Effects of N-methyl-D-aspartate receptor subunit antagonists on regulation of susceptibility to audiogenic seizures in rats

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.

Anticonvulsant effect of dextrometrophan on pentylenetetrazole-induced seizures in mice: Involvement of nitric oxide and N-methyl-d-aspartate receptors

Dextrometrophan (DM), widely used as an antitussive, has recently generated interest as an anticonvulsant drug. Some effects of dextrometrophan are associated with alterations in several pathways, such as inhibition of nitric oxide synthase (NOS) enzyme and N-methyl d-aspartate (NMDA) receptors. In this study, we aimed to investigate the anticonvulsant effect of acute administration of dextrometrophan on pentylenetetrazole (PTZ)-induced seizures and the probable involvement of the nitric oxide (NO) pathway and NMDA receptors in this effect. For this purpose, seizures were induced by intravenous PTZ infusion. All drugs were administrated by intraperitoneal (i.p.) route before PTZ injection. Our results demonstrate that acute DM treatment (10-100mg/kg) increased the seizure threshold. In addition, the nonselective NOS inhibitor L-NAME (10mg/kg) and the neural NOS inhibitor, 7-nitroindazole (40mg/kg), at doses that had no effect on seizure threshold, augmented the anticonvulsant effect of DM (3mg/kg), while the inducible NOS inhibitor, aminoguanidine (100mg/kg), did not affect the anticonvulsant effect of DM. Moreover, the NOS substrate l-arginine (60mg/kg) blunted the anticonvulsant effect of DM (100mg/kg). Also, NMDA antagonists, ketamine (0.5mg/kg) and MK-801 (0.05mg/kg), augmented the anticonvulsant effect of DM (3mg/kg). In conclusion, we demonstrated that the anticonvulsant effect of DM is mediated by a decline in neural nitric oxide activity and inhibition of NMDA receptors.

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.

Administration of lithium and magnesium chloride inhibited tolerance to the anticonvulsant effect of morphine on pentylenetetrazole-induced seizures in mice

Although morphine has an anticonvulsant effect in several animal models of seizures, its potential clinical application in epilepsy may be hindered by its adverse effects like opioid tolerance. The present study evaluated the development of tolerance to the anticonvulsant effect of morphine in a model of clonic seizures induced with pentylenetetrazole (PTZ) in male Swiss mice. We also examined whether administration of either lithium chloride (LiCl) or magnesium chloride (MgCl(2)) was able to prevent the probable tolerance. Our data demonstrated that the anticonvulsant effect of a potent dose of morphine (1mg/kg) was abolished in chronic morphine-treated mice (mice administered the same dose of morphine intraperitoneally twice daily for 4 days). Four days of pretreatment with low and noneffective doses of MgCl(2) (2 and 5mg/kg) and LiCl (5mg/kg) inhibited the development of tolerance to the anticonvulsant effect of morphine (1mg/kg, ip). Moreover, a single acute injection of the aforementioned agents at the same doses reversed the expression of tolerance to the anticonvulsant effects of morphine (1mg/kg, ip). Chronic 17-day treatment with LiCl (600 mg/L in drinking water) also inhibited the development of tolerance to the anticonvulsant effects of 1mg/kg morphine. These results demonstrate that the anticonvulsant effect of morphine is subject to tolerance after repeated administration. Both development and expression of tolerance are inhibited by either LiCl or MgCl(2). As both LiCl and MgCl(2) can modulate the function of N-methyl-d-aspartate (NMDA) receptors, we discuss how NMDA receptor functioning might be involved in the effects of LiCl and MgCl(2) on the development of tolerance to the anticonvulsant effect of morphine.

Voltage-dependent calcium channel and NMDA receptor antagonists augment anticonvulsant effects of lithium chloride on pentylenetetrazole-induced clonic seizures in mice

Although lithium is still a mainstay in the treatment of bipolar disorder, its underlying mechanisms of action have not been completely elucidated. Several studies have shown that lithium can also modulate seizure susceptibility in a variety of models. In the present study, using a model of clonic seizures induced with pentylenetetrazole (PTZ) in male Swiss mice, we investigated whether there is any interaction between lithium and either calcium channel blockers (CCBs: nifedipine, verapamil, and diltiazem) or N-methyl-D-aspartate (NMDA) receptor antagonists (ketamine and MK-801) in modulating seizure threshold. Acute lithium administration (5-100mg/kg, ip) significantly (P<0.01) increased seizure threshold. CCBs and NMDA receptor antagonists also exerted dose-dependent anticonvulsant effects on PTZ-induced seizures. Noneffective doses of CCBs (5mg/kg, ip), when combined with a noneffective dose of lithium (5mg/kg, ip), exerted significant anticonvulsant effects. Moreover, co-administration of a noneffective dose of either MK-801 (0.05mg/kg, ip) or ketamine (5mg/kg, ip) with a noneffective dose of lithium (5mg/kg, ip) significantly increased seizure threshold. Our findings demonstrate that lithium increases the clonic seizure threshold induced by PTZ in mice and interacts with either CCBs or NMDA receptor antagonists in exerting this effect, suggesting a role for Ca(2+) signaling in the anticonvulsant effects of lithium in the PTZ model of clonic seizures in mice.

Non-competitive NMDA receptor antagonists moderate seizure-induced c-fos expression in the rat cerebral cortex

We examined the effects of non-competitive NMDA glutamate receptor antagonists on seizures elicited by 4-aminopyridine (4-AP), and in particular, on the expression of the transcription factor c-fos induced by these seizures. Induction of c-fos mRNA due to 4-AP-elicited seizures was ascertained by reverse transcription polymerase chain reaction in samples of the neocortex. Adult rats were pretreated with the NMDA receptor antagonists amantadine (40 mg/kg), ketamine (3mg/kg), dizocilpine (MK-801; 1mg/kg) or dextrometorphan (40 mg/kg); 4-AP (5mg/kg) was then injected i.p. Controls were treated with either antagonist only or with 4-AP only. Pretreatment with the antagonists (with the exception of amantadine) increased the latency of behavioural seizures, but not all of the antagonists caused symptomatic seizure protection. In the brains which were processed for Fos immunohistochemistry, quantitative evaluation of immunostained cells was performed in the neocortex and hippocampus. Treatment with either antagonist did not induce by itself c-fos expression, with the exception of amantadine, which caused slight Fos induction in the neocortex. Pretreatment with all the antagonists resulted in decrease of seizure-induced Fos immunoreactivity with respect to non-pretreated animals. Decrease of immunostained cells was significant in the neocortex, in the granule cell layer and hilus of the dentate gyrus, in hippocampal areas CA1 and CA2. MK-801, ketamine and dextrometorphan decreased significantly Fos immunoreactivity also in area CA3. The decrease of Fos immunostaining was not directly correlated with a suppression of behavioural seizures. The results support an important role of NMDA receptors in c-fos gene induction in acute 4-AP seizures.

Fos expression in GABAergic cells and cells immunopositive for NMDA receptors in the inferior and superior colliculi following audiogenic seizures in rats

Given the evidence that the inferior colliculus (IC) and superior colliculus (SC) seem to play key roles in connecting auditory pathways and seizure output pathways in the neuronal network for audiogenic seizures (AS) in rats, we examined Fos activation in GABAergic cells and cells immunopositive for glutamate N-methyl-D-aspartate (NMDA) receptors in the IC and SC following AS using the double-labeling procedure. Generalized tonic-clonic seizures (GTCS), which developed as an advanced form of AS in some of the susceptible rats, induced an increase in Fos expression in three IC substructures-the dorsal cortex of IC (DCIC), central nucleus of IC (CIC), and external cortex of IC (ECIC)-and in one SC substructure, the deep gray layer of SC (DpG). Compared with the rats showing GTCS, rats exhibiting wild running (WR) without proceeding to GTCS showed a different pattern of AS-induced Fos expression. The DpG in the WR animals showed no significant increase in the levels of Fos-like immunoreactivity. The degrees of Fos activation that occurred in GABAergic cells and cells immunopositive for NMDA receptors were similar in the DCIC, CIC, ECIC, and DpG following AS. These results suggest that Fos activation in the DpG is involved in the development from WR to GTCS in AS-susceptible rats. They also provide some evidence that some GABAergic neurons in the IC and SC and glutamatergic afferents (via NMDA receptors) to these structures are activated by AS.

Mutation of NMDA receptor subunit epsilon 1: effects on audiogenic-like seizures induced by electrical stimulation of the inferior colliculus in mice

It has been shown that the N-methyl-D-asparate (NMDA) receptor in the inferior colliculus is involved in the induction of audiogenic seizures (AGS). In the present study we examined audiogenic-like seizure susceptibility in GluR epsilon 1 null KO adult mice (n=32) and wild-type adult mice (n=28) by electrically stimulating the inferior colliculus (IC). Threshold current intensities of the GluR epsilon 1 KO mice for wild running, clonic and tonic seizures were higher than those of wild-type mice. In addition, the incidence rates of each seizure syndrome in GluR epsilon 1 KO mice were lower than in wild-type mice at each current intensity. These results show that GluR epsilon 1 KO mice were more resistant to audiogenic-like seizures induced by stimulating the IC. Thus, our findings suggest that the GluR epsilon 1 subunit plays an important role in regulating AGS.