Effects of MK-801 and phenytoin on flurothyl-induced seizures during development

Involvement of N-Methyl-D-Aspartate Receptors in the Anticonvulsive Effects of Licofelone on Pentylenetetrazole-Induced Clonic Seizure in Mice

BACKGROUND AND PURPOSE

Licofelone is a dual 5-lipoxygenase/cyclooxygenase inhibitor, with well-documented anti-inflammatory and analgesic effects, which is used for treatment of osteoarthritis. Recent preclinical studies have also suggested neuroprotective and anti-oxidative properties of this drug in some neurological conditions such as seizure and epilepsy. We have recently demonstrated a role for nitric oxide (NO) signaling in the anti-epileptic activity of licofelone in two seizure models in rodents. Given the important role of N-methyl-D-aspartate receptors (NMDARs) activation in the NO production and its function in the nervous system, in the present study, we further investigated the involvement of NMDAR in the effects of licofelone (1, 3, 5, 10, and 20 mg/kg, intraperitoneal [i.p.]) in an in vivo model of seizure in mice.

METHODS

Clonic seizures were induced in male NMRI mice by intravenous administration of pentylenetetrazol (PTZ).

RESULTS

Acute administration of licofelone exerted anticonvulsant effects at 10 (p<0.01) and 20 mg/kg (p<0.001). A combined treatment with sub-effective doses of the selective NMDAR antagonist MK-801 (0.05 mg/kg, i.p.) and licofelone (5 mg/kg, i.p.) significantly (p<0.001) exerted an anticonvulsant effect on the PTZ-induced clonic seizures in mice. Notably, pre-treatment with the NMDAR co-agonist D-serine (30 mg/kg, i.p.) partially hindered the anticonvulsant effects of licofelone (20 mg/kg).

CONCLUSIONS

Our data suggest a possible role for the NMDAR in the anticonvulsant effects of licofelone on the clonic seizures induced by PTZ in mice.

Dissociation of spontaneous seizures and brainstem seizure thresholds in mice exposed to eight flurothyl-induced generalized seizures

Objective

C57BL/6J mice exposed to eight flurothyl‐induced generalized clonic seizures exhibit a change in seizure phenotype following a 28‐day incubation period and subsequent flurothyl rechallenge. Mice now develop a complex seizure semiology originating in the forebrain and propagating into the brainstem seizure network (a forebrain→brainstem seizure). In contrast, this phenotype change does not occur in seizure‐sensitive DBA/2J mice. The underlying mechanism was the focus of this study.

Methods

DBA/2J mice were exposed to eight flurothyl‐induced seizures (1/day) followed by 24‐h video‐electroencephalographic recordings for 28 days. Forebrain and brainstem seizure thresholds were determined in C57BL/6J and DBA/2J mice following one or eight flurothyl‐induced seizures, or after eight flurothyl‐induced seizures, a 28‐day incubation period, and final flurothyl rechallenge.

Results

Similar to C57BL/6J mice, DBA/2J mice expressed spontaneous seizures. However, unlike C57BL/6J mice, DBA/2J mice continued to have spontaneous seizures without remission. Because DBA/2J mice did not express forebrain→brainstem seizures following flurothyl rechallenge after a 28‐day incubation period, this indicated that spontaneous seizures were not sufficient for the evolution of forebrain→brainstem seizures. Therefore, we determined whether brainstem seizure thresholds were changing during this repeated‐flurothyl model and whether this could account for the expression of forebrain→brainstem seizures. Brainstem seizure thresholds were not different between C57BL/6J and DBA/2J mice on day 1 or on the last induction seizure trial (day 8). However, brainstem seizure thresholds did differ significantly on flurothyl rechallenge (day 28), with DBA/2J mice showing no lowering of their brainstem seizure thresholds.

Significance

These results demonstrate that DBA/2J mice exposed to the repeated‐flurothyl model develop spontaneous seizures without evidence of seizure remission and provide a new model of epileptogenesis. Moreover, these findings indicated that the transition of forebrain ictal discharge into the brainstem seizure network occurs as a result of changes in brainstem seizure thresholds that are independent of spontaneous seizure expression.

Issues related to development of new antiseizure treatments

This report represents a summary of the discussions led by the antiseizure treatment working group of the International League Against Epilepsy (ILAE)/American Epilepsy Society (AES) Working Groups joint meeting in London (London Meeting). We review here what is currently known about the pharmacologic characteristics of current models of refractory seizures, both for adult and pediatric epilepsy. In addition, we address how the National Institute of Neurological Disorders and Stroke (NINDS)-funded Anticonvulsant Screening Program (ASP) is evolving to incorporate appropriate animal models in the search for molecules that might be sufficiently novel to warrant further pharmacologic development. We also briefly address what we believe is necessary, going forward, to achieve the goal of stopping seizures in all patients, with a call to arms for funding agencies, the pharmaceutical industry, and basic researchers.

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.

Regional neural activity within the substantia nigra during peri-ictal flurothyl generalized seizure stages

Structures responsible for the onset, propagation, and cessation of generalized seizures are not known. Lesion and microinfusion studies suggest that the substantia nigra pars reticulata (SNR) seizure-controlling network could play a key role. However, the expression of neural activity within the SNR and its targets during discrete pre- and postictal periods has not been investigated. In rats, we used flurothyl to induce generalized seizures over a controlled time period and 2-deoxyglucose autoradiography mapping technique. Changes in neural activity within the SNR were region-specific. The SNRposterior was selectively active during the pre-clonic period and may represent an early gateway to seizure propagation. The SNRanterior and superior colliculus changed their activity during progression to tonic-clonic seizure, suggesting the involvement in coordinated regional activity that results in inhibitory effects on seizures. The postictal suppression state was correlated with changes in the SNR projection targets, specifically the pedunculopontine tegmental nucleus and superior colliculus.

Cessation of repeated administration of MK-801 changes the anticonvulsant effect against flurothyl-induced seizure in mice

The effects of acute and repeated administration of MK-801 on flurothyl (FE)-induced seizure were investigated in mice. In the acute effect of MK-801 (0.01-0.1 mg/kg ip) in naive and FE-kindled mice, there were no changes on the latencies of clonic seizures. However, MK-801 dose-dependently inhibited both latencies and incidence of tonic seizures in mice and suppressed the grade of seizure severity in FE-kindled mice. Repeated administration of MK-801 at doses of 0.01 and 0.1 mg/kg 2 h prior to each exposure to FE for 8 days did not show any effects on the latencies of clonic seizure. However, seizure severity was significantly exacerbated in the 0.1 mg/kg treated group when mice were re-exposed to FE without MK-801 1 week after the last administration. A week after the repeated administration of MK-801 at a dose of 0.1 mg/kg for 8 days without exposure to FE, mice were exposed to FE 2 h after readministration of MK-801 until tonic seizure occurred. The latencies of clonic seizures were almost the same in the acute experiment in naive controls. The latency of tonic seizure was significantly delayed compared to the acute experiment with MK-801 at a dose of 0.1 mg/kg. These findings suggested that MK-801 possessed an anticonvulsant action against FE-induced tonic seizure. However, the efficacy of this acute effect of MK-801 was impaired at 1 week of withdrawal after repeated administrations. This may be related in part to the changes in sensitivity to NMDA receptors.

An anticonvulsant profile of the ketogenic diet in the rat

The present study was designed to evaluate the anticonvulsant effects of a high-fat ketogenic diet (KD) in rats. Animals were maintained on one of four experimental diets: (1) calorie-restricted ketogenic (KCR); (2) calorie-restricted normal (NCR); (3) ad libitum ketogenic (KAL); or (4) ad libitum normal (NAL). The calorie-restricted diets were fed in quantities such that they were calorically equivalent. All animals began diet treatment at age P37 and each was subjected to one of five chemically-induced seizure tests: bicuculline (BIC; s.c.), picrotoxin (PIC; s.c.), kainate (KA, i.p. or s.c.) and gamma-butyrolactone (GBL, i.p.), strychnine (s.c.). Bipolar epidural electrodes were implanted under ketamine/xylazine anesthesia to permit recording the spike and wave discharges (SWD) characteristic of electroencephalograms during absence seizures. Ketonemia was assayed by measuring blood levels of beta-hydroxybutyrate (BHB) spectrophotometrically prior to induction of seizures in each experiment. Animals fed ketogenic diets (i.e. either calorie restricted or ad libitum) exhibited greater blood levels of BHB compared to control groups. Seizure results show that treatment with a KD: (1) reduced the incidence of bicuculline-induced convulsions; (2) diminished the number of picrotoxin-induced seizures (KCR group only); (3) increased latency to GBL-induced SWD and reduced both the number and duration of SWD; but (4) conferred no protection from strychnine-induced seizures; and (5) made KA-induced seizures more severe. Together these results indicate a spectrum of anticonvulsant action for the KD in rats that includes threshold seizures induced via GABA receptors (BIC, PIC, GBL) but not those induced at glycine (strychnine) or the KA-subclass of glutamate receptors. Uniquely, the KD is the only treatment described that protects against both convulsive and non-convulsive (absence) seizures in rats.

Dietary iron reduces the anti-convulsion activity of phenytoin in electroconvulsion via inhibition of brain penetration

We determined the anti-convulsion activity of phenytoin (PHT) using the maximum electron shock method in mice fed diets containing various concentrations of iron for 18 weeks. Dietary iron reduces the anti-convulsion activity of PHT in a dose-dependent manner (0-6100 ppm). High concentrations of PHT are detected in the plasma of mice fed a high iron diet compared with those fed normal and low iron diets, in contrast to the pharmacological effect. However, the concentration of PHT in the brains of mice fed high amounts of dietary iron decreased significantly 3 h after treatment with PHT, consistent with the anti-convulsion effect of PHT. The relationship between brain and plasma-unbound concentrations of PHT indicates that the penetration of PHT into brain is significantly inhibited by dietary iron.

Special considerations in treating children with epilepsy

The incidence of seizures is high in infants and children. Many epileptic syndromes have their onset early in life. The increase in seizure susceptibility of the immature brain may be due to several factors, including an imbalance between excitatory and inhibitory processes, age-specific differences in ionic transport and clearance systems, high incidence of epileptogenic stimuli early in life, and the age-specific expression of pre- and perinatal brain anomalies. All of these factors must be taken into account when developing safe and effective age-specific antiepileptic drugs (AEDs). The use of developmental epilepsy models, followed by clinical trials in children, may help identify such AEDs.

GluR2 hippocampal knockdown reveals developmental regulation of epileptogenicity and neurodegeneration

In adult rats, kainic acid-induced status epilepticus reduces GluR2 subunit expression prior to neurodegeneration of hippocampal CA3 neurons. Increased formation of Ca2+ permeable AMPA receptors may contribute to the delayed neurodegenerative process. In rat pups, highly prone to seizures but resistant to seizure-induced hippocampal damage, GluR2 mRNA and protein expression remain constant in CA3 neurons possibly contributing to their survival. To investigate whether reduced GluR2 expression in hippocampus may lead to enhanced hippocampal vulnerability in an age-dependent manner and whether changes correspond to altered electroencephalography (EEG) patterns, unilateral microinfusion of GluR2 antisense oligodeoxynucleotides (AS-ODNs) into hippocampus was performed at three ages (postnatal (P8), P13, and adult). At P13, GluR2 knockdown resulted in spontaneous seizure-like behavioral manifestations and neurodegeneration of CA3 neurons lateral and distal from the cannula infusion site. EEG recordings revealed high rhythmic activity associated with seizure-like behavior. In P8 pups and adult rats, there were no behavioral manifestations; distant hippocampal damage of the CA3 was not observed. Results indicate that unilateral knockdown of hippocampal GluR2 subunit expression induces age-dependent seizure-like behavioral manifestations, altered EEG recording patterns, and reduces the survival of CA3 neurons in the hippocampus of young rats during a specific postnatal period (3rd week), when GluR2 expression peaks in development and glutamatergic inputs are maturing.

An indirect method for absorption rate estimation: flurothyl-induced seizures

This paper develops a method to estimate a minimal amount of flurothyl necessary to induce the seizures (the seizure threshold). A simple mathematical model is proposed which permits one to determine the drug absorption rate from the amount which has been administered and from the measured latency to onset of seizure. Experimental animal (rats) were exposed to a continuous intake of flurothyl in two different situations: either being alone in the airtight chamber or sharing it in a pair. In the latter case, we assume that the two rats uniformly share the infused drug. Our calculations estimate that approximately 20 microliters of flurothyl is necessary to induce twitches, whereas 25 microliters of flurothyl is the dose required for the induction of clonic seizures. The model can be used to estimate the threshold amounts of any drug producing obvious behavioral changes irrespective of the route of administration.

Tiagabine

Tiagabine is a novel antiepileptic drug that was designed to block gamma-aminobutyric acid uptake by presynaptic neurons and glial cells. It has been shown to be effective against partial seizures in adults and adolescents. Preliminary pediatric data are also encouraging. This article reviews the available animal and human data on the pharmacokinetics, efficacy, and safety of tiagabine.

Acute effects of MK801 on kainic acid-induced seizures in neonatal rats

Kainic acid (KA) causes behavioral and electrographic status epilepticus (SE) in rats of all ages. In adult rats, the noncompetitive N-methyl-D-aspartate (NMDA) channel blocker MK801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine ) is anticonvulsant against KA-induced seizures: it reduces their severity and protects against neuronal damage, although it may worsen electrographic seizures. Here we examined the effects of MK801 on KA seizures in the immature brain. Neonatal rats (P11-P12) were pretreated with MK801 (0.01, 0.1, 0.5 or 1.0 mg/kg, i.p.) or saline twenty minutes prior to KA (2 mg/kg, i.p.). Clinical seizure behavior was monitored for > 6 hrs, and in some rats the EEG was monitored with an intrahippocampal or intracortical electrode. MK801 caused immobility alternating with hyperactivity, ataxia, scratching and sometimes alternate limb cycling, which correlated with the appearance of spikes on the EEG. Compared to KA alone or KA preceded by 0.01 mg/kg MK801, the higher doses of MK801 (0.1, 0.5 and 1.0 mg/kg) significantly lowered the latency to electrographic seizures (P < 0.001), ictal scratching (P < 0.0001), and status epilepticus (P < 0.0001). MK801 pretreatment did not lower significantly the death rate due to KA seizures. No histologic damage was seen after MK801, KA or both agents together. These results suggest that MK801 exacerbates KA-induced seizures in the neonatal brain, and may even cause ictal behavioral and electrographic manifestations by itself. The findings point to an age-dependency of NMDA antagonist action, and suggest caution in considering the use of NMDA antagonists in neonates.

Objective measure of treatment outcome in epilepsy

Treatment outcome in epilepsy is too often a vague, ill-defined subjective measure. Based on a proposal of Shoffer and Temkin (1987) of "time to kth seizure" as a measure of seizure frequency, we devised the formula: % improvement = 100 - [time to X seizure (i*) x 100]/ [time to X seizure (f*)]. This formula provides an objective measure of treatment outcome and should prove useful in clinical settings and research. We offer examples of practical applications of the formula.