Modification of ionotropic glutamate receptor-mediated processes in the rat hippocampus following repeated, brief seizures

Hippocampal Sclerosis in Pilocarpine Epilepsy: Survival of Peptide-Containing Neurons and Learning and Memory Disturbances in the Adult NMRI Strain Mouse

The present experiments reveal the alterations of the hippocampal neuronal populations in chronic epilepsy. The mice were injected with a single dose of pilocarpine. They had status epilepticus and spontaneously recurrent motor seizures. Three months after pilocarpine treatment, the animals were investigated with the Barnes maze to determine their learning and memory capabilities. Their hippocampi were analyzed 2 weeks later (at 3.5 months) with standard immunohistochemical methods and cell counting. Every animal displayed hippocampal sclerosis. The neuronal loss was evaluated with neuronal-N immunostaining, and the activation of the microglia was measured with Iba1 immunohistochemistry. The neuropeptide Y, parvalbumin, and calretinin immunoreactive structures were qualitatively and quantitatively analyzed in the hippocampal formation. The results were compared statistically to the results of the control mice. We detected neuronal loss and strongly activated microglia populations. Neuropeptide Y was significantly upregulated in the sprouting axons. The number of parvalbumin- and calretinin-containing interneurons decreased significantly in the Ammon’s horn and dentate gyrus. The epileptic animals displayed significantly worse learning and memory functions. We concluded that degeneration of the principal neurons, a numerical decrease of PV-containing GABAergic neurons, and strong peptidergic axonal sprouting were responsible for the loss of the hippocampal learning and memory functions.

Calcium-permeable AMPA receptors are essential to the synaptic plasticity induced by epileptiform activity in rat hippocampal slices

Abnormal neuronal activity during epileptic seizures alters the properties of synaptic plasticity, and, consequently, leads to cognitive impairment. The molecular mechanism of these alterations in synaptic plasticity is still unclear. In the present study, using a 4-aminopyridine (4-AP) in vitro model, we demonstrated that epileptiform activity in rat hippocampal slices initially causes substantial enhancement of field excitatory postsynaptic potential amplitude. However, the potentiation of CA3-CA1 synapses was temporary and switched to long-term depression (LTD) within an hour. Previous studies showed that transient incorporation of calcium-permeable AMPA receptors (CP-AMPARs) is crucial for the consolidation of long-term potentiation (LTP). We confirmed that, in normal conditions, the blockage of CP-AMPARs prevented the consolidation of LTP induced by theta-burst stimulation (TBS). In contrast, the blockage of CP-AMPARs preserved synaptic potentiation induced by epileptiform activity. One hour after a period of epileptiform activity in the hippocampal slices, synaptic plasticity was substantially altered, and the TBS protocol was unable to produce LTP. Moreover, if CP-AMPARs were blocked, the TBS protocol induced LTD. Our results indicate that CP-AMPARs play an essential role in the molecular mechanism of the disturbances of synaptic plasticity caused by epileptiform activity.

Synaptic GluN2A-Containing NMDA Receptors: From Physiology to Pathological Synaptic Plasticity

N-Methyl-d-Aspartate Receptors (NMDARs) are ionotropic glutamate-gated receptors. NMDARs are tetramers composed by several homologous subunits of GluN1-, GluN2-, or GluN3-type, leading to the existence in the central nervous system of a high variety of receptor subtypes with different pharmacological and signaling properties. NMDAR subunit composition is strictly regulated during development and by activity-dependent synaptic plasticity. Given the differences between GluN2 regulatory subunits of NMDAR in several functions, here we will focus on the synaptic pool of NMDARs containing the GluN2A subunit, addressing its role in both physiology and pathological synaptic plasticity as well as the contribution in these events of different types of GluN2A-interacting proteins.

Ultrastructural and functional changes at the tripartite synapse during epileptogenesis in a model of temporal lobe epilepsy

The persistent unresponsiveness of many of the acquired epilepsies to traditional antiseizure medications has motivated the search for prophylactic drug therapies that could reduce the incidence of epilepsy in this at risk population. These studies are based on the idea of a period of epileptogenesis that can follow a wide variety of brain injuries. Epileptogenesis is hypothesized to involve changes in the brain not initially associated with seizures, but which result finally in seizure prone networks. Understanding these changes will provide crucial clues for the development of prophylactic drugs. Using the repeated low-dose kainate rat model of epilepsy, we have studied the period of epileptogenesis following status epilepticus, verifying the latent period with continuous EEG monitoring. Focusing on ultrastructural properties of the tripartite synapse in the CA1 region of hippocampus we found increased astrocyte ensheathment around both the presynaptic and postsynaptic elements, reduced synaptic AMPA receptor subunit and perisynaptic astrocyte GLT-1 expression, and increased number of docked vesicles at the presynaptic terminal. These findings were associated with an increase in frequency of the mEPSCs observed in patch clamp recordings of CA1 pyramidal cells. The results suggest a complex set of changes, some of which have been associated with increasingly excitable networks such as increased vesicles and mEPSC frequency, and some associated with compensatory mechanisms, such as increased astrocyte ensheathment. The diversity of ultrastructural and electrophysiological changes observed during epileptogeneiss suggests that potential drug targets for this period should be broadened to include all components of the tripartite synapse.

Dendritic spine morphology and memory formation depend on postsynaptic Caskin proteins

CASK-interactive proteins, Caskin1 and Caskin2, are multidomain neuronal scaffold proteins. Recent data from Caskin1 knockout animals indicated only a mild role of Caskin1 in anxiety and pain perception. In this work, we show that deletion of both Caskins leads to severe deficits in novelty recognition and spatial memory. Ultrastructural analyses revealed a reduction in synaptic profiles and dendritic spine areas of CA1 hippocampal pyramidal neurons of double knockout mice. Loss of Caskin proteins impaired LTP induction in hippocampal slices, while miniature EPSCs in dissociated hippocampal cultures appeared to be unaffected. In cultured Caskin knockout hippocampal neurons, overexpressed Caskin1 was enriched in dendritic spine heads and increased the amount of mushroom-shaped dendritic spines. Chemically induced LTP (cLTP) mediated enlargement of spine heads was augmented in the knockout mice and was not influenced by Caskin1. Immunocytochemistry and immunoprecipitation confirmed that Shank2, a master scaffold of the postsynaptic density, and Caskin1 co-localized within the same complex. Phosphorylation of AMPA receptors was specifically altered by Caskin deficiency and was not elevated by cLTP treatment further. Taken together, our results prove a previously unnoticed postsynaptic role of Caskin scaffold proteins and indicate that Caskins influence learning abilities via regulating spine morphology and AMPA receptor localisation.

Levetiracetam effect and electrophysiological mechanism of action in rats with cobalt-induced chronic epilepsy

Levetiracetam was initially developed as a nootropic drug, although since 2002 it has been used as anticonvulsant for the treatment of partial and generalized epilepsy syndromes. The purpose of the research was to investigate anti-paroxysmal activity of levetiracetam (LEV) on the model of cobalt-induced chronic epilepsy caused by the application of cobalt to the sensorimotor area of the rat cortex to evaluate LEV impact on the different stages of epileptogenesis. LEV effects were studied at the initial stage of the epileptogenesis (2nd day after the cobalt application) and at the stage of generalized paroxysmal activity (6th day after the cobalt application). The research showed that levetiracetam administration (dosages 50 mg/kg and 200 mg/kg) at the early stage of the epileptogenesis had no statistically significant effect on the development of paroxysmal activity in both primary and secondary epileptic areas: in the ipsi- and contralateral cortex, hypothalamus and hippocampus. LEV administration on 6th day (dosage 50 mg/kg) did not have statistical effect on the epileptogenesis, while at a dosage of 200 mg/kg on 6th day LEV significantly suppressed paroxysmal activity in the studied structures of rats with cobalt epilepsy. The strongest anti-paroxysmal effect was detected in hippocampus and was expressed as the normalization of bioelectrical activity and the appearance of a regular theta rhythm. Thus, LEV effects are mostly directed to the hippocampal area of epileptiform activity and, to a lesser extent, to the cortical area.

Advances in the Potential Biomarkers of Epilepsy

Epilepsy is a group of chronic neurological disorders characterized by recurrent, spontaneous, and unpredictable seizures. It is one of the most common neurological disorders, affecting tens of millions of people worldwide. Comprehensive studies on epilepsy in recent decades have revealed the complexity of epileptogenesis, in which immunological processes, epigenetic modifications, and structural changes in neuronal tissues have been identified as playing a crucial role. This review discusses the recent advances in the biomarkers of epilepsy. We evaluate the possible molecular background underlying the clinical changes observed in recent studies, focusing on therapeutic investigations, and the evidence of their safety and efficacy in the human population. This article reviews the pathophysiology of epilepsy, including recent reports on the effects of oxidative stress and hypoxia, and focuses on specific biomarkers and their clinical implications, along with further perspectives in epilepsy research.

The Reactive Plasticity of Hippocampal Ionotropic Glutamate Receptors in Animal Epilepsies

Ionotropic glutamate receptors (iGluRs) mediate the synaptic and metabolic actions of glutamate. These iGluRs are classified within the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type, kainate-type, and N-methyl-d-aspartate (NMDA)-type functional receptor families. The iGluR assemblies are regulated by transcription, alternative splicing, and cytoplasmic post-translational modifications. The iGluR subunit proteins are transported from the endoplasmic reticulum, inserted into the synaptic membranes, and anchored at their action site by different scaffolding and interacting proteins. The functional properties of iGluRs depend on their subunit composition, the amino acid sequence of the protein domains, and the scaffolding proteins in the synaptic membranes. The iGluRs are removed from the membranes by enzymatic action and endocytosis. Hippocampal iGluRs are rearranged through the upregulation and downregulation of the subunits following deafferentation and epileptic seizures. The rearrangement of iGluRs and the alteration of their subunit composition transform neurons into “pathological” cells, determining the further plasticity or pathology of the hippocampal formation. In the present review, we summarize the expression of AMPA, kainate, and NMDA receptor subunits following deafferentation, repeated mild seizures, and status epilepticus. We compare our results to literature descriptions, and draw conclusions as to the reactive plasticity of iGluRs in the hippocampus.

Changes in Functional Properties of Rat Hippocampal Neurons Following Pentylenetetrazole-induced Status Epilepticus

Pathophysiological remodeling processes following status epilepticus (SE) play a critical role in the pathophysiology of epilepsy but have not yet been not fully investigated. In the present study, we examined changes in intrinsic properties of pyramidal neurons, basal excitatory synaptic transmission, and short-term synaptic plasticity in hippocampal slices of rats after SE. Seizures were induced in 3-week-old rats by an intraperitoneal pentylenetetrazole (PTZ) injection. Only animals with generalized seizures lasting more than 30 min were included in the experiments. We found that CA1 pyramidal neurons became more excitable and started firing at a lower excitatory input due to a significant increase in input resistance. However, basal excitatory synaptic transmission was reduced in CA3-CA1 synapses, thus preventing the propagation of excitation through neural networks. A significant increase in paired-pulse facilitation 1 d after SE pointed to a decrease in the probability of glutamate release. Increased intrinsic excitability of neurons and decreased synaptic transmission differentially affected the excitability of a neural network. In terms of changes in seizure susceptibility after SE, we observed a significant increase in the maximal electroshock threshold 1 day after SE, suggesting a decrease in seizure susceptibility. However, after 1 week, there was no difference in seizure susceptibility between control and post-SE rats. The effects of SE on functional properties of hippocampal neurons were transient in the PTZ model, and most of them had recovered 1 week after SE. However, some minor alterations, such as smaller amplitude field potentials, were observed 1 month after SE.

Seizure-Induced Potentiation of AMPA Receptor-Mediated Synaptic Transmission in the Entorhinal Cortex

Excessive excitation is considered one of the key mechanisms underlying epileptic seizures. We investigated changes in the evoked postsynaptic responses of medial entorhinal cortex (ERC) pyramidal neurons by seizure-like events (SLEs), using the modified 4-aminopyridine (4-AP) model of epileptiform activity. Rat brain slices were perfused with pro-epileptic solution contained 4-AP and elevated potassium and reduced magnesium concentration. We demonstrated that 15-min robust epileptiform activity in slices leads to an increase in the amplitude of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated component of the evoked response, as well as an increase in the polysynaptic γ-aminobutyric acid (GABA) and N-methyl-D-aspartate (NMDA) receptor-mediated components. The increase in AMPA-mediated postsynaptic conductance depends on NMDA receptor activation. It persists for at least 15 min after the cessation of SLEs and is partly attributed to the inclusion of calcium-permeable AMPA receptors in the postsynaptic membrane. The mathematical modeling of the evoked responses using the conductance-based refractory density (CBRD) approach indicated that such augmentation of the AMPA receptor function and depolarization by GABA receptors results in prolonged firing that explains the increase in polysynaptic components, which contribute to overall network excitability. Taken together, our data suggest that AMPA receptor enhancement could be a critical determinant of sustained status epilepticus (SE).

ΔFosB Regulates Gene Expression and Cognitive Dysfunction in a Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is characterized by cognitive decline and 5- to 10-fold increased seizure incidence. How seizures contribute to cognitive decline in AD or other disorders is unclear. We show that spontaneous seizures increase expression of ΔFosB, a highly stable Fos-family transcription factor, in the hippocampus of an AD mouse model. ΔFosB suppressed expression of the immediate early gene c-Fos, which is critical for plasticity and cognition, by binding its promoter and triggering histone deacetylation. Acute histone deacetylase (HDAC) inhibition or inhibition of ΔFosB activity restored c-Fos induction and improved cognition in AD mice. Administration of seizure-inducing agents to nontransgenic mice also resulted in ΔFosB-mediated suppression of c-Fos, suggesting that this mechanism is not confined to AD mice. These results explain observations that c-Fos expression increases after acute neuronal activity but decreases with chronic activity. Moreover, these results indicate a general mechanism by which seizures contribute to persistent cognitive deficits, even during seizure-free periods.

NMDA Receptor GluN2 Subtypes Control Epileptiform Events in the Hippocampus

NMDA receptors (NMDARs) play a key role in synaptic plasticity and excitotoxicity. Subtype-specific role of NMDAR in neural disorders is an emerging area. Recent studies have revealed that mutations in NMDARs are a cause for epilepsy. Hippocampus is a known focal point for epilepsy. In hippocampus, expression of the NMDAR subtypes GluN1/GluN2A and GluN1/GluN2B is temporally regulated. However, the pharmacological significance of these subtypes is not well understood in epileptic context/models. To investigate this, epilepsy was induced in hippocampal slices by the application of artificial cerebrospinal fluid that contained high potassium but no magnesium. Epileptiform events (EFEs) were recorded from the CA1 and DG areas of hippocampus with or without subtype-specific antagonists. Irrespective of the age group, CA1 and DG showed epileptiform activity. The NMDAR antagonist AP5 was found to reduce the number of EFEs significantly. However, the application of subtype-specific antagonists (TCN 201 for GluN1/GluN2A and Ro 25-69811 for GluN1/GluN2B) revealed that EFEs had area-specific and temporal components. In slices from neonates, EFEs in CA1 were effectively reduced by Ro 25-69811, but were largely insensitive to TCN 201. In contrast, EFEs in DG were equally sensitive to both of the subtype-specific antagonists. However, the differential sensitivity for the antagonists observed in neonates was absent in later developmental stages. The study provides a functional insight into the NMDAR subtype-dependent contribution of EFEs in hippocampus of young rats, which may have implications in treating childhood epilepsy and avoiding unnecessary side effects of broad spectrum antagonists.

Alterations in Properties of Glutamatergic Transmission in the Temporal Cortex and Hippocampus Following Pilocarpine-Induced Acute Seizures in Wistar Rats

Temporal lobe epilepsy (TLE) is the most common type of focal epilepsy in humans, and is often developed after an initial precipitating brain injury. This form of epilepsy is frequently resistant to pharmacological treatment; therefore, the prevention of TLE is the prospective approach to TLE therapy. The lithium-pilocarpine model in rats replicates some of the main features of TLE in human, including the pathogenic mechanisms of cell damage and epileptogenesis after a primary brain injury. In the present study, we investigated changes in the properties of glutamatergic transmission during the first 3 days after pilocarpine-induced acute seizures in Wistar rats (PILO-rats). Using RT-PCR and electrophysiological techniques, we compared the changes in the temporal cortex (TC) and hippocampus, brain areas differentially affected by seizures. On the first day, we found a transient increase in a ratio of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl d-aspartate (NMDA) receptors in the excitatory synaptic response in pyramidal neurons of the CA1 area of the dorsal hippocampus, but not in the TC. This was accompanied by an increase in the slope of input-output (I/O) curves for fEPSPs recorded in CA1, suggesting an enhanced excitability in AMPARs in this brain area. There was no difference in the AMPA/NMDA ratio in control rats on the third day. We also revealed the alterations in NMDA receptor subunit composition in PILO-rats. The GluN2B/GluN2A mRNA expression ratio increased in the dorsal hippocampus but did not change in the ventral hippocampus or the TC. The kinetics of NMDA-mediated evoked EPSCs in hippocampal neurons was slower in PILO-rats compared with control animals. Ifenprodil, a selective antagonist of GluN2B-containing NMDARs, diminished the area and amplitude of evoked EPSCs in CA1 pyramidal cells more efficiently in PILO-rats compared with control animals. These results demonstrate that PILO-induced seizures lead to more severe alterations in excitatory synaptic transmission in the dorsal hippocampus than in the TC. Seizures affect the relative contribution of AMPA and NMDA receptor conductances in the synaptic response and increase the proportion of GluN2B-containing NMDARs in CA1 pyramidal neurons. These alterations disturb normal circuitry functions in the hippocampus, may cause neuron damage, and may be one of the important pathogenic mechanisms of TLE.

Changes of AMPA receptor properties in the neocortex and hippocampus following pilocarpine-induced status epilepticus in rats

Temporal lobe epilepsy (TLE) is the most common type of epilepsy in humans. The lithium-pilocarpine model in rodents reproduces some of the main features of human TLE. Three-week-old Wistar rats were used in this study. The changes in AMPA receptor subunit composition were investigated in several brain areas, including the medial prefrontal cortex (mPFC), the temporal cortex (TC), and the dorsal (DH) and ventral hippocampus (VH) during the first week following pilocarpine-induced status epilepticus (PILO-induced SE). In the hippocampus, GluA1 and GluA2 mRNA expression slightly decreased after PILO-induced SE and returned to the initial level on the seventh day. We did not detect any significant changes in mRNA expression of the GluA1 and GluA2 subunits in the TC, whereas in the mPFC we observed a significant increase of GluA1 mRNA expression on the third day and a decrease in GluA2 mRNA expression during the entire first week. Accordingly, the GluA1/GluA2 expression ratio increased in the mPFC, and the functional properties of the pyramidal cell excitatory synapses were disturbed. Using whole-cell voltage-clamp recordings, we found that on the third day following PILO-induced SE, isolated mPFC pyramidal neurons showed an inwardly rectifying current-voltage relation of kainate-evoked currents, suggesting the presence of GluA2-lacking calcium-permeable AMPARs (CP-AMPARs). IEM-1460, a selective antagonist of CP-AMPARs, significantly reduced the amplitude of evoked EPSC in pyramidal neurons from mPFC slices on the first and third days, but not on the seventh day. The antagonist had no effects on EPSC amplitude in slices from control animals. Thus, our data demonstrate that PILO-induced SE affects subunit composition of AMPARs in different brain areas, including the mPFC. SE induces transient (up to few days) incorporation of CP-AMPARs in the excitatory synapses of mPFC pyramidal neurons, which may disrupt normal circuitry functions.

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.

Neonatal seizures alter NMDA glutamate receptor GluN2A and 3A subunit expression and function in hippocampal CA1 neurons

Neonatal seizures are commonly caused by hypoxic and/or ischemic injury during birth and can lead to long-term epilepsy and cognitive deficits. In a rodent hypoxic seizure (HS) model, we have previously demonstrated a critical role for seizure-induced enhancement of the AMPA subtype of glutamate receptor (GluA) in epileptogenesis and cognitive consequences, in part due to GluA maturational upregulation of expression. Similarly, as the expression and function of the N-Methyl-D-aspartate (NMDA) subtype of glutamate receptor (GluN) is also developmentally controlled, we examined how early life seizures during the critical period of synaptogenesis could modify GluN development and function. In a postnatal day (P)10 rat model of neonatal seizures, we found that seizures could alter GluN2/3 subunit composition of GluNs and physiological function of synaptic GluNs. In hippocampal slices removed from rats within 48–96 h following seizures, the amplitudes of synaptic GluN-mediated evoked excitatory postsynaptic currents (eEPSCs) were elevated in CA1 pyramidal neurons. Moreover, GluN eEPSCs showed a decreased sensitivity to GluN2B selective antagonists and decreased Mg²⁺ sensitivity at negative holding potentials, indicating a higher proportion of GluN2A and GluN3A subunit function, respectively. These physiological findings were accompanied by a concurrent increase in GluN2A phosphorylation and GluN3A protein. These results suggest that altered GluN function and expression could potentially contribute to future epileptogenesis following neonatal seizures, and may represent potential therapeutic targets for the blockade of future epileptogenesis in the developing brain.

Protein kinase D promotes plasticity-induced F-actin stabilization in dendritic spines and regulates memory formation

PKD regulates the stabilization of the F-actin network within dendritic spines upon chemically induced plasticity changes and is needed for proper hippocampal LTP and spatial memory formation.

Actin turnover in dendritic spines influences spine development, morphology, and plasticity, with functional consequences on learning and memory formation. In nonneuronal cells, protein kinase D (PKD) has an important role in stabilizing F-actin via multiple molecular pathways. Using in vitro models of neuronal plasticity, such as glycine-induced chemical long-term potentiation (LTP), known to evoke synaptic plasticity, or long-term depolarization block by KCl, leading to homeostatic morphological changes, we show that actin stabilization needed for the enlargement of dendritic spines is dependent on PKD activity. Consequently, impaired PKD functions attenuate activity-dependent changes in hippocampal dendritic spines, including LTP formation, cause morphological alterations in vivo, and have deleterious consequences on spatial memory formation. We thus provide compelling evidence that PKD controls synaptic plasticity and learning by regulating actin stability in dendritic spines.

Analysis of gene expression during aging of CGNs in culture: implication of SLIT2 and NPY in senescence

Senescence is the major key factor that leads to the loss of neurons throughout aging. Cellular senescence is not the consequence of single cause, but there are multiple aspects which may induce senescence in a cell. Various causes such as gene expression, molecular interactions and protein processing and chromatin organization are described as causal factor for senescence. It is well known that the damage to the nuclear or mitochondrial DNA contributes to the aging either directly by inducing the apoptosis/cellular senescence or indirectly by altering cellular functions. The significant nuclear DNA damage with the age is directly associated with the continuous declining in DNA repair. The continuous decline in expression of topoisomerase 2 beta (Topo IIβ) in cultured cerebellar granule neurons over time indicated the decline in the repair of damage DNA. DNA Topo IIβ is an enzyme that is crucial for solving topological problems of DNA and thus has an important role in DNA repair. The enzyme is predominantly present in non-proliferating cells such as neurons. In this paper, we have studied the genes which were differentially expressed over time in cultured cerebellar granule neurons (CGNs) and identified potential genes associated with the senescence. Our results showed that the two genes neuropeptide Y (Npy) and Slit homolog 2 (Drosophila) (Slit2) gradually increase during aging, and upon suppression of these two genes, there was gradual increase in cell viability along with restoration of the expression of Topo IIβ and potential repair proteins.

Repeated application of 4-aminopyridine provoke an increase in entorhinal cortex excitability and rearrange AMPA and kainate receptors

Entorhinal cortex is a highly epilepsy-prone brain region. Effects of repetitive seizures on ionotropic glutamate receptors (iGluRs) were investigated in rat entorhinal cortex slices. Seizures were induced by daily administration of 4-aminopyridine (4-AP). Electrophysiological, pharmacological and histological investigations were carried out to determine changes in synaptic efficacy and in sensitivity of iGluRs due to recurring seizures. Repeated 4-AP-induced seizures increased the amplitude of evoked synaptic field responses in rat entorhinal cortical slices. While vulnerability to inhibition of AMPA receptors by the specific antagonist GYKI 52466 was slightly reduced, responsiveness to NMDA receptor antagonist APV remained unaffected. Testing of bivalent cation permeability of iGluRs revealed reduced Ca(2+)-influx through non-NMDA receptors. According to the semi-quantitative histoblot analysis GluA1-4, GluA1, GluA2, GluK5, GluN1 and GluN2A subunit protein expression differently altered. While there was a marked decrease in the level of GluA1-4, GluA2 and GluK5 receptor subunits, GluA1 and GluN2A protein levels moderately increased. The results indicate that brief convulsions, repeated daily for 10 days can increase overall entorhinal cortex excitability despite a reduction in AMPA/kainate receptor activity, probably through the alteration of local network susceptibility.

In vitro intrinsic optical imaging can be used for source determination in cortical slices

In the last decades intrinsic optical imaging has become a widely used technique for monitoring activity in vivo and in vitro. It is assumed that in brain slices the source of intrinsic optical signals (IOSs) is the change in light scattering caused by cell swelling or shrinkage. The aim of the present study was to find a correlation between electrical activity and parallel optical characteristics, elicited by 4-aminopyridine-containing or Mg(2+) -free medium in rat cortical brain slices. Electrophysiological signals and reflected light alterations were recorded during spontaneous seizure activity. Current source density (CSD) analysis was performed on the electrophysiological records. Direct correlation analysis of IOS to CSD was made, and source distribution provided by IOS and CSD methods was compared by determining Matthews correlation coefficient. The gradual development of seizure-like activity elicited the reduction of light reflectance. The main findings of our experiments are that long-term epileptiform activity resulted in persistent alteration in IOSs of brain slices. The observed IOS pattern remained stable after 1 h incubation in convulsants. The pattern of IOS shows good correlation with the data obtained from the CSD analysis. Persistent IOS changes provide information about the area-specific changes of basic excitability, which can serve as a background for ictal and interictal-like epileptiform activity. We can conclude that changes in IOSs correlate well with electrophysiological recordings under different conditions. Our experiments provide evidence that underlying synchronised neuronal processes produce parallel alterations in IOSs and electrophysiological activity.

3-Mercaptopropionic acid-induced repetitive seizures increase GluN2A expression in rat hippocampus: a potential neuroprotective role of cyclopentyladenosine

The N-methyl-D-aspartate receptor (NMDAR) is involved in synaptic plasticity, learning, memory, and neurological diseases like epilepsy and it is the major mediator of excitotoxicity. Functional NMDARs in the mature brain are heteromeric complexes composed of different subunits: GluN1 and GluN2. There are four different GluN2 subunits (A-D) and each of them critically determines the pharmacological and electrophysiological properties of NMDARs. GluN1 is ubiquitously expressed in the central nervous system while the highest GluN2A expression is in the hippocampus. Adenosine, an endogenous anticonvulsant, is a neuromodulator with a critical role in the regulation of neuronal activity, mediating its effect on specific receptors, among which adenosine A1 receptor is highly expressed in the hippocampus. In the present work hippocampal GluN2A expression after the convulsant drug 3-mercaptopropionic acid (MP) induced seizures and the effect of cyclopentyladenosine (CPA) given alone or prior to MP (CPA + MP) in an acute or repetitive experimental model was studied. CPA administered to rats for one or 4 days increases seizure threshold induced by MP. After one administration of MP, no significant difference in GluN2A expression was observed in CPA and CPA + MP by Western blot, although immunohistochemistry revealed an increase in CA2/3 area. However, repetitive MP administration during 4 days showed a significant increase of GluN2A expression, and the repetitive administration of CPA 30 min prior to MP caused a significant decrease of GluN2A expression with respect to MP treatment, returning to control levels. These results show that GluN2A subunit is involved in repetitive MP-induced seizures, while CPA administration displays a protective effect against it.

Calcium-permeable AMPA receptors are expressed in a rodent model of status epilepticus

OBJECTIVE

A study was undertaken to characterize the plasticity of AMPA receptor (AMPAR)-mediated neurotransmission in the hippocampus during status epilepticus (SE).

METHODS

SE was induced by pilocarpine, and animals were studied 10 minutes (refractory SE) or 60 minutes (late SE) after the onset of the first grade 5 seizures. AMPAR-mediated currents were recorded from CA1 pyramidal neurons and dentate granule cells (DGCs) by voltage clamp technique. The surface expression of GluA2 subunit on hippocampal membranes was determined using a biotinylation assay. GluA2 internalization and changes in intracellular calcium ([Ca](i)) levels were studied in hippocampal cultures using immunocytochemical and live-imaging techniques. AMPAR antagonist treatment of SE was evaluated by video and electroencephalography.

RESULTS

AMPAR-mediated currents recorded from CA1 neurons from refractory and late SE animals were inwardly rectifying, and philanthotoxin-sensitive; similar changes were observed in recordings obtained from DGCs from refractory SE animals. GluA2 subunit surface expression was reduced in the hippocampus during refractory and late SE. In cultured hippocampal pyramidal neurons, recurrent bursting diminished surface expression of the GluA2 subunit and enhanced its internalization rate. Recurrent bursting-induced increase in [Ca](i) levels was reduced by selective inhibition of GluA2-lacking AMPARs. GYKI-52466 terminated diazepam-refractory SE.

INTERPRETATION

During SE, there is rapid, ongoing plasticity of AMPARs with the expression of GluA2-lacking AMPARs. These receptors provide another source of Ca(2+) entry into the principal neurons. Benzodiazepam-refractory SE can be terminated by AMPAR antagonism. The data identify AMPARs as a potential therapeutic target for the treatment of SE.

Thinking of Co²⁺-staining explant tissue or cultured cells? How to make it reliable and specific

Ca(2+) and/or Zn(2+) entry into neurons and glial cells is often a key step driving the processes of neurodevelopment and disease. As a result, a major pre-occupation of many neuroscientists has been in tracking down when and where nervous tissues express ion channels with appreciable divalent ion permeability. The cobalt (Co(2+))-staining technique is one of the few techniques that allow a snapshot of the entire neuronal circuit, and selectively labels cells expressing divalent-permeable ion channels with a brown-black precipitate. Despite this, its use has been remarkably limited in the past decade. Reluctance to employ this approach has largely been related to an earlier concern with obtaining a reliable and reproducible means of visualizing transported Co(2+). Here we show that recent advances have resolved these issues, opening this straightforward and valuable technique to a much larger neuroscience audience.

Propofol effectively inhibits lithium-pilocarpine- induced status epilepticus in rats via downregulation of N-methyl-D-aspartate receptor 2B subunit expression

Status epilepticus was induced via intraperitoneal injection of lithium-pilocarpine. The inhibitory effects of propofol on status epilepticus in rats were judged based on observation of behavior, electroencephalography and 24-hour survival rate. Propofol (12.5-100 mg/kg) improved status epilepticus in a dose-dependent manner, and significantly reduced the number of deaths within 24 hours of lithium-pilocarpine injection. Western blot results showed that, 24 hours after induction of status epilepticus, the levels of N-methyl-D-aspartate receptor 2A and 2B subunits were significantly increased in rat cerebral cortex and hippocampus. Propofol at 50 mg/kg significantly suppressed the increase in N-methyl-D-aspartate receptor 2B subunit levels, but not the increase in N-methyl-D-aspartate receptor 2A subunit levels. The results suggest that propofol can effectively inhibit status epilepticus induced by lithium-pilocarpine. This effect may be associated with downregulation of N-methyl-D-aspartate receptor 2B subunit expression after seizures.

Hippocampus versus entorhinal cortex decoupling by an NR2 subunit-specific block of NMDA receptors in a rat in vitro model of temporal lobe epilepsy

The role of N-methyl-D-aspartate receptors (NMDARs) in the generation and maintenance of epileptic seizures has been widely investigated, however, little is known of possible separate roles played by NMDARs that contain different NR2 subunits. A better comprehension of how distinct NMDARs subtypes participate in seizure generation and/or diffusion may lead to the development of more targeted pharmacologic strategies to treat epilepsy. Therefore, we have performed an electrophysiologic investigation using a multielectrode array device, on slices comprising entorhinal cortex (EC) and hippocampus, continuously perfused in a Mg(2+) -free medium, with added 4-aminopiridine (4AP; 10-15 μm). Two separate rhythmic patterns of interictal-like activity were generated in EC and hippocampus, with EC seizures entrained to those in CA3, so that a significant degree of cross-correlation occurred. Perfusion with the NR2A-containing NMDAR antagonist [(R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid (NVP-AAM077; 50 nm) or Zn(2+) (200 nm), did not affect the rate of interictal-like events in EC and hippocampus; however, it significantly reduced their cross-correlation, causing a substantial decoupling of the two rhythm generators. The same effect was observed with (αR,βS)-α-(4-hydroxyphenyl)-β-methyl-4-(phenylmethyl)-1-piperidinepropanol maleate (Ro25-6981; 1 μm), when coapplied with a subthreshold dose of NVP-AAM077. Our results suggest that NR2 subunits may be crucial in entraining cortical networks, leading to recruitment of wider range oscillations during epilepsy. Therefore, a pharmacologic strategy directed onto NR2 subunits may help to limit seizure diffusion and recruitment of potentially entrained oscillatory networks.

Altered mRNA editing and expression of ionotropic glutamate receptors after kainic acid exposure in cyclooxygenase-2 deficient mice

Kainic acid (KA) binds to the AMPA/KA receptors and induces seizures that result in inflammation, oxidative damage and neuronal death. We previously showed that cyclooxygenase-2 deficient (COX-2(-/-)) mice are more vulnerable to KA-induced excitotoxicity. Here, we investigated whether the increased susceptibility of COX-2(-/-) mice to KA is associated with altered mRNA expression and editing of glutamate receptors. The expression of AMPA GluR2, GluR3 and KA GluR6 was increased in vehicle-injected COX-2(-/-) mice compared to wild type (WT) mice in hippocampus and cortex, whereas gene expression of NMDA receptors was decreased. KA treatment decreased the expression of AMPA, KA and NMDA receptors in the hippocampus, with a significant effect in COX-2(-/-) mice. Furthermore, we analyzed RNA editing levels and found that the level of GluR3 R/G editing site was selectively increased in the hippocampus and decreased in the cortex in COX-2(-/-) compared with WT mice. After KA, GluR4 R/G editing site, flip form, was increased in the hippocampus of COX-2(-/-) mice. Treatment of WT mice with the COX-2 inhibitor celecoxib for two weeks decreased the expression of AMPA/KA and NMDAR subunits after KA, as observed in COX-2(-/-) mice. After KA exposure, COX-2(-/-) mice showed increased mRNA expression of markers of inflammation and oxidative stress, such as cytokines (TNF-α, IL-1β and IL-6), inducible nitric oxide synthase (iNOS), microglia (CD11b) and astrocyte (GFAP). Thus, COX-2 gene deletion can exacerbate the inflammatory response to KA. We suggest that COX-2 plays a role in attenuating glutamate excitotoxicity by modulating RNA editing of AMPA/KA and mRNA expression of all ionotropic glutamate receptor subunits and, in turn, neuronal excitability. These changes may contribute to the increased vulnerability of COX-2(-/-) mice to KA. The overstimulation of glutamate receptors as a consequence of COX-2 gene deletion suggests a functional coupling between COX-2 and the glutamatergic system.

Matrix metalloproteinase-9 activity increased by two different types of epileptic seizures that do not induce neuronal death: a possible role in homeostatic synaptic plasticity

Matrix metalloproteases (MMPs) degrade or modify extracellular matrix or membrane-bound proteins in the brain. MMP-2 and MMP-9 are activated by treatments that result in a sustained neuronal depolarization and are thought to contribute to neuronal death and structural remodeling. At the synapse, MMP actions on extracellular proteins contribute to changes in synaptic efficacy during learning paradigms. They are also activated during epileptic seizures, and MMP-9 has been associated with the establishment of aberrant synaptic connections after neuronal death induced by kainate treatment. It remains unclear whether MMPs are activated by epileptic activities that do not induce cell death. Here we examine this point in two animal models of epilepsy that do not involve extensive cell damage. We detected an elevation of MMP-9 enzymatic activity in cortical regions of secondary generalization after focal seizures induced by 4-aminopyridine (4-AP) application in rats. Pro-MMP-9 levels were also higher in Wistar Glaxo Rijswijk (WAG/Rij) rats, a genetic model of generalized absence epilepsy, than they were in Sprague-Dawley rats, and this elevation was correlated with diurnally occurring spike-wave-discharges in WAG/Rij rats. The increased enzymatic activity of MMP-9 in these two different epilepsy models is associated with synchronized neuronal activity that does not induce widespread cell death. In these epilepsy models MMP-9 induction may therefore be associated with functions such as homeostatic synaptic plasticity rather than neuronal death.