Potential therapeutic uses of N-methyl-D-aspartate antagonists in cerebral ischemia

REVIEW ■ : Neuroprotection in Brain Ischemia: An Update (Part I

Ischemic brain injury produced by stroke or cardiac arrest is a major cause of human neurological disability. Steady advances in the neurosciences have elucidated pathophysiological mechanisms of brain ischemia and have suggested many therapeutic approaches directed at specific injury mechanisms to achieve neuroprotection of the acutely ischemic brain. The first portion of this two-part review highlights the differentiating features and pathological mechanisms of focal and global cerebral ischemic injury and summarizes a wealth of recent evidence as to how antagonism of excitatory amino acid neurotoxicity, mediated via NMDA as well as non-NMDA receptors, may offer a means of diminishing the extent of ischemic injury. The Neuroscientist 1:95-103, 1995

Acute spinal cord injury, part II: contemporary pharmacotherapy

Spinal cord injury (SCI) remains a common and devastating problem of modern society. Through an understanding of underlying pathophysiologic mechanisms involved in the evolution of SCI, treatments aimed at ameliorating neural damage may be developed. The possible pharmacologic treatments for acute spinal cord injury are herein reviewed. Myriad treatment modalities, including corticosteroids, 21-aminosteroids, opioid receptor antagonists, gangliosides, thyrotropin-releasing hormone (TRH) and TRH analogs, antioxidants and free radical scavengers, calcium channel blockers, magnesium replacement therapy, sodium channel blockers, N -methyl-D-aspartate receptor antagonists, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-kainate receptor antagonists, modulators of arachadonic acid metabolism, neurotrophic growth factors, serotonin antagonists, antibodies against inhibitors of axonal regeneration, potassium channel blockers (4-aminopyridine), paclitaxel, clenbuterol, progesterone, gabexate mesylate, activated protein C, caspase inhibitors, tacrolimus, antibodies against adhesion molecules, and other immunomodulatory therapy have been studied to date. Although most of these agents have shown promise, only one agent, methylprednisolone, has been shown to provide benefit in large clinical trials. Given these data, many individuals consider methylprednisolone to be the standard of care for the treatment of acute SCI. However, this has not been established definitively, and questions pertaining to methodology have emerged regarding the National Acute Spinal Cord Injury Study trials that provided these conclusions. Additionally, the clinical significance (in contrast to statistical significance) of recovery after methylprednisolone treatment is unclear and must be considered in light of the potential adverse effects of such treatment. This first decade of the new millennium, now touted as the Decade of the Spine, will hopefully witness the emergence of universal and efficacious pharmacologic therapy and ultimately a cure for SCI.

Modulation of [3H]MK-801 binding to NMDA receptors in vivo and in vitro

[3H]MK-801 binding in vivo was used to determine the occupancy of NMDA receptor ligands shown to allosterically modulate binding in vitro. ED(50) values (mg/kg) were obtained for the channel blockers (+)-5-methyl-10,11-dihydro-5,4-dibenzo[a,d]cyclohepten-5,10-imine maleate ((+)-MK-801, 0.2), 1-(1-phenylcyclohexyl)piperidine (phencyclidine, PCP, 1.7) and ketamine (4.4). Antagonists at the glutamate (DL-(2-carboxypiperazine-4-yl)propyl-1-phosphonate (DL-CPP, 5.7)) and glycine site (7-Chloro-4-hydroxy-3-(3-phenoxy)-phenyl-2(H)quinolinone (L-701,324, 14.1), 3R(+)cis-4-methyl-pyrrollid-2-one (L-687,414, 15.1)) inhibited [3H]MK-801 binding in vivo to varying maximum levels (69%, 103% and 45%, respectively). NR2B subunit-selective compounds acting at the ifenprodil site inhibited [3H]MK-801 in vivo by a maximum of 52-72% and gave ED(50) values (mg/kg) of: (+/-)-(1S*, 2S*)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol ((+/-)CP-101,606), 1.9; (+/-)-(3R, 4S)-3-[4-(4-fluorophenyl)-4-hydroxypiperidin-1-yl]chroman-4,7-diol ((+/-)CP-283,097), 1.8; (+/-)-(R*, S*)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidine propanol ((+/-)Ro 25-6981), 1.0; ifenprodil, 6.0. The glycine site agonist D-serine stimulated binding to 151% of control with an ED(50) of 1.7 mg/kg. Results show that [3H]MK-801 binding in vivo may be used to measure receptor occupancy of ligands acting not only within the ion channel but also at modulatory sites on the NMDA receptor complex.

MRZ 2/579, a novel uncompetitive N-methyl-D-aspartate antagonist, reduces infarct volume and brain swelling and improves neurological deficit after focal cerebral ischemia in rats

The purpose of this study was to evaluate the effects of MRZ 2/579, an uncompetitive N-methyl-D-aspartate antagonist, on infarct size, extent of swelling and neurological deficit in a model of transient middle cerebral artery occlusion in rats. Physiologically controlled Sprague-Dawley rats received 2 h MCAo by retrograde insertion of an intraluminal suture coated with poly-L-lysine. The agent (MRZ 2/579) or vehicle (sodium chloride 0.9%) was administered i.v. immediately after suture removal following a 2-h period of MCAo. Two experimental groups were studied: group A was treated by vehicle (bolus infusion:1 ml/kg for 10 min followed by infusion of 6 ml/kg/h over 6 h). Group B was treated by MRZ 2/579 (bolus infusion:10 mg/kg for 10 min followed by infusion of 6 mg/kg/h over 6 h). The neurological status was evaluated during occlusion (at 60 min) and daily for 3 days after MCAo. Brains were then perfusion-fixed, and infarct volumes and brain swelling were determined. MRZ 2/579 significantly improved the neurological score compared to vehicle-treated rats at 48 h (6.2+/-0.6 and 8.7+/-0.5, respectively; P<0.004) and 72 h after MCAo (5.2+/-0.6 and 8.4+/-0.5, respectively; P<0.001). Treatment with MRZ 2/579 also significantly reduced total infarct volume (29.3+/-11.1 and 83.2+/-16.5 mm(3), respectively; P<0. 01), cortical infarct volume (24.8+/-11.2 and 70.0+/-18.0 mm(3), respectively; P<0.04) and subcortical infarction (21.2+/-4.1 and 49. 6+/-4.5 mm(3), respectively; P<0.0002). Brain swelling was also markedly reduced compared with vehicle-treated rats (4.7+/-1.3 and 10.8+/-2.1%, respectively; P<0.02). These results demonstrate that treatment with MRZ 2/579, when administered promptly after reperfusion, confers neuroprotective effects on infarct volume, brain swelling, and neurological score compared to the vehicle group.

Combined therapy affects outcomes differentially after mild traumatic brain injury and secondary forebrain ischemia in rats

Muscarinic and NMDA receptors contribute to post-traumatic hypersensitivity to secondary ischemia. However, the effect of these receptor antagonists on behavior and CA1 neuronal death after traumatic brain injury (TBI) with acute (1 h after TBI) forebrain ischemia has not been systematically assessed. We examined cognitive and motor dysfunction and the relationship of behavior deficits to neuronal death in this model using muscarinic and NMDA antagonists. Three behavioral groups (n=10/group) of Wistar rats were subjected to mild TBI and 6 min of forebrain ischemia imposed 1 h after TBI with 45 days survival. Motor and spatial memory performance were assessed using the rotarod task and Morris water maze. Seven additional groups (n=6/group) were evaluated only for CA1 death after 7 days survival following sham, individual or combined injury with and without drug treatments. Rats were given 0.3 mg/kg MK-801 (M) and 1.0 mg/kg scopolamine (S) alone or combined (M-S) before or 45 min after TBI. Rotarod performance was tested at days 1-5 and maze performance on days 11-15 and 40-44 after M-S treatment. The 7-day studies showed M-S treatment (p<0.01) reduced CA1 neuronal death better than either S or M alone. Behavioral groups had inadvertent post-ischemic hypothermia that decreased CA1 death and likely influenced behavioral morbidity. M-S given before TBI (p<0.01) decreased memory deficits on day 15, while M-S treatment given after TBI was ineffective. Unexpectedly, M-S treatment before or after TBI produced transient motor deficits (p<0. 01). Memory improvement occurred independent of CA1 death.

Neuroprotective agents in acute ischemic stroke

Recent scientific data suggest that there is a "window of opportunity" between the onset of brain ischemia and irreversible neuronal death. Although neurons can withstand complete absence of blood flow for only minutes, collateral circulation typically surrounds the "core" of the stroke. In these brain regions known as the ischemic penumbra, brain tissue can survive for several hours following stroke onset. Recent studies suggest that neuroprotective agents would give these viable neurons the chance for recovery. This article reviews the mechanisms of ischemic neuronal injury, mechanism of action of neuroprotective agents, current neuroprotective clinical trials, and guidelines for patient care management.

Ethical challenges in stroke research

BACKGROUND

Ethical issues are a critical consideration in the design and conduct of clinical research.

SUMMARY OF REVIEW

A number of completed trials of proposed stroke treatments raise classical ethical issues in challenging ways. The combination of acutely ill and vulnerable patients, the use of potentially toxic drugs, and very short time frames for decision making and drug administration demand an especially careful evaluation of risk and benefit, the process of consent, and the permissible treatment of control patients.

CONCLUSIONS

The study of acute stroke treatments may require more complex safeguards than other neuroclinical trials.

Lisuride prevents learning and memory impairment and attenuates the increase in extracellular dopamine induced by transient global cerebral ischemia in rats

In this experiment, we tested the efficacy of neuroprotection with lisuride, a dopamine agonist, using the 4-vessel occlusion rat model. Functional improvement was evaluated with two behavior tests exploring learning and memorization capacity in the rat, the Morris water maze and the 14-unit T-maze, 18 days after ischemia. Extracellular dopamine levels during ischemia were determined in search of a possible neuroprotection mechanism. Dopamine and its metabolites, DOPAC and HVA, as well as the serotonin metabolite, 5-HIAA, were assayed with HPLC-EC, in striatal extracellular fluid obtained by in vivo microdialysis in the awake rat. Lisuride was administered at a total dose of 10 ng by continuous intrastriatal infusion or at the dose of 0.5 mg/kg by i.p. infusion, 160 minutes before onset of ischemia for the neurochemical study and at the dose of 0.5 mg/kg via i.p. infusion, 1 hour before occlusion of the carotid arteries, for the behavior tests. Behavioral testing showed significantly better recovery in both sets of behavioral tests, with more pronounced positive results with the 14-unit T-maze, in comparison with the saline-treated animals. Microdialysis confirmed a significant attenuation of the ischemia-induced dopamine surge, whatever the mode of administration, compared with saline-treated animals. These results show that lisuride offers significant neuroprotection from the effect of experimental transient global forebrain cerebral ischemia in the rat; the mechanism would imply, at least in part, reduced levels of extracellular dopamine.

Is chlormethiazole neuroprotective in experimental global cerebral ischemia? A microdialysis and behavioral study

Chlormethiazole, an anticonvulsive agent, has been shown to have a possible neuroprotective effect against cerebral ischemia. In addition, chlormethiazole inhibits methamphetamine-induced release of dopamine, protecting against this neurotransmitter's neurotoxicity. The aim of this work was to ascertain whether, in experimental cerebral ischemia, chlormethiazole administration attenuated the ischemia-induced rise of the extracellular concentration of aminergic neurotransmitters and whether it reduces ischemia-induced deficits in memory and learning. Histology for assessment of ischemic damage was a so included. The four-vessel occlusion rat model was used to induce global cerebral ischemia. Aminergic neurotransmitters and their metabolites in the striatal extracellular fluid obtained by microdialysis were assayed by high-performance liquid chromatography-electrochemical detection. The drug was administered either IP (50 mg/kg-1) or directly through the dialysis probe (30 microM) 80 min before ischemia. For the behavioral test and histology, the drug was given IP (100 mg/kg-1) 1 h postischemia. The results obtained did not demonstrate any statistically significant evidence that chlormethiazole has an effect on the ischemia-induced rise in extracellular dopamine and serotonin levels. There was also no variation in metabolite levels. Behavioral measures (learning, recall) were not changed appreciably by the treatment. We observed no significant cell protection in the hippocampus (CA1, CA1), striatum, and entorhinal cortex in animals treated with chlormethiazole. We conclude that, under our experimental conditions, chlormethiazole has little or no effect on the neurochemical, neurobehavioral, and histological consequences of global cerebral ischemia.

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.

Reduction of excitotoxicity-induced brain damage by the competitive NMDA antagonist CGP 40116: a longitudinal study using diffusion-weighted imaging

The cerebroprotective properties of the competitive N-methyl-D-aspartate (NMDA) antagonist CGP 40116 were evaluated in a rat model of excitotoxicity-induced brain damage using direct intrastriatal injection of quinolinic acid and subsequent (5 or 45 min later) i.p. administration of the drug. Diffusion-weighted magnetic resonance imaging (DWI) was used to follow the temporal lesion growth during the acute phase (4 h) and T2-weighted MRI (T2WI) to quantify vasogenic edema extent 2 days later. For control animals, we found a rapid increase in lesion volume during the first hour followed by a moderate growth over the following hours. The DWI-visible hyperintensity was partially reversible after treatment with CGP 40116. The onset of action of CGP 40116 was immediate. The final outcome (63% reduction of lesion volume within 2-4 h post-surgery) was independent of the time of drug administration. DWI data after 4 h correlated well with those obtained by T2WI 2 days later. DWI is a valuable method for early prediction of the outcome of therapeutic interventions of excitotoxic insults.

Neuroprotection by dextromethorphan in acute experimental subdural hematoma in the rat

Experimental acute subdural hematoma in the rat has been shown to produce a zone of apparent infarction under the clot, and excitatory amino acid toxicity appears to play a role in the damage observed. We report the effect of dextromethorphan, a commonly used antitussive and a noncompetitive NMDA-type glutamate receptor antagonist, on the volume of histologic damage seen at 72 h after acute subdural hematoma in the rat. Sixty-five Long-Evans rats underwent placement of acute subdural hematoma using the "cranial window" model. Fourteen animals received oral dextromethorphan, 10 mg/kg/dose, twice daily for 3 days, and an additional 20 animals also received a single 20 mg/kg intraperitoneal dose 15 min after clot placement in addition to the oral regimen. Control animals received equal volumes of sterile water. Brain lesions in all animals were characterized by well-circumscribed infarctions underlying the subdural hematoma. Lesion volume in control animals was 88.3 +/- 9.3 mm3 (mean +/- standard error of the mean), while animals receiving dextromethorphan had significantly smaller lesions, which was independent of dosing schedule (59.9 +/- 9.2 mm3)(p = 0.0403). Animal weight was also found to be a significant covariate (p = 0.038). Because of its safety in humans and efficacy as a neuroprotectant in a variety of models, dextromethorphan may be a promising agent for clinical use, particularly in children.

Normothermic versus hypothermic cardiopulmonary bypass: central nervous system outcomes

The recent advent of "warm heart" surgery has resulted in reexamination of the neuroprotective effects of hypothermia in the setting of cardiopulmonary bypass (CPB). Hypothermia has been shown to confer significant protection in the setting of transient, but not permanent, ischemia. The mechanism of this neuroprotection is unclear at this time. Reduction in cerebral metabolic rate is believed to be less important compared with the effect of hypothermia on the release of excitatory neurotransmitters, catecholamines, or other mediators of cellular injury. It is for this reason that mild hypothermia (33-35 degrees C) is believed to confer significant neuroprotection. Two large randomized trials of warm versus cold heart surgery have been reported. Neither study found a difference in terms of neuropsychologic dysfunction. However, one study identified a threefold increase in strokes in the "warm" patients. The reasons for this difference are not clear; however, there were various differences in technique and patient population that may have been important. There are other reports of large series of patients undergoing normothermic bypass, with no increase in stroke rate over what is reported elsewhere in the literature. To date, the evidence would suggest that neuropsychologic function is not affected by CPB temperature, suggesting that the transient ischemia is not an important mechanism in this injury. Stroke after CPB is usually the result of permanent ischemia, and hypothermia's effect in this setting is minimal. It would seem unlikely that hypothermia offers anything more than modest benefits in the clinical situation where there is no circulatory arrest.

Pretreatment with antisense oligodeoxynucleotides directed against the NMDA-R1 receptor enhances survival and behavioral recovery following traumatic brain injury in rats

Treatment with N-methyl-D-aspartate (NMDA) receptor antagonists limits tissue damage following CNS ischemia or trauma, supporting the hypothesis that NMDA receptors participate in the pathophysiology of such injuries. An alternative approach for evaluating this hypothesis is to examine the effects of selective inhibition of NMDA receptor synthesis, using antisense oligodeoxynucleotides. In the present studies, the effects of antisense oligodeoxynucleotides directed at NMDA-R1 receptor subunit, administered intracerebroventricularly (i.c.v.) prior to injury, were evaluated in a well-defined traumatic brain injury model in rats. Outcome measures included survival, motor recovery, and histological changes. Administration of antisense oligodeoxynucleotides (15 nmol/ml twice daily x 2 days) did not alter physiological variables or motor function prior to trauma. However, such treatment significantly decreased mortality and improved behavioral recovery at 2 weeks after trauma as compared to animals treated with the corresponding sense oligodeoxynucleotides. Although cell counts in hippocampus did not differ between treatment groups, astrocyte activation as reflected by glial fibrillary astrocytic protein (GFAP) immunocytochemistry was significantly reduced in antisense treated animals. These findings provide additional evidence that NMDA receptors contribute to secondary injury after brain trauma and may suggest an alternative treatment approach.

HU-211, a nonpsychotropic cannabinoid, improves neurological signs and reduces brain damage after severe forebrain ischemia in rats

The purpose of the present study was to examine the dose-response relationship and the therapeutic time window for the synthetic nonpsychotropic cannabinoid (HU-211) as a neuroprotective agent in transient, severe forebrain ischemia in the rat. Adult Sprague-Dawley rats were subjected to 20 min common carotid artery occlusion (CCAo) 24 h after coagulation of both vertebral arteries. Thirty minutes after the onset of CCAo, rats received an iv injection of HU-211 2, 4, or 8 mg/kg in HPCD (n = 12, 18, and 11, respectively), or the appropriate vehicle (n = 20). Neurological signs were scored daily for 3 d following ischemia. A significant improvement (p < 0.01, Kruskal-Wallis nonparametric test, followed by Mann-Whitney U-test, p < 0.05) of neurological deficits by the 4 mg/kg dose of HU-211, was observed 24, 48, and 72 h after insult. On the third day post-CCAo, the rat brain was taken for histopathological evaluation of the CA-1 sector of the hippocampus. Counts of viable neurons in the hippocampal CA1 field showed significantly more live cells in the HU-211 (4 mg/kg) treated animals (P < 0.001, ANOVA followed by Duncan's post-hoc test, p < 0.05). The drug was equally effective when given 30 and 60 min after ischemia, but neuroprotection was no longer significant after 3 h. We suggest that HU-211 may be a potential treatment for postischemic brain damage in human beings.

Neuroprotective effects of 619C89, a use-dependent sodium channel blocker, in rat traumatic brain injury

Release of the excitatory amino acid (EAA) neurotransmitter glutamate has been implicated in secondary tissue damage following central nervous system (CNS) trauma and ischemia. The present study evaluated the neuroprotective actions of 619C89, a sodium channel blocker that inhibits ischemia-induced glutamate release, on traumatic brain injury (TBI) in rats using a lateral fluid percussion model. Various motor-related behavioral outcomes were used to evaluate neurologic function. Glial fibrillary acidic protein (GFAP) immunostaining and Cresyl violet staining were used to assess the histological changes. Treatment with 619C89, at a dose of 30 mg/kg administered intravenously 15 min before brain injury, significantly attenuated behavioral deficits at 24 h and 1 week. At 2 weeks, neuronal loss in the CA1 and CA3 pyramidal cell layers of the hippocampus was significantly decreased by 619C89 administration. Treatment with this compound also significantly attenuated increases in GFAP-immunoreactivity in both ipsilateral and contralateral CA1 regions. The present results suggest a potential therapeutic role for sodium channel blockade and/or glutamate release inhibition in the treatment of TBI.

Safety, tolerability, and pharmacokinetics of the N-methyl-D-aspartate antagonist dextrorphan in patients with acute stroke. Dextrorphan Study Group

BACKGROUND AND PURPOSE

Dextrorphan hydrochloride is a noncompetitive N-methyl-D-aspartate antagonist that is neuroprotective in experimental models of focal brain ischemia. The purpose of this study was to determine the maximum loading dose and maintenance infusion of dextrorphan hydrochloride that are well tolerated in patients with an acute stroke.

METHODS

An intravenous infusion of dextrorphan or placebo was begun within 48 hours of onset of a mild-to-moderate hemispheric stroke. Initially, patients were treated with either placebo (n = 15) or dextrorphan (n = 22) using a 1-hour loading dose (60 to 150 mg) followed by a 23-hour ascending-dose maintenance infusion (maximum total dose, 3310 mg). Subsequently, 29 patients were treated with dextrorphan in an open trial using a 1-hour loading dose (145 to 260 mg) followed by an 11-hour constant rate (30 to 70 mg/h) infusion.

RESULTS

Transient and reversible adverse effects, including nystagmus, nausea, vomiting, somnolence, hallucinations, and agitation, commonly occurred in dextrorphan-treated patients. Loading-dose escalation was stopped because of rapid-onset, reversible, symptomatic hypotension in 7 of 21 patients treated with doses of 200 to 260 mg/h. At the highest rates of maintenance infusion (> 90 mg/h), 3 patients developed deep stupor or apnea. The maximum tolerated loading dose was 180 mg/h, and the maximum tolerated maintenance infusion was 70 mg/h. Maximum plasma levels of 750 to 1000 ng/mL were obtained in 9 patients. There was no difference in neurological outcome at 48 hours between the dextrorphan-treated and placebo-treated patients.

CONCLUSIONS

The highest doses of dextrorphan administered were associated with serious adverse experiences in some patients. Lower doses (loading doses of 145 to 180 mg, maintenance infusions of 50 to 70 mg/h) were better tolerated and rapidly produced potentially neuroprotective plasma concentrations of dextrorphan. These doses were associated with well-defined pharmacological effects compatible with N-methyl-D-aspartate receptor antagonism.

Neuroprotective effect of N-methyl-D-aspartate non-competitive antagonists (arylcyclohexylamine derivatives) on human cultured spinal cord cells

The non-competitive N-methyl-D-aspartate antagonist cis-Pip/Me 1-[1-(2-thienyl)-2-methyl-cyclohexyl] piperidine (GK11) either in racemic(+/-) or (-) enantiomeric form has been investigated on survival of human fetal spinal cord cells in culture. The treated cultured cells were processed for immunocytochemical detection of GABA at different time intervals ranging from 3 to 27 weeks. The number of GABA-stained cells was found to be greater in the treated cultures than in the control ones. The qualitative and quantitative morphological features of both control and treated cells were analysed by computer-assisted methods which allowed us to individualize three different populations: (1) all control and treated small neuritic field neurons, (2) young control and old treated large neuritic field neurons and (3) old control neurons with large neuritic fields.

Cognitive effects of milacemide and methylphenidate in healthy young adults

Cognitive effects of the novel glycine prodrug milacemide (400 mg), the catecholaminergic agonist methylphenidate (20 mg), and placebo were evaluated in 48 healthy young adults. Throughout a 6-h drug treatment day, subjects repeatedly performed tests of target-detection vigilance, immediate and delayed verbal free recall, and Buschke Selective Reminding; total free recall and forced-choice recognition tests were administered at the end of the day. Significant improvement in both vigilance reaction time and Selective Reminding Sum Recall was observed in the methylphenidate group. Contrary to expectations, the milacemide group evidenced significant declines in both vigilance perceptual sensitivity and free-recall difference scores (delayed-immediate). Vigilance reaction times significantly decreased over repeat testing in all groups, but only the methylphenidate group differed from placebo. The reaction-time functions for milacemide and placebo were similar, suggesting arousal was not diminished under milacemide and could not account for the cognitive decrements. No significant drug effects obtained for total free recall or recognition performance. Although the glycine prodrug milacemide was ineffective as a cognitive enhancer, the involvement of the NMDA receptor in memory function reported in the literature supports continued exploration of other approaches for manipulating NMDA receptor activity.

Requirements for high affinity binding of glycine analogs to the glycine site of the NMDA receptor complex

Correlation of the isopotential contours of the optimized conformations of a series of alpha-amino acids, in their neutral and zwitterionic forms, with their potencies to inhibit [3H]glycine binding and to enhance [3H]10,11-dihydro-5-methyl-5H-dibenzo[a,d]cyclohepten-5,10-imine ([3H]MK-801) binding, leads to the following conclusions: (a) steric congestion at the amino group is detrimental to binding potency; (b) a zwitterionic amino acid is required for high affinity to the receptor; (c) a conformation in which the carboxylate group is at a 90 degrees dihedral angle to the ammonium nitrogen is preferred for high affinity; and (d) placing the carbon backbone of the zwitterionic alpha-amino acid, in its preferred conformation, above the plane defined by the ammonium nitrogen and the carboxylate oxygen atoms, and viewing the molecule along the nitrogen to carboxylate carbon axis, there is a space forbidden to the ligand (receptor-required-space) to the left.

Glycine site of the excitatory amino acid N-methyl-D-aspartate receptor in neonatal and adult brain

The N-methyl-D-aspartate (NMDA) receptor complex in brain is a glutamate receptor subtype with several recognition sites including a glycine site that is able to modulate and activate allosterically the receptor. This receptor may be important in the regulation of developmental synaptic plasticity. The release of glutamate and consequent overstimulation of NMDA receptors that follows hypoxia-ischaemia leads to brain damage. Brain tissue obtained at necropsy was studied in a total of 16 term infants aged less than 1 week to 22 weeks and in four adults aged from 66 to 84 years. Glycine sites were determined in brain sections by the binding of the selective ligand [3H]5,7-dichloro-kynurenic acid and measured by autoradiography. In infant brains the amount of binding to the glycine site was higher in temporal cortex and hippocampus than in basal ganglia and was also higher than in comparable areas of adult brain. The amount of glycine site binding in infant cortex increased with postnatal age. The data suggest that infant brain acquires a relatively high density of NMDA receptors in temporal lobe due to postnatal proliferation of glutamatergic synapses. These findings have therapeutic implications as drugs that reduce NMDA receptor function by blocking the glycine modulatory site would be pertinent to preventing brain damage after hypoxia-ischaemia.

BW619C89, a glutamate release inhibitor, protects against focal cerebral ischemic damage

BACKGROUND AND PURPOSE

The excitatory amino acid neurotransmitter glutamate is involved in excitotoxic brain injury and neurodegeneration after cerebral ischemia. Therefore, compounds that block the release of glutamate may be useful as cerebroprotective agents. The purpose of this study was to evaluate the cerebroprotective properties of a glutamate release inhibitor, BW619C89.

METHODS

In the studies reported here, the effect of BW619C89 [4-amino-2-(4-methyl-1-piperazinyl)-5-(2,3,5-trichlorophenyl)pyrimidine] on neurotransmitter release (endogenous amino acids, gamma-aminobutyric acid, and acetylcholine) from slices of rat brain cerebral cortex in vitro has been determined. The neuroprotective efficacy of BW619C89 has been evaluated using the middle cerebral artery occlusion model of focal cerebral ischemia in the Fischer 344 rat.

RESULTS

In the in vitro studies, BW619C89 inhibited veratrine- (but not potassium-) evoked release of both endogenous glutamate and aspartate from rat cerebral cortex slices with IC50 values of approximately 5 microM. BW619C89 was approximately 10-fold less potent to inhibit veratrine-evoked 3H-gamma-aminobutyric acid release (IC50 = 51 microM), fourfold less potent to inhibit 3H-acetylcholine release (IC50 = 21 microM), and at 10 microM had only weak activity at excitatory amino acid (N-methyl-D-aspartate, kainate, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) binding sites. When administered intravenously to Fischer 344 rats 5 minutes after permanent middle cerebral artery occlusion, BW619C89 produced marked reductions of both total (cortex and basal ganglia) and cortical infarct volumes. Cortical infarct size was reduced by 20% at a dose of BW619C89 of 5 mg/kg (n = 6, not significant); 43% at 10 mg/kg (n = 8, P < .01); 59% at 20 mg/kg (n = 8, P < .001); 61% at 30 mg/kg (n = 8, P < .001), and 53% at 40 mg/kg (n = 8, P < .001). BW619C89 at doses of 20 and 30 mg/kg also significantly reduced noncortical (basal ganglia) infarct volumes, demonstrating that a proportion of this tissue also appears to be salvageable. Behavioral effects observed were dose related, generally minor, and at doses of 20 mg/kg IV and above consisted of body tremor and mild ataxia lasting approximately 2 hours.

CONCLUSIONS

These results suggest that glutamate release inhibitors such as BW619C89 may provide an alternative to excitatory amino acid receptor antagonists in the treatment of focal cerebral ischemia and stroke.

Adenosine inhibits the NMDA receptor-mediated excitatory postsynaptic potential in the hippocampus

The effect of the stable adenosine analogue 2-chloroadenosine (CADO) on the component of the field excitatory postsynaptic potential (fepsp) mediated by the N-methyl-D-aspartate (NMDA) type of glutamate receptor was studied in the hippocampal CA1 area of the rat. CADO inhibited the NMDA receptor-mediated component of the fepsp (EC50 = 0.10 +/- 0.02 microM), more efficiently than it inhibited the fepsp (EC50 = 0.40 +/- 0.08 microM). The results suggest that adenosine may modulate phenomena associated with the NMDA receptor, such as synaptic plasticity and excitotoxicity.

Therapeutic interventions in acute stroke

1. Potential therapies for ischaemic stroke include agents to reduce oedema, to improve cerebral perfusion, to reduce excitotoxic damage, to minimise free-radical induced injury and to reduce complications such as deep venous thrombosis. 2. Of the anti-oedema drugs, steroids are ineffective and possibly dangerous; intravenous glycerol is unproven. 3. Haemodilution to reduce whole blood viscosity and improve perfusion is ineffective. Thrombolytic drugs have not been adequately tested but several randomised multicentre trials are now commencing. Early treatment and CT scanning are essential. 4. Anticoagulants and antiplatelet drugs may have wide applicability but have not been tested in the acute phase of stroke. A multi-centre trial will address this issue. 5. Neuronal cytoprotection offers exciting prospects for acute stroke treatment. Antagonists of glutamate at the NMDA receptor, calcium and sodium channel blocking agents and free radical scavenging drugs have potent effects experimentally. Several agents are now reaching clinical trials. The calcium antagonist nimodipine has been disappointing in large scale trials but some studies were flawed by late treatment. 6. Successful treatment of acute stroke is likely to combine several approaches. 7. Therapeutic trials in stroke must include CT scanning, early treatment and a multicentre approach to achieve large numbers of patients.

TCP enhances the survival of human fetal spinal cord cells in culture

Dissociated cell cultures were prepared from human spinal cords of 7-10-week-old fetuses. After 10 weeks progressive neuronal necrosis was observed in controls whereas N/1-(2-thienyl)cyclohexyl/piperidine (TCP) enhanced the survival time of the cells. After 21 weeks the number of gamma-aminobutyric acid (GABA)ergic and neuron specific enolase (NSE)-stained neurons was higher in the TCP-treated cultures than in controls. TCP appears to be a promising drug for long term survival of neurons.

Amantadine as N-methyl-D-aspartic acid receptor antagonist: new possibilities for therapeutic applications?

The N-methyl-D-aspartic acid (NMDA) receptor is an intriguing target for the development of drugs with anti-Parkinsonian activity as well as with protective actions against degenerative processes induced by brain ischemia. Amantadine is used in the treatment of Parkinson's disease without a well established mechanism of action. We show here that amantadine inhibits, in a non-competitive way, the NMDA receptor-mediated stimulation of acetylcholine release from rat neostriatum in vitro in "therapeutic" (i.e., low micromolar) concentrations. This indicates that amantadine might exert its anti-Parkinsonian effect via blockade of NMDA receptors. Sustained stimulation of NMDA receptors induces so-called excitotoxicity. Recently, it was demonstrated that amantadine is able to inhibit NMDA induced cell death in a neuronal culture. On the basis of these findings it seems worth investigating if amantadine is also able to protect against neurodegenerative processes caused by brain ischemia in vivo.

Safety and tolerance of oral dextromethorphan in patients at risk for brain ischemia

Experimental ischemia models have shown the antitussive dextromethorphan to be an N-methyl-D-aspartate antagonist with neuroprotective properties. We treated 10 patients with a history of recent stroke or transient ischemic attack with oral dextromethorphan (60 mg q.i.d.) for 3 weeks in a placebo-controlled, double-blind, crossover tolerance study. We documented no clinical evidence of toxicity attributable to dextromethorphan in this preliminary study.