Kainic acid induction of mossy fiber sprouting: dependence on mouse strain

Loss of efferent projections of the hippocampal formation in the mouse intrahippocampal kainic acid model

Unilateral intrahippocampal injection of kainic acid is used as a model of medial temporal lobe epilepsy and provides a platform to study the mechanisms of epilepsy. Here, we used an AAV-9 EYFP-tagged viral vector as an anterograde tracer, injected into the dorsal and ventral hippocampus after kainic acid injection, to map out the efferent hippocampal projections after the development of spontaneous seizures in this model. The purpose of the study was to identify the extent of changes in hippocampal efferent system in several brain regions that receive significant inputs from the hippocampus. Loss of efferent hippocampal fibers was greatest in the retrosplenial cortex where neuronal loss was also observed. Loss of fibers was also observed in the fornix without any specific effect in the lateral mammillary nuclei. Although expected, these observations provide further evidence of the broader network effects as a result of hippocampal cell loss.

Investigating Developmental and Epileptic Encephalopathy Using Drosophila melanogaster

Developmental and epileptic encephalopathies (DEEs) are the spectrum of severe epilepsies characterized by early-onset, refractory seizures occurring in the context of developmental regression or plateauing. Early infantile epileptic encephalopathy (EIEE) is one of the earliest forms of DEE, manifesting as frequent epileptic spasms and characteristic electroencephalogram findings in early infancy. In recent years, next-generation sequencing approaches have identified a number of monogenic determinants underlying DEE. In the case of EIEE, 85 genes have been registered in Online Mendelian Inheritance in Man as causative genes. Model organisms are indispensable tools for understanding the in vivo roles of the newly identified causative genes. In this review, we first present an overview of epilepsy and its genetic etiology, especially focusing on EIEE and then briefly summarize epilepsy research using animal and patient-derived induced pluripotent stem cell (iPSC) models. The Drosophila model, which is characterized by easy gene manipulation, a short generation time, low cost and fewer ethical restrictions when designing experiments, is optimal for understanding the genetics of DEE. We therefore highlight studies with Drosophila models for EIEE and discuss the future development of their practical use.

Dynamic interaction of local and transhemispheric networks is necessary for progressive intensification of hippocampal seizures

The detailed mechanisms of progressive intensification of seizures often occurring in epilepsy are not well understood. Animal models of kindling, with progressive intensification of stimulation-induced seizures, have been previously used to investigate alterations in neuronal networks, but has been obscured by limited recording capabilities during electrical stimulations. Remote networks in kindling have been studied by physical deletions of the connected structures or pathways, inevitably leading to structural reorganisations and related adverse effects. We used optogenetics to circumvent the above-mentioned problems inherent to electrical kindling, and chemogenetics to temporarily inhibit rather than ablate the remote interconnected networks. Progressively intensifying afterdischarges (ADs) were induced by repetitive photoactivation of principal neurons in the hippocampus of anaesthetized transgenic mice expressing ChR2. This allowed, during the stimulation, to reveal dynamic increases in local field potentials (LFPs), which coincided with the start of AD intensification. Furthermore, chemogenetic functional inhibition of contralateral hippocampal neurons via hM4D(Gi) receptors abrogated AD progression. These findings demonstrate that, during repeated activation, local circuits undergo acute plastic changes with appearance of additional network discharges (LFPs), leading to transhemispheric recruitment of contralateral dentate gyrus, which seems to be necessary for progressive intensification of ADs.

Animal models of temporal lobe epilepsy following systemic chemoconvulsant administration

In order to understand the pathophysiology of temporal lobe epilepsy (TLE), and thus to develop new pharmacological treatments, in vivo animal models that present features similar to those seen in TLE patients have been developed during the last four decades. Some of these models are based on the systemic administration of chemoconvulsants to induce an initial precipitating injury (status epilepticus) that is followed by the appearance of recurrent seizures originating from limbic structures. In this paper we will review two chemically-induced TLE models, namely the kainic acid and pilocarpine models, which have been widely employed in basic epilepsy research. Specifically, we will take into consideration their behavioral, electroencephalographic and neuropathologic features. We will also evaluate the response of these models to anti-epileptic drugs and the impact they might have in developing new treatments for TLE.

Animal models in epilepsy research: legacies and new directions

Human epilepsies encompass a wide variety of clinical, behavioral and electrical manifestations. Correspondingly, studies of this disease in nonhuman animals have brought forward an equally wide array of animal models; that is, species and acute or chronic seizure induction protocols. Epilepsy research has a long history of comparative anatomical and physiological studies on a range of mostly mammalian species. Nonetheless, a relatively limited number of rodent models have emerged as the primary choices for most investigations. In many cases, these animal models are selected on the basis of convenience or tradition, although technical or experimental rationale does, and should, factor into these decisions. More complex mammalian brains and genetic model organisms including zebrafish have been studied less, but offer substantial advantages that are becoming widely recognized.

Optogenetic inhibition of chemically induced hypersynchronized bursting in mice

Synchronized activity is common during various physiological operations but can culminate in seizures and consequently in epilepsy in pathological hyperexcitable conditions in the brain. Many types of seizures are not possible to control and impose significant disability for patients with epilepsy. Such intractable epilepsy cases are often associated with degeneration of inhibitory interneurons in the cortical areas resulting in impaired inhibitory drive onto the principal neurons. Recently emerging optogenetic technique has been proposed as an alternative approach to control such seizures but whether it may be effective in situations where inhibitory processes in the brain are compromised has not been addressed. Here we used pharmacological and optogenetic techniques to block inhibitory neurotransmission and induce epileptiform activity in vitro and in vivo. We demonstrate that NpHR-based optogenetic hyperpolarization and thereby inactivation of a principal neuronal population in the hippocampus is effectively attenuating seizure activity caused by disconnected network inhibition both in vitro and in vivo. Our data suggest that epileptiform activity in the hippocampus caused by impaired inhibition may be controlled by optogenetic silencing of principal neurons and potentially can be developed as an alternative treatment for epilepsy.

Kainic Acid-induced neurotoxicity: targeting glial responses and glia-derived cytokines

Glutamate excitotoxicity contributes to a variety of disorders in the central nervous system, which is triggered primarily by excessive Ca²⁺ influx arising from overstimulation of glutamate receptors, followed by disintegration of the endoplasmic reticulum (ER) membrane and ER stress, the generation and detoxification of reactive oxygen species as well as mitochondrial dysfunction, leading to neuronal apoptosis and necrosis. Kainic acid (KA), a potent agonist to the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate class of glutamate receptors, is 30-fold more potent in neuro-toxicity than glutamate. In rodents, KA injection resulted in recurrent seizures, behavioral changes and subsequent degeneration of selective populations of neurons in the brain, which has been widely used as a model to study the mechanisms of neurodegenerative pathways induced by excitatory neurotransmitter. Microglial activation and astrocytes proliferation are the other characteristics of KA-induced neurodegeneration. The cytokines and other inflammatory molecules secreted by activated glia cells can modify the outcome of disease progression. Thus, anti-oxidant and anti-inflammatory treatment could attenuate or prevent KA-induced neurodegeneration. In this review, we summarized updated experimental data with regard to the KA-induced neurotoxicity in the brain and emphasized glial responses and glia-oriented cytokines, tumor necrosis factor-α, interleukin (IL)-1, IL-12 and IL-18.

Receptor protein tyrosine phosphatase sigma regulates synapse structure, function and plasticity

The mechanisms that regulate synapse formation and maintenance are incompletely understood. In particular, relatively few inhibitors of synapse formation have been identified. Receptor protein tyrosine phosphatase σ (RPTPσ), a transmembrane tyrosine phosphatase, is widely expressed by neurons in developing and mature mammalian brain, and functions as a receptor for chondroitin sulfate proteoglycans that inhibits axon regeneration following injury. In this study, we address RPTPσ function in the mature brain. We demonstrate increased axon collateral branching in the hippocampus of RPTPσ null mice during normal aging or following chemically induced seizure, indicating that RPTPσ maintains neural circuitry by inhibiting axonal branching. Previous studies demonstrated a role for pre-synaptic RPTPσ promoting synaptic differentiation during development; however, subcellular fractionation revealed enrichment of RPTPσ in post-synaptic densities. We report that neurons lacking RPTPσ have an increased density of pre-synaptic varicosities in vitro and increased dendritic spine density and length in vivo. RPTPσ knockouts exhibit an increased frequency of miniature excitatory post-synaptic currents, and greater paired-pulse facilitation, consistent with increased synapse density but reduced synaptic efficiency. Furthermore, RPTPσ nulls exhibit reduced long-term potentiation and enhanced novel object recognition memory. We conclude that RPTPσ limits synapse number and regulates synapse structure and function in the mature CNS.

Epileptic tolerance is associated with enduring neuroprotection and uncoupling of the relationship between CA3 damage, neuropeptide Y rearrangement and spontaneous seizures following intra-amygdala kainic acid-induced status epilepticus in mice

Brief, non-harmful seizures can activate endogenous protective programmes which render the brain resistant to damage caused by prolonged seizure episodes. Whether protection in epileptic tolerance is long-lasting or influences the subsequent development of epilepsy is uncertain. Presently, we investigated the relationship between hippocampal pathology, neuropeptide Y rearrangement and spontaneous seizures in sham- and seizure-preconditioned mice after status epilepticus induced by intra-amygdala kainate. Seizure-induced neuronal death at 24 h was significantly reduced in the ipsilateral hippocampal CA3 and hilus of tolerance mice compared to sham-preconditioned animals subject to status epilepticus. Damage to the CA3-hilus remained reduced in tolerance mice 21 days post-status. In sham-preconditioned mice subject to status epilepticus correlative statistics showed there was a strong inverse relationship between CA3, but not hilar, neuron counts and the number of spontaneous seizures. A strong positive association was also found between neuropeptide Y score and spontaneous seizure count in these mice. In contrast, there was no significant association between spontaneous seizure count and CA3 neuron loss or neuropeptide Y rearrangement in the tolerance mice. These data show that tolerance-conferred neuroprotection is long-lasting and that tolerance disrupts the normal association between CA3 damage, synaptic rearrangement and occurrence of spontaneous seizures in this model.

Hippocampal damage after intra-amygdala kainic acid-induced status epilepticus and seizure preconditioning-mediated neuroprotection in SJL mice

Exposure of the brain to a stressful stimulus that is sub-threshold for permanent injury can temporarily protect against cell death during a subsequent and otherwise damaging insult. One or more brief, non-harmful seizure episode(s) (seizure preconditioning) can dramatically reduce hippocampal damage when given prior to status epilepticus (epileptic tolerance). We recently reported that status epilepticus-induced hippocampal damage in C57BL/6 mice could be reduced by approximately 50% when preceded 24h earlier by a brief, non-injurious generalized seizure induced by 15mg/kg systemic kainic acid (KA). Since other mouse strains might display different vulnerability to either seizure preconditioning or status epilepticus, we investigated whether epileptic tolerance could be acquired in another strain. SJL mice, reported to display greater seizure sensitivity to systemic KA, received intra-amygdala microinjection of KA to trigger status epilepticus. Intracerebral recordings confirmed evoked seizures involved the ipsilateral hippocampus. Status epilepticus produced hippocampal damage which mainly affected the ipsilateral CA3 and hilus; a pattern similar to C57BL/6 mice. The damage extended through the full rostro-caudal extent of the hippocampal formation. Seizure preconditioning using 20mg/kg systemic KA, but not 15mg/kg, significantly reduced hippocampal damage after status epilepticus by 37% in the dorsal hippocampus and by 65% in the ventral hippocampus. These studies suggest status epilepticus induced by intra-amygdala KA in SJL mice models aspects of the pathophysiology of human mesial temporal sclerosis. Moreover, seizure preconditioning effectively produces neuroprotection in SJL mice, further establishing epileptic tolerance as a conserved endogenous neuroprotection paradigm.

Overexpression of GAP-43 reveals unexpected properties of hippocampal mossy fibers

The mossy fiber (MF) system targets the apical dendrites of CA3 pyramidal cells in the stratum lucidum (SL). In mice overexpressing the growth-associated protein GAP-43 there is an apparent ectopic growth of these MFs into the stratum oriens (SO) targeting the basal dendrites of these same pyramidal cells (Aigner et al. (1995) Cell 83:269-278). This is the first evidence to our knowledge that links increased GAP-43 expression with growth of central axons. Here we studied the Aigner et al. transgenic mice but were unable to confirm such growth into SO. However, using quantitative methods we did observe enhanced growth within the regions normally targeted by MFs, for example, the SL in the CA3a region. These contrasting results led us to study MFs with double-immunostaining using an immunohistochemical marker for MFs, the zinc transporter, ZnT3, to visualize the colocalization of transgenic GAP-43 within MFs. Unexpectedly, using both fluorescence and confocal microscopy, we were unable to detect colocalization of GAP-43-positive axons with ZnT3-positive MF axons within the MF pathways, either in the region of the MF axons or in the SL, where MF terminals are abundant. In contrast, the plasma membrane-associated presynaptic marker SNAP-25 did colocalize with transgenic GAP-43-positive terminals in the SL. Synaptophysin, the vesicle-associated presynaptic terminal marker, colocalized with ZnT3 but did not appear to colocalize with GAP-43. The present findings raise important questions about the properties of granule cells and the MF mechanisms that differentially regulate axonal remodeling in the adult hippocampus: (1) Because there appears to be at least two populations of granule cells defined by their differential protein expression, this points to the existence of an intrinsic heterogeneity of granule cell expression beyond that contributed by adult neurogenesis; (2) Giventhe present evidence that growth is induced in mice overexpressing GAP-43 in adjacent non-GAP-43 containing MFs, the potential exists for a heretofore unexplored interaxonal communication mechanism.

Fcgamma receptors contribute to pyramidal cell death in the mouse hippocampus following local kainic acid injection

Recent studies have demonstrated the contribution of the gamma subunit of the Fc receptor of IgG (FcRgamma) to neuronal death following ischemic injury and Parkinson's disease. We examined the role of FcRgamma in hippocampal pyramidal cell death induced by kainic acid (KA). FcRgamma-deficient mice (FcRgamma-/-) and their FcRgamma+/+ littermates (wild type, B6) received an injection of KA into the dorsal hippocampus. Pyramidal cell death was quantified 24 and 72 h after the injection. The number of survived pyramidal cells was significantly larger in FcRgamma-/- mice than in B6 mice in both the CA1 and CA3. Immunohistochemical and immunofluorescent studies detected FcgammaRIIB protein in parvalbumin neurons, whereas FcgammaRIII and FcgammaRI proteins were detected in microglial cells. No activated microglial cells were detected 24 h after the KA injection in FcRgamma-/- mice, whereas many activated microglial cells were present in B6 mice. The production of nitrotyrosine as well as of the inducible nitric oxide synthase and cyclooxygenase-2 proteins, increased by 16 h after the KA injection in B6 mice. In addition, tissue plasminogen activator and metalloproteinase-2 proteins increased. By contrast, the magnitude of oxidative stress and the increase in protease expression were mild in FcRgamma-/- mice. Co-injection of a neutralizing antibody against FcgammaRll and FcgammaRlll with KA abolished pyramidal cell death and microglial activation. In addition, the neutralizing antibody reduced oxidative stress and expression of proteases. These observations suggested a role for FcgammaRllB in parvalbumin neurons as well as FcRgamma in microglia in pyramidal cell death.

Excitability changes within transverse lamellae of dentate granule cells and their longitudinal spread following orthodromic or antidromic activation

The functional organization of the perforant path input to the dentate gyrus of the exposed hippocampus was studied in adult rabbits anesthetized with urethane and chloralose. Electrical stimulation of perforant path fibers caused excitation of granule cells along narrow, nearly transverse strips (lamellae) of tissue. Stimulation of granule cell axons (mossy fibers) in CA3 caused antidromic activation of granule cells along similar strips. Paired-pulse stimulation revealed marked changes in granule cell excitability both within a lamella (on-line) and for several mm off-line along the septo-temporal axis of the dentate gyrus. After the first pulse, granule cells were inhibited for up to about 100 ms and then facilitated for up to hundreds of ms. Feedback activity along mossy fiber collaterals exciting local inhibitory and excitatory neurons appeared to dominate in producing on- and off-line inhibition and facilitation. Neurons mediating these effects could be inhibitory basket cells and other inhibitory interneurons targeting granule cells on- and off-line. In addition, excitatory mossy cells with far reaching, longitudinally running axons could affect off-line granule cells by exciting them directly or inhibit them indirectly by exciting local inhibitory interneurons. A scheme for dentate gyrus function is proposed whereby information to the dentate gyrus becomes split into interacting transverse strips of neuronal assemblies along which temporal processing occurs. A matrix of neuronal assemblies thus arises within which fragments of events and experiences is stored through the plasticity of synapses within and between the assemblies. Similar fragments may then be recognized at later times allowing memories of the whole to be created by pattern completion at subsequent computational stages in the hippocampus.

Temporal Profile of Clinical Signs and Histopathologic Changes in an F-344 Rat Model of Kainic Acid–induced Mesial Temporal Lobe Epilepsy

Since there is limited information in the literature, the purpose of this study was to investigate clinical signs, morphology, and temporal progression of lesions from Days 3 to 168 in a kainic acid (KA)-induced Fischer-344 (F-344) rat model of mesial temporal lobe epilepsy (MTLE). Following a single KA subcutaneous dose of 9 mg/kg to young adult male rats, 95% survived, 93% exhibited status epilepticus, and 80% eventually developed spontaneous motor seizures. Histopathology included hematoxylin and eosin (H&E), autofluorescence, Fluoro-Jade B, Timm’s, ED-1/CD68, GFAP, doublecortin, and Ki-67. Neuronal degeneration occurred on Day 3 in the hippocampal CA1, CA3, and dentate hilar regions; amyg-daloid and thalamic nuclei; and frontoparietotemporal, entorhinal and piriform cortices. Degeneration severity peaked on Day 6 and decreased progressively until Day 168. Aberrant mossy fiber (MF) sprouting was present in the inner molecular layer of dentate gyrus on Days 6–168. Microliosis and astrogliosis peaked on Day 28 and generally colocalized with the distribution of neuronal degeneration. Important correlates to human MTLE included induction of spontaneous seizures, more severe neuronal damage of CA1 than CA3 (in contrast to other animal models but similar to humans), hilar neuronal loss, activated microgliosis and astrogliosis, aberrant MF sprouting, and dentate granule cell neurogenesis. Aberrant MF sprouting prior to spontaneous motor seizures and reduced seizure frequency with a decrease in aberrant MF sprouting support the hypothesis that MF sprouts are necessary for spontaneous seizure generation and maintenance.

Genetic susceptibility to traumatic brain injury and apolipoprotein E gene

Traumatic brain injury (TBI) is defined as an injury caused by a blow or jolt to the head or a penetrating head injury that disrupts the normal function of the brain. It is a common emergency and severe case in neurosurgery field. Nowadays, there are more and more evidences showing that TBI, which is apparently similar in pathology and severity in the acute stage, may have different outcomes. The known prognostic factors (such as age, severity of injury and treatments, etc.) explain only part of this variability and the concept of genetic susceptibility of traumatic brain injury has already been accepted by more and more people. It is now demonstrated that genetic polymorphism may play a key role in the susceptibility to TBI, even outcome following TBI. Although there are many genes that may involved in pathophysiological processes influencing TBI, apolipoprotein E gene has become one of the most extensive studied genes in neurotrauma and neurodegenerative disease and seems to take an important part in the neural responses to TBI. In this article, we will review the current understanding of the genetic susceptibility of TBI and the advancements regarding the impact of apolipoprotein E genotype on the severity and/or outcome following TBI.

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.

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.

Learning-induced axonal remodeling: Evolutionary divergence and conservation of two components of the mossy fiber system within Rodentia

Damage to the hippocampal formation results in profound impairments in spatial navigation in rats and mice leading to the widely accepted assumption that the hippocampal cellular and molecular memory mechanisms of both genera are conserved. Recently our group has shown in two rat strains that hippocampal-dependent training in the water maze specifically induces robust 'sprouting' of granule cell suprapyramidal mossy fiber axon terminal fields. Here we sought to investigate whether the pronounced remodeling of adult hippocampal circuitry observed in the rat is also present in the mouse motivated by the thought that subsequent studies using genetically-engineered mice could then be implemented to explore the molecular mechanisms underlying training-dependent axonal growth in adult rodents. However, in contrast to Wistar rats, no changes in the Timm's-stained area of mossy fiber terminal fields (MFTFs) were observed in C57BL/6J or 129Sv/EmsJ inbred wild-type mice after water maze training. Neither extending the duration of training nor scaling down the size of the apparatus was able to induce sprouting in mouse mossy fiber pathways. Though there may be similarities in the ultimate output of the hippocampus of rats and mice as inferred from lesion studies, the current results, as well as differences in learning and memory characteristics between the two genera, suggest that the way in which the component circuitry functions is likely to be different; a not too surprising conclusion given the substantial evolutionary distance between them (>20 million years). The present findings afford an opportunity for uncovering linkages between evolutionarily significant alterations in hippocampal circuitry in relation to genera-specific information storage requirements.

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.

Comparison of seizure phenotype and neurodegeneration induced by systemic kainic acid in inbred, outbred, and hybrid mouse strains

We assessed inbred, outbred and hybrid mouse strains for susceptibility to seizures and neurodegeneration induced by systemic administration of kainic acid (KA). Each strain showed a unique pattern of susceptibility to seizures as assessed by the dose necessary to induce continuous tonic clonic seizures, progression through six seizure levels, the number of mice that failed to satisfy seizure criteria, and seizure-induced mortality. In general, the C57BL/6, ICR, FVB/N, and BALB/c strains were resistant to seizures while the C57BL/10, DBA/2 J, and F1 C57BL/6*CBA/J strains were vulnerable. Neuronal cell death was quantified in four subfields of the hippocampus: CA3, the hilus of the dentate gyrus, CA1, and the dentate granule cell layer. Neurodegeneration was also semiquantitatively assessed in other brain regions including the neocortex, striatum, thalamus, hypothalamus and amygdala. Although there was variability in the extent of cell death within strains, there were significant differences in the amount of hippocampal cell death between strains and also different patterns of neurodegeneration in affected brain areas. In general, the C57BL/6, C57BL/10, and F1 C57BL/6*CBA/J strains were resistant to neurodegeneration while the FVB/N, ICR and DBA/2 J strains were vulnerable. The BALB/c strain was unique in that neurodegeneration was confined to the hippocampus. Consistent with previous findings, the resistant neurodegeneration phenotype was dominant in an F1 cross of resistant and vulnerable inbred strains. Our results, using a large number of mouse strains, definitively demonstrate that a mouse strain's seizure phenotype is not related to its neurodegeneration phenotype.

Kindling and status epilepticus models of epilepsy: rewiring the brain

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

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.

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.

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.

Intact spatial memory in mice with seizure-induced partial loss of hippocampal pyramidal neurons

We generated defined neuronal loss in hippocampus of genetically identical mice by pilocarpine injections and studied the impact of these seizures on the performance of mice in spatial learning and memory. The numbers of TUNEL-positive degenerating cells paralleled the severity of the seizures. When compared to the numbers found for not-seizured control mice, mild, moderate, and severe seizures produced significant increases in TUNEL-positive neurons in CA1 and CA3 regions by 19, 25, and 63%, respectively. Water maze learning was abolished after the severe seizures. However, spatial learning was normal after mild or moderate seizures. Therefore, there was no linear correlation between the impairment of learning and memory performance with the number of degenerating neurons in hippocampus. Our data suggest that normal spatial learning and memory can be achieved without the full number of hippocampal pyramidal neurons in partially lesioned hippocampus.

The protective effect of a ketogenic diet on kainic acid-induced hippocampal cell death in the male ICR mice

This study was designed to evaluate the antiapoptotic effects of a ketogenic diet (KD) through histological (cresyl violet staining, TUNEL staining and immunohistochemistry) and behavioral studies using kainic acid (KA, 25mg/kg i.p.)-induced seizures in male ICR mice. KA-induced seizure in rodents is widely used as an experimental model for human temporal lobe epilepsy because of their behavioral and pathological similarities. A KA-induced seizure causes neuronal damage in hippocampal pyramidal neurons and involves a caspase-3-mediated apoptotic pathway. In this study, the seizure onset time of the KD-fed group was delayed compared to that of the group fed a normal diet (ND) after a systemic KA injection. Histological studies revealed that KA caused pyknosis in most of the hippocampal areas in the ND-fed group, however, well-preserved pyramidal neurons were detected in the hippocampus of mice that had been on KD for 1 month, which began on postnatal day 21. The number of TUNEL-positive cells and caspase-3-positive cells in the hippocampus of the KD-fed group was lower than that of the ND-fed group. These findings indicate that KD has an antiepileptic effect via a neuroprotective action that involves the inhibition of caspase-3-mediated apoptosis of hippocampal neurons.

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.

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

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

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.