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.     

Kainic acid induced limbic seizures: metabolic, behavioral, electroencephalographic and neuropathological correlates

Increasing amounts (0.3–12 mg/kg) of kainic acid (KA) were given intravenously to rats and behavioral, electrographic, 2-deoxyglucose autoradiographic and neuropathologic responses were studied. A dose of 12 mg/kg kainic acid caused a stereo-typed sequence of staring spells, wet dog shakes, automatisms — mild limbic convulsions and severe limbic convulsions (rearing, bilateral upper extremity clonus, falling and salivation) that developed over 1–2 h. Smaller doses showed different thresholds for these behavioral phenomena but a similar time course of development. Electrographic seizures in limbic areas were similar to afterdischarges produced with electrical stimulation and their threshold was 4 mg/kg. The earliest electrical changes occurred in the hippocampus and were accompanied by staring spells. Subsequently seizures were synchronized in limbic centers during which mild convulsive activity was seen. Finally electrical seizures appeared in both limbic and surface leads, coincident with severe convulsions. Quantitative deoxyglucose autoradiography showed that the hippocampul CA₃ region was most sensitive to metabolic changes. Mild limbic convulsions were associated with increased glucose utilization in the hippocampus, subiculum, pyriform and entorhinal cortices, septum and amygdala. Severe limbic convulsions led to even larger changes in these areas as well as the substantia nigra and part of the thalamus. Neuronal damage was seen without systemic metabolic derangements with a threshold of 7 mg/kg. The regions of neuropathology overlapped with areas of intense seizure activity.These data show that systemic kainic acid preferentially activates seizures in the limbic system, particularly the hippocampus, and that different behavioral concomitants of limbic seizures depend on specific patterns of activation of limbic and extra limbic circuits. A scheme for the anatomic spread of seizures in limbic and non-limbic structures is proposed. A state of ‘limbic status’ underlies the neurotoxicity of kainic acid.     

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.     

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.     

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.     

Pilocarpine-induced epileptogenesis in the rat: impact of initial duration of status epilepticus on electrophysiological and neuropathological alterations

This study characterized the electrophysiological and neuropathological changes in rat brains caused by pilocarpine (PILO)-induced status epilepticus (SE) of different duration. SE induced by PILO (375 mg/kg, i.p. adm.) were terminated with a bolus dose of diazepam (10 mg/kg, i.v. adm.) injected 7.5, 15, 30, 60 or 120 min after initiation of the secondary generalization of the SE. Three weeks later, the gain in body weight was significantly reduced in the rats exposed to PILO-induced SE lasting 30 min or more, when compared to controls. Spontaneous seizures were not detected in rats with PILO-induced SE of 7.5 min duration whereas 50 and 25% of the rats exposed to seizure durations of 30 and 120 min expressed motor seizures. Significant alterations reflecting hyperexcitability (increased number of population spikes (PSs)) and reduced paired-pulse inhibition were observed in recordings of hippocampal field potentials from rats with PILO-induced SE of at least 30 min duration. This was substantiated by brain lesions (necrosis in olfactory cortex, hippocampus, amygdala and thalamus) in all rats manifesting a SE of at least 30 min duration. Thus, the results of the present study demonstrate that rats exposed to PILO-induced SE of at least 30 min duration manifest an epileptogenic process, revealed 3 weeks later by several parameters. Among these, hippocampal field potentials appear to represent the most sensitive marker, potentially useful for pharmacological evaluation of drugs with putative antiepileptogenic properties.     

Substantia nigra regulates action of antiepileptic drugs

The cholinergic agonist pilocarpine triggers sustained limbic seizures in rodents. Pilocarpine seizures were blocked by systemic administration of benzodiazepines, barbiturates, valproate and trimethadione, while diphenylhydantoin did not affect, and ethosuximide increased the susceptibility of rats to such seizures. This pattern of action antiepileptic drugs is characteristic for pilocarpine seizures and different from other rodent models of epilepsy. Although the anatomical substrates in the forebrain involved in the expression of anticonvulsant activity are unknown, the basal ganglia are believed to be essential for the motor expression of pilocarpine seizures. Bilateral microinjections into the substantia nigra, a major output station of the basal ganglia, of midazolam (ED₅₀ 38.5 nmol; range 29–52 nmol), phenobarbital (ED₅₀ 16 nmol; range 7–39 nmol) and trimethadione (ED₅₀ 30 nmol; range 16–56 nmol) protected rats against pilocarpine seizures (380 mg/kg i.p.). Diphenylhydantoin (up to 100 nmol) remained inactive, while ethosuximide (ED₅₀ 38 nmol; range 22–65.5 nmol) reduced the threshold for pilocarpine seizures, converting subconvulsant doses of pilocarpine (200 mg/kg i.p.) into convulsant ones. The profiles of action of antiepileptic drugs on pilocarpine seizures were similar following intranigral and systemic administration. These observations suggest that the substantia nigra may mediate some actions of antiepileptic drugs.     

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.     

Influence of nicardipine,nimodipine and flunarizine on the anticonvulsant efficacy of antiepileptics against pentylenetetrazol in mice

Summary Among three calcium channel inhibitors, only nicardipine (10–40mg/kg) significantly inhibited clonic seizures induced by pentylenetetrazol administered at its CD₉₇ (convulsive dose 97%) of 81mg/kg, subcutaneously. Nimodipine and flunarizine (both up to 80mg/kg) did not suppress pentylenetetrazol-induced clonic seizures per se. Co-administration of nicardipine (5 mg/kg) resulted in a significant enhancement of the protective potency of either ethosuximide (50mg/kg) or valproate (100 mg/kg) against clonic seizures in this test. Similar effects were noted in case of combined treatment of nimodipine (20–40mg/kg) with these antiepileptics. On the contrary, flunarizine (up to 20 mg/kg) did not modify the anticonvulsive action of these antiepileptic drugs. Moreover, none of the studied calcium channel inhibitors influenced the protective activity of clonazepam (0.01 mg/kg). The antiepileptic drugs, administered alone in above doses, were ineffective against pentylenetetrazol-induced clonic convulsions. In case of ethosuximide and valproate, the motor performance in the chimney test was worsened by co-administration of nimodipine (40mg/kg). We found no pharmacokinetic interactions (at least in relation to the plasma levels of ethosuximide and valproate) that could explain the observed results. Thus, we conclude that a combination of some calcium channel inhibitors and antiepileptic drugs may provide more efficient protection against experimental seizures which may bear a potential clinical significance.     

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.     

Dopamine control of seizure propagation: intranigral dopamine D1 agonist SKF-38393 enhances susceptibility to seizures

The involvement of dopamine (DA) in human and experimental epilepsy has been discounted as DAergic drugs have little effect on convulsions. This work presents evidence that bilateral microinjection of the DAD1 agonist SKF-38393 into the substantia nigra enhances the susceptibility of rats to seizures, with an ED50 of 20 pmol (range 13-31 pmol), converting subconvulsant doses of the cholinergic agonist pilocarpine (200 mg/kg; i.p.) into convulsant ones. The proconvulsant action of SKF-38393 was reversed by blocking D1-mediated transmission in the substantia nigra with the D1 antagonist SCH-23390. The D2 agonist LY-171555 did not modulate the threshold for limbic seizures when injected into the substantia nigra. In the striatum, the D2 agonist LY-171555 protected rats against limbic seizures induced by systemic administration of pilocarpine (380 mg/kg; i.p.), with an ED50 of 2 pmol (range 1.4-2.8 pmol). The anticonvulsant action of LY-171555 in the striatum was reversed by haloperidol. The D1 agonist SKF-38393 did not affect pilocarpine seizures following administration into the striatum. Systemic administration of DAergic drugs showed that the D1 agonist SKF-38393 decreased the threshold for pilocarpine seizures, with an ED50 of 0.81 mg/kg (range 0.45-1.47 mg/kg), whereas the D2 agonist LY-171555 had no effect on susceptibility of rats to pilocarpine. The proconvulsant action of SKF-38393 was blocked by the D1 antagonist SCH-23390. These results suggest that DA differentially modulates seizure threshold in the forebrain acting via D1 mechanisms in the substantia nigra and D2 mechanisms in the striatum.     

Anticonvulsant and neuroprotective effects of the novel nootropic agent nefiracetam on kainic acid-induced seizures in rats

Nefiracetam is a novel pyrrolidone-type nootropic agent, and it has been reported to possess a potential for antiepileptic therapy as well as cognition-enhancing effects. We investigated the anticonvulsant and neuroprotective effects of nefiracetam in kainic acid-induced seizures of rats, compared with levetiracetam and standard antiepileptic drugs. Subcutaneous injection of kainic acid (10 mg/kg) induced typical behavioral seizures such as wet dog shakes and limbic seizures and histopathological changes in the hippocampus (degeneration and loss of pyramidal cells in CA1 to CA4 areas). Nefiracetam (25, 50 and 100 mg/kg po) had no effect on the behavioral seizures and dose-dependently inhibited the hippocampal damage. In contrast, levetiracetam, a pyrrolidone-type antiepileptic drug, inhibited neither. Valproic acid and ethosuximide prevented the hippocampal damage without attenuating the behavioral seizures as nefiracetam. Zonisamide and phenytoin did not inhibit the behavioral seizures, while zonisamide enhanced the hippocampal damage and phenytoin increased the lethality rate. Carbamazepine inhibited the behavioral seizures at 50 mg/kg and enhanced that at 100 mg/kg, and it completely inhibited the hippocampal damage at both doses. We have previously reported that anticonvulsant spectrum of nefiracetam paralleled that of zonisamide, phenytoin or carbamazepine in standard screening models. However, the pharmacological profile of nefiracetam was closer to valproic acid or ethosuximide than that of zonisamide, phenytoin or carbamazepine in this study. These results suggest that anticonvulsant spectrum and mechanism of nefiracetam are distinct from those of standard antiepileptic drugs, and nefiracetam possesses a neuroprotective effect that is unrelated to seizure inhibition.     

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.     

Stimulant and convulsive effects of kynurenines injected into brain ventricles in mice

Summary Each of six kynurenines tested (DL-kynurenine, quinolinic, 3-hydroxy-anthranilic, xanthurenic, picolinic, and nicotinic acids) injected into brain ventricles in mice in doses of 25–50 mcg produced motor excitement and/or clonic convulsions. Anthranilic acid did not produce these effects. The strongest metabolite was quinolinic acid, which was active in a dose of 1 mcg. It was also the only compound which produced motor excitement and convulsions after intraperitoneal injection (in doses of 400–600 mg/kg,i.e. 10,000–15,000 mcg per mouse). The hypothermic effect of intraventricularly-injected kynurenines was roughly similar to that of intraperitoneally-injected material at 100–1000 times higher doses. These data suggest poor penetration of kynurenines formed in the liver into the brain, and the possible involvement of these metabolites of tryptophan (particularly if they are formed inside the brain) in the mechanism of seizures.     

Effect of temperature on kainic acid-induced seizures

The effects of body temperature on kainic acid-induced seizures and seizure-related brain damage were examined in rats. In rats with status epilepticus induced by intraperitoneal injection of 12 mg/kg of kainic acid (KA), ictal discharges were decreased by 50% when body temperature was lowered to 28°C and nearly abolished when body temperature was lowered to 23°C. In rats with mild hypothermia (28°C), the duration of ictal discharges following KA injection was significantly lower than in rats with normal body temperature. No detectable hippocampal cell loss was observed in rats with hypothermia to 28°C whereas gross cell loss in the hippocampus was observed in all rats with KA injection at normal body temperature. In contract to hypothermia, hyperthermia markedly aggravated the seizures and hippocampal damage induced by KA. Following elevation of body temperature to 42°C KA (12 mg/kg) resulted in severe seizures and all rats died of tonic seizures within 2 h. Furthermore, 6 mg/kg of KA administered to rats with a body temperature of 41–42°C, resulted in up to 4 h of continuous ictal discharges whereas no continuous ictal discharges were observed after the same injections in rats with normal body temperature. Histological examination in rats receiving 6 mg/kg of KA revealed severe cell loss in the hippocampus in rats with hyperthermia but not in rats with normal temperature. These results demonstrate that body temperature plays an important role in the control of epileptic seizures and seizure-related brain damage. These data suggest that hypothermia may be useful in reducing seizures and associated brain damage and that hyperthermia should be avoided in status epilepticus.     

Seizures produced by pilocarpine in mice: A behavioral, electroencephalographic and morphological analysis

Increasing doses of pilocarpine, 100-400 mg/kg, were given intraperitoneally to mice and the resulting behavioral, electroencephalographic and neuropathological alterations were studied. No behavioral phenomena were observed in mice treated with the lowest dose of pilocarpine. Occasional tremor and myoclonus of hindlimbs were found in animals which received pilocarpine in a dose of 200 mg/kg. At doses of 300, 325 and 350 mg/kg, pilocarpine produced a sequence of behavioral alterations including staring spells, limbic gustatory automatisms and motor limbic seizures that developed over 15-30 min and built up progressively into a limbic status epilepticus lasting for several hours. The highest dose of pilocarpine, 400 mg/kg, was generally lethal to mice. Pilocarpine produced both interictal and ictal epileptiform activity in the electroencephalogram (EEG). The earliest EEG alterations appeared in the hippocampus and then spread to cortical areas. EEG seizures started 10-15 min after injection of large doses of pilocarpine, 300-350 mg/kg. Ictal periods lasted for 1-2 min, recurred every 5-10 min and were followed by periods of depression of the EEG activity. By 30-45 min paroxysmal activity resulted in a status epilepticus. Examination of frontal forebrain sections with light microscopy revealed a widespread damage to several brain regions including the hippocampus, amygdala, thalamus, olfactory cortex, neocortex and substantia nigra. Scopolamine, 10 mg/kg, and diazepam, 10 mg/kg, prevented the development of convulsive activity and brain damage produced by pilocarpine. The results emphasize that excessive and sustained stimulation of cholinergic receptors can lead to seizures and seizure-related brain damage in mice. It is proposed that systemic pilocarpine in mice provides a useful animal model for studying mechanisms of and therapeutic approaches to temporal lobe epilepsy.     

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)     

Mitogen-activated protein kinase is increased in the limbic structures of the rat brain during the early stages of status epilepticus

Systemic administration of pilocarpine (PILO) in adult rat produces acute limbic seizures leading to status epilepticus. Recent studies have shown the activation of mitogen-activated protein kinase (MAPK) cascades during experimentally induced seizures. MAPK activation may be triggered by glutamatergic stimulation and may play a key role in signal transduction pathways. In the present study, immunocytochemistry was used to analyze the spatiotemporal distribution pattern of the MAPK protein and its active form (A-MAPK) following PILO-induced status epilepticus. MAPK and A-MAPK immunoreactivities exhibited different patterns of distribution in the brain of normal and epileptic rats. The saline-treated rats, as well as the animals that received PILO but did not evolve to status epilepticus, showed a weak but selective MAPK immunoreactivity, detected in the hippocampal pyramidal neurons, dentate gyrus, hilus, CA3, CA1, and entorhinal, piriform, and cingulate cortices. A-MAPK immunoreactivity was instead observed only in neurites of the CA3 and hilus and in cells of the entorhinal and piriform cortices. In PILO-treated rats, between 30 and 60 min after status epilepticus there was an increase of the immunoreactivity to both antibodies, which were differently distributed throughout several structures of the limbic system. The immunostaining showed a slight decrease after 5 h of status epilepticus. However, MAPK and A-MAPK immunopositivities decreased markedly after 12 h of status epilepticus, returning almost to the basal expression. These findings are consistent with a spatial and time-dependent MAPK expression in selected limbic structures, and its activation could represent an initial trigger for neuronal modifications that may take part in the mechanism underlying acute epileptogenesis and in long-lasting neuropathological changes of the PILO model of epilepsy.     

Contrasting effects of zonisamide and acetazolamide on amygdaloid kindling in rats

PURPOSE: Zonisamide (ZNS) and acetazolamide (AZM) are two antiepileptic drugs (AEDs) that differ in clinical efficacy. To elucidate the mechanisms of action of these compounds, we investigated their therapeutic and prophylactic effects in rats by using a kindling model of partial epilepsy. METHODS: Electrodes were implanted into the left amygdala of adult male Wistar rats. The animals were stimulated at the afterdischarge threshold until five stage 5 seizures were induced. The generalized seizure threshold was then determined. Therapeutic effects were examined in rats manifesting successive convulsions with near-threshold stimulation. To test prophylactic effects, drugs were administered intraperitoneally before daily kindling stimulation until the animal had a stage 5 seizure or reached day 18. RESULTS: ZNS (10-40 mg/kg; n=6) suppressed kindled seizures in a dose-dependent manner. Repeated administration for 7 days produced tolerance to anticonvulsive effects. AZM (25-200 mg/kg; n=7) showed limited therapeutic effect, alleviating only the clonic convulsion in stage 5 seizures and reducing afterdischarge duration. Secondary generalization was not significantly suppressed during repeated treatment (50-200 mg/kg; n=6). ZNS, 25 or 40 mg/kg (n=8), significantly retarded seizure development; 15.0 or 17.0 daily stimulations were required to produce a stage 5 seizure. AZM, 50-200 mg/kg (n=6), also retarded seizure development, with 14.0-14.8 stimulations required. CONCLUSIONS: ZNS exhibited modest therapeutic and prophylactic effects, whereas AZM showed mainly prophylactic effects. Hypotheses are presented that may explain the mechanisms of action of these drugs.     

Kainic acid-induced seizures: Dose-relationship of behavioural neurochemical and histopathological changes

Behavioural, neurochemical and histopathological changes induced by systemic injection of kainic acid were investigated at various doses of the neurotoxin (3, 6 and 10 mg/kg s.c.). There was a positive correlation between the dose of kainic acid and the extent of both the acute neurochemical changes 3 h after the injection (increases of 3, 4-dihydroxyphenylacetic acid and 5-hydroxyindoleacetic acid levels and a decrease in noradrenaline levels in all brain regions investigated), the acute histopathological changes (shrinkage and condensation of nerve cells and brain oedema in the entire forebrain) and the extent of behavioural alterations (immobility, ‘wet dog shakes’ and limbic seizures). However, the slope of the dose-response curves was very steep. Late and irreversible alterations included losses of the enzyme markers glutamic acid decar☐ylase and choline acetyltransferase and, histopathologically, incomplete parenchymal necrosis and haemorrhages. These changes, however, were restricted to a few brain regions, the most important being the hippocampus, amygdala, entorhinal and pyriform cortex, and olfactory bulb, and they were seen only in animals which had undergone severe convulsions. It is suggested that the irreversible brain lesions in this animal model of limbic (temporal lobe) epilepsy are not solely induced by a direct action of kainic acid, but may be caused — at least in part — by additional, secondary pathogenetic mechanisms.