The hippocampus in experimental chronic epilepsy: a morphometric analysis

The case against secondary epileptogenesis

Secondary epileptogenesis is a theory that hypothesizes that uncontrolled seizures in people with epilepsy lead to the development of new sites of seizure onset. This process has often been cited when people experience a new seizure type after a period of poor seizure control. The theory proposes that repeated seizures induce changes in regions of the brain that are regularly recruited into the seizure. These hypothetical changes can then lead to a new, independent seizure onset zone. The concept is based on a number of clinical observations which secondary epileptogenesis could explain. However there are alternative explanations from the clinic as well as from the laboratory that call the process into question. In this review some of the observations that have been used to support the theory will be reviewed, and the many counterarguments will be presented. At this time there is little evidence to support secondary epileptogenesis and much to refute it.

Dynamics of neurodegeneration in the hippocampus of Krushinsky-Molodkina rats correlates with the progression of limbic seizures

Audiogenic seizures (AGS) (audiogenic kindling) in genetically selected audiogenic rodents are a reliable model of temporal lobe epilepsy (TLE). Temporal lobe epilepsy is accompanied with neurodegeneration in the hippocampus, but how the cells die is not fully understood. We analyzed the dynamics and mechanisms of cell loss in the hippocampus of audiogenic Krushinsky-Molodkina (KM) rats during the development of TLE. Audiogenic kindling of different durations was carried out to reproduce TLE progression in KM rats. Behavioral analysis showed the development of post-tonic clonus, the main indicator of TLE, by the 14th AGS. The severity and duration of post-tonic clonus positively correlated with the increase in the number of AGS. Temporal lobe epilepsy development was accompanied with two peaks of cell loss. The first peak was detected after 7 AGS in the dentate gyrus (DG) granular layer and associated with activation of p53- and mitochondria-dependent apoptosis. After a 7-day rest period, activation of autophagy and restoration of cell number were revealed. The second peak occurred after 14 AGS, affected both granular and hilar mossy cells and persisted further after 21 AGS, but no compensation was observed. Thus, activation of autophagy probably plays a neuroprotective role and supports survival of hippocampal cells at the beginning of epileptogenesis, but exacerbation of limbic seizures during TLE development causes irreversible neurodegeneration.

Seizure activity and brain damage in a model of focal non-convulsive status epilepticus

AIMS

Focal non-convulsive status epilepticus (FncSE) is a common emergency condition that may present as the first epileptic manifestation. In recent years, it has become increasingly clear that de novo FncSE should be promptly treated to improve post-status outcome. Whether seizure activity occurring during the course of the FncSE contributes to ensuing brain damage has not been demonstrated unequivocally and is here addressed.

METHODS

We used continuous video-EEG monitoring to characterise an acute experimental FncSE model induced by unilateral intrahippocampal injection of kainic acid (KA) in guinea pigs. Immunohistochemistry and mRNA expression analysis were utilised to detect and quantify brain injury, 3-days and 1-month after FncSE.

RESULTS

Seizure activity occurring during the course of FncSE involved both hippocampi equally. Neuronal loss, blood-brain barrier permeability changes, gliosis and up-regulation of inflammation, activity-induced and astrocyte-specific genes were observed in the KA-injected hippocampus. Diazepam treatment reduced FncSE duration and KA-induced neuropathological damage. In the contralateral hippocampus, transient and possibly reversible gliosis with increase of aquaporin-4 and Kir4.1 genes were observed 3 days post-KA. No tissue injury and gene expression changes were found 1-month after FncSE.

CONCLUSIONS

In our model, focal seizures occurring during FncSE worsen ipsilateral KA-induced tissue damage. FncSE only transiently activated glia in regions remote from KA-injection, suggesting that seizure activity during FncSE without local pathogenic co-factors does not promote long-lasting detrimental changes in the brain. These findings demonstrate that in our experimental model, brain damage remains circumscribed to the area where the primary cause (KA) of the FncSE acts. Our study emphasises the need to use antiepileptic drugs to contain local damage induced by focal seizures that occur during FncSE.

Recurrent limbic seizures do not cause hippocampal neuronal loss: A prolonged laboratory study

PURPOSE

It remains controversial whether neuronal damage and synaptic reorganization found in some forms of epilepsy are the result of an initial injury and potentially contributory to the epileptic condition or are the cumulative affect of repeated seizures. A number of reports of human and animal pathology suggest that at least some neuronal loss precedes the onset of seizures, but there is debate over whether there is further damage over time from intermittent seizures. In support of this latter hypothesis are MRI studies in people that show reduced hippocampal volumes and cortical thickness with longer durations of the disease. In this study we addressed the question of neuronal loss from intermittent seizures using kindled rats (no initial injury) and rats with limbic epilepsy (initial injury).

METHODS

Supragranular mossy fiber sprouting, hippocampal neuronal densities, and subfield area measurements were determined in rats with chronic limbic epilepsy (CLE) that developed following an episode of limbic status epilepticus (n = 25), in kindled rats (n = 15), and in age matched controls (n = 20). To determine whether age or seizure frequency played a role in the changes, CLE and kindled rats were further classified by seizure frequency (low/high) and the duration of the seizure disorder (young/old).

RESULTS

Overall there was no evidence for progressive neuronal loss from recurrent seizures. Compared with control and kindled rats, CLE animals showed increased mossy fiber sprouting, decreased neuronal numbers in multiple regions and regional atrophy. In CLE, but not kindled rats: 1) Higher seizure frequency was associated with greater mossy fiber sprouting and granule cell dispersion; and 2) greater age with seizures was associated with decreased hilar densities, and increased hilar areas. There was no evidence for progressive neuronal loss, even with more than 1000 seizures.

CONCLUSION

These findings suggest that the neuronal loss associated with limbic epilepsy precedes the onset of the seizures and is not a consequence of recurrent seizures. However, intermittent seizures do cause other structural changes in the brain, the functional consequences of which are unclear.

Is Mossy Fiber Sprouting a Potential Therapeutic Target for Epilepsy?

Mesial temporal lobe epilepsy (MTLE) caused by hippocampal sclerosis is one of the most frequent focal epilepsies in adults. It is characterized by focal seizures that begin in the hippocampus, sometimes spread to the insulo-perisylvian regions and may progress to secondary generalized seizures. Morphological alterations in hippocampal sclerosis are well defined. Among them, hippocampal sclerosis is characterized by prominent cell loss in the hilus and CA1, and abnormal mossy fiber sprouting (granular cell axons) into the dentate gyrus inner molecular layer. In this review, we highlight the role of mossy fiber sprouting in seizure generation and hippocampal excitability and discuss the response of alternative treatment strategies in terms of MFS and spontaneous recurrent seizures in models of TLE (temporal lobe epilepsy).

Animal models of status epilepticus and temporal lobe epilepsy: a narrative review

Temporal lobe epilepsy (TLE) is the chronic and pharmacoresistant form of epilepsy observed in humans. The current literature is insufficient in explicating the comprehensive mechanisms underlying its pathogenesis and advancement. Consequently, the development of a suitable animal model mimicking the clinical characteristics is required. Further, the relevance of status epilepticus (SE) to animal models is dubious. SE occurs rarely in people; most epilepsy patients never experience it. The present review summarizes the established animal models of SE and TLE, along with a brief discussion of the animal models that have the distinctiveness and carries the possibility to be developed as effective models for TLE. The review not only covers the basic requirements, mechanisms, and methods of induction of each model but also focuses upon their major limitations and possible modifications for their future use. A detailed discussion on chemical, electrical, and hypoxic/ischemic models as well as a brief explanation on the genetic models, most of which are characterized by development of SE followed by neurodegeneration, is presented.

Glial source of nitric oxide in epileptogenesis: A target for disease modification in epilepsy

Epileptogenesis is the process of developing an epileptic condition and/or its progression once it is established. The molecules that initiate, promote, and propagate remarkable changes in the brain during epileptogenesis are emerging as targets for prevention/treatment of epilepsy. Epileptogenesis is a continuous process that follows immediately after status epilepticus (SE) in animal models of acquired temporal lobe epilepsy (TLE). Both SE and epileptogenesis are potential therapeutic targets for the discovery of anticonvulsants and antiepileptogenic or disease-modifying agents. For translational studies, SE targets are appropriate for screening anticonvulsive drugs prior to their advancement as therapeutic agents, while targets of epileptogenesis are relevant for identification and development of therapeutic agents that can either prevent or modify the disease or its onset. The acute seizure models do not reveal antiepileptogenic properties of anticonvulsive drugs. This review highlights the important components of epileptogenesis and the long-term impact of intervening one of these components, nitric oxide (NO), in rat and mouse kainate models of TLE. NO is a putative pleotropic gaseous neurotransmitter and an important contributor of nitro-oxidative stress that coexists with neuroinflammation and epileptogenesis. The long-term impact of inhibiting the glial source of NO during early epileptogenesis in the rat model of TLE is reviewed. The importance of sex as a biological variable in disease modification strategies in epilepsy is also briefly discussed.

Management of status epilepticus

Status epilepticus is a neurologic and medical emergency manifested by prolonged seizure activity or multiple seizures without return to baseline. It is associated with substantial medical cost, morbidity, and mortality. There is a spectrum of severity dependent on the type of seizure, underlying pathology, comorbidities, and appropriate and timely medical management. This chapter discusses the evolving definitions of status epilepticus and multiple patient and clinical factors which influence outcome. The pathophysiology of status epilepticus is reviewed to provide a better understanding of the mechanisms which contribute to status epilepticus, as well as the potential long-term effects. The clinical presentations of different types of status epilepticus in adults are discussed, with emphasis on the hospital course and management of the most dangerous type, generalized convulsive status epilepticus. Strategies for the evaluation and management of status epilepticus are provided based on available evidence from clinical trials and recommendations from the Neurocritical Care Society and the European Federation of Neurological Societies.

1400W, a highly selective inducible nitric oxide synthase inhibitor is a potential disease modifier in the rat kainate model of temporal lobe epilepsy

Status epilepticus (SE) initiates epileptogenesis to transform normal brain to epileptic state which is characterized by spontaneous recurrent seizures (SRS). Prior to SRS, progressive changes occur in the brain soon after SE, for example, loss of blood-brain barrier (BBB) integrity, neuronal hyper-excitability (epileptiform spiking), neuroinflammation [reactive gliosis, high levels of reactive oxygen/nitrogen species (ROS/RNS)], neurodegeneration and synaptic re-organization. Our hypothesis was that modification of early epileptogenic events will alter the course of disease development and its progression. We tested the hypothesis in the rat kainate model of chronic epilepsy using a novel disease modifying drug, 1400W, a highly selective inhibitor of inducible nitric oxide synthase (iNOS/NOS-II). In an in vitro mouse brain slice model, using a multi-electrode array system, co-application of 1400W with kainate significantly suppressed kainate-induced epileptiform spiking. In the rats, in vivo, 4h after the induction of SE with kainate, 1400W (20mg/kg, i.p.) was administered twice daily for three days to target early events of epileptogenesis. The rats were subjected to continuous (24/7) video-EEG monitoring, remotely, for six months from epidurally implanted cortical electrodes. The 1400W treatment significantly reduced the epileptiform spike rate during the first 12-74h post-SE, which resulted in >90% reduction in SRS in long-term during the six month period when compared to the vehicle-treated control group (257±113 versus 19±10 episodes). Immunohistochemistry (IHC) of brain sections at seven days and six months revealed a significant reduction in; reactive astrogliosis and microgliosis (M1 type), extravascular serum albumin (a marker for BBB leakage) and neurodegeneration in the hippocampus, amygdala and entorhinal cortex in the 1400W-treated rats when compared to the vehicle control. In the seven day group, hippocampal Western blots revealed downregulation of inwardly-rectifying potassium (Kir 4.1) channels and glutamate transporter-1 (GLT-1) levels in the vehicle group, and 1400W treatment partially reversed Kir 4.1 levels, however, GLT-1 levels were unaffected. In the six month group, a significant reduction in mossy fiber staining intensity in the inner molecular layer of the dentate gyrus was observed in the 1400W-treated group. Overall these findings demonstrate that 1400W, by reducing the epileptiform spike rate during the first three days of post-insult, potentially modifies epileptogenesis and the severity of chronic epilepsy in the rat kainate model of TLE.

Relationship between seizure frequency and number of neuronal and non-neuronal cells in the hippocampus throughout the life of rats with epilepsy

The relationship between seizure frequency and cell death has been a subject of controversy. To tackle this issue, we determined the frequency of seizures and the total number of hippocampal cells throughout the life of rats with epilepsy using the pilocarpine model. Seizure frequency varied in animals with epilepsy according to which period of life they were in, with a progressive increase in the number of seizures until 180 days (sixth months) of epileptic life followed by a decrease (330 days-eleventh month) and subsequently stabilization of seizures. Cell counts by means of isotropic fractionation showed a reduction in the number of hippocampal neuronal cells following 30, 90, 180 and 360 days of spontaneous recurrent seizures (SRS) in rats compared to their controls (about 25%-30% of neuronal cell reduction). In addition, animals with 360 days of SRS showed a reduction in the number of neuronal cells when compared with animals with 90 and 180 days of seizures. The total number of hippocampal non-neuronal cells was reduced in rats with epilepsy after 30 days of SRS, but no significant alteration was observed on the 90th, 180th and 360th days. The total number of neuronal cells was negatively correlated with seizure frequency, indicating an association between occurrence of epileptic seizures throughout life and neuronal loss. In sum, our results add novel data to the literature concerning the time-course of SRS and hippocampal cell number throughout epileptic life.

Beyond the CA1 subfield: Local hippocampal shape changes in MRI-negative temporal lobe epilepsy

OBJECTIVE

Hippocampal atrophy in temporal lobe epilepsy (TLE) can indicate mesial temporal sclerosis and predict surgical success. Yet many patients with TLE do not have significant atrophy (magnetic resonance imaging (MRI) negative), which presents a diagnostic challenge. We used a new variant of high-dimensional large-deformation mapping to assess whether patients with apparently normal hippocampi have local shape changes that mirror those of patients with significant hippocampal atrophy.

METHODS

Forty-seven patients with unilateral TLE and 32 controls underwent structural brain MRI. High-dimensional large-deformation mapping provided hippocampal surface and volume estimates for each participant, dividing patients into low versus high hippocampal atrophy groups. A vertex-level generalized linear model compared local shape changes between groups.

RESULTS

Patients with low-atrophy TLE (MRI negative) had significant local hippocampal shape changes compared to controls, similar to those in the contralateral hippocampus of high-atrophy patients. These changes primarily involved the subicular and hilar/dentate regions, instead of the classically affected CA1 region. Disease duration instead co-varied with lateral hippocampal atrophy, co-localizing with the CA1 subfield.

SIGNIFICANCE

These findings show that patients with "MRI-negative" TLE have regions of hippocampal atrophy that cluster medially, sparing the lateral regions (CA1) involved in high-atrophy patients. This suggests an overall effect of temporal lobe seizures manifesting as bilateral medial hippocampal atrophy, and a more selective effect of hippocampal seizures leading to disease-proportional CA1 atrophy, potentially reflecting epileptogenesis.

Influence of neuropathology on convection-enhanced delivery in the rat hippocampus

Local drug delivery techniques, such as convention-enhanced delivery (CED), are promising novel strategies for delivering therapeutic agents otherwise limited by systemic toxicity and blood-brain-barrier restrictions. CED uses positive pressure to deliver infusate homogeneously into interstitial space, but its distribution is dependent upon appropriate tissue targeting and underlying neuroarchitecture. To investigate effects of local tissue pathology and associated edema on infusate distribution, CED was applied to the hippocampi of rats that underwent electrically-induced, self-sustaining status epilepticus (SE), a prolonged seizure. Infusion occurred 24 hours post-SE, using a macromolecular tracer, the magnetic resonance (MR) contrast agent gadolinium chelated with diethylene triamine penta-acetic acid and covalently attached to albumin (Gd-albumin). High-resolution T1- and T2-relaxation-weighted MR images were acquired at 11.1 Tesla in vivo prior to infusion to generate baseline contrast enhancement images and visualize morphological changes, respectively. T1-weighted imaging was repeated post-infusion to visualize final contrast-agent distribution profiles. Histological analysis was performed following imaging to characterize injury. Infusions of Gd-albumin into injured hippocampi resulted in larger distribution volumes that correlated with increased injury severity, as measured by hyperintense regions seen in T2-weighted images and corresponding histological assessments of neuronal degeneration, myelin degradation, astrocytosis, and microglial activation. Edematous regions included the CA3 hippocampal subfield, ventral subiculum, piriform and entorhinal cortex, amygdalar nuclei, middle and laterodorsal/lateroposterior thalamic nuclei. This study demonstrates MR-visualized injury processes are reflective of cellular alterations that influence local distribution volume, and provides a quantitative basis for the planning of local therapeutic delivery strategies in pathological brain regions.

Experimental models of status epilepticus and neuronal injury for evaluation of therapeutic interventions

This article describes current experimental models of status epilepticus (SE) and neuronal injury for use in the screening of new therapeutic agents. Epilepsy is a common neurological disorder characterized by recurrent unprovoked seizures. SE is an emergency condition associated with continuous seizures lasting more than 30 min. It causes significant mortality and morbidity. SE can cause devastating damage to the brain leading to cognitive impairment and increased risk of epilepsy. Benzodiazepines are the first-line drugs for the treatment of SE, however, many people exhibit partial or complete resistance due to a breakdown of GABA inhibition. Therefore, new drugs with neuroprotective effects against the SE-induced neuronal injury and degeneration are desirable. Animal models are used to study the pathophysiology of SE and for the discovery of newer anticonvulsants. In SE paradigms, seizures are induced in rodents by chemical agents or by electrical stimulation of brain structures. Electrical stimulation includes perforant path and self-sustaining stimulation models. Pharmacological models include kainic acid, pilocarpine, flurothyl, organophosphates and other convulsants that induce SE in rodents. Neuronal injury occurs within the initial SE episode, and animals exhibit cognitive dysfunction and spontaneous seizures several weeks after this precipitating event. Current SE models have potential applications but have some limitations. In general, the experimental SE model should be analogous to the human seizure state and it should share very similar neuropathological mechanisms. The pilocarpine and diisopropylfluorophosphate models are associated with prolonged, diazepam-insensitive seizures and neurodegeneration and therefore represent paradigms of refractory SE. Novel mechanism-based or clinically relevant models are essential to identify new therapies for SE and neuroprotective interventions.

Sequel of spontaneous seizures after kainic acid-induced status epilepticus and associated neuropathological changes in the subiculum and entorhinal cortex

Injection of the seaweed toxin kainic acid (KA) in rats induces a severe status epilepticus initiating complex neuropathological changes in limbic brain areas and subsequently spontaneous recurrent seizures. Although neuropathological changes have been intensively investigated in the hippocampus proper and the dentate gyrus in various seizure models, much less is known about changes in parahippocampal areas. We now established telemetric EEG recordings combined with continuous video monitoring to characterize the development of spontaneous seizures after KA-induced status epilepticus, and investigated associated neurodegenerative changes, astrocyte and microglia proliferation in the subiculum and other parahippocampal brain areas. The onset of spontaneous seizures was heterogeneous, with an average latency of 15 ± 1.4 days (range 3–36 days) to the initial status epilepticus. The frequency of late spontaneous seizures was higher in rats in which the initial status epilepticus was recurrent after its interruption with diazepam compared to rats in which this treatment was more efficient. Seizure-induced neuropathological changes were assessed in the subiculum by losses in NeuN-positive neurons and by Fluoro-Jade C staining of degenerating neurons. Neuronal loss was already prominent 24 h after KA injection and only modestly progressed at the later intervals. It was most severe in the proximal subiculum and in layer III of the medial entorhinal cortex and distinct Fluoro-Jade C labeling was observed there in 75% of rats even after 3 months. Glutamatergic neurons, labeled by in situ hybridization for the vesicular glutamate transporter 1 followed a similar pattern of cell losses, except for the medial entorhinal cortex and the proximal subiculum that appeared more vulnerable. Glutamate decarboxylase65 (GAD65) mRNA expressing neurons were generally less vulnerable than glutamate neurons. Reactive astrocytes and microglia were present after 24 h, however, became prominent only after 8 days and remained high after 30 days. In the proximal subiculum, parasubiculum and entorhinal cortex the number of microglia cells was highest after 30 days. Although numbers of reactive astrocytes and microglia were reduced again after 3 months, they were still present in most rats. The time course of astrocyte and microglia proliferation parallels that of epileptogenesis.

Early MR diffusion and relaxation changes in the parahippocampal gyrus precede the onset of spontaneous seizures in an animal model of chronic limbic epilepsy

Structural changes in limbic regions are often observed in individuals with temporal lobe epilepsy (TLE) and in animal models. However, the brain structural changes during the evolution into epilepsy remain largely unknown. Therefore, the purpose of this study was to define the temporal changes in limbic structures after experimental status epilepticus (SE) during the latency period of epileptogenesis in vivo, with quantitative diffusion tensor imaging (DTI) and T2 relaxometry in an animal model of chronic TLE. A pair of fifty micron electrodes was implanted into the ventral hippocampus in twelve male adult rats. Self-sustaining SE was induced with electrical stimulation in eleven rats. Three rats served as age-matched controls. In vivo diffusion tensor and T2 magnetic resonance imaging (MRI) was performed at 11.1 Tesla, pre- and post-implantation of electrodes and 3, 5, 7, 10, 20, 40 and 60 days post-SE to assess structural changes. Spontaneous seizures were identified with continuous time-locked video-monitoring. Following imaging in vivo, fixed, excised brains were MR imaged at 17.6 Tesla. Subsequently, histological analysis was correlated with MRI results. Following SE, 8/11 injured rats developed spontaneous seizures. Unique to these 8 rats, early T2, diffusivity and anisotropy changes were observed in vivo within the parahippocampal gyrus (contralateral) and fimbria (bilateral). In excised brains, bilateral increase in anisotropy was observed in the dentate gyrus, corresponding to mossy fiber sprouting as determined by Timm staining. Using T2 relaxometry and DTI, specific transient and long-term structural changes were observed only in rats that developed spontaneous limbic seizures.

Reduction of seizure thresholds following electrical stimulation of sensorimotor cortex is dependent on stimulation intensity and is not related to synaptic potentiation

Epilepsy is characterized as a chronic brain state with a very low seizure threshold, and the occurrence of repeated seizure activity. Currently, there is no animal model of induced epilepsy that allows for the exploration of the brain mechanisms underlying a low seizure threshold without the elicitation of seizures. In this study, we employed repeated application of different intensities of electrical stimulation in an attempt to reduce afterdischarge (seizure) thresholds without eliciting seizures. We utilized an in vivo model of neocortical activation via stimulation of the corpus callosum of the adult rat. The intensities were chosen to be subthreshold (20, 30, 40, 50 microA), near threshold (150 microA), and suprathreshold (250, 500 microA) relative to the mean initial afterdischarge threshold (ADT). We also examined changes in the evoked field responses of the transcallosal pathway to the sensorimotor cortex as a measure of synaptic efficacy. Our results indicated that stimulation at 50 microA was effective at reducing the ADT, while minimizing the number of seizures elicited. Stimulation at 150 microA resulted in the concomitant reduction of ADT and repeated seizures typical of most electrical kindling studies. Finally, the 500 microA group showed repeated seizures, but no reduction of afterdischarge threshold. These stimulation intensities (50 microA, 150 microA, 500 microA and 0 microA-control) can be used to independently determine the brain mechanisms responsible for 1) the acquisition of a low afterdischarge threshold independent of the reorganizing effect of repeated seizures, and 2) the elicitation of repeated seizures independent of stimulation induced reduction of afterdischarge threshold.

Diagnosis and management of nonconvulsive status epilepticus in children

Nonconvulsive status epilepticus (NCSE) encompasses a wide range of diagnoses with variable outcomes and treatment recommendations. In children, NCSE can be observed in various conditions, including acute neurological injuries, specific childhood epilepsy syndromes and other neurological conditions, and can also be observed in individuals with learning difficulties. NCSE in children is thought to be under-recognized, and further studies examining the electrographic characteristics of very young children in NCSE would aid the prompt recognition of additional patients. Some subtypes of NCSE are probably more harmful than others, and long-term prospective studies are needed to evaluate the damaging potential of NCSE itself as opposed to that of the underlying circumstances in which it occurs. Specific data in childhood are clearly lacking, but extrapolation from adult studies indicates that aggressive treatment is most warranted in comatose patients. By contrast, a cautious approach seems to be indicated for absence status epilepticus, complex partial status epilepticus and electrical status epilepticus during sleep.

The relevance of kindling for human epilepsy

Kindling is one of the most widely used models of seizures and epilepsy, and it has been used in its more than three decade history to provide many key insights into seizures and epilepsy. It remains a mainstay of epilepsy related research, but the question remains how the results from kindling experiments further our understanding of the underlying neurobiology of human epilepsy. In this article we compare the basic features of kindling and human epilepsy, especially human limbic or temporal lobe epilepsy. In this review we focus on a limited number of topics that may show areas in which kindling has been often cited as a tool for better understanding of human epilepsy. These areas include the underlying circuits, the importance of seizure spontaneity, the associated neuropathology, the contribution of genetics, seizure susceptibility, and the underlying pathophysiology of epilepsy. In the course of this article we will show that there are many features that kindling can teach us by direct comparison or implication about human temporal epilepsy. We will also see that not all findings associated with kindling may be applicable to the human condition. Ultimately we wish to encourage critical thinking about kindling and the similarities that it shares and does not share with the human epilepsy so the results from studies using this model are applied rationally to further our insights the mechanisms of human epilepsy.

Evolving into epilepsy: Multiscale electrophysiological analysis and imaging in an animal model

Epilepsy research for the design of seizure detection/prediction neuroprosthetics has been faced with the search for electrophysiologic control parameters that can be used to infer the epileptic state of the animal and be leveraged at a later time to deliver neurotherapeutic feedback. The analysis presented here uses multi-microelectrode array technology to provide an electrophysiologic quantification of a hippocampal neural ensemble during the latent period of epileptogenesis. Through the use of signal processing system identification methodologies, we were able to assess the spatial and temporal interrelations of ensembles of hippocampal neurons and relate them to the evolution of the epileptic condition. High-field magnetic resonance (MR) imaging was used to determine the location of electrode placement and to evaluate hippocampal pyramidal cell structural damage. Long-term single unit activity analysis suggests that hippocampal neurons in both CA1-2 and dentate regions increase the number of occurrences and duration of their bursting activity after injury to the contra-lateral hippocampus. The trends inferred from both single neuron and ensemble analysis suggests that the evolution into epilepsy is not abrupt but modulates gradually from the time of injury.

Altered expression of GABA(A) and GABA(B) receptor subunit mRNAs in the hippocampus after kindling and electrically induced status epilepticus

Epilepsy may result from altered transmission of the principal inhibitory transmitter GABA in the brain. Using in situ hybridization in two animal models of epileptogenesis, we investigated changes in the expression of nine major GABA(A) receptor subunits (alpha1, alpha2, alpha4, alpha5, beta1-beta3, gamma2 and delta) and of the GABA(B) receptor species GABA(B)R1a, GABA(B)R1b and GABA(B)R2 in 1) hippocampal kindling and 2) epilepsy following electrically-induced status epilepticus (SE). Hippocampal kindling triggers a decrease in seizure threshold without producing spontaneous seizures and hippocampal damage, whereas the SE model is characterized by spontaneous seizures and hippocampal damage. Changes in the expression of GABA(A) and GABA(B) receptor mRNAs were observed in both models, and compared with those seen in other models and in human temporal lobe epilepsy. The most prominent changes were a relatively fast (24 h after kindling and electrically-induced SE) and lasting (7 and 30 days after termination of kindling and SE, respectively) reduction of GABA(A) receptor subunit delta mRNA levels (by 43-78%) in dentate granule cells, accompanied by increases in mRNA levels of all three beta-subunits (by 8-79%) and subunit gamma2 (by 11-43%). Levels of the minor subunit alpha4 were increased by up to 60% in dentate granule cells in both animal models, whereas those of subunit alpha5 were decreased 24 h and 30 days after SE, but not after kindling. In cornu ammonis 3 pyramidal cells, downregulation of subunits alpha2, alpha4, alpha5, and beta1-3 was observed in the ventral hippocampus and of alpha2, alpha5, beta3 and gamma2 in its dorsal extension 24 h after SE. Similar but less pronounced changes were seen in sector cornu ammonis 1. Persistent decreases in subunit alpha2, alpha4 and beta2 transcript levels were presumably related to SE-induced cell loss. GABA(B) receptor expression was characterized by increases in GABA(B)R2 mRNA levels at all intervals after kindling and SE. The observed changes suggest substantial and cell specific rearrangement of GABA receptors. Lasting downregulation of subunits delta and alpha5 in granule cells and transient decreases in subunit alpha2 and beta1-3 mRNA levels in cornu ammonis 3 pyramidal cells are suggestive of impaired GABA(A) receptor-mediated inhibition. Persistent upregulation of subunits beta1-3 and gamma2 of the GABA(A) receptor and of GABA(B)R2 mRNA in granule cells, however, may result in activation of compensatory anticonvulsant mechanisms.

Seizures, not hippocampal neuronal death, provoke neurogenesis in a mouse rapid electrical amygdala kindling model of seizures

PURPOSE

Proliferation of neural precursors adjacent to the granule cell layer of the dentate gyrus has been identified in previous epilepsy models. Convincingly demonstrating that seizure activity is the stimulant for neurogenesis, rather than neuronal death or other insults inherent to seizure models, is difficult. To address this we derived a rapid electrical amygdala kindling model in mice known to be resistant to seizure-induced neuronal death as an experimental model of focal seizures and to analyze subsequent neurogenesis.

METHODS

Mice were implanted with bipolar electrodes in the left amygdala and given electrical stimulation (3 s, 100 Hz, 1 ms monophasic square wave pulses every 5 min, 40 in total) while being observed and graded for the development of seizures. Neurogenesis in the hippocampus was assessed by counting bromodeoxyuridine-immunoreactive cells co-labeled for astrocyte (glial fibrillary acidic protein) and neuronal nuclear markers.

RESULTS

Bromodeoxyuridine-reactive cell numbers were three-fold higher in stimulated mice compared with controls at 1 week in the subgranular region and at three weeks extensive co-labeling with neuronal nuclear was noted in cells which had migrated into the body of the granule cell layer, while mice receiving stimulation but failing to kindle did not differ significantly from controls. No increase in neuronal death was detected by terminal deoxynucleotidyl transferase-mediated digoxigenin-11-dUTP nick end labeling, Fluorojade or fluorescent examination of hematoxylin and eosin-stained sections in any inter-group comparison.

CONCLUSIONS

We propose that this kindling paradigm, not previously applied to mice, demonstrates more convincingly than previously the surge in neurogenesis in response to seizures, and the effects of seizures alone in regard to neuronal injury and regeneration.

AAV-mediated hippocampal expression of short and long Homer 1 proteins differentially affect cognition and seizure activity in adult rats

Homer proteins mediate molecular rearrangements leading to changes in spine morphology. This points to a role of Homer in learning and memory. Homer 1c features both the ligand binding domain and a coiled-coiled domain for self-multimerization. Homer 1a lacks the coiled-coiled domain. Here, we report a new isoform which we termed 1g, lacking the Homer ligand binding domain. We dissected the functional roles of the individual Homer 1 domains, encoded by Homer 1a, 1c, and 1g, in vivo. Recombinant adeno-associated virus (AAV)-mediated overexpression of these forms in the hippocampus of adult rats has opposing effects on learning behavior. Increased levels of Homer 1a impaired hippocampal-dependent memory, while Homer 1g and 1c slightly enhanced memory performance. Homer 1g induced anxiety. Moreover, AAV-Homer 1a animals showed attenuation of electrographic seizures in a model of status epilepticus. These results suggest that Homer 1 proteins play an active role in behavioral plasticity.

Acute diffusion abnormalities in the hippocampus of children with new-onset seizures: the development of mesial temporal sclerosis

We studied the role of early diffusion-weighted imaging DWI in the investigation of children with new-onset prolonged seizures which eventually result in unilateral hippocampal sclerosis (HS). We carried out MRI on five children aged 17 months to 7 years including conventional and diffusion-weighted sequences. We calculated apparent diffusion coefficients (ADC) for the affected and the normal opposite hippocampus. Follow-up examinations were performed, including DWI and ADC measurements in four. We studied four children within 3 days of the onset of prolonged psychomotor seizures and showed increased signal on T2-weighted images, and DWI, indicating restricted diffusion, throughout the affected hippocampus. The ADC were reduced by a mean of 14.4% in the head and by 15% in the body of the hippocampus. In one child examined 15 days after the onset of seizures, the ADC were the same on both sides. All five patients showed hippocampal atrophy on follow-up 2-18 months later. In the four patients in whom ADC were obtained on follow-up, they were increased by 19% in the head and 17% in the body. DWI may represent a useful adjunct to conventional MRI for identifying acute injury to the hippocampus which results in sclerosis.

Administration of caspase 3 inhibitor during and after status epilepticus in rat: effect on neuronal damage and epileptogenesis

Symptomatic temporal lobe epilepsy typically develops in three phases: brain damage --> epileptogenesis --> spontaneous seizures (epilepsy). The challenge is to prevent epileptogenesis after injury. We hypothesized that alleviation of damage by caspase inhibitors will reduce epileptogenesis or at least have disease-modifying effects (less severe epilepsy, milder cognitive decline). Epileptogenesis was triggered by amygdala stimulation-induced status epilepticus (SE) in rats and spontaneous seizures were monitored with video-electroencephalography (EEG). First, we tested the neuroprotective effect of a 1-week treatment with caspase 1, 3 or 9 inhibitors (3 micro g/d/i.c.v., started 3 h after the beginning of SE). The least damage to the hippocampus was observed in animals treated with the caspase 3 inhibitor (z-DEVD-fmk) which reduced the enzyme activity to 6% of that in the vehicle group. Thus, z-DEVD-fmk was chosen for long-term studies, in which the treatment regime remained the same except the dose was doubled (6 micro g/d/i.c.v.). Video-EEG monitoring was performed for 3 to 4 weeks, starting either 8 or 14 weeks after SE. One group of animals was tested in water-maze and fear-conditioning tests, and all animals were perfused for histological analysis. Treatment with the caspase 3 inhibitor neither prevented the development of epilepsy, nor had any disease-modifying effects. Mossy fibre sprouting, however, was reduced. The present data indicate that administration of z-DEVD-fmk monotherapy was not antiepileptogenic despite its short-term neuroprotective effects. These findings challenge the idea that prevention of cell death is the primary target for the development of antiepileptogenic compounds.

Neuronal cell death in a rat model for mesial temporal lobe epilepsy is induced by the initial status epilepticus and not by later repeated spontaneous seizures

PURPOSE

To determine whether repeated seizures contribute to hippocampal sclerosis, we investigated whether cell loss in the (para) hippocampal region was related to the severity of chronic seizure activity in a rat model for temporal lobe epilepsy (TLE).

METHODS

Chronic epilepsy developed after status epilepticus (SE) that was electrically induced 3-5 months before. The presence of neuronal damage was assessed by using Fluoro-Jade and dUTP nick end-labeling (TUNEL) of brain sections counterstained with Nissl.

RESULTS

We found a negative correlation between the numbers of surviving hilar cells and the duration of the SE (r = -0.66; p < 0.01). In the chronic phase, we could discriminate between rats with occasional seizures (0.15 +/- 0.05 seizures per day) without progression and rats with progressive seizure activity (8.9 +/- 2.8 seizures/day). In both groups, the number of TUNEL-positive cells in parahippocampal regions was similar and higher than in controls. In the hippocampal formation, this was not significantly different from controls. Fluoro-Jade staining showed essentially the same pattern at 1 week and no positive neurons in chronic epileptic rats.

CONCLUSIONS

Cell death in this rat model is related to the initial SE rather than to the frequency of spontaneous seizures. These results emphasize that it is of crucial importance to stop the SE as soon as possible to prevent extended cell loss and further progression of the disease. They also suggest that neuroprotectants can be useful during the first week after SE, but will not be very useful in the chronic epileptic phase.

Neural growth, neural damage and neurotrophins in the kindling model of epilepsy

Do seizures change the brain? Studies on the kindling model--a widely used animal model of epilepsy--suggest that they do. Dr. Racine, one of the pioneers in the kindling field, describes the basic phenomena of kindling, and discusses the possible roles of cell growth and cell death in this model.

Activation of presynaptic group III metabotropic glutamate receptors depresses spontaneous inhibition in layer V of the rat entorhinal cortex

Whole cell voltage clamp recording was used to investigate neurotransmitter release onto neurones in deep and superficial layers of rat entorhinal cortex in vitro. Activation of metabotropic glutamate receptors with the agonist (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid depressed spontaneous release of the inhibitory neurotransmitter GABA in layer V, but not in layer II. Depression of transmitter release did not persist in the presence of the sodium channel blocker tetrodotoxin. It seems likely that activation of presynaptic glutamate heteroreceptors inhibits action potential dependent release of neurotransmitter via a direct action at the presynaptic terminal. We confirmed that depression of inhibitory neurotransmission in layer V was mediated by group III metabotropic glutamate receptors using a specific group III antagonist, (RS)-cyclopropyl-4-phosphonophenylglycine. Application of the antagonist alone did not alter the frequency of spontaneous neurotransmitter release, suggesting that the metabotropic glutamate receptor is not tonically active. In layer V of the entorhinal cortex, activation of presynaptic metabotropic glutamate receptors enhances spontaneous glutamate release, and inhibits spontaneous release of GABA. These effects may combine to increase random action potential firing in this layer, thereby reducing its capacity for synchrony generation. Our results are consistent with an anticonvulsant action for group III metabotropic glutamate receptors in the entorhinal cortex.

The midline thalamus: alterations and a potential role in limbic epilepsy

PURPOSE

In limbic or mesial temporal lobe epilepsy, much attention has been given to specific regions or cell populations (e.g., the hippocampus or dentate granule cells). Epileptic seizures may involve broader changes in neural circuits, and evidence suggests that subcortical regions may play a role. In this study we examined the midline thalamic regions for involvement in limbic seizures, changes in anatomy and physiology, and the potential role for this region in limbic seizures and epilepsy.

METHODS

Using two rat models for limbic epilepsy (hippocampal kindled and chronic spontaneous limbic epilepsy) we examined the midline thalamus for evidence of involvement in seizure activity, alterations in structure, changes in the basic in vitro physiology of the thalamic neurons. We also explored how this region may influence limbic seizures.

RESULTS

The midline thalamus was consistently involved with seizure activity from the onset, and there was significant neuronal loss in the medial dorsal and reuniens/rhomboid nuclei. In addition, thalamic neurons had changes in synaptically mediated and voltage-gated responses. Infusion of lidocaine into the midline thalamus significantly shortened afterdischarge duration.

CONCLUSIONS

These observations suggest that this thalamic region is part of the neural circuitry of limbic epilepsy and may play a significant role in seizure modulation. Local neuronal changes can enhance the excitability of the thalamolimbic circuits.

Pathogenic significance of neuronal migration disorders in temporal lobe epilepsy

To assess the epileptogenic lesions, a series of 202 cases with temporal lobectomy were analyzed histopathologically. The severity of hippocampal neuronal loss in patients with temporal lobe epilepsy was quantitatively analyzed and compared against autopsy controls of patients who died of nonneurologic disorders. For the histopathologic diagnosis of neuronal migration disorder (NMD), immunohistochemical stains for neurofilament protein (NF-M/H) and microtubule-associated protein 2 (MAP2) and Bielschowsky silver stains were routinely performed. Histopathology of NMD was classified by the 4-grade system. MAP2 immunoreactivity was useful in the identification of loss of normal polarization of dendrites in the abnormal neurons. NF-M/H immunohistochemistry and silver stains effectively labeled microscopic or occult lesions of NMD (grade II and III). Ammon hom sclerosis (AHS) was identified in 73.3% and NMD in 57.9%. There was more than 50% neuronal cell loss in 82.8% of AHS, and variable degrees of cell loss were observed in the dual-pathology groups. The frequency of dual pathology (both AHS and NMD) was 65.0% and showed relatively equal distributions in grades I, II, III, whereas the pure NMD group were classified predominantly as grades II and III. NMD might be a basic pathogenic substrate causing temporal lobe epilepsy. The dual pathology may indicate the presence of epileptogenic lesions in the neocortical and temporolimbic areas.

Neuronal microdysgenesis and acquired lesions of the hippocampal formation connected with seizure activities in Ihara epileptic rat

The present study was designed to examine the morphological features of the hippocampal formation in the Ihara epileptic rat (IER), and to characterize genetically programmed lesions and acquired lesions connected with seizure activities. Neuropathological investigation of the hippocampal formation was performed in four separate groups, 2-month-old IERs with neither abnormal behaviors nor any seizure activity, and 12-month-old IERs of both sexes with abnormal behaviors, circling seizures or generalized tonic-clonic convulsions. In every IER examined, there were invariable and fundamental neuropathological findings consisting of abnormal neuronal clusters in the CA1 of the hippocampal formation. Moreover, disarrangement of neuronal cells, such as dispersion and gaps in lamination of pyramidal neurons, were observed. These changes were thought to represent genetically programmed lesions, neuronal microdysgenesis, because they were common findings in 2-month-old and 12-month-old IERs of both sexes. An enlargement of the dentate gyrus was also found in rats that experienced generalized tonic-clonic convulsions or circling seizures. This enlargement of the dentate gyrus, on the other hand, was categorized as a secondary and acquired lesion connected with seizure activities. It is suggested that the neuronal microdysgenesis in the hippocampal formation of IER has an intimate relationship with epileptogenesis and/or an enhancement of seizure susceptibility.

Aberrant neuronal physiology in the basal nucleus of the amygdala in a model of chronic limbic epilepsy

Limbic epilepsy is a chronic condition associated with a broad zone of seizure onset and pathology. Studies have focused mainly on the hippocampus, but there are indications that changes occur in other regions of the limbic system. This study used in vitro intracellular recording and histology to examine alterations to the physiology and anatomy of the basal nucleus of the amygdala in a rat model of chronic limbic epilepsy characterized by spontaneously recurring seizures. Epileptic pyramidal neuron responses evoked by stria terminalis stimulation revealed hyperexcitability characterized by multiple action potential bursts and no evident inhibitory potentials. In contrast, no hyperexcitability was observed in amygdalar neurons from kindled (included as a control for seizure activity) or control rats. Blockade of ionotropic glutamate receptors unmasked inhibitory postsynaptic potentials in epileptic pyramidal neurons. Control, kindled and epileptic inhibitory potentials were predominantly biphasic, with fast and slow components, but a few cells exhibited only the fast component (2/12 in controls, 0/3 in kindled, 3/10 in epileptic). Epileptic fast inhibitory potentials had a more rapid onset and shorter duration than control and kindled. Approximately 40% of control neurons exhibited spontaneous inhibitory potentials; no spontaneous inhibitory potentials were observed in neurons from kindled or epileptic rats. A preliminary histological examination revealed no gross alterations in the basal amygdala from epileptic animals. These results extend previous findings from this laboratory that hyperexcitability is found in multiple epileptic limbic regions and may be secondary to multiple alterations in excitatory and inhibitory efficacy. Because there were no differences between control and kindled animals, the changes observed in the epileptic animals are unlikely to be secondary to recurrent seizures.

Laminar differences in recurrent excitatory transmission in the rat entorhinal cortex in vitro

Paired intracellular recordings were used to investigate recurrent excitatory transmission in layers II, III and V of the rat entorhinal cortex in vitro. There was a relatively high probability of finding a recurrent connection between pairs of pyramidal neurons in both layer V (around 12%) and layer III (around 9%). In complete contrast, we have failed to find any recurrent synaptic connections between principal neurons in layer II, and this may be an important factor in the relative resistance of this layer in generating synchronized epileptiform activity. In general, recurrent excitatory postsynaptic potentials in layers III and V of the entorhinal cortex had similar properties to those recorded in other cortical areas, although the probabilities of connection are among the highest reported. Recurrent excitatory postsynaptic potentials recorded in layer V were smaller with faster rise times than those recorded in layer III. In both layers, the recurrent potentials were mediated by glutamate primarily acting at alpha-amino-3-hydroxy-5-methyl-4-isoxazole receptors, although there appeared to be a slow component mediated by N-methyl-D-aspartate receptors. In layer III, recurrent transmission failed on about 30% of presynaptic action potentials evoked at 0.2Hz. This failure rate increased markedly with increasing (2, 3Hz) frequency of activation. In layer V the failure rate at low frequency was less (19%), and although it increased at higher frequencies this effect was less pronounced than in layer III. Finally, in layer III, there was evidence for a relatively high probability of electrical coupling between pyramidal neurons. We have previously suggested that layers IV/V of the entorhinal cortex readily generate synchronized epileptiform discharges, whereas layer II is relatively resistant to seizure generation. The present demonstration that recurrent excitatory connections are widespread in layer V but not layer II could support this proposal. The relatively high degree of recurrent connections and electrical coupling between layer III cells may be a factor in it's susceptibility to neurodegeneration during chronic epileptic conditions.

Anticonvulsant effects of gamma surgery in a model of chronic spontaneous limbic epilepsy in rats

OBJECT

The management of intractable epilepsy remains a challenge, despite advances in its surgical and nonsurgical treatment. The identification of low-risk, low-cost therapeutic strategies that lead to improved outcome is therefore an important ongoing goal of basic and clinical research. Single-dose focal ionizing beam radiation delivered at necrosis-inducing and subnecrotic levels was investigated for its effects on seizure activity by using an established model of chronic recurrent spontaneous limbic seizures in rats.

METHODS

A single 90-minute period of repetitive electrical stimulation (inducing stimulus) of the hippocampus in rats elicited a single episode of status epilepticus, followed by a 2- to 4-week seizure-free period. Spontaneous recurrent seizures developed subsequently and persisted for the duration of monitoring (2-10 months). Simultaneous computerized electroencephalography and video recording were used to monitor the animals. After the establishment of spontaneous recurrent seizures, bilateral radiation centered in the ventral hippocampal formation was administered with the Leksell gamma knife, aided by a stereotactic device custom made for small animals. A center dose of 10, 20, or 40 Gy was administered using a 4-mm collimator. Control animals were subjected to the same seizure-inducing stimulus but underwent a sham treatment instead of gamma irradiation. In a second experiment, the authors examined the effects of gamma irradiation on the proclivity of hippocampal neurons to display epileptiform discharges. Naive animals were irradiated with a single 40-Gy dose, as already described. Slices of the hippocampus were prepared from animals killed between 1 and 178 days postirradiation. Sensitivity to penicillin-induced epileptiform spiking was examined in vitro in slices prepared from control and irradiated rat brains.

CONCLUSIONS

In the first experiment, single doses of 20 or 40 Gy (but not 10 Gy) reduced substantially, and in some cases eliminated, behaviorally and electrographically recognized seizures. Significant reductions in both the frequency and duration of spontaneous seizures were observed during a follow-up period of up to 10 months postradiation. Histological examination of the targeted region did not reveal signs of necrosis. These findings indicate that single-dose focal ionizing beam irradiation at subnecrotic dosages reduces or eliminates repetitive spontaneous seizures in a rat model of temporal lobe epilepsy. In the second experiment, synaptically driven neuronal firing was shown to be intact in hippocampal neurons subjected to 40-Gy doses. However, the susceptibility to penicillin-induced epileptiform activity was reduced in the brain slices of animals receiving 40-Gy doses, compared with those from control rats that were not irradiated. The results provide rational support for the utility of subnecrotic gamma irradiation as a therapeutic strategy for treating epilepsy. These findings also provide evidence that a functional increase in the seizure threshold of hippocampal neurons contributes to the anticonvulsant influence of subnecrotic gamma irradiation.

Effect of theophylline and trimethobenzamide when given during kainate-induced status epilepticus: an improved histopathologic rat model of human hippocampal sclerosis

PURPOSE

The most common pathology in temporal lobe epilepsy (TLE) is hippocampal sclerosis. It is controversial whether status epilepticus (SE) or prolonged seizures plus secondary cerebral injuries are pathogenic mechanisms of hippocampal sclerosis. This study addressed this question in rat models of TLE.

METHODS

Hippocampal neuron densities and supragranular mossy fiber sprouting were determined in adult rats subjected to systemic kainate-induced SE (KA-only) and KA-induced SE followed 75 minutes later by theophylline (KA/Theo) or trimethobenzamide (KA/Tri). These drugs probably decrease seizure-induced cerebral hyperemia or hypertension.

RESULTS

Compared with controls and KA-only rats, KA/Tri and KA/Theo rats showed decreased CA3b and CA1 neuron densities (i.e., greater Sommer's sector injury). In addition, KA/Tri rats showed that increased trimethobenzamide dosages were associated with decreased hilar, CA3c, CA3b, CA1, and subiculum neuron densities. There were no significant differences in supragranular mossy fiber sprouting between KA-only, KA/Tri, and KA/Theo rats.

CONCLUSIONS

Pharmacologic manipulations during KA-induced SE are associated with differences in hippocampal pathology, especially in Sommer's sector, and the final pattern of damage and axon sprouting shows histopathologic similarities to that in patients with hippocampal sclerosis. Our findings support the hypothesis that secondary physiologic insults during SE that are likely to decrease seizure-induced cerebral hyperemia and hypertension may generate greater hippocampal neuronal injury compared with SE alone, and this may be a pathogenic mechanism of human hippocampal sclerosis in patients with TLE.

Presentation, evaluation, and treatment of nonconvulsive status epilepticus

Nonconvulsive status epilepticus (NCSE) is much more common than is generally appreciated. It is certainly underdiagnosed, but its presentation is protean. Diagnostic criteria and treatment are controversial. Absence status is characterized by confusion or diminished responsiveness, with occasional blinking or twitching, lasting hours to days, with generalized spike and slow wave discharges on the EEG. Complex partial status consists of prolonged or repetitive complex partial seizures (with a presumed focal onset) and produces an "epileptic twilight state" with fluctuating lack of responsiveness or confusion. There is a clear overlapping of syndromes. Other confused, stuporous, or comatose patients with rapid, rhythmic, epileptiform discharges on the EEG may have "electrographic" status and should be considered in the same diagnostic category. NCSE typically occurs following supposedly controlled convulsions or other seizures, but with persistent neurologic dysfunction despite apparently adequate treatment. Confusion in the elderly or among emergency room patients is also a typical setting. The diagnosis of NCSE usually involves an abnormal mental status with diminished responsiveness, a supportive EEG, and often a response to anticonvulsant medication. All patients have clinical neurologic deficits, but the EEG findings and response to seizure medication are variable and are more controversial criteria. The response to drugs can be delayed for up to days. Experimental models and pathologic studies showing neuronal damage from status epilepticus pertain primarily to generalized convulsive status. Most morbidity from NCSE appears due to the underlying illness rather than to the NCSE itself. Some cases of prolonged NCSE or those with concomitant systemic illness, focal lesions, or very rapid epileptiform discharges may suffer more long-lasting damage. Although clinical studies show little evidence of permanent neurologic injury, the prolonged memory dysfunction in several cases and the similarities to convulsive status suggest that NCSE should be treated expeditiously. The diagnosis is important to make because NCSE impairs the patient's health significantly, and it is often a treatable and completely reversible condition.

Status epilepticus: risk factors and complications

Status epilepticus is common and associated with significant mortality and complications. It affects approximately 50 patients per 100,000 population annually and recurs in >13%. History of epilepsy is the strongest single risk factor for generalized convulsive status epilepticus. More than 15% of patients with epilepsy have at least one episode of status epilepticus and low antiepileptic drug levels are a potentially modifiable risk factor. Other risks include young age, genetic predisposition, and acquired brain insults. Fever is a very common risk in children, as is stroke in adults. Mortality rates are 15% to 20% in adults and 3% to 15% in children. Acute complications result from hyperthermia, pulmonary edema, cardiac arrhythmias, and cardiovascular collapse. Long-term complications include epilepsy (20% to 40%), encephalopathy (6% to 15%), and focal neurologic deficits (9% to 11%). Neuronal injury leading to temporal lobe epilepsy is probably mediated by excess excitation via activation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors and consequent elevated intracellular calcium that causes acute necrosis and delayed apoptotic cell death. Some forms of nonconvulsive status epilepticus may also lead to neuronal injury by this mechanism, but others may not. Based on clinical and experimental observations, complex partial status epilepticus is more likely to result in neuronal injury similar to generalized convulsive status epilepticus. Absence status epilepticus is much less likely to result in neuronal injury, and complications because it may be mediated primarily through excess inhibition. Future research strategies to prevent complications of status epilepticus include the study of new drugs (including NMDA antagonists, new drug delivery systems, and drug combinations) to stop seizure activity and prevent acute and delayed neuronal injury that leads to the development of epilepsy.

Activation of presynaptic group III metabotropic receptors enhances glutamate release in rat entorhinal cortex

The role of group III metabotropic glutamate receptors (mGluRs) in modulating excitatory synaptic transmission was investigated in the rat entorhinal cortex (EC) in vitro. AMPA receptor-mediated excitatory postsynaptic currents (EPSCs) were recorded in the whole cell configuration of the patch-clamp technique from visually identified neurons in layers V and II. In layer V, bath application of the specific group III mGluR agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4, 500 microM) resulted in a marked facilitation of both spontaneous and activity-independent "miniature" (s/mEPSC) event frequency. The facilitatory effect of L-AP4 (100 microM) on sEPSC frequency prevailed in the presence of DL-2-amino-5-phosphonopentanoic acid (100 microM) but was abolished by the group III antagonist (RS)-cyclopropyl-4-phosphonophenylglycine (20 microM). These data confirmed that group III mGluRs, and not N-methyl-D-aspartate (NMDA) receptors were involved in the response to L-AP4. Bath application of the specific mGluR4a agonist (1S,3R,4S)-1-aminocyclopentane-1,2, 4-tricarboxylic acid (20 microM) also had a facilitatory effect on sEPSC frequency, suggesting involvement of mGluR4a. In layer II neurons, L-AP4 caused a reduction in sEPSC frequency but did not affect mEPSCs recorded in the presence of tetrodotoxin. These findings suggest that a group III mGluR with mGluR4a-like pharmacology is involved in modulating synaptic transmission in layer V cells of the EC. The effect on mEPSCs suggests that this receptor is located presynaptically and that its activation results in a direct facilitation of glutamate release. This novel facilitatory effect is specific to layer V and, to our knowledge, is the first report of a direct facilitatory action of group III mGluRs on synaptic transmission. In layer II, L-AP4 had an inhibitory effect on glutamate release similar to that reported in other brain regions.

Effect of long-term spontaneous recurrent seizures or reinduction of status epilepticus on the development of supragranular mossy fiber sprouting

In a recent report we have shown that a protein synthesis inhibitor, cycloheximide (CHX), is able to block the mossy fiber sprouting (MFS) that would otherwise be triggered by pilocarpine (Pilo)-induced status epilepticus (SE), and also gives relative protection against hippocampal neuronal death. Under this condition animals still showed spontaneous recurrent seizures (SRS) which led us to question the role played by sprouted mossy fibers in generating those seizures. In both patients and animal models of epilepsy the relative contribution of SE (when present) and/or SRS for the development of MFS is not known. In the present study we investigated the relationship between MFS, SE and SRS, and evaluated whether the CHX-induced blockade of MFS was transient or permanent in nature. We performed a chronic study which included animals subject to Pilo-induced SE in the presence of CHX and sacrificed between 8 and 10 months later, and animals that were subject to Pilo-induced SE in the presence of CHX and underwent a reinduction of SE with Pilo, 45 days after the first induction, but this time in the absence of CHX. Re-induction of SE or a long period of chronic seizures, were able to trigger supragranular MFS even in animals where the first (or only) SE event was triggered in the presence of CHX. MFS did not show any association with the frequency of SRS, and thus seemed to depend more critically on time. Our current findings allow us to suggest that MFS are neither the cause nor the consequence of SRS in the pilocarpine model.

Evidence against permanent neurologic damage from nonconvulsive status epilepticus

Nonconvulsive status epilepticus (NCSE) is much more common than is generally appreciated and is certainly underdiagnosed, but its long-term effects are largely undetermined and remain controversial. There is increasing experimental evidence that generalized convulsive status epilepticus produces lasting neuropathologic damage in the hippocampus, but experimental models often include provocation of status epilepticus (SE) by physical (e.g., electrical stimulation) and chemical (including excitotoxic) agents that may induce damage independent of the epileptiform discharges. Also, damage appears to be related to the intensity and duration of electrical stimulation. Such models usually include high-frequency discharges sustained over long periods, somewhat different from the electrical activity of typical human NCSE. Pathologic studies in humans pertain primarily to patients who have had generalized convulsive status epilepticus. Clinical studies of the effects of NCSE are mandatory, but conclusions are difficult to come by, in part because of diverse definitions of NCSE. An altered mental status is obligatory, but the pertinent EEG and medication response criteria are controversial. Response to medication can be delayed by many hours or even days. Absence SE appears to cause no lasting effects. Complex partial SE is less uniform. Most reported cases have returned to baseline neurologic function, but several well-described patients have had prolonged memory deficits. The significance of other deficits is difficult to interpret in light of concomitant vascular and other diseases causing neurologic dysfunction. Clinical series usually lack premorbid neurologic and neuropsychologic assessment. The few exceptions are complicated by preexisting mental retardation and other deficits, by the coexistence of progressive illness, by the later effects of recurrent seizures, and almost always by the confounding influence of anticonvulsant medications. Most morbidity appears attributable to the underlying illnesses rather than to the NCSE itself. It is possible that relatively infrequent cases of prolonged NCSE or those with the synergistic effect of concomitant systemic illness, focal lesions, or very rapid excitatory epileptiform discharges may suffer more long-lasting damage, but these observations are still preliminary. NCSE should be treated expeditiously because of the acute neurologic impairment of the patients, because of the attendant morbidity including physical injury, and because it may go on to generalized convulsions. There is reasonable concern about possible long-term effects, but permanent neurologic damage from NCSE has not yet been established as a mandate for urgent treatment.

Animal models of nonconvulsive status epilepticus

Nonconvulsive status epilepticus includes three clinical situations: complex partial status epilepticus; absence status epilepticus: and obtundation in the presence of electrographic status epilepticus. Animal models that provide information helpful to clinical management exist for both complex partial and absence status epilepticus. In models of complex partial status epilepticus (pilocarpine, kainic acid, and various protocols using electrical stimulation), neuronal damage in discrete neuronal populations follows an episode of status epilepticus. Hippocampal populations are particularly susceptible to neuropathologic sequelae. Although it is difficult in some cases to distinguish whether the inducing agent or the status epilepticus causes neuropathology, the similar patterns of damage caused by different inducing stimuli provide converging lines of evidence suggesting that the neuropathologic consequences stem at least in part from status epilepticus. In models of absence status epilepticus (genetic mutants, pentylenetetrazole), there is relatively scarce neuropathology that can be attributed directly to status epilepticus. Together these data from animal models suggest that neuropathologic consequences from complex partial status epilepticus may be more severe than those from absence status epilepticus. If these findings translate to patients, then nonconvulsive status epilepticus of the complex partial type should be managed more aggressively than nonconvulsive status epilepticus of the absence type.

Status epilepticus causes long-term NMDA receptor-dependent behavioral changes and cognitive deficits

PURPOSE

The role of N-methyl-D-aspartate (NMDA)-receptor activation on behavioral and cognitive changes after status epilepticus (SE) is unknown. In this study, behavioral and cognitive changes after SE were evaluated in the short and long term and in rats in which the NMDA receptor was inactivated during SE.

METHODS

Pilocarpine (350 mg/kg) was injected to induce SE. Inhibition of the NMDA receptor during SE was achieved with MK-801 (4 mg/kg). Seizure intensity during SE was monitored by electroencephalography (EEG). After SE, behavioral studies were performed to identify abnormal behavior by using behavioral tests adapted from Moser's functional observational battery. Cognitive changes were assessed by using the Morris Water Maze (MWM).

RESULTS

Pilocarpine-treated animals scored significantly higher on two of the behavioral tests: the Touch test and the Pick-Up test. These behavioral changes occurred very soon after SE, with the earliest changes observed 2 days after SE and persisting for the life of the animal. Inhibition of the NMDA receptor with MK-801 completely inhibited these behavioral changes under conditions that did not alter the duration of SE. In addition, pilocarpine-treated animals exhibited cognitive deficits as determined by using the MWM. Six weeks after SE, the animals displayed significantly longer latencies to locate the hidden platform on this test. The impaired performance on the MWM also occurred as early as 5 days after SE. These cognitive deficits were prevented in animals treated with MK-801 during SE.

CONCLUSIONS

The results indicate that behavioral and cognitive changes occur soon after SE, are permanent, and are dependent on NMDA-receptor activation during SE. NMDA-receptor activation may play an important role in causing cognitive and behavioral morbidity after recovery from SE.

Functional anatomy of limbic epilepsy: a proposal for central synchronization of a diffusely hyperexcitable network

The limbic/mesial temporal lobe epilepsy syndrome has been defined as a focal epilepsy, with the implication that there is a well defined focus of onset, traditionally centered around the hippocampus. The pathology of the hippocampus in this syndrome has been well described and a number of physiological abnormalities have been defined in this structure in animal models and humans with epilepsy. However, anatomical and physiological abnormalities have also been described in other limbic sites in this form of epilepsy. Previous studies have shown broadly synchronized or multifocal seizure onset within the limbic system of the animal models and human patients. We hypothesized that the epileptogenic circuit for the initiation of seizures was distributed throughout the limbic system with a possible central synchronizing process. In vitro studies showed that multiple limbic sites in epileptic animals (hippocampus, entorhinal cortex, piriform cortex and amygdala) have epileptiform changes with prolonged depolarizations and multiple superimposed action potentials. In vivo studies revealed that thalamic stimulation yields short latency excitatory responses in the entorhinal cortex and hippocampus. In addition, in epileptic animals, thalamic stimulation caused epileptiform responses in the hippocampus. Based on the findings of this study and on previous anatomy and physiology reports, we hypothesize that the process of seizure initiation involves broad circuit interactions involving multiple independent limbic structures, and that the midline thalamus may act as a physiological synchronizer. We offer a new proposal for the functional anatomy of limbic epilepsy that takes widespread hyperexcitability in the limbic system and the potential for thalamic synchronization into consideration.

Hippocampal AMPA and NMDA mRNA levels and subunit immunoreactivity in human temporal lobe epilepsy patients and a rodent model of chronic mesial limbic epilepsy

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 NMDAR2(a&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 NMDAR2(a&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 NMDAR2(a&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.

Ontogeny of altered synaptic function in a rat model of chronic temporal lobe epilepsy

In the limbic status model of chronic temporal lobe epilepsy, hippocampal stimulation induces acute status epilepticus in rats; recurrent, spontaneous seizures develop following an asymptomatic silent period lasting several weeks. Previous work has shown increased excitability and decreased inhibition in CA1 pyramidal neurons in chronically epileptic animals. To determine the relationship of altered cellular responses to seizure onset, in vitro intracellular recording was used to follow the evolution of changes in synaptic physiology occurring during the seizure-free silent period. Pyramidal cells displayed increasing epileptiform activity throughout the period investigated, 3-14 days following status; the mean number of evoked action potentials from 1.1+/-0.05 in control cells to 2.4+/-0.4 early (3 days after status) and 4. 3+/-0.7 late (14 days) in the silent period. Monosynaptic inhibitory postsynaptic potentials mediated by gamma-aminobutyric acid-A receptors in silent period cells differed markedly from controls. Area, rise time, and duration of these potentials decreased by 40-60% within 3 days following status and to values commensurate with chronically epileptic animals in 7 to 10 days. gamma-Aminobutyric acid-B receptor-mediated IPSPs diminished more gradually in the silent period, reaching a minimum at day 14. In contrast, presynaptic gamma-aminobutyric acid-B receptor function showed maximum impairment 3 days after status. The benzodiazepine type 1 receptor agonist zolpidem reduced hyperexcitability in both silent period and chronically epileptic cells, but was more effective at unmasking the underlying IPSP in silent period neurons. The results indicate that changes in different components of pyramidal cell inhibitory synaptic physiology associated with chronic epilepsy in this model evolve individually at different rates, but are all complete before seizure onset. Although the results do not imply causality, they do suggest that the development of physiological changes in CA1 pyramidal cells may contribute to the lag period preceding the onset of chronic seizures.

Responses of deep entorhinal cortex are epileptiform in an electrogenic rat model of chronic temporal lobe epilepsy

We investigated whether entorhinal cortex (EC) layer IV neurons are hyperexcitable in the post-selfsustaining limbic status epilepticus (post-SSLSE) animal model of temporal lobe epilepsy. We studied naive rats (n = 44), epileptic rats that had experienced SSLSE resulting in spontaneous seizures (n = 45), and electrode controls (n = 7). There were no differences between electrode control and naive groups, which were pooled into a single control group. Intracellular and extracellular recordings were made from deep layers of EC, targeting layer IV, which was activated by stimulation of the superficial layers of EC or the angular bundle. There were no differences between epileptic and control neurons in basic cellular characteristics, and all neurons were quiescent under resting conditions. In control tissue, 77% of evoked intracellular responses consisted of a short-duration [8.6 +/- 1.3 (SE) ms] excitatory postsynaptic potential and a single action potential followed by gamma-aminobutyric acid-A (GABAA) and GABAB inhibitory post synaptic potentials (IPSPs). Ten percent of controls did not contain IPSPs. In chronically epileptic tissue, evoked intracellular responses demonstrated prolonged depolarizing potentials (256 +/- 39 ms), multiple action potentials (13 +/- 4), and no IPSPs. Ten percent of epileptic responses were followed by rhythmic "clonic" depolarizations. Epileptic responses exhibited an all-or-none response to progressive increases in stimulus intensity and required less stimulation to elicit action potentials. In both epileptic and control animals, intracellular responses correlated precisely in morphology and duration with extracellular field potentials. Severing the hippocampus from the EC did not alter the responses. Duration of intracellular epileptic responses was reduced 22% by the N-methyl--aspartate (NMDA) antagonist (-)-2-amino-5-phosphonovaleric acid (APV), but they did not return to normal and IPSPs were not restored. Epileptic and control responses were abolished by the non-NMDA antagonist 6, 7-dinitroquinoxaline-2-3-dione (DNQX). A monosynaptic IPSP protocol was used to test connectivity of inhibitory interneurons to primary cells by direct activation of interneurons with a stimulating electrode placed near the recording electrode in the presence of APV and DNQX. Using this protocol, IPSPs similar to control (P > 0.05) were seen in epileptic cells. The findings demonstrate that deep layer EC cells are hyperexcitable or "epileptiform" in this model. Hyperexcitability is not due to interactions with the hippocampus. It is due partially to augmented NMDA-mediated excitation. The lack of IPSPs in epileptic neurons may suggest inhibition is impaired, but we found evidence that inhibitory interneurons are connected to their target cells and are capable of inducing IPSPs.