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.     

抗痫灵对大鼠杏仁核点燃效应的拮抗作用

用大鼠杏仁核点燃模型研究了抗痫灵的抗癫痫作用。每日1次电刺激杏仁基底外侧核可在第15d产生点燃效应的5期反应,刺激后放电显著延长。抗痫灵在未产生中枢镇静的剂量就能抑制点燃效应的5期反应,ED_(50)为84.7mg/kg,对刺激后放电影响较少。丙戊酸钠抑制5期反应作用较强,并能显著缩短刺激后放电时程。实验表明抗痫灵具有对抗慢性杏仁核点燃效应的作用。     

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.     

The effects of LY-201116 [4-amino-N-(2,6-dimethylphenyl) benzamide] on the amygdala-kindled rat

The effects of LY-201116, a 4-aminobenzamide, were examined in rats using the amygdala kindling model, both during acquisition of the kindled response and in fully kindled animals. Dose-response and time-response studies for efficacy and rotorod toxicity were completed following intraperitoneal injection of the drug. Afterdischarge duration, behavioral seizure response, kindled seizure threshold and EEG recordings were used to assess efficacy and toxicity of the drug. In the acquisition trial, the drug (7.5 mg/Kg) did not significantly alter the number of stimulations required to produce the first stage 5 kindled response nor did it modify afterdischarge durations. Doses of 11.25 and 15 mg/Kg suppressed afterdischarge and diminished behavioral responses significantly in fully kindled rats, but these doses were also neurotoxic as judged by rotorod performance. The non-selective anticonvulsant effect of 11.25 mg/Kg lasted at least 90 min. A dose of 15 mg/Kg raised kindled seizure threshold and diminished afterdischarge duration. Doses of 20, 30 and 40 mg/Kg produced spontaneous EEG spikes and seizures accompanied by behavioral convulsions. The drug thus exhibited non-selective anticonvulsant effects in fully kindled rats following doses of 11.25 or 15 mg/Kg, but exhibited proconvulsant activity following doses in the range of 20-40 mg/Kg.     

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.     

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.     

Role of AMPA and GluR5 kainate receptors in the development and expression of amygdala kindling in the mouse

The role of AMPA and GluR5-containing kainate receptors in the development and expression of amygdala kindling was examined using the selective 2,3-benzodiazepine AMPA receptor antagonist GYKI 52466 [(1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2, 3-benzodiazepine] and the decahydroisoquinoline mixed AMPA receptor and GluR5 kainate receptor antagonist LY293558 {(3S,4aR,6R, 8aR)-6-[2-(1(2)H-tetrazole-5-yl)ethyl]decahydroisoquinoline- 3-carboxy lic acid)}. Administration of GYKI 52466 (5-40 mg/kg, intraperitoneally) and LY293558 (10-40 mg/kg, intraperitoneally) prior to daily kindling stimulation in mice produced a dose-dependent suppression of the rate of development of behavioral kindled seizure activity and reduced the duration of the stimulation-induced electrographic afterdischarge. In drug-free stimulation sessions after the initial drug-treatment sessions, there was an acceleration in the rate of kindling development compared with the rate during the preceding drug-administration period; the "rebound" rate was also greater than the kindling rate in saline-treated control animals. In fully kindled animals, both GYKI 52466 and LY293558 produced a dose-dependent suppression of evoked seizures (ED(50), 19.3 and 16.7 mg/kg, respectively). Although AMPA receptors appear to be critical to the expression of kindled seizures, since kindling development progressed despite the suppression of behavioral seizure activity, AMPA receptors are less important to the kindling process. LY293558 was modestly less effective at suppressing behavioral seizures during kindling and was not superior to GYKI 52466 in retarding the overall extent of kindling development, indicating that GluR5 kainate receptors do not contribute to epileptogenesis in this model.     

Caffeine modification of kindled amygdaloid seizures

Rats were kindled during exposure to caffeine (50 mg/kg) or saline given IP twenty minutes before daily electrical stimulation of the amygdala until 3 kindled amygdaloid seizures (KAS) occured. They were then stimulated for 3 days without drug pretreatment followed by 5 additional days with drug pretreatment. There were no significant differences between the two groups in the number of daily stimulations or in the total seconds of cumulative afterdischarge (AD) needed to reach the first KAS. During kindling, the daily average AD tended to be longer in the caffeine treated group. This difference became significant (>200% saline) when the KAS was reached. When KAS animals were stimulated without caffeine pretreatment, the average AD returned to control lengths. When put back on caffeine pretreatment, the average AD was again increased. Caffeine (6–50 mg/kg, IP) was also evaluated in previously kindled rats using suprathreshold (400 μAMP) and threshold (20 μA increments) seizures. Caffeine had no consistent effect on threshold values. However, 12–50 mg/kg of caffeine increased seizure severity and AD durations after threshold stimulation. With suprathreshold stimulation, the length of the AD was significantly increased only after the highest dose of caffeine. It would appear that caffeine lengthens induced afterdischarges both during the acquisition phase of kindling and in the fully kindled subject. Caffeine does not appear to lower seizure thresholds or increase the rate of aquisition of the KAS in the doses tested in this model. It is postulated that caffeine may modify the KAS through an inhibition of the mechanisms which terminate the elicited AD.     

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.     

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.     

A pharmacological study in the kindling model of epilepsy

The anticonvulsant properties of carbamazepine were evaluated in the kindled amygdaloid seizure model in rats. Carbamazepine significantly raised the threshold for seizures, reduced the duration of elicited afterdischarges and attenuated the severity of seizures in previously-kindled rats, at doses that did not cause sedation or ataxia. A similar reduction in the duration of elicited afterdischarges and severity of seizures was seen after suprathreshold stimulation (400 mu A) with doses of carbamazepine that were without obvious sedative or ataxic effects. After acute intraperitoneal injections (solvent = 2% Tween-80 and 70% propylene glycol), the maximum anticonvulsant effectiveness against suprathreshold stimulation was seen at 30 min. When administered daily (13 days) during acquisition or development of kindling, carbamazepine (25 and 50 mg/kg, i.p.) had variable effects on kindling. Neither dose consistently reduced the duration of elicited afterdischarges during the acquisition phase. Both groups tended to reduce the developing seizure, with the smaller dose of carbamazepine (25 mg/kg) resulting in a more consistent and significant reduction in severity of seizures. No significant differences in number of daily stimulations needed to reach fully kindled seizures were found. Previous studies have reported variable results with carbamazepine and the kindled amygdaloid seizure in rats. The present study provides a comprehensive evaluation of carbamazepine in this model of epilepsy and discusses the results with regard to the finding reported previously.     

A characterization of chemical kindling with the pesticide endosulfan.

In a companion article (see this issue), the proconvulsant properties of the pesticide endosulfan in electrical kindling of the amygdala are described. In the present report, an evaluation of the chemical kindling properties of endosulfan is presented. Repeated administration (3 times per week for a total of 21 doses) of endosulfan (5 and 10 mg/kg in corn oil, PO) was found to induce behavioral seizures in rats. Behavioral seizure development was most apparent in the high dose group (10 mg/kg). Heightened seizure responsiveness to a challenge dose was maintained following a two-week, drug-free period, arguing against cumulative toxicity as a mechanism for seizure induction. Electrical kindling induced by once daily stimulation of the amygdala began approximately 4 weeks after the final dose of endosulfan. In the absence of further dosing, a significant facilitation in the rate of kindling development was evident as a function of prior treatment with endosulfan. An enhancement in the rate of kindling was also evident in the low dose group (5 mg/kg) in the absence of clonic seizure development during dosing. A history of endosulfan treatment failed to affect threshold for inducing an afterdischarge (AD) and had equivocal effects on the development of AD with repeated stimulation. Pretreatment with a single high dose of endosulfan 2 weeks prior to electrical kindling was without effect on the development of the kindled response. Endosulfan has been reported to decrease binding of gamma-amino-butyric acid (GABA). Chemical kindling with endosulfan may result from the interaction of this pesticide with GABA-mediated neurotransmission in the central nervous system (CNS).(ABSTRACT TRUNCATED AT 250 WORDS)     

香荚兰素对抗大鼠杏仁核点燃效应

用大鼠杏仁核点燃模型研究了香荚兰素的抗癫痫作用。每日一次低电流刺激杏仁基底外侧核可在第15天使大鼠产生典型的五期反应,刺激后放电显著延长。香荚兰素在不产生中枢抑制作用的剂量即可抑制点燃效应的五期反应,ED₅₀为286 mg/kg(ip),同时使刺激后放电时程比对照组下降28.6%。苯妥英钠抑制五期反应的ED₅₀为56mg/kg,也能明显缩短刺激后放电时程。本文提供了一种新型慢性实验性癲痫模型。     

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.     

Anticonvulsant and antiepileptogenic actions of MK-801 in the kindling and electroshock models

The actions of MK-801, a noncompetitive antagonist at the N-methyl-d-aspartate subtype of excitatory amino acid receptor, were investigated on the development of kindling and on seizures in the electroshock and kindling models. The drug MK-801 potently and effectively suppressed the tonic hindlimb extension component of electroshock-induced seizures; it also suppressed both the electrophysiological and behavioral manifestations of the development of kindling. In contrast to its effects on electroshock-induced seizures and the development of kindling, MK-801 only partly reduced the duration of seizures in fully kindled animals and did not elevate the threshold for afterdischarge despite the use of a large dose, associated with profound untoward behavioral effects. Together with previous findings, these results support the idea that noncompetitive blockade of NMDA receptors markedly inhibits the development of kindling. The diminished effectiveness of MK-801 against kindled seizures suggests that MK-801 will not be a clinically-useful anticonvulsant against complex partial seizures.     

Effect of opiates on the parameters of seizures in rats with full amygdaloid-kindled convulsions

The effect on the parameters of seizures of opiates, administered in doses used clinically for analgesic effects, was studied in rats with full amygdaloid-kindled seizures. The largest dose of fentanyl studied (100 micrograms/kg) had a pronounced inhibitory effect on kindled seizures: severity of seizures, duration of seizures and duration of afterdischarge were significantly reduced to 36, 40 and 37% of controls, respectively, and the latency of seizures was significantly increased to 168% of untreated animals. The largest dose of pentazocine (16 mg/kg) also significantly inhibited the duration of seizures and duration of afterdischarge. Morphine (1-4 mg/kg) and meperidine (4-16 mg/kg) had a tendency to inhibit the duration of seizure and afterdischarges but did not significantly affect any of the measured parameters of seizures. Fentanyl, meperidine and pentazocine resulted in a lowering, whereas morphine caused a slight elevation, of the threshold for initiation of kindled seizures. The data suggest that fentanyl, in relatively small doses, may cause an inhibition of the intensity of behavioural and electrographic seizures but, paradoxically, an increased sensitivity to induction of seizures in rats with full amygdaloid-kindled seizures.     

Proconvulsant activity of endosulfan in amygdala kindling.

The proconvulsant properties of the chlorinated hydrocarbon insecticide, endosulfan, were investigated using electrical kindling of the amygdala. Male rats were implanted with electrodes in the amygdala and stimulated once daily with a standard kindling stimulus 60-90 min following endosulfan (0, 2.5, 5.0 mg/kg, PO). No alterations were observed in either the threshold to induce an afterdischarge (AD) or the duration of clonus upon seizure generalization. Endosulfan significantly reduced the number of stimulations required to produce Stage 5 generalized seizures. Seizures prior to stimulation were evident in a subset of animals from both dosage groups and were never observed in controls. The presence of kindled seizures was maintained in the absence of further dosing, as amygdala stimulation 2-4 weeks after the last endosulfan treatment resulted in generalized seizures in all animals. These results suggest that faster kindling rates induced by endosulfan are not readily attributable to transient toxicant-related increases in excitability of the nervous system. It was concluded that endosulfan has proconvulsant properties that may be related to an action on GABA within the central nervous system.     

琥珀酸对大鼠化学性点燃和杏仁核电刺激性点燃的抑制作用

目的:研究琥珀酸对大鼠戊四哇化学性点燃(kindling)发作及杏仁核电刺激点燃发作的影响及作用机制.方法:建立大鼠戊四唑化学性点燃模型和杏仁核电刺激点燃癫痫模型,测定琥珀酸对点燃发作的脑电活动及行为变化指标的影响.测定琥珀酸对GABA_A受体拮抗剂印防己毒素诱发小鼠惊厥的影响.结果:琥珀酸(100-400 mg/kg,iP)对两种点燃模型有显著抑制作用,降低发作强度和全身性发作百分率(P<0.05,P<0.01),可升高杏仁核电刺激点燃大鼠的局灶性后放电阈值(P<0.05,P<0.01),以上反应呈剂量效应关系。琥珀酸可延长印防己毒素诱发小鼠惊厥的潜伏期(P<0.05,P<0.01).结论:琥珀酸对大鼠戊四唑化学性点燃和脑杏仁核电刺激点燃发作有抑制作用,其机制可能与增强GABA_A受体功能有关.     

Gabapentin-lactam, a close analogue of the anticonvulsant gabapentin, exerts convulsant activity in amygdala kindled rats

The cyclic GABA analogue gabapentin (GBP), which recently has been marketed for treatment of epilepsy, is particularly effective against complex-partial seizures as occurring in temporal lobe epilepsy. In the present study, we compared the effects of GBP and its lactam analogue (GBP-L) in the amygdala kindling model of temporal lobe epilepsy. In fully kindled rats, GBP (50 mg/kg and 100 mg/kg i.p.) dose-dependently increased the threshold for focal seizures and inhibited the progression from focal to generalized seizures. This effect was not associated with any marked adverse effects. In contrast, GBP-L (10-50 mg/kg) induced myoclonic activity and generalized clonic seizures in kindled rats, demonstrating a striking qualitative difference between the two compounds. By comparison with non-kindled rats it was shown that kindling markedly enhanced the sensitivity of rats to the convulsant activity of GBP-L. The finding that the anticonvulsant efficacy of GBP is lost by lactam formation indicates that GBP and GBP-L differ in their mechanism(s) of action.     

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