Genetic dissection of the signals that induce synaptic reorganization

Preferential pruning of inhibitory synapses by microglia contributes to alteration of the balance between excitatory and inhibitory synapses in the hippocampus in temporal lobe epilepsy

BACKGROUND

A consensus has formed that neural circuits in the brain underlie the pathogenesis of temporal lobe epilepsy (TLE). In particular, the synaptic excitation/inhibition balance (E/I balance) has been implicated in shifting towards elevated excitation during the development of TLE.

METHODS

Sprague Dawley (SD) rats were intraperitoneally subjected to kainic acid (KA) to generate a model of TLE. Next, electroencephalography (EEG) recording was applied to verify the stability and detectability of spontaneous recurrent seizures (SRS) in rats. Moreover, hippocampal slices from rats and patients with mesial temporal lobe epilepsy (mTLE) were assessed using immunofluorescence to determine the alterations of excitatory and inhibitory synapses and microglial phagocytosis.

RESULTS

We found that KA induced stable SRSs 14 days after status epilepticus (SE) onset. Furthermore, we discovered a continuous increase in excitatory synapses during epileptogenesis, where the total area of vesicular glutamate transporter 1 (vGluT1) rose considerably in the stratum radiatum (SR) of cornu ammonis 1 (CA1), the stratum lucidum (SL) of CA3, and the polymorphic layer (PML) of the dentate gyrus (DG). In contrast, inhibitory synapses decreased significantly, with the total area of glutamate decarboxylase 65 (GAD65) in the SL and PML diminishing enormously. Moreover, microglia conducted active synaptic phagocytosis after the formation of SRSs, especially in the SL and PML. Finally, microglia preferentially pruned inhibitory synapses during recurrent seizures in both rat and human hippocampal slices, which contributed to the synaptic alteration in hippocampal subregions.

CONCLUSIONS

Our findings elaborately characterize the alteration of neural circuits and demonstrate the selectivity of synaptic phagocytosis mediated by microglia in TLE, which could strengthen the comprehension of the pathogenesis of TLE and inspire potential therapeutic targets for epilepsy treatment.

Electrographic seizures and ictal-interictal continuum (IIC) patterns in critically ill patients

Critical care long-term continuous electroencephalogram (cEEG) monitoring has expanded dramatically in the last several decades spurned by technological advances in EEG digitalization and several key clinical findings: 1-Seizures are relatively common in the critically ill-large recent observational studies suggest that around 20% of critically ill patients placed on cEEG have seizures. 2-The majority (~75%) of patients who have seizures have exclusively "electrographic seizures", that is, they have no overt ictal clinical signs. Along with the discovery of the unexpectedly high incidence of seizures was the high prevalence of EEG patterns that share some common features with archetypical electrographic seizures but are not uniformly considered to be "ictal". These EEG patterns include lateralized periodic discharges (LPDs) and generalized periodic discharges (GPDs)-patterns that at times exhibit ictal-like behavior and at other times behave more like an interictal finding. Dr. Hirsch and colleagues proposed a conceptual framework to describe this spectrum of patterns called the ictal-interictal continuum (IIC). In the following years, investigators began to answer some of the key pragmatic clinical concerns such as which patients are at risk of seizures and what is the optimal duration of cEEG use. At the same time, investigators have begun probing the core questions for critical care EEG-what is the underlying pathophysiology of these patterns, at what point do these patterns cause secondary brain injury, what are the optimal treatment strategies, and how do these patterns affect clinical outcomes such as neurological disability and the development of epilepsy. In this review, we cover recent advancements in both practical concerns regarding cEEG use, current treatment strategies, and review the evidence associating IIC/seizures with poor clinical outcomes.

Epileptogenesis

Epileptogenesis is a chronic process that can be triggered by genetic or acquired factors, and that can continue long after epilepsy diagnosis. In 2015, epileptogenesis is not a treatment indication, and there are no therapies available in clinic to treat individuals at risk of epileptogenesis. However, thanks to active research, a large number of animal models have become available for search of molecular mechanisms of epileptogenesis. The first glimpses of treatment targets and biomarkers that could be developed to become useful in clinic are in sight. However, the heterogeneity of the epilepsy condition, and the dynamics of molecular changes over the course of epileptogenesis remain as challenges to overcome.

Rapamycin reveals an mTOR-independent repression of Kv1.1 expression during epileptogenesis

Changes in ion channel expression are implicated in the etiology of epilepsy. However, the molecular leading to long-term aberrant expression of ion channels are not well understood. The mechanistic/mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase that mediates activity-dependent protein synthesis in neurons. mTOR is overactive in epilepsy, suggesting that excessive protein synthesis may contribute to the neuronal pathology. In contrast, we found that mTOR activity and the microRNA miR-129-5p reduce the expression of the voltage-gated potassium channel Kv1.1 in an animal model of temporal lobe epilepsy (TLE). When mTOR activity is low, Kv1.1 expression is high and the frequency of behavioral seizures is low. However, as behavioral seizure activity rises, mTOR activity increases and Kv1.1 protein levels drop. In CA1 pyramidal neurons, the reduction in Kv1.1 lowers the threshold for action potential firing. Interestingly, blocking mTOR activity with rapamycin reduces behavioral seizures and temporarily keeps Kv1.1 levels elevated. Overtime, seizure activity increases and Kv1.1 protein decreases in all animals, even those treated with rapamycin. Notably, the concentration of miR-129-5p, the negative regulator of Kv1.1 mRNA translation, increases by 21days post-status epilepticus (SE), sustaining Kv1.1 mRNA translational repression. Our results suggest that following kainic-acid induced status epilepticus there are two phases of Kv1.1 repression: (1) an initial mTOR-dependent repression of Kv1.1 that is followed by (2) a miR-129-5p persistent reduction of Kv1.1.

Microarray-assisted fine mapping of quantitative trait loci on chromosome 15 for susceptibility to seizure-induced cell death in mice

Prior studies with crosses of the FVB/NJ (FVB; seizure-induced cell death-susceptible) mouse and the C57BL/6J (B6; seizure-induced cell death-resistant) mouse revealed the presence of a quantitative trait locus (QTL) on chromosome 15 that influenced susceptibility to kainic acid-induced cell death (Sicd2). In an earlier study, we confirmed that the Sicd2 interval harbors gene(s) conferring strong protection against seizure-induced cell death through the creation of the FVB.B6-Sicd2 congenic strain, and created three interval-specific congenic lines (ISCLs) that encompass Sicd2 on chromosome 15 to fine-map this locus. To further localise this Sicd2 QTL, an additional congenic line carrying overlapping intervals of the B6 segment was created (ISCL-4), and compared with the previously created ISCL-1-ISCL-3 and assessed for seizure-induced cell death phenotype. Whereas all of the ISCLs showed reduced cell death associated with the B6 phenotype, ISCL-4, showed the most extensive reduction in seizure-induced cell death throughout all hippocampal subfields. In order to characterise the susceptibility loci on Sicd2 by use of this ISCL and identify compelling candidate genes, we undertook an integrative genomic strategy of comparing exon transcript abundance in the hippocampus of this newly developed chromosome 15 subcongenic line (ISCL-4) and FVB-like littermates. We identified 10 putative candidate genes that are alternatively spliced between the strains and may govern strain-dependent differences in susceptibility to seizure-induced excitotoxic cell death. These results illustrate the importance of identifying transcriptomics variants in expression studies, and implicate novel candidate genes conferring susceptibility to seizure-induced cell death.

Hilar mossy cell circuitry controlling dentate granule cell excitability

Glutamatergic hilar mossy cells of the dentate gyrus can either excite or inhibit distant granule cells, depending on whether their direct excitatory projections to granule cells or their projections to local inhibitory interneurons dominate. However, it remains controversial whether the net effect of mossy cell loss is granule cell excitation or inhibition. Clarifying this controversy has particular relevance to temporal lobe epilepsy, which is marked by dentate granule cell hyperexcitability and extensive loss of dentate hilar mossy cells. Two diametrically opposed hypotheses have been advanced to explain this granule cell hyperexcitability—the “dormant basket cell” and the “irritable mossy cell” hypotheses. The “dormant basket cell” hypothesis proposes that mossy cells normally exert a net inhibitory effect on granule cells and therefore their loss causes dentate granule cell hyperexcitability. The “irritable mossy cell” hypothesis takes the opposite view that mossy cells normally excite granule cells and that the surviving mossy cells in epilepsy increase their activity, causing granule cell excitation. The inability to eliminate mossy cells selectively has made it difficult to test these two opposing hypotheses. To this end, we developed a transgenic toxin-mediated, mossy cell-ablation mouse line. Using these mutants, we demonstrated that the extensive elimination of hilar mossy cells causes granule cell hyperexcitability, although the mossy cell loss observed appeared insufficient to cause clinical epilepsy. In this review, we focus on this topic and also suggest that different interneuron populations may mediate mossy cell-induced translamellar lateral inhibition and intralamellar recurrent inhibition. These unique local circuits in the dentate hilar region may be centrally involved in the functional organization of the dentate gyrus.

The effects of glycemic control on seizures and seizure-induced excitotoxic cell death

Background

Epilepsy is the most common neurological disorder after stroke, affecting more than 50 million persons worldwide. Metabolic disturbances are often associated with epileptic seizures, but the pathogenesis of this relationship is poorly understood. It is known that seizures result in altered glucose metabolism, the reduction of intracellular energy metabolites such as ATP, ADP and phosphocreatine and the accumulation of metabolic intermediates, such as lactate and adenosine. In particular, it has been suggested that the duration and extent of glucose dysregulation may be a predictor of the pathological outcome of status. However, little is known about neither the effects of glycemic control on brain metabolism nor the effects of managing systemic glucose concentrations in epilepsy.

Results

In this study, we examined glycemic modulation of kainate-induced seizure sensitivity and its neuropathological consequences. To investigate the relationship between glycemic modulation, seizure susceptibility and its neuropathological consequences, C57BL/6 mice (excitotoxin cell death resistant) were subjected to hypoglycemia or hyperglycemia, followed by systemic administration of kainic acid to induce seizures. Glycemic modulation resulted in minimal consequences with regard to seizure severity but increased hippocampal pathology, irrespective of whether mice were hypoglycemic or hyperglycemic prior to kainate administration. Moreover, we found that exogenous administration of glucose following kainic acid seizures significantly reduced the extent of hippocampal pathology in FVB/N mice (excitotoxin cell death susceptible) following systemic administration of kainic acid.

Conclusion

These findings demonstrate that modulation of the glycemic index can modify the outcome of brain injury in the kainate model of seizure induction. Moreover, modulation of the glycemic index through glucose rescue greatly diminishes the extent of seizure-induced cell death following kainate administration. Our data support the hypothesis that deficient insulin signaling may represent a critical contributing factor in the susceptibility to seizure-induced cell death and this may be an important therapeutic target.

Strain differences in seizure-induced cell death following pilocarpine-induced status epilepticus

Mouse strains differ from one another in their susceptibility to seizure-induced excitotoxic cell death. Previously, we have demonstrated that mature inbred strains of mice show remarkable genetic differences in susceptibility to the neuropathological consequences of seizures in the kainate model of status epilepticus. At present, while the cellular mechanisms underlying strain-dependent differences in susceptibility remain unclear, some of this variation is assumed to have a genetic basis. However, it remains unclear whether strain differences in susceptibility to seizure-induced cell death observed following kainate administration are observed following systemic administration of other chemoconvulsants. In rodents, the cholinomimetic convulsant pilocarpine is widely used to induce status epilepticus (SE), followed by hippocampal damage and spontaneous recurrent seizures, resembling temporal lobe epilepsy. This model has initially been described in rats, but is increasingly used in mice. We characterized neuronal pathologies after pilocarpine-induced status epilepticus (SE) in eight inbred strains of mice focusing on the hippocampus. A ramping-up dose protocol for pilocarpine was used and behavior was monitored for 4-5 h. While we did not observe any significant differences in seizure latency or duration to pilocarpine among the inbred strains, we did observe a significant difference in susceptibility to the neuropathological consequences of pilocarpine-induced SE. Of the eight genetically diverse mouse strains screened for pilocarpine-induced status, BALB/cJ and BALB/cByJ were the only two strains that were resistant to the neuropathological consequences of seizure-induced cell death. Additional studies of these murine strains may be useful for investigating genetic influences on pilocarpine-induced status epilepticus.

Galanin receptor 1 deletion exacerbates hippocampal neuronal loss after systemic kainate administration in mice

Background

Galanin is a neuropeptide with a wide distribution in the central and peripheral nervous systems and whose physiological effects are mediated through three G protein-coupled receptor subtypes, GalR1, GalR2, and GalR3. Several lines of evidence indicate that galanin, as well as activation of the GalR1 receptor, is a potent and effective modulator of neuronal excitability in the hippocampus.

Methodology/Principal Findings

In order to test more formally the potential influence of GalR1 on seizure-induced excitotoxic cell death, we conducted functional complementation tests in which transgenic mice that exhibit decreased expression of the GalR1 candidate mRNA underwent kainate-induced status epilepticus to determine if the quantitative trait of susceptibility to seizure-induced cell death is determined by the activity of GalR1. In the present study, we report that reduction of GalR1 mRNA via null mutation or injection of the GalR1 antagonist, galantide, prior to kainate-induced status epilepticus induces hippocampal damage in a mouse strain known to be highly resistant to kainate-induced neuronal injury. Wild-type and GalR1 knockout mice were subjected to systemic kainate administration. Seven days later, Nissl and NeuN immune- staining demonstrated that hippocampal cell death was significantly increased in GalR1 knockout strains and in animals injected with the GalR1 antagonist. Compared to GalR1-expressing mice, GalR1-deficient mice had significantly larger hippocampal lesions after status epilepticus.

Conclusions/Significance

Our results suggest that a reduction of GalR1 expression in the C57BL/6J mouse strain renders them susceptible to excitotoxic injury following systemic kainate administration. From these results, GalR1 protein emerges as a new molecular target that may have a potential therapeutic value in modulating seizure-induced cell death.

Congenic strains provide evidence that a mapped locus on chromosome 15 influences excitotoxic cell death

Inbred strains of mice differ in their susceptibility to excitotoxin-induced cell death, but the genetic basis of individual variation is unknown. Prior studies with crosses of the FVB/NJ (seizure-induced cell death susceptible) mouse and the seizure-induced cell death resistant mouse, C57BL/6J, showed the presence of three quantitative trait loci (QTLs), named seizure-induced cell death 1 (Sicd1) to Sicd3. To better localize and characterize the Sicd2 locus, two reciprocal congenic mouse strains were created. While the B6.FVB-Sicd2 congenic mouse was without effect on modifying susceptibility to seizure-induced excitotoxic cell death, the FVB.B6-Sicd2 congenic mouse, in which the chromosome (Chr) 15 region of C57BL/6J was introgressed into FVB/NJ, showed reduced seizure-induced excitotoxic cell death following kainate administration. Phenotypic comparison between FVB and the congenic FVB.B6-Sicd2 strain confirmed that the Sicd2 interval harbors gene(s) conferring strong protection against seizure-induced excitotoxic cell death. Interval-specific congenic lines (ISCLs) that encompass Sicd2 on Chr 15 were generated and were used to fine-map this QTL. Resultant progeny were treated with kainate and examined for the extent of seizure-induced cell death in order to deduce the Sicd2 genotypes of the recombinants through linkage analysis. All of the ISCLs exhibited reduced cell death associated with the C57BL/6J phenotype; however, ISCL-2 showed the most dramatic reduction in seizure-induced cell death in both area CA3 and in the dentate hilus. These findings confirm the existence of polymorphic loci within the reduced critical region of Sicd2 that regulate the severity of seizure-induced cell death.

Unexpected survival of neurons of origin of the pyramidal tract after spinal cord injury

There is continuing controversy about whether the cells of origin of the corticospinal tract (CST) undergo retrograde cell death after spinal cord injury (SCI). All previous attempts to assess this have used imaging and/or histological techniques to assess upper motoneurons in the cerebral cortex. Here, we address the question in a novel way by assessing Wallerian degeneration and axon numbers in the medullary pyramid of Sprague Dawley rats after both acute SCI, either at cervical level 5 (C5) or thoracic level 9 (T9), and chronic SCI at T9. Our findings demonstrate that only a fraction of a percentage of the total axons in the medullary pyramid exhibit any sign of degeneration at any time after SCI--no more so than in uninjured control rats. Moreover, design-based counts of myelinated axons revealed no decrease in axon number in the medullary pyramid after SCI, regardless of injury level, severity, or time after injury. Spinal cord-injured rats had fewer myelinated axons in the medullary pyramid at 1 year after injury than aged matched controls, suggesting that injury may affect ongoing myelination of axons during aging. We conclude that SCI does not cause death of the CST cell bodies in the cortex; therefore, therapeutic strategies aimed at promoting axon regeneration of the CST in the spinal cord do not require a separate intervention to prevent retrograde degeneration of upper motoneurons in the cortex.

Interictal spikes and epileptogenesis

Interictal spikes are widely accepted diagnostically as a sign of epilepsy, but reasons for the presence of interictal activity in the epileptic brain are unknown. Interictal spikes are easily generated in normal brain by pharmacologically reducing inhibition, and experimental studies of acquired epilepsy indicate that spikes precede seizures. These data lead to the hypothesis that interictal spikes are correlated with epilepsy because they play a fundamental role in epileptogenesis following brain injury. Spikes may guide sprouting axons back to their network of origin, increase and sustain the strength of the synapses formed by sprouted axons, and alter the balance of ion channels in the epileptic focus, such that seizures become possible. This hypothesis has implications that are testable: altering spiking or the calcium signals generated by spikes should alter epileptogenesis and spikes should precede seizures in brain-injured human patients.

Neuroprotection by glutamate receptor antagonists against seizure-induced excitotoxic cell death in the aging brain

We previously have identified phenotypic differences in susceptibility to hippocampal seizure-induced cell death among two inbred strains of mice. We have also reported that the age-related increased susceptibility to the neurotoxic effects of seizure-induced injury is regulated in a strain-dependent manner. In the present study, we wanted to begin to determine the pharmacological mechanism that contributes to variability in the response to the neurotoxic effects of kainate. Thus, we compared the effects of the NMDA receptor antagonist, MK-801 and of the AMPA receptor antagonist NBQX on hippocampal damage in the kainate model of seizure-induced excitotoxic cell death in young, middle-aged, and aged C57BL/6 and FVB/N mice, when given 90 min following kainate-induced status epilepticus. Following kainate injections, mice were scored for seizure activity and brains from mice in each age and antagonist group were processed for light microscopic histopathologic evaluation 7 days following kainate administration to evaluate the severity of seizure-induced injury. Administration of MK-801 significantly reduced the extent of hippocampal damage in young, mature and aged FVB/N mice, while application of NBQX was only effective at attenuating cell death in young and aged mice throughout all hippocampal subfields. Our results suggest that both NMDA and non-NMDA receptors are involved in kainate-induced cell death in the mouse and suggest that aging may differentially affect the ability of neuroprotectants to protect against hippocampal damage. Differences in the effectiveness of these two antagonists could result from differential regulation of glutamatergic neurotransmitter systems or ion channel specificity.

Hippocampal injury, atrophy, synaptic reorganization, and epileptogenesis after perforant pathway stimulation-induced status epilepticus in the mouse

Prolonged dentate granule cell discharges produce hippocampal injury and chronic epilepsy in rats. In preparing to study this epileptogenic process in genetically altered mice, we determined whether the background strain used to generate most genetically altered mice, the C57BL/6 mouse, is vulnerable to stimulation-induced seizure-induced injury. This was necessary because C57BL/6 mice are reportedly resistant to the neurotoxic effects of kainate-induced seizures, which we hypothesized to be related to strain differences in kainate's effects, rather than genetic differences in intrinsic neuronal vulnerability. Bilateral perforant pathway stimulation-induced granule cell discharge for 4 hours under urethane anesthesia produced degeneration of glutamate receptor subunit 2 (GluR2)-positive hilar mossy cells and peptide-containing interneurons in both FVB/N (kainate-vulnerable) and C57BL/6 (kainate-resistant) mice, indicating no strain differences in neuronal vulnerability to seizure activity. Granule cell discharge for 2 hours in C57BL/6 mice destroyed most GluR2-positive dentate hilar mossy cells, but not peptide-containing hilar interneurons, indicating that mossy cells are the neurons most vulnerable to this insult. Stimulation for 24 hours caused extensive hippocampal neuron loss and injury to the septum and entorhinal cortex, but no other detectable damage. Mice stimulated for 24 hours developed hippocampal sclerosis, granule cell mossy fiber sprouting, and chronic epilepsy, but not the granule cell layer hypertrophy (granule cell dispersion) produced by intrahippocampal kainate. These results demonstrate that perforant pathway stimulation in mice reliably reproduces the defining features of human mesial temporal lobe epilepsy with hippocampal sclerosis. Experimental studies in transgenic or knockout mice are feasible if electrical stimulation is used to produce controlled epileptogenic insults.

Alterations in excitotoxicity and prostaglandin metabolism in a transgenic mouse model of Alzheimer's disease

To address the potential impact of presenilin mutations on the prostaglandin metabolism in a neurodegenerative model of glutamatergic excitotoxicity, we injected kainic acid intraperitoneally (30mg/kg body weight) into mice over-expressing the human N141I mutation of presenilin-2, which is known to cause an early-onset form of Alzheimer's disease. We compared the seizure activity as well as seizure lethality in 2- and 6-month-old mice, transgenic for the above-mentioned point mutation, and their wildtype littermates and found that mice harboring the hN141I mutation showed a relative resistance to excitotoxic treatment. This was associated with a constituitively reduced expression of the cyclooxygenases COX-1 and COX-2 in the hippocampus of N141I presenilin-2 mice and a reduced induction of COX-2 expression post-kainate injection. In the past, clinical trials have suggested that both non-steroidal anti-inflammatory drugs, which impact upon a cell's prostaglandin metabolism, and glutamatergic antagonists might be of benefit to patients suffering from Alzheimer's-type dementias. Yet, the exact mechanism by which these drugs are beneficial remains unclear, although it seems possible that presenilins might be implicated in the process, at least in the case of early-onset forms. The data presented here strongly support the notion of an implication of presenilins in the alterations in the prostaglandin system, which have been observed in Alzheimer's disease and may contribute to the underlying pathogenesis of the disease.

Neuroprotection against excitotoxic brain injury in mice after ovarian steroid depletion

Ovarian steroid hormones influence not only seizure phenomena, but also the neuronal cell death that follows. In the present study, we applied two models of ovarian steroid loss, ovariectomy and chemically-induced ovarian failure, to evaluate kainate-induced seizure activity and the susceptibility of hippocampal neurons to seizure-induced neurodegeneration. Young adult female FVB/NJ mice were ovariectomized with (OVX+E, n=6) or without (OVX, n=8) estrogen replacement. A separate group of females received the ovotoxin, 4-vinylcyclohexene diepoxide (VCD, n=8) to deplete ovarian follicles. Mice underwent kainate-induced status epilepticus and were evaluated for seizure activity (3 h) and delayed hippocampal neuronal injury (7 days). While there were no differences in latency or duration of severe seizures among control, OVX and VCD-treated mice, OVX+E mice exhibited seizures of a significantly longer duration. However, both VCD-induced ovarian failure and OVX led to a dramatic reduction in the extent of excitotoxic cell death, with slightly greater effects observed in VCD-treated mice. Estradiol administration to OVX mice also exerted a significant neuroprotective effect against kainate-induced cell death. These results support and extend earlier findings suggesting that the hormonal milieu may have differential effects on seizure susceptibility that are separate and distinct from those influencing hippocampal neuronal vulnerability. Collectively, these findings highlight the complex interactions among the loss of ovarian steroid hormones, estrogen replacement, seizures, and seizure-induced cell death.

Increased susceptibility to kainic acid-induced seizures in Engrailed-2 knockout mice

The En2 gene, coding for the homeobox-containing transcription factor Engrailed-2 (EN2), has been associated to autism spectrum disorder (ASD). Due to neuroanatomical and behavioral abnormalities, which partly resemble those observed in ASD patients, En2 knockout (En2(-/-)) mice have been proposed as a model for ASD. In the mouse embryo, En2 is involved in the specification of midbrain/hindbrain regions, being predominantly expressed in the developing cerebellum and ventral midbrain, and its expression is maintained in these structures until adulthood. Here we show that in the adult mouse brain, En2 mRNA is expressed also in the hippocampus and cerebral cortex. Hippocampal En2 mRNA content decreased after seizures induced by kainic acid (KA). This suggests that En2 might also influence the functioning of forebrain areas during adulthood and in response to seizures. Indeed, a reduced expression of parvalbumin and somatostatin was detected in the hippocampus of En2(-/-) mice as compared to wild-type (WT) mice, indicating an altered GABAergic innervation of limbic circuits in En2(-/-) mice. In keeping with these results, En2(-/-) mice displayed an increased susceptibility to KA-induced seizures. KA (20 mg/kg) determined more severe and prolonged generalized seizures in En2(-/-) mice, when compared to WT animals. Seizures were accompanied by a widespread c-fos and c-jun mRNA induction in the brain of En2(-/-) but not WT mice. Long-term histopathological changes (CA1 cell loss, upregulation of neuropeptide Y) also occurred in the hippocampus of KA-treated En2(-/-) but not WT mice. These findings suggest that En2(-/-) mice might be used as a novel tool to study the link between epilepsy and ASD.

Variation in Galr1 expression determines susceptibility to exocitotoxin-induced cell death in mice

Inbred strains of mice differ in their susceptibility to excitotoxin-induced cell death, but the genetic basis of individual variation in differential susceptibility is unknown. Previously, we identified a highly significant quantitative trait locus (QTL) on chromosome 18 that influenced susceptibility to kainic acid-induced cell death (Sicd1). Comparison of susceptibility to seizure-induced cell death between reciprocal congenic lines for Sicd1 and parental background mice indicates that genes influencing this trait were captured in both strains. Two positional gene candidates, Galr1 and Mbp, map to 55 cM, where the Sicd1 QTL had been previously mapped. Thus, this study was undertaken to determine if Galr1 and/or Mbp could be considered as candidate genes. Genomic sequence comparison of these two functional candidate genes from the C57BL/6J (resistant at Sicd1) and the FVB/NJ (susceptible at Sicd1) strains showed no single-nucleotide polymorphisms. However, expression studies confirmed that Galr1 shows significant differential expression in the congenic and parental inbred strains. Galr1 expression was downregulated in the hippocampus of C57BL/6J mice and FVB.B6-Sicd1 congenic mice when compared with FVB/NJ or B6.FVB-Sicd1 congenic mice. A survey of Galr1 expression among other inbred strains showed a significant effect such that 'susceptible' strains showed a reduction in Galr1 expression as compared with 'resistant' strains. In contrast, no differences in Mbp expression were observed. In summary, these results suggest that differential expression of Galr1 may contribute to the differences in susceptibility to seizure-induced cell death between cell death-resistant and cell death-susceptible strains.

Effect of age on kainate-induced seizure severity and cell death

While the onset and extent of epilepsy increases in the aged population, the reasons for this increased incidence remain unexplored. The present study used two inbred strains of mice (C57BL/6J and FVB/NJ) to address the genetic control of age-dependent neurodegeneration by building upon previous experiments that have identified phenotypic differences in susceptibility to hippocampal seizure-induced cell death. We determined if seizure induction and seizure-induced cell death are affected differentially in young adult, mature, and aged male C57BL/6J and FVB/NJ mice administered the excitotoxin, kainic acid. Dose response testing was performed in three to four groups of male mice from each strain. Following kainate injections, mice were scored for seizure activity and brains from mice in each age group were processed for light microscopic histopathologic evaluation 7 days following kainate administration to evaluate the severity of seizure-induced brain damage. Irrespective of the dose of kainate administered or the age group examined, resistant strains of mice (C57BL/6J) continued to be resistant to seizure-induced cell death. In contrast, aged animals of the FVB/NJ strain were more vulnerable to the induction of behavioral seizures and associated neuropathology after systemic injection of kainic acid than young or middle-aged mice. Results from these studies suggest that the age-related increased susceptibility to the neurotoxic effects of seizure induction and seizure-induced injury is regulated in a strain-dependent manner, similar to previous observations in young adult mice.

Acute and chronic responses to the convulsant pilocarpine in DBA/2J and A/J mice

Characterizing the responses of different mouse strains to experimentally-induced seizures can provide clues to the genes that are responsible for seizure susceptibility, and factors that contribute to epilepsy. This approach is optimal when sequenced mouse strains are available. Therefore, we compared two sequenced strains, DBA/2J (DBA) and A/J. These strains were compared using the chemoconvulsant pilocarpine, because pilocarpine induces status epilepticus, a state of severe, prolonged seizures. In addition, pilocarpine-induced status is followed by changes in the brain that are associated with the pathophysiology of temporal lobe epilepsy (TLE). Therefore, pilocarpine can be used to address susceptibility to severe seizures, as well as genes that could be relevant to TLE. A/J mice had a higher incidence of status, but a longer latency to status than DBA mice. DBA mice exhibited more hippocampal pyramidal cell damage. DBA mice developed more ectopic granule cells in the hilus, a result of aberrant migration of granule cells born after status. DBA mice experienced sudden death in the weeks following status, while A/J mice exhibited the most sudden death in the initial hour after pilocarpine administration. The results support previous studies of strain differences based on responses to convulsants. They suggest caution in studies of seizure susceptibility that are based only on incidence or latency. In addition, the results provide new insight into the strain-specific characteristics of DBA and A/J mice. A/J mice provide a potential resource to examine the progression to status. The DBA mouse may be valuable to clarify genes regulating other seizure-associated phenomena, such as seizure-induced neurogenesis and sudden death.

A quantitative trait locus on chromosome 18 is a critical determinant of excitotoxic cell death susceptibility

C57BL/6J (B6) and FVB/NJ (FVB) mice are phenotypically distinct in their susceptibility to seizure-induced cell death after kainate administration. Previous studies using quantitative trait loci (QTLs) mapping established that the distal region of mouse chromosome 18 contains a gene(s) that is probably responsible for the difference in seizure-induced cell death susceptibility between two inbred strains, B6 and FVB, that are relatively resistant and susceptible, respectively, to seizure-induced cell death. The genetic locus has been mapped to a approximately 12-centimorgan region of chromosome 18, designated as seizure-induced cell death 1 (Sicd1). In order to confirm the Sicd1 QTL, we have developed congenic mouse strains containing the relevant donor segment from the resistant B6 strain on the susceptible FVB background, also referred to as the FVB.B6-Sicd1 congenic strain. Congenic and FVB littermate controls were tested in a seizure-induced cell death paradigm. The presence of B6 chromosome 18 alleles on an FVB genetic background conferred protection against seizure-induced cell death, as compared with FVB littermate controls. To further localize the Sicd1 QTL, new congenic lines carrying overlapping intervals of the B6 segment were created [interval-specific congenic lines (ISCLs)-1-4] and assessed for seizure-induced cell death phenotype. All of the ISCLs exhibited reduced cell death associated with the B6 phenotype, as compared with the parental FVB strain. The most dramatic of these, ISCL-4, showed a nearly four-fold reduction in the extent of seizure-induced cell death. This suggests that ISCL-4 contains the putative gene(s) of the Sicd1 QTL.

Dissociation of the immunoreactivity of synaptophysin and GAP-43 during the acute and latent phases of the lithium–pilocarpine model in the immature and adult rat

RATIONALE

Lithium-pilocarpine-induced status epilepticus (SE) generates neuronal lesions in the limbic forebrain, cerebral cortex and thalamus that lead to circuit reorganization and spontaneous recurrent seizures. The process of reorganization in regions with neuronal damage is not fully clarified.

METHODS

In the present study, we evaluated by immunohistochemistry the early reorganization during the latent period with two neuronal markers, synaptophysin and growth-associated protein 43 (GAP-43) in rats subjected to SE at PN21 and as adults.

RESULTS

Synaptophysin immunoreactivity increased between 24 h and 3 weeks post-SE in regions with severe and rapidly occurring neuronal loss, namely thalamus, amygdala, piriform and entorhinal cortices. GAP-43 expression decreased at 1 and 3 weeks in the same regions. The immunoreactivity of synaptophysin and GAP-43 increased in the inner molecular layer of dentate gyrus from 24 h after SE, and decreased in the outer molecular layer from 72 h after SE. These changes likely result from the death of hilar neurons and the reduction of the input from the entorhinal cortex. In 21-day-old rats that experience less SE-induced neuronal loss, increased immunoreactivity of synaptophysin was only found in piriform and entorhinal cortex while no changes occurred in GAP-43 expression.

CONCLUSION

These findings suggest that there is an age-related relation between the extent and rapidity of the process of neuronal death and the expression of these markers. Synaptophysin appears to be a more sensitive marker of plasticity induced by SE than GAP-43.

Role of Endogenous Neural Stem Cells in Neurological Disease and Brain Repair

These examples show that stem-cell-based therapy of neuro-psychiatric disorders will not follow a single scheme, but rather include widely different approaches. This is in accordance with the notion that the impact of stem cell biology on neurology will be fundamental, providing a shift in perspective, rather than introducing just one novel therapeutic tool. Stem cell biology, much like genomics and proteomics, offers a "view from within" with an emphasis on a theoretical or real potential and thereby the inherent openness, which is central to the concept of stem cells. Thus, stem cell biology influences many other, more traditional therapeutic approaches, rather than introducing one distinct novel form of therapy. Substantial advances have been made i n neural stemcell research during the years. With the identification of stem and progenitor cells in the adult brain and the complex interaction of different stem cell compartments in the CNS--both, under physiological and pathological conditions--new questions arise: What is the lineage relationship between t he different progenitor cells in the CNS and how much lineage plasticity exists? What are the signals controlling proliferation and differentiation of neural stem cells and can these be utilized to allow repair of the CNS? Insights in these questions will help to better understand the role of stem cells during development and aging and the possible relation of impaired or disrupted stem cell function and their impact on both the development and treatment of neurological disease. A number o f studies have indicated a limited neuronal and glial regeneration certain pathological conditions. These fundamental observations have already changed our view on understanding neurological disease and the brain's capacity for endogenous repair. The following years will have to show how we can influence andmodulate endogenous repair nisms by increasing the cellular plasticity in the young and aged CNS.

Anatomic and electrophysiologic evidence for a proconvulsive circuit in the dentate gyrus of reeler mutant mice, an animal model of diffuse cortical malformation

Although cortical malformations (CMs) are often associated with epilepsy, the underlying mechanisms are unknown. The reeler mouse is a model of CM with enhanced susceptibility to epileptiform activity, including the in vitro dentate gyrus, a region normally resistant to seizures. In this study, field potential recordings in hippocampal slices and the Timm stain were used to examine mossy fiber distribution in the dentate gyrus. In artificial cerebrospinal fluid containing bicuculline, 100% of reeler slices and 0% of control slices had spontaneous and antidromic evoked prolonged negative field potential shifts that were blocked by glutamate receptor antagonists. Sections from reeler mice, but not controls, exhibited a dark band of Timm's stain at the molecular layer/granule cell layer border. These data reveal that mossy fiber distribution is altered in reeler mice and coincides with the presence of an abnormal proconvulsive glutamatergic circuit.

Relationship of calpain-mediated proteolysis to the expression of axonal and synaptic plasticity markers following traumatic brain injury in mice

The role of neuronal plasticity and repair on the final functional outcome following traumatic brain injury (TBI) remains poorly understood. Moreover, the relationship of the magnitude of post-traumatic secondary injury and neurodegeneration to the potential for neuronal repair has not been explored. To address these questions, we employed Western immunoblotting techniques to examine how injury severity affects the spatial and temporal expression of markers of axonal growth (growth-associated protein GAP-43) and synaptogenesis (pre-synaptic vesicular protein synaptophysin) following either moderate (0.5 mm, 3.5 M/s) or severe (1.0 mm, 3.5 M/s) lateral controlled cortical impact traumatic brain injury (CCI-TBI) in young adult male CF-1 mice. Moderate CCI increased GAP-43 levels at 24 and 48 h post-insult in the ipsilateral hippocampus relative to sham, non-injured animals. This increase in axonal plasticity occurred prior to maximal hippocampal neurodegeneration, as revealed by de Olmos silver staining, at 72 h. However, moderate CCI-TBI did not elevate GAP-43 expression in the ipsilateral cortex where neurodegeneration was extensive by 6 h post-TBI. In contrast to moderate injury, severe CCI-TBI failed to increase hippocampal GAP-43 levels and instead resulted in depressed GAP-43 expression in the ipsilateral hippocampus and cortex at 48 h post-insult. In regards to injury-induced changes in synaptogenesis, we found that moderate CCI-TBI elevated synaptophysin levels in the ipsilateral hippocampus at 24, 48, 72 h and 21 days, but this effect was not present after severe injury. Together, these data highlights the adult brain's ability for axonal and synaptic plasticity following a focal cortical injury, but that severe injuries may diminish these endogenous repair mechanisms. The differential effects of moderate versus severe TBI on the post-traumatic plasticity response may be related to the calpain-mediated proteolytic activity occurring after a severe injury preventing increased expression of proteins required for plasticity. Supporting this hypothesis is the fact that GAP-43 is a substrate for calpain along with our data demonstrating that calpain-mediated degradation of the cytoskeletal protein, alpha-spectrin, is approximately 10 times greater in ipsilateral hippocampal tissue following severe compared to moderate CCI-TBI. Thus, TBI severity has a differential effect on the injury-induced neurorestorative response with calpain activation being one putative factor contributing to neuroregenerative failure following severe CCI-TBI. If true, then calpain inhibition may lead to both neuroprotective effects and an enhancement of neuronal plasticity/repair mechanisms post-TBI.

In vivo post-transcriptional regulation of GAP-43 mRNA by overexpression of the RNA-binding protein HuD

HuD is a neuronal-specific RNA-binding protein that binds to and stabilizes the mRNAs of growth-associated protein-43 (GAP-43) and other neuronal proteins. HuD expression increases during brain development, nerve regeneration, and learning and memory, suggesting that this protein is important for controlling gene expression during developmental and adult plasticity. To examine the function of HuD in vivo, we generated transgenic mice overexpressing human HuD under the control of the calcium-calmodulin-dependent protein kinase IIalpha promoter. The transgene was expressed at high levels throughout the forebrain, including the hippocampal formation, amygdala and cerebral cortex. Using quantitative in situ hybridization, we found that HuD overexpression led to selective increases in GAP-43 mRNA in hippocampal dentate granule cells and neurons in the lateral amygdala and layer V of the neorcortex. In contrast, GAP-43 pre-mRNA levels were unchanged or decreased in the same neuronal populations. Comparison of the levels of mature GAP-43 mRNA and pre-mRNA in the same neurons of transgenic mice suggested that HuD increased the stability of the transcript. Confirming this, mRNA decay assays revealed that the GAP-43 mRNA was more stable in brain extracts from HuD transgenic mice than non-transgenic littermates. In conclusion, our results demonstrate that HuD overexpression is sufficient to increase GAP-43 mRNA stability in vivo.

Do interictal spikes drive epileptogenesis?

Interictal spikes are periodic, very brief bursts of neuronal activity that are observed in the electroencephalogram of patients with chronic epilepsy. These spikes are useful diagnostically, but we do not know why they are so strongly associated with the spontaneous seizures that characterize chronic epilepsy. Interictal spikes appear before the first spontaneous seizures in animal models of acquired epilepsy, and spikes are sufficient to induce long-term changes in synaptic connections between neurons. Thus, spikes may guide the development of the neuronal circuits that initiate spontaneous seizures. If so, then attempts to prevent or cure epilepsy may best be directed at spikes rather than seizures.

Reciprocal changes of CD44 and GAP-43 expression in the dentate gyrus inner molecular layer after status epilepticus in mice

Mossy fiber sprouting (MFS), a common feature of human temporal lobe epilepsy and many epilepsy animal models, contributes to hippocampal hyperexcitability. The molecular events responsible for MFS are not well understood, although the growth-associated protein GAP-43 has been implicated in rats. Here, we focus on the hyaluronan receptor CD44, which is involved in routing of retinal axons during development and is upregulated after injury in many tissues including brain. After pilocarpine-induced status epilepticus (SE) in mice most hilar neurons died and neuropeptide Y (NPY) immunoreactivity appeared in the dentate inner molecular layer (IML) after 10-31 days indicative of MFS. Strong CD44 immunoreactivity appeared in the IML 3 days after pilocarpine, then declined over the next 4 weeks. Conversely, GAP-43 immunoreactivity was decreased in the IML at 3-10 days after pilocarpine-induced SE. After SE induced by repeated kainate injections, mice did not show any hilar cell loss or changes in CD44 or GAP-43 expression in the IML, and MFS was absent at 20-35 days. Thus, after SE in mice, early loss of GAP-43 and strong CD44 induction in the IML correlated with hilar cell loss and subsequent MFS. CD44 is one of the earliest proteins upregulated in the IML and coincides with early sprouting of mossy fibers, although its function is still unknown. We hypothesize that CD44 is involved in the response to axon terminal degeneration and/or neuronal reorganization preceding MFS.

Exacerbated status epilepticus and acute cell loss, but no changes in epileptogenesis, in mice with increased brain-derived neurotrophic factor signaling

Several studies suggest that brain-derived neurotrophic factor (BDNF) can exacerbate seizure development during status epilepticus (S.E.) and subsequent epileptogenesis in the adult brain. On the other hand, evidence exists for the protective effect of BDNF. To study this controversy, we induced S.E. with kainate in transgenic mice with increased BDNF signaling due to trkB overexpression. Transgenic mice experienced a more severe S.E. than wild type animals did. Furthermore, they had increased acute hippocampal neuronal loss when assessed at 48 h after S.E. The effect of trkB overexpression on the development of epilepsy, chronic neuronal death, mossy fiber sprouting, and neurogenesis were studied at 4.5 months after kainate-induced S.E. No differences were found in the rate of epileptogenesis, severity of epilepsy, or cellular markers of network reorganization between transgenic and wild type mice. No differences between genotypes were observed in TUC-4 staining, indicating no effect of trkB overexpression to immature neuron numbers. Instead, in Cresyl Violet-stained preparations, the highest density of neurons was found in untreated transgenic mice suggesting a favorable effect of trkB overexpression on the survival of neurons in the hippocampus. Our data support the role of BDNF and trkB signaling in seizure generation and acute cellular damage after S.E. Long-term outcome was not, however, exacerbated by trkB overexpression.

Recurrent excitation in the dentate gyrus of a murine model of temporal lobe epilepsy

Similar to rats, systemic pilocarpine injection causes status epilepticus (SE) and the eventual development of spontaneous seizures and mossy fiber sprouting in C57BL/6 and CD1 mice, but the physiological correlates of these events have not been identified in mice. Population responses in granule cells of the dentate gyrus were examined in transverse slices of the ventral hippocampus from pilocarpine-treated and untreated mice. In Mg(2+)-free bathing medium containing bicuculline, conditions designed to increase excitability in the slices, electrical stimulation of the hilus resulted in a single population spike in granule cells from control mice and pilocarpine-treated mice that did not experience SE. In SE survivors, similar stimulation resulted in a population spike followed, at a variable latency, by negative DC shifts and repetitive afterdischarges of 3-60 s duration, which were blocked by ionotropic glutamate receptor antagonists. Focal glutamate photostimulation of the granule cell layer at sites distant from the recording pipette resulted in population responses of 1-30 s duration in slices from SE survivors but not other groups. These data support the hypothesis that SE-induced mossy fiber sprouting and synaptic reorganization are relevant characteristics of seizure development in these murine strains, resembling rat models of human temporal lobe epilepsy.

Do seizures cause irreversible cognitive damage? Evidence from animal studies

Data from experimental models provide evidence that both prolonged and brief seizures can cause irreversible impairment in spatial and emotional learning and memory. Factors related to the severity of the behavioral impairments include genetic background, age at the time of the epileptogenic insult, extent of brain lesion, location of seizure focus, seizure duration, seizure number, brain reserve, and environmental and social living conditions. Further, as in humans, the interval between the last seizure and behavioral testing as well as treatment with antiepileptic drugs can affect the test results.

Plasticity following Injury to the Adult Central Nervous System: Is Recapitulation of a Developmental State Worth Promoting?

The adult central nervous system (CNS) appears to initiate a transient increase in plasticity following injury, including increases in growth-related proteins and generation of new cells. Recent evidence is reviewed that the injured adult CNS exhibits events and patterns of gene expression that are also observed during development and during regeneration following damage to the mature peripheral nervous system (PNS). The growth of neurons during development or regeneration is correlated, in part, with a coordinated expression of growth-related proteins, such as growth-associated-protein-43 (GAP-43), microtubule-associated-protein-1B (MAP1B), and polysialylated-neural-cell-adhesion-molecule (PSA-NCAM). For each of these proteins, evidence is discussed regarding its specific role in neuronal development, signals that modify its expression, and reappearance following injury. The rate of adult hippocampal neurogenesis is also affected by numerous endogenous and exogenous factors including injury. The continuing study of developmental neurobiology will likely provide further gene and protein targets for increasing plasticity and regeneration in the mature adult CNS.

Genetic basis of kainate-induced excitotoxicity in mice: phenotypic modulation of seizure-induced cell death

Excitotoxicity, a process in which excessive excitation of glutamate receptors results in cell death, has been implicated in a number of neurological disorders. However, the genetic characteristics and molecular mechanisms that can modulate the extent of cell death are unclear. Previously, we had reported that the extent of excitotoxic cell death is conferred by differences in the genetic background of several mouse strains. As a first step in the identification of loci that can modulate the extent of excitotoxin-induced cell death, we tested C57BL/6 and FVB/N mice, their F1 hybrids and backcross progeny for differences in apparent excitotoxic cell death induced by kainic acid (KA). While no strain dependent differences in seizure duration were observed, phenotypic analysis of cell death indicated that C57BL/6 mice showed no seizure-induced cell death, while FVB/N mice exhibited extensive cell death. Studies of seizure-induced cell death in hybrid and backcross progeny revealed an association between seizure-induced cell death and genotype. Mice from the F1 cross exhibited little to no seizure-induced cell death, indicative that the extent of cell death is conferred as a dominant genetic trait. Phenotypic assessment of cell death in backcross progeny suggests that differences in apparent cell death are conferred by a single gene locus. These findings implicate genetic factors in individual differences in excitotoxin-induced cell death.

Protection provided by cyclosporin A against excitotoxic neuronal death is genotype dependent

PURPOSE

Previous studies have shown that the immunosuppressant cyclosporin A (CsA), a specific blocker of the mitochondrial permeability transition (MPT) pore, can dramatically ameliorate the selective neuronal necrosis resulting from ischemia-reperfusion, traumatic brain injury, and N-methyl-d-aspartate (NMDA)-evoked neurotoxicity. The purpose of this study was to determine whether two different immunosuppressants, CsA and FK-506, could ameliorate the neuronal damage observed after kainate-induced seizures in strains that are differentially susceptible to excitotoxin-induced cell death.

METHODS

Excitotoxin-resistant (C57BL/6) or -susceptible (FVB/N) mice were administered kainate alone (30 mg/kg), CsA alone (5, 10, or 20 mg/kg), or one of the immunosuppressants (CsA, 5 mg/kg or 10 mg/kg; FK-506, 0.5 mg/kg) followed by kainate. After drug administration, mice were monitored continuously for the onset and extent of seizure activity. After a survival of 7 days, animals were assessed for hippocampal damage.

RESULTS

Whereas CsA alone induced no epileptogenic effects and both immunosuppressants were without effect on the induction of kainate-induced seizures, administration of CsA to excitotoxin-susceptible mice (FVB/N) virtually eliminated neuronal cell death. In contrast, induction of neuronal cell death was evident when CsA was administered to excitotoxin-resistant mice (C57BL/6). Administration of FK-506, another commonly used immunosuppressant, which lacks an effect on the MPT, had no effect on modification of susceptibility to kainate-induced cell death in either strain.

CONCLUSIONS

As our data show differential protection of hippocampal neurons against excitotoxic cell death by pretreatment with CsA, these results suggest that strain-dependent differences in mitochondrial integrity and function may exist.

GAP-43 mRNA and protein expression in the hippocampal and parahippocampal region during the course of epileptogenesis in rats

In order to reveal axonal rewiring in the hippocampal and parahippocampal regions after status epilepticus, we investigated the temporal evolution of growth-associated protein-43 (GAP-43) mRNA and protein expression in two rat models of mesial temporal lobe epilepsy (MTLE). Status epilepticus (SE) was induced by electrical stimulation of the angular bundle or by intraperitoneal kainic acid (KA) injections. Despite increased GAP-43 mRNA expression in dentate granule cells at 24 h after SE, GAP-43 protein expression in the inner molecular layer (IML) of the dentate gyrus decreased progressively after 24 h after SE in both models. Nevertheless robust mossy fiber sprouting (MFS) was evident in the IML of chronic epileptic rats. Remaining GAP-43 protein expression in the IML in chronic epileptic rats did not correlate with the extent of MFS, but with the number of surviving hilar neurons. In the parahippocampal region, GAP-43 mRNA expression was decreased in layer III of the medial entorhinal area (MEAIII) in parallel with extensive neuronal loss in this layer. There was a tendency of GAP-43 mRNA up-regulation in the presubiculum, a region that projects to MEAIII. With regard to this parahippocampal region, however, changes in GAP-43 mRNA expression were not followed by protein changes. The presence of the presynaptic protein GAP-43 in a neurodegenerated MEAIII indicates that fibers still project to this layer. Whether reorganization of fibers has occurred in this region after SE needs to be investigated with tools other than GAP-43.

Differences in ionotropic glutamate receptor subunit expression are not responsible for strain-dependent susceptibility to excitotoxin-induced injury

Systemic administration of kainic acid in C57BL/6 and FVB/N mice induces a comparable level of seizure induction yet results in differential susceptibility to seizure-induced cell death. While kainate administration causes severe hippocampal damage in mice of the FVB/N strain, C57BL/6 mice display no demonstrable cell loss or damage. At present, while the cellular mechanisms underlying strain-dependent differences in susceptibility remain unclear, some of this variation is assumed to have a genetic basis. As glutamate receptors are thought to participate in seizure induction and the subsequent neuronal degeneration that ensues, previous studies have proposed that variation in the precise subunit composition of glutamate receptors may result in differential susceptibility to excitotoxic cell death. Thus, we chose to examine the relationship between the cellular distribution and expression of glutamate receptor subunit proteins and cell loss within the hippocampus in mouse strains resistant and susceptible to kainate-induced excitotoxicity. Using semi-quantitative Western blot techniques and immunohistochemistry with the use of antibodies that recognize subunits of the KA (GluR5,6,7), AMPA (GluR1, GluR2, and GluR4), and NMDA (NMDAR1 and NMDAR2A/2B) receptors, we found no significant strain-dependent differences in the expression or distribution of these glutamate receptor subunits in the intact hippocampus. Following kainate administration, expression changes in ionotropic glutamate receptor subunits paralleled the development of susceptibility to cell death in the FVB/N strain only. Strain differences in hippocampal vulnerability to kainate-induced status epilepticus are not due to glutamate receptor protein expression.

Mouse strain differences in kainic acid sensitivity, seizure behavior, mortality, and hippocampal pathology

Genetic influences contribute to susceptibility to seizures and to excitotoxic injury, but it is unclear if/how these susceptibilities are linked. This study assessed the impact of genetic background on mouse strain seizure susceptibility, seizure phenotype, mortality, and hippocampal histopathology. A subcutaneous (s.c.) kainic acid multiple injection protocol was developed. Five mouse strains were tested: a and b) C57BL/6J and 129/SvJ, strains commonly used in gene targeting experiments; c) C3HeB/FeJ, a strain with reported sensitivity to the convulsant effects of kainic acid (KA); d) 129/SvEms, a strain reportedly susceptible to hippocampal excitotoxic cell death; and e) a mixed genetic background strain (129/SvJXC57BL/6J) from which targeted gene deletion experiments have been carried out. Histopathological features were examined at early (7-10 day), delayed (2-4 month), and late (6-13 month) time points.Mouse background strains can be genetically segregated based on excitotoxin sensitivity, seizure phenotype, mortality, and hippocampal histopathology. When injected with KA, C3HeB/FeJ and C57BL/6J strains were resistant to cell death and synaptic reorganization despite severe behavioral seizures, while 129/SvEms mice developed marked pyramidal cell loss and mossy fiber sprouting despite limited seizure activity. The mixed background 129/SvJXC57BL/6J group exhibited features of both parental strains. In the mouse strains tested, the duration or severity of seizure activity was not predictive of subsequent hippocampal pyramidal cell death and/or synaptic reorganization. Unlike rats, mice exhibiting prolonged high-grade KA-induced seizure activity did not develop subsequent spontaneous behavioral seizures.

Reassessment of the effects of cycloheximide on mossy fiber sprouting and epileptogenesis in the pilocarpine model of temporal lobe epilepsy

A feature of animal models of temporal lobe epilepsy and the human disorder is hippocampal sclerosis and Timm stain in the inner molecular layer (IML) of the dentate gyrus, which represents synaptic reorganization and may be important in epileptogenesis. We reassessed the hypothesis that pre-treatment with cycloheximide (CHX) prevents Timm staining in the IML following pilocarpine (PILO)-induced status epilepticus (a multifocal model of temporal lobe epilepsy), but allows epileptogenesis (i.e., chronic spontaneous seizures) after a latent period. Hippocampal slices from PILO-treated rats without Timm stain in the IML after CHX treatment were hypothesized to lack the electrophysiological abnormalities suggestive of recurrent excitation. The primary experimental groups were as follows: 1) CHX (1 mg/kg) 30-45 min prior to administration of PILO (320 mg/kg ip, 2) only PILO, and 3) only saline (0.5 ml, IP). The CHX pre-treatment significantly decreased the number of rats that responded to PILO with status epilepticus compared to rats that received only PILO. Pre-treatment with CHX did not significantly alter the spontaneous motor seizure rate post-treatment compared to treatment with PILO alone in those animals from each group that developed status epilepticus during PILO treatment. Timm stain in the IML was not significantly different between the PILO- and PILO+CHX-treated rats. Using quantitative methods, CHX did not prevent hilar, CA1, or CA3 neuronal loss compared to the PILO-treated rats. Extracellular responses to hilar stimulation in 30 microM bicuculline and 6 mM [K(+)](o) demonstrated all-or-none bursting in both the CHX+PILO- and PILO-treated rats but not in control rats. Whole cell recordings from granule cells, using glutamate flash photolysis to activate other granule cells, showed that both the CHX+PILO- and PILO-treated rats had excitatory synaptic interactions in the granule cell layer, which were not found after saline treatment. Some rats responded to PILO (with or without CHX pre-treatment) with only one or a few seizures at treatment, and some of these animals (n = 4) demonstrated spontaneous motor seizures within 2 mo after treatment. Timm staining and neuron loss in this group were not clearly different from saline-treated rats. These results suggest that in the PILO model, pre-treatment with CHX does not affect mossy fiber sprouting in the IML of epileptic rats and does not prevent the formation of recurrent excitatory circuits. However, the develoment of spontaneous motor seizures, in a small number of rats, could occur without detectable hippocampal neuron loss or mossy fiber sprouting, as assessed by the Timm stain method.

Complications associated with genetic background effects in models of experimental epilepsy

To elucidate the genetic influences contributing to susceptibility to seizure disorders, researchers have long used selected lines and inbred strains of rodents. In recent years, the use of genetically altered mice as models of complex human disease has revolutionized biomedical research into the genetics of disease pathogenesis and potential therapeutic interventions. In particular, the study of transgenic and gene-deleted (knockout) mice can provide important insights into the in vivo function and interaction of specific gene products. While a variety of inbred mouse mutations have been used to directly evaluate the genetic basis of seizure disorders, data obtained from such genetically altered mice must be interpreted carefully. An increasing number of scientific articles have reported that the phenotype of a given single gene mutation in mice can be modulated by the genetic background of the inbred strain in which the mutation is maintained. This effect is attributable to so-called modifier genes, which act in combination with the causative gene. In this review, the author points out the importance of considering the genetic background of the strain used to create these animal models, the potential problems with interpretation of phenotype, and solutions to selecting an appropriate mouse model of experimental epilepsy. Despite these potential limitations, knockout mice provide a powerful tool for understanding the genetic and neurobiological mechanisms contributing to experimental epilepsy.

Neural (N-) cadherin, a synaptic adhesion molecule, is induced in hippocampal mossy fiber axonal sprouts by seizure

Aberrant mossy fiber sprouting and synaptic reorganization are plastic responses in human temporal lobe epilepsy, and in pilocarpine-induced epilepsy in rodents. Although the morphological features of the hippocampal epileptic reaction have been well documented, the molecular mechanisms underlying these structural changes are not understood. The classic cadherins, calcium-dependent cell adhesion molecules, are known to function in development in neurite outgrowth, synapse formation, and stabilization. In pilocarpine-induced status epilepticus, the expression of N-cadherin mRNA was sharply upregulated and reached a maximum level (1- to 2.5-fold) at 1- to 4 weeks postseizure in the granule cell layer and the pyramidal cell layer of CA3. N-cadherin protein was correspondingly increased and became concentrated in the inner molecular layer of the dentate gyrus, consistent with the position of mossy fiber axonal sprouts. Moreover, N-cadherin labeling was punctate; colocalized with definitive synaptic markers, and partially localized on polysialated forms of neural cell adhesion molecule (PSA-NCAM)-positive dendrites of granule cells in the inner molecular layer. Our findings show that N-cadherin is likely to be a key factor in responsive synaptogenesis following status epilepticus, where it functions as a mediator of de novo synapse formation.

Pilocarpine-induced status epilepticus results in mossy fiber sprouting and spontaneous seizures in C57BL/6 and CD-1 mice

Several rodent models are available to study the cellular mechanisms associated with the development of temporal lobe epilepsy (TLE), but few have been successfully transferred to inbred mouse strains commonly used in genetic mutation studies. We examined spontaneous seizure development and correlative axon sprouting in the dentate gyrus of CD-1 and C57BL/6 mice after systemic injection of pilocarpine. Pilocarpine induced seizures and status epilepticus (SE) after systemic injection in both strains, although SE onset latency was greater for C57BL/6 mice. There were also animals of both strains which did not experience SE after pilocarpine treatment. After a period of normal behavior for several days after the pilocarpine treatment, spontaneous tonic-clonic seizures were observed in most CD-1 mice and all C57BL/6 that survived pilocarpine-induced SE. Robust mossy fiber sprouting into the inner molecular layer was observed after 4-8 weeks in mice from both strains which had experienced SE, and cell loss was apparent in the hippocampus. Mossy fiber sprouting and spontaneous seizures were not observed in mice that did not experience a period of SE. These results indicate that pilocarpine induces spontaneous seizures and mossy fiber sprouting in both CD-1 and C57BL/6 mouse strains. Unlike systemic kainic acid treatment, the pilocarpine model offers a potentially useful tool for studying TLE development in genetically modified mice raised on the C57BL/6 background.

Dopamine D2 receptor signaling controls neuronal cell death induced by muscarinic and glutamatergic drugs

Dopamine (DA), through D1/D2 receptor-mediated signaling, plays a major role in the control of epileptic seizures arising in the limbic system. Excitotoxicity leading to neuronal cell death in the affected areas is a major consequence of seizures at the cellular level. In this respect, little is known about the role of DA receptors in the occurrence of epilepsy-induced neuronal cell death. Here we analyze the occurrence of seizures and neurotoxicity in D2R -/- mice treated with the cholinergic agonist pilocarpine. We compared these results with those previously obtained with kainic acid (KA), a potent glutamate agonist. Importantly, D2R -/- mice develop seizures at doses of both drugs that are not epileptogenic for WT littermates and show greater neurotoxicity. However, pilocarpine-induced seizures result in a more widespread neuronal death in both WT and D2R -/- brains in comparison to KA. Thus, the absence of D2R lowers the threshold for seizures induced by both glutamate and acetylcholine. Moreover, the dopaminergic control of epilepsy-induced neurodegeneration seems to be mediated by distinct interactions of D2R signaling with these two neurotransmitters.

Decreased BDNF signalling in transgenic mice reduces epileptogenesis

Brain derived neurotrophic factor (BDNF) has been suggested to be involved in epileptogenesis. Both pro- and antiepileptogenic effects have been reported, but the exact physiological role is still unclear. Here, we investigated the role of endogenous BDNF in epileptogenesis by using transgenic mice overexpressing truncated trkB, a dominant negative receptor of BDNF. After induction of status epilepticus (SE) by kainic acid, the development of spontaneous seizures was monitored by video-EEG system. Hilar cell loss, and the number of neuropeptide Y immunoreactive cells were studied as markers of cellular damage, and mossy fibre sprouting was investigated as a plasticity marker. Our results show that transgenic mice had significantly less frequent interictal spiking than wild-type mice, and the frequency of spontaneous seizures was lower. Furthermore, compared to wild-type animals, transgenic mice had less severe seizures with later onset and mortality was lower. In contrast, no differences between genotypes were observed in any of the cellular or plasticity markers. Our results suggest that transgenic mice with decreased BDNF signalling have reduced epileptogenesis.

Is neuronal death required for seizure-induced epileptogenesis in the immature brain?

Do seizures cause neuronal death? At least in the immature hippocampus, this may not be the critical question for determining the mechanisms of epileptogenesis. Neuronal injury and death have clearly been shown to occur in most epilepsy models in the mature brain, and are widely considered a prerequisite to seizure-induced epilepsy. In contrast, little neuronal death occurs after even a severe and prolonged seizure prior to the third postnatal week. However, seizures early in life, for example prolonged experimental febrile seizures, can profoundly and permanently change the hippocampal circuit in a pro-epileptogenic direction. These seizure-induced alterations of limbic excitability may require transient structural injury, but are mainly due to functional changes in expression of gene coding for specific receptors and channels, leading to altered functional properties of hippocampal neurons. Thus, in some pro-epileptogenic models in the developing brain, neither the death of neurons nor death-induced abnormalities of surviving neurons may underlie the formation of an epileptic circuit. Rather, findings in the experimental prolonged febrile seizure model suggest that persistent functional alterations of gene expression ('neuroplasticity') in diverse hippocampal neuronal populations may promote pro-epileptogenic processes induced by these seizures. These findings also suggest that during development, relatively short, intense bursts of neuronal activity may disrupt 'normal' programmed maturational processes to result in permanent, selective alterations of gene expression, with profound functional consequences. Therefore, determining the cascade of changes in the programmed expression of pertinent genes, including their temporal and cell-specific spatial profiles, may provide important information for understanding the process of transformation of an evolving, maturing hippocampal network into one which is hyperexcitable.

Seizure-induced neuronal death is associated with induction of c-Jun N-terminal kinase and is dependent on genetic background

Previous studies have shown that expression of c-Jun protein, as well as the c-Jun amino-terminal kinase (JNK) group of mitogen-activated protein kinases, may play a critical role in the pathogenesis of glutamate neurotoxicity. In order to define the molecular cascade that leads to c-Jun activation following excitotoxic injury and delineate whether induction of protein synthesis is related to cell death signaling cascades or those changes associated with increased seizure activity, we examined the expression of JNK-1, as well as its substrate, c-Jun and N-terminal phosphorylated c-Jun following kainic acid (KA) administration in two strains of mice. In the present study, we assessed the immunohistochemical expression of these proteins at time points between 2 h and 7 days, in excitotoxic cell death-resistant (C57BL/6) and -susceptible (FVB/N) mouse strains that were systemically injected with saline or kainic acid. No strain-related differences in the immunohistochemical expression of any of the proteins were observed in intact control mice. However, following KA administration, the magnitude and period of induction of JNK-1 protein was associated with impending cell death, while increased phosphorylation of c-Jun protein was associated with resistance to cell death. In contrast, expression of c-Jun protein does not appear to be a reliable indicator of impending cell death, as it was expressed in resistant and vulnerable subfields in mice susceptible to kainate injury. These results provide the first evidence that JNK-1 expression may be involved in producing the neuronal cell death response following excitotoxin-induced injury.

Differential regulation of primary protein kinase C substrate (MARCKS, MLP, GAP-43, RC3) mRNAs in the hippocampus during kainic acid-induced seizures and synaptic reorganization

In the mature hippocampus, kainic acid seizures lead to excitotoxic cell death and synaptic reorganization in which granule cell axons (mossy fibers) form ectopic synapses on granule cell dendrites. In the present study, we examined the expression of four major, developmentally regulated protein kinase C (PKC) substrates (MARCKS, MLP, GAP-43, RC3), which have different subcellular and regional localizations in the hippocampus at several time points (6 hr, 12 hr, 18 hr, 24 hr, 48 hr, 5 days, or 15 days) following kainic acid seizures using in situ hybridization. Consistent with previous reports, following kainate seizures, GAP-43 mRNA expression exhibited a delayed and protracted elevation in the granule cell layer, which peaked at 24 hr, whereas expression in fields CA1 and CA3 remained relatively unchanged. Conversely, RC3 mRNA expression exhibited a delayed reduction in the granule cell layer that was maximal at 18 hr, as well as a reduction CA1 at 48 hr, whereas CA3 levels did not change. MARCKS mRNA expression in the granule cell layer and CA1 remained stable following kainate, although an elevation was observed in subfield CA3c at 12 hr. Similarly, MLP mRNA expression did not change in the granule cell layer or CA1 following kainate but exhibited a protracted elevation in subfields CA3b,c beginning at 6 hr post-kainate. Collectively these data demonstrate that different PKC substrate mRNAs exhibit unique expression profiles and regulation in the different cell fields of the mature hippocampus following kainic acid seizures and during subsequent synaptic reorganization. The expression profiles following kainate seizures bear resemblance to those observed during postnatal hippocampal development, which may indicate the recruitment of common regulatory mechanisms.