NMDA receptor-dependent plasticity of granule cell spiking in the dentate gyrus of normal and epileptic rats

Rapidly repeated visual stimulation induces long-term potentiation of VEPs and increased content of membrane AMPA and NMDA receptors in the V1 cortex of cats

Studies report that rapidly repeated sensory stimulation can evoke LTP-like improvement of neural response in the sensory cortex. Whether this neural response potentiation is similar to the classic LTP induced by presynaptic electrical stimulation remains unclear. This study examined the effects of repeated high-frequency (9 Hz) versus low-frequency (1 Hz) visual stimulation on visually-evoked field potentials (VEPs) and the membrane protein content of AMPA / NMDA receptors in the primary visual cortex (V1) of cats. The results showed that repeated high-frequency visual stimulation (HFS) caused a long-term improvement in peak-to-peak amplitude of V1-cortical VEPs in response to visual stimuli at HFS-stimulated orientation (SO: 90°) and non-stimulated orientation (NSO: 180°), but the effect exhibited variations depending on stimulus orientation: the amplitude increase of VEPs in response to visual stimuli at SO was larger, reached a maximum earlier and lasted longer than at NSO. By contrast, repeated low-frequency visual stimulation (LFS) had not significantly affected the amplitude of V1-cortical VEPs in response to visual stimuli at both SO and NSO. Furthermore, the membrane protein content of the key subunit GluA1 of AMPA receptors and main subunit NR1 of AMPA receptors in V1 cortex was significantly increased after HFS but not LFS when compared with that of control cats. Taken together, these results indicate that HFS can induce LTP-like improvement of VEPs and an increase in membrane protein of AMPA and NMDA receptors in the V1 cortex of cats, which is similar to but less specific to stimulus orientation than the classic LTP.

Termination of convulsion seizures by destabilizing and perturbing seizure memory engrams

Epileptogenesis, arising from alterations in synaptic strength, shares mechanistic and phenotypic parallels with memory formation. However, direct evidence supporting the existence of seizure memory remains scarce. Leveraging a conditioned seizure memory (CSM) paradigm, we found that CSM enabled the environmental cue to trigger seizure repetitively, and activating cue-responding engram cells could generate CSM artificially. Moreover, cue exposure initiated an analogous process of memory reconsolidation driven by mammalian target of rapamycin-brain-derived neurotrophic factor signaling. Pharmacological targeting of the mammalian target of rapamycin pathway within a limited time window reduced seizures in animals and interictal epileptiform discharges in patients with refractory seizures. Our findings reveal a causal link between seizure memory engrams and seizures, which leads us to a deeper understanding of epileptogenesis and points to a promising direction for epilepsy treatment.

Kainate receptors modulate the microstructure of synchrony during dentate gyrus epileptiform activity

Temporal Lobe Epilepsy (TLE) is the most common form of epilepsy in adults. In TLE, recurrent mossy fiber (rMF) sprouting from dentate gyrus granule cells (DGCs) forms an aberrant epileptogenic network between dentate granule cells (DGCs) that operates via ectopically expressed kainate receptors (KARs). It was previously shown that KARs expressed at the rMF-DGC synapses play a prominent role in epileptiform network events in TLE. However, it is not well understood how KARs influence neuronal network dynamics and contribute to the generation of epileptiform network activity in the dentate gyrus. To address this question, we monitored the activity of DGCs using single-cell resolution calcium imaging performed in a reliable in vitro model of TLE. Under our experimental conditions, the most prominent DGC activity patterns were interictal-like epileptiform network events, which were correlated with high levels of neuronal synchronization. The pharmacological blockade of KARs reduced the frequency as well as the number of neurons involved in these events, without altering their spatiotemporal dynamics. Analysis of the microstructure of synchrony showed that blockade of KARs diminished the fraction of neurons forming the main functional cluster. Therefore, we propose that KARs act as modulators in the epileptic network by facilitating the recruitment of neurons into coactive cell assemblies, thereby contributing to the occurrence of epileptiform network events.

TREM2 and Microglia Contribute to the Synaptic Plasticity: from Physiology to Pathology

Synapses are bridges for information transmission in the central nervous system (CNS), and synaptic plasticity is fundamental for the normal function of synapses, contributing substantially to learning and memory. Numerous studies have proven that microglia can participate in the occurrence and progression of neurodegenerative diseases (NDDs), such as Alzheimer's disease (AD), by regulating synaptic plasticity. In this review, we summarize the main characteristics of synapses and synaptic plasticity under physiological and pathological conditions. We elaborate the origin and development of microglia and the two well-known microglial signaling pathways that regulate synaptic plasticity. We also highlight the unique role of triggering receptor expressed on myeloid cells 2 (TREM2) in microglia-mediated regulation of synaptic plasticity and its relationship with AD. Finally, we propose four possible ways in which TREM2 is involved in regulating synaptic plasticity. This review will help researchers understand how NDDs develop from the perspective of synaptic plasticity.

Epileptic seizures and link to memory processes

Epileptogenesis is a complex and not well understood phenomenon. Here, we explore the hypothesis that epileptogenesis could be "hijacking" normal memory processes, and how this hypothesis may provide new directions for epilepsy treatment. First, we review similarities between the hypersynchronous circuits observed in epilepsy and memory consolidation processes involved in strengthening neuronal connections. Next, we describe the kindling model of seizures and its relation to long-term potentiation model of synaptic plasticity. We also examine how the strengthening of epileptic circuits is facilitated during the physiological slow wave sleep, similarly as episodic memories. Furthermore, we present studies showing that specific memories can directly trigger reflex seizures. The neuronal hypersynchrony in early stages of Alzheimer's disease, and the use of anti-epileptic drugs to improve the cognitive symptoms in this disease also suggests a connection between memory systems and epilepsy. Given the commonalities between memory processes and epilepsy, we propose that therapies for memory disorders might provide new avenues for treatment of epileptic patients.

Deficits in Behavioral and Neuronal Pattern Separation in Temporal Lobe Epilepsy

In temporal lobe epilepsy, the ability of the dentate gyrus to limit excitatory cortical input to the hippocampus breaks down, leading to seizures. The dentate gyrus is also thought to help discriminate between similar memories by performing pattern separation, but whether epilepsy leads to a breakdown in this neural computation, and thus to mnemonic discrimination impairments, remains unknown. Here we show that temporal lobe epilepsy is characterized by behavioral deficits in mnemonic discrimination tasks, in both humans (females and males) and mice (C57Bl6 males, systemic low-dose kainate model). Using a recently developed assay in brain slices of the same epileptic mice, we reveal a decreased ability of the dentate gyrus to perform certain forms of pattern separation. This is because of a subset of granule cells with abnormal bursting that can develop independently of early EEG abnormalities. Overall, our results linking physiology, computation, and cognition in the same mice advance our understanding of episodic memory mechanisms and their dysfunction in epilepsy.SIGNIFICANCE STATEMENT People with temporal lobe epilepsy (TLE) often have learning and memory impairments, sometimes occurring earlier than the first seizure, but those symptoms and their biological underpinnings are poorly understood. We focused on the dentate gyrus, a brain region that is critical to avoid confusion between similar memories and is anatomically disorganized in TLE. We show that both humans and mice with TLE experience confusion between similar situations. This impairment coincides with a failure of the dentate gyrus to disambiguate similar input signals because of pathologic bursting in a subset of neurons. Our work bridges seizure-oriented and memory-oriented views of the dentate gyrus function, suggests a mechanism for cognitive symptoms in TLE, and supports a long-standing hypothesis of episodic memory theories.

Short-Term Depression of Sprouted Mossy Fiber Synapses from Adult-Born Granule Cells

Epileptic seizures potently modulate hippocampal adult neurogenesis, and adult-born dentate granule cells contribute to the pathologic retrograde sprouting of mossy fiber axons, both hallmarks of temporal lobe epilepsy. The characteristics of these sprouted synapses, however, have been largely unexplored, and the specific contribution of adult-born granule cells to functional mossy fiber sprouting is unknown, primarily due to technical barriers in isolating sprouted mossy fiber synapses for analysis. Here, we used DcxCreER^T2 transgenic mice to permanently pulse-label age-defined cohorts of granule cells born either before or after pilocarpine-induced status epilepticus (SE). Using optogenetics, we demonstrate that adult-born granule cells born before SE form functional recurrent monosynaptic excitatory connections with other granule cells. Surprisingly, however, although healthy mossy fiber synapses in CA3 are well characterized "detonator" synapses that potently drive postsynaptic cell firing through their profound frequency-dependent facilitation, sprouted mossy fiber synapses from adult-born cells exhibited profound frequency-dependent depression, despite possessing some of the morphological hallmarks of mossy fiber terminals. Mature granule cells also contributed to functional mossy fiber sprouting, but exhibited less synaptic depression. Interestingly, granule cells born shortly after SE did not form functional excitatory synapses, despite robust sprouting. Our results suggest that, although sprouted mossy fibers form recurrent excitatory circuits with some of the morphological characteristics of typical mossy fiber terminals, the functional characteristics of sprouted synapses would limit the contribution of adult-born granule cells to hippocampal hyperexcitability in the epileptic hippocampus.SIGNIFICANCE STATEMENT In the hippocampal dentate gyrus, seizures drive retrograde sprouting of granule cell mossy fiber axons. We directly activated sprouted mossy fiber synapses from adult-born granule cells to study their synaptic properties. We reveal that sprouted synapses from adult-born granule cells have a diminished ability to sustain recurrent excitation in the epileptic hippocampus, which raises questions about the role of sprouting and adult neurogenesis in sustaining seizure-like activity.

Dendrites of dentate gyrus granule cells contribute to pattern separation by controlling sparsity

The hippocampus plays a key role in pattern separation, the process of transforming similar incoming information to highly dissimilar, nonverlapping representations. Sparse firing granule cells (GCs) in the dentate gyrus (DG) have been proposed to undertake this computation, but little is known about which of their properties influence pattern separation. Dendritic atrophy has been reported in diseases associated with pattern separation deficits, suggesting a possible role for dendrites in this phenomenon. To investigate whether and how the dendrites of GCs contribute to pattern separation, we build a simplified, biologically relevant, computational model of the DG. Our model suggests that the presence of GC dendrites is associated with high pattern separation efficiency while their atrophy leads to increased excitability and performance impairments. These impairments can be rescued by restoring GC sparsity to control levels through various manipulations. We predict that dendrites contribute to pattern separation as a mechanism for controlling sparsity. © 2016 The Authors Hippocampus Published by Wiley Periodicals, Inc.

Mechanisms Responsible for Cognitive Impairment in Epilepsy

Epilepsy is often associated with cognitive and behavioral impairments that can have profound impact on the quality of life of patients. Although the mechanisms of cognitive impairment are not completely understood, we make an attempt to describe, from a systems perspective, how information processing is affected in epilepsy disorders. The aim of this review is to (1) define the nature of cognitive deficits associated with epilepsy, (2) review fundamental systems-level mechanisms underlying information processing, and (3) describe how information processing is dysfunctional in epilepsy and investigate the relative contributions of etiology, seizures, and interictal discharges (IDs). We conclude that these mechanisms are likely to be important and deserve more detailed scrutiny in the future.

A Spontaneous Recurrent Seizure Bioassay for Anti-Epileptogenic Molecules

Drug design in epilepsy is now tackling a new target - epileptogenesis. This is the process whereby a normal brain becomes susceptible to recurrent seizures. One of the stumbling blocks in the design and discovery of new chemical entities as antiepileptogenics is the implementation of an appropriate biological model. Current models, such as the maximal electroshock model, are models of seizures, not models of epileptogenesis. To develop such a model, we have extended and modified a chronic pilocarpine spontaneous recurrent seizure (SRS) model for the purposes of developing a bioassay with which to screen new compounds for putative antiepileptogenic bioactivity.

Effects of high-speed railway noise on the synaptic ultrastructure and phosphorylated-CaMKII expression in the central nervous system of SD rats

To investigate the toxic effects of high-speed railway noise on learning and memory function, Sprague-Dawley (SD) rats were exposed to high-speed railway noise for 90 days. The noise was recorded from an actual environment and adjusted to a day-night equivalent continuous A-weighted sound pressure level (L(dn)) of 70dB(A). Transmission electron microscopy (TEM) and Western blot analysis were used to observe the synaptic ultrastructure and detect the level of phosphorylated-Ca(2+)/calmodulin-dependent protein kinase II (p-CaMKII), respectively, in the hippocampus, temporal lobe and amygdala. Compared with the sham control group, the results of the TEM showed that the width of the noise model group's synaptic cleft increased markedly in the hippocampus, amygdala (P<0.05) and temporal lobe (P<0.01); the thickness of postsynaptic density (PSD) decreased significantly (P<0.01). The results of the TEM suggest that the synaptic plasticity of structure and function were abnormal and that this abnormality resulted in a reduction in synaptic transmission efficiency. This reduction may have led to dysfunctions in learning and memory. Additionally, the Western blot analyses revealed that the level of p-CaMKII decreased significantly in the temporal lobe of the noise model group compared with the sham control group (P<0.05). The results of the Western blot analysis indicate a reduction in synaptic transmission efficiency, which resulted in impairments in learning and memory function in the temporal lobe. Both of the above conclusions are consistent with each other.

Synaptic plasticity of NMDA receptors: mechanisms and functional implications

Beyond their well-established role as triggers for LTP and LTD of fast synaptic transmission mediated by AMPA receptors, an expanding body of evidence indicates that NMDA receptors (NMDARs) themselves are also dynamically regulated and subject to activity-dependent long-term plasticity. NMDARs can significantly contribute to information transfer at synapses particularly during periods of repetitive activity. It is also increasingly recognized that NMDARs participate in dendritic synaptic integration and are critical for generating persistent activity of neural assemblies. Here we review recent advances on the mechanisms and functional consequences of NMDAR plasticity. Given the unique biophysical properties of NMDARs, synaptic plasticity of NMDAR-mediated transmission emerges as a particularly powerful mechanism for the fine tuning of information encoding and storage throughout the brain.

Synaptic kainate receptors in interplay with INaP shift the sparse firing of dentate granule cells to a sustained rhythmic mode in temporal lobe epilepsy

Dentate granule cells, at the gate of the hippocampus, use coincidence detection of synaptic inputs to code afferent information under a sparse firing regime. In both human patients and animal models of temporal lobe epilepsy, mossy fibers sprout to form an aberrant glutamatergic network between dentate granule cells. These new synapses operate via long-lasting kainate receptor-mediated events, which are not present in the naive condition. Here, we report that in chronic epileptic rat, aberrant kainate receptors in interplay with the persistent sodium current dramatically expand the temporal window for synaptic integration. This introduces a multiplicative gain change in the input-output operation of dentate granule cells. As a result, their sparse firing is switched to an abnormal sustained and rhythmic mode. We conclude that synaptic kainate receptors dramatically alter the fundamental coding properties of dentate granule cells in temporal lobe epilepsy.

A selective interplay between aberrant EPSPKA and INaP reduces spike timing precision in dentate granule cells of epileptic rats

Spike timing precision is a fundamental aspect of neuronal information processing in the brain. Here we examined the temporal precision of input–output operation of dentate granule cells (DGCs) in an animal model of temporal lobe epilepsy (TLE). In TLE, mossy fibers sprout and establish recurrent synapses on DGCs that generate aberrant slow kainate receptor–mediated excitatory postsynaptic potentials (EPSP_KA) not observed in controls. We report that, in contrast to time-locked spikes generated by EPSP_AMPA in control DGCs, aberrant EPSP_KA are associated with long-lasting plateaus and jittered spikes during single-spike mode firing. This is mediated by a selective voltage-dependent amplification of EPSP_KA through persistent sodium current (I_NaP) activation. In control DGCs, a current injection of a waveform mimicking the slow shape of EPSP_KA activates I_NaP and generates jittered spikes. Conversely in epileptic rats, blockade of EPSP_KA or I_NaP restores the temporal precision of EPSP–spike coupling. Importantly, EPSP_KA not only decrease spike timing precision at recurrent mossy fiber synapses but also at perforant path synapses during synaptic integration through I_NaP activation. We conclude that a selective interplay between aberrant EPSP_KA and I_NaP severely alters the temporal precision of EPSP–spike coupling in DGCs of chronic epileptic rats.

Co-localization of excitatory and inhibitory transmitters in the brain

OBJECTIVES

To review the understanding about co-localisation of amino acid transmitters in the brain.

RESULTS

The idea that neurons release the same transmitter at all their synapses is associated with Henry Dale and formulated as Dale's principle by John Eccles. This idea has been modified during the last years based on several studies showing that transmitters can co-exist at the same synapse. First, a large body of evidence was presented showing that a classical transmitter can be co-localized with different types of neuropeptides. Then, several studies showed that a synapse could co-release two classical transmitters.

CONCLUSION

This review presents and discusses data from our laboratory showing co-release of glutamate/gamma-aminobutyric acid (GABA), aspartate/glutamate and aspartate/GABA from different types of hippocampal synapses.

Dentate filter function is altered in a proepileptic fashion during aging

PURPOSE

The elderly have an increased incidence and prevalence for seizure disorders. Further, since up to 50% of these cases have no identifiable antecedent, it has been hypothesized that aging of the central nervous system itself may be epileptogenic. Aged rats, compared to adults, exhibit a greater susceptibility to and severity of seizures associated with hippocampal activation. Whether this aging-related change reflects proconvulsive changes in limbic circuitry is unknown and thus was the focus of this study.

METHODS

Hippocampal slices from adult and aged Fischer 344 rats were examined using electrophysiological techniques. The dentate gyrus was our model region since it is involved with both wet-dog shakes and limbic seizures, and it is affected preferentially with age.

RESULTS

No differences were noted between groups in field potential activity elicited with low frequency stimulation. In contrast, 5-Hz molecular layer stimulation could evoke multiple population spikes in approximately 40% of aged versus 0% of adult slices. Further, recording in CA3 revealed that this stimulation paradigm could elicit multiple spikes in aged, but not adult, slices that frequently evolved into spontaneous epileptiform bursts. This change in the capacity of the dentate to respond to and filter afferent input was associated with an aging-related decrease in the frequency of spontaneous IPSPs and an increased propensity for large amplitude prolonged EPSPs following disinhibition.

CONCLUSIONS

These epileptogenic changes in dentate function and circuitry could contribute to the exacerbated susceptibility for hippocampal seizures in aged rodents, as well as the aging-related decline in spatial learning and memory.

Do seizures affect the developing brain? Lessons from the laboratory

Laboratory models of prolonged seizures and status epilepticus in developing animals demonstrate age- and model-dependent propensity for brain injury. Even in models without overt brain injury, plasticity, which leads to epileptogenicity as well as to behavioral and cognitive effects, has been demonstrated. Brief, recurrent seizures in the neonatal period not only appear to exhibit plasticity that can be anatomically and physiologically meaningful but also seem to produce cognitive deficits. Translation of these findings into clinical practice is limited by the effects chronic therapy may have on brain development. There is little evidence that available treatments can effectively alter epileptogenesis. However, it is widely agreed that prolonged seizures and status epilepticus can carry negative consequences. Preventing epileptogenesis remains an important goal to modify the development of comorbidities, and it represents an area of research in need of much progress. For now, prevention of prolonged seizures with early intervention is important and is the most effective available option to minimize the potential short- and long-term adverse effects of prolonged seizures and optimize patient outcomes.

The intracerebroventricular kainic acid-induced damage affects animal nociceptive behavior

In the present study, we examined nociceptive behaviors on various pain models after the pretreatment of kainic acid intracerebroventricularly. We found that intracerebroventricular administration of kainic acid shows significant neuronal damage on the hippocampal CA3 region in the brain slices stained with cresyl violet. Compared to the control group, intracerebroventricular pretreatment of kainic acid significantly attenuated nocifensive behaviors induced by intraplantar formalin (only in the 2nd phase), intrathecal glutamate, TNF-alpha or IL-1beta. However, nocifensive behaviors induced by intraperitoneal acetic acid (writhing test), intrathecal substance P or IFN-gamma were not affected by the pretreatment of kainic acid. These results suggest that (1) KA-induced alterations of nocifensive behaviors are related to the neuronal death of the hippocampal formation, especially CA3 pyramidal neurons and (2) nocifensive behaviors induced by formalin, acetic acid, SP, glutamate, and pro-inflammatory cytokines were modulated in a different manner.

The effects of aging on dentate circuitry and function

The central nervous system (CNS) undergoes a variety of anatomic, physiologic, and behavioral changes during aging. One region that has received a great deal of attention is the hippocampal formation due to the increased incidence of impaired spatial learning and memory with age. The hippocampal formation is also highly susceptible to Alzheimer's disease, ischemia/hypoxia, and seizure generation, the three most common aging-related neurological disorders. While data reveal that the dentate gyrus plays a key role in hippocampal function and dysfunction, the majority of electrophysiological studies that have examined the effects of age on the hippocampal formation have focused on CA3 and CA1. We perceive this to be an oversight and consequently will highlight data in this review which demonstrate an age-related disruption in dentate circuitry and function, and propose that these changes contribute to the decline in hippocampal-dependent behavior seen with "normal" aging.

Unmasking recurrent excitation generated by mossy fiber sprouting in the epileptic dentate gyrus: an emergent property of a complex system

Seizure-induced sprouting of the mossy fiber pathway in the dentate gyrus has been observed nearly universally in experimental models of limbic epilepsy and in the epileptic human hippocampus. The observation of progressive mossy fiber sprouting induced by kindling demonstrated that even a few repeated seizures are sufficient to alter synaptic connectivity and circuit organization. As it is now recognized that seizures induce synaptic reorganization in hippocampal and cortical pathways, the implications of seizure-induced synaptic reorganization for circuit properties and function have been subjects of intense interest. Detailed anatomical characterization of the sprouted mossy fiber pathway has revealed that the overwhelming majority of sprouted synapses in the inner molecular layer of the dentate gyrus form recurrent excitatory connections, and are thus likely to contribute to recurrent excitation and potentially to enhanced susceptibility to seizures. Nevertheless, difficulties in detecting functional abnormalities in circuits reorganized by mossy fiber sprouting and the fact that some sprouted axons appear to form synapses with inhibitory interneurons have been cited as evidence that sprouting may not contribute to seizure susceptibility, but could form recurrent inhibitory circuits and be a compensatory response to prevent seizures. Quantitative analysis of the synaptic connections of the sprouted mossy fiber pathway, assessment of the functional features of sprouted circuitry using reliable physiological measures, and the perspective of complex systems analysis of neural circuits strongly support the view that the functional effects of the recurrent excitatory circuits formed by mossy fiber sprouting after seizures or injury emerge only conditionally and intermittently, as observed with spontaneous seizures in human epilepsy. The recognition that mossy fiber sprouting is induced after hippocampal injury and seizures and contributes conditionally to emergence of recurrent excitation has provided a conceptual framework for understanding how injury and seizure-induced circuit reorganization may contribute to paroxysmal network synchronization, epileptogenesis, and the consequences of repeated seizures, and thus has had a major influence on understanding of fundamental aspects of the epilepsies.

Seizures and Antiepileptic Drugs: Does Exposure Alter Normal Brain Development?

Seizures and antiepileptic drugs (AEDs) affect brain development and have long-term neurological consequences. The specific molecular and cellular changes, the precise timing of their influence during brain development, and the full extent of the long-term consequences of seizures and AEDs exposure have not been established. This review critically assesses both the basic and clinical science literature on the effects of seizures and AEDs on the developing brain and finds that evidence exists to support the hypothesis that both seizures and antiepileptic drugs influence a variety of biological process, at specific times during development, which alter long-term cognition and epilepsy susceptibility. More research, both clinical and experimental, is needed before changes in current clinical practice, based on the scientific data, can be recommended.

Kynurenate and 7-chlorokynurenate formation in chronically epileptic rats

PURPOSE

The tryptophan metabolite kynurenic acid (KYNA) and its synthetic derivative, 7-chlorokynurenic acid (7-Cl-KYNA), are antagonists of the glycine co-agonist ("glycine(B)") site of the N-methyl-D-aspartate (NMDA)-receptor. Both compounds have neuroprotective and anticonvulsive properties but do not readily penetrate the blood-brain barrier. However, KYNA and 7-Cl-KYNA can be formed in, and released from, astrocytes after the peripheral administration of their transportable precursors kynurenine and 4-chlorokynurenine, respectively. The present study was designed to examine these biosynthetic processes, as well as astrogliosis, in animals with spontaneously recurring seizures.

METHODS

The fate and formation of KYNA and 7-Cl-KYNA was studied in vivo (microdialysis) and in vitro (tissue slices) in rats exhibiting chronic seizure activity (pilocarpine model) and in appropriate controls. Neuronal loss and gliosis in these animals were examined immunohistochemically.

RESULTS

In vivo microdialysis revealed higher ambient extracellular KYNA levels and enhanced de novo formation of 7-Cl-KYNA in the entorhinal cortex and hippocampus in epileptic rats. Complementary studies in tissue slices showed increased neosynthesis of KYNA and 7-Cl-KYNA in the same two brain areas. Microscopic analysis revealed pronounced astrocytic reactions in entorhinal cortex and hippocampus in epileptic animals.

CONCLUSIONS

These results demonstrate that the epileptic brain can synthesize glycine(B) receptor antagonists in situ. Astrogliosis probably accounts for their enhanced production in chronically epileptic rats. These results bode well for the use of 4-chlorokynurenine in the treatment of chronic seizure disorders.

Glutamate Receptor Antagonists and Benzodiazepine Inhibit the Progression of Granule Cell Dispersion in a Mouse Model of Mesial Temporal Lobe Epilepsy

PURPOSE

Unilateral intrahippocampal injection of kainic acid (KA) in adult mice induces the progressive dispersion of dentate granule cells, one of the characteristic pathologic changes of mesial temporal lobe epilepsy. However, little is known about the mechanisms that trigger this dispersion. In this study, the possible involvement of glutamatergic and gamma-aminobutyric acid (GABA)ergic neurotransmissions in the development of granule cell dispersion (GCD) was examined in this model.

METHODS

Antagonists of N-methyl-d-aspartate (NMDA) receptor (MK-801) and non-NMDA receptor (GYKI52466), and an agonist of benzodiazepine-GABA(A) receptor (midazolam) were injected before and after KA in various ways, and the morphologic changes of the hippocampus, especially GCD, were examined.

RESULTS

MK-801 (5 mg/kg, i.p.) did not reduce GCD when injected 2 h before KA injection but inhibited GCD almost completely for <or=14 days, when injected 4 h after KA. However, mild to moderate dispersion was observed at 28 days, indicating that MK-801 may delay the progression of GCD. Similarly, daily treatment with MK-801 (2 x 1 mg/kg i.p./day) for the first 26 days after KA significantly reduced GCD. In contrast, GYKI52466 (30 mg/kg, s.c.) was effective only when it was injected before KA. A significant reduction of GCD was also observed after continuous administration of midazolam (10 mg/kg/h) after KA.

CONCLUSIONS

These data show that GCD in this mouse model is triggered by either the stimulation of the NMDA receptor or reduction of GABA(A)-mediated inhibition after intrahippocampal injection of KA. It is suggested that the increased excitation or the reduced inhibition or both could be one of the factors triggering or maintaining or both the process of GCD.

Kindling and status epilepticus models of epilepsy: rewiring the brain

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

GABAergic synapses in hippocampus exocytose aspartate on to NMDA receptors: quantitative immunogold evidence for co-transmission

We previously found evidence for the exocytosis of aspartate from excitatory nerve terminals in hippocampus [J. Neurosci. 18, (1998) 6059]. Here we show, by immunogold electron microscopy in hippocampal slices that aspartate is co-localized and co-exocytosed with GABA from synaptic vesicles in nerve endings assumed to be inhibitory on dentate granule cells and CA1 pyramidal cells. By immunogold double labeling cytochemistry in perfusion fixed hippocampus, we further find that GABA-positive terminals forming symmetric synaptic specializations on perikarya of granule and pyramidal cells express NMDA receptors. In addition, NMDA receptors are present at synapses on granule cell bodies that have asymmetric synaptic specializations and contain high levels of GABA. Glutamate levels are low in the described types of GABA-positive nerve terminals, but high in terminals making asymmetric synapses on dendritic spines, whereas aspartate is localized with high levels in all of these types of terminal. We propose that aspartate is exocytotically released not only from glutamatergic terminals, but also from GABAergic terminals to act on NMDA receptors and may have a role in the regulation of synaptic transmission. Under pathological conditions, release of an excitatory transmitter at an inhibitory synapse could contribute to the development of, for example, epilepsy.

The recurrent mossy fiber pathway of the epileptic brain

The dentate gyrus is believed to play a key role in the pathogenesis of temporal lobe epilepsy. In normal brain the dentate granule cells serve as a high-resistance gate or filter, inhibiting the propagation of seizures from the entorhinal cortex to the hippocampus. The filtering function of the dentate gyrus depends in part on the near absence of monosynaptic connections among granule cells. In humans with temporal lobe epilepsy and in animal models of temporal lobe epilepsy, dentate granule cells form an interconnected synaptic network associated with loss of hilar interneurons. This recurrent mossy fiber pathway mediates reverberating excitation that can reduce the threshold for granule cell synchronization. Factors that augment activity in this pathway include modest increases in [K+]o; loss of GABA inhibition; short-term, frequency-dependent facilitation (frequencies of 1-2 Hz); feedback activation of kainate autoreceptors; and release of zinc from recurrent mossy fiber boutons. Factors that diminish activity include short-term, frequency-dependent depression (frequencies < 1 Hz); feedback activation of type II metabotropic glutamate receptors; and the potential release of GABA, neuropeptide Y, adenosine, and dynorphin from recurrent mossy fiber boutons. The axon sprouting and reactive synaptogenesis that follow seizure-related brain damage can also create or strengthen recurrent excitation in other brain regions. These changes are expected to facilitate participation of these regions in seizures. Thus, reactive processes that are often considered important for recovery of function after most brain injuries probably contribute to neurological dysfunction in epilepsy.

Electrophysiological evidence of monosynaptic excitatory transmission between granule cells after seizure-induced mossy fiber sprouting

Mossy fiber sprouting is a form of synaptic reorganization in the dentate gyrus that occurs in human temporal lobe epilepsy and animal models of epilepsy. The axons of dentate gyrus granule cells, called mossy fibers, develop collaterals that grow into an abnormal location, the inner third of the dentate gyrus molecular layer. Electron microscopy has shown that sprouted fibers from synapses on both spines and dendritic shafts in the inner molecular layer, which are likely to represent the dendrites of granule cells and inhibitory neurons. One of the controversies about this phenomenon is whether mossy fiber sprouting contributes to seizures by forming novel recurrent excitatory circuits among granule cells. To date, there is a great deal of indirect evidence that suggests this is the case, but there are also counterarguments. The purpose of this study was to determine whether functional monosynaptic connections exist between granule cells after mossy fiber sprouting. Using simultaneous recordings from granule cells, we obtained direct evidence that granule cells in epileptic rats have monosynaptic excitatory connections with other granule cells. Such connections were not obtained when age-matched, saline control rats were examined. The results suggest that indeed mossy fiber sprouting provides a substrate for monosynaptic recurrent excitation among granule cells in the dentate gyrus. Interestingly, the characteristics of the excitatory connections that were found indicate that the pathway is only weakly excitatory. These characteristics may contribute to the empirical observation that the sprouted dentate gyrus does not normally generate epileptiform discharges.

Brain tumor and seizures: pathophysiology and its implications for treatment revisited

Seizures affect approximately 50% of patients with primary and metastatic brain tumors. Partial seizures have the highest incidence, followed by secondarily generalized, depending on histologic subtype, location, and tumor extent. The underlying pathophysiologic mechanisms of tumor-associated seizures are poorly understood and include theories of altered peritumoral amino acids, regional metabolism, pH, neuronal or glial enzyme and protein expression, as well as immunologic activity. An involvement of changed distribution and function of N-methyl-d-aspartate subclass of glutamate receptors also has been suggested. The often unpredictable responses to seizures after surgical tumor removal add substantial evidence that multiple factors are involved. The therapy of tumor-related seizures is far from perfect. Several factors contribute to these treatment difficulties, such as tumor growth and drug interactions; however, one of the main reasons for poor seizure control may result from the insufficient or even absent influence of the currently available antiepileptic drugs (AEDs) on most of the pathophysiologic mechanisms of tumor-related seizures. Studies are needed to elucidate more clearly the pathophysiologic mechanisms of tumor-related seizures and to identify and develop the optimal AEDs.

Contribution of NMDA receptor channels to the expression of LTP in the hippocampal dentate gyrus

The role of glutamatergic NMDA receptor channels (NMDARs) in the induction of long-term potentiation (LTP) has been well established. In contrast, whether or not NMDARs contribute to the expression of LTP has been an issue of debate. In this study, we investigated the contribution of NMDARs to LTP expression in the hippocampal dentate gyrus (DG) by stimulating perforant path afferents with short bursts of pulses delivered at a moderate frequency (40 Hz), instead of using the traditional protocol of a single stimulus at a low frequency (<0.1 Hz). The synaptic summation provided by the "burst" protocol enabled us to measure the NMDAR-mediated component of synaptic responses (NMDA component), defined as the NMDAR antagonist D-2-amino-5-phosphonovalerate (APV2+)-sensitive component, in the presence of physiological concentrations of Mg (1 mM). Intracellular recordings were obtained from DG granule cells of rabbit hippocampal slices, and excitatory postsynaptic potentials (EPSPs) were measured in terms of the integrated area of their profiles. At 40 Hz, frequency facilitation of the evoked EPSPs was observed. The NMDA component gradually increased during the five-pulse train and frequency facilitation was significantly reduced after the application of APV. We tested the hypothesis that NMDARs undergo potentiation in LTP by comparing the NMDA/non-NMDA ratio of the synaptic responses in control and LTP groups. An increase in the ratio was observed in the LTP group, strongly suggesting potentiation of NMDARs. To infer changes in conductance at individual synapses based on EPSPs recorded at the soma, we constructed a compartmental model of a morphologically reconstructed DG granule cell. The effect on the NMDA/non-NMDA ratio of changes in AMPA and NMDA component synaptic conductance, and of differences in the distribution of activated synapses, was studied with computer simulations. The results confirmed that NMDARs are potentiated after the induction of LTP and contribute significantly to the expression of potentiation under physiological conditions.

Physiological and structural evidence for hippocampal involvement in persistent seizure susceptibility after traumatic brain injury

Epilepsy is a common outcome of traumatic brain injury (TBI), but the mechanisms of posttraumatic epileptogenesis are poorly understood. One clue is the occurrence of selective hippocampal cell death after fluid-percussion TBI in rats, consistent with the reported reduction of hippocampal volume bilaterally in humans after TBI and resembling hippocampal sclerosis, a hallmark of temporal-lobe epilepsy. Other features of temporal-lobe epilepsy, such as long-term seizure susceptibility, persistent hyperexcitability in the dentate gyrus (DG), and mossy fiber synaptic reorganization, however, have not been examined after TBI. To determine whether TBI induces these changes, we used a well studied model of TBI by weight drop on somatosensory cortex in adult rats. First, we confirmed an early and selective cell loss in the hilus of the DG and area CA3 of hippocampus, ipsilateral to the impact. Second, we found persistently enhanced susceptibility to pentylenetetrazole-induced convulsions 15 weeks after TBI. Third, by applying GABA(A) antagonists during field-potential and optical recordings in hippocampal slices 3 and 15 weeks after TBI, we unmasked a persistent, abnormal APV-sensitive hyperexcitability that was bilateral and localized to the granule cell and molecular layers of the DG. Finally, using Timm histochemistry, we detected progressive sprouting of mossy fibers into the inner molecular layers of the DG bilaterally 2-27 weeks after TBI. These findings are consistent with the development of posttraumatic epilepsy in an animal model of impact head injury, showing a striking similarity to the enduring behavioral, functional, and structural alterations associated with temporal-lobe epilepsy.

Glutamate receptor involvement in dentate granule cell epileptiform activity evoked by mossy fiber stimulation

In many persons with temporal lobe epilepsy, dentate granule cells form an interconnected synaptic network. This recurrent mossy fiber circuit mediates reverberating excitation that may facilitate seizure propagation by synchronizing granule cell discharge. The involvement of specific glutamate receptors in granule cell epileptiform activity evoked by stimulating the mossy fibers was investigated with use of rat hippocampal slices superfused with bicuculline, with or without increasing [K+](o) to 6 mM. The occurrence of short-latency mossy fiber-evoked granule cell epileptiform activity in slices from pilocarpine-treated rats correlated with the presence and extent of recurrent mossy fiber growth. Blockade of AMPA receptors nearly abolished the orthodromic component of the response; subsequent antagonism of kainate receptors as well appeared to have no further action. Antagonism of NMDA receptors reduced the duration of epileptiform discharge, but increased the amplitude of population spikes within the evoked burst. Thus AMPA and NMDA, but perhaps not kainate, receptors play an important role in this type of epileptiform activity. Activation of type II metabotropic glutamate receptors, which inhibits the release of glutamate from mossy fiber boutons, reduced the magnitude of epileptiform discharge. This action was reversed by a partial agonist of these receptors. However, neither an agonist nor an antagonist of type III metabotropic glutamate receptors significantly altered the response. Considering the importance of synchronous granule cell discharge for seizure propagation from the entorhinal cortex to the hippocampus, agonists of type II metabotropic glutamate receptors may be useful in suppressing such discharge both experimentally and clinically.