Limbic seizures produced by pilocarpine in rats: behavioural, electroencephalographic and neuropathological study

Behavioural, electroencephalographic and neuropathological responses to increasing doses of pilocarpine (100-400 mg/kg) administered intraperitoneally to rats were studied. At the dose of 400 mg/kg pilocarpine produced a sequence of behavioural alterations including staring spells, olfactory and gustatory automatisms and motor limbic seizures that developed over 1-2 h and built up progressively into limbic status epilepticus. Smaller doses showed different threshold for these behavioural phenomena but a similar time course of development. The earliest electrographic alterations occurred in the hippocampus and then epileptiform activity propagated to amygdala and cortex. Subsequently electrographic seizures appeared in both limbic and cortical leads. The ictal periods recurred each 5-15 min and were followed by variable periods of depression of the electrographic activity. The sequence of electrographic changes correlated well with the development of behavioural phenomena. Histological examination of frontal forebrain sections revealed disseminated, apparently seizure-mediated pattern of brain damage. Neuropathological alterations were observed in the olfactory cortex, amygdaloid complex, thalamus, neocortex, hippocampal formation and substantia nigra. Pretreatment of animals with scopolamine (20 mg/kg) and diazepam (10 mg/kg) prevented the development of convulsive activity and brain damage. These results show that systemic pilocarpine in rats selectively elaborates epileptiform activity in the limbic structures accompanied by motor limbic seizures, limbic status epilepticus and widespread brain damage. It is suggested that a causative relationship between excessive stimulation of cholinergic receptors in the brain and epileptic brain damage may exist.     

Effects of aminophylline and 2-chloroadenosine on seizures produced by pilocarpine in rats: Morphological and electroencephalographic correlates

The effects of 2-chloroadenosine, aminophylline, bicuculline, beta-carboline-3-carboxylic acid methylester and Ro 15-1788 on seizures produced by pilocarpine were examined in rats. In animals pretreated with aminophylline at doses of 25-100 mg/kg, non-convulsant dose of pilocarpine, 100 mg/kg, resulted in severe motor limbic seizures, which rapidly developed into the status epilepticus. Electroencephalographic monitoring showed progressive evolution of seizure activity with initial high-voltage fast activity followed by high-voltage spiking and electrographic seizures. Morphological analysis of frontal forebrain sections with light microscopy demonstrated widespread damage to the hippocampal formation, thalamus, amygdala, olfactory cortex, substantia nigra and neocortex. Bicuculline, 2 mg/kg, beta-carboline-3-carboxylic acid methylester, 5 mg/kg, and Ro 15-1788, 50 mg/kg, did not augment seizures produced by pilocarpine, 100 mg/kg. 2-Chloroadenosine, 5 and 10 mg/kg, blocked the appearance of behavioral and electrographic seizures produced by pilocarpine, 380 mg/kg, and prevented the occurrence of brain damage. The results indicate that purinergic mechanisms are involved in the buildup of pilocarpine-induced convulsions and seizure-related brain damage in rats.     

Only certain antiepileptic drugs prevent seizures induced by pilocarpine

Seizures produced in rats by systemically administered pilocarpine (PILO) provide a model for studying the generation and spread of convulsive activity in the forebrain. PILO, 380 mg/kg, induces a sequence of behavioral and electroencephalographic alterations indicative of motor limbic seizures and status epilepticus which is followed by widespread damage to the limbic forebrain resembling that occurring subsequent to prolonged intractable seizures in humans. The present study was undertaken to determine whether clinically utilized antiepileptic drugs share an ability to suppress seizures and brain damage elicited by PILO in rats. Clonazepam, ED50 0.35 mg/kg (0.25-0.49), phenobarbital, 23.4 mg/kg (18.5-29.6), and valproic acid, 286 mg/kg (202-405), prevented the buildup of limbic seizures and protected against seizure-related brain damage. Pretreatment with trimethadione, 179 mg/kg (116-277), resulted in a moderate protection against PILO-induced seizures, whereas carbamazepine, 10-50 mg/kg, and diphenylhydantoin, 10-200 mg/kg, blocked neither convulsions nor brain damage produced by the drug. Surprisingly, ethosuximide, 196 mg/kg (141-272), and acetazolamide, 505 mg/kg (332-766), both lowered the threshold for seizures induced by PILO and converted a non-convulsant dose of PILO, 200 mg/kg, into a convulsant one. These results indicate that only certain anticonvulsant drugs elevate the threshold for PILO-induced seizures and prevent the occurrence of epilepsy-related brain damage. The resistance of seizures produced by PILO in rats to antiepileptic drugs reaffirms the clinically obvious lack of effective treatments for limbic convulsions.     

Only certain antiepileptic drugs prevent seizures induced by pilocarpine

Seizures produced in rats by systemically administered pilocarpine (PILO) provide a model for studying the generation and spread of convulsive activity in the forebrain. PILO, 380 mg/kg, induces a sequence of behavioral and electroencephalographic alterations indicative of motor limbic seizures and status epilepticus which is followed by widespread damage to the limbic forebrain resembling that occurring subsequent to prolonged intractable seizures in humans. The present study was undertaken to determine whether clinically utilized antiepileptic drugs share an ability to suppress seizures and brain damage elicited by PILO in rats. Clonazepam, ED₅₀ 0.35 mg/kg (0.25–0.49), phenobarbital, 23.4 mg/kg (18.5–29.6), and valproic acid, 286 mg/kg (202–405), prevented the buildup of limbic seizures and protected against seizure-related brain damage. Pretreatment with trimethadione, 179 mg/kg (116–277), resulted in a moderate protection against PILO-induced seizures, whereas carbamazepine, 10–50 mg/kg, and diphenylhydantoin, 10–200 mg/kg, blocked neither convulsions nor brain damage produced by the drug. Surprisingly, ethosuximide, 196 mg/kg (141–272), and acetazolamide, 505 mg/kg (332–766), both lowered the threshold for seizures induced by PILO and converted a non-convulsant dose of PILO, 200 mg/kg, into a convulsant one. These results indicate that only certain anticonvulsant drugs elevate the threshold for PILO-induced seizures and prevent the occurrence of epilepsy-related brain damage. The resistance of seizures produced by PILO in rats to antiepileptic drugs reaffirms the clinically obvious lack of effective treatments for limbic convulsions.     

The susceptibility of rats to pilocarpine-induced seizures is age-dependent

Behavioral, electroencephalographic and morphological changes induced by systemic administration of pilocarpine hydrochloride were studied in 3-90-day-old rats. Pilocarpine, 100, 200 and 380 mg/kg, presented a characteristic array of behavioral patterns in developing rats. Hyper- or hypoactivity, tremor, loss of postural control, scratching, head bobbing and myoclonic movements of the limbs dominated the behavior in 3-9-day-old rats. No overt motor seizures were observed in this age group. More intense behavioral signs evolving in some animals to limbic seizures and status epilepticus occurred when pilocarpine was administered in 12-day-old-rats. The electrographic activity in these animals progressed from low voltage spiking registered concurrently in the hippocampus and cortex during the first week of life into localized epileptic activity in the hippocampus, which spread to cortical recordings during the second week of life. No morphological alterations were detected in the brains of 3-12-day-old rats subjected to the action of pilocarpine, 100-380 mg/kg. The adult pattern of behavioral and electroencephalographic sequelae after pilocarpine was encountered in 15-21-day-old rats. Akinesia, tremor and head bobbing progressed in 15-21-day-old rats given pilocarpine, 100-380 mg/kg, to motor limbic seizures and status epilepticus. The lethal toxicity of pilocarpine reached 50% during the third week of life. This increased susceptibility to the convulsant action of pilocarpine was characterized by a shortened latency for behavioral and electrographic signs, and an increased severity of seizures relative to older and younger rats. In 15-21-day-old rats subjected to pilocarpine-induced convulsions high voltage fast activity superposed over hippocampal theta-rhythm, progressed into high voltage spiking and spread to cortical records. The electrographic activity became well synchronized and then developed into seizures and status epilepticus. Morphological analysis of frontal forebrain sections in 15-21-day-old rats which underwent status epilepticus after pilocarpine revealed no damage or an attenuated pattern of damage. In 15-21-day-old rats which presented epilepsy-related brain damage, morphological breakdown was seen in the hippocampus, amygdala, olfactory cortex, neocortex and certain thalamic nuclei. No damage was detected in the substantia nigra and lateral thalamic nucleus. An adult pattern of the damage to the brain, in terms of extent and topography, was present in 4-5-week-old rats.(ABSTRACT TRUNCATED AT 400 WORDS)     

Ontogenic study of lithium-pilocarpine-induced status epilepticus in rats.

Lithium is known to potentiate the ability of pilocarpine to induce status epilepticus in rats. The goal of this study was to determine whether lithium could potentiate pilocarpine-induced seizures in developing animals. Behavioral, electroencephalographic (EEG), and histopathological changes induced by systemic administration of lithium (3 meq/kg) followed 20 h later by pilocarpine (3, 10, 30, 60 mg/kg) were studied in 3-30-day-old rats. Lithium followed by pilocarpine (30 and 60 mg/kg) induced hyperactivity, tremor, loss of postural control and scratching but no electrographic seizures in 3-8-day-old rats. In the 7-10-day-old animals pretreatment with lithium and pilocarpine 60 mg/kg induced status epilepticus with sustained myoclonus and continuous bilateral synchronous spike and sharp wave, but doses of pilocarpine lower than 60 mg/kg had no effect. The susceptibility to lithium-pilocarpine-induced status epilepticus increased markedly during the third postnatal week of life. During this time period, rats treated with lithium (3 meq/kg) plus pilocarpine 10 mg/kg exhibited behavioral and EEG manifestations of status epilepticus. The same combination of lithium and pilocarpine failed to induce status epilepticus either before or after the third week of life. Histopathological analysis of the brains of the animals used in these studies failed to demonstrate the widespread damage reported in adult rats that have undergone lithium-pilocarpine-induced status epilepticus.     

Susceptibility to seizures produced by pilocarpine in rats after microinjection of isonnazid or γ-vinyl-GABA into the substantia nigra

Pilocarpine, given intraperitoneally to rats, reproduces the neuropathological sequelae of temporal lobe epilepsy and provides a relevant animal model for studying mechanisms of buildup of convulsive activity and pathways operative in the generalization and propagation of seizures within the forebrain. In the present study, the effects of manipulating the activity of the γ-aminobutyric acid (GABA)-mediated synaptic inhibition within the substantia nigra on seizures produced by pilocarpine in rats, were investigated. In animals pretreated with microinjections of isoniazid, 150 μg, an inhibitor of activity of the GABA-synthesizing enzyme, l-glutamic acid decar☐ylase, into the substantia nigra pars reticulata (SNR), bilaterally, non-convulsant doses of pilocarpine, 100 and 200 mg/kg, resulted in severe motor limbic seizures and status epilepticus. Electroencephalographic and behavioral monitoring revealed a profound reduction of the threshold for pilocarpine-induced convulsions. Morphological analysis of frontal forebrain sections with light microscopy revealed seizure-related damage to the hippocampal formation, thalamus, amygdala, olfactory cortex, substantia nigra and neocortex, which is typically observed with pilocarpine in doses exceeding 350 mg/kg. Bilateral intrastriatal injections of isoniazid did not augment seizures produced by pilocarpine, 200 mg/kg. Application of an irreversible inhibitor of GABA transminase, γ-vinyl-GABA (d, l,-4-amino-hex-5-enoic acid), 5 μg, into the SNR, bilaterally, suppressed the appearance of electrographic and behavioral seizures produced by pilocarpine, 380 mg/kg. This treatment was also sufficient to protect animals from the occurrence of brain damage. Microinjections of γ-vinyl-GABA, 5 μg, into the dorsal striatum, bilaterally, failed to prevent the development of convulsions produced by pilocarpine, 380 mg/kg. The results demonstrate that the threshold for pilocarpine-induced seizures in rats is subjected to the regulation of the GABA-mediated synaptic inhibition within the substantia nigra.     

Differential effects of non-steroidal anti-inflammatory drugs on seizures produced by pilocarpine in rats

The muscarinic cholinergic agonist pilocarpine induces in rats seizures and status epilepticus followed by widespread damage to the forebrain. The present study was designed to investigate the effect of 5 non-steroidal anti-inflammatory drugs, sodium salicylate, phenylbutazone, indomethacin, ibuprofen and mefenamic acid, on seizures produced by pilocarpine. Pretreatment of rats with sodium salicylate, ED50 103 mg/kg (60-174), and phenylbutazone, 59 mg/kg (50-70) converted the non-convulsant dose of pilocarpine, 200 mg/kg, to a convulsant one. Indomethacin, 1-10 mg/kg, and ibuprofen, 10-100 mg/kg, failed to modulate seizures produced by pilocarpine. Mefenamic acid, 26 (22-30) mg/kg, prevented seizures and protected rats from seizure-related brain damage induced by pilocarpine, 380 mg/kg. These results indicate that non-steroidal anti-inflammatory drugs differentially modulate the threshold for pilocarpine-induced seizures.     

Limbic seizures without brain damage after injection of low doses of kainic acid into the amygdala of freely moving rats

Kainic acid (KA, 8-15 ng) was injected into the amygdala of conscious freely moving rats via chronically implanted fused silica cannulas. At 15-25 min after the injection, most rats suffered a limbic seizure attack of short duration, consisting of mastication, forelimb clonus, and raising on hind limbs, behaviorally indistinguishable from kindled seizures. Typically, the attack was followed by stereotypies, intense exploration, and by 1 or 2 more attacks. About 60 min after the injection, most rats appeared normal again and histopathological changes in their brains did not exceed those seen in vehicle-injected rats. In 3 cases, however, recurrent seizures culminated in behavioral status epilepticus 60-90 min after the injection. The status epilepticus was stopped by i.p. injection of diazepam (10 mg/kg) after a duration of 10 min (1 case) and 30 min (2 cases), respectively. After 10 min status epilepticus, we observed marginal neuronal damage with slight gliosis in both hippocampi (CA3 and CA1); after 30 min, hippocampal histopathology was more pronounced, with additional necrosis of the ipsilateral piriform cortex. After 0.8 microgram KA, a hundredfold higher dose, the incidence of limbic seizures during the first 40 min was not significantly higher (9/12) than after the lower KA doses (13/19). However, a significantly higher proportion of rats exhibited long-lasting seizure activity, associated with confluent destruction of CA3 pyramidal cells and additional seizure-related brain damage. Our results show that limbic motor seizures do not inevitably lead to histopathological changes in the brain, provided they do not culminate in a state of permanent seizure activity.     

Effects of morphine and naloxone on pilocarpine-induced convulsions in rats

Systemic administration of morphine hydrochloride (MF; 5-80 mg/kg; i.p.) in rats enhanced the epileptogenic potential of pilocarpine hydrochloride (PIL) in a dose-dependent manner. PIL, 100 mg/kg; i.p., which did not result in convulsions by itself, produced sustained limbic seizures and epileptic brain damage in MF-pretreated rats. MF-induced enhancement of PIL neurotoxicity was blocked by naloxone hydrochloride (NAL; 2 and 10 mg/kg; i.p.). Administration of NAL (2-20 mg/kg) prior to PIL in the dose of 380 mg/kg moderately decreased the incidence of convulsions, brain damage and lethal toxicity produced by this agent. These results support the hypothesis that opiate mechanisms are involved in the maintenance of the threshold for propagation of seizure activity within limbic circuits.     

An animal model of nonconvulsive status epilepticus: a contribution to clinical controversies

PURPOSE: To characterize electroencephalographic and behavioral effects as well as electrophysiologic and morphologic consequences of a subconvulsive dose of pilocarpine in lithium chloride-pretreated rats. METHODS: Pilocarpine (15 mg/kg) was administered intraperitoneally to adult rats pretreated with lithium chloride (3 mEq/kg, i.p.). Behavior was observed for 2 h and videotaped in three consecutive sessions. At the same time, EEG was recorded from the sensorimotor cortex and the dorsal hippocampus. Threshold intensities of currents necessary to elicit hippocampal afterdischarges were determined 24 h and 1 week after the pilocarpine administration. The brains were histologically examined 1 week after pilocarpine administration using Nissl stain. RESULTS: Pilocarpine induced time-limited nonconvulsive status epilepticus (NCSE). Epileptic EEG activity concurrent with prominent behavioral features was observed both in the neocortex and, predominantly, in the hippocampus. No changes in afterdischarge thresholds were observed in the dorsal hippocampus 24 h and 1 week after NCSE. One week after NCSE, seizure-related brain damage was found mainly in the motor neocortical fields. CONCLUSIONS: Pilocarpine-induced NCSE in rats strongly resembles a short-term human complex partial status epilepticus. Our animal model is suitable for studying the possible adverse effects of prolonged nonconvulsive seizures.     

Acute behavioral and EEG effects of NW-1015 on electrically-induced afterdischarge in conscious monkeys

NW-1015 is a novel Na+ and Ca2+ channel blocker with broad spectrum anticonvulsant activity and an excellent safety margin. As the compound also shows sigma-1 receptor ligand properties it was deemed important to determine whether it possesses anticonvulsant properties in primates without causing behavioral and EEG abnormalities. Thus, the effects of NW-1015 on limbic electrically-induced afterdischarge (AD) were evaluated in four cynomolgus monkeys, and its activity compared to a single effective dose of phenytoin (PHT). The four male cynomolgus monkeys were chronically implanted for EEG recordings, from cortex and limbic structures. AD was induced in limbic areas by electrical stimulation. The effects of NW-1015 on the duration and the behavioral component of the AD were randomly tested at doses from 25 to 75 mg/kg and compared with the effects of PHT 50 mg/kg. Similarly to PHT, 50 mg/kg of NW-1015 significantly shortened the EEG AD and almost abolished AD elicited behavioral seizure. Only the behavioral effects of AD were reduced after administration of 25 mg/kg p.o. NW-1015 did not cause EEG or interictal behavioral alterations at doses up to 75 mg/kg p.o. These data further confirm the broad-spectrum anticonvulsant activity and a good safety profile of NW-1015 even in a primate model of complex partial seizures and suggest that its affinity for sigma-1 receptors is behaviorally irrelevant.     

Repeated low-dose treatment of rats with pilocarpine: low mortality but high proportion of rats developing epilepsy.

Systemic administration of pilocarpine in rats can result in a chronic behavioral state that is similar to human temporal lobe epilepsy. The pilocarpine model of epilepsy is widely used for studying the factors that contribute to the development of epilepsy as a consequence of status epilepticus (SE). For this purpose, pilocarpine is either administered alone at a high systemic dose or in combination with lithium, which markedly potentiates the convulsant effect of pilocarpine. Both experimental protocols, however, are associated with high mortality rates. In the present study, we evaluated whether mortality rate in rats can be decreased by repeated administration of low doses of pilocarpine. The time the rats spent in SE was limited by diazepam. Preliminary experiments in lithium-free rats indicated that repeated low-dose administration of pilocarpine is too time-consuming to produce SE compared to single high-dose administration. All subsequent experiments were performed in lithium-pretreated rats. Single-dose injection of 30 mg/kg pilocarpine produced SE in approximately 70% of the animals, but 45% of the rats died although SE was interrupted by diazepam after 90 min. Repeated i.p. administration of 10 mg/kg pilocarpine at 30-min intervals resulted in SE after 2-4 injections; the mean dose of pilocarpine needed to induce SE was 26 mg/kg. When SE was interrupted after 90 min, mortality rate was below 10%, which was significantly lower compared to the protocol with one single administration of 30 mg/kg pilocarpine. In contrast to mortality rate, the development of spontaneous recurrent seizures did not differ between experimental protocols. Almost all rats which had experienced a SE of at least 60 min developed chronic epilepsy. Average latency to the first spontaneous seizure was approximately 40 days. The frequency and severity of spontaneous seizures was not significantly different between protocols, although animal groups with repeated low-dose treatment tended to have higher frequencies of spontaneous seizures compared to single-dose administration. The present study demonstrates that systemic treatment of lithium-pretreated rats with several low doses of pilocarpine efficiently produces SE and chronic epilepsy with much lower mortality rates than single-dose pilocarpine.     

Effects of drugs on the initiation and maintenance of status epilepticus induced by administration of pilocarpine to lithium-pretreated rats

The ability of various drugs to prevent the onset of status epilepticus induced by administration of the muscarinic agonist, pilocarpine, to lithium-pretreated rats was determined. Motor limbic seizures and status epilepticus occurred in 100% of rats administered pilocarpine (30 mg/kg, s.c.) 20 h after pretreatment with lithium (3 meq/kg, i.p.). The latency to spike activity and to status epilepticus was 20 +/- 1 min and 24 +/- 1 min, respectively. Atropine, diazepam, phenytoin, carbamazepine, phenobarbital, paraldehyde, and L-phenylisopropyladenosine (L-PIA) prevented all phases of seizure activity induced by lithium/pilocarpine treatment. The initiation of status epilepticus was significantly prolonged by pretreatment with sodium valproate. These findings indicate that the seizures induced by administration of lithium and pilocarpine accurately model generalized tonic-clonic epilepsy. The anticonvulsant activity of L-PIA was prevented by prior treatment with the adenosine antagonist, theophylline. The latency to spike and seizure activity was decreased by theophylline, indicating that endogenous adenosine may have a tonic inhibitory influence on cholinergic neurons. Atropine, diazepam, phenobarbital, phenytoin, sodium valproate, L-PIA, and carbamazepine did not interrupt seizure activity when administered 60 min after pilocarpine (approximately 35 min after initiation of status epilepticus). When rats were administered paraldehyde at this time, status epilepticus was rapidly terminated and all rats survived. Thus, status epilepticus induced by lithium and pilocarpine provides a seizure model that is not responsive to conventional anticonvulsants.     

Seizures produced by pilocarpine: Neuropathological sequelae and activity of glutamate decarboxylase in the rat forebrain

Morphological analysis of brains from rats receiving a convulsant dose of the muscarinic cholinergic agonist, pilocarpine hydrochloride (380 mg/kg), revealed a widespread damage to the forebrain as assessed by light microscopy 5–7 days after seizures. The substantia nigra, olfactory cortex, amygdala, hippocampus, septum, temporal cortex and thalamus underwent prominent morphological injury and cell loss. A concurrent assessment of the activity of -glutamate decarboxylase (GAD), the γ-aminobutryrate (GABA) synthesizing enzyme, demonstrated marked deficits in GAD activity in the brain regions undergoing morphological insult. Diazepam, 10 mg/kg, and scopolamine hydrochloride, 10 mg/kg, administered 30 min prior to the injection of pilocarpine, 380 mg/kg, prevented acute behavioral and electrographic, and long-term morphological and biochemical sequelae of seizures. These findings suggest that the muscarinic antagonist, scopolamine, and the anticonvulsant benzodiazepine, diazepam, may aid in preventing extensive brain damage related to pathological muscarinic cholinergic overactivity. The similarity of the topography of the damage and deficits in the GAD activity in brains of rats treated with pilocarpine indicates that GABAergic neurons are lost in the subregions of the brain preferentially sensitive to the convulsant action of pilocarpine.     

Antioxidant response analysis in the brain after pilocarpine treatments

Cholinergic and gabaergic systems play an important role generating electroencephalographic activity and regulating vigilance states. Pilocarpine is a cholinergic agonist commonly used to induce seizures and an epilepticus-like state in rodents. A relationship between status epilepticus and reactive oxygen species has been also suggested which could result in seizure-induced neurodegeneration. The aim of this study was to evaluate the existence of oxidative damage as well as the antioxidant enzyme response in cortex and hippocampus after the administration of an intraperitoneal (350 mg/kg) and an intracerebroventricular (360 microg, 1 microl) pilocarpine injection in rats. The GABA agonist muscimol (1 mg/kg, i.p.), with described neuroprotective properties, was used as a negative control. Only systemic pilocarpine induced oxidative damage. Malondialdehyde levels, as a marker of lipid peroxidation (LP), increased in both regions (55-56%). Catalase (52-80%) and superoxide dismutase (53-60%) activities also rose in both regions but glutathione peroxidase activity only increased in cortex (45%). Glutathione reductase and caspase-3 activity did not change. In conclusion, systemic pilocarpine produced oxidative brain damage, whereas local pilocarpine brain injection had no effects. Moreover, the enzymatic determinations performed in this study are a good tool to study brain injury in pharmacological manipulations such as the ones used in short recording EEG studies.     

Effects of thyrotropin-releasing hormone (TRH) on status epilepticus in rats

Recent studies have demonstrated that intramuscular administration of thyrotropin-releasing hormone (TRH) or its analogue improves various clinical aspects of intractable epilepsy such as Lennox-Gastaut syndrome, West syndrome, and myoclonus epilepsy. Other clinical studies reported efficient property of intravenous TRH against status epilepticus. However, it is also true that intravenous TRH produces epileptic seizures in patients with epilepsy or organic brain damage. Thus, the utility of intravenous TRH for the treatment of status epilepticus seems to be equivocal. To further explore the problem in this regard, we examined the effect of TRH on limbic status epilepticus in rats. Thirty-eight male Wistar rats weighing 180-220g were used. Status epilepticus was induced by intracerebral injection of a combination of 200 micrograms of dibutyryl-cAMP (db-cAMP) and 67.2ng of ethylenediaminetetraacetic acid (EDTA) into the amygdala (AM) through an implanted cannula. 30 min later, TRH or vehicle (distilled water) was administered intravenously (i.v.) or intracerebroventricularly (i.c.v.). Although 3 mg/kg of TRH (n = 9), when injected i.v., did not alter the pattern of electroclinical ictal responses induced by db-cAMP/EDTA, 25 mg/kg (n = 5) and 50 mg/kg (n = 5) of TRH significantly exaggerated EEG and/or behavioral ictal seizures, beginning immediately after the injection and lasting for more than 30 min. With 50 mg/kg of TRH, the exaggerated seizure patterns were followed by marked suppression of electroclinical seizures. 50 micrograms of i.c.v. TRH (n = 5), like higher doses of i.v. TRH, caused a slight, but not a significant, build up of electroclinical ictal seizures, beginning immediately after the injection and lasting for about 30 min.(ABSTRACT TRUNCATED AT 250 WORDS)     

Excitatory neurotransmitters in the lateral habenula and pedunculopontine nucleus of rat modulate limbic seizures induced by pilocarpine.

The involvement of the excitatory neurotransmitter system in the lateral habenula and pedunculopontine nucleus in the initiation and propagation of limbic seizures induced by pilocarpine has been investigated in the rat. Limbic seizures occur in animals following bilateral microinjection into the lateral habenula of N-methyl-D-aspartate (NMDA) (5 and 12.5 nmol) or kainate (100 and 200 pmol), 15 min prior to a subconvulsant dose of pilocarpine (150 mg/kg, i.p.). In the absence of pilocarpine NMDA (5 and 12.5 nmol) or kainate (100 and 200 pmol), injected focally into the lateral habenula or pedunculopontine nucleus, produced sniffing, grooming and tremor but no electrographic or behavioural seizures. Limbic seizures also occur after a subconvulsant dose of pilocarpine when it is preceded by injection of NMDA (5 and 12.5 nmol) or kainate (50, 100 and 200 pmol) into the pedunculopontine nucleus. Behavioural and electrographic signs of limbic seizures following pilocarpine (380 mg/kg, i.p.) were attenuated or completely antagonized by focal injection into the lateral habenula of the NMDA antagonist, 2-amino-7-phosphonoheptanoate (AP7) (10 and 50 pmol) or kainate antagonist, gamma-D-glutamylaminomethylsulphonate (GAMS) (20 nmol). In addition, AP7 (0.05, 0.1 and 1.0 nmol) or GAMS (40 nmol) injected into the pedunculopontine nucleus suppressed limbic seizures induced by i.p. administration of pilocarpine (380 mg/kg). The relative efficacy of NMDA and non-NMDA receptor antagonists revealed that the selective NMDA antagonist, AP7, was more potent in its anticonvulsant activity in comparison to GAMS, a non-NMDA receptor antagonist.     

Status epilepticus induced by lithium-pilocarpine in the immature rat does not change the long-term susceptibility to seizures

The causal relationship between early seizures and subsequent temporal lobe epilepsy has not yet been established. Prospective clinical studies reported that seizures occurring early in life rarely result in hippocampal sclerosis. Likewise, in most experimental models, early seizures occurring before the end of the second postnatal week do not lead to neuronal damage and subsequent epilepsy. In some models, this early event decreases latency sensitivity and threshold to seizures. In the present study, we induced lithium and pilocarpine status epilepticus (SE) in 10-day-old (P10) rats. The goal of this study was to determine whether this early life SE altered the sensitivity to convulsants such as pentylenetetrazol (20 and 25 mg/kg), picrotoxin (2.5 and 4.0 mg/kg) and kainate (5 and 8 mg/kg) during adulthood. The occurrence of electrographic seizures (spike-and-wave discharges, SWD) and/or of behavioral seizures was monitored. There was no difference in latency to and duration of SWDs and seizures between lithium-saline and lithium-pilocarpine exposed rats. Thus, SE induced by lithium and pilocarpine early in life does not change the sensitivity to limbic seizures or seizures induced by GABA(A) antagonists during adulthood.     

Review: cholinergic mechanisms and epileptogenesis. The seizures induced by pilocarpine: a novel experimental model of intractable epilepsy

High-dose treatment with pilocarpine hydrochloride, a cholinergic muscarinic agonist, induces seizures in rodents following systemic or intracerebral administration. Pilocarpine seizures are characterized by a sequential development of behavioral patterns and electrographic activity. Hypoactivity, tremor, scratching, head bobbing, and myoclonic movements of the limbs progress to recurrent myoclonic convulsions with rearing, salivation, and falling, and status epilepticus. The sustained convulsions induced by pilocarpine are followed by widespread damage to the forebrain. The amygdala, thalamus, olfactory cortex, hippocampus, neocortex, and substantia nigra are the most sensitive regions to epilepsy-related damage following convulsions produced by pilocarpine. Spontaneous seizures are observed in the long-term period following the administration of convulsant doses of pilocarpine. Developmental studies show age-dependent differences in the response of rats to pilocarpine. Seizures are first noted in 7-12 day-old rats, and the adult pattern of behavioral and electroencephalographic sequelae of pilocarpine is seen in 15-21-day-old rats. During the third week of life the rats show an increased susceptibility to the convulsant action of pilocarpine relative to older and younger animals. The developmental progress of the convulsive response to pilocarpine does not correlate with evolution of the brain damage. The adult pattern of the damage is seen after a delay of 1-2 weeks in comparison with the evolution of seizures and status epilepticus. The susceptibility to seizures induced by pilocarpine increases in rats aged over 4 months. The basal ganglia curtail the generation and spread of seizures induced by pilocarpine. The caudate putamen, the substantia nigra, and the entopeduncular nucleus govern the propagation of pilocarpine-induced seizures. The antiepileptic drugs diazepam, clonazepam, phenobarbital, valproate, and trimethadione protect against pilocarpine-induced convulsions, while diphenylhydantoin and carbamazepine are ineffective. Ethosuximide and acetazolamide increase the susceptibility to convulsant action of pilocarpine. Lithium, morphine, and aminophylline also increase the susceptibility of rats to pilocarpine seizures. The pilocarpine seizure model may be of value in designing new therapeutic approaches to epilepsy.