Generalized Seizure Triggering Threshold Research Articles

Suppression of fully kindled seizure and retardation of kindling acquisition by YM928 in the rat kindling model of epilepsy

We investigated the effects of 2-[ N-(4-chlorophenyl)- N-methylamino]-4 H-pyrido[3.2-e]-1,3-thiazin-4-one (YM928), a selective α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist, in the rat kindling model of complex partial seizures. YM928 (10 and 30 mg/kg p.o.) markedly suppressed the motor seizures and afterdischarge induced by electrical stimulation of the amygdala at generalized seizure-triggering threshold intensity. YM928 (10 mg/kg p.o.) did not induce apparent abnormal behavior, but did induce sedation at a dose of 30 mg/kg p.o. YM928 (30 mg/kg p.o.) showed a similar anticonvulsant effect at twice the threshold intensity as it did at threshold intensity. Diazepam (10 mg/kg p.o.) and phenobarbital (60 mg/kg p.o.) also exerted anticonvulsant activities. Diazepam (10 mg/kg) showed a similar effect at twice the threshold as at threshold, but the anticonvulsant effect of phenobarbital (60 mg/kg p.o.) was reversed when the stimulus was doubled. When YM928 (10 mg/kg p.o.) was administered 60 min before daily stimulation of the amygdala, the development of kindling seizure was significantly retarded. These results indicate that YM928 has anticonvulsant effects and suppresses kindling acquisition without sedative effects, and may be suitable as an antiepileptic drug for the treatment of complex partial seizures in humans.

Effects of YM90K, a selective AMPA receptor antagonist, on amygdala-kindling and long-term hippocampal potentiation in the rat

To investigate the role of α-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) type glutamate receptors in epileptic seizures, we examined the antiepileptogenic and anticonvulsant effects of YM90K [6-(1 H-imidazol-1-yl)-7-nitro-2,3-(1 H,4 H)-quinoxalinedione hydrochloride], a potent and selective new AMPA receptor antagonist, in the rat amygdala-kindling model of epilepsy. Pretreatment with YM90K (7.5–30 mg/kg i.p.) markedly retarded the evolution of kindling. Once kindling was established, administration of YM90K (7.5–30 mg/kg i.p.) significantly and dose-dependently suppressed fully kindled seizures. The maximal effects were observed 15–30 min after injection. When the intensity of electrical stimulation was increased to twice the generalized seizure-triggering threshold, the anticonvulsant effects of YM90K were reversed, suggesting that they were due to elevation of the generalized seizure-triggering threshold. Furthermore, an anticonvulsant dose (15 mg/kg) of YM90K affected neither field potentials nor long-term potentiation in the hippocampus in vivo. These results indicate that AMPA receptors play an important role in the seizure expression mechanism and the development of kindling-induced epileptogenesis, and suggest the possible clinical usefulness of AMPA receptor antagonists as antiepileptic drugs.

Antiepileptic effects of topiramate on amygdaloid kindling in rats.

We examined the antiepileptic properties of topiramate (TPM) in amygdaloid (AM) kindling in rats. Electrodes were implanted into the left AM of adult male Wistar rats. The animals were kindled at the after-discharge (AD) threshold. After the completion of kindling, the generalized seizure triggering threshold was determined. The drugs were administered intraperitoneally in animals which showed stable generalized convulsions at near-threshold stimulation. Intraperitoneal administration of TPM at doses of 25 mg/kg or more produced an anticonvulsive effect, but did not readily suppress limbic seizures. Complete suppression of AD was observed in only 3/8 rats at the highest dose of 200 mg/kg, which was not statistically significant. On the other hand, TPM at 100 and 200 mg/kg significantly delayed AM kindling. Thus, TPM showed modest therapeutic properties of conventional antiepileptic drugs in kindling model, those of TPM more closely resemble those of phenobarbital and the benzodiazepines than those of phenytoin and carbamazepine.

Effects of lamotrigine and conventional antiepileptic drugs on amygdala- and hippocampal-kindled seizures in rats.

We investigated the anticonvulsant and adverse behavioral effects of lamotrigine (LTG), a novel antiepileptic drug (AED), as well as other conventional AEDs on kindled seizures in rats. We also applied an anticonvulsive dose of LTG in vivo to rats in which the hippocampus had been subjected to long-term potentiation (LTP). LTG potently attenuated limbic-kindled seizures in a dose-dependent fashion, at doses at which animals showed no adverse behavioral effects. LTG was effective in preventing kindled seizures for up to 24 h after a single i.p. administration. The anticonvulsant effects of LTG were reversed when the stimulus current was raised to two or three times the generalized seizure-triggering threshold. Among the AEDs examined, valproate and LTG were the only drugs that engendered a potent anticonvulsant effect without concomitant adverse behavioral effects. Although all of the other AEDs exhibited anticonvulsant effects with various potencies, they produced adverse effects such as sedation or motor ataxia. Furthermore, an anticonvulsant dose of LTG did not affect either the induction or maintenance of tetanus-induced LTP in the hippocampus. These results indicate that LTG potently suppresses limbic-kindled seizures by raising the seizure triggering-threshold in the kindling focus at doses that do not affect LTP in the hippocampus.

Antiepileptogenic and Anticonvulsant Effects of YM90K, a Selective AMPA‐Receptor Antagonist, in the Rat Kindling Model of Epilepsy

Purpose: We previously demonstrated that NBQX, a competitive AMPA‐receptor antagonist, significantly retards the development of kindling and suppresses fully kindled seizures. In contrast, however, it has also been reported that NBQX does not retard the development of kindling. In this study, to investigate the role of AMPA receptors in epileptic seizures, we examined the antiepileptogenic and anticonvulsant effects of YM90K [6‐(1H‐imidazol‐1‐yl)‐7‐nitro‐2,3‐(1H,4H)‐quinoxalinedione hydrochloride], a selective and potent AMPA‐receptor antagonist, in the rat amygdala kindling model of epilepsy. Methods: Experiment 1: Investigation of the antiepileptogenic action of YM90K on the development of kindling. Twenty‐five Sprague‐Dawley rats, each bearing a tripolar electrode implanted into the left amygdala, were used. From days 2 to 9, rats received i.p. injections of either YM90K (7.5, 15, or 30 mg/kg) or saline 30 min before each electrical stimulation. From days 10 to 30, daily kindling stimulations were continued without drug administration. Experiment 2: Investigation of the anticonvulsant action of YM90K on fully kindled amygdaloid seizures. Eight rats previously kindled from the left amygdala were used. To determine whether the effect of YM90K was dose dependent, the rats were subjected to stimulation at the generalized seizure‐triggering threshold (GST) intensity, 30 min after i.p. administration of YM90K (7.5, 15, or 30 mg/kg) or saline. The effects of YM90K (15 mg/kg, i.p.) at various times (15 min‐4 h) between the injection and the stimulation also were examined. Furthermore, to determine the effect on GST, rats received the stimulation at twice the GST intensity, 30 min after i.p. administration of YM90K (15 mg/kg). Results: Experiment 1: Pretreatment with YM90K markedly retarded the evolution of kindling during drug sessions in a dose‐dependent manner, as assessed in terms of both seizure stage and after discharge (AD) duration. In the YM90K‐treated group, the number of stimulations required to induce the first stage 2 or generalized stage 4 or 5 kindled seizure was greater than that in the saline‐treated group.Experiment 2: The administration of YM90K significantly and dose‐dependently suppressed fully kindled seizures. None of the rats treated with 15 or 30 mg/kg of YM90K showed generalized seizures. The maximal effects were observed at 15–30 min after the injection, and the effects disappeared 1 h after the injection. At the GST intensity, only one rat showed generalized seizures, and five rats did not show ADS after i.p. administration of YM90K (15 mg/kg). On the other hand, the effects of YM9OK were reversed at twice the GST intensity. Six rats showed generalized seizures, and all rats showed ADS. Conclusions: The role of AMPA receptors in the development of kindling is not yet fully understood. However, the effects of YM90K suggest that AMPA receptors participate in the development of kindling, as we previously reported with NBQX. It was also shown that YM90K has an anticonvulsant effect as potent as that of NBQX, on previously kindled seizures from the amygdala in rats. It appears that the mechanism underlying the anticonvulsant effect differs between the two AMPA receptor antagonists (i.e., YM90K increases the GST, whereas NBQX does not). These results indicate that AMPA receptors play an important role in the seizure‐expression mechanism and the development of kindling‐induced epileptogenesis.

Effects of Antiepileptie Drugs on Seizure Thresholds in the Rat Kindling Model of Temporal Lobe Epilepsy

Purpose: To determine the antiepileptic profiles of antiepileptic drugs (AEDs), we compared the effects of various AEDs, including phenytoin (PHT), carbamazepine (CBZ), valproate (VPA), and diazepam (DZP), on seizure thresholds in the rat kindling model of temporal lobe epilepsy. Methods: Male Sprague‐Dawley rats were used. A tripolar electrode was implanted into the amygdala under pentobarbital anesthesia. All rats underwent daily kindling stimulation (100 Hz, 2 s) until stable generalized seizures were induced. The generalized seizure‐triggering threshold (GST) was then determined for each rat by the application of stimulation, which increased in intensity by 50 μA steps at intervals of 15 min. After intraperitoneal administration of PHT (180 mg/kg), CBZ (40 mg/kg), VPA (200 mg/kg), or DZP (2 mg/kg), electrical stimulation at the intensity of the GST, or 2 or 3 times the GST, was delivered to the amygdala, and reversal of the anticonvulsant effects was measured. Results: All AEDs had potent anticonvulsant effects on amygdala‐kindled seizures induced by stimulation at the GST. After treatment with PHT and CBZ, when the stimulus intensity was increased to 2 or 3 times the GST, the anticonvulsant effects were completely eliminated. In contrast, the effects of VPA were partially reversed and those of DZP were only slightly reversed by increases in the stimulus intensity. Conclusions: These findings suggest that the prinicipal mechanism for the anticonvulsant effects of PHT and CBZ, which act primarily on voltage‐sensitive Na+ channels, is the significant increase of seizure thresholds in epileptogenic foci. In our previous study, lamotrigine had the same effects as PHT and CBZ on seizure thresholds. In contrast, the fundamental profile of VPA and DZP, which enhance a‐aminobutyric acid (GABA)/benzodiazepine (BZD) systems, differs from that of PHT/CBZ‐type AEDs, and the main mechanism for the anticonvulsant effects of these two drugs is blockade of seizure propagation and generalization. This conclusion is consistent with our recent findings for tiagabine (a selective GABA uptake inhibitor) in the kindling model (Morimoto et al. Epilepsia 1997;38:966).

Effects of single and repeated administration of sulthiame on amygdaloid kindled seizures in rats.

In this study, we assessed the anti-convulsive effects of sulthiame (SUL) in amygdaloid (AM) kindled rats. Electrodes were implanted into the left AM of adult male Wistar rats. The animals were kindled at the after-discharge (AD) threshold. Upon completion of kindling, a generalized seizure triggering threshold was determined. The drugs were administered intraperitoneally in rats which reproducibly exhibited generalized convulsions at the near-threshold stimulation. Single administration of SUL (25-200 mg; n = 7-9) reduced the forelimb clonus (FCL) duration, but only the highest dose significantly regressed the secondarily generalized convulsion. During repeated administration of SUL, 50 mg/kg for 8 days, FCL duration was significantly alleviated until the fifth treatment day. With the dose of 200 mg/kg, significant suppression of secondary generalization was noted only until the second test day. On the other hand, significant reductions of FCL and AD duration were preserved afterwards. The anti-convulsive effects of SUL indicated in this study were not comparable to those of other standard anti-epileptic drugs reported from our laboratory.

The effects of N-methyl- d-aspartate (NMDA) and its competitive antagonist, 3-(2-carboxypiperazine-4-yl)propyl-l-phosphonic acid (CPP), injected into caudate-putamen on kindled amygdaloid seizures in rats

N-methyl- d-aspartate (NMDA) is an agonist of NMDA receptors and 3-(2-carboxypiperazine-4-yl)propyl-l-phosphonic acid (CPP) is an NMDA receptor antagonist. NMDA (1 or 2 nmol per side) or CPP (2.5 or 10 nmol per side) was injected into the bilateral caudate-putamen of amygdaloid-kindled rats. In addition, CPP (10 nmol) was ipsilaterally or contralaterally injected into the unilateral caudate-putamen. Either 20 min after NMDA or 60 min after CPP, the kindled amygdala was stimulated at the generalized seizure triggering threshold. In a few animals tested, injection of NMDA into the bilateral caudate-putamen produced transient spiking activity, with no clinical manifestations. This feature began about 5 min after the injection and lasted about 10 s. When these animals were excluded from the statistical analysis, NMDA in the caudate-putamen showed a weak and non-significant anticonvulsant action. Injection of CPP into the bilateral caudate-putamen caused no ictal change, but markedly suppressed the kindled seizures. Injection of CPP into the unilateral caudate-putamen, regardless of the site, did not cause any ictal change, or affect the stimulation of the amygdala. These findings suggest that: (1) NMDA receptors in the caudate-putamen facilitate the development of kindled amygdaloid seizures; (2) activation of NMDA receptors in the bilateral, but not in the unilateral, caudate-putamen is required for the generalization and expression of kindled amygdaloid seizures.

Comparison of effects of bilateral injections of bicuculline and muscimol into the caudate-putamen of amygdaloid-kindled rats

Bicuculline is an antagonist of γ-aminobutyric acid (GABA) receptors, and muscimol is an agonist of GABA receptors. In this study, the effects of bilateral injections of bicuculline and muscimol into the caudate-putamen (CP) were compared in amygdaloid-kindled rats. Thirty minutes after the injection of bicuculline (1, 10 and 100 pmol per CP) or muscimol (10, 50 and 100 nmol per CP), the kindled amygdala was stimulated at the previously established generalized seizure triggering threshold (GST). Most doses of bicuculline caused no significant alteration either in the seizure stage or in the afterdischarge duration. Only the 100-pmol dose produced a marked reduction in the afterdischarge duration. With 10 nmol of muscimol, there was no significant change in the kindled seizure stage or in the afterdischarge duration. However, 50 and 100 nmol of muscimol markedly suppressed both parameters. These findings suggest that CP efferent pathways are involved in the mechanism that underlies the development of kindled amygdaloid seizures, and support the concept that GABA acts as an anticonvulsant in the brain.

Antiepileptogenic and anticonvulsant effects of NBQX, a selective AMPA receptor antagonist, in the rat kindling model of epilepsy

To investigate the role of non-NMDA receptors in epileptic seizures, we examined the antiepileptogenic and anticonvulsant effects of NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)-quinoxaline), a potent and selective AMPA receptor antagonist, in the rat kindling model. Systemic administration of 10–40 mg/kg NBQX significantly and dose dependently suppressed previously kindled seizures from the amygdala (AM), assessed in terms of the motor seizure stage and afterdischarge (AD) duration. The maximal effects were observed at 0.5–1 h after drug injection. When the intensity of electrical stimulation was increased to twice the generalized seizure-triggering threshold (GST), the anticonvulsant effects of NBQX on AM-kindled seizures were not reversed, suggesting that the effects were not due to non-specific elevation of the GST. In contrast to AM-kindled seizures, 20–40 mg/kg NBQX significantly suppressed only the motor seizure stage without reducing the AD duration of previously hippocampal-kindled seizures. Daily administration of 15 or 30 mg/kg NBQX prior to each electrical stimulation of the AM markedly and significantly suppressed the development of kindling. During drug sessions, the growth of the AD duration was blocked almost completely, while the waveform of ADs became more complex. These results indicate that NBQX has potent antiepileptogenic and anticonvulsant actions on kindling, at least more from the AM and that non-NMDA receptors have an important role in seizure propagation.

Experimental study of the mechanism of seizure induction: changes in the concentrations of excitatory amino acids in the epileptic focus of the cat amygdaloid kindling model.

L-glutamate (Glu) and L-aspartate (Asp) are two major excitatory amino acids that may be involved in seizure susceptibility and seizure induction. The concentrations of Glu and Asp were measured by microdialysis in the epileptic focus in a cat amygdaloid kindling model. Sequential changes in Glu and Asp (before, during, and after seizure) were measured in the partial seizure (S4) and generalized seizure (S6) groups. By stimulation at and 50 microA below the partial seizure-triggering threshold in the S4 group and the generalized seizure-triggering threshold in the S6 group, Glu was released from the epileptic focus in the S4 group, and both Glu and Asp were released in the S6 group after seizure and stimulation (below threshold), and the amount of Glu and/or Asp release determined seizure induction. Excitatory amino acids may be the trigger of seizure induction in the cat amygdaloid kindling model.

Ipsilateral hemiconvulsive seizure after unilateral injection of procaine into the substantia innominata of amygdaloid-kindled rats

The effects of procaine injected into the substantia innominata (SI) and the amygdala (AM) were compared in AM-kindled rats. Intra-SI injection of procaine (ipsilateral to the kindled AM) caused immediate contralateral hemiparesis lasting for about 30 min, associated with EEG amplitude reduction. When the kindled AM was stimulated at the previously established generalized seizure triggering threshold (GST), 8/15 animals had ipsilateral hemiconvulsive seizure 5 min after the injection. In all the animals, afterdischarge was readily produced with a significant reduction in its duration. When procaine was injected into the kindled AM, there was no paresis or EEG change. AM stimulation at the GST did not evoke afterdischarge 5 min after the injection, although the kindled seizure was reactivated by a more intense stimulus. After 24 hr, all the animals, regardless of the site of procaine injection, responded to GST stimulation with a kindled seizure. The data suggest that (1) the effect of intra-SI procaine is not due to drug diffusion to the kindled AM, and (2) the kindled AM can gain access to the contralateral hemispheric motor mechanism without participation of the ipsilateral SI.

γ-Glutamyltaurine has potent and long-lasting antiepileptic action as demonstrated by intra-amygdaloid injection in amygdala-kindled rats

Intra-amygdaloid injections of γ-glutamyltaurine, a recently identified brain dipeptide, strongly suppressed seizures in amygdala-kindled rats stimulated with intensities 10 ro 50 μA above the generalized seizure triggering threshold (mean 82 μA). The suppressive effect persisted as long as 3 days. Taurine had relatively weak suppressive effects. Thus γ-glutamyltaurine seems much more potent than taurine in the suppression of epileptic seizures when injected directly into the kindling site.

Kindling of the massa intermedia of the thalamus in rats

The kindling response of the massa intermedia (MI) was assessed in rats. Clinical manifestation of the MI kindling was generally similar to that of limbic kindling, and positive transfer to the amygdala (AM) was obtained following MI kindling. However, MI kindling showed (1) a relatively high afterdischarge threshold which sometimes increased during the course of kindling. (2) a seizure stage instability with frequent regression to earlier stages, (3) a failure to establish a generalized seizure triggering threshold with an ‘all-or-none’ property, and (4) a generalized tonic-clonic seizure, which was quite different from a kindledlimbic seizure, during early phase of kindling. Furthermore, the MI stimulation caused violent beating movement of the forelimbs, jumping, or running regardless of presence/absence of afterdischarge. The findings suggest that mechanisms other than a simple activation of limbic structures are involved in the process of MI kindling.

Anticonvulsant effect of liposome-entrapped superoxide dismutase in amygdaloid-kindled rats

Amygdaloid-kindled rats received intravenous human copper-zinc superoxide dismutase (CuZn-SOD) either in free form or entrapped within liposomes (SOD-L), at 5, 10 or 20 mg/kg. The animals were stimulated at the generalized seizure-triggering threshold 5 min, 2 h and then every 24 h after the drug was given, until 5 consecutive stage 5 seizures were induced. Free CuZn-SOD had little or no effect. However, SOD-L, particularly at 10 mg/kg, had a prolonged anticonvulsant effect, although there was great individual variation in the onset and duration of seizure suppression. This effect of SOD-L may be due to the ability of liposomes to act as a depot for the sustained release of drugs.

Anticonvulsant Effect of Systemically Administered γ-Aminobutyric Acid (GABA)-Containing Liposomes in Rats Induced to Seizure by Amygdaloid Electrical Stimulation

AbstractThe effects of γ-aminobutyric acid (GABA)-containing liposomes were examined in amygdaloid kindled rats. One hr after intraperitoneal administration of GABA, liposomes (LIPO), or GABA-containing LIPO (GABA-L), the animals were stimulated at the generalized seizure triggering threshold. GABA-L had a dose-dependent anticonvulsant effect, whereas no effect was seen with either GABA or LIPO alone. The effect of GABA-L may be due to the ability of LIPO to act as a sustained release reservoir.

An important role of non-NMDA receptors in the expression of kindled amygdaloid seizure in rats.

gamma-D-glutamylaminomethylsulphonic acid (GAMS), a preferential antagonist of non-NMDA receptors (kainate and quisqualate receptors), was injected into the kindled amygdala (AM) of rats. When the kindled AM was stimulated at the previously established generalized seizure triggering threshold (GST) one hour after the GAMS (1 or 2 mumol) injection, afterdischarge (AD) generation was completely suppressed. However, a re-stimulation at the intensity of 40-200 microA above the GST generated AD associated with Stage 1 or 5 seizure. Our result suggests an important role played by non-NMDA receptors in the expression and generalization of AM-onset seizures.

Comparison between substantia innominata and amygdala kindling in rats

Kindling was induced in rats by electrical stimulation of the lateral portion of the substantia innominata (SI). The pattern of seizure development was similar to that of amygdala (AM) kindling. However, lateral SI kindling was associated with ipsilateral head turning as an initial manifestation. In addition, lateral SI kindling had a higher afterdischarge threshold than AM kindling, and the generalized seizure triggering threshold was more unstable in SI kindling than in AM kindling. These findings suggest that lateral SI participates in, but is not essential for, AM seizure development in rats.

Suppression of amygdaloid kindled convulsion following unilateral injection of 2-amino-7-phosphonoheptanoic acid (2-APH) into the substantia innominata of rats

The comparative effect of intracerebral injection of 2-APH, a selective antagonist for N-methyl- d-aspartate (NMDA) receptors, into the substantia innominata (SI) and the amygdala (AM) of AM-kindled rats was examined. The intra-SI injection (ipsilateral to the kindled AM) induced a transient incoordination followed by immobility with loss of the rightening reflex, beginning at about 5 min following the injection and lasting for about 3 h. When the animals were stimulated at the previously established generalized seizure triggering threshold (GST) 45 min after the injection, the kindled seizure regressed to earlier stages although the afterdischarge (AD) duration remained unchanged. At 24 h, kindled seizure was readily activated at the GST. When 2-APH was injected into the kindled AM, no behavioural change occurred but AM stimulation at the GST failed to produce AD 45 min after the injection. Kindled seizure could be elicited, however, when the stimulus intensity was increased. This elevation of the GST lasted for 1–18 days. The findings suggest that NMDA receptors in the AM and SI play a differential role in AM seizure initiation and propagation, respectively. They also provide further support to the role presumed to be played by the SI in transforming the limbic seizure into motor seizure.

Generalized Seizure Triggering Threshold and Serum Phenobarbital Levels During Pregnancy in Rats

In experiments using pregnant rats, the following results were noted: (a) Of the amygdaloid kindled rats, the generalized seizure triggering threshold of the pregnant group was significantly higher than that of the nonpregnant group. (b) Of the rats taking oral phenobarbital, 4 mg/kg once a day, there was no significant difference of serum levels between the pregnant group and the nonpregnant group. (c) Of the amygdaloid kindled rats taking oral phenobarbital, 4 mg/kg once a day, the generalized seizure triggering threshold of the pregnant group was significantly higher than that of the nonpregnant group. It may be suggested that, when the drug dose remains the same, the rise in threshold by gestational physiological changes antagonizes the fall in threshold by decreased serum levels of anticonvulsants because of increasing distribution volume due to increasing body weight during pregnancy.