Differential susceptibility to seizures induced by systemic kainic acid treatment in mature DBA/2J and C57BL/6J mice

CXCR5 Regulates Neuronal Polarity Development and Migration in the Embryonic Stage via F-Actin Homeostasis and Results in Epilepsy-Related Behavior

Epilepsy is a common, chronic neurological disorder that has been associated with impaired neurodevelopment and immunity. The chemokine receptor CXCR5 is involved in seizures via an unknown mechanism. Here, we first determined the expression pattern and distribution of the CXCR5 gene in the mouse brain during different stages of development and the brain tissue of patients with epilepsy. Subsequently, we found that the knockdown of CXCR5 increased the susceptibility of mice to pentylenetetrazol- and kainic acid-induced seizures, whereas CXCR5 overexpression had the opposite effect. CXCR5 knockdown in mouse embryos via viral vector electrotransfer negatively influenced the motility and multipolar-to-bipolar transition of migratory neurons. Using a human-derived induced an in vitro multipotential stem cell neurodevelopmental model, we determined that CXCR5 regulates neuronal migration and polarization by stabilizing the actin cytoskeleton during various stages of neurodevelopment. Electrophysiological experiments demonstrated that the knockdown of CXCR5 induced neuronal hyperexcitability, resulting in an increased number of seizures. Finally, our results suggested that CXCR5 deficiency triggers seizure-related electrical activity through a previously unknown mechanism, namely, the disruption of neuronal polarity.

Strain-dependent effects on neurobehavioral and seizure phenotypes in Scn2aK1422E mice

Pathogenic variants in SCN2A are associated with a range of neurodevelopmental disorders (NDD). Despite being largely monogenic, SCN2A-related NDD show considerable phenotypic variation and complex genotype-phenotype correlations. Genetic modifiers can contribute to variability in disease phenotypes associated with rare driver mutations. Accordingly, different genetic backgrounds across inbred rodent strains have been shown to influence disease-related phenotypes, including those associated with SCN2A-related NDD. Recently, we developed a mouse model of the variant SCN2A-p.K1422E that was maintained as an isogenic line on the C57BL/6J (B6) strain. Our initial characterization of NDD phenotypes in heterozygous Scn2aK1422E mice revealed alterations in anxiety-related behavior and seizure susceptibility. To determine if background strain affects phenotype severity in the Scn2aK1422E mouse model, phenotypes of mice on B6 and [DBA/2J×B6]F1 hybrid (F1D2) strains were compared. Convergent evidence from neurobehavioral assays demonstrated lower anxiety-like behavior in Scn2aK1422E mice compared to wild-type and further suggested that this effect is more pronounced on the B6 background compared to the F1D2 background. Although there were no strain-dependent differences in occurrence of rare spontaneous seizures, response to the chemoconvulsant kainic acid revealed differences in seizure generalization and lethality risk, with variation based on strain and sex. Continued examination of strain-dependent effects in the Scn2aK1422E mouse model could reveal genetic backgrounds with unique susceptibility profiles that would be relevant for future studies on specific traits and enable the identification of highly penetrant phenotypes and modifier genes that could provide clues about the primary pathogenic mechanism of the K1422E variant.

The Deletion of GluK2 Alters Cholinergic Control of Neuronal Excitability

Kainate receptors (KARs) are key regulators of synaptic circuits by acting at pre- and postsynaptic sites through either ionotropic or metabotropic actions. KARs can be activated by kainate, a potent neurotoxin, which induces acute convulsions. Here, we report that the acute convulsive effect of kainate mostly depends on GluK2/GluK5 containing KARs. By contrast, the acute convulsive activity of pilocarpine and pentylenetetrazol is not alleviated in the absence of KARs. Unexpectedly, the genetic inactivation of GluK2 rather confers increased susceptibility to acute pilocarpine-induced seizures. The mechanism involves an enhanced excitability of GluK2-/- CA3 pyramidal cells compared with controls upon pilocarpine application. Finally, we uncover that the absence of GluK2 increases pilocarpine modulation of Kv7/M currents. Taken together, our findings reveal that GluK2-containing KARs can control the excitability of hippocampal circuits through interaction with the neuromodulatory cholinergic system.

Touchscreen Cognitive Deficits, Hyperexcitability, and Hyperactivity in Males and Females Using Two Models of Cdkl5 Deficiency

Many neurodevelopmental disorders (NDDs) are the result of mutations on the X chromosome. One severe NDD resulting from mutations on the X chromosome is CDKL5 deficiency disorder (CDD). CDD is an epigenetic, X-linked NDD characterized by intellectual disability (ID), pervasive seizures and severe sleep disruption, including recurring hospitalizations. CDD occurs at a 4:1 ratio, with a female bias. CDD is driven by the loss of cyclin-dependent kinase-like 5 (CDKL5), a serine/threonine kinase that is essential for typical brain development, synapse formation and signal transmission. Previous studies focused on male subjects from animal models, likely to avoid the complexity of X mosaicism. For the first time, we report translationally relevant behavioral phenotypes in young adult (8–20 weeks) females and males with robust signal size, including impairments in learning and memory, substantial hyperactivity and increased susceptibility to seizures/reduced seizure thresholds, in both sexes, and in two models of CDD preclinical mice, one with a general loss-of-function mutation and one that is a patient-derived mutation.

A novel mouse model for pyridoxine-dependent epilepsy due to antiquitin deficiency

Pyridoxine-dependent epilepsy (PDE) is a rare autosomal recessive disease caused by mutations in the ALDH7A1 gene leading to blockade of the lysine catabolism pathway. PDE is characterized by recurrent seizures that are resistant to conventional anticonvulsant treatment but are well-controlled by pyridoxine (PN). Most PDE patients also suffer from neurodevelopmental deficits despite adequate seizure control with PN. To investigate potential pathophysiological mechanisms associated with ALDH7A1 deficiency, we generated a transgenic mouse strain with constitutive genetic ablation of Aldh7a1. We undertook extensive biochemical characterization of Aldh7a1-KO mice consuming a low lysine/high PN diet. Results showed that KO mice accumulated high concentrations of upstream lysine metabolites including ∆1-piperideine-6-carboxylic acid (P6C), α-aminoadipic semialdehyde (α-AASA) and pipecolic acid both in brain and liver tissues, similar to the biochemical picture in ALDH7A1-deficient patients. We also observed preliminary evidence of a widely deranged amino acid profile and increased levels of methionine sulfoxide, an oxidative stress biomarker, in the brains of KO mice, suggesting that increased oxidative stress may be a novel pathobiochemical mechanism in ALDH7A1 deficiency. KO mice lacked epileptic seizures when fed a low lysine/high PN diet. Switching mice to a high lysine/low PN diet led to vigorous seizures and a quick death in KO mice. Treatment with PN controlled seizures and improved survival of high-lysine/low PN fed KO mice. This study expands the spectrum of biochemical abnormalities that may be associated with ALDH7A1 deficiency and provides a proof-of-concept for the utility of the model to study PDE pathophysiology and to test new therapeutics.

The Kainic Acid Models of Temporal Lobe Epilepsy

Experimental models of epilepsy are useful to identify potential mechanisms of epileptogenesis, seizure genesis, comorbidities, and treatment efficacy. The kainic acid (KA) model is one of the most commonly used. Several modes of administration of KA exist, each producing different effects in a strain-, species-, gender-, and age-dependent manner. In this review, we discuss the advantages and limitations of the various forms of KA administration (systemic, intrahippocampal, and intranasal), as well as the histologic, electrophysiological, and behavioral outcomes in different strains and species. We attempt a personal perspective and discuss areas where work is needed. The diversity of KA models and their outcomes offers researchers a rich palette of phenotypes, which may be relevant to specific traits found in patients with temporal lobe epilepsy.

Dexmedetomidine - Commonly Used in Functional Imaging Studies - Increases Susceptibility to Seizures in Rats But Not in Wild Type Mice

Functional MRI (fMRI) utilizes changes in metabolic and hemodynamic signals to indirectly infer the underlying local changes in neuronal activity. To investigate the mechanisms of fMRI responses, spontaneous fluctuations, and functional connectivity in the resting-state, it is important to pursue fMRI in animal models. Animal studies commonly use dexmedetomidine sedation. It has been demonstrated that potent sensory stimuli administered under dexmedetomidine are prone to inducing seizures in Sprague-Dawley (SD) rats. Here we combined optical imaging of intrinsic signals and cerebral blood flow with neurophysiological recordings to measure responses in rat area S1FL to electrical forepaw stimulation administered at 8 Hz. We show that the increased susceptibility to seizures starts no later than 1 h and ends no sooner than 3 h after initiating a continuous administration of dexmedetomidine. By administering different combinations of anesthetic and sedative agents, we demonstrate that dexmedetomidine is the sole agent necessary for the increased susceptibility to seizures. The increased susceptibility to seizures prevails under a combination of 0.3–0.5% isoflurane and dexmedetomidine anesthesia. The blood-oxygenation and cerebral blood flow responses to seizures induced by forepaw stimulation have a higher amplitude and a larger spatial extent relative to physiological responses to the same stimuli. The epileptic activity and the associated blood oxygenation and cerebral blood flow responses stretched beyond the stimulation period. We observed seizures in response to forepaw stimulation with 1–2 mA pulses administered at 8 Hz. In contrast, responses to stimuli administered at 4 Hz were seizure-free. We demonstrate that such seizures are generated not only in SD rats but also in Long-Evans rats, but not in C57BL6 mice stimulated with similar potent stimuli under dexmedetomidine sedation. We conclude that high-amplitude hemodynamic functional imaging responses evoked by peripheral stimulation in rats sedated with dexmedetomidine are possibly due to the induction of epileptic activity. Therefore, caution should be practiced in experiments that combine the administration of potent stimuli with dexmedetomidine sedation. We propose stimulation paradigms that elicit seizure-free, well detectable neurophysiological and hemodynamic responses in rats. We further conclude that the increased susceptibility to seizures under dexmedetomidine sedation is species dependent.

A comparative study of the phenotype with kainic acid-induced seizure in DBA/2 mice from three different sources

The kainic acid-induced seizure mouse model is widely used in epilepsy research. In this study, we applied kainic acid to the subcutaneous injections of three different sources of DBA/2 mice to compare and evaluate the seizure response. The three mouse sources consisted of DBA/2Kor1 (Korea FDA source), DBA/2A (USA source), and DBA/2 (Japan source), and were purchased from different vendors. To compare the responses of DBA/2 mice to kainic acid injections, we examined the survival rate, seizure phenotype scoring, and behavioral changes. We also evaluated brain lesions using histopathological analysis. Following the administration of kainic acid, almost half of the cohort survived, and the seizure phenotype displayed a moderate level of sensitivity (2 ~ 4 out of 6). In the histopathologic analysis, there was no change in morphological features, and levels of glial fibrillary acidic protein (GFAP) and ionized calcium binding adaptor molecule 1 (Iba-1) increased in the kainic acid-treated groups. However, there was no difference in the neuronal nuclei (NeuN) expression level. All the data showed that the responses in the kainic acid-treated group were similar across the three strains. In conclusion, our results suggest that the three sources of DBA/2 mice (DBA/2Kor1, DBA/2A, and DBA/2B) have similar pathological responses to kainic acid-induced seizures.

Collaborative Cross mice reveal extreme epilepsy phenotypes and genetic loci for seizure susceptibility

OBJECTIVE

Animal studies remain essential for understanding mechanisms of epilepsy and identifying new therapeutic targets. However, existing animal models of epilepsy do not reflect the high level of genetic diversity found in the human population. The Collaborative Cross (CC) population is a genetically diverse recombinant inbred panel of mice. The CC offers large genotypic and phenotypic diversity, inbred strains with stable genomes that allow for repeated phenotypic measurements, and genomic tools including whole genome sequence to identify candidate genes and candidate variants.

METHODS

We evaluated multiple complex epileptic traits in a sampling of 35 CC inbred strains using the flurothyl-induced seizure and kindling paradigm. We created an F2 population of 297 mice with extreme seizure susceptibility and performed quantitative trait loci (QTL) mapping to identify genomic regions associated with seizure sensitivity. We used quantitative RNA sequencing from CC hippocampal tissue to identify candidate genes and whole genome sequence to identify genetic variants likely affecting gene expression.

RESULTS

We identified new mouse models with extreme seizure susceptibility, seizure propagation, epileptogenesis, and SUDEP (sudden unexpected death in epilepsy). We performed QTL mapping and identified one known and seven novel loci associated with seizure sensitivity. We combined whole genome sequencing and hippocampal gene expression to pinpoint biologically plausible candidate genes (eg, Gabra2) and variants associated with seizure sensitivity.

SIGNIFICANCE

New mouse models of epilepsy are needed to better understand the complex genetic architecture of seizures and to identify therapeutics. We performed a phenotypic screen utilizing a novel genetic reference population of CC mice. The data we provide enable the identification of protective/risk genes and novel molecular mechanisms linked to complex seizure traits that are currently challenging to study and treat.

Genetic backgrounds have unique seizure response profiles and behavioral outcomes following convulsant administration

Three highly utilized strains of mice, common for preclinical genetic studies, were evaluated for seizure susceptibility and behavioral outcomes common to the clinical phenotypes of numerous psychiatric disorders following repeated low-dose treatment with either a gamma-aminobutyric acid (GABA) receptor antagonist (pentylenetetrazole (PTZ)) or a glutamate agonist (kainic acid (KA)). Effects of strain and treatment were evaluated with classic seizure scoring and a tailored behavior battery focused on behavioral domains common in neuropsychiatric research: learning and memory, social behavior, and motor abilities, as well as seizure susceptibility and/or resistance. Seizure response was induced by a single daily treatment of either PTZ (30 mg/kg, intraperitoneally (i.p.)) or KA (5 mg/kg, i.p.) for 10 days. Pentylenetetrazole-treated FVB/NJ and C57BL/6NJ strains of mice showed strong, clear seizure responses. This also resulted in cognitive and social deficits, and increased susceptibility to a high dose of PTZ. Kainic acid-treated FVB/NJ and C57BL/6NJ strains of mice had a robust seizure response, which resulted in hyperactivity. Pentylenetetrazole-treated C57BL/6J mice demonstrated mild hyperactivity, while KA-treated C57BL/6J displayed cognitive deficits and resistance to a high dose of KA but no social deficits. Overall, a uniquely different seizure response profile was detected in the C57BL/6J strain with few observable instances of seizure response despite repeated convulsant administration by two mechanisms. This work illustrated that differing background genetic strains have unique seizure susceptibility profiles and distinct social and cognitive behavior following PTZ and/or KA treatment and that it is, therefore, necessary to consider strain differences before attributing behavioral phenotypes to gene(s) of interest during preclinical evaluations of genetic mouse models, especially when outcome measures are focused on cognitive and/or social behaviors common to the clinical features of numerous neurological disorders.

Status Epilepticus: Behavioral and Electroencephalography Seizure Correlates in Kainate Experimental Models

Various etiological factors, such as head injury, chemical intoxication, tumors, and gene mutation, can induce epileptogenesis. In animal models, status epilepticus (SE) triggers epileptogenesis. In humans, convulsive SE for >30 min can be a life-threatening medical emergency. The duration and severity of convulsive SE are highly variable in chemoconvulsant animal models. A continuous video-electroencephalography (EEG) recording, and/or diligent direct observation, facilitates quantification of exact duration of different stages of convulsive seizures (Racine stages 3–5) to determine the severity of SE. A continuous convulsive SE for >30 min usually causes high mortality in some rodents and results in widespread brain damage in the surviving animals, in spite of treating with antiepileptic drugs (AEDs). AEDs control behavioral seizures but not EEG seizures. The severity of initial SE impacts epileptogenesis and cognitive function; therefore, quantitative assessment of behavioral SE and EEG in animal models will help to understand the impact of SE severity on epileptogenesis. There are several excellent reviews on experimental models of seizure/SE/epilepsy. This review focusses on the comparison of induction and characterization of behavioral SE and EEG correlates in mice and rats induced by kainate. We also discuss the advantages of repeated low dose of kainate (i.p. route), which minimizes variability in the initial SE severity between animals and reduces mortality rate. A refined approach to induce SE with kainate also addresses the two of the 3Rs (i.e., refinement and reduction), the guiding principles for ethical and scientific standpoint of animal research.

BAD knockout provides metabolic seizure resistance in a genetic model of epilepsy with sudden unexplained death in epilepsy

Metabolic alteration, either through the ketogenic diet (KD) or by genetic alteration of the BAD protein, can produce seizure protection in acute chemoconvulsant models of epilepsy. To assess the seizure-protective role of knocking out (KO) the Bad gene in a chronic epilepsy model, we used the Kcna1^-/- model of epilepsy, which displays progressively increased seizure severity and recapitulates the early death seen in sudden unexplained death in epilepsy (SUDEP). Beginning on postnatal day 24 (P24), we continuously video monitored Kcna1^-/- and Kcna1^-/- Bad^-/- double knockout mice to assess survival and seizure severity. We found that Kcna1^-/- Bad^-/- mice outlived Kcna1^-/- mice by approximately 2 weeks. Kcna1^-/- Bad^-/- mice also spent significantly less time in seizure than Kcna1^-/- mice on P24 and the day of death, showing that BadKO provides seizure resistance in a genetic model of chronic epilepsy.

The role of genetic background in susceptibility to chemical warfare nerve agents across rodent and non-human primate models

Genetics likely play a role in various responses to nerve agent exposure, as genetic background plays an important role in behavioral, neurological, and physiological responses to environmental stimuli. Mouse strains or selected lines can be used to identify susceptibility based on background genetic features to nerve agent exposure. Additional genetic techniques can then be used to identify mechanisms underlying resistance and sensitivity, with the ultimate goal of developing more effective and targeted therapies. Here, we discuss the available literature on strain and selected line differences in cholinesterase activity levels and response to nerve agent-induced toxicity and seizures. We also discuss the available cholinesterase and toxicity literature across different non-human primate species. The available data suggest that robust genetic differences exist in cholinesterase activity, nerve agent-induced toxicity, and chemical-induced seizures. Available cholinesterase data suggest that acetylcholinesterase activity differs across strains, but are limited by the paucity of carboxylesterase data in strains and selected lines. Toxicity and seizures, two outcomes of nerve agent exposure, have not been fully evaluated for genetic differences, and thus further studies are required to understand baseline strain and selected line differences.

The relevance of inter- and intrastrain differences in mice and rats and their implications for models of seizures and epilepsy

It is becoming increasingly clear that the genetic background of mice and rats, even in inbred strains, can have a profound influence on measures of seizure susceptibility and epilepsy. These differences can be capitalized upon through genetic mapping studies to reveal genes important for seizures and epilepsy. However, strain background and particularly mixed genetic backgrounds of transgenic animals need careful consideration in both the selection of strains and in the interpretation of results and conclusions. For instance, mice with targeted deletions of genes involved in epilepsy can have profoundly disparate phenotypes depending on the background strain. In this review, we discuss findings related to how this genetic heterogeneity has and can be utilized in the epilepsy field to reveal novel insights into seizures and epilepsy. Moreover, we discuss how caution is needed in regards to rodent strain or even animal vendor choice, and how this can significantly influence seizure and epilepsy parameters in unexpected ways. This is particularly critical in decisions regarding the strain of choice used in generating mice with targeted deletions of genes. Finally, we discuss the role of environment (at vendor and/or laboratory) and epigenetic factors for inter- and intrastrain differences and how such differences can affect the expression of seizures and the animals' performance in behavioral tests that often accompany acute and chronic seizure testing.

Differential Activation of Calpain-1 and Calpain-2 following Kainate-Induced Seizure Activity in Rats and Mice

Systemic injection of kainate produces repetitive seizure activity in both rats and mice. It also results in short-term synaptic modifications as well as delayed neurodegeneration. The signaling cascades involved in both short-term and delayed responses are not clearly defined. The calcium-dependent protease calpain is activated in various brain structures following systemic kainate injection, although the precise involvement of the two major brain calpain isoforms, calpain-1 and calpain-2, remains to be defined. It has recently been reported that calpain-1 and calpain-2 play opposite roles in NMDA receptor-mediated neuroprotection or neurodegeneration, with calpain-1 being neuroprotective and calpain-2 being neurodegenerative. In the present study, we determined the activation pattern of calpain-1 and calpain-2 by analyzing changes in levels of different calpain substrates, including spectrin, drebrin, and PTEN (phosphatase and tensin homolog; a specific calpain-2 substrate) in both rats, and wild-type and calpain-1 knock-out mice. The results indicate that, while calpain-2 is rapidly activated in pyramidal cells throughout CA1 and CA3, rapid calpain-1 activation is restricted to parvalbumin-positive and to a lesser extent CCK-positive, but not somatostatin-positive, interneurons. In addition, calpain-1 knock-out mice exhibit increased long-term neurodegeneration in CA1, reinforcing the notion that calpain-1 activation is neuroprotective.

Oriental Medicine Woohwangchungsimwon Attenuates Kainic Acid-Induced Seizures and Neuronal Cell Death in the Hippocampus

Woohwangchungsimwon (WCW) is an oriental medicine that has been extensively prescribed in Asia to patients with apoplexy, high blood pressure, acute/chronic convulsion, and so on. However, the potential therapeutic value of WCW in treating the pathologic brain has not yet been fully investigated. In the present study, we evaluated whether WCW has beneficial effects on kainic acid (KA)-induced excitotoxicity. An intraperitoneal injection of KA (40 mg/kg) and an intracerebroventricular injection of KA (0.2 μg) produced typical seizure behavior and neuronal cell death in the CA1 and CA3 pyramidal layers of the hippocampus, respectively. However, the systemic administration of WCW significantly attenuated the seizure behavior and neuronal cell death. WCW was found to exert the best protective effect when it was administrated 2 hours before a KA injection. Moreover, this WCW-induced neuroprotection was accompanied by a reduction in microglia activation and tumor necrosis factor-alpha, interleukin (IL)-1β, IL-6, inducible nitric oxide synthase, and cyclooxyganase-2 in the hippocampus. These results suggest that WCW has therapeutic potential to suppress KA-induced pathogenesis in the brain by inhibiting inflammation.

Scopolamine-induced convulsions in fasted animals after food intake: sensitivity of C57BL/6J mice and Sprague-Dawley rats

Food intake triggers convulsions in fasted BALB/c mice and Wistar albino rats treated with antimuscarinic drugs, scopolamine or atropine. Inbred strain studies have yielded considerable information regarding genetic influences on seizure susceptibility and factors contribute to epileptogenesis in rodents. This study, therefore, investigated sensitivity to antimuscarinic-induced seizures in C57BL/6J mice and Sprague-Dawley rats. Food deprivation for 48h in mice and 52h in rats did not produce strain differences in body weight loss. Fasted animals treated i.p. with 3mg/kg scopolamine developed convulsions after food intake. The incidence of convulsions was indifferent in comparison to BALB/c mice and Wistar albino rats. Number of animals developing stage 5 was more and onset of convulsions was longer in C57BL/6J mice than in BALB/c mice. Strain-related differences in sensitivity to seizures in C57BL/6J mice may need further evaluation for investigating genetic influences on scopolamine-induced seizures.

Spatiotemporal differences in the c-fos pathway between C57BL/6J and DBA/2J mice following flurothyl-induced seizures: A dissociation of hippocampal Fos from seizure activity

Significant differences in seizure characteristics between inbred mouse strains highlight the importance of genetic predisposition to epilepsy. Here, we examined the genetic differences between the seizure-resistant C57BL/6J (B6) mouse strain and the seizure-susceptible DBA/2J (D2) strain in the phospho-Erk and Fos pathways to examine seizure-induced neuronal activity to uncover potential mechanistic correlates to these disparate seizure responsivities. Expression of neural activity markers was examined following 1, 5, or 8 seizures, or after 8 seizures, a 28 day rest period, and a final flurothyl rechallenge. Two brain regions, the hippocampus and ventromedial nucleus of the hypothalamus (VMH), had significantly different Fos expression profiles following seizures. Fos expression was highly robust in B6 hippocampus following one seizure and remained elevated following multiple seizures. Conversely, there was an absence of Fos (and phospho-Erk) expression in D2 hippocampus following one generalized seizure that increased with multiple seizures. This lack of Fos expression occurred despite intracranial electroencephalographic recordings indicating that the D2 hippocampus propagated ictal discharge during the first flurothyl seizure suggesting a dissociation of seizure discharge from Fos and phospho-Erk expression. Global transcriptional analysis confirmed a dysregulation of the c-fos pathway in D2 mice following 1 seizure. Moreover, global analysis of RNA expression differences between B6 and D2 hippocampus revealed a unique pattern of transcripts that were co-regulated with Fos in D2 hippocampus following 1 seizure. These expression differences could, in part, account for D2's seizure susceptibility phenotype. Following 8 seizures, a 28 day rest period, and a final flurothyl rechallenge, ∼85% of B6 mice develop a more complex seizure phenotype consisting of a clonic-forebrain seizure that uninterruptedly progresses into a brainstem seizure. This seizure phenotype in B6 mice is highly correlated with bilateral Fos expression in the VMH and was not observed in D2 mice, which always express clonic-forebrain seizures upon flurothyl retest. Overall, these results illustrate specific differences in protein and RNA expression in different inbred strains following seizures that precede the reorganizational events that affect seizure susceptibility and changes in seizure semiology over time.

Susceptibility to seizure-induced excitotoxic cell death is regulated by an epistatic interaction between Chr 18 (Sicd1) and Chr 15 (Sicd2) loci in mice

Seizure-induced cell death is believed to be regulated by multiple genetic components in addition to numerous external factors. We previously defined quantitative trait loci that control susceptibility to seizure-induced cell death in FVB/NJ (susceptible) and C57BL/6J (resistant) mice. Two of these quantitative trait loci assigned to chromosomes 18 (Sicd1) and 15 (Sicd2), control seizure-induced cell death resistance. In this study, through the use of a series of novel congenic strains containing the Sicd1 and Sicd2 congenic strains and different combinations of the Sicd1 or Sicd2 sub region(s), respectively, we defined these genetic interactions. We generated a double congenic strain, which contains the two C57BL/6J differential segments from chromosome 18 and 15, to determine how these two segments interact with one another. Phenotypic comparison between FVB-like littermates and the double congenic FVB.B6-Sicd1/Sicd2 strain identified an additive effect with respect to resistance to seizure-induced excitotoxic cell death. It thus appears that C57BL/6J alleles located on chromosomes 18 and 15 interact epistatically in an additive manner to control the extent of seizure-induced excitotoxic cell death. Three interval-specific congenic lines were developed, in which either segments of C57BL/6J Chr 18 or C57BL/6J Chr 15 were introduced in the FVB/NJ genetic background, and progeny were treated with kainate and examined for the extent of seizure-induced cell death. All of the interval-specific congenic lines exhibited reduced cell death in both area CA3 and the dentate hilus, associated with the C57BL/6J phenotype. These experiments demonstrate functional interactions between Sicd1 and Sicd2 that improve resistance to seizure-induced excitotoxic cell death, validating the critical role played by gene-gene interactions in excitotoxic cell death.

A locus on mouse Ch10 influences susceptibility to limbic seizure severity: fine mapping and in silico candidate gene analysis

Identification of genes contributing to mouse seizure susceptibility can reveal novel genes or pathways that provide insight into human epilepsy. Using mouse chromosome substitution strains and interval-specific congenic strains (ISCS), we previously identified an interval conferring pilocarpine-induced limbic seizure susceptibility on distal mouse chromosome 10 (Ch10). We narrowed the region by generating subcongenics with smaller A/J Ch10 segments on a C57BL/6J (B6) background and tested them with pilocarpine. We also tested pilocarpine-susceptible congenics for 6-Hz ECT (electroconvulsive threshold), another model of limbic seizure susceptibility, to determine whether the susceptibility locus might have a broad effect on neuronal hyperexcitability across more than one mode of limbic seizure induction. The ISCS Line 1, which contained the distal 2.7 Mb segment from A/J (starting at rs29382217), was more susceptible to both pilocarpine and ECT. Line 2, which was a subcongenic of Line 1 (starting at rs13480828), was not susceptible, thus defining a 1.0 Mb critical region that was unique to Line 1. Bioinformatic approaches identified 45 human orthologs within the unique Line 1 susceptibility region, the majority syntenic to human Ch12. Applying an epilepsy network analysis of known and suspected excitability genes and examination of interstrain genomic and brain expression differences revealed novel candidates within the region. These include Stat2, which plays a role in hippocampal GABA receptor expression after status epilepticus, and novel candidates Pan2, Cdk2, Gls2 and Cs, which are involved in neural cell differentiation, cellular remodeling and embryonic development. Our strategy may facilitate discovery of novel human epilepsy genes.

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.

Kainic acid-induced neuronal degeneration in hippocampal pyramidal neurons is driven by both intrinsic and extrinsic factors: analysis of FVB/N↔C57BL/6 chimeras

The excitotoxic effects of kainic acid (KA) in the mouse hippocampus is strain dependent. Following KA administration, the large majority of hippocampal pyramidal cells die in the FVB/N (FVB) mouse, while the pyramidal cells of the C57BL/6 (B6) strain are largely spared. We generated aggregation chimeras between the sensitive FVB and the resistant B6 strains to investigate whether intrinsic or extrinsic features of a neuron confer cell vulnerability or resistance to KA. The constitutive expression of transgenic green fluorescence protein (GFP) or β-galactosidase expressed from the ROSA26 locus was used to mark cells in FVB or B6 mice, respectively. These makers enable the identification of cells from each parental genotype while TUNEL (terminal deoxynucleotidyl transferase-mediated biotinylated dUTP nick end labeling)-staining labeled dying cells. The analysis of the percentage of dying cells in FVB-GFP ↔ B6-ROSA chimeras yielded an intriguing mix of both intrinsic and extrinsic factors in the readout of cell phenotype. Thus, normally resistant B6-ROSA pyramidal neurons demonstrated an increasing sensitivity to KA, in a linear fashion, when the percentage of FVB-GFP cells was increased, either across chimeras or in different regions of the same chimera. However, the death of B6-ROSA pyramidal cells never exceeded ∼70% of the total amount of B6 neurons regardless of the amount of FVB cells in the chimeric hippocampus. In a similar manner, FVB-GFP cells show lower amounts of cell death in chimeras that are colonized by B6-ROSA cells, but again, are never fully rescued. These data indicate that both intrinsic and extrinsic factors modulate the sensitivity of hippocampal pyramidal cells to kainic acid.

Transgenic models of spinocerebellar ataxia type 10: modeling a repeat expansion disorder

Spinocerebellar ataxia type 10 (SCA10) is an autosomal dominant neurodegenerative disease with a spectrum of phenotypes. SCA10 is caused by a pentanucleotide repeat expansion of the ATTCT motif within intron 9 of ATAXIN 10 (ATXN10). Patients present with cerebellar ataxia; however, a subset also develops epileptic seizures which significantly contribute to the morbidity and mortality of the disease. Past research from our lab has demonstrated that epileptic SCA10 patients predominantly originate from or have ancestral ties to Mexico. In addition, a large proportion of epileptic SCA10 patients carry repeat interruptions within their SCA10 expansion. This paper outlines the variability in SCA10 phenotypes and our attempts to model these phenotypes using transgenic mouse models and highlights the benefits of using a transgenic model organism to understand the pathological mechanisms of a human disease.

Metabolic dysfunction associated with adiponectin deficiency enhances kainic acid-induced seizure severity

Metabolic syndrome has deleterious effects on the CNS, and recent evidence suggests that obesity rates are higher at presentation in children who develop epilepsy. Adiponectin is secreted by adipose tissue and acts in the brain and peripheral organs to regulate glucose and lipid metabolism. Adiponectin deficiency predisposes toward metabolic syndrome, characterized by obesity, insulin resistance, impaired glucose tolerance, hyperlipidemia, and cardiovascular morbidity. To investigate the relationship between metabolic syndrome and seizures, wild-type C57BL/6J and adiponectin knock-out mice were fed a high-fat diet, followed by treatment with low doses of kainic acid to induce seizures. Adiponectin deficiency in mice fed a high-fat diet resulted in greater fat accumulation, impaired glucose tolerance, hyperlipidemia, increased seizure severity, and increased hippocampal pathology. In contrast, there were no adverse effects of adiponectin deficiency on metabolic phenotype or seizure activity in mice fed a normal (low-fat) chow diet. These findings demonstrate that metabolic syndrome modulates the outcome of seizures and brain injury.

The relevance of individual genetic background and its role in animal models of epilepsy

Growing evidence has indicated that genetic factors contribute to the etiology of seizure disorders. Most epilepsies are multifactorial, involving a combination of additive and epistatic genetic variables. However, the genetic factors underlying epilepsy have remained unclear, partially due to epilepsy being a clinically and genetically heterogeneous syndrome. Similar to the human situation, genetic background also plays an important role in modulating both seizure susceptibility and its neuropathological consequences in animal models of epilepsy, which has too often been ignored or not been paid enough attention to in published studies. Genetic homogeneity within inbred strains and their general amenability to genetic manipulation have made them an ideal resource for dissecting the physiological function(s) of individual genes. However, the inbreeding that makes inbred mice so useful also results in genetic divergence between them. This genetic divergence is often unaccounted for but may be a confounding factor when comparing studies that have utilized distinct inbred strains. The purpose of this review is to discuss the effects of genetic background strain on epilepsy phenotypes of mice, to remind researchers that the background genetics of a knockout strain can have a profound influence on any observed phenotype, and outline the means by which to overcome potential genetic background effects in experimental models of epilepsy.

Aquaporin-4-dependent edema clearance following status epilepticus

We investigated the role of aquaporin-4 in the development of cerebral edema following kainic acid-induced status epilepticus (SE) using specific gravimetry and T2 MRI techniques at 6 h, 1 day, 4 days and 7 days after SE. Our results indicate significantly greater tissue edema and T2 MRI changes in AQP4(-/-) compared to AQP4(+/+) mice that peaks at about 1 day after SE (greater in hippocampus relative to cortex). These results have implications for the mechanisms of edema formation and clearance following intense seizure activity.

Comparison of focally induced epileptiform activities in C57BL/6 and BALB/c mice by using in vivo EEG recording

Epilepsy characterized by repeated seizures is influenced by genetic factors. Seizure response of inbred mouse strains changes depending on the variety of stimuli including chemical (e.g., pentylenetetrazole, nicotine, cocaine, NMDA, kainate), physical (e.g., auditory) or electrical. In this study, we compared the susceptibilities of C57BL/6 and BALB/c mice strains to penicillin induced epileptiform activity (a focally induced, experimental epilepsy model), by analyzing the spike onset latency, spike amplitude and spike frequency. The power spectrums of baseline EEGs were also investigated. We found no alterations of spike onset latencies between the C57 and BALB mice. However, spike amplitudes and spike frequencies were found to be higher in BALB mice than C57 mice. With regard to EEG power spectrum, absolute power of investigated bands was not different between the two strains. Interestingly, the relative power of all investigated bands differed significantly between two strains. The relative power of delta and theta was lower whereas relative power of alpha, beta and gamma was higher in C57 mice compared to BALB mice. In conclusion, our findings showed that BALB mice are more sensitive to penicillin induced epileptiform activity when compared to C57 strain.

Mapping a mouse limbic seizure susceptibility locus on chromosome 10

PURPOSE

Mapping seizure susceptibility loci in mice provides a framework for identifying potentially novel candidate genes for human epilepsy. Using C57BL/6J × A/J chromosome substitution strains (CSS), we previously identified a locus on mouse chromosome 10 (Ch10) conferring susceptibility to pilocarpine, a muscarinic cholinergic agonist that models human temporal lobe epilepsy by inducing initial limbic seizures and status epilepticus (status), followed by hippocampal cell loss and delayed-onset chronic spontaneous limbic seizures. Herein we report further genetic mapping of pilocarpine quantitative trait loci (QTLs) on Ch10.

METHODS

Seventy-nine Ch10 F(2) mice were used to map QTLs for duration of partial status epilepticus and the highest stage reached in response to pilocarpine. Based on those results we created interval-specific congenic lines to confirm and extend the results, using sequential rounds of breeding selectively by genotype to isolate segments of A/J Ch10 genome on a B6 background.

KEY FINDINGS

Analysis of Ch10 F(2) genotypes and seizure susceptibility phenotypes identified significant, overlapping QTLs for duration of partial status and severity of pilocarpine-induced seizures on distal Ch10. Interval-specific Ch10 congenics containing the susceptibility locus on distal Ch10 also demonstrated susceptibility to pilocarpine-induced seizures, confirming results from the F(2) mapping population and strongly supporting the presence of a QTL between rs13480781 (117.6 Mb) and rs13480832 (127.7 Mb).

SIGNIFICANCE

QTL mapping can identify loci that make a quantitative contribution to a trait, and eventually identify the causative DNA-sequence polymorphisms. We have mapped a locus on mouse Ch10 for pilocarpine-induced limbic seizures. Novel candidate genes identified in mice can be investigated in functional studies and tested for their role in human epilepsy.

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.

Quantitative trait loci for electrical seizure threshold mapped in C57BLKS/J and C57BL/10SnJ mice

We mapped the quantitative trait loci (QTL) that contribute to the robust difference in maximal electroshock seizure threshold (MEST) between C57BLKS/J (BKS) and C57BL10S/J (B10S) mice. BKS, B10S, BKS × B10S F1 and BKS × B10S F2 intercross mice were tested for MEST at 8-9 weeks of age. Results of F2 testing showed that, in this cross, MEST is a continuously distributed trait determined by polygenic inheritance. Mice from the extremes of the trait distribution were genotyped using microarray technology. MEST correlated significantly with body weight and sex; however, because of the high correlation between these factors, the QTL mapping was conditioned on sex alone. A sequential series of statistical analyses was used to map QTLs including single-point, multipoint and multilocus methods. Two QTLs reached genome-wide levels of significance based upon an empirically determined permutation threshold: chromosome 6 (LOD = 6.0 at ∼69 cM) and chromosome 8 (LOD = 5.7 at ∼27 cM). Two additional QTLs were retained in a multilocus regression model: chromosome 3 (LOD = 2.1 at ∼68 cM) and chromosome 5 (LOD = 2.7 at ∼73 cM). Together the four QTLs explain one third of the total phenotypic variance in the mapping population. Lack of overlap between the major MEST QTLs mapped here in BKS and B10S mice and those mapped previously in C57BL/6J and DBA/2J mice (strains that are closely related to BKS and B10S) suggest that BKS and B10S represent a new polygenic mouse model for investigating susceptibility to seizures.

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.

Neurobehavioral consequences of stressor exposure in rodent models of epilepsy

Both normal, non-epileptic as well as seizure-prone rodents exhibit a spectrum of anxiogenic-like behaviors in response to stressor exposure. Comparative analysis reveals that the same set of emotionality dependent measures is sensitive to both stress reactivity in normal rodents as well as stress hyperreactivity typically seen in seizure-prone rodents. A variety of unconditioned, exploratory tasks reflect global sensitivity to stressor exposure in the form of behavioral inhibition of locomotor output. Moreover, well chosen stressors can trigger de novo seizures with or without a history of seizure incidence. Seizures may be elicited in response to stressful environmental stimuli such as noxious noises, tail suspension handling, or home cage disturbance. Stress reactivity studies in rodents with a genetic predisposition to seizures have yielded important clues regarding brain substrates that mediate seizure ontogeny and modulate ictogenesis. Brains of seizure susceptible rodents reflect elevated content of the stress-related neuropeptide, corticotropin-releasing factor (CRF) in several nuclei relative to non-susceptible controls and neutralization of brain CRF attenuates seizure sensitivity. Findings outlined in this review support a diathesis-stress hypothesis in which behavioral- and neuro-pathologies of genetically seizure susceptible rodents arise in part due to multifaceted hyperreactivity to noxious environmental stimuli.

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.

Characterization of febrile seizures and febrile seizure susceptibility in mouse inbred strains

Febrile seizures (FS) are the most prevalent seizures in children. Although FS are largely benign, complex FS increase the risk to develop temporal lobe epilepsy (TLE). Studies in rat models for FS have provided information about functional changes in the hippocampus after complex FS. However, our knowledge about the genes and pathways involved in the causes and consequences of FS is still limited. To enable molecular, genetic and knockout studies, we developed and characterized an FS model in mice and used it as a phenotypic screen to analyze FS susceptibility. Hyperthermia was induced by warm air in 10- to 14-day-old mice and induced FS in all animals. Under the conditions used, seizure-induced behavior in mice and rats was similar. In adulthood, treated mice showed increased hippocampal Ih current and seizure susceptibility, characteristics also seen after FS in rats. Of the seven genetically diverse mouse strains screened for FS susceptibility, C57BL/6J mice were among the most susceptible, whereas A/J mice were among the most resistant. Strains genetically similar to C57BL/6J also showed a susceptible phenotype. Our phenotypic data suggest that complex genetics underlie FS susceptibility and show that the C57BL/6J strain is highly susceptible to FS. As this strain has been described as resistant to convulsants, our data indicate that susceptibility genes for FS and convulsants are distinct. Insight into the mechanisms underlying seizure susceptibility and FS may help to identify markers for the early diagnosis of children at risk for complex FS and TLE and may provide new leads for treatment.

Serotonin hyperinnervation abolishes seizure susceptibility in Otx2 conditional mutant mice

The homeobox-containing transcription factor Otx2 is crucially involved in fate determination of midbrain neurons. Mutant mice, in which Otx2 was conditionally inactivated by a Cre recombinase expressed under the transcriptional control of the Engrailed1 (En1) gene (En1(cre/+); Otx2(flox/flox)), show a reduced number of dopaminergic neurons and an increased number of serotonergic neurons in the ventral midbrain. Despite these developmental anatomical alterations, En1(cre/+); Otx2(flox/flox) adult mice display normal motor function. Here, we further investigated the neurological consequences of Otx2 inactivation in adult En1(cre/+); Otx2(flox/flox) mice. Adult En1(cre/+); Otx2(flox/flox) mice showed increased serotonin (5-HT) levels in the pons, ventral midbrain, hippocampus (CA3 subfield), and cerebral cortex, as indicated by HPLC and immunohistochemistry. Conversely, SERT (5-HT transporter) levels were decreased in conditional mutant brains. As a consequence of this increased 5-HT hyperinnervation, En1(cre/+); Otx2(flox/flox) mice were resistant to generalized seizures induced by the glutamate agonist kainic acid (KA). Indeed, prolonged pretreatment of En1(cre/+); Otx2(flox/flox) mice with the 5-HT synthesis inhibitor para-chlorophenylalanine (pCPA) restored brain 5-HT content to control levels, fully reestablishing KA seizure susceptibility. Accordingly, c-fos mRNA induction after KA was restricted to the hippocampus in En1(cre/+); Otx2(flox/flox) mice, whereas a widespread c-fos mRNA labeling was observed throughout the brain of En1(cre/+); Otx2(flox/flox) mice pretreated with pCPA. These results clearly show that increased brain 5-HT levels are responsible for seizure resistance in En1(cre/+); Otx2(flox/flox) mice and confirm the important role of 5-HT in the control of seizure spread.

Chemoconvulsant-induced seizure susceptibility: toward a common genetic basis?

Despite the efforts employed, understanding the genetic architecture underlying epilepsy remains difficult. To reach this aim, convulsive epilepsies are classically modeled in mice, where genetic studies are less constricting than in humans. Pharmacogenetic approaches are one major source of investigation where kainic acid, pentylenetetrazol, and the ss-carboline family represent compounds that are used extensively. Several quantitative trait loci (QTLs) influencing the convulsant effects of these drugs have been mapped using either recombinant inbred strains (RIS) or segregating F2 populations (or both). In our laboratory, we have recently mapped two QTLs for methyl 6, 7-dimethoxy-4-ethyl-ss-carboline-3-carboxylate (DMCM), and seizure response using an F2 method. One is located on the distal part of Chromosome 1, a region implicated in a number of other studies. Here, we address the general importance of this chromosomal fragment for influencing seizure susceptibility.

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.

Differences in two mice strains on kainic acid-induced amygdalar seizures

Seizure susceptibility and neurodegeneration induced by microinjection of kainic acid (KA) into the left amygdala (LA) in C57BL/6 and FVB/N mouse strains were compared. Mice were administered 0.5 microg of KA into the LA. Electroencephalograms (EEGs) and behavior were monitored for 48 h after KA microinjection. Neuronal cell damages in the CA1 and CA3 regions of hippocampus were assessed at 2 and 48 h after KA microinjection. We demonstrated that the FVB/N mice showed longer duration of seizure activity than the C57BL/6 mice in EEGs and behavioral observation. The C57BL/6 mice were resistant to neurodegeneration while the FVB/N mice were vulnerable as measured 2 and 48 h after KA microinjection. These differences might be associated with genetic background.

Use of chromosome substitution strains to identify seizure susceptibility loci in mice

Seizure susceptibility varies among inbred mouse strains. Chromosome substitution strains (CSS), in which a single chromosome from one inbred strain (donor) has been transferred onto a second strain (host) by repeated backcrossing, may be used to identify quantitative trait loci (QTLs) that contribute to seizure susceptibility. QTLs for susceptibility to pilocarpine-induced seizures, a model of temporal lobe epilepsy, have not been reported, and CSS have not previously been used to localize seizure susceptibility genes. We report QTLs identified using a B6 (host) x A/J (donor) CSS panel to localize genes involved in susceptibility to pilocarpine-induced seizures. Three hundred fifty-five adult male CSS mice, 58 B6, and 39 A/J were tested for susceptibility to pilocarpine-induced seizures. Highest stage reached and latency to each stage were recorded for all mice. B6 mice were resistant to seizures and slower to reach stages compared to A/J mice. The CSS for Chromosomes 10 and 18 progressed to the most severe stages, diverging dramatically from the B6 phenotype. Latencies to stages were also significantly shorter for CSS10 and CSS18 mice. CSS mapping suggests seizure susceptibility loci on mouse Chromosomes 10 and 18. This approach provides a framework for identifying potentially novel homologous candidate genes for human temporal lobe epilepsy.

Inbred mouse strains as a tool to analyze hippocampal neuronal loss after brain injury: a stereological study

Traumatic brain injury (TBI) damages the hippocampus both in experimental animal models and in humans. In particular, the mechanical injury in combination with the genetic susceptibility to injury may result in neuronal loss from the hippocampus. This report explores the time-course of neuronal loss in the four primary subregions of the mouse hippocampus after a lateral fluid percussion injury (FPI) to the brain, and how subtle genetic differences between C57BL/6J and C57BL/10J mouse strains influence the extent and time course of neuronal loss. Using design-based stereological procedures, our results indicate negligible neuronal loss ipsilateral to the injury at 2 days postinjury in C57BL/6J mice, whereas a significant number (30-40%) of neurons are lost across all subregions of the hippocampus (dentate, hilus, area CA3, and area CA1) by 1 week, which does not appear to progress at 1 month, compared to sham. Additionally, neuronal counts after lateral FPI in a genetically similar, yet kainic acid-sensitive, mouse strain (C57BL/10J) showed no statistically significant differences in neuron number compared to the C57BL/6J strain in response to brain injury. Hippocampal neuronal loss after lateral FPI and its consequent circuit disruption may depend more on factors related to the mechanics and secondary consequences of the injury, as opposed to subtle genetic variations between inbred mouse strains. The loss of neurons appears to be restricted to the first week post-injury, and the remaining neurons may serve as a substrate for recovery.