Microarray profile of seizure damage-refractory hippocampal CA3 in a mouse model of epileptic preconditioning

Fundamentally different roles of neuronal TNF receptors in CNS pathology: TNFR1 and IKKβ promote microglial responses and tissue injury in demyelination while TNFR2 protects against excitotoxicity in mice

Background

During inflammatory demyelination, TNF receptor 1 (TNFR1) mediates detrimental proinflammatory effects of soluble TNF (solTNF), whereas TNFR2 mediates beneficial effects of transmembrane TNF (tmTNF) through oligodendroglia, microglia, and possibly other cell types. This model supports the use of selective inhibitors of solTNF/TNFR1 as anti-inflammatory drugs for central nervous system (CNS) diseases. A potential obstacle is the neuroprotective effect of solTNF pretreatment described in cultured neurons, but the relevance in vivo is unknown.

Methods

To address this question, we generated mice with neuron-specific depletion of TNFR1, TNFR2, or inhibitor of NF-κB kinase subunit β (IKKβ), a main downstream mediator of TNFR signaling, and applied experimental models of inflammatory demyelination and acute and preconditioning glutamate excitotoxicity. We also investigated the molecular and cellular requirements of solTNF neuroprotection by generating astrocyte-neuron co-cultures with different combinations of wild-type (WT) and TNF and TNFR knockout cells and measuring N-methyl-d-aspartate (NMDA) excitotoxicity in vitro.

Results

Neither neuronal TNFR1 nor TNFR2 protected mice during inflammatory demyelination. In fact, both neuronal TNFR1 and neuronal IKKβ promoted microglial responses and tissue injury, and TNFR1 was further required for oligodendrocyte loss and axonal damage in cuprizone-induced demyelination. In contrast, neuronal TNFR2 increased preconditioning protection in a kainic acid (KA) excitotoxicity model in mice and limited hippocampal neuron death. The protective effects of neuronal TNFR2 observed in vivo were further investigated in vitro. As previously described, pretreatment of astrocyte-neuron co-cultures with solTNF (and therefore TNFR1) protected them against NMDA excitotoxicity. However, protection was dependent on astrocyte, not neuronal TNFR1, on astrocyte tmTNF-neuronal TNFR2 interactions, and was reproduced by a TNFR2 agonist.

Conclusions

These results demonstrate that neuronal TNF receptors perform fundamentally different roles in CNS pathology in vivo, with neuronal TNFR1 and IKKβ promoting microglial inflammation and neurotoxicity in demyelination, and neuronal TNFR2 mediating neuroprotection in excitotoxicity. They also reveal that previously described neuroprotective effects of solTNF against glutamate excitotoxicity in vitro are indirect and mediated via astrocyte tmTNF-neuron TNFR2 interactions. These results consolidate the concept that selective inhibition of solTNF/TNFR1 with maintenance of TNFR2 function would have combined anti-inflammatory and neuroprotective properties required for safe treatment of CNS diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02200-4.

Involvement of monocarboxylate transporters in the cross-tolerance between epilepsy and cerebral infarction: A promising choice towards new treatments

Noxious stimuli applied at doses close to but below the threshold of cell injury induce adaptative responses that provide a defense against additional stress from the same (tolerance) or other (cross-tolerance) stimuli. Such endogenous modulators mediate the tolerance induced by numerous sublethal physical and chemical stress factors, of which epileptic preconditioning (EPC) and mild global ischemia are two most important mutually protective actions. However, the evidence for the complicated underlying mechanisms involved in this neuroprotective effects are lacking. During hypoxia/ischemia (H/I) and intense neural activity, lactate released from astrocytes is taken up by neurons and is stored for energy, a process mediated by monocarboxylate transporters (MCTs) in central nervous system (CNS). The present study investigated whether ischemic preconditioning (IPC) or EPC can provide protection to CNS against epilepsy or cerebral infarction respectively through regulation of MCTs expression in vivo. Rats were subjected to transitory middle cerebral artery occlusion (MCAO) or kainic acid (KA) preconditioning protocol respectively, followed by KA induced epilepsy or lethal MCAO as well as lactate transportation inhibitor injection, with a subsequent evaluation of behavior and infarct volume as well as MCTs expression in rats brain. IPC reduced the severity of status epilepticus induced by KA injection and EPC reduce infarct volume resulted from lethal MCAO. However, lactate transport blocking attenuated this neuroprotective effect and MCTs expression followed the same variation trends. These findings demonstrate that MCTs dependent mechanism is involved in the cross-tolerance between epilepsy and cerebral infarction, provide a potential basis for the clinical treatment of patients with brain diseases characterized by epilepsy and H/I.

Exposing immature hippocampal neurons to excitotoxins reveals distinct transcriptome and protein regulation with induction of common survival signaling pathways

Early life traumas lead to neuroprotection by preconditioning mechanisms. To determine which genes and pathways are most likely involved in specific adaptive effects, immature hippocampal cultures were exposed to a single high dose of glutamate (250 μM), NMDA (100 μM), or KA (300 μM) for 48 h (5-7 DIV) based on our prior "two hit" in vitro model of preconditioning. Transcriptome profiling and immunocytochemistry of gene candidates were performed 7 days later when cultured neurons mature (14 DIV). Many genes were up- and down- regulated involving distinct Ca²⁺-binding protein families, G-coupled proteins, various growth factors, synaptic vesicle docking factors, certain neurotransmitter receptors, heat shock, oxidative stress, and certain anti-apoptotic Bcl-2 gene members that influence neuronal survival. Immunohistochemistry showed a marked decrease in the number of Calb1 and Calm2 positive neurons following NMDA but not after glutamate exposure whereas ryanodine and Cav1.2 voltage gated channel expression was less affected. Survivors had marked increases in Calm2 immunostaining; however, high-density neural clusters observed in controls, were depleted after NMDA and partly diminished after glutamate. While NR1 mRNA expression was decreased in the microarray, specific antibodies revealed selective loss of the NR1C1 splice variant. Calm2 which can inactivate NMDA receptors by binding to C1 but not C2 regions of its NR1 subunit suggests that loss of the C1 splice variant will reduce co-regulation with Calm2 and alter NR1 trafficking, phosphorylation, and NMDA currents following early life NMDA exposure. A dramatic reduction in the density of GABAAα5 and GABAB receptor expressing neurons was observed after NMDA exposure but immunodensity measurements were unchanged as was the expression of the GABA synthesizing enzyme, GAD, suggesting that fast inhibitory neurotransmission and response to benzodiazepines and GABAB-mediated IPSPs may be preserved in matured survivors. Selective upregulation of Chat and CNRIP was detected after glutamate treatment suggesting this condition would decrease cholinergic and excitatory neurotransmission by decreasing Ach content and CB1 interacting protein function. This decrease likely contributes to memory and attention tasks deficits that follow a single early neurological insult. Diverse changes that follow overactivation of excitatory networks of immature neurons appear long-lasting or permanent and are expected to have profound effects on network function and adaptive responses to further insult.

Bi-directional genetic modulation of GSK-3β exacerbates hippocampal neuropathology in experimental status epilepticus

Glycogen synthase kinase-3 (GSK-3) is ubiquitously expressed throughout the brain and involved in vital molecular pathways such as cell survival and synaptic reorganization and has emerged as a potential drug target for brain diseases. A causal role for GSK-3, in particular the brain-enriched GSK-3β isoform, has been demonstrated in neurodegenerative diseases such as Alzheimer’s and Huntington’s, and in psychiatric diseases. Recent studies have also linked GSK-3 dysregulation to neuropathological outcomes in epilepsy. To date, however, there has been no genetic evidence for the involvement of GSK-3 in seizure-induced pathology. Status epilepticus (prolonged, damaging seizure) was induced via a microinjection of kainic acid into the amygdala of mice. Studies were conducted using two transgenic mouse lines: a neuron-specific GSK-3β overexpression and a neuron-specific dominant-negative GSK-3β (GSK-3β-DN) expression in order to determine the effects of increased or decreased GSK-3β activity, respectively, on seizures and attendant pathological changes in the hippocampus. GSK-3 inhibitors were also employed to support the genetic approach. Status epilepticus resulted in a spatiotemporal regulation of GSK-3 expression and activity in the hippocampus, with decreased GSK-3 activity evident in non-damaged hippocampal areas. Consistent with this, overexpression of GSK-3β exacerbated status epilepticus-induced neurodegeneration in mice. Surprisingly, decreasing GSK-3 activity, either via overexpression of GSK-3β-DN or through the use of specific GSK-3 inhibitors, also exacerbated hippocampal damage and increased seizure severity during status epilepticus. In conclusion, our results demonstrate that the brain has limited tolerance for modulation of GSK-3 activity in the setting of epileptic brain injury. These findings caution against targeting GSK-3 as a treatment strategy for epilepsy or other neurologic disorders where neuronal hyperexcitability is an underlying pathomechanism.

Induction of Type 2 Iodothyronine Deiodinase After Status Epilepticus Modifies Hippocampal Gene Expression in Male Mice

Status epilepticus (SE) is an abnormally prolonged seizure that results from either a failure of mechanisms that terminate seizures or from initiating mechanisms that inherently lead to prolonged seizures. Here we report that mice experiencing a 3 hours of SE caused by pilocarpine exhibit a rapid increase in expression of type 2 iodothyronine deiodinase gene (Dio2) and a decrease in the expression of type 3 iodothyronine deiodinase gene in hippocampus, amygdala and prefrontal cortex. Type 3 iodothyronine deiodinase in hippocampal sections was seen concentrated in the neuronal nuclei, typical of ischemic injury of the brain. An unbiased analysis of the hippocampal transcriptome of mice undergoing 3 hours of SE revealed a number of genes, including those involved with response to oxidative stress, cellular homeostasis, cell signaling, and mitochondrial structure. In contrast, in mice with targeted disruption of Dio2 in astrocytes (Astro D2KO mouse), the highly induced genes in the hippocampus were related to inflammation, apoptosis, and cell death. We propose that Dio2 induction caused by SE accelerates production of T3 in different areas of the central nervous system and modifies the hippocampal gene expression profile, affecting the balance between adaptive and maladaptive mechanisms.

Proteomic Analysis After Status Epilepticus Identifies UCHL1 as Protective Against Hippocampal Injury

Brief, non-harmful seizures (preconditioning) can temporarily protect the brain against prolonged, otherwise injurious seizures. Following focal-onset status epilepticus (SE) in preconditioned (tolerance) and sham-preconditioned (injury) mice, we screened for protein changes using a proteomic approach and identified several putative candidates of epileptic tolerance. Among SE-induced changes to both proteomic screens, proteins clustered in key regulatory pathways, including protein trafficking and cytoskeletal regulation. Downregulation of one such protein, ubiquitin carboxyl-terminal hydrolase isozyme L1 (UCHL1), was unique to injury and not evident in tolerance. UCHL1 inhibition decreased hippocampal ubiquitin, disrupted UPS function, interfered with seizure termination and exacerbated seizure-induced cell death. Though UCHL1 transcription was maintained after SE, we observed downregulation of the pro-translational antisense Uchl1 (AsUchl1) and confirmed that both AsUchl1 and rapamycin can increase UCHL1 expression in vivo. These data indicate that the post-transcriptional loss of UCHL1 following SE is deleterious to neuronal survival and may contribute to hyperexcitability, and are suggestive of a novel modality of rapamycin therapy.

Spatiotemporal progression of ubiquitin-proteasome system inhibition after status epilepticus suggests protective adaptation against hippocampal injury

BACKGROUND

The ubiquitin-proteasome-system (UPS) is the major intracellular pathway leading to the degradation of unwanted and/or misfolded soluble proteins. This includes proteins regulating cellular survival, synaptic plasticity and neurotransmitter signaling; processes controlling excitability thresholds that are altered by epileptogenic insults. Dysfunction of the UPS has been reported to occur in a brain region- and cell-specific manner and contribute to disease progression in acute and chronic brain diseases. Prolonged seizures, status epilepticus, may alter UPS function but there has been no systematic attempt to map when and where this occurs in vivo or to determine the consequences of proteasome inhibition on seizure-induced brain injury.

METHOD

To determine whether seizures lead to an impairment of the UPS, we used a mouse model of status epilepticus whereby seizures are triggered by an intra-amygdala injection of kainic acid. Status epilepticus in this model causes cell death in selected brain areas, in particular the ipsilateral CA3 subfield of the hippocampus, and the development of epilepsy after a short latent period. To monitor seizure-induced dysfunction of the UPS we used a UPS inhibition reporter mouse expressing the ubiquitin fusion degradation substrate ubiquitin^G76V-green fluorescent protein. Treatment with the specific proteasome inhibitor epoxomicin was used to establish the impact of proteasome inhibition on seizure-induced pathology.

RESULTS AND CONCLUSIONS

Our studies show that status epilepticus induced by intra-amygdala kainic acid causes select spatio-temporal UPS inhibition which is most evident in damage-resistant regions of the hippocampus, including CA1 pyramidal and dentate granule neurons then appears later in astrocytes. In support of this exerting a beneficial effect, injection of mice with the proteasome inhibitor epoxomicin protected the normally vulnerable hippocampal CA3 subfield from seizure-induced neuronal death in the model. These studies reveal brain region- and cell-specific UPS impairment occurs after seizures and suggest UPS inhibition can protect against seizure-induced brain damage. Identifying networks or pathways regulated through the proteasome after seizures may yield novel target genes for the treatment of seizure-induced cell death and possibly epilepsy.

A calcium-sensitive feed-forward loop regulating the expression of the ATP-gated purinergic P2X7 receptor via specificity protein 1 and microRNA-22

Cells have developed complex transcriptional regulatory mechanisms to maintain intracellular homeostasis and withstand pathophysiological stressors. Feed-forward loops comprising transcription factors that drive expression of both target gene and a microRNA as negative regulator, are gaining increasing recognition as key regulatory elements of cellular homeostasis. The ATP-gated purinergic P2X7 receptor (P2X7R) is an important driver of inflammation and has been implicated in the pathogenesis of numerous brain diseases including epilepsy. Changes in P2X7R expression have been reported in both experimental models and in epilepsy patients but the mechanism(s) controlling P2X7R levels remain incompletely understood. The specificity protein 1 (Sp1) has been shown to induce P2X7R transcription in vitro and recent data has identified microRNA-22 as a post-transcriptional repressor of P2X7R expression after seizures. In the present study we show that Sp1 can induce the transcription of both microRNA-22 and P2X7R in vitro during increased neuronal activity and in vivo in a mouse model of status epilepticus. We further show that Sp1-driven microRNA-22 transcription is calcium-sensitive and Sp1 occupancy of the microRNA-22 promoter region is blocked under conditions of seizure activity sufficient to elicit neuronal death. Taken together, our results suggest a neuronal activity-dependent P2X7R expression which is induced by the transcription factor Sp1 and repressed in a calcium-dependent manner by microRNA-22.

Brief low [Mg(2+)]o-induced Ca(2+) spikes inhibit subsequent prolonged exposure-induced excitotoxicity in cultured rat hippocampal neurons

Reducing [Mg²⁺]_o to 0.1 mM can evoke repetitive [Ca²⁺]_i spikes and seizure activity, which induces neuronal cell death in a process called excitotoxicity. We examined the issue of whether cultured rat hippocampal neurons preconditioned by a brief exposure to 0.1 mM [Mg²⁺]_o are rendered resistant to excitotoxicity induced by a subsequent prolonged exposure and whether Ca²⁺ spikes are involved in this process. Preconditioning by an exposure to 0.1 mM [Mg²⁺]_o for 5 min inhibited significantly subsequent 24 h exposure-induced cell death 24 h later (tolerance). Such tolerance was prevented by both the NMDA receptor antagonist D-AP5 and the L-type Ca²⁺ channel antagonist nimodipine, which blocked 0.1 mM [Mg²⁺]_o-induced [Ca²⁺]_i spikes. The AMPA receptor antagonist NBQX significantly inhibited both the tolerance and the [Ca²⁺]_i spikes. The intracellular Ca²⁺ chelator BAPTA-AM significantly prevented the tolerance. The nonspecific PKC inhibitor staurosporin inhibited the tolerance without affecting the [Ca²⁺]_i spikes. While Gö6976, a specific inhibitor of PKCα had no effect on the tolerance, both the PKCε translocation inhibitor and the PKCζ pseudosubstrate inhibitor significantly inhibited the tolerance without affecting the [Ca²⁺]_i spikes. Furthermore, JAK-2 inhibitor AG490, MAPK kinase inhibitor PD98059, and CaMKII inhibitor KN-62 inhibited the tolerance, but PI-3 kinase inhibitor LY294,002 did not. The protein synthesis inhibitor cycloheximide significantly inhibited the tolerance. Collectively, these results suggest that low [Mg²⁺]_o preconditioning induced excitotoxic tolerance was directly or indirectly mediated through the [Ca²⁺]_i spike-induced activation of PKCε and PKCξ, JAK-2, MAPK kinase, CaMKII and the de novo synthesis of proteins.

Dynamic changes in DNA methylation in ischemic tolerance

Epigenetic mediators of gene expression are hypothesized to regulate transcriptomic responses to preconditioning ischemia and ischemic tolerance. Here, we utilized a methyl-DNA enrichment protocol and sequencing (ChIP-seq) to identify patterns of DNA methylation in an established model of ischemic tolerance in neuronal cultures (oxygen and glucose deprivation: OGD). We observed an overall decrease in global DNA methylation at 4 h following preconditioning ischemia (30 min OGD), harmful ischemia (120 min OGD), and in ischemic tolerant neuronal cultures (30 min OGD, 24 h recovery, 120 min OGD). We detected a smaller cohort of hypermethylated regions following ischemic conditions, which were further analyzed revealing differential chromosomal localization of methylation, and a differential concentration of methylation on genomic regions. Together, these data show that the temporal profiles of DNA methylation with respect to chromatin hyper- and hypo-methylation following various ischemic conditions are highly dynamic, and may reveal novel targets for neuroprotection.

Role of MicroRNAs in innate neuroprotection mechanisms due to preconditioning of the brain

Insults to the brain that are sub-threshold for damage activate endogenous protective pathways, which can temporarily protect the brain against a subsequent harmful episode. This mechanism has been named as tolerance and its protective effects have been shown in experimental models of ischemia and epilepsy. The preconditioning-stimulus can be a short period of ischemia or mild seizures induced by low doses of convulsant drugs. Gene-array profiling has shown that both ischemic and epileptic tolerance feature large-scale gene down-regulation but the mechanism are unknown. MicroRNAs are a class of small non-coding RNAs of ~20-22 nucleotides length which regulate gene expression at a post-transcriptional level via mRNA degradation or inhibition of protein translation. MicroRNAs have been shown to be regulated after non-harmful and harmful stimuli in the brain and to contribute to neuroprotective mechanisms. This review focuses on the role of microRNAs in the development of tolerance following ischemic or epileptic preconditioning.

De-repression of myelin-regulating gene expression after status epilepticus in mice lacking the C/EBP homologous protein CHOP

The C/EBP homologous protein CHOP is normally present at low levels in cells but increases rapidly after insults such as DNA damage or endoplasmatic reticulum stress where it contributes to cellular homeostasis and apoptosis. By forming heterodimers with other transcription factors, CHOP can either act as a dominant-negative regulator of gene expression or to induce the expression of target genes. Recent work demonstrated that seizure-induced hippocampal damage is significantly worse in mice lacking CHOP and these animals go on to develop an aggravated epileptic phenotype. To identify novel CHOP-controlled target genes which potentially influence the epileptic phenotype, we performed a bioinformatics analysis of tissue microarrays from chop-deficient mice after prolonged seizures. GO analysis revealed genes associated with biological membranes were prominent among those in the chop-deficient array dataset and we identified myelin-associated genes to be particularly de-repressed. These data suggest CHOP might act as an inhibitor of myelin-associated processes in the brain and could be targeted to influence axonal regeneration or reorganisation.

Early-life seizures in predisposing neuronal preconditioning: a critical review

Although seizures are known to be harmful, recent evidence indicates that they can also lead to adaptations that protect neurons from further insult. For example, a history of two episodes of status epilepticus during a critical period of early development can prolong the time period of resistance to hippocampal injury during the postnatal period. Neonatal seizures may lead to this neuroprotection via a preconditioning mechanism that could be attributed to attenuation of Ca(2+) currents, reduction of inflammation, and induction of survival signaling pathways. Understanding mechanisms underlying neuroprotective preconditioning may elucidate new therapeutic targets and improve outcomes and quality of life for pediatric epilepsy patients. This review will detail the specific cellular and molecular findings involved in neuronal preconditioning predisposed by early-life seizures.

Transcriptome profiling of hippocampal CA1 after early-life seizure-induced preconditioning may elucidate new genetic therapies for epilepsy

Injury of the CA1 subregion induced by a single injection of kainic acid (1 × KA) in juvenile animals (P20) is attenuated in animals with two prior sustained neonatal seizures on P6 and P9. To identify gene candidates involved in the spatially protective effects produced by early-life conditioning seizures we profiled and compared the transcriptomes of CA1 subregions from control, 1 × KA- and 3 × KA-treated animals. More genes were regulated following 3 × KA (9.6%) than after 1 × KA (7.1%). Following 1 × KA, genes supporting oxidative stress, growth, development, inflammation and neurotransmission were upregulated (e.g. Cacng1, Nadsyn1, Kcng1, Aven, S100a4, GFAP, Vim, Hrsp12 and Grik1). After 3 × KA, protective genes were differentially over-expressed [e.g. Cat, Gpx7, Gad1, Hspa12A, Foxn1, adenosine A1 receptor, Ca(2+) adaptor and homeostasis proteins, Cacnb4, Atp2b2, anti-apoptotic Bcl-2 gene members, intracellular trafficking protein, Grasp and suppressor of cytokine signaling (Socs3)]. Distinct anti-inflammatory interleukins (ILs) not observed in adult tissues [e.g. IL-6 transducer, IL-23 and IL-33 or their receptors (IL-F2 )] were also over-expressed. Several transcripts were validated by real-time polymerase chain reaction (QPCR) and immunohistochemistry. QPCR showed that casp 6 was increased after 1 × KA but reduced after 3 × KA; the pro-inflammatory gene Cox1 was either upregulated or unchanged after 1 × KA but reduced by ~70% after 3 × KA. Enhanced GFAP immunostaining following 1 × KA was selectively attenuated in the CA1 subregion after 3 × KA. The observed differential transcriptional responses may contribute to early-life seizure-induced pre-conditioning and neuroprotection by reducing glutamate receptor-mediated Ca(2+) permeability of the hippocampus and redirecting inflammatory and apoptotic pathways. These changes could lead to new genetic therapies for epilepsy.

CHOP regulates the p53-MDM2 axis and is required for neuronal survival after seizures

Hippocampal sclerosis is a frequent pathological finding in patients with temporal lobe epilepsy and can be caused by prolonged single or repeated brief seizures. Both DNA damage and endoplasmic reticulum stress have been implicated as underlying molecular mechanisms in seizure-induced brain injury. The CCAAT/enhancer-binding protein homologous protein (CHOP) is a transcriptional regulator induced downstream of DNA damage and endoplasmic reticulum stress, which can promote or inhibit apoptosis according to context. Recent work has proposed inhibition of CHOP as a suitable neuroprotective strategy. Here, we show that transcript and protein levels of CHOP increase in surviving subfields of the hippocampus after prolonged seizures (status epilepticus) in mouse models. CHOP was also elevated in the hippocampus from epileptic mice and patients with pharmacoresistant epilepsy. The hippocampus of CHOP-deficient mice was much more vulnerable to damage in mouse models of status epilepticus. Moreover, compared with wild-type animals, CHOP-deficient mice subject to status epilepticus developed more spontaneous seizures, displayed protracted hippocampal neurodegeneration and a deficit in a hippocampus-dependent object-place recognition task. The absence of CHOP was associated with a supra-maximal induction of p53 after status epilepticus, and inhibition of p53 abolished the cell death-promoting consequences of CHOP deficiency. The protective effect of CHOP could be partly explained by activating transcription of murine double minute 2 that targets p53 for degradation. These data demonstrate that CHOP is required for neuronal survival after seizures and caution against inhibition of CHOP as a neuroprotective strategy where excitotoxicity is an underlying pathomechanism.

Tolerance to ischemia - an increasingly complex biology

In this review we identify and discuss some of the genomics studies of preconditioning and the ischemic tolerance phenomenon. Such studies have been attempted in multiple species, using different array technologies and with different preconditioning and tolerance models. In addition, studies are starting to reveal epigenetic mechanisms and modifiers of tolerance and preconditioning. Together these studies are starting to reveal some of the immense complexity of the ischemic tolerance phenomenon, yet further studies await to be performed.

Investigating gene promoter methylation in a mouse model of status epilepticus

Epigenetic modification of DNA by methylation of the cytosine present in CG dinucleotides constitutes a key regulatory mechanism in the control of gene expression in neurological diseases. In this chapter, we describe an in-depth methodology of methylated DNA immunoprecipitation used in combination with tiling microarrays (MeDIP-chip) in order to analyze genome-wide gene promoter methylation in the hippocampus of mice following status epilepticus (prolonged seizure). While a specific mouse model and array format are described, the method can be applied to DNA from many tissues to analyze the methylation status of promoter regions across whole genomes, using a wide range of available array formats (both custom designed and commercially catalogued). We conclude the chapter with the description of bisulfite sequencing validation of MeDIP-chip results.

NMDA preconditioning attenuates cortical and hippocampal seizures induced by intracerebroventricular quinolinic acid infusion

Searching for new therapeutic strategies through modulation of glutamatergic transmission using effective neuroprotective agents is essential. Glutamatergic excitotoxicity is a common factor to neurodegenerative diseases and acute events such as cerebral ischemia, traumatic brain injury, and epilepsy. This study aimed to evaluate behavioral and electroencephalographic (EEG) responses of mice cerebral cortex and hippocampus to subconvulsant and convulsant application of NMDA and quinolinic acid (QA), respectively. Moreover, it aimed to evaluate if EEG responses may be related to the neuroprotective effects of NMDA. Mice were preconditioned with NMDA (75 mg/kg, i.p.) and EEG recordings were performed for 30 min. One day later, QA was injected (36.8 nmol/site) and EEG recordings were performed during 10 min. EEG analysis demonstrated NMDA preconditioning promotes spike-wave discharges (SWDs), but it does not display behavioral manifestation of seizures. Animals that were protected by NMDA preconditioning against QA-induced behavioral seizures, presented higher number of SWD after NMDA administration, in comparison to animals preconditioned with NMDA that did display behavioral seizures after QA infusion. No differences were observed in latency for the first seizure or duration of seizures. EEG recordings after QA infusion demonstrated there were no differences in the number of SWD, latency for the first seizure or duration of seizures in animals pretreated with saline or in animals preconditioned by NMDA that received QA. A negative correlation was identified between the number of NMDA-induced SWD and QA-induced seizures severity. These results suggest a higher activation during NMDA preconditioning diminishes mice probability to display behavioral seizures after QA infusion.

Expression profiling the microRNA response to epileptic preconditioning identifies miR-184 as a modulator of seizure-induced neuronal death

Brief seizures (epileptic/seizure preconditioning) are capable of activating endogenous protective pathways in the brain which can temporarily generate a damage-refractory state against subsequent and otherwise harmful episodes of prolonged seizures (tolerance). Altered expression of microRNAs, a class of non-coding RNAs that function post-transcriptionally to regulate mRNA translation has recently been implicated in the molecular mechanism of epileptic tolerance. Here we characterized the effect of seizure preconditioning induced by low-dose systemic kainic acid on microRNA expression in the hippocampus of mice. Seizure preconditioning resulted in up-regulation of 25 mature microRNAs in the CA3 subfield of the mouse hippocampus, with the highest levels detected for miR-184. This finding was supported by real time PCR and in situ hybridization showing increased neuronal miR-184 levels and a reduction in protein levels of a miR-184 target. Inhibiting miR-184 expression in vivo resulted in the emergence of neuronal death after preconditioning seizures and increased seizure-induced neuronal death following status epilepticus in previously preconditioned animals, without altered electrographic seizure durations. The present study suggests miRNA up-regulation after preconditioning may contribute to development of epileptic tolerance and identifies miR-184 as a novel contributor to neuronal survival following both mild and severe seizures.

Pilocarpine-induced status epilepticus increases Homer1a and changes mGluR5 expression

Homer1a regulates expression of group I metabotropic glutamate receptors type I (mGluR1 and mGluR5) and is involved in neuronal plasticity. It has been reported that Homer1a expression is upregulated in the kindling model and hypothesized to act as an anticonvulsant. In the present work, we investigated whether pilocarpine-induced status epilepticus (SE) would alter Homer1a and mGluR5 expression in hippocampus. Adult rats were subjected to pilocarpine-model and analyzed at 2h, 8h, 24h and 7 d following SE. mRNA analysis showed the highest expression of Homer1a at 8h after SE onset, while immunohistochemistry demonstrated that Homer1a protein expression was significantly increased in hippocampus, amygdala and piriform and entorhinal cortices at 24h after SE onset when compared to control animals. The increased Homer1a expression coincided with a significant decrease of mGluR5 protein expression in amygdala and piriform and entorhinal cortices. The data suggest that during the critical periods of epileptogenesis, overexpression of Homer1a occurs to counteract hyperexcitability and thus Homer1a may be a molecular target in the treatment of epilepsy.

Differential DNA methylation patterns define status epilepticus and epileptic tolerance

Prolonged seizures (status epilepticus) produce pathophysiological changes in the hippocampus that are associated with large-scale, wide-ranging changes in gene expression. Epileptic tolerance is an endogenous program of cell protection that can be activated in the brain by previous exposure to a non-harmful seizure episode before status epilepticus. A major transcriptional feature of tolerance is gene downregulation. Here, through methylation analysis of 34,143 discrete loci representing all annotated CpG islands and promoter regions in the mouse genome, we report the genome-wide DNA methylation changes in the hippocampus after status epilepticus and epileptic tolerance in adult mice. A total of 321 genes showed altered DNA methylation after status epilepticus alone or status epilepticus that followed seizure preconditioning, with >90% of the promoters of these genes undergoing hypomethylation. These profiles included genes not previously associated with epilepsy, such as the polycomb gene Phc2. Differential methylation events generally occurred throughout the genome without bias for a particular chromosomal region, with the exception of a small region of chromosome 4, which was significantly overrepresented with genes hypomethylated after status epilepticus. Surprisingly, only few genes displayed differential hypermethylation in epileptic tolerance. Nevertheless, gene ontology analysis emphasized the majority of differential methylation events between the groups occurred in genes associated with nuclear functions, such as DNA binding and transcriptional regulation. The present study reports select, genome-wide DNA methylation changes after status epilepticus and in epileptic tolerance, which may contribute to regulating the gene expression environment of the seizure-damaged hippocampus.

miRNA Expression profile after status epilepticus and hippocampal neuroprotection by targeting miR-132

When an otherwise harmful insult to the brain is preceded by a brief, noninjurious stimulus, the brain becomes tolerant, and the resulting damage is reduced. Epileptic tolerance develops when brief seizures precede an episode of prolonged seizures (status epilepticus). MicroRNAs (miRNAs) are small, noncoding RNAs that function as post-transcriptional regulators of gene expression. We investigated how prior seizure preconditioning affects the miRNA response to status epilepticus evoked by intra-amygdalar kainic acid in mice. The miRNA was extracted from the ipsilateral CA3 subfield 24 hours after focal-onset status epilepticus in animals that had previously received either seizure preconditioning (tolerance) or no preconditioning (injury), and mature miRNA levels were measured using TaqMan low-density arrays. Expression of 21 miRNAs was increased, relative to control, after status epilepticus alone, and expression of 12 miRNAs was decreased. Increased miR-132 levels were matched with increased binding to Argonaute-2, a constituent of the RNA-induced silencing complex. In tolerant animals, expression responses of >40% of the injury-group-detected miRNAs differed, being either unchanged relative to control or down-regulated, and this included miR-132. In vivo microinjection of locked nucleic acid-modified oligonucleotides (antagomirs) against miR-132 depleted hippocampal miR-132 levels and reduced seizure-induced neuronal death. Thus, our data strongly suggest that miRNAs are important regulators of seizure-induced neuronal death.

Suppression of TNF receptor-1 signaling in an in vitro model of epileptic tolerance

Tumor necrosis factor-α (TNFα) is a pleiotropic cytokine that can regulate cell survival, inflammation or, under certain circumstances, trigger cell death. Previous work in rat seizure models and analysis of temporal lobe samples from epilepsy patients has suggested seizures activate TNF receptor 1 (TNFR1). Here we explored the activation and functional significance of TNFR1 signaling in the mouse hippocampus using in vitro and in vivo models of seizure-induced neuronal injury. Focal-onset status epilepticus in mice upregulated TNFR1 levels and led to formation of TNFR1-TNFR-associated death domain (TRADD) and TRADD-Fas-associated death domain (FADD) binding. Seizure-like injury modeled in vitro by removal of chronic excitatory blockade in mouse hippocampal neurons also activated this TNFR1 signaling pathway. Prior exposure of hippocampal neurons to a non-harmful seizure episode, via NMDA receptor blockade, 24 h prior to injurious seizures significantly reduced cell death and modeled epileptic tolerance in vitro. TNFR1 complex formation with TRADD and TRADD-FADD binding were reduced in tolerant cells. Finally, TNFR1 signaling and cell death were reduced by PKF-242-484, a dual matrix metaloproteinase/TNFα converting enzyme inhibitor. The present study shows that TNFR1 signaling is activated in mouse seizure models and may contribute to neuropathology in vitro and in vivo while suppression of this pathway may underlie neuroprotection in epileptic tolerance.

Classic hippocampal sclerosis and hippocampal-onset epilepsy produced by a single "cryptic" episode of focal hippocampal excitation in awake rats

In refractory temporal lobe epilepsy, seizures often arise from a shrunken hippocampus exhibiting a pattern of selective neuron loss called "classic hippocampal sclerosis." No single experimental injury has reproduced this specific pathology, suggesting that hippocampal atrophy might be a progressive "endstage" pathology resulting from years of spontaneous seizures. We posed the alternative hypothesis that classic hippocampal sclerosis results from a single excitatory event that has never been successfully modeled experimentally because convulsive status epilepticus, the insult most commonly used to produce epileptogenic brain injury, is too severe and necessarily terminated before the hippocampus receives the needed duration of excitation. We tested this hypothesis by producing prolonged hippocampal excitation in awake rats without causing convulsive status epilepticus. Two daily 30-minute episodes of perforant pathway stimulation in Sprague-Dawley rats increased granule cell paired-pulse inhibition, decreased epileptiform afterdischarge durations during 8 hours of subsequent stimulation, and prevented convulsive status epilepticus. Similarly, one 8-hour episode of reduced-intensity stimulation in Long-Evans rats, which are relatively resistant to developing status epilepticus, produced hippocampal discharges without causing status epilepticus. Both paradigms immediately produced the extensive neuronal injury that defines classic hippocampal sclerosis, without giving any clinical indication during the insult that an injury was being inflicted. Spontaneous hippocampal-onset seizures began 16-25 days postinjury, before hippocampal atrophy developed, as demonstrated by sequential magnetic resonance imaging. These results indicate that classic hippocampal sclerosis is uniquely produced by a single episode of clinically "cryptic" excitation. Epileptogenic insults may often involve prolonged excitation that goes undetected at the time of injury.

Functional, metabolic, and synaptic changes after seizures as potential targets for antiepileptic therapy

Little is known about how the brain limits seizure duration and terminates seizures. Depending on severity and duration, a single seizure is followed by various functional, metabolic, and synaptic changes that may form targets for novel therapeutic strategies. It is long known that most seizures are followed by a period of postictal refractoriness during which the threshold for induction of additional seizures is increased. The endogenous anticonvulsant mechanisms involved in this phenomenon may be relevant for both spontaneous seizure arrest and increase of seizure threshold after seizure arrest. Postictal refractoriness has been extensively studied in various seizure and epilepsy models, including electrically and chemically induced seizures, kindling, and genetic animal models of epilepsy. During kindling development, two antagonistic processes occur simultaneously, one responsible for kindling-like events and the other for terminating ictus and postictal refractoriness. Frequently occurring seizures may lead to an accumulation of postictal refractoriness that may last weeks. The mechanisms involved in seizure termination and postictal refractoriness include changes in ionic microenvironment, in pH, and in various endogenous neuromodulators such as adenosine and neuropeptides. In animal models, the anticonvulsant efficacy of several antiepileptic drugs (AEDs) is increased during postictal refractoriness, which is a logical consequence of the interaction between endogenous anticonvulsant processes and the mechanism of AEDs. As discussed in this review, enhanced understanding of these endogenous processes may lead to novel targets for AED development.

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.

Deletion of Puma protects hippocampal neurons in a model of severe status epilepticus

Prolonged seizures (status epilepticus) can activate apoptosis-associated signaling pathways. The extent to which such pathways contribute to cell death might depend on the insult intensity, whereby the programmed or apoptotic cell death component is reduced when seizures are more severe or protracted. We recently showed that mice lacking the pro-apoptotic Bcl-2 homology domain 3-only protein Puma (Bbc3) were potently protected against damage caused by status epilepticus. In the present study we examined whether Puma deficiency was protective when the seizure episode was more severe. Intra-amygdala microinjection of 1 microg kainic acid (KA) into C57BL/6 mice triggered status epilepticus that lasted about twice as long as with 0.3 microg KA prior to lorazepam termination. Hippocampal damage was also significantly greater in the higher-dose group. Over 80% of degenerating neurons after seizures were positive for DNA fragmentation assessed by terminal deoxynucleotidyl dUTP nick end labeling (TUNEL). Microscopic analysis of neuronal nuclear morphology in TUNEL-positive cells revealed the proportion displaying large rounded clumps of condensed chromatin was approximately 50% lower in the high-dose versus low-dose KA group. Nevertheless, compared to heterozygous and wild-type mice subject to status epilepticus by high-dose KA, neuronal death was reduced by approximately 50% in the hippocampus of Puma-deficient mice. These data suggest aspects of the apoptotic component of seizure-induced neuronal death are insult duration- or severity-dependent. Moreover, they provide further genetic evidence that seizure-induced neuronal death is preventable by targeting so-called apoptosis-associated signaling pathways and Puma loss likely disrupts caspase-independent or non-apoptotic seizure-induced neuronal death.

Status epilepticus affects the gigantocellular network of the pontine reticular formation

BACKGROUND

The impairment of the pontine reticular formation (PRF) has recently been revealed to be histopathologically connected with focal-cortical seizure induced generalized convulsive status epilepticus. To elucidate whether the impairment of the PRF is a general phenomenon during status epilepticus, the focal-cortical 4-aminopyridine (4-AP) application was compared with other epilepsy models. The presence of "dark" neurons in the PRF was investigated by the sensitive silver method of Gallyas in rats sacrificed at 3 h after focal 4-AP crystal or systemic 4-AP, pilocarpine, or kainic acid application. The behavioral signs of the developing epileptic seizures were scored in all rats. The EEG activity was recorded in eight rats.

RESULTS

Regardless of the initiating drug or method of administration, "dark" neurons were consistently found in the PRF of animals entered the later phases of status epilepticus. EEG recordings demonstrated the presence of slow oscillations (1.5-2.5 Hz) simultaneously with the appearance of giant "dark" neurons in the PRF.

CONCLUSION

We argue that the observed slow oscillation corresponds to the late periodic epileptiform discharge phase of status epilepticus, and that the PRF may be involved in the progression of status epilepticus.

Seizure preconditioning and epileptic tolerance: models and mechanisms

One or more brief seizures can serve to activate endogenous protective programmes which render brain regions temporarily less susceptible to damage following an otherwise harmful episode of status epilepticus (a prolonged seizure). Epileptic tolerance has been demonstrated using a variety of seizure preconditioning paradigms, including electroconvulsive shocks and low doses of excitotoxins such as kainic acid. The cell and molecular mechanisms underlying the protection are not fully understood but proposed mediators include the transcription factor NfκB, altered ion channel expression, upregulation of growth factors and other protective genes, and suppression of pro-apoptotic Bcl-2 family proteins. Application of microarrays to profile the transcriptome of seizure-preconditioning and tolerance has provided further insights, including roles for chromatin remodeling and evidence that preconditioning generates an anti-excitotoxicity phenotype by reprogramming the transcriptional response to status epilepticus. This review summarizes the various animal models of epileptic tolerance, reviews the key effector(s) and the utility of this experimental paradigm for identifying novel targets for neuroprotection and anti-epileptogenesis.

Early life stress as an influence on limbic epilepsy: an hypothesis whose time has come?

The pathogenesis of mesial temporal lobe epilepsy (MTLE), the most prevalent form of refractory focal epilepsy in adults, is thought to begin in early life, even though seizures may not commence until adolescence or adulthood. Amongst the range of early life factors implicated in MTLE causation (febrile seizures, traumatic brain injury, etc.), stress may be one important contributor. Early life stress is an a priori agent deserving study because of the large amount of neuroscientific data showing enduring effects on structure and function in hippocampus and amygdala, the key structures involved in MTLE. An emerging body of evidence directly tests hypotheses concerning early life stress and limbic epilepsy: early life stressors, such as maternal separation, have been shown to aggravate epileptogenesis in both status epilepticus and kindling models of limbic epilepsy. In addition to elucidating its influence on limbic epileptogenesis itself, the study of early life stress has the potential to shed light on the psychiatric disorder that accompanies MTLE. For many years, psychiatric comorbidity was viewed as an effect of epilepsy, mediated psychologically and/or neurobiologically. An alternative – or complementary – perspective is that of shared causation. Early life stress, implicated in the pathogenesis of several psychiatric disorders, may be one such causal factor. This paper aims to critically review the body of experimental evidence linking early life stress and epilepsy; to discuss the direct studies examining early life stress effects in current models of limbic seizures/epilepsy; and to suggest priorities for future research.

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

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

Excitotoxic-mediated transcriptional decreases in HCN2 channel function increase network excitability in CA1

Changes in the conductance of the hyperpolarization-activated, cyclic nucleotide-gated (HCN) channel that mediates Ih are proposed to contribute to increased network excitability. Synchronous neuronal burst activity is a good reflection of network excitability and can be generated in isolated hippocampal slice cultures by removing Mg2+ from the extracellular fluid. We demonstrate that Ih contributes to this activity by increasing both the frequency and duration of bursting events. Changes in HCN channel function are also implicated in altered seizure susceptibility. Short-term application of kainic acid (KA) is known to initiate long lasting changes in neuronal networks that result in seizures, and in slice cultures was found to alter HCN mRNA levels in an isoform and hippocampal sub-region specific manner. These changes correlate with the ability of each sub-region to develop synchronous burst activity following KA that we have previously reported. Specifically, a loss of synchronous activity in the CA3 correlated with an increase in HCN2 mRNA levels that normalized concomitantly with the restoration of CA3 burst activity 7 days post insult. In contrast, in CA1 an increase in synchronous burst duration correlated with a reduction in HCN2 mRNA levels and both changes were still evident for 7 days post insult. Lamotrigine, known to increase Ih, reversed the impact of KA on burst duration in CA1 at both time-points linking a transcriptional reduction in HCN2 function to increased burst duration.

Electroencephalographic and behavioral convulsant effects of hydrobromide and hydrochloride salts of bupropion in conscious rodents

A novel bromide salt of the antidepressant bupropion (bupropion HBr) has recently been developed and approved for use in the United States. Given previous use of bromides to treat seizures, and that the existing chloride salt of bupropion (HCl) can cause seizures, it is important to determine if the HBr salt may be less likely to cause seizures than the HCl salt. In the present animal studies this was evaluated by means of quantified electroencephalogram (EEG), observation, and the rotarod test in mice and rats. Both bupropion salts were tested at increasing equimolar doses administered intraperitoneally. The results in mice showed that bupropion HCl 125 mg/kg induced a significantly higher ten-fold increase in the mean number of cortical EEG seizures compared to bupropion HBr (7.50 +/- 2.56 vs 0.75 +/- 0.96; p = 0.045), but neither drug caused any brain injuries. In rats bupropion HBr 100 mg/kg induced single EEG seizure activity in the cortical and hippocampal (depth) electrodes and in significantly (p < 0.05) fewer rats (44%) compared to bupropion HCl, which induced 1 to 4 convulsions per rat in all rats (100%) dosed. The total duration of cortical seizures in bupropion HCl-treated rats was significantly longer than the corresponding values obtained in bupropion HBr-treated rats (424.6 seconds vs 124.5 seconds respectively, p < 0.05). Bupropion HCl consistently induced more severe convulsions at each dose level compared to bupropion HBr. Both treatments demonstrated a similar dose-dependent impairment of rotarod performance in mice. In conclusion, these findings suggest that bupropion HBr may have a significantly lower potential to induce seizures in mice and rats, particularly at higher doses, compared to bupropion HCl. Determination of this potential clinical advantage will require human studies. If confirmed by such studies, it is likely that this potential beneficial clinical benefit would be due to the presence of the bromide salt given the long history of the use of bromide to treat seizure disorders.

Hippocampal transcriptome after status epilepticus in mice rendered seizure damage-tolerant by epileptic preconditioning features suppressed calcium and neuronal excitability pathways

Preconditioning brain with a sub-lethal stressor can temporarily generate a damage-refractory state. Microarray analyses have defined the changes in hippocampal gene expression that follow brief preconditioning seizures, but not the transcriptome after a prolonged and otherwise injurious seizure in previously preconditioned brain. Presently, microarray analysis was performed 24 h after status epilepticus in mice that had received previously either seizure preconditioning (tolerance) or sham-preconditioning (injury). Transcriptional changes in the hippocampal CA3 subfield of >or=2 fold were detected for 1357 genes in the tolerance group compared to a non-seizure control group, with 54% up-regulated. Of these regulated genes, 792 were also regulated in the injury group. Among the remaining 565 genes regulated only in tolerance, 73% were down-regulated. Analysis of the genes differentially suppressed in tolerance identified calcium signaling, ion channels and excitatory neurotransmitter receptors, and the synapse as over-represented among pathways, functions and compartments. Finally, 12 days continuous EEG recordings determined mice with induced tolerance had fewer spontaneous electrographic seizures compared to the injury group. Our data suggest the transcriptional phenotype of neuroprotection in tolerance may be dictated by the biology of the preconditioning stressor, functions by transcriptional reduction of vulnerability to excitotoxicity, and has anti-epileptogenic effects.

Differential changes in mGlu2 and mGlu3 gene expression following pilocarpine-induced status epilepticus: a comparative real-time PCR analysis

Group II metabotropic glutamate (mGlu II) receptors subtype 2 and 3 (mGlu2 and mGlu3) are subtle regulators of neuronal excitability and synaptic plasticity in the hippocampus. In recent years, researchers have investigated the potential neuroprotective and anticonvulsant effects of compounds acting on mGlu II receptors. However, abnormal expression and function of mGlu2 and mGlu3 have been reported in temporal lobe epilepsy, a phenomena that may limit the therapeutic effectiveness of these potentially new antiepileptic drugs. Here, we investigated seizure-induced changes in mGlu2 and mGlu3 mRNA following pilocarpine-inducted status epilepticus (SE) and subsequent epileptogenesis. Relative changes in gene expression were assessed by comparative analysis of quantitative real-time PCR (qrtPCR) by the delta-delta CT method. Pilocarpine-treated and control rats were sacrificed at different periods (24 h, 10 days, one month and more than two months) following SE. Total RNA was isolated from microdissected dentate gyrus and processed for RT-PCR and qrtPCR using glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as an endogenous control gene. Analysis of relative quantification (RQ) ratios of mGlu2 and mGlu3 mRNA expression revealed a significant down-regulation of both targets at 24 h after SE. Gene expression partially recovered at 10 days following SE reaching control levels at one month after SE. Two month after SE, mGlu2 mRNA expression was significantly down-regulated to approximately 41% of control expression whereas mGlu3 mRNA was comparable to control levels. Our data indicate that mGlu2 and mGlu3 expression is dynamically down-regulated or selectively enhanced during critical periods of epileptogenesis. Seizure-induced differential dysregulation of mGlu2 and mGlu3 receptors may affect the availability of these molecular targets for therapeutic compounds in epilepsy.