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

Molecular Biomarkers of Neuronal Injury in Epilepsy Shared with Neurodegenerative Diseases

In neurodegenerative diseases, changes in neuronal proteins in the cerebrospinal fluid and blood are viewed as potential biomarkers of the primary pathology in the central nervous system (CNS). Recent reports suggest, however, that level of neuronal proteins in fluids also alters in several types of epilepsy in various age groups, including children. With increasing evidence supporting clinical and sub-clinical seizures in Alzheimer's disease, Lewy body dementia, Parkinson's disease, and in other less common neurodegenerative conditions, these findings call into question the specificity of neuronal protein response to neurodegenerative process and urge analysis of the effects of concomitant epilepsy and other comorbidities. In this article, we revisit the evidence for alterations in neuronal proteins in the blood and cerebrospinal fluid associated with epilepsy with and without neurodegenerative diseases. We discuss shared and distinctive characteristics of changes in neuronal markers, review their neurobiological mechanisms, and consider the emerging opportunities and challenges for their future research and diagnostic use.

Brain pathology in focal status epilepticus: evidence from experimental models

Status Epilepticus (SE) is often a neurological emergency characterized by abnormally sustained, longer than habitual seizures. The new ILAE classification reports that SE "…can have long-term consequences including neuronal death, neuronal injury…depending on the type and duration of seizures". While it is accepted that generalized convulsive SE exerts detrimental effects on the brain, it is not clear if other forms of SE, such as focal non-convulsive SE, leads to brain pathology and contributes to long-term deficits in patients. With the available clinical and experimental data, it is hard to discriminate the specific action of the underlying SE etiologies from that exerted by epileptiform activity. This information is highly relevant in the clinic for better treatment stratification, which may include both medical and surgical intervention for seizure control. Here we review experimental studies of focal SE, with an emphasis on focal non-convulsive SE. We present a repertoire of brain pathologies observed in the most commonly used animal models and attempt to establish a link between experimental findings and human condition(s). The extensive literature on focal SE animal models suggest that the current approaches have significant limitations in terms of translatability of the findings to the clinic. We highlight the need for a more stringent description of SE features and brain pathology in experimental studies in animal models, to improve the accuracy in predicting clinical translation.

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.

Neurodegeneration and Pathology in Epilepsy: Clinical and Basic Perspectives

Epilepsy is commonly associated with a number of neurodegenerative and pathological alterations in those areas of the brain that are involved in repeated electrographic seizures. These most prominently include neuron loss and an increase in astrocyte number and size but may also include enhanced blood-brain barrier permeability, the formation of new capillaries, axonal sprouting, and central inflammation. In animal models in which seizures are either repeatedly elicited or are self-generated, a similar set of neurodegenerative and pathological alterations in brain anatomy are observed. The primary causal agent responsible for these alterations may be the cascade of events that follow a seizure and lead to an hypoperfusion/hypoxic episode. While epilepsy has long and correctly been considered an electrical disorder, the vascular system likely plays an important causal role in the neurodegeneration and pathology that occur as a consequence of repeated seizures.

A new rapid kindling variant for induction of cortical epileptogenesis in freely moving rats

Kindling, one of the most used models of experimental epilepsy is based on daily electrical stimulation in several brain structures. Unlike the classic or slow kindling protocols (SK), the rapid kindling types (RK) described until now require continuous stimulation at suprathreshold intensities applied directly to the same brain structure used for subsequent electrophysiological and immunohistochemical studies, usually the hippocampus. However, the cellular changes observed in these rapid protocols, such as astrogliosis and neuronal loss, could be due to experimental manipulation more than to epileptogenesis-related alterations. Here, we developed a new RK protocol in order to generate an improved model of temporal lobe epilepsy (TLE) which allows gradual progression of the epilepsy as well as obtaining an epileptic hippocampus, thus avoiding direct surgical manipulation and electric stimulation over this structure. This new protocol consists of basolateral amygdala (BLA) stimulation with 10 trains of biphasic pulses (10 s; 50 Hz) per day with 20 min-intervals, during 3 consecutive days, using a subconvulsive and subthreshold intensity, which guarantees tissue integrity. The progression of epileptic activity was evaluated in freely moving rats through electroencephalographic (EEG) recordings from cortex and amygdala, accompanied with synchronized video recordings. Moreover, we assessed the effectiveness of RK protocol and the establishment of epilepsy by evaluating cellular alterations of hippocampal slices from kindled rats. RK protocol induced convulsive states similar to SK protocols but in 3 days, with persistently lowered threshold to seizure induction and epileptogenic-dependent cellular changes in amygdala projection areas. We concluded that this novel RK protocol introduces a new variant of the chronic epileptogenesis models in freely moving rats, which is faster, highly reproducible and causes minimum cell damage with respect to that observed in other experimental models of epilepsy.

Are there really "epileptogenic" mechanisms or only corruptions of "normal" plasticity?

Plasticity in the nervous system, whether for establishing connections and networks during development, repairing networks after injury, or modifying connections based on experience, relies primarily on highly coordinated patterns of neural activity. Rhythmic, synchronized bursting of neuronal ensembles is a fundamental component of the activity-dependent plasticity responsible for the wiring and rewiring of neural circuits in the CNS. It is therefore not surprising that the architecture of the CNS supports the generation of highly synchronized bursts of neuronal activity in non-pathological conditions, even though the activity resembles the ictal and interictal events that are the hallmark symptoms of epilepsy. To prevent such natural epileptiform events from becoming pathological, multiple layers of homeostatic control operate on cellular and network levels. Many data on plastic changes that occur in different brain structures during the processes by which the epileptogenic aggregate is constituted have been accumulated but their role in counteracting or promoting such processes is still controversial. In this chapter we will review experimental and clinical evidence on the role of neural plasticity in the development of epilepsy. We will address questions such as: is epilepsy a progressive disorder? What do we know about mechanism(s) accounting for progression? Have we reliable biomarkers of epilepsy-related plastic processes? Do seizure-associated plastic changes protect against injury and aid in recovery? As a necessary premise we will consider the value of seizure-like activity in the context of normal neural development.

Morphological changes among hippocampal dentate granule cells exposed to early kindling-epileptogenesis

Temporal lobe epilepsy is associated with changes in the morphology of hippocampal dentate granule cells. These changes are evident in numerous models that are associated with substantial neuron loss and spontaneous recurrent seizures. By contrast, previous studies have shown that in the kindling model, it is possible to administer a limited number of stimulations sufficient to produce a lifelong enhanced sensitivity to stimulus evoked seizures without associated spontaneous seizures and minimal neuronal loss. Here we examined whether stimulation of the amygdala sufficient to evoke five convulsive seizures (class IV or greater on Racine's scale) produce morphological changes similar to those observed in models of epilepsy associated with substantial cell loss. The morphology of GFP-expressing granule cells from Thy-1 GFP mice was examined either 1 day or 1 month after the last evoked seizure. Interestingly, significant reductions in dendritic spine density were evident 1 day after the last seizure, the magnitude of which had diminished by 1 month. Further, there was an increase in the thickness of the granule cell layer 1 day after the last evoked seizure, which was absent a month later. We also observed an increase in the area of the proximal axon, which again returned to control levels a month later. No differences in the number of basal dendrites were detected at either time point. These findings demonstrate that the early stages of kindling epileptogenesis produce transient changes in the granule cell body layer thickness, molecular layer spine density, and axon proximal area, but do not produce striking rearrangements of granule cell structure.

Seizure-induced changes in neuropeptide Y-containing cortical neurons: Potential role for seizure threshold and epileptogenesis

Seizure activity induces transient changes in the levels of neuropeptide Y (NPY) and somatostatin (SS) in various brain regions, but it remains unclear whether this effect can persist for long periods and whether it is relevant to epileptogenesis. We report that brief seizures evoked by electroshock produced an increase in the number of NPY neurons in the dentate hilus and retrosplenial cortex, an effect that lasted 10 weeks. The number of hilar SS neurons remained unchanged. However, the pentylenetetrazole seizure threshold was somewhat decreased in electroshock-treated rats. Despite this, no spontaneous seizures were detected in this group. In contrast, status epilepticus (pilocarpine model) produced loss of the hilar NPY and SS cells. Moreover, all rats with status epilepticus showed spontaneous behavioral seizures and their seizure threshold was markedly decreased. These findings support the notion that sustained NPY overexpression induced by brief seizures can be important in preventing epileptogenesis.

Cytogenesis in the adult rat dentate gyrus is increased following kindled seizures but is unaltered in pharmacological models of absence seizures

The production of new neurons continues throughout adulthood in the dentate gyrus of the hippocampal formation, and is believed to play a role in hippocampus-dependent learning and memory. Seizure-induced changes in adult neurogenesis have been examined primarily in convulsive rodent seizure models, but not in models of nonconvulsive absence seizures. This study examined progenitor cell proliferation in the gamma-hydroxybutyrate (GHB) model of typical absence seizures and the AY-9944 model of atypical absence seizures, and compared these results with changes seen in the rat amygdala kindling model. Kindled subjects were found to have 189% more proliferating cells than sham-kindled control subjects, whereas no significant difference was found between the GHB or AY-9944 model and control subjects. These results suggest that changes in adult neurogenesis in models of absence seizures do not occur, and that seizure-induced enhancement of neurogenesis could depend on the characteristics of the seizure discharge.

An investigation of EEG dynamics in an animal model of temporal lobe epilepsy using the maximum Lyapunov exponent

Analysis of intracranial electroencephalographic (iEEG) recordings in patients with temporal lobe epilepsy (TLE) has revealed characteristic dynamical features that distinguish the interictal, ictal, and postictal states and inter-state transitions. Experimental investigations into the mechanisms underlying these observations require the use of an animal model. A rat TLE model was used to test for differences in iEEG dynamics between well-defined states and to test specific hypotheses: 1) the short-term maximum Lyapunov exponent (STL(max)), a measure of signal order, is lowest and closest in value among cortical sites during the ictal state, and highest and most divergent during the postictal state; 2) STL(max) values estimated from the stimulated hippocampus are the lowest among all cortical sites; and 3) the transition from the interictal to ictal state is associated with a convergence in STL(max) values among cortical sites. iEEGs were recorded from bilateral frontal cortices and hippocampi. STL(max) and T-index (a measure of convergence/divergence of STL(max) between recorded brain areas) were compared among the four different periods. Statistical tests (ANOVA and multiple comparisons) revealed that ictal STL(max) was lower (p<0.05) than other periods, STL(max) values corresponding to the stimulated hippocampus were lower than those estimated from other cortical regions, and T-index values were highest during the postictal period and lowest during the ictal period. Also, the T-index values corresponding to the preictal period were lower than those during the interictal period (p<0.05). These results indicate that a rat TLE model demonstrates several important dynamical signal characteristics similar to those found in human TLE and support future use of the model to study epileptic state transitions.

Temporal lobe epilepsy: anatomical and effective connectivity

While temporal lobe epilepsy (TLE) has been treatable with anti-seizure medications over the past century, there still remain a large percentage of patients whose seizures remain untreatable pharmacologically. To better understand and treat TLE, our laboratory uses several in vivo analytical techniques to estimate connectivity in epilepsy. This paper reviews two different connectivity-based approaches with an emphasis on application to the study of epilepsy. First, we present effective connectivity techniques, such as Granger causality, that has been used to assess the dynamic directional relationships among brain regions. These measures are used to better understand how seizure activity initiates, propagates, and terminates. Second, structural techniques, such as magnetic resonance imaging, can be used to assess changes in the underlying neural structures that result in seizure. This paper also includes in vivo epilepsy-centered examples of both effective and anatomical connectivity analysis. These analyses are performed on data collected in vivo from a spontaneously seizing animal model of TLE. Future work in vivo on epilepsy will no doubt benefit from a fusion of these different techniques. We conclude by discussing the interesting possibilities, implications, and challenges that a unified analysis would present.

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.

Molecular neuropathology of temporal lobe epilepsy: complementary approaches in animal models and human disease tissue

Patients with temporal lobe epilepsies (TLE) frequently develop pharmacoresistance to antiepileptic treatment. In individuals with drug-refractory TLE, neurosurgical removal of the epileptogenic focus provides a therapy option with high potential for seizure control. Biopsy specimens from TLE patients constitute unique tissue resources to gain insights in neuropathological and molecular alterations involved in human TLE. Compared to human tissue specimens in most neurological diseases, where only autopsy material is available, the bioptic tissue samples from pharmacoresistant TLE patients open rather exceptional preconditions for molecular biological, electrophysiological as well as biochemical experimental approaches in human brain tissue, which cannot be carried out in postmortem material. Pathological changes in human TLE tissue are multiple and relate to structural and cellular reorganization of the hippocampal formation, selective neurodegeneration, and acquired changes of expression and distribution of neurotransmitter receptors and ion channels, underlying modified neuronal excitability. Nevertheless, human TLE tissue specimens have some limitations. For obvious reasons, human TLE tissue samples are only available from advanced, drug-resistant stages of the disease. However, in many patients, a transient episode of status epilepticus (SE) or febrile seizures in childhood can induce multiple structural and functional alterations that after a latency period result in a chronic epileptic condition. This latency period, also referred to as epileptogenesis, cannot be studied in human TLE specimens. TLE animal models may be particularly helpful in order to shed characterize new molecular pathomechanisms related to epileptogenesis and open novel therapeutic strategies for TLE. Here, we will discuss experimental approaches to unravel molecular-neuropathological aspects of TLE and highlight characteristics and potential of molecular studies in human and/or experimental TLE.

Stereological methods reveal the robust size and stability of ectopic hilar granule cells after pilocarpine-induced status epilepticus in the adult rat

Following status epilepticus in the rat, dentate granule cell neurogenesis increases greatly, and many of the new neurons appear to develop ectopically, in the hilar region of the hippocampal formation. It has been suggested that the ectopic hilar granule cells could contribute to the spontaneous seizures that ultimately develop after status epilepticus. However, the population has never been quantified, so it is unclear whether it is substantial enough to have a strong influence on epileptogenesis. To quantify this population, the total number of ectopic hilar granule cells was estimated using unbiased stereology at different times after pilocarpine-induced status epilepticus. The number of hilar neurons immunoreactive for Prox-1, a granule-cell-specific marker, was estimated using the optical fractionator method. The results indicate that the size of the hilar ectopic granule cell population after status epilepticus is substantial, and stable over time. Interestingly, the size of the population appears to be correlated with the frequency of behavioral seizures, because animals with more ectopic granule cells in the hilus have more frequent behavioral seizures. The hilar ectopic granule cell population does not appear to vary systematically across the septotemporal axis, although it is associated with an increase in volume of the hilus. The results provide new insight into the potential role of ectopic hilar granule cells in the pilocarpine model of temporal lobe 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.

[Extra-hippocampal temporal lesions inducing symptomatic drug-resistant epilepsies. Which surgical procedure?]

In partial symptomatic epilepsy due to discrete brain lesion, total removal of the epileptogenic lesion generally yields major reduction of seizures, achieved in 85% of the patients. However, prognosis is worse in patients with symptomatic temporal lobe epilepsy. Implication of the temporo-mesial structures in the seizures genesis is generally considered. Careful electroclinical and radiological analysis can provide useful but insufficient information. In order to evaluate the criteria we used to guide our surgical strategy, we analyzed retrospectively a series of 47 patients with drug-resistant symptomatic extra hipocampic temporal epilepsy surgically treated either by isolated lesionectomy (group 1, n=17) or by resection of temporo-mesial structures and associated lesionectomy (group 2, n=30). Patients with extrahippocampal lesions and hippocampal sclerosis (dual pathology) were excluded from this study. With a mean follow-up of 72 months, overall results showed that 84% of group 2 patients (Engel's grade Ia) were seizure-free compared with only 47% of group 1 patients. Statistical analysis showed that the type of surgical procedure was the main prognostic factor. In conclusion, the optimal surgical procedure cannot be defined only with the criteria usually retained for temporo-mesial involvement in seizure genesis. Taking into account the prognostic value of such implication, although complex, is of paramount importance. Our results could be explained by the presence of an acquired dual functional pathology.

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.

Chronic temporal lobe epilepsy: spatial extent and degree of metabolic dysfunction studied with magnetic resonance spectroscopy (MRS)

INTRODUCTION

Proton magnetic resonance spectroscopy ((1)H MRS) has been proposed as a lateralizing method for the presurgical evaluation of patients with medically intractable temporal lobe epilepsy (TLE). Studies have shown correlations between temporal lobe (TL) NAA and seizure frequency, and TL NAA/Cr and the duration of epilepsy in patients with TLE. This latter finding may suggest that progressive neuronal dysfunction may occur in both temporal lobes in patients with TLE, even when the seizures originate in only one temporal lobe. We analyzed our data in an attempt to find a possible correlation between extension of neuronal dysfunction based on NAA measures and duration of epilepsy.

METHODS

We studied 45 consecutive patients with the diagnosis of TLE, who were referred for presurgical evaluation. Duration of epilepsy was defined as the interval between the age of seizure onset and the time of the MRS examination. All studies were performed in the inter-ictal state, prior to intracranial monitoring or resection. We performed two-tailed Pearson correlation analysis between ipsilateral NAA/Cr and extension of the abnormality (voxels involved) and the duration of the seizure disorder in years.

RESULTS

The average duration of epilepsy in this group was 20 years. No significant correlation was found between duration of epilepsy and mean hippocampal NAA/Cr (r=-.131, p=.390); nor was a correlation found between duration of epilepsy in years or the extent of metabolic lesion (voxels involved) (r=-.264, p=.079).

CONCLUSIONS

Hippocampal NAA/Cr does not correlate with duration of epilepsy in TLE. Our findings suggest that cross-sectional group measures of hippocampal neuronal function do not suggest damage progression.

Epilepsy induced by extended amygdala-kindling in rats: lack of clear association between development of spontaneous seizures and neuronal damage

Most patients with temporal lobe epilepsy (TLE), the most common type of epilepsy, show pronounced loss of neurons in limbic brain regions, including the hippocampus, amygdala, and parahippocampal regions. Hippocampal damage in patients with TLE is characterized by extensive neuronal loss in the CA3 and CA1 sectors and the hilus of the dentate gyrus. There is a long and ongoing debate on whether this type of hippocampal damage, referred to as hippocampal sclerosis, is the cause or consequence of TLE. Furthermore, hippocampal damage may contribute to the progressive features of TLE. The present study was designed to determine whether development of spontaneous recurrent seizures (SRS) after extended kindling of the amygdala in rats is associated with neuronal damage. The kindling model of TLE was chosen because previous studies have shown that only part of the rats develop SRS after extended kindling, thus allowing to compare the brain pathology of rats that received the same number of amygdala stimulation but did or did not develop SRS. For extended kindling, rats were stimulated twice daily 3-5 days a week for up to about 280 stimulations. During long-term EEG/video monitoring, SRS were observed in 50% of the rats over the period of extended kindling. SRS often started with myoclonic jerks or focal seizures and subsequently progressed into secondarily generalized seizures, so that the development of SRS recapitulated the earlier kindling of elicited seizures. No obvious neurodegeneration was observed in the CA1 and CA3 sectors of the hippocampus, the amygdala, parahippocampal regions or thalamus. A significant bilateral reduction in neuronal density was determined in the dentate hilus after extended kindling, but this reduction in hilar cell density did not significantly differ between rats with and without observed SRS. Determination of the total number of hilar neurons and of hilar volume indicated that the reduced neuronal density in the dentate hilus was due to expansion of hilar area but not to neuronal damage. The data demonstrate that extended kindling does not cause any hippocampal damage resembling hippocampal sclerosis, but that SRS develop in the absence of such damage.

Kindling and status epilepticus models of epilepsy: rewiring the brain

This review focuses on the remodeling of brain circuitry associated with epilepsy, particularly in excitatory glutamate and inhibitory GABA systems, including alterations in synaptic efficacy, growth of new connections, and loss of existing connections. From recent studies on the kindling and status epilepticus models, which have been used most extensively to investigate temporal lobe epilepsy, it is now clear that the brain reorganizes itself in response to excess neural activation, such as seizure activity. The contributing factors to this reorganization include activation of glutamate receptors, second messengers, immediate early genes, transcription factors, neurotrophic factors, axon guidance molecules, protein synthesis, neurogenesis, and synaptogenesis. Some of the resulting changes may, in turn, contribute to the permanent alterations in seizure susceptibility. There is increasing evidence that neurogenesis and synaptogenesis can appear not only in the mossy fiber pathway in the hippocampus but also in other limbic structures. Neuronal loss, induced by prolonged seizure activity, may also contribute to circuit restructuring, particularly in the status epilepticus model. However, it is unlikely that any one structure, plastic system, neurotrophin, or downstream effector pathway is uniquely critical for epileptogenesis. The sensitivity of neural systems to the modulation of inhibition makes a disinhibition hypothesis compelling for both the triggering stage of the epileptic response and the long-term changes that promote the epileptic state. Loss of selective types of interneurons, alteration of GABA receptor configuration, and/or decrease in dendritic inhibition could contribute to the development of spontaneous seizures.

Status epilepticus severity influences the long-term outcome of neurogenesis in the adult dentate gyrus

Status epilepticus (SE) is characterized by continual seizure activity that can vary widely in the intensity of convulsions. We induced seizures by applying continuous electrical stimulation to the hippocampus in adult rats to explore the effects of three different SE states on neurogenesis and neuronal death in the hippocampus. Rats exhibiting the most severe SE state (fully convulsive) demonstrated profound increases in cell proliferation in the dentate gyrus (DG) at 1 week post-insult, but the majority of the new neurons had died at 4 weeks. In contrast, rats exhibiting less severe SE states (ambulatory or masticatory, partial convulsive) had the same degree of cell proliferation at 1 week, but most new neurons survived at 4 weeks. As compared to partially convulsive SE rats, fully convulsive SE rats had significantly greater DG pathology. Our data indicate that SE of varying severity triggers similar short-term proliferation of neural progenitors, but that the long-term outcome of neurogenesis is influenced by the degree of insult-induced degeneration in the DG tissue environment.

MRI and in situ hybridization reveal early disturbances in brain size and gene expression in the megencephalic (mceph/mceph) mouse

The mouse model for megencephaly, mceph/mceph, carries a truncating deletion in the Shaker-related voltage gated potassium channel gene 1. Affected mice display neurological disturbances and motor dysfunctions. Symptoms begin to show at 3-4 weeks of age. The cause of the brain enlargement is not clear. To elucidate early events in the development of the disease we used magnetic resonance imaging (MRI) to quantify, over time, the increase in size of several discrete brain regions in the same mice at 3, 8 and 12 weeks of age. We also analysed markers for neuropeptides and growth factors to explore their possible involvement at an early stage. The results show an enlargement of the total coronal area of the brain already at 3 weeks of age. Total brain volume, ventral cortex, hippocampal formation and cerebral cortex were enlarged at 8 weeks and onwards. Thalamus, brainstem, cerebellum and spinal cord did not differ from controls even at 12 weeks. We also report distinct disturbances in the expression of brain-derived neurotrophic factor, insulin-like growth factor binding protein 6 and several neuropeptides at 2 and 3 weeks of age, that is, before an obvious behavioural phenotype can be observed. These results provide an objective description of the size changes in different brain regions of the mceph/mceph mouse, and suggest that certain molecules could be involved in the early processes underlying these changes.

Truncation of the Shaker-like voltage-gated potassium channel, Kv1.1, causes megencephaly

The megencephaly mouse, mceph/mceph, displays dramatically increased brain volume and hypertrophic brain cells. Despite overall enlargement, the mceph/mceph brain appears structurally normal, without oedema, hydrocephaly or leukodystrophy, and with only minor astrocytosis. Furthermore, it presents striking disturbances in expression of trophic and neuromodulating factors within the hippocampus and cortex. Using a positional cloning approach we have identified the mceph mutation. We show that mceph/mceph mice carry an 11-base-pair deletion in the gene encoding the Shaker-like voltage-gated potassium channel subtype 1, Kcna1. The mutation leads to a frame shift and the predicted MCEPH protein is truncated at amino acid 230 (out of 495), terminating with six aberrant amino acids. The expression of Kcna1 mRNA is increased in the mceph/mceph brain. However, the C-terminal domains of the corresponding Kv1.1 protein are absent. The putative MCEPH protein retains only the N-terminal domains for channel assembly and may congregate nonfunctional complexes of multiple Shaker-like subunits. Indeed, whereas Kcna2 and Kcna3 mRNA expression is normal, the mceph/mceph hippocampus displays decreased amounts of Kv1.2 and Kv1.3 proteins, suggesting interactions at the protein level. We show that mceph/mceph mice have disturbed brain electrophysiology and experience recurrent behavioural seizures, in agreement with the abnormal electrical brain activity found in Shaker mutants. However, in contrast to the commonly demonstrated epilepsy-induced neurodegeneration, we find that the mceph mutation leads to seizures with a concomitant increase in brain size, without overt neural atrophy.

A ligand of the p65/p95 receptor suppresses perforant path kindling, kindling-induced mossy fiber sprouting, and hilar area changes in adult rats

Kindling, an animal model of epilepsy, results in an increased volume of the hilus of the dentate gyrus and sprouting of the mossy fiber pathway in the hippocampus. Our previous studies have revealed that chronic infusion of neurotrophins can regulate not only seizure development, but also these kindling-induced structural changes. Kindling, in turn, can alter the expression of neurotrophins and their receptors. We previously showed that intraventricular administration of a synthetic peptide that interferes with nerve growth factor stability and thus its binding to TrkA and p75(NTR) receptors suppressed kindling and sprouting. However, the precise involvement of TrkA, p75(NTR), and downstream signaling effectors of neurotrophins on kindling, sprouting and hilar changes are unknown. One of these downstream effectors is Ras. In the present study, we find that intraventricular infusion of the synthetic peptide Reo3Y, which binds to p65/p95 receptors and causes a rapid inactivation of Ras protein, impairs development of perforant path kindling, reduces the growth in afterdischarge duration, blocks kindling-induced mossy fiber sprouting in area CA3 of hippocampus and in inner molecular layer of the dentate gyrus, and prevents kindling-induced increases in hilar area. These results are consistent with a mediation of neurotrophin effects on kindling, hilar area, and axonal sprouting via Trk receptors, and suggest important roles for Ras in kindling and in kindling-induced structural changes.

Hippocampal cell proliferation and epileptogenesis after audiogenic kindling are not accompanied by mossy fiber sprouting or Fluoro-Jade staining

Repetitive sound-induced seizures, known as audiogenic kindling (AK), gradually induce the transference of epileptic activity from brainstem to forebrain structures along with behavioral changes. The aim of our work was to correlate the behavioral changes observed during the AK with possible alterations in neuronal proliferation, cell death, hippocampal mossy fiber sprouting and in the EEG pattern of Wistar audiogenic rats, a genetically susceptible strain from our laboratory. Susceptible and non-susceptible animals were submitted to repeated sound stimulations for 14-16 days and hippocampal mitotic activity was studied through the incorporation of bromodeoxyuridine (BrdU). Cell death and mossy fiber sprouting were assessed, respectively, by using Fluoro-Jade and Timm staining, 2 and 32 days after the last kindling stimulation. In addition, we used immunofluorescent double labeling for a glial and a mitotic marker to evaluate newly born cell identity. Some animals had hippocampus and amygdala electrodes for EEG recordings. Our results show that kindled animals with 6-11 generalized limbic seizures (class IV-V) had increased cell proliferation in the dentate gyrus when compared with animals with zero or one to three seizures. BrdU-positive cells labeled on day 2 and on day 32 were both GFAP negative. In the later group, rounded and well-defined BrdU-positive/GFAP-negative nuclei were seen in different portions of the granule cell layer. We did not observe any Fluoro-Jade or differential Timm staining in kindled animals at both killing times. However, EEG recordings showed intense epileptic activity in the hippocampus and amygdala of all animals with limbic seizures.Therefore, our data indicate that AK-induced limbic epileptogenicity is able to increase the hippocampal mitotic rate, even though it does not seem to promote neuronal death or mossy fiber sprouting in the supragranular layer of the dentate gyrus.

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.

Continuous infusion of neurotrophin-3 triggers sprouting, decreases the levels of TrkA and TrkC, and inhibits epileptogenesis and activity-dependent axonal growth in adult rats

Neurotrophin-3 (NT-3), a member of the neurotrophin family of neurotrophic factors, is important for cell survival, axonal growth and neuronal plasticity. Epileptiform activation can regulate the expression of neurotrophins, and increases or decreases in neurotrophins can affect both epileptogenesis and seizure-related axonal growth. Interestingly, the expression of nerve growth factor and brain-derived neurotrophic factor is rapidly up-regulated following seizures, while NT-3 mRNA remains unchanged or undergoes a delayed down-regulation, suggesting that NT-3 might have a different function in epileptogenesis. In the present study, we demonstrate that continuous intraventricular infusion of NT-3 in the absence of kindling triggers mossy fiber sprouting in the inner molecular layer of the dentate gyrus and the stratum oriens of the CA3 region. Furthermore, despite this NT-3-related sprouting effect, continuous infusion of NT-3 retards the development of behavioral seizures and inhibits kindling-induced mossy fiber sprouting in the inner molecular layer of the dentate gyrus. We also show that prolonged infusion of NT-3 leads to a decrease in kindling-induced Trk phosphorylation and a down-regulation of the high-affinity Trk receptors, TrkA and TrkC, suggesting an involvement of both cholinergic nerve growth factor receptors and hippocampal NT-3 receptors in these effects. Our results demonstrate an important inhibitory role for NT-3 in seizure development and seizure-related synaptic reorganization.

Morphologic study of neuronal death, glial activation, and progenitor cell division in the hippocampus of rat models of epilepsy

PURPOSE

To clarify the relationship of neuronal death to cellular responses, we studied neuronal death as well as reactions of glia and progenitor cells in the hippocampus of two rat models of epilepsy.

METHODS

Seizures were induced by either kainic acid (KA) administration or electrical kindling. Neuronal degeneration was assessed by in situ DNA fragmentation analysis. Reactions of glial cells were studied by immunohistochemistry. Progenitor cell division was evaluated using the bromodeoxyuridine (BrdU) labeling method.

RESULTS

DNA fragmentation and reactive microglia were observed in the CA1, CA3, and hilus region for 24 h to 4 weeks after KA injection, but not detected in the kindling model. Reactive astrocytes and enhancement of progenitor cell division were seen in both animal models. The number of BrdU-positive cells began to increase on day 3 after KA injection, peaked on day 5, and returned to baseline on day 10. After kindling, the number of BrdU-positive cells began to increase after five consecutive experience of stage I seizures.

CONCLUSIONS

These observations show that neuronal degeneration is not necessary for triggering the upregulation. Microglial activation is closely related to the neuronal death process induced by KA.

Glial cell line-derived neurotrophic factor modulates kindling and activation-induced sprouting in hippocampus of adult rats

Kindling, a phenomenon in which repeated electrical stimulation of certain forebrain structures leads to an increase in the evoked epileptogenic response, is widely used to investigate the mechanisms of epilepsy. Kindling also results in sprouting of the dentate gyrus mossy fiber pathway and triggers astrocyte hypertrophy and increased volume of the hilus of the dentate gyrus. Our previous studies showed that infusion of the neurotrophin nerve growth factor accelerated the behavioral progression of amygdala kindling and affected kindling-induced structural changes in the brain, whereas intrahilar infusion of another neurotrophin, brain-derived neurotrophic factor, delayed amygdala kindling-induced seizure development and reduced the growth in afterdischarge duration, but had little effect on kindling-induced structural changes. In this paper, we report the effects of infusion of glial cell line-derived neurotrophic factor, a neurotrophic factor of the TGF-beta superfamily having similar central nervous system neuronal targets as brain-derived neurotrophic factor. We show that continuous intraventricular infusion of glial cell line-derived neurotrophic factor inhibits the behavioral progression of perforant path kindling-induced seizures without affecting afterdischarge duration. In addition, we demonstrate that intraventricular administration of glial cell line-derived neurotrophic factor prevents kindling-induced increases in hilar area and blocks mossy fiber sprouting in the CA3 region of the hippocampus. Glial cell line-derived neurotrophic factor did not have a statistically significant effect on the mossy fiber density in the inner molecular layer. Our results raise the possibility that glial cell line-derived neurotrophic factor plays a role in kindling and activation-induced neural growth via mechanisms distinct from those of the neurotrophins.

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.

Network and pharmacological mechanisms leading to epileptiform synchronization in the limbic system in vitro

Seizures in patients presenting with mesial temporal lobe epilepsy result from the interaction among neuronal networks in limbic structures such as the hippocampus, amygdala and entorhinal cortex. Mesial temporal lobe epilepsy, one of the most common forms of partial epilepsy in adulthood, is generally accompanied by a pattern of brain damage known as mesial temporal sclerosis. Limbic seizures can be mimicked in vitro using preparations of combined hippocampus-entorhinal cortex slices perfused with artificial cerebrospinal fluid containing convulsants or nominally zero Mg(2+), in order to produce epileptiform synchronization. Here, we summarize experimental evidence obtained in such slices from rodents. These data indicate that in control animals: (i) prolonged, NMDA receptor-dependent epileptiform discharges, resembling electrographic limbic seizures, originate in the entorhinal cortex from where they propagate to the hippocampus via the perforant path-dentate gyrus route; (ii) the initiation and maintenance of these ictal discharges is paradoxically contributed by GABA (mainly type A) receptor-mediated mechanisms; and (iii) CA3 outputs, which relay a continuous pattern of interictal discharge at approximately 1Hz, control rather than sustain ictal discharge generation in entorhinal cortex. Recent work indicates that such a control is weakened in the pilocarpine model of epilepsy (presumably as a result of CA3 cell damage). In addition, in these experiments electrographic seizure activity spreads directly to the CA1-subiculum regions through the temporoammonic pathway. Studies reviewed here indicate that these changes in network interactions, along with other mechanisms of synaptic plasticity (e.g. axonal sprouting, decreased activation of interneurons, upregulation of bursting neurons) can confer to the epileptic, damaged limbic system, the ability to produce recurrent limbic seizures as seen in patients with mesial temporal lobe epilepsy.

Functional genomics in experimental and human temporal lobe epilepsy: powerful new tools to identify molecular disease mechanisms of hippocampal damage

The human genome project is a milestone for molecular genetic studies on complex, sporadic disorders in the human central nervous system (CNS). Functional analysis and tissue-/cell-specific expression profiles will be of particular importance anticipating the magnitude of expressed genes in the brain and their dynamic epigenetic modifications. The recent progress in microarray technologies allows expression studies for a large number of genes. In combination with laser-microdissection and quantitative reverse transcription-polymerase chain reaction technologies, such large-scale expression analyses can be successfully addressed in well-defined tissue specimens or cellular subpopulations. Complex, sporadic diseases, such as temporal lobe epilepsy (TLE), are challenging for functional genomics. Issues of particular importance in this field include molecular mechanisms of neurodevelopmental abnormalities, neuronal plasticity and hyperexcitability as well as neuronal cell damage in affected CNS areas. The availability of anatomically well-preserved surgical specimens, i.e. hippocampus obtained from epilepsy patients with Ammon's horn sclerosis or focal lesions not affecting the hippocampus proper as well as comparisons with experimental TLE models may help to elucidate specific molecular-pathological mechanisms during epileptogenesis and in chronic conditions of the disease.

Repeated brief seizures induce progressive hippocampal neuron loss and memory deficits

The long-term effects of repeated brief seizures on spatial memory and hippocampal neuronal populations were assessed in kindled rats. Rats that experienced a range of 3 afterdischarges to 134 secondary generalized tonic-clonic (Class V) seizures evoked by stimulation of the olfactory bulb were evaluated in a radial arm maze task that is a measure of spatial memory and is disrupted by hippocampal damage. After completion of the memory task and a minimum of approximately 3 months after the last evoked seizure, stereological methods were used to assess neuronal populations at septal and temporal locations of the hippocampus and dentate gyrus. Repeated brief seizures induced a long-lasting deficit in spatial memory performance that was detected after a cumulative total of approximately 6 partial and 30 secondary generalized seizures. The memory deficit progressively increased as a function of the number of seizures, and was not observed in age-matched, electrode-implanted, unstimulated, but otherwise similarly handled paired controls. Neuronal loss was detected in the temporal hilus of the dentate gyrus, CA1, and CA3 of the hippocampus after 69 or more secondary generalized tonic-clonic seizures, and was associated with the progressive memory dysfunction. Repeated brief seizures induced progressive, permanent functional and structural abnormalities in the hippocampus, which included spatial memory deficits accompanied by gradually evolving neuronal loss in a pattern resembling human hippocampal sclerosis. These experimental results support the view that hippocampal sclerosis and associated memory dysfunction are induced by repeated seizures, and imply that seizure control could prevent adverse long-term consequences of seizures on hippocampal dependent functions.

Seizures induce phase shifts of rat circadian rhythms

RATIONALE

Epileptic seizures may alter neuroendocrinological cycles. Light pulses induce phase shifts in circadian rhythms. Using hippocampal-kindled rats to ensure maximal clinical expression, we determined if seizures likewise induce phase shifts.

METHODS

We monitored the circadian rhythm of temperature (CRT) with intraperitoneal radiotelemetry in rats (n=21) isolated from time cues and light for 3-week trials. Seizures were triggered with hippocampal electrical stimulation at different circadian phases. Optimized, least-error phase shifts were calculated from preictal and postictal CRTs. Induced seizures were referenced to CRT (t(max)=00:00, 24-h circadian cycle).

RESULTS

Phase shifts (individual responses=57) differed across the circadian cycle. Rather than forming a clear phase-response curve, phase shifts were especially variable between 00:00 and 06:00 h.

CONCLUSIONS

This study demonstrates that electrically-induced seizures induce advances and delays in CRT in a phase-dependent fashion but in a pattern different from typical light-induced phase shifts. Disorders of circadian regulation may contribute to some of the altered endogenous cycles associated with epilepsy.

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.

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.

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.

Oxidative stress is involved in seizure-induced neurodegeneration in the kindling model of epilepsy

Reactive oxygen species have been implicated in the development of seizures under pathological conditions and linked to seizure-induced neurodegeneration. There has been little direct evidence, however, of free radical production resulting from seizures. Using amygdala-kindled rats, we have examined the generation of reactive oxygen species following seizures, and their possible contribution to seizure development and seizure-induced neuronal loss. The concentrations of two products of free radical-induced lipid peroxidation, malonaldehyde and 4-hydroxy-2(E)-nonenal, were measured using colorimetric assays. Lipid peroxidation was increased in both hemispheres of kindled rats as compared to sham-operated controls. Cell death was also significantly increased in all hippocampal areas. Antioxidants (vitamin E and glutathione) prevented the rise in lipid peroxides and hippocampal neuronal death during kindling, but did not arrest the development of seizures.Thus, epileptiform activity can result in free radical production which may be one of the factors leading to cell death.

Effects of circadian regulation and rest-activity state on spontaneous seizures in a rat model of limbic epilepsy

PURPOSE

Circadian regulation via the suprachiasmatic nuclei and rest-activity state may influence expression of limbic seizures.

METHODS

Male rats (n = 14) were made epileptic by electrical stimulation of the hippocampus, causing limbic status epilepticus and subsequent seizures. We monitored seizures with intrahippocampal electrodes in 12-12-h light/dark (LD) cycles and in continuous dark (DD). We used radiotelemetry monitoring of activity to measure state and body temperature to determine circadian phase. Cosinor analysis and chi2 tests determined whether seizures occurred rhythmically when plotted by phase. State was defined as inactive or active in 10-min epochs based on whether activity count was below or above a cut-off value validated from video observation.

RESULTS

In LD, the peak seizure occurrence was 14:59 h after circadian temperature peak (95% confidence limit, 13:37-16:19). Phasic seizure occurrence persisted in DD for 14:05 (12:31-15:38), p < 0.0001, against uniform mean distribution. In LD, 14,787 epochs contained 1, 268 seizures; seizures preferentially occurred during inactive epochs (965 observed, 878 expected in proportion to the overall distribution of inactive versus active epochs; p < 0.001). In DD, 20, 664 epochs contained 1,609 seizures; seizures had no preferential occurrence by state (999 observed, 1,025 expected; p = 0.16).

CONCLUSIONS

Limbic seizures occurred with an endogenous circadian rhythm. Seizures preferentially struck during inactivity during entrainment to the light-dark cycle.

Enhancement of progenitor cell division in the dentate gyrus triggered by initial limbic seizures in rat models of epilepsy

PURPOSE

Mitogenic effects of seizures on granule cell progenitors in the dentate gyrus were studied in two rat models of epilepsy. We investigated which stage of epileptogenesis is critical for eliciting progenitor cell division and whether seizure-induced neuronal degeneration is responsible for the enhancement of progenitor cell division.

METHODS

Seizures were induced by either kainic acid (KA) administration or electrical kindling. Neurogenesis of dentate granule cells was evaluated using the bromodeoxyuridine (BrdU) labeling method, and neuronal degeneration was assessed by in situ DNA fragmentation analysis.

RESULTS

After injection of KA, the number of BrdU-positive granule cells began to increase at day 3 after the treatment, peaked at day 5, and returned to baseline at day 10. By day 13, the values were lower than control. After kindling, the number of BrdU-positive cells began to increase after five consecutive experiences of stage I seizures. The increase occurred from day 1 to day 3 after the last electrical stimulation, but returned to baseline by day 7. After generalized seizures were well established, repeated stimulation did not facilitate division of granule cell progenitors. DNA fragmentation was noted in pyramidal neurons in the CA1, CA3, and hilus regions at 18 h after KA injection, but not in the kindling model.

CONCLUSIONS

These observations indicate that a mechanism in epileptogenesis boosts dentate progenitor cell division, but progenitor cells may become unreactive to prolonged generalized seizures. Pyramidal neuronal degeneration is not necessary for triggering the upregulation. It is suggested that newly born granule cells may play a role in the network reorganization that occurs during epileptogenesis.

Hypothalamic neuronal loss and altered circadian rhythm of temperature in a rat model of mesial temporal lobe epilepsy

PURPOSE

Numerous dysfunctions in endogenous hypothalamic function have been associated with mesial temporal lobe epilepsy (MTLE). One endogenous activity is the circadian rhythm of temperature (CRT). In this study we examined whether hypothalamically mediated function is altered in the electrically induced, self-sustained, limbic status epilepticus model of MTLE. We then wished to determine whether there was a structural basis for regulatory alterations.

METHODS

We measured CRT with peritoneal temperature telemetry obtained in light-entrained (LD) and in free-running, constant-dark (DD) conditions. CRT from epileptic and controls of normal animals and kindled animals were quantized by fast Fourier transform-nonlinear least squares analysis to determine rhythmic complexity.

RESULTS

The circadian component of CRT was preserved in all animals. In DD, CRTs of epileptic animals were more complex than those of normal animals. CRT of kindled animals showed no increased complexity after electrically induced seizures. Neuronal density was decreased in regions of the anterior and posterior hypothalamus but not in the suprachiasmatic nuclei from the epileptic rats.

CONCLUSIONS

Alterations in CRT due to the epileptic state were independent of isolated seizures. Altered circadian thermoregulation in epileptic rats corresponded to regional hypothalamic neuronal loss. Structural changes of the hypothalamus may explain alterations in endogenous rhythms in MTLE.

Mossy fiber sprouting induced by repeated electroconvulsive shock seizures

The elicitation of repeated focal seizures (kindling) induces mossy fiber sprouting in the hippocampus of the rat. The present study investigated whether repeated generalized seizures also induce mossy fiber sprouting. Human psychiatric patients receive repeated generalized seizures during electroconvulsive therapy (ECT). Male Long-Evans rats received a course of eight electroconvulsive shock (ECS) seizures administered on a 48-h schedule over a course of 2 1/2 weeks. Control subjects received matched handling, but no stimulation. Fourteen days after the last ECS trial, all subjects were sacrificed and their brains subjected to Timm staining. Cell counts and area measures were also taken in the hilus. Significant sprouting, but not significant cell loss, was seen in the fascia dentata of the subjects that had received ECS.

Molecular neuropathology of human mesial temporal lobe epilepsy

With the recent progress in surgical treatment modalities, human brain tissue from patients with intractable focal epilepsies will increasingly become available for studies on the molecular pathology, electrophysiological changes and pathogenesis of human focal epilepsies. An inherent problem for studies on human temporal lobe epilepsy (TLE) is the lack of suitable controls. Strategies to alleviate this obstacle include the use of human post mortem samples, hippocampus from experimental animals and, in particular, the comparative analysis of surgical specimens from patients with Ammon's horn sclerosis (AHS) and with focal temporal lesions but anatomically preserved hippocampal structures. In this review we focus on selected aspects of the molecular neuropathology of TLE: (1) the potential impact of persisting calretinin-immunoreactive neurons with Cajal-Retzius cell morphology, (2) astrocytic tenascin-C induction and redistribution as potential regulator of aberrant axonal sprouting and (3) alterations of Ca2+ -mediated hippocampal signalling pathways. The diverse and complex changes described so far in human TLE specimens require a systematic interdisciplinary approach to distinguish primary, epileptogenic alterations and secondary, compensatory mechanisms in the pathogenesis of human temporal lobe epilepsies.

Vulnerability of the dentate gyrus to aging and intracerebroventricular administration of kainic acid

The hippocampal formation is highly vulnerable to the aging process, demonstrating functional alterations in circuitry with aging. Aging may also change the sensitivity of the hippocampal formation to excitotoxic lesions. In this study, using young adult, middle aged, and aged Fischer 344 rats, we evaluated morphometric changes in the dentate gyrus as a function of age and also in response to an administration of an excitotoxin (kainic acid) into the right lateral ventricle. The dentate gyrus was measured for changes in the area of dentate hilus and the dentate granule cell layer, alterations in the width of the dentate granule cell layer, and degree of dentate hilar cell loss. With aging, the hilar area increased in size while the area and width of the dentate granule cell layer remained constant. However, the most striking change with aging was a significant reduction in the number of dentate hilar neurons. Intracerebroventricular kainic acid produced consistent lesions in the entire ipsilateral CA3 region, and the size of CA3 lesion was identical in all three ages of animals. Following the lesion, areas of both the dentate hilus and the granule cell layer were significantly decreased in only young adult and middle aged animals whereas the width of the dentate granule cell layer was significantly increased only in the middle aged group. In contrast, dentate hilar neurons were significantly reduced in all ages of animals with the maximum reductions in neuron number observed in the aged group. Thus, aging in the dentate gyrus is characterized by a significantly decreased number of dentate hilar neurons and also a significantly increased susceptibility of dentate hilar neurons to excitotoxic damage.

Brain-derived neurotrophic factor infusion delays amygdala and perforant path kindling without affecting paired-pulse measures of neuronal inhibition in adult rats

Kindling is an animal model of human temporal lobe epilepsy in which excitability in limbic structures is permanently enhanced by repeated stimulations. Kindling also increases the expression of nerve growth factor, brain-derived neurotrophic factor, and brain-derived neurotrophic factor receptor messenger RNAs in both the hippocampus and cerebral cortex and causes structural changes in the hippocampus including hilar hypertrophy. We have recently shown that intraventricular nerve growth factor infusion enhances the development of kindling, whereas blocking nerve growth factor activity retards amygdaloid kindling. Furthermore, we have shown that nerve growth factor protects against kindling-induced hilar hypertrophy. The physiological role of brain-derived neurotrophic factor in kindling is not as clear. Acute injection of brain-derived neurotrophic factor increases neuronal excitability and causes seizures, whereas chronic brain-derived neurotrophic factor infusion in rats slows hippocampal kindling. In agreement with the latter, we show here that intrahilar brain-derived neurotrophic factor infusion delays amygdala and perforant path kindling. In addition, we show that brain-derived neurotrophic factor, unlike nerve growth factor, does not protect against kindling-induced increases in hilar area. To test the hypothesis that brain-derived neurotrophic factor suppresses kindling by increasing inhibition above normal levels, we performed paired-pulse measures in the perforant path-dentate gyrus pathway. Brain-derived neurotrophic factor infused into the hippocampus had no effect on the stimulus intensity function (input/output curves); there was also no significant effect on paired-pulse inhibition. We then kindled the perforant path 10 days after the end of brain-derived neurotrophic factor treatment. Once again, kindling was retarded, showing that the brain-derived neurotrophic factor effect is long-lasting. These results indicate that prolonged in vivo infusion of brain-derived neurotrophic factor reduces, rather than increases, excitability without increasing inhibitory neuron function, at least as assessed by paired-pulse protocols. This effect may be mediated by long-lasting effects on brain-derived neurotrophic factor receptor regulation.

Acute cellular alterations in the hippocampus after status epilepticus

The critical, fundamental mechanisms that determine the emergence of status epilepticus from a single seizure and the prolonged duration of status epilepticus are uncertain. However, several general concepts of the pathophysiology of status epilepticus have emerged: (a) the hippocampus is consistently activated during status epilepticus; (b) loss of GABA-mediated inhibitory synaptic transmission in the hippocampus is critical for emergence of status epilepticus; and, finally (c) glutamatergic excitatory synaptic transmission is important in sustaining status epilepticus. This review focuses on the alteration of GABAergic inhibition in the hippocampus that occurs during the prolonged seizures of status epilepticus. If reduction in GABAergic inhibition leads to development of status epilepticus, enhancement of GABAergic inhibition would be expected to interrupt status epilepticus. Benzodiazepines and barbiturates are both used in the treatment of status epilepticus and both drugs enhance GABA(A) receptor-mediated inhibition. However, patients often become refractory to benzodiazepines when seizures are prolonged, and barbiturates are often then used for these refractory cases of status epilepticus. Recent evidence suggests the presence of multiple GABA(A) receptor isoforms in the hippocampus with different sensitivity to benzodiazepines but similar sensitivity to barbiturates, thus explaining why the two drug classes might have different clinical effects. In addition, rapid functional plasticity of GABA(A) receptors has been demonstrated to occur during status epilepticus in rats. During status epilepticus, there was a substantial reduction of diazepam potency for termination of the seizures. The loss of sensitivity of the animals to diazepam during status epilepticus was accompanied by an alteration in the functional properties of hippocampal dentate granule cell GABA(A) receptors. Dentate granule cell GABA(A) receptor currents from rats undergoing status epilepticus had reduced sensitivity to diazepam and zinc but normal sensitivity to GABA and pentobarbital. Therefore, the prolonged seizures of status epilepticus rapidly altered the functional properties of hippocampal dentate granule cell GABA(A) receptors, possibly explaining why benzodiazepines and barbiturates may not be equally effective during treatment of the prolonged seizures of status epilepticus. A comprehensive understanding of the cellular and molecular events leading to the development, maintenance, and cytotoxicity of status epilepticus should permit development of more effective treatment strategies and reduction in the mortality and morbidity of status epilepticus.