Mitogen-activated protein kinase is increased in the limbic structures of the rat brain during the early stages of status epilepticus

The lncRNA H19 binding to let-7b promotes hippocampal glial cell activation and epileptic seizures by targeting Stat3 in a rat model of temporal lobe epilepsy

Objectives

Glial cell activation contributes to the inflammatory response and occurrence of epilepsy. Our preliminary study demonstrated that the long non‐coding RNA, H19, promotes hippocampal glial cell activation during epileptogenesis. However, the precise mechanisms underlying this effect remain unclear.

Materials and methods

H19 and let‐7b were overexpressed or silenced using an adeno‐associated viral vector in vivo. Their expression in a kainic acid‐induced epilepsy model was evaluated by real‐time quantitative PCR, fluorescence in situ hybridization, and cytoplasmic and nuclear RNA isolation. A dual‐luciferase reporter assay was used to evaluate the direct binding of let‐7b to its target genes and H19. Western blot, video camera monitoring and Morris water maze were performed to confirm the role of H19 and let7b on epileptogenesis.

Results

H19 was increased in rat hippocampus neurons after status epilepticus, which might be due to epileptic seizure‐induced hypoxia. Increased H19 aggravated the epileptic seizures, memory impairment and mossy fibre sprouting of the epileptic rats. H19 could competitively bind to let‐7b to suppress its expression. Overexpression of let‐7b inhibited hippocampal glial cell activation, inflammatory response and epileptic seizures by targeting Stat3. Moreover, overexpressed H19 reversed the inhibitory effect of let‐7b on glial cell activation.

Conclusions

LncRNA H19 could competitively bind to let‐7b to promote hippocampal glial cell activation and epileptic seizures by targeting Stat3 in a rat model of temporal lobe epilepsy.

Neurosteroid regulation of GABAA receptors: A role in catamenial epilepsy

The female reproductive hormones progesterone and estrogen regulate network excitability. Fluctuations in the circulating levels of these hormones during the menstrual cycle cause frequent seizures during certain phases of the cycle in women with epilepsy. This seizure exacerbation, called catamenial epilepsy, is a dominant form of drug-refractory epilepsy in women of reproductive age. Progesterone, through its neurosteroid derivative allopregnanolone, increases γ-aminobutyric acid type-A receptor (GABAR)-mediated inhibition in the brain and keeps seizures under control. Catamenial seizures are believed to be a neurosteroid withdrawal symptom, and it was hypothesized that exogenous administration of progesterone to maintain its levels high during luteal phase will treat catamenial seizures. However, in a multicenter, double-blind, phase III clinical trial, progesterone treatment did not suppress catamenial seizures. The expression of GABARs with reduced neurosteroid sensitivity in epileptic animals may explain the failure of the progesterone clinical trial. The expression of neurosteroid-sensitive δ subunit-containing GABARs is reduced, and the expression of α4γ2 subunit-containing GABARs is upregulated, which alters the inhibition of dentate granule cells in epilepsy. These changes reduce the endogenous neurosteroid control of seizures and contribute to catamenial seizures.

Gene Expression Profiling of Two Epilepsy Models Reveals the ECM/Integrin signaling Pathway is Involved in Epiletogenesis

The molecular mechanisms underlying the development of epilepsy, i.e., epileptogenesis, are due to altered expression of a series of genes. Global expression profiling of temporal lobe epilepsy is confounded by a number of factors, including the variability among animal species, animal models, and tissue sampling time-points. In this study, we pooled two microarray datasets of the most used pilocarpine and kainic acid epilepsy models from the Gene Expression Omnibus database. A total of 567 known and novel genes were commonly differentially expressed across the two models. Pathway analyses demonstrated that the dysregulated genes were involved in 46 pathways. Real-time PCR and western blot analysis confirmed the activation of extracellular matrix (ECM)/integrin signaling pathways. Moreover, targeting ECM/integrin signaling inhibits astrocyte activation and promotes neuron injury in the hippocampus of epileptic mice. This study may provide a "gene/pathway database" that with further investigation can determine the mechanisms underlining epileptogenesis and the possible targets for neuron protection in the hippocampus after status epilepticus.

Mitogen- and Stress-Activated Protein Kinase 1 Regulates Status Epilepticus-Evoked Cell Death in the Hippocampus

Mitogen-activated protein kinase (MAPK) signaling has been implicated in a wide range of neuronal processes, including development, plasticity, and viability. One of the principal downstream targets of both the extracellular signal-regulated kinase/MAPK pathway and the p38 MAPK pathway is Mitogen- and Stress-activated protein Kinase 1 (MSK1). Here, we sought to understand the role that MSK1 plays in neuroprotection against excitotoxic stimulation in the hippocampus. To this end, we utilized immunohistochemical labeling, a MSK1 null mouse line, cell viability assays, and array-based profiling approaches. Initially, we show that MSK1 is broadly expressed within the major neuronal cell layers of the hippocampus and that status epilepticus drives acute induction of MSK1 activation. In response to the status epilepticus paradigm, MSK1 KO mice exhibited a striking increase in vulnerability to pilocarpine-evoked cell death within the CA1 and CA3 cell layers. Further, cultured MSK1 null neurons exhibited a heighted level of N-methyl-D-aspartate-evoked excitotoxicity relative to wild-type neurons, as assessed using the lactate dehydrogenase assay. Given these findings, we examined the hippocampal transcriptional profile of MSK1 null mice. Affymetrix array profiling revealed that MSK1 deletion led to the significant (>1.25-fold) downregulation of 130 genes and an upregulation of 145 genes. Notably, functional analysis indicated that a subset of these genes contribute to neuroprotective signaling networks. Together, these data provide important new insights into the mechanism by which the MAPK/MSK1 signaling cassette confers neuroprotection against excitotoxic insults. Approaches designed to upregulate or mimic the functional effects of MSK1 may prove beneficial against an array of degenerative processes resulting from excitotoxic insults.

RNA Polymerase 1 Is Transiently Regulated by Seizures and Plays a Role in a Pharmacological Kindling Model of Epilepsy

Ribosome biogenesis, including the RNA polymerase 1 (Pol1)-mediated transcription of rRNA, is regulated by the pro-epileptogenic mTOR pathway. Therefore, hippocampal Pol1 activity was examined in mouse models of epilepsy including kainic acid- and pilocarpine-induced status epilepticus (SE) as well as a single seizure in response to pentylenetetrazole (PTZ). Elevated 47S pre-rRNA levels were present acutely after induction of SE suggesting activation of Pol1. Conversely, after a single seizure, 47S pre-rRNA was transiently downregulated with increased levels of unprocessed 18S rRNA precursors in the cornu Ammonis (CA) region. At least in the dentate gyrus (DG), the pilocarpine SE-mediated transient activation of Pol1 did not translate into long-term changes of pre-rRNA levels or total ribosome content. Unaltered hippocampal ribosome content was also found after a 20-day PTZ kindling paradigm with increasing pro-convulsive effects of low dose PTZ that was injected every other day. However, after selectively deleting the essential Pol1 co-activator, transcription initiation factor-1A (Tif1a/Rrn3) from excitatory neurons, PTZ kindling was impaired while DG total ribosome content was moderately reduced and no major neurodegeneration was observed throughout the hippocampus. Therefore, Pol1 activity of excitatory neurons is required for PTZ kindling. As seizures affect ribosome biogenesis without long-term effects on the total ribosome content, such a requirement may be associated with a need to produce specialized ribosomes that promote pro-epileptic plasticity.

Plasticity of intrinsic firing response gain in principal hippocampal neurons following pilocarpine-induced status epilepticus

OBJECTIVE

In experimental models of temporal lobe epilepsy (TLE), brain neurons manifest multiple changes in intrinsic excitability that contribute to neuronal network hyperexcitability. We have investigated whether the intrinsic firing response gain, quantified by the slope of the function relating the number of evoked spikes (Ns) to input excitatory current intensity (I), is modified in principal rat hippocampal neurons in the pilocarpine-status epilepticus (SE) model of TLE.

METHODS

Intracellular recordings were made in CA3 and CA1 pyramidal cells (PCs) and dentate granule cells (GCs) in acute hippocampal slices obtained 7-36days after pilocarpine-SE. Firing response gains were determined empirically from Ns/I relationships and compared to other measured neuronal properties.

RESULTS

The firing response gain in all three types of principal neurons, particularly in CA3 PCs, was markedly multiplied following pilocarpine-SE. Analyses of persistent changes in active and passive properties of CA3 PCs suggested that this increase is multifactorial in origin, the major factors being a reduction in amplitude of the slow afterhyperpolarization and an increase in the fraction of bursting neurons.

SIGNIFICANCE

Here we show that pilocarpine-SE causes multiplication of the firing response gain in the three principal neurons in the hippocampal trisynaptic pathway. This alteration undoubtedly would contribute to hippocampal hyperexcitability in SE-induced TLE.

Progressive Seizure Aggravation in the Repeated 6-Hz Corneal Stimulation Model Is Accompanied by Marked Increase in Hippocampal p-ERK1/2 Immunoreactivity in Neurons

The 6-Hz corneal stimulation test is used to screen novel antiepileptic molecules to overcome the problem of drug refractoriness. Although recognized as a standard test, it has been evaluated only recently in the attempt to characterize the putative neuronal networks involved in seizures caused by corneal stimulation. In particular, by recording from the CA1 region we previously established that the hippocampus participates to propagation of seizure activity. However, these findings were not corroborated by using markers of neuronal activation such as FosB/ΔFosB antigens. In view of this discrepancy, we performed new experiments to characterize the changes in levels of phosphorylated extracellular signal-regulated kinases1/2 (p-ERK1/2), which are also used as markers of neuronal activation. To this aim, mice underwent corneal stimulation up to three different times, in three sessions separated by an interval of 3 days. To characterize a group in which seizures could be prevented by pharmacological treatment, we also considered pretreatment with the ghrelin receptor antagonist EP-80317 (330 μg/kg). Control mice were sham-treated. Video electrocorticographic (ECoG) recordings were obtained from mice belonging to each group of treatment. Animals were finally used to characterize the immunoreactivity for FosB/ΔFosB and p-ERK1/2 in the hippocampus. As previously shown, FosB/ΔFosB levels were highly increased throughout the hippocampus by the first induced seizure but, in spite of the progressively increased seizure severity, they were restored to control levels after the third stimulation. At variance, corneal stimulation caused a progressive increase in p-ERK1/2 immunoreactivity all over the hippocampus, especially in CA1, peaking in the third session. Predictably, EP-80317 administration reduced both duration and severity of seizures, prevented the increase in FosB/ΔFosB levels in the first session, and partially counteracted the increase in p-ERK1/2 levels in the third session. The vast majority of p-ERK1/2 immunopositive cells were co-labeled with FosB/ΔFosB antibodies, suggesting the existence of a relationship between the investigated markers in a subpopulation of neurons activated by seizures. These findings suggest that p-ERK1/2 are useful markers to define the aggravation of seizures and the response to anticonvulsant treatments. In particular, p-ERK1/2 expression clearly identified the involvement of hippocampal regions during seizure aggravation in the 6-Hz model.

Mouse hippocampal phosphorylation footprint induced by generalized seizures: Focus on ERK, mTORC1 and Akt/GSK-3 pathways

Exacerbated hippocampal activity has been associated to critical modifications of the intracellular signaling pathways. We have investigated rapid hippocampal adaptive responses induced by maximal electroshock seizure (MES). Here, we demonstrate that abnormal and exacerbated hippocampal activity induced by MES triggers specific and temporally distinct patterns of phosphorylation of extracellular signal-related kinase (ERK), mammalian target of rapamycin complex (mTORC) and Akt/glycogen synthase kinase-3 (Akt/GSK-3) pathways in the mouse hippocampus. While the ERK pathway is transiently activated, the mTORC1 cascade follows a rapid inhibition followed by a transient activation. This rebound of mTORC1 activity leads to the selective phosphorylation of p70S6K, which is accompanied by an enhanced phosphorylation of the ribosomal subunit S6. In contrast, the Akt/GSK-3 pathway is weakly altered. Finally, MES triggers a rapid upregulation of several plasticity-associated genes as a consequence exacerbated hippocampal activity. The results reported in the present study are reminiscent of the one observed in other models of generalized seizures, thus defining a common molecular footprint induced by intense and aberrant hippocampal activities.

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.

Profiling status epilepticus-induced changes in hippocampal RNA expression using high-throughput RNA sequencing

Status epilepticus (SE) is a life-threatening condition that can give rise to a number of neurological disorders, including learning deficits, depression, and epilepsy. Many of the effects of SE appear to be mediated by alterations in gene expression. To gain deeper insight into how SE affects the transcriptome, we employed the pilocarpine SE model in mice and Illumina-based high-throughput sequencing to characterize alterations in gene expression from the induction of SE, to the development of spontaneous seizure activity. While some genes were upregulated over the entire course of the pathological progression, each of the three sequenced time points (12-hour, 10-days and 6-weeks post-SE) had a largely unique transcriptional profile. Hence, genes that regulate synaptic physiology and transcription were most prominently altered at 12-hours post-SE; at 10-days post-SE, marked changes in metabolic and homeostatic gene expression were detected; at 6-weeks, substantial changes in the expression of cell excitability and morphogenesis genes were detected. At the level of cell signaling, KEGG analysis revealed dynamic changes within the MAPK pathways, as well as in CREB-associated gene expression. Notably, the inducible expression of several noncoding transcripts was also detected. These findings offer potential new insights into the cellular events that shape SE-evoked pathology.

Antiepileptic Effect of Uncaria rhynchophylla and Rhynchophylline Involved in the Initiation of c-Jun N-Terminal Kinase Phosphorylation of MAPK Signal Pathways in Acute Seizures of Kainic Acid-Treated Rats

Seizures cause inflammation of the central nervous system. The extent of the inflammation is related to the severity and recurrence of the seizures. Cell surface receptors are stimulated by stimulators such as kainic acid (KA), which causes intracellular mitogen-activated protein kinase (MAPK) signal pathway transmission to coordinate a response. It is known that Uncaria rhynchophylla (UR) and rhynchophylline (RP) have anticonvulsive effects, although the mechanisms remain unclear. Therefore, the purpose of this study is to develop a novel strategy for treating epilepsy by investigating how UR and RP initiate their anticonvulsive mechanisms. Sprague-Dawley rats were administered KA (12 mg/kg, i.p.) to induce seizure before being sacrificed. The brain was removed 3 h after KA administration. The results indicate that pretreatment with UR (1.0 g/kg), RP (0.25 mg/kg), and valproic acid (VA, 250 mg/kg) for 3 d could reduce epileptic seizures and could also reduce the expression of c-Jun aminoterminal kinase phosphorylation (JNKp) of MAPK signal pathways in the cerebral cortex and hippocampus brain tissues. Proinflammatory cytokines interleukin (IL)-1β, IL-6, and tumor necrosis factor-α remain unchanged, indicating that the anticonvulsive effect of UR and RP is initially involved in the JNKp MAPK signal pathway during the KA-induced acute seizure period.

N-methyl-D-aspartic acid receptor activation downregulates expression of δ subunit-containing GABAA receptors in cultured hippocampal neurons

Neurosteroids are endogenous allosteric modulators of GABAA receptors (GABARs), and they enhance GABAR-mediated inhibition. However, GABARs expressed on hippocampal dentate granule neurons of epileptic animals are modified such that their neurosteroid sensitivity is reduced and δ subunit expression is diminished. We explored the molecular mechanisms triggering this GABAR plasticity. In the cultured hippocampal neurons, treatment with N-methyl-D-aspartic acid (NMDA) (10 μM) for 48 hours reduced the surface expression of δ and α4 subunits but did not increase the expression of γ2 subunits. The tonic current recorded from neurons in NMDA-treated cultures was reduced, and its neurosteroid modulation was also diminished. In contrast, synaptic inhibition and its modulation by neurosteroids were preserved in these neurons. The time course of NMDA's effects on surface and total δ subunit expression was distinct; shorter (6 hours) treatment decreased surface expression, whereas longer treatment reduced both surface and total expression. Dl-2-amino-5-phosphonopentanoic acid (APV) blocked NMDA's effects on δ subunit expression. Chelation of calcium ions by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid tetrakis (acetoxymethyl ester) (BAPTA-AM) or blockade of extracellular signal-regulated kinase (ERK) 1/2 activation by UO126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio] butadiene) also prevented the effects of NMDA. Thus, prolonged activation of NMDA receptors in hippocampal neurons reduced GABAR δ subunit expression through Ca(2+) entry and at least in part by ERK1/2 activation.

Frontier of epilepsy research - mTOR signaling pathway

Studies of epilepsy have mainly focused on the membrane proteins that control neuronal excitability. Recently, attention has been shifting to intracellular proteins and their interactions, signaling cascades and feedback regulation as they relate to epilepsy. The mTOR (mammalian target of rapamycin) signal transduction pathway, especially, has been suggested to play an important role in this regard. These pathways are involved in major physiological processes as well as in numerous pathological conditions. Here, involvement of the mTOR pathway in epilepsy will be reviewed by presenting; an overview of the pathway, a brief description of key signaling molecules, a summary of independent reports and possible implications of abnormalities of those molecules in epilepsy, a discussion of the lack of experimental data, and questions raised for the understanding its epileptogenic mechanism.

Convulsant doses of a dopamine D1 receptor agonist result in Erk-dependent increases in Zif268 and Arc/Arg3.1 expression in mouse dentate gyrus

Activation of dopamine D1 receptors (D1Rs) has been shown to induce epileptiform activity. We studied the molecular changes occurring in the hippocampus in response to the administration of the D1-type receptor agonist, SKF 81297. SKF 81297 at 2.5 and 5.0 mg/kg induced behavioural seizures. Electrophysiological recordings in the dentate gyrus revealed the presence of epileptiform discharges peaking at 30-45 min post-injection and declining by 60 min. Seizures were prevented by the D1-type receptor antagonist, SCH 23390, or the cannabinoid CB1 receptor agonist, CP 55,940. The effect of SKF 81297 was accompanied by increased phosphorylation of the extracellular signal-regulated protein kinases 1 and 2 (ERK), in the granule cells of the dentate gyrus. This effect was also observed in response to administration of other D1-type receptor agonists, such as SKF83822 and SKF83959. In addition, SKF 81297 increased the phosphorylation of the ribosomal protein S6 and histone H3, two downstream targets of ERK. These effects were prevented by genetic inactivation of D1Rs, or by pharmacological inhibition of ERK. SKF 81297 was also able to enhance the levels of Zif268 and Arc/Arg3.1, two immediate early genes involved in transcriptional regulation and synaptic plasticity. These changes may be involved in forms of activity-dependent plasticity linked to the manifestation of seizures and to the ability of dopamine to affect learning and memory.

Activation of ERK by spontaneous seizures in neural progenitors of the dentate gyrus in a mouse model of epilepsy

Cellular changes that are associated with spontaneous seizures in temporal lobe epilepsy are not well understood but could influence ongoing epilepsy-related processes. In order to identify cell signaling events that could occur at the time of spontaneous seizures, the localization of phosphorylated extracellular signal-regulated kinase (pERK) was studied in a pilocarpine mouse model of epilepsy at very short intervals (1.5-2.5 min) after detection of a spontaneous seizure. Within the hippocampal formation, immunolabeling of pERK was evident in a subpopulation of cells in the subgranular zone (SGZ) of the dentate gyrus at these short intervals. Many of these cells had a long vertical process and resembled radial glia, while others had short processes and were oriented horizontally. Labeling with a series of developmental markers demonstrated that virtually all pERK-labeled cells were neural progenitor cells (NPCs). A high percentage ( approximately 80%) of the pERK-labeled cells was labeled with either glial fibrillary acidic protein or brain lipid binding protein, indicating that these cells were radial glia-like NPCs. A smaller percentage of labeled cells expressed NeuroD, suggesting that they were later-developing NPCs that were assuming a neuronal identity. Early expression of pERK was not detected in immature neurons. Double labeling with proliferation markers demonstrated that approximately 30% of pERK-labeled NPCs expressed Mcm2, indicating that they were actively proliferating. Furthermore, virtually all radial glia-like NPCs that were in the proliferative cycle expressed pERK. These findings suggest that spontaneous seizures and associated ERK activation could contribute to the proliferation of radial glia-like NPCs in this epilepsy model.

The pilocarpine model of epilepsy: what have we learned?

The systemic administration of a potent muscarinic agonist pilocarpine in rats promotes sequential behavioral and electrographic changes that can be divided into 3 distinct periods: (a) an acute period that built up progressively into a limbic status epilepticus and that lasts 24 h, (b) a silent period with a progressive normalization of EEG and behavior which varies from 4 to 44 days, and (c) a chronic period with spontaneous recurrent seizures (SRSs). The main features of the SRSs observed during the long-term period resemble those of human complex partial seizures and recurs 2-3 times per week per animal. Therefore, the pilocarpine model of epilepsy is a valuable tool not only to study the pathogenesis of temporal lobe epilepsy in human condition, but also to evaluate potential antiepileptogenic drugs. This review concentrates on data from pilocarpine model of epilepsy.

Expression and localization of voltage dependent potassium channel Kv4.2 in epilepsy associated focal lesions

An increasing number of observations suggest an important role for voltage-gated potassium (Kv) channels in epilepsy. We studied the cell-specific distribution of Kv4.2, phosphorylated (p) Kv4.2 and the Kv4.2 interacting protein NCS-1 using immunocytochemistry in different epilepsy-associated focal lesions. In hippocampal sclerosis (HS), Kv4.2 and pKv4.2 immunoreactivity (IR) was reduced in the neuropil in regions with prominent neuronal cell loss. In both HS and malformations of cortical development (MCD), intense labeling was found in neuronal somata, but not in dendrites. Strong NCS-1 IR was observed in neurons in all lesion types. Western blot analysis demonstrated an increase of total Kv4.2 in all lesions and activation of the ERK pathway in HS and ganglioglioma. These findings indicate that Kv4.2 is expressed in both neuronal and glial cells and its regulation may involve potassium channel interacting proteins, alterations in the subcellular localization of the channel, as well as phosphorylation-mediated posttranslational modifications.

Quantitative transcriptional neuroanatomy of the rat hippocampus: evidence for wide-ranging, pathway-specific heterogeneity among three principal cell layers

We have used laser-capture microdissection and microarray hybridization to characterize gene expression in the three principal neuron layers of rat hippocampus. Correlative and clustering analyses revealed all three layers to be easily differentiated from one another based on gene expression profile alone. A greater disparity in gene expression exists between dentate granule and pyramidal cell layers, reflecting phenotypic and ontological differences between those cell populations. Remarkably, the level of more than 45% of expressed transcripts was significantly different among the three neuron populations, with more than a third of those (>1,000 transcripts) being at least twofold different between layers. Even CA1 and CA3 pyramidal cell layers were dramatically different on a transcriptional level with a separate analysis indicating that nearly 20% of transcripts are differentially expressed between them. Only a small number of transcripts were specific for a given hippocampal cell layer, suggesting that functional differences are more likely secondary to wide-ranging expression differences of modest magnitude rather than very large disparities in a few genes. Categorical analysis of transcript abundance revealed concerted differences in gene expression among the three cell layers referable to specific cellular pathways. For instance, transcripts encoding proteins involved in glucose metabolism are most highly expressed in the CA3 pyramidal layer, which may reflect an underlying greater metabolic rate of these neurons and partially explain their exquisite vulnerability to seizure-induced damage. Conversely, transcripts related to MAP kinase signaling pathways and transcriptional regulator activity are prominent in the dentate granule cell layer, which could contribute to its resistance to damage following seizure activity by positioning these neurons to respond to external stimuli by altering transcription. Taken together, these data suggest that unique physiological characteristics of major cell layers, such as neuronal activity, neuronal plasticity, and vulnerability to neurodegeneration, are reflected in substantial transcriptional heterogeneity within the hippocampus.

Altered phosphorylation and localization of the A-type channel, Kv4.2 in status epilepticus

Extracelluar signal-regulated kinase (ERK) pathway activation has been demonstrated following convulsant stimulation; however, little is known about the molecular targets of ERK in seizure models. Recently, it has been shown that ERK phosphorylates Kv4.2 channels leading to down-regulation of channel function, and substantially alters dendritic excitability. In the kainate model of status epilepticus (SE), we investigated whether ERK phosphorylates Kv4.2 and whether the changes in Kv4.2 were evident at a synaptosomal level during SE. Western blotting was performed on rat hippocampal whole cell, membrane, synaptosomal, and surface biotinylated extracts following systemic kainate using an antibody generated against the Kv4.2 ERK sites and for Kv4.2, ERK, and phospho-ERK. ERK activation was associated with an increase in Kv4.2 phosphorylation during behavioral SE. During SE, ERK activation and Kv4.2 phosphorylation were evident at the whole cell and synaptosomal levels. In addition, while whole-cell preparations revealed no alterations in total Kv4.2 levels, a decrease in synaptosomal and surface expression of Kv4.2 was evident after prolonged SE. These results demonstrate ERK pathway coupling to Kv4.2 phosphorylation. The finding of decreased Kv4.2 levels in hippocampal synaptosomes and surface membranes suggest additional mechanisms for decreasing the dendritic A-current, which could lead to altered intrinsic membrane excitability during SE.

Dynamic seizure-related changes in extracellular signal-regulated kinase activation in a mouse model of temporal lobe epilepsy

Extracellular signal-regulated kinase (ERK) is highly sensitive to regulation by neuronal activity and is critically involved in several forms of synaptic plasticity. These features suggested that alterations in ERK signaling might occur in epilepsy. Previous studies have described increased ERK phosphorylation immediately after the induction of severe seizures, but patterns of ERK activation in epileptic animals during the chronic period have not been determined. Thus, the localization and abundance of phosphorylated extracellular signal-regulated kinase (pERK) were examined in a pilocarpine model of recurrent seizures in C57BL/6 mice during the seizure-free period and at short intervals after spontaneous seizures. Immunolabeling of pERK in control animals revealed an abundance of distinctly-labeled neurons within the hippocampal formation. However, in pilocarpine-treated mice during the seizure-free period, the numbers of pERK-labeled neurons were substantially decreased throughout much of the hippocampal formation. Double labeling with a general neuronal marker suggested that the decrease in pERK-labeled neurons was not due primarily to cell loss. The decreased ERK phosphorylation in seizure-prone animals was interpreted as a compensatory response to increased neuronal excitability within the network. Nevertheless, striking increases in pERK labeling occurred at the time of spontaneous seizures and were evident in large populations of neurons at very short intervals (as early as 2 min) after detection of a behavioral seizure. These findings suggest that increased pERK labeling could be one of the earliest immunohistochemical indicators of neurons that are activated at the time of a spontaneous seizure.

An enriched environment improves cognitive performance after early-life status epilepticus accompanied by an increase in phosphorylation of extracellular signal-regulated kinase 2

An enriched environment can enhance brain recovery in animals with early-life status epilepticus (SE). The purpose of this study was to determine the effects of early-life SE on spatial memory and hippocampal extracellular signal-regulated kinase (ERK) level, and the possible therapeutic effects of the enriched environment. Rats were assigned randomly to four groups: (1) control rats (nonenriched control); (2) control rats housed in an enriched environment from Postnatal Day (P) 25 to P40 (enriched control); (3) rats in which SE was induced with lithium-pilocarpine (Li-PC) at P21 (nonenriched SE); and (4) rats in which SE was induced with Li-PC at P21 and then housed in an enriched environment from P25 to P40 (enriched SE). As adults, the rats underwent spatial learning and memory tests in the Morris water maze between P50 and P55. At P55, subsets of animals were evaluated for expression of hippocampal ERK1/2 phosphorylation immediately following completion of the Morris water maze. At ~P100, another set of animals was tested for seizure threshold. When studied as adults, only the nonenriched SE group had a spatial memory deficit. The nonenriched SE group also exhibited lower levels of phosphorylated ERK2 as compared with the nonenriched control, enriched control, and enriched SE groups. Both the nonenriched SE and enriched SE groups had reduced seizure thresholds as compared with the nonenriched control and enriched control groups. Results from this study demonstrate that an enriched environment improves spatial memory in rats subjected to early-life SE, possibly through upregulation of phosphorylated ERK2 in the hippocampus. However, an enriched environment has no effect on seizure threshold.

Involvement of extracellular regulated kinase and p38 kinase in hippocampal seizure tolerance

The mechanisms underlying brain seizure tolerance, a phenomenon in which brief periods of seizures protect brain against the lethal effects of subsequent sustained seizures, are poorly understood. Because brain seizure tolerance and brain ischemia tolerance likely share certain common mechanisms, the recent evidence that activation of extracellular regulated kinase (ERK) and p38 kinase pathways plays a critical role in ischemic preconditioning suggests that a similar mechanism may underlie brain seizure tolerance. We investigated the hypothesis in a rat kainic acid preparation of seizure preconditioning and tolerance, which was established by induction of one episode of priming epileptic status lasting for 20 min on the first day and another episode of sustained epileptic status lasting for 2 hr on the second day. We observed that acute seizures lead to a rapid activation of ERK and p38 in the hippocampal CA3 area, the brain region most susceptible to the lethal effects of epileptic status. Pretreatment with the ERK inhibitor PD98059 and the p38 inhibitor SB203580 selectively reduces seizure-elicited activation of ERK and p38, respectively, and significantly reduces priming seizure-induced protection of CA3 neurons. These findings indicate that, similar to brain ischemia tolerance, brain seizure tolerance also involves the ERK and p38 signaling pathways.

Expression of apoptosis inhibitor protein Mcl1 linked to neuroprotection in CNS neurons

Mcl1 is a Bcl2-related antiapoptotic protein originally isolated from human myeloid leukemia cells. Unlike Bcl2, expression has not been reported in CNS neurons. We isolated Mcl1 in a direct screen for candidate modifier genes of neuronal vulnerability by differential display of mRNAs upregulated following prolonged seizures in two mouse strains with contrasting levels of hippocampal cell death. Mcl1 is widely expressed in neurons, and transcription is rapidly induced in both strains. In resistant C57Bl/6J mice, Mcl1 protein levels remain persistently elevated in hippocampal pyramidal neurons after seizures, but fall rapidly in C3H/HeJ hippocampus, coinciding with extensive neuronal apoptosis. DNA damage and caspase-mediated cell death were strikingly increased in Mcl1-deficient mice when compared to +/+ littermates after similar seizures. We identify Mcl1 as a neuronal gene responsive to excitotoxic insult in the brain, and link relative levels of Mcl1 expression to inherited differences in neuronal thresholds for apoptosis.

Plasticity of dendritic excitability

Dendrites are equipped with a plethora of voltage-gated ion channels that greatly enrich the computational and storage capacity of neurons. The excitability of dendrites and dendritic function display plasticity under diverse circumstances such as neuromodulation, adaptation, learning and memory, trauma, or disorders. This adaptability arises from alterations in the biophysical properties or the expression levels of voltage-gated ion channels-induced by the activity of neurotransmitters, neuromodulators, and second-messenger cascades. In this review we discuss how this plasticity of dendritic excitability could alter information transfer and processing within dendrites, neurons, and neural networks under physiological and pathological conditions.

Role of kinin B1 and B2 receptors in the development of pilocarpine model of epilepsy

The tissue sclerosis found in epilepsy of limbic origin is characterized by shrunken gliotic hippocampus, granule cell loss in the dentate gyrus and extensive pyramidal cell loss in Ammon's horn. Evidence has indicated that sprouting of dentate granule cell axons into the inner molecular layer of the dentate gyrus is related to hyperexcitability. Trying to understand the role of kinin B1 and B2 receptors in the physiopathology of temporal lobe epilepsy (TLE), the present work was delineated to study the development of the epilepsy model induced by pilocarpine in B1 and B2 knockout mice (B1KO and B2KO, respectively). Behavior parameters, cell death and mossy fiber sprouting were analyzed. B1KO mice showed increased latency for the first seizure, associated to a decreased frequency of spontaneous seizures, when compared with wild-type mice. In addition, B1KO mice showed less cell death in all hippocampal formation associated to a reduced grade of mossy fiber sprouting. Furthermore, B2KO mice presented minor duration of the silent period and an increased frequency of spontaneous seizures, when compared with wild-type mice. B2KO and their control lineage showed similar pattern of cell death in the hippocampus, which was very intense when compared with saline-treated animals. The mossy fiber sprouting was also increased in B2KO mice, when compared to wild-type mice and saline-treated animals. Taken together, these data suggest a deleterious effect for kinin B1 receptor and a protective effect for kinin B2 receptor during the development of the temporal lobe epilepsy.

Acquired Dendritic Channelopathy in Temporal Lobe Epilepsy

Inherited channelopathies are at the origin of many neurological disorders. Here we report a form of channelopathy that is acquired in experimental temporal lobe epilepsy (TLE), the most common form of epilepsy in adults. The excitability of CA1 pyramidal neuron dendrites was increased in TLE because of decreased availability of A-type potassium ion channels due to transcriptional (loss of channels) and posttranslational (increased channel phosphorylation by extracellular signal-regulated kinase) mechanisms. Kinase inhibition partly reversed dendritic excitability to control levels. Such acquired channelopathy is likely to amplify neuronal activity and may contribute to the initiation and/or propagation of seizures in TLE.

The synthesis and distribution of the kinin B1 and B2 receptors are modified in the hippocampus of rats submitted to pilocarpine model of epilepsy

Kinins, a special class of polypeptides, are represented by bradykinin (BK), kallidin (Lys-BK), as well as their metabolites. The biological actions of these polypeptides binding on their receptors (B1 and B2) have been related to inflammation process, cytokines action, glutamate release and prostaglandins production. Usually, kinin B1 receptor is not expressed at a significant level under physiologic conditions in most tissues, but its expression is induced by injury, or upon exposure in vivo or in vitro to pro-inflammatory mediators. The kinin B2 receptor subtype is constitutively and widely expressed throughout the central and peripheral nervous system. These data raise the possibility for de novo expression of those receptors during the temporal lobe epilepsy (TLE), which has been related to cell death, gliosis and hippocampal reorganization. To correlate kinin system and TLE, adult male Wistar rats were submitted to pilocarpine model of epilepsy. The hippocampi were removed 6 h, 5 and 60 days after status epilepticus (SE) onset. The collected tissues were used to study the expression of kinin B1 and B2 mRNA receptors, using Real-Time PCR. Immunohistochemistry assay was also employed to visualize kinin B1 and B2 distribution in the hippocampus. The results show increased kinin B1 and B2 mRNA levels during acute, silent and chronic periods and changes in the kinin B1 and B2 receptors distribution. In addition, the immunoreactivity against kinin B1 receptor was increased mainly during the silent period, where neuron clusters of could be visualized. The kinin B2 receptor immunoreactivity also showed augmentation but mainly during the acute and silent periods. Our results suggest that kinin B1 and B2 receptors play an important role in the epileptic phenomena.

Extracellular signal-regulated kinase 1/2 is required for the induction of group I metabotropic glutamate receptor-mediated epileptiform discharges

Transient stimulation of group I metabotropic glutamate receptors (mGluRs) induces persistent prolonged epileptiform discharges in hippocampal slices via a protein synthesis-dependent process. At present, the signaling process underlying the induction of these epileptiform discharges remains unknown. We examined the possible role of extracellular signal-regulated kinases (ERK1 and ERK2) because these kinases can be activated by group I mGluRs, and their activation may regulate gene expression and alter protein synthesis. The group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG; 50 microm) induced activation of ERK1/2 in hippocampal slices. 2-(2-Diamino-3-methoxyphenyl-4H-1-benzopyran-4-one (PD98059) (50 microm) a specific inhibitor of mitogen-activated protein kinase kinase (MEK), suppressed ERK1/2 activation by DHPG. PD98059 or another MEK inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene (10 microm), also prevented the induction of the prolonged epileptiform discharges by DHPG. In the presence of ionotropic glutamate receptor inhibitors and tetrodotoxin (blockers), DHPG-induced epileptiform discharges were suppressed, whereas ERK1/2 activation persisted. Protein kinase C inhibitors (2-[1-(3-dimethylaminopropyl)-5-methoxyindol-3-yl]-3-(1H-indol-3-yl) maleimide, 1 microm; or chelerythrine, 10 microm) did not prevent the generation of DHPG-induced epileptiform discharges, nor did they suppress the activation of ERK1/2 by DHPG in slices pretreated with the blockers. Genistein (30 microm), a broad-spectrum tyrosine kinase inhibitor, suppressed the DHPG-induced epileptiform discharges and the ERK1/2 activation in the presence of blockers. Induction of DHPG-mediated epileptiform discharges was also suppressed by 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-D]pyrimidine (10 microm), an Src-family tyrosine kinase inhibitor. The study shows that group I mGluRs activate ERK1/2 through a tyrosine kinase-dependent process and that this activation of ERK1/2 is necessary for the induction of prolonged epileptiform discharges in the hippocampus.

Expression of ICAM-1, TNF-alpha, NF kappa B, and MAP kinase in tubers of the tuberous sclerosis complex

Individuals affected with tuberous sclerosis complex (TSC) develop cortical tubers characterized by disorganized cytoarchitecture and morphologically abnormal cell types, such as dysplastic neurons (DNs) and giant cells (GCs). As part of ongoing cDNA array analysis to study the molecular pathogenesis of tuber formation, we detected increased expression of intercellular adhesion molecule-1 (ICAM-1) mRNA, a cell adhesion molecule (CAM) that functions in cytokine signaling, in tubers. Western and immunohistochemical analyses revealed that ICAM-1 protein was selectively expressed in tubers, but was only minimally expressed in control cortex, adjacent nontuberal cortex, or in non-TSC focal cortical dysplasia. Increased expression of ICAM-1 was found in mice in which the Tsc1 gene was conditionally inactivated in astrocytes. Expression of molecules involved in ICAM-1 activation and cytokine signaling were increased in tubers, including tumor necrosis factor alpha (TNF-alpha), mitogen activated protein kinase (MAPK), and nuclear factor kappa B (NF-kappaB). Numerous CD68-immunoreactive macrophages were observed clustered around GCs further supporting an inflammatory response in tubers. Expression of caspase 8 and Fas support cytokine activation and detection of TUNEL reactivity suggests ongoing cell death in tubers. Specific alterations in ICAM-1, TNF-alpha, NF-kappaB1, and MAPK expression coupled with the detection of numerous CD68-immunoreactive macrophages suggests activation of proinflammatory cytokine signaling pathways in tubers that may culminate in cell death.

Levels of the synaptic protein X11 alpha/mint1 are increased in hippocampus of rats with epilepsy

X11 alpha or Mint1 is a protein containing an N-terminal sequence, which binds to Munc-18 protein, a middle phosphotyrosine-binding domain (PTB) and two C-terminal PDZ (Post-synaptic density/Discs large/Zone Occludens-1) domains. The PDZ domains, which mediate protein-protein interactions have been shown to be involved in the organization of synaptic signaling pathways. Mint1 plays an important role in vesicle synaptic transport toward the active zone at the pre-synaptic site, and also participates in the transport of NR2B subunit of the NMDA receptor, to the post-synaptic site. To investigate the participation and distribution of this protein in the hippocampal subfield of rats submitted to the pilocarpine model of epilepsy, Mint1 was analyzed using Western blotting and immunohistochemistry. Animals of 5 h of status epilepticus showed decreased levels of this protein in the hippocampus when compared to the control animals. In contrast, animals from seizure-free period (silent group) and during spontaneous seizures phase (chronic group) showed increased Mint1 immunostaining in all hippocampal subfields, mainly in the dentate gyrus, when compared to the control group. The blotting confirmed the results obtained by immunohistochemistry. The present work suggests that Mint1 may be related to hippocampal plasticity during epileptogenesis in the pilocarpine model of temporal lobe epilepsy.

Kinase-dependent loss of Na+ channel slow-inactivation in rat CA1 hippocampal pyramidal cell dendrites after brief exposure to convulsants

Na+ channels in the dendrites of rat CA1 pyramidal neurons display a profound activity-dependent inactivation, termed slow inactivation, that limits excitability in the dendrites even at low physiological rates of firing. The magnitude of this slow inactivation is powerfully modulated by a protein kinase C-dependent process. Because activation of kinases is a rapid and common feature of a number of seizure models, we hypothesized that a loss of slow inactivation of Na+ channels might exacerbate other changes in excitability. Thus, we observed the effects of a brief (5 min) chemical convulsant treatment on Na+ currents and action potentials in hippocampal slices. We found that slow inactivation decreased significantly and remained decreased for at least 30 min after return to control conditions. Pretreatment with either chelerythrine, a protein kinase C inhibitor, or U0126, a mitogen-activated protein kinase/extracellular signal regulated kinase kinase (MEK) inhibitor, blocked this reduction of slow inactivation. These results demonstrate that a brief period of hyperexcitability leads to a rapid, protein kinase-dependent loss of slow inactivation of Na+ channels that would contribute to and perhaps prolong the hyperexcitable state.

Activation of ERK1/2 after neonatal rat cerebral hypoxia-ischaemia

Activation of extracellular signal-related kinases (ERK1/2), also known as p42/44 mitogen-activated protein kinase (MAPK), is considered important for neuronal survival, cell proliferation and apoptosis. In the present study, activation (phosphorylation) of ERK1/2 (P-ERK) was investigated in brains of 7-day-oldrats after hypoxia-ischaemia (HI). In damaged areas, P-ERK-positive neurons appeared immediately after HI and the staining remained for at least 8 h. At later time points, 24 and 72 h post-HI, P-ERK-positive neurons were found in the core of the infarct and in the border zone to undamaged tissue. These cells also showed signs of DNA damage and calpain-induced fodrin breakdown, indicative of injury. At 72 h post-HI, P-ERK was also observed in microglia in the border zone to the damaged area and in astrocytes and oligodendrocytes in white matter of both hemispheres. P-ERK was strongly expressed in the subventricular zone in both hemispheres after HI at most time points, although the staining in the ipsilateral (damaged hemisphere) was stronger than in the contralateral (non-damaged hemisphere). In summary, ERK1/2 activation occurred early in neurons after HI in the neonatal brain, and mainly in cells displaying signs of damage.

Transient neuronal but persistent astroglial activation of ERK/MAP kinase after focal brain injury in mice

Astrogliosis is a nearly ubiquitous response to a variety of insults to the central nervous system (CNS). This reaction is triggered rapidly, but can persist for years after the initial trauma. Little is known about the signaling mechanisms responsible for this activation and its chronic maintenance. Extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) activation is implicated in several functions important to the reactive glial phenotype such as cellular proliferation and motility. Here we utilize immunohistochemistry with a phosphorylation state-specific antibody (pERK) to characterize the temporal and spatial pattern of ERK/MAPK activation in neurons and glia following a forebrain stab lesion (FSL) in mice. Early activation (1 h) was primarily in perilesional neuronal elements, particularly of the hippocampus. Occasional perilesional glia were also positive for pERK. Additionally, ependymal cells bilaterally stained prominently for pERK. These patterns of pERK immunoreactivity at 1 h were abolished by pretreatment with the selective MEK inhibitor, SL327. ERK/MAPK activation at later time points between 1 day (d) and 30 d was primarily restricted to perilesional astrocytes with maximum labeling at 3 d. However, pERK-positive astrocytes represented only a subset of total GFAP-positive cells and were found more proximal to the lesion suggesting specific functional activation of these cells. Finally, immunostaining for the phosphorylated form of cAMP response element-binding (CREB) protein, a downstream target of the ERK/MAPK cascade, was increased in perilesional glia 7 d after FSL. Sustained activation of the ERK/MAPK signaling pathway in perilesional reactive glia suggests a critical role for this cascade in astrogliosis.

ERK regulation in chronic ethanol exposure and withdrawal

The extracellular signal regulated protein kinases (ERKs), also known as mitogen-activated protein kinases (MAPK) of 42 and 44 kd, play a crucial role in the induction of various forms of neural plasticity. Ethanol induces long-lasting functional changes that are more severe following repeated exposure and may involve intracellular signal transduction mechanisms. Therefore, we investigated the regulation of the ERK signal transduction pathway in models of continuous and intermittent ethanol exposure and withdrawal. Moderate blood alcohol levels (BALs) reduced ERK activation in most of the brain regions studied. Conversely, during withdrawal, activation of ERK was increased in most areas with some regional variations in the levels and kinetics of induction. The most dramatic effects were observed in the amygdala, the cerebellum, the striatum and the hippocampus. In the amygdala and the cerebellum, the activation of ERK observed during withdrawal was significantly higher after intermittent ethanol exposure than after continuous exposure, suggesting the establishment of a form of sensitization to the effects of withdrawal on ERK regulation. Thus the dysregulation of the ERK pathway could contribute to escalation of withdrawal symptoms induced by repeated withdrawal and possibly to the neuroadaptative changes believed to underlie progression towards addiction.

The role of muscarinic acetylcholine receptor-mediated activation of extracellular signal-regulated kinase 1/2 in pilocarpine-induced seizures

Pilocarpine-induced seizures are mediated by the M(1) subtype of muscarinic acetylcholine receptor (mAChR), but little is known about the signaling mechanisms linking the receptor to seizures. The extracellular signal-regulated kinase (ERK) signaling cascade is activated by M(1) mAChR and is elevated during status epilepticus. Yet, the role of ERK activation prior to seizure has not been evaluated. Here, we examine the role of pilocarpine-induced ERK activation in the induction of seizures in mice by pharmacological and behavioral approaches. We show that pilocarpine induces ERK activation prior to the induction of seizures by both western blot and immunocytochemistry with an antibody to phosphorylated ERK. In addition, we show that the ERK pathway inhibitor SL327 effectively blocks the pilocarpine-induced ERK activation. However, SL327 pretreatment has no effect on the initiation of seizures. In fact, animals treated with SL327 had higher seizure-related mortality than vehicle-treated animals, suggesting activated ERK may serve a protective role during seizures. In addition, ERK inhibition had no effect on the development of the long-term sequelae of status epilepticus (SE), including mossy fiber sprouting, neuronal death and spontaneous recurrent seizures.

Phosphorylation of P42/P44 MAP kinase and DNA fragmentation in the rat perforant pathway stimulation model of limbic epilepsy

The intracellular signaling pathways associated with neuronal injury after perforant pathway stimulation of the rodent hippocampus have not been examined. To determine whether activation of the p42/p44 (Erk1/2) MAP kinase (MAPK) phosphorylation cascade is linked to neuronal injury after perforant pathway stimulation (PPS), we stained for phosphorylated Erk1/2 (P-Erk1/2) and for DNA fragmentation, a marker of cell death after PPS. Eighteen Sprague-Dawley rats underwent PPS for 6 (n=6), 12 (n=6), or 24 (n=6) h and were sacrificed either immediately (n=9) or 48 h (n=9) after stimulation. Sham-operated non-stimulated control animals (n=2) and control animals receiving low frequency stimulation only (n=4) were also examined. Brain sections were stained for DNA fragmentation and P-Erk1/2. DNA fragmentation was evident only in granule cells and CA3 pyramidal cells of the stimulated side 48 h after 24 h of PPS. PPS resulted in robust phosphorylation of Erk1/2 that displayed a stereotyped timecourse, appearing first in hilar neurons on the ipsilateral side and later in hilar neurons, granule cells, hippocampal pyramidal and non-neuronal cell populations on both the stimulated and contralateral sides. Both Erk1/2 phosphorylation and DNA fragmentation show definite and reproducible staining patterns after PPS that vary based on duration of stimulation. Populations displaying Erk1/2 activation appeared to differ from those showing DNA fragmentation and neuronal injury.

Prolonged nuclear retention of activated extracellular signal-regulated protein kinase promotes cell death generated by oxidative toxicity or proteasome inhibition in a neuronal cell line

In the HT22 mouse hippocampal cell line and primary immature embryonic rat cortical neurons, glutamate-induced oxidative toxicity is associated with a delayed but chronic activation of extracellular signal-regulated kinase-1/2 (ERK-1/2). ERK-1/2 is also activated in HT22 cells that undergo caspase-dependent cell death upon inhibition of proteasome-dependent protein degradation brought about by MG132 treatment. As in glutamate-treated HT22 cells and primary neurons, inhibition of MEK-1, an upstream activator of ERK-1/2 protects against MG132-induced toxicity. Furthermore, activated ERK-1/2 is retained within the nucleus in glutamate- and MG132-treated HT22 cells. Although previous studies suggested that ERK-1/2 activation was downstream of many cell death-inducing signals in HT22 cells, we show here that cycloheximide, the Z-vad caspase inhibitor, and a nonlethal heat shock protect against glutamate- and MG132-induced toxicity without diminishing ERK-1/2 activation. In these cases, ERK-1/2, although chronically activated, is not retained within the nucleus but accumulates within the cytoplasm. Thus, persistent nuclear retention of activated ERK-1/2 may be a critical factor in eliciting proapoptotic effects in neuronal cells subjected to oxidative stress or proteasome inhibition.

Evoked epileptiform discharges in the rat anterior piriform cortex: generation and local propagation

The purpose of this study was to identify cellular and synaptic properties of neurons in a small region within the anterior piriform cortex (aPC), termed the area tempestas (AT), responsible for triggering forebrain seizures in rats. Using a brain slice preparation, we performed whole-cell patch recordings from neurons in the regions overlapping the functionally defined AT. Local electrical stimulation activated synaptic inputs to neurons in these regions, collectively termed the deep aPC (daPC). Synaptic inputs were blocked by selective ionotropic glutamate receptor antagonists. Excitatory bursts were evoked from 59% of daPC neurons as the stimulus intensity was raised above a precise threshold. Secondary bursts (6-15 Hz) occurred in 34% of daPC neurons. Evoked bursts were synaptically driven, as they were blocked by TTX (1 microM) or 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX, 1 microM), but not by inclusion of cesium and N-(2, 6-dimethylphenylcarbamoylmethyl) triethylammonium (QX-314) in the internal patch solution. Neither augmentation of excitatory nor suppression of inhibitory transmission were required to evoke bursts from daPC neurons. However, bicuculline (20 microM) lowered the threshold intensity for evoking discharges and increased the incidence and duration of evoked bursts, indicating active inhibitory control of daPC neurons. Stimulation in the daPC evoked epileptiform field potentials from layer II of the adjacent PC and bursts from layer II pyramidal neurons. This work demonstrates that synaptically dependent excitatory burst discharges can be evoked from daPC neurons without altering the balance between synaptic excitation and inhibition. Stimuli that trigger bursts in daPC neurons also generate epileptiform activity in layer II pyramidal cells, indicating that propagation of excitatory activity triggered from the daPC to the pyramidal neurons of the aPC can contribute to the initiation of seizures induced by disinhibition of the AT in vivo.

Selective alterations of glycosaminoglycans synthesis and proteoglycan expression in rat cortex and hippocampus in pilocarpine-induced epilepsy

Proteoglycans and glycosaminoglycans are elements of matrix. In the nervous system, glycosaminoglycans modulate neurite outgrowth and are co-receptors for growth factors playing a crucial role in cell differentiation and synaptogenesis. The receptor of protein tyrosine phosphatase beta (RPTPbeta) is a chondroitin sulphate proteoglycan which plays an important role in neural morphogenesis and axon guidance mechanisms. Pilocarpine-treated rats present status epilepticus, which is followed by a seizure-free period (silent), by a period of spontaneous recurrent seizures (chronic), and the hippocampus of these animals exhibits cell loss and mossy fiber sprouting. Thus, the synthesis of heparan sulphate and chondroitin sulphate and the time course of RPTPbeta immunoreactivity were studied in the hippocampus and cerebral cortex during these phases of pilocarpine-induced epilepsy. The results showed decreased synthesis of heparan sulphate during the acute phase and an increased synthesis of chondroitin sulphate during the silent period in the cortex and hippocampus. In control rats RPTPbeta immunoreactivity was detected only in glial cells. After 6 h of status epilepticus the RPTPbeta immunoreactivity was no longer detectable in the glial cells in both tissues and intense staining became evident in the matrix, surrounding CA3 and dentate gyrus and piriform cortex neurones. In the silent and chronic periods RPTPbeta immunoreactivity was mainly detected in neuronal somata and fibers of neurones of hippocampus and cortex. These changes show a selective variation of synthesis and expression of glycosaminoglycans and RPTPbeta in relation to epilepsy suggesting a molecular interplay between glia and neurones during seizures.