Pentylenetetrazol augmentation of developing kindled amygdaloid seizures

The effects of daily administration of pentylenetetrazol (PTZ) on the rate of the development of kindled amygdaloid seizures (KAS) were tested in the rat. Rats were pretreated for 6 days with 25 mg/kg PTZ or normal saline 15 min. prior to each daily amygdaloid stimulation. The PTZ group exhibited fully developed KAS, including a maximal behavioral ranking (BR) and an afterdischarge duration (AD) of over 90 sec, in 3.8 ± 0.3 stimulations compared to 8.0 ± 0.6 stimulations (ave. ± SEM) for the saline controls. Within 10 min after dosing, PTZ consistently induced intermittent spiking of 5 sec or less in the EEG and behaviorally induced head nodding and sniffing. However, chemical kindling due to the 6 repetitive doses of PTZ was not observed. When the PTZ pretreatment was stopped on the 7th day, the BR and AD of the PTZ pretreatment group decreased from 187% and 161% of control values, respectively, to 118% and 100% of control. It appears that PTZ augments both the AD and BR of the developing KAS but does not accelerate the actual rate of the development of the KAS.     

Opiate modification of amygdaloid-kindled seizures in rats

Male Long-Evans rats were stereotaxically implanted bilaterally with bipolar electrodes in the central amygdala. Rats were then kindled once daily for 1 sec until 3 consecutive Stage V [25] kindled seizures were elicited. On the following day, animals were injected (IP) with either saline, naloxone (10 mg/kg), naltrexone (10 mg/kg) or morphine sulfate (10 mg/kg) and again stimulated parameters the kindling stimulation parameters. Saline injected animals continued to show long bilateral AD's and behaviors (i.e., forelimb clonus, rearing, falling) typical of Stage V kindled animals. In contrast, rats injected with naloxone or naltrexone showed reduced behavioral seizures. Potentiation of post-ictal spiking by morphine in amygdaloid-kindled rats was also observed supporting previous reports [7,21]. In a second experiment, the reduction of kindled seizure severity by naloxone was systematically replicated. It is concluded that opiates can significantly modify amygaloid-kindled seizures, and that brain endorphins may play a role in the development of maintenance of an amygdaloid-kindled seizure focus.     

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.     

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.     

NAALADase (GCP II) inhibition prevents cocaine-kindled seizures

The prediction that inhibition of NAALADase, an enzyme catalyzing the cleavage of glutamate from N-acetyl-aspartyl-glutamate, would produce antiepileptogenic effects against cocaine was tested. Cocaine kindled seizures were developed in male, Swiss-Webster mice by daily administration of 60 mg/kg cocaine for 5 days. The NAALADase inhibitor 2-(phosphonomethyl)pentanedioic acid (2-PMPA) produced dose-dependent protection (10-100 mg/kg) against both the development of seizure kindling and the occurrence of seizures during the kindling process without observable behavioral side-effects. It is not likely that 2-PMPA produced protection against cocaine kindling by altering the potency of the convulsant stimulus as daily administration of 2-PMPA did not alter the convulsant thresholds for cocaine. Lower daily doses of cocaine (40 mg/kg) did not increase the incidence of seizures but produced kindling, as evidenced by the increase in seizure susceptibility when mice were probed with a higher dose of cocaine. 2-PMPA was also effective in preventing the development of sensitization to this covert kindling process. In contrast to its efficacy against cocaine kindled seizures, 2-PMPA failed to attenuate the convulsions engendered by acute challenges with pentylenetetrazole, bicuculline, N-methyl-D-aspartate, maximal electroshock or cocaine. Similarly, acutely-administered 2-PMPA did not block cocaine seizures in fully-kindled mice. NAALADase inhibition thus provides a novel means of attenuating the development of cocaine seizure kindling.     

Caffeine augmentation of electroconvulsive seizures

Caffeine has been used clinically to increase seizure length in electroconvulsive treatment (ECT). The present study was designed to establish an animal model of caffeine-augmented seizures for further study of mechanisms and effects of pharmacological manipulation of seizure length. Increasing doses of caffeine (0–200 mg/kg, IP) were given before electroconvulsive stimulation (ECS) in rats and resulting seizure lengths were quantified by timing of classical tonic-clonic convulsive movements. With this paradigm, caffeine led to a dose-dependent increase in seizure duration. This proconvulsant action of caffeine was detectable within 1 min after dosing, persisted for at least 230 min and was reversible. The results suggest that seizure length is a practicable measure in pharmacological modification of electroconvulsive seizures. They also suggest that pharmacologically-modified ECS can be modeled effectively in animals.     

Repeated administration of subconvulsant doses of GABA antagonist drugs

Repeated subconvulsant doses of the GABA antagonist drugs picrotoxin (5 mg/kg), pentylenetetrazol (PTZ) (30 mg/kg), and bicuculline (3.5 mg/kg), were given IP once daily to rats. Picrotoxin produced rapid kindling to full seizures in about 5 days, PTZ produced sporadic myoclonic seizures after 17 days, PTZ produced sporadic myoclonic seizures after 17 days whereas bicuculline only produced occasional mild jerking. Following these treatments, seizure thresholds to these drugs were measured by an IV infusion method to minimise problems of systemic uptake and metabolism of the drugs. Picrotoxin- and PTZ-treated animals showed no alteration in seizure threshold. However, following bicuculline pretreatment, seizure threshold was raised. Methodological problems in the interpretation of pharmacological kindling are discussed.     

Formamidine pesticides enhance susceptibility to kindled seizures in amygdala and hippocampus of the rat

Electrical kindling of the amygdala and hippocampus was used to evaluate the effects of two formamidines, chlordimeform (CDF) and amitraz (AMZ), upon seizures susceptibility in the rat. Male Long-Evans rats were implanted with electrodes in the amygdala or dorsal dentate gyrus, and injected IP daily with 40 mg/kg CDF, 50 mg/kg AMZ, or equal volumes of their respective vehicles. Afterdischarge (AD) thresholds were determined after the first injection. Animals were then stimulated twice daily, 2 and 4 hours postinjeciton, at a standard 200 microA stimulus intensity until three stage 5 generalized seizures ensued. Both CDF and AMZ significantly facilitated amygdaloid kindling rate, and CDF also facilitated hippocampal kindling rate. The effects of AMZ on hippocampal kindling were not assessed. AD durations were prolonged in the formamidine-treated groups, but there was no effect on AD thresholds. The alpha-2 adrenergic agonist and/or local anesthetic-like properties of these compounds may be responsible for these seizure enhancing effects.     

Effects of alcohol on kindled seizure thresholds in rats

Seizures were kindled in the amygdala and ventral hippocampus of rats until a stage 5 (clonic convulsion) was elicited. Stage 5 thresholds were then determined. Animals were then injected with either saline, or a 600 mg/kg, or 1600 mg/kg dose of 25% ethanol. The effect of each of these injections on seizure thresholds was assessed. The 1600 mg/kg dose caused a significant elevation in both AD and motor seizure thresholds, relative to the 600 mg/kg dose and saline, which did not differ. The elevation of seizure thresholds was significantly greater for animals with seizures kindled in the ventral hippocampus.     

Nifedipine has paradoxical effects on the development of kindling but not on kindled seizures in amygdala-kindled rats

The effects of nifedipine, an antagonist of voltage-operated calcium channels, on the development of amygdala kindling and on the production of fully kindled seizures, stimulated from the amygdala, were investigated. Rats were treated daily with two doses (5 and 50 mg/kg, i.p.) of nifedipine during the development of kindling. Both doses of nifedipine retarded the development of kindled seizures and 50 mg/kg of nifedipine prolonged the latency to the occurrence of bilateral forelimb clonus. In contrast to these antiepileptogenic effects, however, both doses also increased the duration of afterdischarge. This resulted in a striking increase in the cumulative duration of afterdischarge, required to reach stage 4 and 5 seizures. Contrary to the results of a previous study, 50 mg/kg of nifedipine did not produce any significant effect on fully kindled seizures, regardless of the interval (5 min-24 hr) between injection and stimulation of kindling. These results suggested that although nifedipine inhibited the propagation processes of seizures during development of kindling, it appeared to increase the duration of epileptic activity at the kindling focus.     

The effect of exogenous GM1 ganglioside on kindled-amygdaloid seizures

The effects of 12 daily doses of 30 mg/kg GM1 ganglioside i.p. on the acquisition of kindled-amygdaloid seizures in the rat was studied. No modification in the rate of kindling or the expression of the elicited seizures was noted during the acquisition phase. Further studies with additional fully amygdaloid kindled rats failed to show significant modification of suprathreshold or threshold elicited seizures after single 30-60 mg/kg i.p. doses of GM1 ganglioside. Despite previous studies which have shown antibodies to GM1 ganglioside to be convulsive, no anticonvulsant activity was demonstrated in this study with exogenous GM1 ganglioside using a battery of kindled-amygdaloid seizure tests in the rat.     

Effect of cocaine and pentylenetetrazol on cortical kindling

The effect of drug-induced convulsions on subsequent cortical kindling was studied in male Long-Evans rats. Animals experienced three intravenous infusions of physiological saline at 3 day intervals, or three convulsions induced by the infusion of cocaine or pentylenetetrazol (PTZ). Beginning eight days after the last infusion, all animals were kindled by stimulation of the anterior neocortex (area 6). PTZ-induced convulsions facilitated the development of both the behavioral convulsion and the electrographic seizure during cortical kindling, while cocaine-induced convulsions facilitated only the development of the electrographic seizure. Comparison of these results with previous research indicates that convulsions induced by these two drugs have long-lasting effects on brain function which differ both in their anatomical distribution and in the nature of the effects produced. These drugs also differed in their acute effects at subconvulsant doses on the expression of cortically kindled seizures. Cocaine (and lidocaine, another local anesthetic) substantially elevated afterdischarge (AD) threshold and inhibited the focal component of the cortically kindled seizure. PTZ had no significant effect on either of these variables but significantly increased AD duration. In addition to these drug effects, a substantial inhibitory effect on seizure expression was observed, both during kindling and afterwards, when ADs were elicited daily but not when they were separated by 3 days or more. This finding suggests that the large number of ADs typically required for cortical kindling may be due in part to daily stimulation.     

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.     

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.     

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)     

Differential noradrenergic influence on seizure expression in genetically Fast and Slow kindling rat strains during massed trial stimulation of the amygdala

The involvement of alpha(2) noradrenergic receptors during amygdala 'massed' stimulation (MS) was examined in rats that were selectively bred to be seizure-prone (Fast) or seizure-resistant (Slow) to amygdala kindling. The selective alpha(2) noradrenergic agonist guanfacine, or the antagonist idazoxan, was intraperitoneally injected during the MS procedure to study subsequent changes in afterdischarge (AD) threshold, AD duration and behavioral seizure expression. These measurements were again assessed weekly for 2 weeks after the MS treatment. Daily kindling began immediately thereafter. Following 6 stage-5 once daily convulsive seizures, guanfacine or idazoxan were re-administered. With idazoxan, the Slow rats expressed greater numbers of convulsive seizures and longer AD durations compared to guanfacine or saline controls during MS treatment. This pro-convulsive property of idazoxan was absent in Fast rats. By contrast, Fast rats showed enhanced convulsive expression in the presence of guanfacine. In the fully kindled rat, idazoxan and guanfacine differentially impacted seizure duration and severity in the Slow rats, but again not in the Fast rats. These data suggest that some aspect(s) of the alpha(2) noradrenergic system in the Fast and Slow rats are dissimilar and the mechanisms by which these receptors govern seizure genesis and propagation may be genetically controlled and distinct.     

Modification of excitation and inhibition evoked in dentate gyrus by perforant path stimulation: effects of aminophylline and kindling

Rats were implanted with chronic electrodes to stimulate the perforant path and record the elicited monosynaptic evoked potentials from the dentate gyrus of the hippocampal formation. Dentate responses were examined in awake and anesthetized animals after exposure to saline and aminophylline (100 mg/kg, IP). In the awake animal, aminophylline treatment did not significantly alter the threshold or elicited amplitude of either the excitatory post-synaptic potential (EPSP) or the population spike (PS). Aminophylline pretreatment markedly enhanced the length and severity of elicited seizures from hippocampal (dentate gyrus) or perforant pathway stimulation. After daily perforant pathway stimulations which established "kindled" seizures, aminophylline significantly increased only the amplitude of the evoked PS in awake animals. In animals anesthetized with chloropent, aminophylline increased significantly before kindling the amplitude of both the EPSP and PS without effecting thresholds for each. After perforant pathway kindling, only the PS amplitude was increased significantly by aminophylline. Inhibition, thought to be from GABA-mediated recurrent collaterals, was found to be increased rather than decreased by kindling. Further, aminophylline treatment did not result in reduction of this inhibition before or after kindling. These data suggest that at this dose of aminophylline neither enhanced transmitter release at this synapse as measured by the amplitude of the EPSP, nor reduced recurrent collateral inhibition significantly contributed to the prolongation of elicited seizure afterdischarge. The increase in PS amplitude reflecting an increased number of granule cells excited to discharge with perforant path stimulation after aminophylline was noted in awake animals but was greatest in the anesthetized animals. Although the number of granule cells excited to discharge was increased by aminophylline, the small increase in amplitude seen compared to the effects of other neurotoxins on this synapse makes this an unlikely explanation for the profound increased seizure response seen after aminophylline.     

The effects of doxapram, diazepam, phenobarbital and pentylenetetrazol on suprathreshold and threshold stimulations in amygdaloid kindled rats

Amygdaloid kindled rats were utilized to determine the effects of the respiratory stimulant, doxapram (7.5-60 mg/kg) on seizures elicited by suprathreshold or threshold electrical stimulation. For comparison, a single dose of pentylenetetrazol (30 mg/kg), phenobarbital (40 mg/kg) and diazepam (2 mg/kg) were also tested in the same paradigm. With suprathreshold stimulation, doxapram did not increase the intensity of seizures. Neither the afterdischarge duration (AD) nor the behavioral rank (BR) were increased with doses high enough to cause systemic signs of toxicity. Similarly, the convulsant pentylenetetrazol did not increase AD or BR after suprathreshold stimulation. The anticonvulsants phenobarbital and diazepam significantly reduced both AD and BR after suprathreshold stimulation. Seizure thresholds tended to be higher after the various doses of doxapram with no consistent effect on elicited AD or BR. Pentylenetetrazol did not change threshold values, but was found to increase AD and BR at threshold. Both phenobarbital and diazepam raised thresholds with a significant decrease in elicited AD and BR. It would appear that the respiratory stimulant doxapram has little proconvulsant activity in this model of epilepsy.     

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.     

Changes in immunoreactive somatostatin in brain following lidocaine-induced kindling in rat

The kindling phenomenon has become a useful model for studying epileptogenesis. The present authors have previously reported increased levels of immunoreactive somatostatin (IR-SRIF) in various regions of the brain of electrically-amygdaloid kindled (EAK) rats. In this study, an examination was made of immunoreactive somatostatin in pharmacologically-kindled (PK) rats. Sixteen male Sprague-Dawley rats were injected intraperitoneally (i.p.) with a subthreshold dose of lidocaine (60 mg/kg), once daily. Once the kindling phenomenon was established, kindled rats (7), non-kindled rats (9) and controls (6) were sacrificed by microwave irradiation. Another group of 5 rats was injected with a single suprathreshold dose of lidocaine (110 mg/kg) and killed 10 min after the resultant seizure. Various brain areas were removed and assayed for immunoreactive somatostatin in kindled rats. Immunoreactive somatostatin was significantly greater than in controls in the amygdala (56%; P less than 0.02), entorhinal + piriform cortex (50%; P less than 0.05) and hypothalamus (29%; P less than 0.02). In non-kindled rats, immunoreactive somatostatin increased only in the amygdala (58%; P less than 0.02). No difference was found in the immunoreactive somatostatin content of rats injected with an suprathreshold dose of lidocaine compared to controls. The alteration of immunoreactive somatostatin, in both lidocaine-kindled and electrically-amygdaloid kindled rats suggests a possible role of this neuropeptide in kindling.