascular procedures were given ascending doses of oral DM prior to and 24 hours after surgery. Serum, CSF, and brain levels of DM and its active metabolite, dextrorphan, were measured. One hundred eighty-one patients received a total of 212 courses of DM treatment in dose ranges of 0.8 to 9.64 mg/kg. Serum DM levels correlated highly with CSF and brain DM levels. Brain levels were 68-fold higher than serum levels, whereas CSF levels were fourfold lower than serum levels. The maximum DM levels attained were 1514 ng/ml (serum) 118 ng/ml (CSF), and 92,700 ng/g (brain). The maximum dextrorphan levels were 501 ng/ml (serum), 167 ng/ml (CSF), and 6840 ng/g (brain). In 11 patients, brain and plasma levels of DM were comparable to levels that have been shown to be neuroprotective in animal studies. Frequent side effects occurring at neuroprotective levels of DM included nystagmus (64%), nausea and vomiting (27%) distorted vision (27%), feeling "drunk" (27%), ataxia (27%), and dizziness (27%). All symptoms were reversible and no patient suffered severe adverse reactions. This study demonstrates that potentially neuroprotective doses of DM can be administered safely to neurosurgical patients. Brain and CSF levels of DM can be estimated from serum levels of the drug. Side effects, even at the highest levels, proved to be tolerable and reversible. Administration of DM to patients at risk for cerebral injury should be further explored.

Adverse effects of dextromethorphan on the spatial learning of rats in the Morris water maze

The effects of the non-competitive NMDA receptor antagonist dextromethorphan on spatial learning were assessed using the Morris water maze. Dextromethorphan was administered to 4 groups of rats in 10, 20, 30, and 40 mg/kg doses. An additional group of rats was administered saline to serve as a vehicle control group. Dextromethorphan impaired learning dose dependently in the initial training phase of the experiment. During the probe trial, dose-dependent performance deficits were noted in the first 15 s of the trial only. Search strategy differences between the lowest and highest dose groups were also observed during the probe trial. During the reversal training phase, when the platform was moved to a new location, the dose-dependent impairment was seen again, but the 40 mg/kg group perseverated to the former location longer than the other groups. A cued control trial indicated that in addition to the learning impairment produced, the highest dose of dextromethorphan may also impair sensory-motor coordination.

Stroke is an emergency

Stroke is an emergency. Ischemic stroke is similar to myocardial infarction in that the pathogenesis is loss of blood supply to the tissue, which can result in irreversible damage if blood flow is not restored quickly. Public education is needed to emphasize the warning signs of stroke. Patients should seek medical help immediately, using emergency transport systems. Therapy geared toward minimizing the damage from an acute stroke should be started without delay in the emergency room. This includes measures to protect brain tissue, support perfusion pressure, and minimize cerebral edema. Strategies for improving recovery should also begin immediately. All major medical centers need stroke teams and stroke units. Stroke prevention should be given high priority as a public health strategy. Risk factor management should be part of general health care and should begin in childhood, with emphasis on nutrition, exercise, weight control, and avoidance of tobacco. Health screening and early treatment of hypertension and hypercholesterolemia has decreased the incidence of stroke and heart disease, but these efforts need to be expanded to reach all segments of the population. Basic research has opened the door to new therapies aimed at re-establishing blood flow and limiting tissue damage. Clinical trials have already led to changes in stroke prevention, including studies of carotid endarterectomy and ticlopidine and warfarin therapy (for patients with atrial fibrillation). Trials in progress are testing the usefulness of ancrod, neuroprotective agents, antioxidant agents, anti-inflammatory agents, low-molecular-weight heparin, thrombolytic drugs, and angioplasty. Any delay starting therapy after an acute stroke will result in progressive, irreversible loss of brain tissue. Clinicians should remember that for a stroke patient, time is brain tissue.

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.

Electrophysiological characterisation of the antagonist properties of two novel NMDA receptor glycine site antagonists, L-695,902 and L-701,324

The pharmacological effects of two novel N-methyl-D-aspartate (NMDA) receptor glycine site antagonists, L-701,324 and L-695,902 were examined on whole-cell voltage-clamped cells and compared to a prototypic antagonist, 7-chlorokynurenic acid. Both L-701,324 and L-695,902 non-competitively antagonised NMDA responses elicited in rat cultured cortical neurones, this was shown to be due to a competitive interaction at the glycine co-agonist site on the receptor complex (Kb values: 19 nM and 2.6 microM, respectively). Inhibition curves for the antagonism of responses to combined applications of NMDA and glycine showed that both antagonists were devoid of any intrinsic activity i.e. "full" antagonists and were, therefore, capable of completely abolishing inward currents. Despite this fact, both of these novel antagonists apparently modulated glutamate affinity for its recognition site-a property hitherto associated only with glycine site partial agonists. Human recombinant NMDA receptors comprising NR1a/NR2A and NR1a/NR2B subunits expressed in mouse fibroblast cells were also used to determine whether L-701,324 and L-695,902 were capable of discriminating between subtypes of NMDA receptor. Inhibition curves to each antagonist showed no difference in affinity for either of these subunit assemblies (mK1 values L-701,324 = 0.005 microM on both assemblies; L-695,902 = 4.37 and 3.7 microM on NR1a/NR2A and NR1a/NR2B, respectively). Kinetic analysis of the off-rates of antagonism with L-701,324 revealed that the high affinity of this compound compared to 7-chlorokynurenic acid were attributable to an exceptionally slow dissociation of the antagonist from the receptor.

The antidepressant metapramine is a low-affinity antagonist at N-methyl-D-aspartic acid receptors

Metapramine, a pharmacological compound with antidepressant activity in humans, was tested for possible antiglutamatergic activity, in vitro. We investigated the effects of metapramine on the N-methyl-D-aspartic acid (NMDA) receptor complex, by determining whether this compound would interfere with the binding of [3H]N-[1-(2-thienyl)cyclohexyl]-3,4-piperidine ([3H]TCP) to rat cortical membranes in the presence of either glycine NMDA, or both. Metapramine in the micromolar range inhibited the binding of [3H]TCP in the presence of both NMDA and glycine (IC50 = 1.4 +/- 0.2 microM). That very similar affinities were observed when either NMDA or glycine was present suggests that metapramine exerted a direct action at the PCP site. The affinity of metapramine for this site was about 25 and 350 times lower than that of PCP and MK-801, respectively. Metapramine inhibited the NMDA-evoked increase in guanosine 3',5'-cyclic monophosphate (cGMP) levels of neonatal rat cerebellar slices (IC50 = 13 microM). These results suggest that metapramine is a low-affinity antagonist of the NMDA receptor complex channel. This paper discusses the potential application of metapramine to the treatment of diseases linked to excessive stimulation of glutamatergic NMDA receptors.

Excitatory amino acids in health and disease

PURPOSE

To review the role of excitatory neurotransmitters in normal mammalian brain function, the concept of excitotoxic neuronal death as an important final common path in a variety of diseases, and modification of excitatory synaptic transmission as an important new pharmacological principle. These principles are discussed, with special emphasis on diseases of importance to older adults.

DATA SOURCES

A MEDLINE search from 1966 to May 1995 was undertaken, as well as a manual search of current issues of clinical and basic neuroscience journals, for articles that addressed glutamate N-methyl-D-aspartate and/or excitotoxicity.

STUDY SELECTION

A total of 5398 original and 68 review articles were identified that addressed animal and human experimentation relevant to excitotoxic neuronal death. There were 364 articles with potential significance for clinical application identified; 132 of the most recent references are provided.

DATA EXTRACTION

All articles were classified into three categories: general receptor, biology pathogenesis of disease, and pharmacotherapy.

RESULTS

Glutamic and aspartic acids are the physiological mediators of most excitatory synaptic transmission. This is critical to several normal nervous system functions, including memory and long-term modification of synaptic transmission and nociception. Activation of the inotropic NMDA and non-NMDA receptors increases transmembrane calcium and sodium fluxes, and the metabotropic glutamate receptor activation results in generation of inositol triphosphate and inhibition of adenylate cyclase. Numerous modulatory sites exist, especially on the NMDA receptor. Nitric oxide, arachidonic acid, superoxide, and intracellular calcium overload are the ultimate mediators of neuronal death. Glutamate re-uptake transporters belong to a unique family of amino acid transport systems, the malfunction of which is intricately involved in disease pathogenesis. Ischemic stroke, hypoglycemia, Parkinson's disease, alcohol intoxication and withdrawal, Alzheimer's disease, epilepsy, and chronic pain syndromes are only some of the important clinical neurological disorders with a major pathogenic role for the excitatory amino acids.

CONCLUSIONS

Pharmacological manipulation of the excitatory amino acid receptors is likely to be of benefit in important and common diseases of the nervous system. Only a few of the currently available drugs that modify excitatory neurotransmission, such as remacemide, lamotrigine, and tizanidine, have an acceptable therapeutic index. The identification of numerous receptor subtypes, topographic variabilities of distribution, and multiple modulatory sites will provide a true challenge to the neuropharmacologist.