Mossy fiber plasticity and enhanced hippocampal excitability, without hippocampal cell loss or altered neurogenesis, in an animal model of prolonged febrile seizures

Cell cycle reentry and cell proliferation as candidates for the seizure predispositions in the hippocampus of EL mouse brain

We have recently found that there was DNA fragmentation without cell loss in the hippocampus in EL mice, an epileptic mutant. Neurotrophic factors are also expressed at high levels during the early developmental stages. In the present study, we used EL mice to examine how altered cyclin and the corresponding cyclin dependent kinase (CDK) family are related to cell proliferation during development and during epileptogenesis. Developmental changes of cyclin family and corresponding CDK family (cyclin D/CDK-4, cyclin E/CDK-2, cyclin A/CDK-2, cyclin A/CDK-1, cyclin B/CDK-1) were examined by Western blotting in the hippocampus of EL mice and in nonepileptic control animals (DDY mice). In addition, we attempted to quantify cell proliferation during this period. The developmental changes in cell proliferation were determined by using systemic injections of Bromo-deoxyUridine (BrdU) to label dividing cells. As compared with the control DDY mice, EL mice show an upregulation of cell cycle specific Cyclins/CDKs during early developmental stages suggesting that reentry into the cell cycle is enhanced prior to the onset of seizure activity, possibly due to the abundance of neurotrophic factors. These results show that Cyclins/CDKs are activated during early stages of development in an epileptic animal, before the mouse exhibits seizures. These results suggest that reentry of cells into the cell cycle, with consequent cell proliferation in the hippocampus, contribute to the seizure predispositions of EL mice.

Can fever treat epileptic encephalopathies?

PURPOSE

To describe resistant epileptic encephalopathies that significantly improved after an acute febrile episode (FE).

METHODS

We reviewed the clinical history of patients with daily pharmacoresistant seizures referred to the Saint-Vincent de Paul Hospital in the last 5 years. Four patients experienced seizure arrest in relation with a febrile episode.

RESULTS

The four patients suffered from epileptic encephalopathy. Three were symptomatic, one cryptogenic. They presented spasms and atypical absences, beginning after the age of 1 year. All seizures stopped at the onset of fever, and significant EEG improvement was observed. The seizure-free period ranged from 2 to 24 months.

DISCUSSION AND CONCLUSION

The close link between the occurrence of FE and the disappearance of seizures and EEG improvement, contrasting with the previous pharmacoresistance of this epileptic encephalopathy, supports a non fortuitous association. Several mechanisms could explain this phenomenon, including viral etiology, hyperthermia, inflammatory-immune reaction and ACTH release. Better understanding this phenomenon could open new therapeutic perspectives.

Fever, febrile seizures and epilepsy

Seizures induced by fever (febrile seizures) are the most common type of pathological brain activity in infants and children. These febrile seizures and their potential contribution to the mechanisms of limbic (temporal lobe) epilepsy have been a topic of major clinical and scientific interest. Key questions include the mechanisms by which fever generates seizures, the effects of long febrile seizures on neuronal function and the potential contribution of these seizures to epilepsy. This review builds on recent advances derived from animal models and summarizes our current knowledge of the mechanisms underlying febrile seizures and of changes in neuronal gene expression and function that facilitate the enduring effects of prolonged febrile seizures on neuronal and network excitability. The review also discusses the relevance of these findings to the general mechanisms of epileptogenesis during development and points out gaps in our knowledge, including the relationship of animal models to human febrile seizures and epilepsy.

Age-Dependent Consequences of Status Epilepticus: Animal Models

Status epilepticus (SE) is a significant neurological emergency that occurs most commonly in children. Although SE has been associated with an elevated risk of brain injury, it is unclear from clinical studies in whom and under what circumstances brain injury will occur. The purpose of this review is to evaluate the effects of age on the consequences of SE. In this review, we focus mainly on the animal data that describe the consequences of a single episode of SE induced in the adult and immature rat brain. The experimental data suggest that the risk of developing SE-induced brain damage, subsequent epilepsy and cognitive deficits in large part depends on the age in which the SE occurs. Younger rats are more resistant to seizure-induced brain damage than older rats; however, when SE occurs in immature rats with abnormal brains, there is an increase in the severity of seizure-induced brain injury. Better understanding of the pathophysiologic mechanisms underlying the age-specific alterations to the brain induced by SE will lead to the development of novel and effective strategies to improve the deleterious consequences.

Stereological analysis of GluR2-immunoreactive hilar neurons in the pilocarpine model of temporal lobe epilepsy: correlation of cell loss with mossy fiber sprouting

Mossy fiber sprouting and the genesis of ectopic granule cells contribute to reverberating excitation in the dentate gyrus of epileptic brain. This study determined whether the extent of sprouting after status epilepticus in rats correlates with the seizure-induced degeneration of GluR2-immunoreactive (GluR2+) hilar neurons (presumptive mossy cells) and also quantitated granule cell-like GluR2-immunoreactive hilar neurons. Stereological cell counting indicated that GluR2+ neurons account for 57% of the total hilar neuron population. Prolonged pilocarpine-induced status epilepticus killed 95% of these cells. A smaller percentage of GluR2+ neurons (74%) was killed when status epilepticus was interrupted after 1-3.5 h with a single injection of phenobarbital, and the number of residual GluR2+ neurons varied among animals by a factor of 6.2. GluR2+ neurons were not necessarily more vulnerable than other hilar neurons. In rats administered phenobarbital, the extent of recurrent mossy fiber growth varied inversely and linearly with the number of GluR2+ hilar neurons that remained intact (P=0.0001). Thus the loss of each GluR2+ neuron was associated with roughly the same amount of sprouting. These findings support the hypothesis that mossy fiber sprouting is driven largely by the degeneration of and/or loss of innervation from mossy cells. Granule cell-like GluR2-immunoreactive neurons were rarely encountered in the hilus of control rats, but increased 6- to 140-fold after status epilepticus. Their number did not correlate with the extent of hilar cell death or mossy fiber sprouting in the same animal. The morphology, number, and distribution of these neurons suggested that they were hilar ectopic granule cells.

Neuropeptide Y: potential role in recurrent developmental seizures

Seizures induce profound plastic changes in the brain, including altered expression of neuropeptide Y (NPY) and its receptors. Here, I discuss a potential role of NPY plasticity in the developmental brain: in a rat model of febrile seizures (FS), the most common type of seizures in infants and young children, NPY expression was up-regulated in hippocampus after experimentally induced FS. Interestingly, NPY up-regulation was associated with an increased seizure threshold for additional (recurrent) FS, and this effect was abolished when an antagonist against NPY receptor type 2 was applied. These findings suggest that inhibitory actions of NPY, released after seizures, exert a protective effect that reduces the risk of seizure recurrence in the developing brain.

Prevention of plasticity of endocannabinoid signaling inhibits persistent limbic hyperexcitability caused by developmental seizures

Depolarization-induced suppression of inhibition (DSI) is an endocannabinoid-mediated short-term plasticity mechanism that couples postsynaptic Ca2+ rises to decreased presynaptic GABA release. Whether the gain of this retrograde synaptic mechanism is subject to long-term modulation by glutamatergic excitatory inputs is not known. Here, we demonstrate that activity-dependent long-term DSI potentiation takes place in hippocampal slices after tetanic stimulation of Schaffer collateral synapses. This activity-dependent, long-term plasticity of endocannabinoid signaling was specific to GABAergic synapses, as it occurred without increases in the depolarization-induced suppression of excitation. Induction of tetanus-induced DSI potentiation in vitro required a complex pathway involving AMPA/kainate and metabotropic glutamate receptor as well as CB1 receptor activation. Because DSI potentiation has been suggested to play a role in persistent limbic hyperexcitability after prolonged seizures in the developing brain, we used these mechanistic insights into activity-dependent DSI potentiation to test whether interference with the induction of DSI potentiation prevents seizure-induced long-term hyperexcitability. The results showed that the in vitro, tetanus-induced DSI potentiation was occluded by previous in vivo fever-induced (febrile) seizures, indicating a common pathway. Accordingly, application of CB1 receptor antagonists during febrile seizures in vivo blocked the seizure-induced persistent DSI potentiation, abolished the seizure-induced upregulation of CB1 receptors, and prevented the emergence of long-term limbic hyperexcitability. These results reveal a new form of activity-dependent, long-term plasticity of endocannabinoid signaling at perisomatic GABAergic synapses, and demonstrate that blocking the induction of this plasticity abolishes the long-term effects of prolonged febrile seizures in the developing brain.

Unmasking recurrent excitation generated by mossy fiber sprouting in the epileptic dentate gyrus: an emergent property of a complex system

Seizure-induced sprouting of the mossy fiber pathway in the dentate gyrus has been observed nearly universally in experimental models of limbic epilepsy and in the epileptic human hippocampus. The observation of progressive mossy fiber sprouting induced by kindling demonstrated that even a few repeated seizures are sufficient to alter synaptic connectivity and circuit organization. As it is now recognized that seizures induce synaptic reorganization in hippocampal and cortical pathways, the implications of seizure-induced synaptic reorganization for circuit properties and function have been subjects of intense interest. Detailed anatomical characterization of the sprouted mossy fiber pathway has revealed that the overwhelming majority of sprouted synapses in the inner molecular layer of the dentate gyrus form recurrent excitatory connections, and are thus likely to contribute to recurrent excitation and potentially to enhanced susceptibility to seizures. Nevertheless, difficulties in detecting functional abnormalities in circuits reorganized by mossy fiber sprouting and the fact that some sprouted axons appear to form synapses with inhibitory interneurons have been cited as evidence that sprouting may not contribute to seizure susceptibility, but could form recurrent inhibitory circuits and be a compensatory response to prevent seizures. Quantitative analysis of the synaptic connections of the sprouted mossy fiber pathway, assessment of the functional features of sprouted circuitry using reliable physiological measures, and the perspective of complex systems analysis of neural circuits strongly support the view that the functional effects of the recurrent excitatory circuits formed by mossy fiber sprouting after seizures or injury emerge only conditionally and intermittently, as observed with spontaneous seizures in human epilepsy. The recognition that mossy fiber sprouting is induced after hippocampal injury and seizures and contributes conditionally to emergence of recurrent excitation has provided a conceptual framework for understanding how injury and seizure-induced circuit reorganization may contribute to paroxysmal network synchronization, epileptogenesis, and the consequences of repeated seizures, and thus has had a major influence on understanding of fundamental aspects of the epilepsies.

Increased number of febrile seizures in children born very preterm: relation of neonatal, febrile and epileptic seizures and neurological dysfunction to seizure outcome at 16 years of age

PURPOSE

In prematurely born population, a cascade of events from initial injury in the developing brain to morbidity may be followed. The aim of our study was to assess seizures in prematurely born children from birth up to 16 years and to evaluate the contribution of different seizures, and of neurological dysfunction to the seizure outcome.

METHODS

Pre- and neonatal data and data from neurodevelopmental examination at 5 years of 60 prospectively followed children born at or before 32 weeks of gestation, and of 60 matched term controls from the 2 year birth cohort were available from earlier phases of the study. Later seizure data were obtained from questionnaires at 5, 9, and 16 years, and from hospital records and parent interviews.

RESULTS

In the preterm group, 16 children (27%) exhibited neonatal seizures, 10 children (17%) had seizures during febrile illness and 5 children had epilepsy. Eight children had only febrile seizures, and 3 of these had both multiple simple and complex febrile seizures and neurodevelopmental dysfunction. None of the 8 children had experienced neonatal seizures, 6 had a positive family history of seizures, but none developed epilepsy. The children with epilepsy had CP and neurocognitive problems, and all but one had experienced neonatal seizures; two of them had also had fever-induced epileptic seizures. In controls 3 children (5%) had simple febrile seizures.

CONCLUSION

Children born very preterm have increased rate of febrile seizures compared to the controls. However, no cascade from initial injury via febrile seizures to epilepsy could be shown during the follow-up of 16 years. Symptomatic epilepsy in prematurely born children is characterised by neonatal seizures, major neurological disabilities and early onset of epilepsy.

Hippocampal volume in childhood complex partial seizures

PURPOSE

This study compared hippocampal volume in children with cryptogenic epilepsy, all of whom had complex partial seizures (CPS), and age and gender matched normal children controlling for between group differences in IQ and demographic variables (e.g., age, gender, ethnicity, socioeconomic status). It also examined the relationship between hippocampal volumes and seizure variables in the patients.

METHODS

Using quantitative magnetic resonance imaging (MRI), we compared the hippocampal volumes of 19 medically treated children with CPS, aged 6-14 years, to 21 age and gender matched normal children.

RESULTS

The children with CPS had significantly smaller total hippocampal volumes than the normal children. This finding was accounted for primarily by significantly smaller anterior hippocampal volumes. Within the CPS group, smaller total and posterior hippocampus volumes were significantly associated with longer duration of illness. Anterior hippocampal volumes, however, were unrelated to seizure variables.

CONCLUSIONS

These findings imply impaired development of the hippocampus, particularly the anterior hippocampus, and a differential effect of the underlying illness and on-going seizures on hippocampal development in medically controlled pediatric CPS.

Hippocampal neurodegeneration, spontaneous seizures, and mossy fiber sprouting in the F344 rat model of temporal lobe epilepsy

The links among the extent of hippocampal neurodegeneration, the frequency of spontaneous recurrent motor seizures (SRMS), and the degree of aberrant mossy fiber sprouting (MFS) in temporal lobe epilepsy (TLE) are a subject of contention because of variable findings in different animal models and human studies. To understand these issues further, we quantified these parameters at 3-5 months after graded injections of low doses of kainic acid (KA) in adult F344 rats. KA was administered every 1 hr for 4 hr, for a cumulative dose of 10.5 mg/kg bw, to induce continuous stages III-V motor seizures for >3 hr. At 4 days post-KA, the majority of rats (77%) exhibited moderate bilateral neurodegeneration in different regions of the hippocampus; however, 23% of rats exhibited massive neurodegeneration in all hippocampal regions. All KA-treated rats displayed robust SRMS at 3 months post-KA, and the severity of SRMS increased over time. Analyses of surviving neurons at 5 months post-KA revealed two subgroups of rats, one with moderate hippocampal injury (HI; 55% of rats) and another with widespread HI (45%). Rats with widespread HI exhibited greater loss of CA3 pyramidal neurons and robust aberrant MFS than rats with moderate HI. However, the frequency of SRMS (approximately 3/hr) was comparable between rats with moderate and widespread HI. Thus, in comparison with TLE model using Sprague-Dawley rats (Hellier et al. [1998] Epilepsy Res. 31:73-84), a much lower cumulative dose of KA leads to robust chronic epilepsy in F344 rats. Furthermore, the occurrence of SRMS in this model is always associated with considerable bilateral hippocampal neurodegeneration and aberrant MFS. However, more extensive hippocampal CA3 cell loss and aberrant MFS do not appear to increase the frequency of SRMS. Because most of the features are consistent with mesial TLE in humans, the F344 model appears ideal for testing the efficacy of potential treatment strategies for mesial TLE.

Ketamine appears associated with better word recall than etomidate after a course of 6 electroconvulsive therapies

Ten patients treated with electroconvulsive therapy (ECT) for depressive illness received anesthesia with either etomidate or ketamine. Three patients received both etomidate and ketamine anesthesia for ECT during separate episodes of depression. Patients anesthetized with ketamine for ECT had significantly less impairment of short-term memory function than did patients who received ECT with etomidate anesthesia. All patients who received both anesthetics for ECT during 2 different episodes had less memory loss during ECT with ketamine than with etomidate. These results show the importance of studying the effects of all anesthetic agents used during ECT on cognitive functions. The results imply that the effect of ECT on memory may be largely caused by effects mediated by glutamate at N-methyl-d-aspartate receptors and suggest that N-methyl-d-aspartate antagonists may offer protection from memory dysfunction during ECT.

The role of synaptic reorganization in mesial temporal lobe epilepsy

The mechanisms underlying mesial temporal lobe epilepsy (MTLE) remain uncertain. Putative mechanisms should account for several features characteristic of the clinical presentation and the neurophysiological and neuropathological abnormalities observed in patients with intractable MTLE. Synaptic reorganization of the mossy fiber pathway has received considerable attention over the past two decades as a potential mechanism that increases the excitability of the hippocampal network through the formation of new recurrent excitatory collaterals. Morphological plasticity beyond the mossy fiber pathway has not been as thoroughly investigated. Recently, plasticity of the CA1 pyramidal axons has been demonstrated in acute and chronic experimental models of MTLE. As the hippocampal formation is topographically organized in stacks of slices (lamellae), synaptic reorganization of CA1 axons projecting to subiculum appears to increase the connectivity between lamellae, providing a mechanism for translamellar synchronization of cellular hyperexcitability, leading to pharmacologically intractable seizures.

Repair of the injured adult hippocampus through graft-mediated modulation of the plasticity of the dentate gyrus in a rat model of temporal lobe epilepsy

Intracerebroventricular kainate administration in rat, a model of temporal lobe epilepsy (TLE), causes degeneration of the hippocampal CA3 pyramidal and dentate hilar neurons. This leads to a robust but aberrant sprouting of the granule cell axons (mossy fibers) into the dentate supragranular layer and the CA3 stratum oriens. Because this plasticity is linked to an increased seizure susceptibility in TLE, strategies that restrain the aberrant mossy fiber sprouting (MFS) are perceived to be important for preventing the TLE development after the hippocampal injury. We ascertained the efficacy of fetal hippocampal CA3 or CA1 cell grafting into the kainate-lesioned CA3 region of the adult rat hippocampus at early post-kainic acid injury for providing a lasting inhibition of the aberrant MFS. Analyses at 12 months after grafting revealed that host mossy fibers project vigorously into CA3 cell grafts but avoid CA1 cell grafts. Consequently, in animals receiving CA3 cell grafts, the extent of aberrant MFS was minimal, in comparison with the robust MFS observed in both "lesion-only" animals and animals receiving CA1 cell grafts. Analyses of the graft axon growth revealed strong graft efferent projections into the dentate supragranular layer with CA3 cell grafting but not with CA1 cell grafting. Thus, the formation of reciprocal circuitry between the dentate granule cells and the grafted CA3 pyramidal neurons is likely the basis of inhibition of the aberrant MFS by CA3 cell grafts. The results also underscore that grafting of cells capable of differentiating into CA3 pyramidal neurons is highly efficacious for a lasting inhibition of the abnormal mossy fiber circuitry development in the injured hippocampus.

Temporal lobe epilepsy after experimental prolonged febrile seizures: prospective analysis

Experimental prolonged febrile seizures (FS) lead to structural and molecular changes that promote hippocampal hyperexcitability and reduce seizure threshold to further convulsants. However, whether these seizures provoke later-onset epilepsy, as has been suspected in humans, has remained unclear. Previously, intermittent EEGs with behavioural observations for motor seizures failed to demonstrate spontaneous seizures in adult rats subjected to experimental prolonged FS during infancy. Because limbic seizures may be behaviourally subtle, here we determined the presence of spontaneous limbic seizures using chronic video monitoring with concurrent hippocampal and cortical EEGs, in adult rats (starting around 3 months of age) that had sustained experimental FS on postnatal day 10. These subjects were compared with groups that had undergone hyperthermia but in whom seizures had been prevented (hyperthermic controls), as well as with normothermic controls. Only events that fulfilled both EEG and behavioural criteria, i.e. electro-clinical events, were considered spontaneous seizures. EEGs (over 400 recorded hours) were normal in all normothermic and hyperthermic control rats, and none of these animals developed spontaneous seizures. In contrast, prolonged early-life FS evoked spontaneous electro-clinical seizures in 6 out of 17 experimental rats (35.2%). These seizures consisted of sudden freezing (altered consciousness) and typical limbic automatisms that were coupled with polyspike/sharp-wave trains with increasing amplitude and slowing frequency on EEG. In addition, interictal epileptiform discharges were recorded in 15 (88.2%) of the experimental FS group and in none of the controls. The large majority of hippocampally-recorded seizures were heralded by diminished amplitude of cortical EEG, that commenced half a minute prior to the hippocampal ictus and persisted after seizure termination. This suggests a substantial perturbation of normal cortical neuronal activity by these limbic spontaneous seizures. In summary, prolonged experimental FS lead to later-onset limbic (temporal lobe) epilepsy in a significant proportion of rats, and to interictal epileptifom EEG abnormalities in most others, and thus represent a model that may be useful to study the relationship between FS and human temporal lobe epilepsy.

Newly born granule cells in the dentate gyrus rapidly extend axons into the hippocampal CA3 region following experimental brain injury

We investigated whether new neurons generated in the adult rat brain following lateral fluid percussion traumatic brain injury (TBI) are capable of projecting axons along the mossy fiber pathway to the CA3 region of the hippocampus. Dividing cells were labeled by intraperitoneal injection of bromodeoxyuridine (BrdU) on the day of surgery/injury, and neurons that extended axons to the CA3 region were retrogradely labeled by fluorescent tracers (FluoSpheres), stereotactically injected into the CA3 region of both the ipsi- and contralateral hippocampus at 1 or 12 days following TBI (n = 12) or sham injury (n = 12) in anaesthetized rats. Animals (n = 6 injured and n = 6 sham-injured controls per time point) were sacrificed at either 3 or 14 days post-injury. Another group of animals (n = 3) was subjected to experimental TBI and BrdU administration and sacrificed 3 days after TBI to be processed for BrdU and immunohistochemistry for polysialylated neural cell adhesion molecule (PSA-NCAM), a growth-related protein normally observed during CNS development. A fivefold bilateral increase in the number of mitotically active (BrdU+) cells was noted within the dentate gyrus when compared to uninjured controls as early as 3 days following TBI. A subgroup of dividing cells was also immunoreactive for PSA-NCAM at 3 days following TBI. By 2 weeks post-injury the number of BrdU+ cells within the dentate gyrus was increased twofold compared to the uninjured counterparts and a proportion of these newly generated cells showed cytoplasmic staining for the fluorescent tracer. These findings document rapid neurogenesis following TBI and show, for the first time, that newly generated granule neurons are capable of extending projections along the hippocampal mossy fiber pathway in the acute post-traumatic period.

Disruption of the neurogenic potential of the dentate gyrus in a mouse model of temporal lobe epilepsy with focal seizures

Adult hippocampal neurogenesis is enhanced in response to multiple stimuli including seizures. However, the relationship between neurogenesis and the development of temporal lobe epilepsy (TLE) remains unclear. Unilateral intrahippocampal injection of kainate in adult mice models the morphological characteristics (e.g. neuronal loss, gliosis, granule cell dispersion and hypertrophy) and occurrence of chronic, spontaneous recurrent partial seizures observed in human TLE. We investigated the influence of a kainate-induced epileptogenic focus on hippocampal neurogenesis, comparing neural stem cell proliferation following status epilepticus and spontaneous recurrent partial seizures. Cell proliferation in the subgranular zone was transiently increased bilaterally after kainate treatment. As a result, neurogenesis was stimulated in the contralateral dentate gyrus. In contrast, the epileptic hippocampus exhibited a strongly reduced neurogenic potential, even after onset of spontaneous recurrent partial seizures, possibly due to an alteration of the neurogenic niche in the subgranular zone. These results show that neurogenesis does not contribute to the formation of the epileptic focus and may be affected when dispersion of dentate gyrus granule cells occurs. Therefore, in patients with TLE, hippocampal sclerosis and granule cell dispersion may play a significant role in disrupting the potential for hippocampal neurogenesis.

Gender differences in febrile seizure-induced proliferation and survival in the rat dentate gyrus

PURPOSE

Febrile seizures are fever-associated early-life seizures that are thought play a role in the development of epilepsy. Seizure-induced proliferation of dentate granule cells has been demonstrated in several adult animal models and is thought to be an integral part of epileptogenesis. The aim of the present study was to investigate proliferation and survival of dentate gyrus (DG) cells born after early-life hyperthermia (HT)-induced seizures in male and female rats.

METHODS

At postnatal day (PN) 10, male and female rats were exposed to heated air to induce seizures. Littermates were used as normothermia controls. Convulsive behavior was observed by two researchers. From PN11 to PN16, rats were injected with bromodeoxyuridine (BrdU) to label dividing cells. The number of BrdU-immunoreactive cells in the DG was counted at PN17 and PN66.

RESULTS

At PN17, male as well as female HT rats had the same amount of BrdU-positive cells compared with controls. At PN66, significantly more BrdU-positive cells were left in HT females (53%) than in controls (44%, percentage of BrdU-positive cells at PN17), whereas no difference was found between HT males and male controls. The net result of proliferation and survival at PN66 was that female HT rats had the same number of BrdU-immunoreactive cells as controls, whereas male HT rats had 25% more BrdU-immunoreactive cells than did controls (p < 0.05).

CONCLUSIONS

Early-life seizures cause a sexually dimorphic cytogenic response that results in an increased population of newborn DG cells in young adult males, while leaving that of young adult females unaltered.

Losing neurons: selective vulnerability and mesial temporal sclerosis

Mesial temporal sclerosis (MTS) is found in about two-thirds of patients with refractory temporal lobe epilepsy (TLE), and surgical removal of the sclerotic structures eliminates seizures in the majority of cases undergoing surgical resection. Although multiple factors have been implicated in the genesis of MTS, it is still unclear why some individuals are more likely to develop hippocampal sclerosis than others. Epileptologists have proposed that there must be at least two factors involved-an initial precipitating injury (IPI), such as a prolonged febrile seizure, CNS infection, or head trauma, and a second factor that increases vulnerability to neuronal injury. This has been termed the "two-hit hypothesis." Three of the many factors that could possibly heighten susceptibility to neuronal injury and MTS are discussed here. These are microdysgenesis, hippocampal dysgenesis, prior seizures, and genetic predisposition. We conclude that there is currently no compelling evidence to support a role for microdysgenesis in MTS. Hippocampal dysgenesis, on the other hand, may account for febrile seizures and possibly MTS in a small subpopulation of patients with TLE. Additional larger studies are needed to confirm these findings. Experimental evidence indicates that an epileptogenic hippocampus can result from prolonged febrile seizures in infant rats, even though these seizures do not cause MTS in the rat. It is not known if this pathophysiological sequence occurs in humans. Lastly, there appears to be a strong genetic component that predisposes some individuals to MTS, regardless of whether they experience an IPI.

Progressive hippocampal and extrahippocampal atrophy in drug resistant epilepsy

PURPOSE OF REVIEW

This article reviews recent experimental and clinical evidence for seizure-related progressive brain damage and discusses possible mechanisms of ongoing brain atrophy in epilepsy.

RECENT FINDINGS

Experimental data indicate that seizures induce brain plasticity that may result in either damage or protection. Brief seizures or status epilepticus may promote resistance to additional damage but also induce cumulative neuronal loss and increase susceptibility to network synchronization. Some experimental studies indicated that, following the initial damage caused by status epilepticus, further brief seizures may not produce significant continuing neuronal loss and hippocampal atrophy, whereas other studies showed the contrary. There is recent evidence that progressive damage and atrophy may occur after an acute insult but are not directly associated with recurrent seizures. Clinical research data continue to show discrepancies regarding whether ongoing seizures cause progressive atrophy. Some cross-sectional and longitudinal magnetic resonance imaging studies in patients with partial epilepsies have shown progressive hippocampal and extrahippocampal atrophy, the severity of which correlated with the duration of epilepsy, seizure frequency, or lifetime seizure number, whereas others have failed to show a clear association.

SUMMARY

Experimental data indicate that epileptogenesis in developing brain may not require significant neuronal loss, which is in keeping with clinical observations that progressive cognitive and behavioural impairment may occur in patients with no detectable brain atrophy. A better understanding of why, when and how progressive brain atrophy occurs will lead to better clinical management, earlier surgical intervention when necessary and, ultimately, prevention of epileptogenesis.

Advances in the pathophysiology of developmental epilepsies

Pediatric epilepsies display unique characteristics that differ significantly from epilepsy in adults. The immature brain exhibits a decreased seizure threshold and an age-specific response to seizure-induced brain injury. Many idiopathic epilepsy syndromes and symptomatic epilepsies commonly present during childhood. This review highlights recent advances in the pathophysiology of developmental epilepsies. Cortical development involves maturational regulation of multiple cellular and molecular processes, such as neurogenesis, neuronal migration, synaptogenesis, and expression of neurotransmitter receptors and ion channels. These normal developmental changes of the immature brain also contribute to the increased risk for seizures and unique responses to seizure-induced brain injury in pediatric epilepsies. Recent technological advances, especially in genetics and imaging, have yielded exciting discoveries about the pathophysiology of specific pediatric epilepsy syndromes, such as the emergence of channelopathies as the cause of many idiopathic epilepsies and identification of malformations of cortical development as a major source of symptomatic epilepsies in children.

Maturation of cultured hippocampal slices results in increased excitability in granule cells

The preparation of hippocampal slices results in loss of input neurons to dentate granule cells, which leads to the reorganization of their axons, the mossy fibers, and alters their functional properties in long-term cultures, but its temporal aspects in the immature hippocampus are not known. In this study, we have focused on the early phase of this plastic reorganization process by analyzing granule cell function with field potential and whole cell recordings during the in vitro maturation of hippocampal slices (from 1 to 17 days in vitro, prepared from 6 to 7-day-old rats), and their morphology using extracellular biocytin labelling technique. Acute slices from postnatal 14-22-day-old rats were analyzed to detect any differences in the functional properties of granule cells in these two preparations. In field potential recordings, small synaptically-evoked responses were detected at 2 days in vitro, and their amplitude increased during the culture time. Whole cell voltage clamp recordings revealed intensive spontaneous excitatory postsynaptic currents, and the susceptibility to stimulus-evoked bursting increased with culture time. In acutely prepared slices, neither synaptically-evoked responses in field potential recordings nor any bursting in whole cell recordings were detected. The excitatory activity was under the inhibitory control of gamma-aminobutyric acid type A receptor. Extracellularily applied biocytin labelled dentate granule cells, and revealed sprouting and aberrant targeting of mossy fibers in cultured slices. Our results suggest that reorganization of granule cell axons takes place during the early in vitro maturation of hippocampal slices, and contributes to their increased excitatory activity resembling that in the epileptic hippocampus. Cultured immature hippocampal slices could thus serve as an additional in vitro model to elucidate mechanisms of synaptic plasticity and cellular reactivity in response to external damage in the developing hippocampus.

The mossy fiber system of the hippocampal formation is decreased by chronic and postnatal but not by prenatal protein malnutrition in rats

We tested in 70-day-old Sprague-Dawley rats, whether malnutrition imposed during different periods of hippocampal development produced deleterious effects on the total reference volume of the mossy fiber system. Animals were treated under four nutritional conditions: (a) well nourished; (b) prenatal protein malnourished; (c) chronic protein malnourished and (d) postnatal protein malnourished. Timm's stained material was used in coronal hippocampal sections (40 microm) to estimate--using the Principle of Cavalieri--the total reference volume of the mossy fiber system in each experimental group. Our results show that chronic and postnatal protein malnourished, but not prenatal malnourished rats, decrease the mossy fiber system and the total reference volume of the mossy fiber system are selectively vulnerable to the type of dietary restriction. Thus, chronic and posnatal protein malnutrition produce deleterious effects, but only rats under prenatal protein malnutrition were able to reorganize synapses in this plexus. These findings raise the possibility that chronic malnutrition, as a long-term stressful factor, might be an important paradigm to test structural hippocampal changes that produce physiological and pathophysiological effects, or the possibility to recover its function for nutritional rehabilitation.

Serial MRI after experimental febrile seizures: altered T2 signal without neuronal death

Whereas most febrile seizures (FSs) carry a benign outcome, a subpopulation of individuals with prolonged FSs are at risk for later temporal lobe epilepsy. Signal changes on magnetic resonance imaging (MRI) may provide early markers for changes in neuronal integrity that may promote epileptogenesis in such individuals. Here, we used serial MRIs, obtained before and at several time points after experimental prolonged FSs, to determine the prevalence and distribution of signal changes on T2-weighted images and to investigate the pathological substrates leading to these changes. Seventy-five percent of immature rats with experimental prolonged FSs had abnormal T2 signal enhancement at 24 hours, and 87.5% at 8 days after the seizures. The altered T2 values involved the dorsal hippocampus (75%), the piriform cortex (87.5%), and the amygdala (25%). However, these changes were not accompanied by evidence of neuronal injury or death in these regions, as assessed using the Fluoro-Jade method. Thus, experimental prolonged FSs lead to relatively frequent abnormal MRI signal in "temporal lobe" structures. Although these changes do not signify cell death, they may denote pathological cellular processes that promote epileptogenesis. .

Endogenous neuropeptide Y prevents recurrence of experimental febrile seizures by increasing seizure threshold

Febrile seizures (FSs) typically occur at the onset of fever and do not recur within the same febrile episode despite enduring or increased hyperthermia. Recurrent seizures during the same febrile episode are considered "complex," with potentially altered prognosis. A characterized immature rat model of FS was used to test the hypotheses that (1) a first FS influences the threshold temperature for subsequent ones, and (2) the underlying mechanisms involve the release and actions of the endogenous inhibitory hippocampal neuropeptide Y (NPY). Experimental FSs were induced two or three times, at 3- to 4-h intervals, and threshold temperatures measured. To determine the potential effects of seizure-induced endogenous NPY on thresholds for subsequent seizures, an antagonist of the major hippocampal NPY receptor (type 2) was infused prior to induction of the second seizure. As an indicator of NPY release, NPY expression was determined 4 and 24 h later. Threshold core and brain temperatures for hyperthermic seizures were consistent with those observed during human fever. Threshold temperatures for a second and third seizure were significantly and progressively higher than those required for the first. This "protective" effect involved induction of endogenous NPY because it was abolished by the NPY antagonist. In addition, NPY mRNA expression was increased in dentate gyrus, CA3 and CA1, after an experimental FS, consistent with peptide release. Collectively these data indicate that the absence of repetitive seizures during a febrile episode involves the inhibitory actions of endogenous NPY, suggesting that the signaling cascade triggered by this peptide might provide targets for therapeutic intervention.

Effects of GABAergic transmissions on the immunoreactivities of calcium binding proteins in the gerbil hippocampus

Although reduced calcium binding protein (CBP) immunoreactivities in the epileptic hippocampus have been well established, it has been controversial that these changes may directly indicate neuronal degeneration. In the present study, therefore, we investigated CBP expressions in the gerbil hippocampus following treatment with gamma-aminobutyric acid (GABA) receptor antagonists in order to assess whether altered CBP expressions are the result of either abnormal excitation or indicative of neuronal damage/degeneration. Seizure-sensitive (SS) gerbils showed a loss/decline of CBP immunoreactivities in some hippocampal neurons as compared with seizure-resistant (SR) gerbils. In muscimol (GABA(A) receptor agonist) treated SS gerbils, expression levels of CBP were enhanced as compared with saline-treated SS gerbils. Bicuculline (a GABA(A) receptor antagonist) treatment markedly reduced CBP immunoreactivities in hippocampal neurons of the SR gerbil. Baclofen (a GABA(B) receptor agonist) treatment increased CBP immunoreactivities in the hippocampus of SS gerbils, although its effect was lower than that of muscimol treatment. Moreover, phaclofen (GABA(B) receptor antagonist) treated SR gerbil showed reduction in calbindin D-28K immunoreactivity, not parvalbumin immunoreactivity, in the hippocampus. These findings therefore suggest that reduced CBP immunoreactivities may be the consequence of abnormal discharge caused by loss of GABAergic inhibition rather than an indication of the neuronal damage/degeneration.

The effects of the immature rat model of febrile seizures on the occurrence of later generalized tonic-clonic and absence epilepsy

This study was designed to investigate whether the intensity of experimental febrile seizures reduces the threshold to generalized tonic-clonic epilepsy and its effects on the development of generalized absence epilepsy in adulthood. For the evaluation of absence epilepsy, WAG/Rij rats and for the tonic-clonic seizures, PTZ injected Wistar rats were used. Our results showed that while the frequency of the experimental febrile seizures facilitates PTZ-induced generalized tonic-clonic seizures, it does not influence the properties of absence epileptic seizures in adulthood.

Susceptibility of immature and adult brains to seizure effects

The extent that status epilepticus (SE), but also brief seizures, affects neuronal structure and function has been the subject of much clinical and experimental research. There is a reliance on findings from animal research because there have been few prospective clinical studies. This review suggests that the features of seizure-induced injury in the immature brain compared with the adult brain are different and that duration of seizures (SE versus brief), number of seizures, cause of seizures, presence of pre-existing abnormalities, and genetics affect the injury. Increased awareness of age-specific injuries from seizure has promoted research to determine the circumstances under which seizures may produce permanent detrimental effects. Together with recent advances in functional neuroimaging, genomic investigation, and prospective human data, these studies are likely to substantially increase our knowledge of seizure-induced injury, leading to the development of improved algorithms for prevention and treatment of epilepsy.

Psychopathology and Pediatric Complex Partial Seizures: Seizure-related, Cognitive, and Linguistic Variables

PURPOSE

This study examined the role of cognition, language, seizure-related, and demographic variables in the psychopathology of children with complex partial seizure disorder (CPS) of average intelligence.

METHODS

One-hundred one CPS and 102 normal children, aged 5.1 to 16.9 years, had a structured psychiatric interview and cognitive and language testing. Parents provided demographic, perinatal, and seizure-related information, as well as behavioral information through the Child Behavior Checklist (CBCL) and a structured psychiatric interview about the child.

RESULTS

Significantly more CPS patients had psychopathology, cognitive deficits, and linguistic deficits than did those in the normal group. Among the patients, Verbal IQ predicted the presence of a psychiatric diagnosis, as well as CBCL scores in the borderline/clinical range. Seizure, linguistic, and demographic variables were unrelated to psychopathology. The cognitive and linguistic deficits of the CPS group, however, were predicted by seizure factors (e.g., prolonged seizures/febrile convulsions; seizure frequency/number of antiepileptic drugs) and demographic factors (e.g., minority status).

CONCLUSIONS

Because subtle verbal cognitive deficits predict behavioral disturbances in pediatric CPSs, the study's findings highlight the importance of assessing behavior, cognition, and language in these children. They also underscore the negative impact of prolonged seizures, febrile convulsions, seizure frequency, and antiepileptic drug polytherapy on cognition and language in pediatric CPSs.

Febrile seizures and mesial temporal sclerosis

PURPOSE OF REVIEW

The sequence of febrile seizures followed by intractable temporal lobe epilepsy is rarely seen from a population perspective. However, several studies have shown a significant relationship between a history of prolonged febrile seizures in early childhood and mesial temporal sclerosis. The interpretation of these observations remains quite controversial. One possibility is that the early febrile seizure damages the hippocampus and is therefore a cause of mesial temporal sclerosis. Another possibility is that the child has a prolonged febrile seizure because the hippocampus was previously damaged by a prenatal or perinatal insult or by genetic predisposition.

RECENT FINDINGS

Imaging studies have shown that prolonged and focal febrile seizures can produce acute hippocampal injury that evolves to hippocampal atrophy, and that complex febrile seizures can originate in the temporal lobes in some children. Several lines of evidence now indicate that genetic predisposition is an important causal factor of febrile seizures and mesial temporal sclerosis. From recent clinical and molecular genetic studies, it appears that the relationship between febrile seizures and later epilepsy is frequently genetic, and there are several syndrome-specific genes for febrile seizures.

SUMMARY

Mesial temporal sclerosis probably has different causes. A number of retrospective studies showed that complex febrile seizures are a causative factor for the later development of mesial temporal sclerosis and temporal lobe epilepsy. However, contradictory results have come from several prospective and retrospective studies. The association between febrile seizures and temporal lobe epilepsy probably results from complex interactions between several genetic and environmental factors.

Febrile Seizures and Mechanisms of Epileptogenesis: Insights from an Animal Model

Temporal lobe epilepsy (TLE) is the most prevalent type of human epilepsy, yet the causes for its development, and the processes involved, are not known. Most individuals with TLE do not have a family history, suggesting that this limbic epilepsy is a consequence of acquired rather than genetic causes. Among suspected etiologies, febrile seizures have frequently been cited. This is due to the fact that retrospective analyses of adults with TLE have demonstrated a high prevalence (20-->60%) of a history of prolonged febrile seizures during early childhood, suggesting an etiological role for these seizures in the development of TLE. Specifically, neuronal damage induced by febrile seizures has been suggested as a mechanism for the development of mesial temporal sclerosis, the pathological hallmark of TLE. However, the statistical correlation between febrile seizures and TLE does not necessarily indicate a causal relationship. For example, preexisting (genetic or acquired) 'causes' that result independently in febrile seizures and in TLE would also result in tight statistical correlation. For obvious reasons, complex febrile seizures cannot be induced in the human, and studies of their mechanisms and of their consequences on brain molecules and circuits are severely limited. Therefore, an animal model was designed to study these seizures. The model reproduces the fundamental key elements of the human condition: the age specificity, the physiological temperatures seen in fevers of children, the length of the seizures and their lack of immediate morbidity. Neuroanatomical, molecular and functional methods have been used in this model to determine the consequences of prolonged febrile seizures on the survival and integrity of neurons, and on hyperexcitability in the hippocampal-limbic network. Experimental prolonged febrile seizures did not lead to death of any of the seizure-vulnerable populations in hippocampus, and the rate of neurogenesis was also unchanged. Neuronal function was altered sufficiently to promote synaptic reorganization of granule cells, and transient and long-term alterations in the expression of specific genes were observed. The contribution of these consequences of febrile seizures to the epileptogenic process is discussed.

Enhanced expression of a specific hyperpolarization-activated cyclic nucleotide-gated cation channel (HCN) in surviving dentate gyrus granule cells of human and experimental epileptic hippocampus

Changes in the expression of ion channels, contributing to altered neuronal excitability, are emerging as possible mechanisms in the development of certain human epilepsies. In previous immature rodent studies of experimental prolonged febrile seizures, isoform-specific changes in the expression of hyperpolarization-activated cyclic nucleotide-gated cation channels (HCNs) correlated with long-lasting hippocampal hyperexcitability and enhanced seizure susceptibility. Prolonged early-life seizures commonly precede human temporal lobe epilepsy (TLE), suggesting that transcriptional dysregulation of HCNs might contribute to the epileptogenic process. Therefore, we determined whether HCN isoform expression was modified in hippocampi of individuals with TLE. HCN1 and HCN2 expression were measured using in situ hybridization and immunocytochemistry in hippocampi from three groups: TLE with hippocampal sclerosis (HS; n = 17), epileptic hippocampi without HS, or non-HS (NHS; n = 10), and autopsy material (n = 10). The results obtained in chronic human epilepsy were validated by examining hippocampi from the pilocarpine model of chronic TLE. In autopsy and most NHS hippocampi, HCN1 mRNA expression was substantial in pyramidal cell layers and lower in dentate gyrus granule cells (GCs). In contrast, HCN1 mRNA expression over the GC layer and in individual GCs from epileptic hippocampus was markedly increased once GC neuronal density was reduced by >50%. HCN1 mRNA changes were accompanied by enhanced immunoreactivity in the GC dendritic fields and more modest changes in HCN2 mRNA expression. Furthermore, similar robust and isoform-selective augmentation of HCN1 mRNA expression was evident also in the pilocarpine animal model of TLE. These findings indicate that the expression of HCN isoforms is dynamically regulated in human as well as in experimental hippocampal epilepsy. After experimental febrile seizures (i.e., early in the epileptogenic process), the preserved and augmented inhibition onto principal cells may lead to reduced HCN1 expression. In contrast, in chronic epileptic HS hippocampus studied here, the profound loss of interneuronal and principal cell populations and consequent reduced inhibition, coupled with increased dendritic excitation of surviving GCs, might provoke a "compensatory" enhancement of HCN1 mRNA and protein expression.

Enhanced expression of a specific hyperpolarization-activated cyclic nucleotide-gated cation channel (HCN) in surviving dentate gyrus granule cells of human and experimental epileptic hippocampus

Changes in the expression of ion channels, contributing to altered neuronal excitability, are emerging as possible mechanisms in the development of certain human epilepsies. In previous immature rodent studies of experimental prolonged febrile seizures, isoform-specific changes in the expression of hyperpolarization-activated cyclic nucleotide-gated cation channels (HCNs) correlated with long-lasting hippocampal hyperexcitability and enhanced seizure susceptibility. Prolonged early-life seizures commonly precede human temporal lobe epilepsy (TLE), suggesting that transcriptional dysregulation of HCNs might contribute to the epileptogenic process. Therefore, we determined whether HCN isoform expression was modified in hippocampi of individuals with TLE. HCN1 and HCN2 expression were measured using in situ hybridization and immunocytochemistry in hippocampi from three groups: TLE with hippocampal sclerosis (HS; n = 17), epileptic hippocampi without HS, or non-HS (NHS; n = 10), and autopsy material (n = 10). The results obtained in chronic human epilepsy were validated by examining hippocampi from the pilocarpine model of chronic TLE. In autopsy and most NHS hippocampi, HCN1 mRNA expression was substantial in pyramidal cell layers and lower in dentate gyrus granule cells (GCs). In contrast, HCN1 mRNA expression over the GC layer and in individual GCs from epileptic hippocampus was markedly increased once GC neuronal density was reduced by >50%. HCN1 mRNA changes were accompanied by enhanced immunoreactivity in the GC dendritic fields and more modest changes in HCN2 mRNA expression. Furthermore, similar robust and isoform-selective augmentation of HCN1 mRNA expression was evident also in the pilocarpine animal model of TLE. These findings indicate that the expression of HCN isoforms is dynamically regulated in human as well as in experimental hippocampal epilepsy. After experimental febrile seizures (i.e., early in the epileptogenic process), the preserved and augmented inhibition onto principal cells may lead to reduced HCN1 expression. In contrast, in chronic epileptic HS hippocampus studied here, the profound loss of interneuronal and principal cell populations and consequent reduced inhibition, coupled with increased dendritic excitation of surviving GCs, might provoke a "compensatory" enhancement of HCN1 mRNA and protein expression.