A slow intravenous infusion of N-methyl-DL-aspartate as a seizure model in the mouse

Sigma-1 receptor and seizures

Over the last decade, sigma-1 receptor (Sig1R) has been recognized as a valid target for the treatment of seizure disorders and seizure-related comorbidities. Clinical trials with Sig1R ligands are underway testing therapies for the treatment of drug-resistant seizures, developmental and epileptic encephalopathies, and photosensitive epilepsy. However, the direct molecular mechanism by which Sig1R modulates seizures and the balance between excitatory and inhibitory pathways has not been fully elucidated. This review article aims to summarize existing knowledge of Sig1R and its involvement in seizures by focusing on the evidence obtained from Sig1R knockout animals and the anti-seizure effects of Sig1R ligands. In addition, this review article includes a discussion of the advantages and disadvantages of the use of existing compounds and describes the challenges and future perspectives on the use of Sig1R as a target for the treatment of seizure disorders.

Targeting NMDA Receptor Complex in Management of Epilepsy

N-methyl-D-aspartate receptors (NMDARs) are widely distributed in the central nervous system (CNS) and play critical roles in neuronal excitability in the CNS. Both clinical and preclinical studies have revealed that the abnormal expression or function of these receptors can underlie the pathophysiology of seizure disorders and epilepsy. Accordingly, NMDAR modulators have been shown to exert anticonvulsive effects in various preclinical models of seizures, as well as in patients with epilepsy. In this review, we provide an update on the pathologic role of NMDARs in epilepsy and an overview of the NMDAR antagonists that have been evaluated as anticonvulsive agents in clinical studies, as well as in preclinical seizure models.

Acute seizure tests in epilepsy research: electroshock- and chemical-induced convulsions in the mouse

Epilepsy is a common (50 million patients worldwide) neurological disorder characterized by seizures that are caused by episodic abnormal electrical activity in the brain. Animal models play an essential role in epilepsy research including the discovery and development of new antiepileptic drugs. Described in this unit are protocols for traditional acute tests in which seizures are induced by either an electrical stimulation or a convulsant agent in non-epileptic mice. Specifically, protocols for the following acute seizure tests are provided: the maximal electroshock induced test (MES), the maximal electroshock seizure threshold (MEST) test, the 6-Hz seizure test, the subcutaneous pentylenetetrazol (s.c. PTZ) seizure test, and the intravenous pentylenetetrazol (i.v. PTZ) seizure test. These tests can be used to characterize anticonvulsant and/or proconvulsant properties of compounds in mice. The MES, s.c. PTZ, and 6-Hz seizure tests represent the three most widely used animal tests in drug-screening programs. Although the parameters of these tests are optimized for mice, the same tests (except for the 6-Hz seizure test), with some modifications, can be used with rats.

The NMDA receptor complex as a therapeutic target in epilepsy: a review

A substantial amount of research has shown that N-methyl-D-aspartate receptors (NMDARs) may play a key role in the pathophysiology of several neurological diseases, including epilepsy. Animal models of epilepsy and clinical studies demonstrate that NMDAR activity and expression can be altered in association with epilepsy and particularly in some specific seizure types. NMDAR antagonists have been shown to have antiepileptic effects in both clinical and preclinical studies. There is some evidence that conventional antiepileptic drugs may also affect NMDAR function. In this review, we describe the evidence for the involvement of NMDARs in the pathophysiology of epilepsy and provide an overview of NMDAR antagonists that have been investigated in clinical trials and animal models of epilepsy.

Glycine transporter 1 (GlyT1) inhibitors exhibit anticonvulsant properties in the rat maximal electroshock threshold (MEST) test

Glycine can act as either an inhibitory neurotransmitter or as a potentiator of NMDA-dependent excitatory neurotransmission. There is some evidence that glycine can have both pro- and anticonvulsant properties in various rodent models of epilepsy. In the present study we tested several glycine transporter 1 (GlyT1) inhibitors including NFPS, SSR 504734, Lu AA21279, Org 25935, SB-710622, GSK931145, as well as the glycine agonist d-serine, in the maximal electroshock threshold (MEST) test in the rat. In a series of experiments, male Sprague-Dawley rats (n=12/group) were pre-treated with a compound of interest and then received an electric shock delivered via corneal electrodes. A cohort of satellite animals (n=3/group) was also used to measure blood and brain levels of Org 25935. All GlyT1 inhibitors increased seizure thresholds dose-dependently, indicative of anticonvulsant activity. SB-710622 and GSK931145 had lower minimum effective doses (MEDs) in the MEST test than other GlyT1 inhibitors. At estimated t(max), increases in dose administered were paralleled by increases in blood and brain concentrations of Org 25935. Thus, increasing extracellular concentration of glycine via inhibition of its uptake protects from electroshock-induced seizures in the rat. Whether strychnine-sensitive or strychnine-insensitive glycine binding sites are involved in this effect remains to be determined.

Modification of ionotropic glutamate receptor-mediated processes in the rat hippocampus following repeated, brief seizures

The seizure-induced molecular and functional alterations of glutamatergic transmission in the hippocampus have been investigated. Daily repeated epileptic seizures were induced for 12 days by intraperitoneal administration of 4-aminopyridine (4-AP; 4.5 mg/kg) in adult Wistar rats. The seizure symptoms were evaluated on the Racine's scale. One day after the last injection, the brains were removed for in vitro electrophysiological experiments and immunohistochemical analysis. The glutamate receptor subunits NR1, NR2A, NR2B, GluR1, GluR1(flop), GluR2, and KA-2 were studied using the histoblotting method. The semi-quantitative analysis of subunit immunoreactivities in hippocampal layers was performed with densitometry. In the hippocampus, increase of GluR1, GluR1(flop) and NR2B immunostaining was observed in most of the areas and layers. The significant decrease of GluR2 staining intensity was observed in the CA1 and dentate gyrus. Calcium permeability of hippocampal neurons was tested by a cobalt uptake assay in hippocampal slices. The uptake of cobalt increased in the CA1 area and dentate gyrus, but not in the CA3 region following 4-AP treatment. Effects of AMPA and NMDA (N-methyl-d-aspartate) glutamate receptor antagonists (1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466) and D-APV respectively) were measured in hippocampal slices using extracellular recording. Analysis of the population spikes revealed the reduced effectiveness of the AMPA receptor antagonist GYKI 52466, while the effect of the NMDA receptor antagonist d-(2R)-amino-5-phosphonovaleric acid was similar to controls. The results demonstrated that repeated convulsions induced structural and functional changes in AMPA receptor-mediated transmission, while NMDA and kainate receptor systems displayed only alterations in receptor subunit composition.

Topiramate selectively protects against seizures induced by ATPA, a GluR5 kainate receptor agonist

Although the mechanism of action of topiramate is not fully understood, its anticonvulsant properties may result, at least in part, from an interaction with AMPA/kainate receptors. We have recently shown that topiramate selectively inhibits postsynaptic responses mediated by GluR5 kainate receptors. To determine if this action of topiramate is relevant to the anticonvulsant effects of the drug in vivo, we determined the protective activity of topiramate against seizures induced by intravenous infusion of various ionotropic glutamate receptor agonists in mice. Topiramate (25-100 mg/kg, i.p.) produced a dose-dependent elevation in the threshold for clonic seizures induced by infusion of ATPA, a selective agonist of GluR5 kainate receptors. Topiramate was less effective in protecting against clonic seizures induced by kainate, a mixed agonist of AMPA and kainate receptors. Topiramate did not affect clonic seizures induced by AMPA or NMDA. In contrast, the thresholds for tonic seizures induced by higher doses of these various glutamate receptor agonists were all elevated by topiramate. Unlike topiramate, carbamazepine elevated the threshold for AMPA- but not ATPA-induced clonic seizures. Our results are consistent with the possibility that the effects of topiramate on clonic seizure activity are due to functional blockade of GluR5 kainate receptors. Protection from tonic seizures may be mediated by other actions of the drug. Together with our in vitro cellular electrophysiological results, the present observations strongly support a unique mechanism of action of topiramate, which involves GluR5 kainate receptors.

Concurrent chronic administration of a CCK(B) antagonist can decrease tolerance to the ataxic effects of ethanol

The effects of chronic administration of a CCK(B) antagonist, CAM1028, were examined on the development of tolerance to ethanol and the appearance of withdrawal signs. Ethanol was given chronically by liquid diet, and none of the dose schedules of CAM1028 affected the amount of ethanol taken in during the chronic treatment. Brain ethanol concentrations were not altered by the administration of CAM1028.The ataxic effects of ethanol were tested on a rotorod, 3 hours after cessation of the ethanol administration, and the last injections of CAM1028 were given 8 hours before withdrawal from the ethanol treatment. When administered at 0.03, 0.1 or 0.3 mg/kg, CAM1028 decreased the extent of tolerance development. Higher doses, 1 and 3 mg/kg, did not alter the tolerance development. Chronic administration of CAM1028 had a small effect in protecting against the effects of ethanol withdrawal that, in contrast to the effects on the tolerance, was seen only at the highest dose, 10 mg/kg, of CAM1028.

CCK(B) antagonists protect against some aspects of the ethanol withdrawal syndrome

Effects of the CCK(B) antagonists, CAM1028 and C1988, and the CCK(A) antagonist, CAM1481, were studied on the ethanol withdrawal syndrome. When handling-induced behavior was measured hourly for 12 h from withdrawal of ethanol, a small, but significant, protective effect was seen with 3 mg/kg CAM1028, but not with 0.3, 1, or 10 mg/kg. C1988 (0.3 1,3, or 10 mg/kg), or CAM1481 (0.1 or 1 mg/kg), had no effects. At 16 h from ethanol withdrawal, these ratings were significantly decreased by 3 mg/kg CAM1028 or C1988, but not by lower doses. At 16 h, CAM1481 had very small, but significant, protective effects. At 3 mg/kg, CAM1028, increased the latencies to audiogenic seizures, but had only small effects on convulsion incidence. CAM1481 did not alter the audiogenic convulsions. The decrease in convulsion thresholds to NMDLA, at 16 h from ethanol withdrawal, was completely prevented by CAM1028 or C1988, at 1 and at 3 mg/kg, but not by lower doses; CAM1481 had no significant effects. The results suggest change in CCK(B) receptors may be involved in the later stages of the ethanol withdrawal syndrome.

Effects of dihydropyridines on the components of the ethanol withdrawal syndrome: possible evidence for involvement of potassium, as well as calcium?

Comparison was made of the ability of two dihydropyridine calcium channel antagonists, nitrendipine and felodipine, to prevent a range of signs of ethanol withdrawal. The increases in handling-induced behavior seen in mice during withdrawal from chronic ethanol treatment were prevented by administration of nitrendipine, 50 mg/kg, but not by, felodipine, 10 mg/kg, a dose that caused a similar displacement of dihydropyridine binding in central nervous system tissue, in vivo and in vitro. A higher dose of felodipine, 20 mg/kg, also had no effects. Nitrendipine, but not felodipine, prevented audiogenic seizures during the withdrawal phase. Similarly, nitrendipine prevented both the decrease in thresholds for N-methyl-DL-aspartate seizures and the increase in thresholds for convulsions due to 4-aminopyridine, which were seen during the withdrawal period, while felodipine did not alter either of these changes. Withdrawal from the ethanol chronic treatment increased the thresholds to seizures produced by intravenous aminophylline; this change was also prevented by nitrendipine. The significance of this increase in thresholds was lost after felodipine administration. In naive mice (not treated with ethanol) the doses of nitrendipine and felodipine used in the withdrawal studies were tested against the effects of convulsant drugs. Both dihydropyridines increased, to similar extents, the thresholds for seizures produced by bicuculline, pentylenetetrazol, and by N-methyl-DL-aspartate. The thresholds for aminophylline were unaltered by either dihydropyridine. In contrast, the thresholds for seizures due to 4-aminopyridine in the naive animals were not changed by felodipine, but were increased by nitrendipine. The results suggest that changes in potassium, as well as calcium, may possibly be involved in some of the stages of the ethanol withdrawal syndrome.

Infusion of NMDA antagonists into the nucleus reticularis pontis oralis inhibits the maximal electroshock seizure response

The nucleus reticularis pontis oralis (RPO) is necessary for the expression of tonic hindlimb extension (THE) in maximal electroshock (MES) seizures of rats. Previous work in this laboratory has demonstrated that both systemic administration and focal RPO microinfusion of D-cycloserine inhibits THE. The purpose of the present study was to characterize specific components of the NMDA receptor/ionophore complex that regulate the anticonvulsant activity mediated by the RPO. Bilateral RPO microinfusion of the competitive NMDA antagonists (-)AP7 and D-CPP as well as the uncompetitive antagonist dizocilpine ((+)MK-801) inhibited THE in a dose-related fashion. Bilateral RPO microinfusion of NMDA did not affect the THE response to MES but did induce convulsions resembling audiogenic seizures in genetically epilepsy prone rats. Bilateral RPO microinfusion of the strychnine-insensitive glycine site partial agonist D-cycloserine and the antagonist 5,7-dichlorokynurenic acid inhibited THE. The strychnine-insensitive glycine partial agonists (+)HA-966 and ACPC, as well as the agonists glycine and D-serine, did not significantly affect the THE response. Strychnine microinfusions in the RPO had no effect on THE. The results support a hypothesis that the RPO is a site of anticonvulsant drug action in MES and indicate that either competitive or uncompetitive NMDA antagonist action regulates the anticonvulsant activity mediated by the RPO. The role of the strychnine-insensitive glycine site in the regulation of the anticonvulsant activity medicated by the RPO is uncertain.

Tolerance to competitive NMDA antagonists, but no crosstolerance with barbiturates

Tolerance occurred to the sedative actions of the competitive NMDA antagonists, CGP39551 and CGP37849, as measured by a decrease in spontaneous locomotor activity after 1 week or 2 weeks of administration, respectively, in studies using the TO strain of mice. Crosstolerance was seen between these compounds. When CGP37849 was given after 2 weeks treatment with CGP39551, an increase in locomotor activity was seen. Chronic barbiturate treatment, producing tolerance to the actions of pentobarbitone, did not affect the sedative properties of CGP39551 or CGP37849. Chronic treatment with CGP39551 did not alter the ataxic actions of pentobarbitone. Seven days of treatment with HA966 caused complete tolerance to its sedative actions, but no crosstolerance was seen to pentobarbitone, CGP39551, or CGP37849. A small but significant decrease was seen in the convulsion thresholds to NMDA after 15 days of treatment with CGP39551, and a small significant increase in ratings of convulsive behavior after 16 days injections of CGP37849. No significant changes were found in either Bmax or Kd for [3H]-MK-801 binding in cerebrocortical tissue 24 h after the last chronic treatment with either of the NMDA antagonists.

Localization of an anatomic substrate for the anticonvulsant activity induced by D-cycloserine

D-Cycloserine is a partial agonist of the strychnine-insensitive glycine site that inhibits the tonic hindlimb extension (THE) component of maximal electroshock seizures (MES). This study determined the effect of focal D-clycoserine microinfusion into nucleus reticularis pontis oralis (RPO) on the THE component of MES in rats. Bilateral microinfusion of D-cycloserine (50 nmol per side) into the RPO region 5.4 and 5.6 mm posterior to bregma inhibited THE in 80% of rats tested. Unilateral D-cycloserine (50 nmol) RPO microinfusions were ineffective. Likewise, RPO microinfusion of vehicle, L-cycloserine (50 nmol per side), or the strychnine-insensitive glycine site antagonist 7-chlorokynurenic acid (10 and 50 nmol per side) did not alter THE incidence. However, coinfusion of 7-chlorokynurenic acid (50 nmol per side) with D-cycloserine (50 nmol per side) completely antagonized the anticonvulsant activity induced by D-cycloserine (8 of 8 rats with THE). These data indicate that the anticonvulsant activity of D-cycloserine is mediated by RPO. Because the anticonvulsant effect is stereospecific and is reversible by 7-chlorokynurenic acid, these results also indicate that D-cycloserine acts through the strychnine-insensitive glycine site to inhibit THE.

The NMDA antagonist procyclidine, but not ifenprodil, enhances the protective efficacy of common antiepileptics against maximal electroshock-induced seizures in mice

Procyclidine (up to 20 mg/kg i.p.) did not influence the electroconvulsive threshold per se, but when given in a dose of 10 mg/kg, it potentiated the protective activity of carbamazepine, diphenylhydantoin, phenobarbital and valproate, and in a dose of 20 mg/kg, that of diazepam against maximal electroshock-induced convulsions in mice. Ifenprodil increased the threshold for electroconvulsions when applied at 20 and 40 mg/kg (i.p.), but surprisingly, when combined with all antiepileptics tested, it did not influence their anticonvulsant actions. The chimney test in mice revealed, that application of procyclidine at 10 mg/kg together with phenobarbital and valproate, and procyclidine at 20 mg/kg with diazepam resulted in motor impairment. However, when procyclidine was applied at 10 mg/kg together with carbamazepine or diphenylhydantoin, no motor impairment was noted. The combined treatment of procyclidine (10 mg/kg) with carbamazepine, diphenylhydantoin, phenobarbital or valproate, as well as procyclidine (20 mg/kg) with diazepam caused significant worsening of long-term memory. Finally, procyclidine did not alter the total plasma levels of carbamazepine, diazepam, diphenylhydantoin, phenobarbital and valproate. It may be concluded that not all agents interfering with NMDA receptor complex-mediated events lead to the potentiation of the anticonvulsant activity of antiepileptic drugs.

Possible involvement of NMDA receptor-mediated transmission in barbiturate physical dependence

1. The competitive antagonists at the N-methyl-D-aspartate (NMDA) receptor, CGP39551 and CGP37849, protected against the barbiturate withdrawal syndrome in mice, as measured by ratings of convulsive behaviour on handling. 2. The effective doses of these compounds were lower than those required to prevent seizures due to NMDA in naive animals; these were in turn lower than those needed to prevent the convulsive effects of the alpha-aminobutyric acid (GABA) antagonist, bicuculline. 3. The NMDA-receptor antagonists did not alter the increase in the incidence of convulsions due to the GABAA antagonist, bicuculline, that is seen during barbiturate withdrawal, although the latencies to these convulsions during barbital withdrawal were significantly increased after CGP39551. 4. Barbiturate withdrawal did not affect the convulsive actions of NMDA, whether measured by the incidence of convulsions or by intravenous infusion. 5. The Bmax for [3H]-dizocilpine ([3H]-MK801) binding was significantly increased by chronic barbital treatment in cerebrocortical but not in hippocampal tissues, while the Kd remained unaltered in either case. 6. At 1 h and 24 h after administration of a single dose of barbitone, the Bmax for [3H]-dizocilpine binding was unaltered in cerebrocortical tissue. Acute addition of barbitone in vitro did not alter [3H]-dizocilpine binding or the displacement of binding of thienylcyclohexylpyridine.

Effect of glutamate receptor antagonists on N-methyl-D-aspartate- and (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced convulsant effects in mice and rats

Selected antagonists of N-methyl-D-aspartate (NMDA) and (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists, acting through different recognition sites were studied in three in vivo experimental procedures: systemic administration of NMDA or AMPA to mice and 7-day-old rats or i.c.v. injection in adult rats. Antagonists were given i.p. before the agonists. Of the substances tested (+)-5-methyl-10,11- dihydro-5H-dibenzocyclohepten-5,10-imine maleate ((+)-MK-801, an uncompetitive NMDA receptor antagonist) and DL-(E)-2-amino-4-methyl-5- phosphono-3-pentanoic acid (CGP-37849, a competitive NMDA receptor antagonist) were the most potent and selective NMDA receptor antagonists, having ED50s below 1 mg/kg in all three tests. 1-Amino-3,5-dimethyladamantane (memantine, an uncompetitive NMDA receptor antagonist) was less potent and, additionally, inhibited AMPA-induced seizures in adult rats. Aminocyclopropane carboxylic acid--a partial agonist at the glycine site coupled to NMDA receptors (GlyB)--was a weak antagonist (ED50 > 150 mg/kg) in mice. Other partial GlyB receptor agonists, aminocyclobutane carboxylic acid, (+,R)-3-amino-1-hydroxy-2-pyrrolidine ((+,R)-HA-966) and d-cycloserine, and antagonists, 5,7-dinitroquinoxaline-2,3-dione (MNQX) and 5,7-dichlorokynurenic acid, were ineffective in mice after systemic administration. The last two agents however were active in adult rats when given i.c.v. Thus affinity, intrinsic activity (GlyB receptor partial agonists) and/or penetration into the brain (GlyB receptor antagonists) seem to be important factors in determining the effectiveness of these agents.(ABSTRACT TRUNCATED AT 250 WORDS)

The anticonvulsant activity of D-cycloserine is specific for tonic convulsions

D-Cycloserine has been shown to exert anticonvulsant activity in maximal electroshock seizures. The purpose of the present study was to evaluate the spectrum of D-cycloserine anticonvulsant activity using other experimental models of epilepsy. D-Cycloserine induced a dose-related decrease in the incidence of tonic convulsions induced by 120 mg/kg of pentylenetetrazol. The ED50 of D-cycloserine for the inhibition of the tonic convulsions was 109 mg/kg. The anticonvulsant activity was specific for the D-isomer at L-cycloserine (400 mg/kg) had no effect on the tonic convulsions. D-Cycloserine had no effect on the pentylenetetrazol-induced clonic convulsions induced by either 70 or 120 mg/kg pentylenetetrazol, electrically induced nonkindled hippocampal seizures or kindled amygdala seizures. D-Cycloserine had no effect on strychnine-induced tonic convulsions. These results indicate that D-cycloserine is inactive against clonic convulsions and may be active only against tonic convulsions mediated by brainstem sites.

New antiepileptic drugs: from serendipity to rational discovery

Antiepileptic drug discovery has made enormous progress from the serendipity and screening processes of earlier days to the rational drug development of today. The modern era of research began with the recognition that enhancement of inhibitory processes in the brain might favorably influence the propensity for seizures, gamma-aminobutyric acid (GABA) being the main inhibitory transmitter. Work in this field led to the development of vigabatrin, which inhibits the enzyme responsible for the degradation of GABA. More recently, research has focused on the therapeutic potential of blocking excitatory amino acids--in particular glutamate. Of the three receptors for glutamate, the N-methyl-D-aspartate (NMDA) receptor is considered the one of most interest in epilepsy, and research on a series of competitive NMDA receptor antagonists--especially those that are orally active--is in the forefront of antiepileptic drug development today. A further alternative for diminishing neuronal excitability is to modulate sodium, potassium, or calcium channels. The latter are especially implicated in absence seizures.