Limbic Status Epilepticus Research Articles

Anatomical studies of DNA fragmentation in rat brain after systemic kainate administration

Rats treated systemically with kainate develop stereotyped epileptic seizures involving mainly limbic structures that may last for hours. This model of limbic status epilepticus has been widely studied using classical neuropathological techniques. We used in situ nick translation histochemistry to examine patterns of DNA fragmentation in this model. We found a stereotyped and reproducible pattern of neuronal populations that demonstrate evidence of DNA fragmentation from 24 h to one week after kainate treatment. Neither blockade of new protein synthesis nor blockade of the N-methyl- d-aspartate-type glutamate receptors significantly altered this response. Moreover, we saw no evidence of the regular internucleosomal cleavage of DNA that produces a characteristic laddered appearance of 180–200 bp DNA fragments after gel electrophoresis in samples obtained from microdissected affected regions. These studies suggest that DNA fragmentation after systemic kainate-induced seizures is not the result of programmed cell death. This assay may be useful for quantitative testing of both neuroprotective agents and mechanistic hypotheses.

P-21-1 - Changes of the lymphocyte subset in major depressive disorder and schizophrenia

This study compared temporal lobe epilepsy patients, along with kindled animals and self sustained limbic status epilepticus (SSLSE) rats for parallels in hippocampal AMPA and NMDA receptor subunit expression. Hippocampal sclerosis patients (HS), non-HS cases, and autopsies were studied for: hippocampal AMPA GluR1-3 and NMDAR1&2b mRNA levels using in situ hybridization; GluR1, GluR2/3, NMDAR1, and NMDAR2a&b immunoreactivity (IR); and neuron densities. Similarly, spontaneously seizing rats after SSLSE, kindled rats, and control animals were studied for: fascia dentata neuron densities; GluR1 and NMDAR2a&b IR; and neo-Timm's staining. In HS and non-HS cases, the mRNA hybridization densities per granule cell, as well as molecular layer IR, showed increased GluR1 (relative to GluR2/3) and increased NMDAR2b (relative to NMDAR1) compared to autopsies. Likewise, the molecular layer of SSLSE rats with spontaneous seizures demonstrated more neo-Timm's staining, and higher levels of GluR1 and NMDAR2a&b IR compared to kindled animals and controls. These results indicate that hippocampal AMPA and NMDA receptor subunit mRNAs and their proteins are differentially increased in association with spontaneous, but not kindled, seizures. Furthermore, there appears to be parallels in fascia dentata AMPA and NMDA receptor subunit expression between HS (and non-HS) epileptic patients and SSLSE rats. This finding supports the hypothesis that spontaneous seizures in humans and SSLSE rats involve differential alterations in hippocampal ionotrophic glutamate receptor subunits. Moreover, non-HS hippocampi were more like HS cases than hippocampi from kindled animals with respect to glutamate receptors; therefore, hippocampi from kindled rats do not accurately model human non-HS cases, despite some similarities in neuron densities and mossy fiber axon sprouting.

Experimental status epilepticus alters gamma-aminobutyric acid type A receptor function in CA1 pyramidal neurons.

There is a reduction of gamma-aminobutyric acid (GABA)-mediated inhibition of the CA1 pyramidal region of the hippocampus during status epilepticus (SE). The cellular basis of this loss of GABA-mediated inhibition is not known. This study tested the possibility that GABA type A (GABAA) receptor function in CA1 pyramidal neurons was reduced or blocked during SE, at least in part by postsynaptic cellular mechanisms. GABAA receptor currents (IGABA) were studied by whole-cell patch-clamp techniques in CA1 pyramidal neurons acutely dissociated from rats undergoing lithium/pilocarpine-induced limbic status epilepticus (SE neurons) and from naive rats (naive neurons). SE neurons had more depolarized resting membrane potential (-17.3 mV) compared with naive neurons (-56 mV). IGABA was absent in 47% of SE neurons and reduced in 55% of the remainder, compared with naive neurons. The reduction in IGABA in SE neurons resulted from a combination of factors, including reduced potency and reduced efficacy of GABA in activating chloride channels, and diminished driving force for the GABA-induced chloride currents once activated. These postsynaptic cellular mechanisms resulted in a net reduction or loss in GABA-mediated inhibition and may explain previous in vivo findings reporting a loss of inhibition in hippocampus during limbic SE.

Changes in inhibitory neurotransmission in the CA1 region and dentate gyrus in a chronic model of temporal lobe epilepsy.

1. In this report we compare changes in inhibitory neurotransmission within the CA1 region and the dentate gyrus (DG) in a model of chronic temporal lobe epilepsy (TLE). Extracellular and intracellular recordings were obtained in combined hippocampal-parahippocampal slices > or = 1 mo after a period of self-sustaining limbic status epilepticus (SSLSE) induced by continuous hippocampal stimulation. 2. Polysynaptic inhibitory postsynaptic potentials (IPSPs) were induced by positioning electrodes to activate specific afferent pathways and evoking responses in the absence of glutamate receptor antagonists [D(-)-2-amino-5-phosphonovaleric acid (APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)]. Polysynaptic IPSPs were evoked in CA1 pyramidal cells from electrodes positioned in stratum radiatum and in stratum lacunosum/moleculare. Polysynaptic IPSPs were evoked in DG granule cells from electrodes positioned over the perforant path located in the subiculum. Monosynaptic IPSPs were induced by positioning electrodes within 200 microns of the intracellular recording electrode (near site stimulation) and stimulating in the presence of APV and CNQX to block ionotropic glutamate receptors. Monosynaptic IPSPs were evoked in CA1 pyramidal cells with electrodes positioned in the stratum lacunosum/moleculare and stratum pyramidale. Monosynaptic IPSPs were evoked in DG granule cells with electrodes positioned in the stratum moleculare. 3. Population spike (PS) amplitudes were employed to assure that a full range of stimulus strengths, from subthreshold for action potentials to an intensity giving maximal-amplitude PSs, was used to elicit polysynaptic IPSPs in CA1 pyramidal cells in both post-SSLSE and control slices. In control tissue, polysynaptic IPSPs were biphasic, composed of early and late events. In post-SSLSE tissue, polysynaptic IPSPs were markedly diminished. The diminution of polysynaptic IPSPs was detected at all levels of stimulus intensity. Both early IPSPs [mediated by gamma-aminobutyric acid-A (GABAA) receptors] and late IPSPs (mediated by GABAB receptors) were diminished. Polysynaptic IPSPs were diminished with both stratum radiatum and with stratum lacunosum/moleculare stimulation. 4. Reversal potentials for either polysynaptic early or polysynaptic late IPSPs evoked in CA1 pyramidal cells by stratum radiatum stimulation were not different in slices from post-SSLSE animals as compared with control animals. Likewise, reversal potentials for either polysynaptic early or polysynaptic late IPSPs evoked by stratum lacunosum/moleculare stimulation did not differ in the two groups. These findings excluded changes in driving force as an explanation for the diminished amplitude of IPSPs in CA1 pyramidal cells in the post-SSLSE model.(ABSTRACT TRUNCATED AT 400 WORDS)

Regional heterogeneity of pathophysiological alterations in CA1 and dentate gyrus in a chronic model of temporal lobe epilepsy.

1. Extracellular and intracellular recording techniques were employed in brain slice preparations to characterize responses of hippocampal tissue in the post-self sustaining limbic status epilepticus (post-SSLSE) model of chronic temporal lobe epilepsy (TLE) as compared with responses in slices from control animals. Experiments were performed > or = 1 mo, and up to 7 mo, after status epilepticus. Two regions of the hippocampal formation linked to different aspects of epileptogenesis, the CA1 region and the dentate gyrus (DG), were studied. In any given experiment, CA1 and DG were examined in different slices from the same animal. 2. Pyramidal cells in CA1 were activated by means of electrodes positioned over fiber bundles that monosynaptically project to these cells, either those located in the stratum lacunosum/moleculare or those in the stratum radiatum. Granule cells were similarly activated by electrodes positioned in the perforant path. Full input-output curves were determined by varying stimulus strength and charting the amplitudes of population spikes (PSs). 3. Two indexes, stimulus sensitivity and responsiveness, were quantified in control tissue and in post-SSLSE tissue by means of input-output curves to provide comparisons between normal and epileptic tissue. There were no changes in stimulus sensitivity, defined as the stimulus intensity required to evoke comparable responses in input-output curves, between control and post-SSLSE tissue. However, responsiveness, defined as the number of extracellular PSs or intracellular action potentials (APs) elicited by a stimulus strength giving rise to maximal-amplitude PSs, proved a reliable method for identifying and categorizing epileptic responses. This index allowed for comparisons between anatomic regions within an experiment as well as among experiments for the same region. Both CA1 pyramidal cells and DG granule cells from post-SSLSE tissue showed hyperresponsiveness relative to control tissue. 4. Control tissue never exhibited > 2 PSs in either CA1 or DG in response to stimuli that produced maximal-amplitude PSs. Therefore a criterion of > or = 3 PSs was adopted to delineate tissue as hyperresponsive on the basis of extracellular responses. In CA1 about one half of the post-SSLSE slices displayed > or = 3 PSs with stimuli giving maximal-amplitude PSs, meeting the criterion for hyperresponsiveness; in DG about one fifth of the slices showed hyperresponsiveness. 5. CA1 and DG differed with respect to the spectrum of hyperresponsiveness they exhibited, this being more robust in CA1. The two regions studied also showed heterogeneity with respect to maximal PS amplitudes.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in excitatory neurotransmission in the CA1 region and dentate gyrus in a chronic model of temporal lobe epilepsy.

1. In this report we compare changes of excitatory neurotransmission within the CA1 region and the dentate gyrus (DG) in a model of chronic temporal lobe epilepsy (TLE). Extracellular and intracellular recordings were obtained from in vitro hippocampal-parahippocampal slices > or = 1 mo after a period of self-sustaining limbic status epilepticus (SSLSE) induced by continuous hippocampal stimulation. Pyramidal cells in CA1 were activated by electrodes in the stratum lacunosum/moleculare or stratum radiatum. Granule cells in DG were similarly activated by electrodes positioned in the perforant path. 2. Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked in CA1 pyramidal cells in post-SSLSE tissue were always longer than those evoked in control tissue, irrespective of whether hyperresponsiveness was present or not. EPSPs elicited by stimulus subthreshold for action potentials (APs) in post-SSLSE and in control slices and matched in amplitude had a statistically greater duration in the post-SSLSE slices. Durations of monosynaptic EPSPs elicited by stimuli subthreshold for APs in DG granule cells in post-SSLSE slices were not longer than EPSPs of equal amplitude elicited in control slices. 3. Higher-intensity stimuli produced EPSPs with associated APs and, in certain cases in the post-SSLSE tissue, hyperresponsive events with multiple (> or = 3) APs. Durations of depolarizing profiles with stimuli producing APs were overall longer in both CA1 pyramidal cells and DG granule cells and correlated with the degree of hyperresponsiveness. 4. Neither the amplitudes nor the durations of monosynaptic EPSPs evoked in CA1 pyramidal cells in slices from control animals were affected by the addition of D(-)-2-amino-5-phosphonovaleric acid (APV), a blocker of the N-methyl-D-aspartate (NMDA) receptor, to the artificial cerebrospinal fluid (ACSF) bathing the slices. In contrast to the situation in control tissue, in post-SSLSE tissue APV shortened EPSPs evoked in CA1 pyramidal cells while not changing their amplitudes. After APV, inhibitory postsynaptic potentials (IPSPs) remained greatly diminished or absent in CA1 pyramidal cells. APV did not statistically decrease amplitudes of monosynaptic EPSPs evoked in DG granule cells in either control slices or post-SSLSE slices. APV decreased EPSP durations in both types of slices, more so in the post-SSLSE tissue. 5. In control slices, APV did not change the amplitudes or durations of depolarizing profiles of responses evoked by stimuli producing APs in CA1. Similarly, APV did not change the amplitudes of such responses in DG. However, APV did reduce the durations of such responses in DG in control slices.(ABSTRACT TRUNCATED AT 400 WORDS)

Opioid peptide expression in models of chronic temporal lobe epilepsy.

Expression of the opioid peptides dynorphin and enkephalin is altered within the first 24 h after acutely induced seizures in certain experimental models of epilepsy. Using in situ hybridization, we examined the expression of prodynorphin and preproenkephalin messenger RNA acutely following induction of kindling with recurrent seizures and in two models of chronic temporal lobe epilepsy: (i) rats fully kindled with rapidly recurring hippocampal seizures; and (ii) rats surviving after self-sustaining limbic status epilepticus induced with focal electrical stimulation of the hippocampus. In naive animals, a ventral-dorsal gradient was identified in the expression of both prodynorphin and preproenkephalin messenger RNA in the dentate gyrus and expression of prodynorphin message was demonstrated for the first time in the ventral portion of cornu Ammonis regio superior. After stimulation producing rapidly recurring hippocampal seizures, acute decreases in prodynorphin messenger RNA were seen in the dentate gyrus and cornu Ammonis regio superior at 24 h after the last seizure. In contrast, increases in preproenkephalin messenger RNA expression were seen acutely in the dentate gyrus, with a decrease seen in the entorhinal cortex. The change in prodynorphin message expression in cornu Ammonis regio superior persisted in kindled animals that were studied after one month seizure-free period. There were no changes in preproenkephalin message in kindled animals studied after the one month seizure-free interval. No statistically significant changes were found for either prodynorphin or preproenkephalin message in the post-self-sustaining limbic status epilepticus group at one month following induced seizures. Acute changes in peptide expression may contribute to increased excitation in the dentate gyrus during induction of kindling, while the chronic change identified in cornu Ammonis regio superior may contribute directly to persistently increased excitability in this region.

The pilocarpine model of epilepsy.

The systemic administration of a potent muscarinic agonist pilocarpine in rats promotes sequential behavioural and electrographic changes that can be divided in three distinct periods: (a) an acute period that built up progressively into a limbic status epilepticus and that lasts 24 h, (b) a silent period with a progressive normalization of EEG and behaviour which varies from 4 to 44 days, and (c) a chronic period with spontaneous recurrent seizures (SRSs). The main features of the SRSs observed during the long-term period resemble those of human complex partial seizures and recurs 2-3 times per week per animal. Therefore, this novel and unique experimental approach may serve as a model of epilepsy mimicking the human condition.

Periodic acid-Schiff (PAS)-positive deposits in brain following kainic acid-induced seizures: relationships to fos induction, neuronal necrosis, reactive gliosis, and blood-brain barrier breakdown.

Periodic acid-Schiff (PAS)-positive deposits have been demonstrated in the central nervous system (CNS) of patients suffering from a wide variety of neurodegenerative disorders including Alzheimer's disease, presenile dementia, Parkinson's disease, diabetes mellitus, myoclonic epilepsy, and cerebral palsy. The etiology of these deposits and their relationship to mechanisms of progressive neurodegeneration is unknown. In the present study, we demonstrate that the kainic acid model of limbic status epilepticus provides a useful system for the study of PAS-positive staining. The relationship between PAS-positive deposition, induction of fos-like immunoreactivity (FLI), neuronal necrosis, reactive gliosis, and blood-brain barrier breakdown following the kainic acid induction of status epilepticus was investigated. Epileptiform activity was elicited in rats by intraperitoneal administration of 10 mg/kg kainic acid and brains were examined 3, 5, 12, 24, 72, and 168 h after drug injection. Four distinct types of PAS-positive staining in rat brain were observed: type 1, extracellular matrix (ECM) or blood vessel associated-material; type 2, granular deposits; type 3, glial labelling; and type 4, neuronal labelling. Results demonstrated that the four types of PAS-positive staining were differentially associated with specific markers of neuropathology: (1) type 1 ECM staining and type 3 glia were preferentially localized to edematous tissue; (2) the majority of type 3 glia were identified as reactive astrocytes, while a minority of appeared to be proliferating microglia; (3) type 1 blood vessels labelled hemorrhaging vasculature; (4) early deposition of type 2 granules was predictive of subsequent cell loss; (5) chronic type 2 granular deposits and type 4 neuronal labelling not associated with cell death could be predicted by early changes in FLI; and (6) chronic deposition of all four forms of PAS-positive material was correlated with earlier, transient blood-brain barrier compromise. The results support the growing literature that local carbohydrate metabolism may be one of a constellation of parameters important to the development of progressive neurodegeneration.

The evolution of a rat model of chronic spontaneous limbic seizures

The evolution of untreated partial epilepsy is unknown. This study uses a newly developed model of chronic limbic epilepsy to determine whether seizures inexorably worsen in duration, frequency and behavioral accompaniment. The seizures begin following an episode of limbic status epilepticus induced by continuous electrical stimulation of the hippocampus, and they persist for more than a year (longest duration followed). We monitored 10 rats continuously with combined EEG and closed circuit television for 24 weeks following the first recorded spontaneous seizure. Seizure duration, behavioral accompaniment and frequency all intensified during the early stages, but the last 12–16 weeks of the study were characterized by a plateau for all measures. The results showed significant increases that occured over the first 12 weeks only ( P < 0.01 for duration and behavioral accompaniment, P < 0.05 for seizure frequency). These findings suggest that untreated epilepsy will undergo an early maturation process, but that once the seizures mature they remain stable over a prolonged period. It was also noted that 67% ( P < 0.00001) of the seizures occured during the day, suggesting that the sleep-wake cycle has a strong influence on the occurence of seizures in this model of limbic epilepsy.

Anticonvulsant effects of the experimental induction of hippocampal theta activity

Epileptic seizures occur less during wakefulness or paradoxical sleep, conditions during which hippocampal theta rhythm is seen. This leads to the hypothesis that this rhythm indicates a physiological state of the hippocampal formation which opposes its recruitment into seizures. This was tested by determining the effects of experimental induction or suppression of hippocampal theta activity on seizures. Hippocampal theta activity can be induced by chemical or electrical stimulation of the medial septal nucleus and adjacent nucleus of the diagonal band of Broca. Microinjections of the muscarinic agonist carbachol in the medial septum during pentylenetetrazol (PTZ) induced facial-forelimb seizures stopped behavioral seizures and EEG spiking within five seconds, and caused hippocampal theta activity. Medial septal electrical stimulation at 4–8 Hz had similar effects. Electrolytic medial septal lesions abolished hippocampal theta activity and lowered myoclonic and facial-forelimb PTZ seizure thresholds. Medial septal carbachol injections were also made during electrically kindled limbic status epilepticus. Within ten seconds, ictal behavior stopped and the EEG spike rate decreased by half with a gradual return to the baseline rate over three minutes. These results demonstrate that the hippocampal theta rhythm corresponds to a seizure-resistant condition, providing a possible explanation for the seizure promoting properties of slow wave sleep.

Some neuropharmacological effects of an ethanolic extract of Maprounea africana in rodents

The ethanolic extract of the leaves of Maprounea africana (Euphorbiaceae), a medicinal plant in the Congolese traditional medicine, induced hypothermia and reduced the latency to the loss of the righting reflex and prolonged the sleeping time induced by pentobarbital in mice. It also significantly delayed the onset of clonic convulsions induced by pentylenetetrazole in mice. However, the extract did not affect either generalized convulsions induced by maximal electroshock and picrotoxin or limbic status epilepticus produced by pilocarpine and kainic acid.

The effect of thyrotropin-releasing hormone (TRH) on limbic status epilepticus in rats

We produced limbic status epilepticus in rats by injecting a combination of dibutyryl-cAMP (db-cAMP) and ethylenediaminetetraacetic acid (EDTA) into the amygdala (AM). Thirty minutes after intra-AM db-cAMP/EDTA injection, thyrotropin-releasing hormone (TRH) was administered intravenously or intracerebroventricularly. Intravenous TRH (3, 25, 50 mg/kg) produced immediate activation of electroclinical seizures, lasting for 25–45 min. In some animals which showed this seizure activation, complete seizure suppression occurred 55–70 min after the TRH treatment. Similar activation of ictal seizures with delayed seizure suppression was obtained after intracerebroventricular TRH (25, 50 μg). The findings suggest that the effects of intravenous TRH are due to its central action and that the use of intravenous TRH is not a promising approach for the treatment of status epilepticus.

Effect of an adenosine antagonist and an adenosine agonist on status entry and severity in a model of limbic status epilepticus

Adenosine is an endogenous neuromodulator that suppresses excitatory neurotransmission. We postulated that adenosine-mediated mechanisms resist status epilepticus (SE) entry and limit SE severity. In the first experiment rats were given an adenosine agonist (2-chloroadenosine), an adenosine antagonist (aminophylline), or saline vehicle, prior to SE induction with pulsed-train current delivered to amygdala in successive 5-min current-on sessions. Saline-treated animals entered limbic SE, with predominantly exploratory behavior, after 6.0 ± 0.9 current-on sessions. Aminophylline increased major convulsive activity during stimulation and resulted in entry into convulsive SE after only 2.1±0.1 sessions. 2-Chloroadenosine, in contrast, suppressed major convulsive activity during stimulation, and blocked (in 3 7 ) or delayed ( 4 7 ) SE entry, with successes requiring 12.8± 0.9 stimulation sessions. In a second experiment, animals already in exploratory SE were administered a single injection of saline vehicle, aminophylline, or 2-chloroadenosine. Aminophylline converted exploratory SE into lethally severe convulsive SE. 2-Chloroadenosine suppressed SE behaviorally and electrographically, and protected recipients from the seizure-associated cerebral damage seen in saline-administered SE controls. These results support the hypothesis that endogenous adenosine mechanisms resist SE entry, modulate the severity of ongoing SE, and limit the anatomic spread of seizure activity.

Status epilepticus results in reversible neuronal injury in infant rat hippocampus: novel use of a marker

Despite ready induction of severe limbic status epilepticus by systemic kainic acid (KA) in infant rats, excitotoxic neuronal injury has not been observed. The mechanisms of this resistance of the immature hippocampus to excitotoxicity are unknown. Acid fuchsin stain has been used as a marker of irreversibly injured neurons in the adult brain. We speculated that the dye might map reversibly injured neurons in the infant. Subsequent to KA-induced status epilepticus in 11-day-old rats, acid fuchsin stain was evident in the hippocampal CA3, CA1, dentate gyrus and hilus by 24 h, peaked at 48 h and disappeared by 6 days, without evidence for neuronal loss. Acid fuchsin may be a useful tool for delineating the distribution of reversibly injured immature neurons in experimental seizure paradigms.

The functional anatomy of limbic status epilepticus in the rat. I. Patterns of 14C-2-deoxyglucose uptake and Fos immunocytochemistry

Limbic status epilepticus was induced in awake, unrestrained rats by electrically stimulating the olfactory cortex or the basal amygdaloid nucleus for about 40 min. One of four stable forms of status was induced, which were distinguished on the basis of their behavioral and EEG manifestations, and their distinct patterns of 14C-2-deoxyglucose uptake and Fos-like immunoreactivity. Type I status was characterized by sporadic EEG discharges and the activation of the amygdalohippocampal area, but had no overt behavioral manifestation. Type II status involved incessant exploratory behaviors, single EEG discharges, and the additional activation of the basal amygdaloid nucleus, some of its efferent projections, and parts of the olfactory cortex. Type III status included all of these same patterns, plus the episodic development of ictal EEG activity associated with facial and forelimb clonus, and the concurrent recruitment of the entire amygdala, ventral hippocampal formation, prefrontal, insular, and olfactory cortices, and related subcortical structures. Type IV status was characterized by generalized clonus, unremitting ictal EEG discharges, and the additional activation of most of the dorsolateral neocortex, neostriatum, and thalamus. In each case of status type I, II, or III, the same anatomical structures that displayed high levels of 14C-2-deoxyglucose uptake also contained many cells that were immunoreactive for Fos, with the exception of the parataenial and mediodorsal thalamic nuclei and the substantia nigra pars reticularis. Thus, the overall patterns of 14C-2-deoxyglucose uptake and Fos-like immunoreactivity from the same animals displayed a remarkable degree of correspondence. The major results indicate that different levels of status are related to the activation of discrete epileptogenic foci, and the capacity of such foci to interact with a distinct set of interconnected anatomical structures. It is suggested that the behavioral manifestations of limbic status epilepticus may be explained by influences of limbic structures in the ventral forebrain upon lower motor elements in the brainstem and spinal cord, without the participation of the "pyramidal" motor system.

The functional anatomy of limbic status epilepticus in the rat. II. The effects of focal deactivation

Limbic status epilepticus was induced in awake, unrestrained rats by electrically stimulating the anterior piriform cortex or the basal amygdaloid nucleus for about 40 min. As described in the preceding article (White and Price, 1993), one of four stable forms of status may be induced. Each form is characterized on the basis of its behavioral and electroencephalographic manifestations, and its distinct patterns of 14C-2-deoxyglucose uptake and Fos-like immunoreactivity. This study was directed at identifying the epileptogenic foci of the two major forms of status, types II and III, by deactivating the basal amygdaloid nucleus, ventral hippocampal formation, amygdalohippocampal area, or anterior piriform cortex during these seizure states. Infusions of the local anesthetic lidocaine, the GABA agonist muscimol, or a vehicle solution alone were made into each of these structures during ongoing type II or type III status. The major finding is that deactivation of the basal amygdaloid nucleus terminated both types of status. This indicates that the basal nucleus is primarily responsible for the generation of widespread status epilepticus activity. Deactivation of the ventral hippocampal formation did not terminate the subconvulsive levels of status, but did prevent the recurrent development of sustained seizures with facial and forelimb clonus that characterize type III status. These models of status epilepticus may be particularly important for understanding seizure mechanisms that are not dependent upon the hippocampal formation. The possible clinical relevance of these findings is discussed in relation to temporal lobe epilepsy.

Non-NMDA glutamate receptors are involved in the maintenance of status epilepticus.

The role of N-methyl-D-aspartate (NMDA), non-NMDA glutamate, metabotropic and muscarinic receptors in the maintenance of status epilepticus (SE) was investigated. SE induced in rat brain by continuous electrical stimulation to the hippocampus was terminated by intracerebroventricular (i.c.v.) injection of the non-NMDA antagonists DNQX and NBQX, but not by the muscarinic antagonists scopolamine or atropine, or the metabotropic antagonist AP3. The NMDA antagonist, MK-801 suppressed motor seizure activity but did not terminate electrographic seizures when generalized SE was induced, suggesting that both non-NMDA and NMDA receptors maintain generalized convulsive SE. However, when limbic SE was induced, MK-801 also had an anticonvulsant effect suggesting differences in the mechanisms maintaining limbic SE and generalized SE.

Kynurenine pathway enzymes in a rat model of chronic epilepsy: Immunohistochemical study of activated glial cells

The kynurenine pathway metabolites quinolinic acid and kynurenic acid have been hypothetically linked to the occurrence of seizure phenomena. The present immunohistochemical study reports the activation of astrocytes containing three enzymes responsible for the metabolism of quinolinic acid and kynurenic acid in a rat model of chronic epilepsy. Rats received 90 min of patterned electrical stimulation through a bipolar electrode stereotaxically positioned in one hippocampus. This treatment induces non-convulsive limbic status epilepticus that leads to chronic, spontaneous, recurrent seizures. One month after the status epilepticus, the rats showed neuronal loss and gliosis in the piriform cortex, thalamus, and hippocampus, particularly on the side contralateral to the stimulation. Astrocytes containing the kynurenic acid biosynthetic enzyme (kynurenine aminotransferase) and the enzymes for the biosynthesis and degradation of quinolinic acid (3-hydroxyanthranilic acid oxygenase and quinolinic acid phosphoribosyltransferase, respectively) became highly hypertrophied in brain areas where neurodegeneration occurred. Detailed qualitative and quantitative analyses were performed in the hippocampus. In CA1 and CA3 regions, the immunostained surface area of reactive astrocytes increased up to five-fold as compared to controls. Enlarged cells containing the three enzymes were mainly observed in the stratum radiatum, whereas the stratum pyramidale, in which neuronal somata degenerated, showed relatively fewer reactive glial cells. Hypertrophied kynurenine aminotransferase- and 3-hydroxyanthranilic acid oxygenase-immunoreactive cells were comparable in their morphology and distribution pattern. In contrast, reactive quinolinic acid phosphoribosyl transferase-positive glial cells displayed diversified sizes and shapes. Some very large quinolinic acid phosphoribosyl transferase-immunoreactive cells were noticed in the molecular layer of the dentate gyrus. In the hippocampus, the number of immunoreactive glial cells increased in parallel to the hypertrophic responses. In addition, pronounced increases in immunoreactivities, associated with hypertrophied astrocytes, occurred around lesioned sites in the thalamus and piriform cortex. These findings indicate that kynurenine metabolites derived from glial cells may play a role in chronic epileptogenesis.

Morphometric effects of intermittent kindled seizures and limbic status epilepticus in the dentate gyrus of the rat

The effect of recurrent seizures on the hippocampus has been controversial for many years. To determine the effect different seizure paradigms had on the structure of the dentate gyrus, we conducted histological studies on the dentate gyrus (DG) from three groups of rats: (1) those that had experienced 1500 intermittent kindled seizures; (2) those that had experienced a single episode of limbic status epilepticus (SE); and (3) control rats that had been implanted with electrodes. When compared to controls the DG of SE rats was overall slightly, but non-significantly, smaller, but the DG of rats with 1500 kindled seizures was significantly larger. The decrease of size following SE was attributable to a significant atrophy of the molecular layer. The increase in area associated with kindling was the result of an enlargement of the molecular layer and the hilus. Absolute neuronal counts showed a decrease in the hilus after SE but no change following kindling, but both groups had decreased neuronal densities in the hilus when compared to controls. The decreased density after SE was secondary to neuronal loss, but the decrease in neuronal density following kindling was the result of the expansion of the hilar neuropil without change in the number of neurons. This study extends our previous findings in Ammon's horn and indicates that SE induces significant neuronal loss, but numerous intermittent kindled seizures have no effect on neuronal numbers in the DG.