The anticonvulsant effects of diazepam and phenobarbital in prek1ndled and kindled cortical seizures

The effects of various doses of diazepam (0.5–4 mg/kg) and phenobarbital (15–60 mg/kg) were determined on prekindled (focal) and kindled (generalized) cortical seizures in the same rats. Only high sedating doses of diazepam or phenobarbital reduced the elicited afterdischarge duration (ADD) and behavioral response in the prekindled focal cortical seizure. Against the kindled seizure, both diazepam and phenobarbital showed a marked and statistically significant increase in effectiveness in all but the smallest doses tested. The ADD of the kindled cortical seizures was reduced to prekindled lengths by diazepam (1–4 mg/kg) or phenobarbital (30–60 mg/kg). The increased anticonvulsant effectiveness found in this study is similar to previous findings with diazepam and phenobarbital against prekindled and kindled amygdaloid seizures, but stands in contrast to findings with prekindled and kindled pentylenetetrazol seizures.     

The effect of a glycine derivative (CP 1552-S) on kindled seizures in rats

The effects of the glycine derivative, CP 1552-S (2-N-pentylaminoacetamide hydrochloride) were evaluated for potential anticonvulsant activity in rats which were cortically- or amygdaloid-kindled. Large doses (300-600 mg/kg, i.p.) of CP 1552 given 30 min before stimulation resulted in significant reductions in duration of afterdischarge after both partially-developed and fully-developed cortically-kindled seizures. The largest dose tested (600 mg/kg, i.p.) markedly reduced the duration of the elicited afterdischarge and the severity of seizure. This dose was associated with prestimulation sedation and a 50% incidence of post-afterdischarge spontaneous, electrical seizure activity. Against kindled amygdaloid seizures, CP 1552-S significantly reduced the duration of afterdischarge at 300 mg/kg (i.p.) without modifying the seizure and without prestimulation behavioral or electrical effects. The largest dose tested (600 mg/kg, i.p.) resulted in a significant reduction of the elicited duration of afterdischarge but was associated with a 25% incidence of prestimulation spontaneous electrical seizure activity and a 45% incidence of post-afterdischarge electrical seizure activity. When CP 1552-S (30-300 mg/kg, i.p.) was administered daily, prior to the amygdaloid kindling stimulus, no difference was noted in the rate of acquisition of the kindled amygdaloid response. It is concluded that the glycine derivative CP 1552-S, has little anticonvulsant activity against the acquisition or development of kindled amygdaloid seizures. It appears to have significant anticonvulsant effects against both cortically- and amygdaloid-kindled afterdischarges with little effect on the behavioral severity of the seizure. Further, large doses of CP 1552-S appeared to result in paradoxical post-afterdischarge and possibly prestimulation electrical seizure activity.     

The effects of pentylenetetrazol, bicuculline and strychnine on the development of kindled seizures

Modification of the rate of acquisition of the kindled amygdaloid seizure by the convulsants pentylenetetrazol, bicuculline and strychnine was studied. Injections of saline, 25 mg/kg of pentylenetetrazol, 2 mg/kg of bicuculline or 1 mg/kg of strychnine were given 15 min prior to the daily electrical stimulation of the amygdala. The drug doses selected were capable of producing some behavioral and electrical epileptoid activity prior to stimulation without inducing generalized seizures. To determine whether pentylenetetrazol or bicuculline accelerated the rate of development of the kindled amygdaloid seizure or merely augmented the expression of each seizure, a crossover design was implemented. The crossover studies involved switching animals during the acquisition phase (between stimulations 3–6) from prestimulation saline to drug or drug to saline injections. It was found that pentylenetetrazol markedly augmented the expression of seizures during kindling development but the results of the crossover studies showed a less dramatic acceleration in the actual rate of the development of the fully generalized kindled amygdaloid seizure. The bicuculline-treated animals showed little augmentation in the expression of seizures during the kindling acquisition phase and in the actual rate of development of the kindled amygdaloid seizure. The strychnine treated animals showed no augmentation in expression of the seizures nor in the rate of development. The effects of prestimulation injections of bicuculline (1, 2 and 3 mg/kg) and strychnine (0.5, 1 and 2 mg/kg) on fully developed kindled amygdaloid seizures were also evaluated. Pretreatment with bicuculline minimally increased seizure afterdischarge duration at the highest dose. When fully kindled animals were pretreated with strychnine, a paradoxical decrease in afterdischarge length and an increase in severity (tonic hindlimb extension) was seen with the largest dose tested. This study emphasizes the potential importance of crossover studies in evaluating pharmacological manipulations of the rate of acquisition of the kindled seizure.     

吲哚醌对大鼠杏仁核点燃的抑制作用

目的：研究吲哚醌对大鼠杏仁核点燃发作的影响及其抗惊厥作用。方法：建立大鼠杏仁核点燃模型,观察发作的电生理指标和行为学变化;在小鼠最大电休克惊厥、戊四唑惊厥和氨基脲惊厥模型计数惊厥发生率。结果： ip吲哚醌50～200 mg.kg-1均可升高杏仁核点燃大鼠的局灶性后放电阈值,降低发作强度和全身性发作 (stage 5) 百分率;可剂量依赖性地对抗小鼠最大电休克发作,并能取消戊四唑惊厥和氨基脲惊厥的强直相,降低戊四唑惊厥的死亡率。结论：吲哚醌对癫痫发作有抑制作用,其机制与抑制MAO-B活性、升高发作阈值有关。     

The anticonvulsant effects of diazepam and phenobarbital in prekindled and kindled seizures in rats

The effects of various doses of diazepam (0.5–4 mg/kg) and phenobarbital (7.5–60 mg/kg) were determined on prekindled and kindled amygdaloid seizures in the same rats. Diazepam was ineffective against the prekindled focal seizures, but demonstrated profound and statistically significant control of the kindled seizures. In the kindled state, diazepam reduced the afterdischarge duration and seizure rank score to prekindled levels. Only the largest sedating dose of phenobarbital produced a reduction of both prekindled afterdischarge duration and seizure rank score. Against the kindled seizure, phenobarbital showed a marked and statistically significant increase in effectiveness in all but the smallest dose tested. The afterdischarge duration of kindled seizures was reduced to prekindled levels by 15–60 mg/kg of phenobarbital, while seizure rank score was reduced to prekindled levels by 30 and 60 mg/kg phenobarbital. The effects of two doses of diazepam (0.5 and 2.5 mg/kg) and phenobarbital (7.5 and 30 mg/kg) were tested against prekindled and kindled pentylenetetrazol (PTZ)-induced seizures. Preliminary work with 3 doses of pentylenetetrazol (30, 40 and 60 mg/kg) demonstrated that repeated doses of 30 mg/kg readily kindled seizures without the significant mortality seen with larger doses. Both diazepam and phenobarbital were less effective against seizures kindled with 30 mg/kg pentylenetetrazol compared to prekindled seizures. The comparative lack of effect that was seen with diazepam and phenobarbital against the pentylenetetrazol kindled seizure at doses associated with control of the kindled amygdaloid seizure may reflect an underlying difference in the pathogenesis of kindling between these seizure models. Further, the lack of suppression of the prekindled amygdaloid afterdischarge duration by large doses of diazepam, in contrast to large doses of phenobarbital, may also reflect differences between the mechanisms of action of these two drugs. This paradigm provides a model for testing the effectiveness of anticonvulsants during the progressive development of various epileptogenic seizures.     

Anticonvulsant and proconvulsant effects of inhibitors of GABA degradation in the amygdala-kindling model

The effects of three drugs, namely gamma-vinyl GABA (vigabatrin), gamma-acetylenic GABA, and aminooxyacetic acid, which increase brain GABA concentrations by irreversible inhibition of GABA degradation, were studied in amygdala-kindled rats. Vigabatrin 800 or 1,200 mg/kg i.p. 4 h after its administration, caused prolongation of behavioural seizures and electrographic afterdischarges recorded from the stimulated amygdala. One to three days after administration it dose dependently reduced seizure severity, seizure duration and afterdischarge duration in most animals. Determination of GABA levels in synaptosomes isolated from 12 brain regions of kindled rats 4 or 48 h after injection of 1,200 mg/kg vigabatrin indicated that the variable effects of this drug at different times after its administration could be related to differences in the time course of nerve terminal GABA increases in selective brain regions such as amygdala and corpus striatum. In contrast to vigabatrin, gamma-acetylenic GABA, 100 mg/kg i.p., reduced seizure severity in kindled rats as early as 4 h after its administration but afterdischarge duration increased significantly on subsequent days. Similar late increases in afterdischarge duration (and limbic seizure activity) after the time of maximum anticonvulsant effect had elapsed were also observed with vigabatrin, which could suggest that the anticonvulsant effect of such drugs is followed by withdrawal hyperexcitability. Aminooxyacetic acid, 20 mg/kg i.p., exerted no significant anticonvulsant effect in kindled rats but prolonged afterdischarge duration in several of the animals studied. The data suggest that GABA-T inhibitors, such as vigabatrin, differ from most antiepileptic drugs previously tested in the kindling model in that they may produce both anticonvulsant and proconvulsant effects at the same dose in the same animal as a function of time after administration.     

Dose-dependent proconvulsant and anticonvulsant actions of the alpha 2 adrenergic agonist, xylazine, on kindled seizures in the rat

The effects of the alpha 2 adrenergic agonist, xylazine, was evaluated on kindling acquisition and on kindled seizure expression in rats. Dose-dependent proconvulsant and anticonvulsant properties were found. The proconvulsant effects were observed at low (0.3 mg/kg) doses. In previously kindled rats these consisted of a decrease in afterdischarge threshold and an increase in the length and severity of the accompanying seizure. This dose also facilitated the rate of kindling in naive subjects. The anticonvulsant effects were observed at higher dose levels (3-20 mg/kg) which also produced sedation and ataxia. If these effects upon kindling are related to the adrenergic actions of xylazine, then it is proposed that the proconvulsant effects are associated with alpha 2 receptor activation and the anticonvulsant effects with alpha 1 receptor activation.     

Comparison of the anticonvulsant effects of two novel GABA uptake inhibitors and diazepam in amygdaloid kindled rats

Summary Two novel, specific inhibitors of GABA uptake, namely SKF 89976-A (N-[4,4-diphenyl-3-butenyl]-nipecotic acid) and SKF 100330-A (N-[4,4-diphenyl-3-butenyl]-guvacine) were tested for anticonvulsant effects in amygdaloid kindled female rats. The anticonvulsant effectiveness of the compounds was compared with that of diazepam. SKF 89976-A and SKF 100330-A produced dosedependent anticonvulsant effects on all seizure parameters measured in fully kindled rats, i.e. they inhibited seizure severity, increased seizure latency, and decreased the duration of motor seizures and EEG afterdischarges. ED 50s for inhibition of seizure severity were 4.6 and 15.1 mg/kg (0.014 and 0.045 mmol/kg) i.p. for SKF 100330-A and SKF 89976-A, respectively. For comparison, the ED 50 of diazepam was 1.9 mg/kg (0.0067 mmol/kg) i.p. Observation of behaviour indicated that the novel GABA uptake blockers exerted no side-effects in anticonvulsant doses, whereas diazepam produced sedative effects at all active dosage levels. The data demonstrate that SKF 100330-A and SKF 89976-A are potent, non-sedative anticonvulsant drugs in the kindling model of epilepsy, and these compounds thus may deserve interest as potential antiepileptic drugs with a very selective mechanism of action.     

Glycine potentiates the anticonvulsant action of diazepam and phenobarbital in kindled amygdaloid seizures of rats

The effect of glycine on the anticonvulsant activity of diazepam and phenobarbital in fully developed kindled amygdaloid seizures in rats was determined. Glycine alone had no significant effect on the seizure response, either when administered orally 1 hr prior to the seizure test or when given chronically in a 0.5 M solution as the source of water.

Administration of glycine (10 mmol/kg, oral) together with diazepam produced a significant reduction in both cortical epileptiform afterdischarge and the severity of seizures at doses of diazepam which had no significant effect on the seizures when administered alone. Glycine potentiated the effects of phenobarbital on the cortical afterdischarge but not the severity of the seizures. The observed potentiation of the effects of diazepam and phenobarbital suggests a glycinergic mechanism in the anticonvulsant action of these drugs which may be mediated in part by the inhibitory γ-aminobutyric acid (GABA) systems.     

The effect of lamotrigine upon development of cortical kindled seizures in the rat.

The effect of lamotrigine, a novel potential antiepileptic drug, upon the development of kindled cortical seizures was investigated in rats. Although lamotrigine, at all doses tested, failed to block or reduce the rate of development of kindling, it did have a profound effect upon the production of both non-kindled and kindled responses. All doses (3, 6, 12 and 18 mg/kg) produced a significant increase in the number of nil responses (where stimulation failed to evoke a behavioural clonus or afterdischarge) and a decrease in non-kindled responses. Doses of 12 and 18 mg/kg also significantly reduced the number of kindled responses and the duration of the kindled seizure. It is suggested that these effects of lamotrigine result from its ability to inhibit the release of glutamate, an excitatory amino acid which has been implicated in the production of kindled seizures. In contrast to previous studies on the development of kindling, it was found that in the groups which received either 12 or 18 mg/kg lamotrigine, it was possible to produce kindling without evoking any nonkindled afterdischarge. This finding is discussed in the light of the current theories surrounding the kindling process. This study suggests that lamotrigine, as well as possibly being of value in the treatment of complex partial and generalised (tonic-clonic) seizures, may also be of value in the treatment of elementary (simple) partial seizures.     

The new antiepileptic drugs lamotrigine and felbamate are effective in phenytoin-resistant kindled rats

We evaluated the anticonvulsant efficacy of the antiepileptic drugs (AEDs) lamotrigine (LTG) and felbamate (FBM) in amygdala kindled rats that had been preselected with respect to their response to phenytoin. Anticonvulsant response was tested by determining the afterdischarge threshold (ADT), i.e., a sensitive measure for drug effects on focal seizure activity. By repeated testing with the phenytoin prodrug fosphenytoin, 3 groups of kindled rats were separated: rats in which consistent anticonvulsant effects were obtained (phenytoin responders), rats which showed no anticonvulsant response (phenytoin nonresponders), and rats with variable responses (variable phenytoin responders). The latter, largest group was used to evaluate at which doses LTG and FBM exerted significant anticonvulsant effects on ADT 1 h after i.p. drug administration. Effective doses were then used for drug testing in phenytoin responders and nonresponders. Both LTG and FBM proved to be effective anticonvulsant drugs in the kindling model by markedly increasing the ADT. Seizure severity and duration recorded at ADT currents were hardly reduced, indicating that both drugs predominantly affect induction of focal seizures and not seizure spread from the focus. In phenytoin nonresponders, LTG and FBM significantly increased ADT, which is in line with their proven efficacy in patients with refractory partial epilepsy in whom phenytoin has failed. However, LTG and, more markedly, FBM were clearly more efficacious in increasing ADT in phenytoin responders than in nonresponders, substantiating that the difference in phenytoin response between these groups of kindled rats extends to other AEDs. The data in this study reveal that phenytoin nonresponders are a unique model for the search for new AEDs with improved efficacy in refractory partial epilepsy.     

Effects of etomidate,ketamine or propofol,and their combinations with conventional antiepileptic drugs on amygdala-kindled convulsions in rats

Ketamine, etomidate and propofol modified behavioral and electrographic correlates of kindled seizures in rats. In detail, ketamine (5 mg/kg) and propofol (15 mg/kg) significantly increased afterdischarge threshold, reduced seizure severity and shortened seizure and afterdischarge durations. Etomidate (7.5 mg/kg) was effective in terms of seizure and afterdischarge durations. Moreover, the combinations of ketamine (2.5 mg/kg) with carbamazepine (15 mg/kg) or valproate (50 mg/kg; all drugs at their subeffective doses), reduced the severity and duration of kindled seizures. The antiseizure potency of the ketamine/carbamazepine combination was comparable to that of carbamazepine alone administered at 20 mg/kg, while the effect of ketamine/valproate was comparable to the efficacy of valproate alone at 100 mg/kg. However, the combinations of ketamine with phenobarbital or diphenylhydantoin did not exert any protective action. Propofol and etomidate entirely failed to interact with conventional antiepileptics. The combinations of ketamine with carbamazepine or valproate did not induce any significant motor impairment in the chimney test or memory deficit in the passive avoidance task. A pharmacokinetic interaction, at least in plasma, can be excluded, because ketamine (2.5 mg/kg) did not affect the free plasma concentrations of carbamazepine or valproate. Results of the present study may suggest that there may be no risk of negative interactions between injectable anesthetics and antiepileptics in cases of partial epilepsy.     

Influence of LY 300164, an antagonist of AMPA/kainate receptors, on the anticonvulsant activity of clonazepam

LY 300164 [7-acetyl-5-(4-aminophenyl)-8,9-dihydro-8-methyl-7H-1,3-dioxolo(4,5H)-2,3-benzodiazepine], an antagonist of AMPA/kainate receptors, at 5 mg/kg exerted a significant anticonvulsant effect, as regards seizure and afterdischarge durations in amygdala-kindled seizures in rats. At lower doses, LY 300164 did not exert anticonvulsant activity. Clonazepam alone (0.003–0.1 mg/kg) significantly diminished seizure severity, seizure and afterdischarge durations. Coadministration of LY 300164 (2 mg/kg) with clonazepam (0.001 mg/kg) resulted in the significant anticonvulsant activity. Seizure severity score, seizure and afterdischarge durations were reduced from 5 to 4, from 32.6 s to 12.3 s, and 42.7 s to 23.2 s. LY 300164 (2 mg/kg), clonazepam (0.001–0.1 mg/kg) and the combination of clonazepam (0.001 mg/kg) with LY 300164 (2 mg/kg) did not affect long-term memory evaluated in the passive avoidance task in rats. LY 300164 (at the subprotective dose of 2 mg/kg) significantly potentiated the anticonvulsant action of clonazepam against maximal electroshock but not against pentylenetetrazol-induced convulsions in mice. The results indicate that blockade of glutamate-mediated events at AMPA/kainate receptors may differently affect the protection offered by clonazepam, which seems dependent upon the model of experimental seizures.     

Effect of aminophylline on amygdaloid-kindled postictal inhibition

Previous studies have shown that methylxanthines such as aminophylline increase the clinical severity and length of electrically elicited limbic afterdischarges in naive and kindled rats without lowering seizure threshold. When fully amygdaloid-kindled rats are electrically stimulated at intertrial stimulation intervals of less than 60 minutes, significant residual inhibition can be demonstrated. The present study examines the effect of three doses of aminophylline (25, 50 and 100 mg/kg) on repeated daily stimulations of fully amygdaloid-kindled rats. After 100 mg/kg aminophylline, the first elicited amygdaloid-kindled seizure afterdischarge was doubled in length compared to saline controls. The second elicited seizure 15 minutes later resulted in status epilepticus and hindlimb extension in the majority of the aminophylline-treated animals with death occurring in 28%. When 25 or 50 mg/kg of aminophylline was given daily for five days before the first of five daily stimulation trials, each separated by 15 minutes, no significant reduction in postictal inhibition was demonstrated compared to saline controls. The 50 mg/kg aminophylline dose consistently and significantly lengthened only the first afterdischarge of each day without affecting the postictal inhibition seen with repeated stimulations. The neural substrate that governs immediate postictal inhibition of amygdaloid-kindled seizures appears to be resistant to modification by aminophylline at low doses. At high doses of aminophylline (100 mg/kg), sustained epileptical activity occurred. The sustained seizure activity seen at the high dose of aminophylline may be secondary to blockade of the processes which normally terminate seizure activity, or it may represent actual inhibition of the immediate postictal inhibitory processes.     

Ketamine-induced changes in kindled amygdaloid seizures

The effects of ketamine on seizures kindled by repetitive electrical stimulation of the amygdala were determined in the rat. The response of fully developed kindled amygdaloid seizures (KAS) to 20, 40, 80 and 120 mg/kg (i.p.) ketamine, administered from 5 to 60 min prior to elicitation of seizures was examined. Ketamine reduced the afterdischarge duration (AD) and behavioral response (BR) in a dose-dependent fashion. However, the effect of ketamine on the afterdischarge duration and behavioral response was not clearly time-dependent for each dose (20–120 mg/kg). A dose-dependent increase in the seizure spiking frequencies in the amygdala and cortex during kindled amygdaloid seizures was also induced by ketamine. Blood plasma and brain levels of ketamine and its metabolites were determined 15 min after 20, 40, 80 and 120 mg/kg ketamine as well as 60 min after 80 mg/kg ketamine. Brain and plasma levels of ketamine and nor-ketamine were similar to those previously reported. Low plasma levels of dehydro-nor-ketamine were seen only at 60 min after 80 mg/kg ketamine. The decrease in afterdischarge duration and behavioral response and the increase in afterdischarge duration spiking frequency seen at 15 min correlated with elevated levels of ketamine and nor-ketamine in brain and plasma. However, by 60 min plasma levels of ketamine remained high, yet the brain levels of both ketamine and nor-ketamine had decreased. This is despite the fact that afterdishcarge duration and behavioral response were still attenuated and afterdischarge duration spiking frequency was still increased. Thus, the exact contribution by ketamine and nor-ketamine to the alteration of afterdischarge duration, behavioral response and afterdischarge spiking frequency cannot be made at this time. It was apparent that inhibition of the afterdischarge duration and behavioral response along with an increase in spiking frequency was not dependent on dehydro-nor-ketamine. The possibility that an unidentified metabolite may contribute to the modification of kindled amygdaloid seizures by ketamine is discussed.     

The effects of various anesthetics on amygdaloid kindled seizures

The effects of various doses of cataleptic anesthetics, gamma-butyrolactone (GBL), phencyclidine (PCP), and ketamine (KET), and the depressant anesthetics, pentobarbital (PB) and chloral hydrate (CH), on amygdaloid kindled seizures were tested in the rat. The seizure activity was monitored by behavioral observation and EEG recording. Anesthetic doses of the cataleptic anesthetics with the exception of KET had minimal effects on the afterdischarge duration (AD) and behavioral ranking (BR) of the elicited seizures. On the other hand, they were more inhibitory to the AD and BR than was the convulsant pentylenetetrazol (PTZ). The only cataleptic that induced spontaneous seizure activity at anesthetic doses was PCP, although KET induced epileptoid activity at supranesthetic doses. Ketamine, PB, and CH completely inhibited elicited seizure activity at anesthetic doses. In addition, rats were kindled by repetitive electrical stimulation during GBL-induced anesthesia or catalepsy. Although both these GBL groups averaged more stimulations to reach generalized seizures than the saline controls, GBL did not block the kindling process.     

Effects of the benzodiazepine antagonists RO 15-1788, CGS-8216 and PK-11195 on amygdaloid kindled seizures and the anticonvulsant efficacy of diazepam

The anticonvulsant effectiveness of the benzodiazepine antagonists RO 15-1788, CGS-8216 and PK-11195 were evaluated against threshold and suprathreshold (400 microA) stimulation in fully amygdaloid-kindled rats. Pretreatment with either RO 15-1788 (3, 10 and 30 mg/kg), CGS-8216 (3, 10 and 30 mg/kg) or PK-11195 (10 and 60 mg/kg) failed in this study to modify consistently either the afterdischarge thresholds or elicited suprathreshold seizures or duration of afterdischarge. Using a double injection paradigm, the effectiveness of these three benzodiazepine antagonists to reverse the anti-convulsant and behavioral effects of diazepam were studied. When diazepam (3 mg/kg) was injected 15 min before or after a second injection of the vehicle control DMSO (0.25 ml/kg), a significant reduction in the duration of afterdischarge and seizure rank, elicited by a suprathreshold stimulation in amygdaloid-kindled rats, occurred. When either CGS 8216 (10 mg/kg) or RO 15-1788 (10 mg/kg) were given 15 min before diazepam (3 mg/kg) prior to stimulation, the anticonvulsant properties of diazepam were blocked. When RO 15-1788 (10 mg/kg) was given 15 min after diazepam, antagonism of the anticonvulsant effects on diazepam was shown. However, when either CGS-8216 (10 mg/kg) or PK-11195 (10 and 60 mg/kg) were given 15 min after diazepam (3 mg/kg), the anticonvulsant properties of diazepam were not blocked. The anticonvulsant effects of diazepam were reversed when CGS-8216 (10 mg/kg) was given 5 min after diazepam (3 mg/kg) or when a larger dose (30 mg/kg) was given at the same 15 min interval.(ABSTRACT TRUNCATED AT 250 WORDS)     

Development of tolerance to clobazam in fully kindled rats: Effects of intermittent flumazenil administration

The effects of acute and chronic treatment with the 1,5-benzodiazepine, clobazam, were studied on fully kindled amygdaloid seizures in rats. After acute dosing, clobazam significantly reduced all parameters of kindled seizures (seizures severity, seizure duration, duration of amygdalar afterdischarges) at doses of 7.5 or 10 mg/kg i.p. ‘Active’ plasma concentrations of clobazam ranged between 300–800 ng/ml. The elimination half-life of clobazam in plasma was about 1 h. Only very low (10–75 ng/ml) levels of the major metabolite, N-desmethylclobazam, were detected in rats. Administration of N-desmethylclobazam indicated that plasma concentrations of at least 300 ng/ml were necessary for anticonvulsant effects. During chronic administration of clobazam, 10 mg/kg 3 times daily, marked tolerance developed to the anticonvulsant and adverse (ataxiogenic and sedative) effects of the benzodiazepine. The experiment was repeated using a different protocol with minimized environmental stimuli and no amygdala stimulation during chronic clobazam administration. The loss of effects on seizure severity and motor function was similar to the first chronic experiment, whereas the loss of effects on seizure and afterdischarge duration was less marked. This indicates that conditioning of ‘learned tolerance’ is partly involved in clobazam tolerance in kindled rats. Intermittent injection of the benzodiazepine receptor antagonist, flumazenil, 5 mg/kg i.p. every third day, did not alter the loss of pharmacodynamic effects during chronic treatment with clobazam, but seemed to prevent hyperexcitation and other abstinence symptoms in the withdrawal period. The data indicate that periodic injection of a benzodiazepine receptor antagonist does not represent a possible therapeutic approach for preventing the development of tolerance during long-term benzodiazepine exposure.     

Phenytoin’s effect on the spread of seizure activity in the amygdala kindling model

Phenytoin is a major antiepileptic drug for treatment of limbic seizures. The effect of phenytoin on the generation and spread of seizure activity was studied in a rat model of this type of seizures. Sprague-Dawley and Wistar rats were implanted with a stimulation and recording electrode in the basolateral amygdala. Naive Sprague-Dawley rats showed an increase in current intensity necessary for eliciting afterdischarges (afterdischarge threshold) of about 200% after administration of phenytoin (75 mg/kg i.p.), while seizure severity at threshold was increased compared to controls. Afterdischarge and seizure durations were significantly prolonged under phenytoin. This result suggests that phenytoin can exert a potent anticonvulsant effect on the generation of focal seizure activity, but it does not suppress or may even increase on-going afterdischarge activity once it occurs. Following amygdala kindling in Wistar rats, administration of phenytoin again resulted in an increase in the afterdischarge threshold. However, all rats still showed generalized seizures, and epileptic afterdischarges could be recorded in various limbic brain regions at threshold current. This result suggests that phenytoin can increase the threshold for generation of epileptic discharges in kindled rats, but is not able to prevent the development of generalized seizure activity and the spread of afterdischarges within the limbic system when focal activity is initiated. We conclude that phenytoin is able to suppress focal seizure activity in the amygdala kindling model of the rat. However, it does not prevent the spread of seizure activity originating in the limbic system. Therefore, a decrease in focal seizure susceptibility seems to be the primary target for phenytoin’s anticonvulsant action. Received: 7 March 1997 / Accepted: 17 June 1997     

2-Methyl-6-phenylethynyl-pyridine (MPEP), a non-competitive mGluR5 antagonist,differentially affects the anticonvulsant activity of four conventional antiepileptic drugs against amygdala-kindled seizures in rats

2-Methyl-6-phenylethynyl-pyridine (MPEP), a selective noncompetitive mGluR5 antagonist, influences the action of conventional antiepileptic drugs in amygdala-kindled seizures in rats. MPEP alone (up to 40 mg/kg) did not affect any seizure parameter. Moreover, the common treatment of MPEP with either carbamazepine or phenytoin (administered at subeffective doses) did not result in any anticonvulsant action in kindled rats. However, when combined with subprotective doses of valproate or phenobarbital, MPEP significantly shortened seizure and afterdischarge durations. Importantly, combinations of MPEP with the two antiepileptics did not have the adverse effects of impaired motor performance or long-term memory in rats. Our data indicate that MPEPmay positively interact with some conventional antiepileptic drugs in the amygdala-kindling model of complex partial seizures.