Increased excitatory synaptic activity and local connectivity of hippocampal CA1 pyramidal cells in rats with kainate-induced epilepsy

Excitotoxic glutamate levels cause the secretion of resident endoplasmic reticulum proteins

Dysregulation of synaptic glutamate levels can lead to excitotoxicity such as that observed in stroke, traumatic brain injury, and epilepsy. The role of increased intracellular calcium (Ca2+ ) in the development of excitotoxicity is well established. However, less is known regarding the impact of glutamate on endoplasmic reticulum (ER)-Ca2+ -mediated processes such as proteostasis. To investigate this, we expressed a secreted ER Ca2+ modulated protein (SERCaMP) in primary cortical neurons to monitor exodosis, a phenomenon whereby ER calcium depletion causes the secretion of ER-resident proteins that perform essential functions to the ER and the cell. Activation of glutamatergic receptors (GluRs) led to an increase in SERCaMP secretion indicating that normally ER-resident proteins are being secreted in a manner consistent with ER Ca2+ depletion. Antagonism of ER Ca2+ channels attenuated the effects of glutamate and GluR agonists on SERCaMP release. We also demonstrate that endogenous proteins containing an ER retention/retrieval sequence (ERS) are secreted in response to GluR activation supporting that neuronal activation by glutamate promotes ER exodosis. Ectopic expression of KDEL receptors attenuated the secretion of ERS-containing proteins caused by GluR agonists. Taken together, our data indicate that excessive GluR activation causes disruption of neuronal proteostasis by triggering the secretion of ER-resident proteins through ER Ca2+ depletion and describes a new facet of excitotoxicity.

Distinct Features of Interictal Activity Predict Seizure Localization and Burden in a Mouse Model of Childhood Epilepsy

The epileptic brain is distinguished by spontaneous seizures and interictal epileptiform discharges (IEDs). Basic patterns of mesoscale brain activity outside of seizures and IEDs are also frequently disrupted in the epileptic brain and likely influence disease symptoms, but are poorly understood. We aimed to quantify how interictal brain activity differs from that in healthy individuals, and identify what features of interictal activity influence seizure occurrence in a genetic mouse model of childhood epilepsy. Neural activity across the majority of the dorsal cortex was monitored with widefield Ca2+ imaging in mice of both sexes expressing a human Kcnt1 variant (Kcnt1m/m ) and wild-type controls (WT). Ca2+ signals during seizures and interictal periods were classified according to their spatiotemporal features. We identified 52 spontaneous seizures, which emerged and propagated within a consistent set of susceptible cortical areas, and were predicted by a concentration of total cortical activity within the emergence zone. Outside of seizures and IEDs, similar events were detected in Kcnt1m/m and WT mice, suggesting that the spatial structure of interictal activity is similar. However, the rate of events whose spatial profile overlapped with where seizures and IEDs emerged was increased, and the characteristic global intensity of cortical activity in individual Kcnt1m/m mice predicted their epileptic activity burden. This suggests that cortical areas with excessive interictal activity are vulnerable to seizures, but epilepsy is not an inevitable outcome. Global scaling of the intensity of cortical activity below levels found in the healthy brain may provide a natural mechanism of seizure protection.SIGNIFICANCE STATEMENT Defining the scope and structure of an epilepsy-causing gene variant's effects on mesoscale brain activity constitutes a major contribution to our understanding of how epileptic brains differ from healthy brains, and informs the development of precision epilepsy therapies. We provide a clear roadmap for measuring how severely brain activity deviates from normal, not only in pathologically active areas, but across large portions of the brain and outside of epileptic activity. This will indicate where and how activity needs to be modulated to holistically restore normal function. It also has the potential to reveal unintended off-target treatment effects and facilitate therapy optimization to deliver maximal benefit with minimal side-effect potential.

Enhanced burst discharges in the CA1 area of the immature versus adult hippocampus: patterns and cellular mechanisms

Burst discharges in the immature brain may contribute to its enhanced seizure susceptibility. The cellular mechanisms underlying burst discharges in the CA1 area of the immature versus adult hippocampus were investigated with simultaneous whole-cell and field-potential recordings. When GABAA receptors were blocked pharmacologically, bursts in CA1 were either graded or all-or-none (or mixed) as a function of electrical stimulation intensity. Most CA1 minislices from immature rats displayed all-or-none or mixed bursts, whereas the slices from adult rats predominantly elicited graded bursts. The frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) were greater in CA1 pyramidal cells from the immature than the adult slices. The developmental differences in CA1 bursting were also detected in slices adjusted for maturational changes in brain volume (i.e., 350 µm thick for immature vs. 450 µm thick for adult rats). Neither N-methyl-d-aspartate (NMDA) nor group I metabotropic glutamate (mGlu1) receptor antagonists blocked the network-driven bursts in immature CA1, but an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor blocker abolished them. Robust excitatory postsynaptic potentials (EPSPs) occurred after bursts in some immature CA1 slices (23%) but never in slices from the adult. The input-output (amount of current injected vs. number of action potentials generated) relationship was markedly greater in CA1 pyramidal cells in the immature compared with the adult hippocampus. These data suggest that the CA1 area of the immature brain is capable of generating network-driven bursts, which declines in adult rats. The increased propensity of burst generation in immature CA1 appears to involve a greater AMPA receptor-mediated synaptic network and an increased intrinsic spike-generating ability.NEW & NOTEWORTHY Burst discharges in the developing brain can provide valuable insights into epileptogenesis. We show that the immature hippocampal CA1 area is capable of generating all-or-none (i.e., network) bursts, which transitions to graded (i.e., nonnetwork) bursts in the mature brain via both synaptic and intrinsic mechanisms. Our results provide new clues to help understand possible mechanisms that may be shared in the immature and epileptic brain and how the normal brain becomes seizure prone (i.e., epileptogenesis).

Hippocampal interlamellar cell-cell connectome that counts

The hippocampus is regarded as a cognition hub, particularly for learning and memory. Previously, neuronal mechanisms underlying various cognitive functions are delineated with the lamellar hippocampal circuitry, dentate gyrus-CA3 or CA2-CA1, within the transverse plane. More recently, interlamellar (often referred to as longitudinal) projections have received intensive attention to help understand signal convergence and divergence in cognition and behavior. Signal propagation along the longitudinal axis is evidenced by axonal arborization patterns and synaptic responses to electro- and photo-stimulation, further demonstrating that information flow is more enriched in the longitudinal plane than the transverse plane. Here, we review the significance of longitudinal connections for cognition, discuss a putative circuit mechanism of place coding, and suggest the reconceptualization of the hippocampal circuitry.

Excitatory synaptic transmission in hippocampal area CA1 is enhanced then reduced as chronic epilepsy progresses

This study examines changes in synaptic transmission with progression of the chronic epileptic state. Male Sprague-Dawley rats (P40-45) were injected with either saline or pilocarpine. In rats injected with pilocarpine, status epilepticus ensued. Hippocampal slices were cut 20-60 days or 80-110 days post-treatment. Evoked and miniature EPSCs (mEPSCs) were recorded from CA1 pyramidal neurons using whole-cell voltage-clamp. Fiber volleys were also recorded from stratum radiatum. Evoked EPSCs from the pilocarpine-treated cohort showed enhanced amplitudes 20-60 days post-treatment compared to the saline-treated cohort, whereas mEPSCs recorded from the same age group showed no change in event frequency and a slight but significant decrease in mEPSC amplitude distribution. In contrast, comparing evoked EPSCs and mEPSCs recorded 80-110 days after treatment indicated reduced amplitudes from pilocarpine-treated animals compared to controls. mEPSC inter-event interval decreased. This could be explained by a partial depletion of the ready releasable pool of neurotransmitter vesicles in Schaffer collateral presynaptic terminals of the pilocarpine-treated rats. In both saline- and pilocarpine-treated cohorts, concomitant decreases in mEPSC amplitudes as time after treatment progressed suggest that age-related changes in CA1 circuitry may be partially responsible for changes in synaptic transmission that may influence the chronic epileptic state.

Prolonged prophylactic effects of gabapentin on status epilepticus-induced neocortical injury

Long-term consequences of status epilepticus (SE) occur in a significant proportion of those who survive the acute episode. We developed an in vivo model of acute focal neocortical SE (FSE) to study long-term effects on local cortical structure and function and potential strategies to mitigate adverse consequences of SE. An acute 2 h episode of FSE was induced in anesthetized mice by epidural application of gabazine +4-aminopyridine over sensorimotor neocortex. Ten and 30 days later, the morphological and functional consequences of this single episode of FSE were studied using immunocytochemical and electrophysiological techniques. Results, focused on cortical layer V, showed astrogliosis, microgliosis, decreased neuronal density, and increased excitatory synapses, along with increased immunoreactivity for thrombospondin 2 (TSP2) and α2δ-1 proteins. In addition, neocortical slices, obtained from the area of prior focal seizure activity, showed abnormal epileptiform burst discharges along with increases in the frequency of miniature and spontaneous excitatory postsynaptic currents in layer V pyramidal cells, together with decreases in both parvalbumin immunoreactivity (PV-IR) and the frequency of miniature inhibitory postsynaptic currents in layer V pyramidal cells. Treatment with an approved drug, gabapentin (GBP) (ip 100 mg/kg/day 3 × /day for 7 days following the FSE episode), prevented the gliosis, the enhanced TSP2- and α2δ-1- IR and the increased excitatory synaptic density in the affected neocortex. This model provides an approach for assessing adverse effects of FSE on neocortical structure and function and potential prophylactic treatments.

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.

Gabapentin Prevents Progressive Increases in Excitatory Connectivity and Epileptogenesis Following Neocortical Trauma

Neocortical injury initiates a cascade of events, some of which result in maladaptive epileptogenic reorganization of surviving neural circuits. Research focused on molecular and organizational changes that occur following trauma may reveal processes that underlie human post-traumatic epilepsy (PTE), a common and unfortunate consequence of traumatic brain injury. The latency between injury and development of PTE provides an opportunity for prophylactic intervention, once the key underlying mechanisms are understood. In rodent neocortex, injury to pyramidal neurons promotes axonal sprouting, resulting in increased excitatory circuitry that is one important factor promoting epileptogenesis. We used laser-scanning photostimulation of caged glutamate and whole-cell recordings in in vitro slices from injured neocortex to assess formation of new excitatory synapses, a process known to rely on astrocyte-secreted thrombospondins (TSPs), and to map the distribution of maladaptive circuit reorganization. We show that this reorganization is centered principally in layer V and associated with development of epileptiform activity. Short-term blockade of the synaptogenic effects of astrocyte-secreted TSPs with gabapentin (GBP) after injury suppresses the new excitatory connectivity and epileptogenesis for at least 2 weeks. Results reveal that aberrant circuit rewiring is progressive in vivo and provide further rationale for prophylactic anti-epileptogenic use of gabapentinoids following cortical trauma.

Long term potentiation, but not depression, in interlamellar hippocampus CA1

Synaptic plasticity in the lamellar CA3 to CA1 circuitry has been extensively studied while interlamellar CA1 to CA1 connections have not yet received much attention. One of our earlier studies demonstrated that axons of CA1 pyramidal neurons project to neighboring CA1 neurons, implicating information transfer along a longitudinal interlamellar network. Still, it remains unclear whether long-term synaptic plasticity is present within this longitudinal CA1 network. Here, we investigate long-term synaptic plasticity between CA1 pyramidal cells, using in vitro and in vivo extracellular recordings and 3D holography glutamate uncaging. We found that the CA1-CA1 network exhibits NMDA receptor-dependent long-term potentiation (LTP) without direction or layer selectivity. By contrast, we find no significant long-term depression (LTD) under various LTD induction protocols. These results implicate unique synaptic properties in the longitudinal projection suggesting that the interlamellar CA1 network could be a promising structure for hippocampus-related information processing and brain diseases.

Absence of Claudin 11 in CNS Myelin Perturbs Behavior and Neurotransmitter Levels in Mice

Neuronal origins of behavioral disorders have been examined for decades to construct frameworks for understanding psychiatric diseases and developing useful therapeutic strategies with clinical application. Despite abundant anecdotal evidence for white matter etiologies, including altered tractography in neuroimaging and diminished oligodendrocyte-specific gene expression in autopsy studies, mechanistic data demonstrating that dysfunctional myelin sheaths can cause behavioral deficits and perturb neurotransmitter biochemistry have not been forthcoming. At least in part, this impasse stems from difficulties in identifying model systems free of degenerative pathology to enable unambiguous assessment of neuron biology and behavior in a background of myelin dysfunction. Herein we examine myelin mutant mice lacking expression of the Claudin11 gene in oligodendrocytes and characterize two behavioral endophenotypes: perturbed auditory processing and reduced anxiety/avoidance. Importantly, these behaviors are associated with increased transmission time along myelinated fibers as well as glutamate and GABA neurotransmitter imbalances in auditory brainstem and amygdala, in the absence of neurodegeneration. Thus, our findings broaden the etiology of neuropsychiatric disease to include dysfunctional myelin, and identify a preclinical model for the development of novel disease-modifying therapies.

Methionine increases BDNF DNA methylation and improves memory in epilepsy

OBJECTIVE

Temporal lobe epilepsy (TLE) patients exhibit signs of memory impairments even when seizures are pharmacologically controlled. Surprisingly, the underlying molecular mechanisms involved in TLE-associated memory impairments remain elusive. Memory consolidation requires epigenetic transcriptional regulation of genes in the hippocampus; therefore, we aimed to determine how epigenetic DNA methylation mechanisms affect learning-induced transcription of memory-permissive genes in the epileptic hippocampus.

METHODS

Using the kainate rodent model of TLE and focusing on the brain-derived neurotrophic factor (Bdnf) gene as a candidate of DNA methylation-mediated transcription, we analyzed DNA methylation levels in epileptic rats following learning. After detection of aberrant DNA methylation at the Bdnf gene, we investigated functional effects of altered DNA methylation on hippocampus-dependent memory formation in our TLE rodent model.

RESULTS

We found that behaviorally driven BdnfDNA methylation was associated with hippocampus-dependent memory deficits. Bisulfite sequencing revealed that decreased BdnfDNA methylation levels strongly correlated with abnormally high levels of BdnfmRNA in the epileptic hippocampus during memory consolidation. Methyl supplementation via methionine (Met) increased BdnfDNA methylation and reduced BdnfmRNA levels in the epileptic hippocampus during memory consolidation. Met administration reduced interictal spike activity, increased theta rhythm power, and reversed memory deficits in epileptic animals. The rescue effect of Met treatment on learning-induced BdnfDNA methylation, Bdnf gene expression, and hippocampus-dependent memory, were attenuated by DNA methyltransferase blockade.

INTERPRETATION

Our findings suggest that manipulation of DNA methylation in the epileptic hippocampus should be considered as a viable treatment option to ameliorate memory impairments associated with TLE.

Status epilepticus triggers early and late alterations in brain-derived neurotrophic factor and NMDA glutamate receptor Grin2b DNA methylation levels in the hippocampus

Status epilepticus (SE) triggers abnormal expression of genes in the hippocampus, such as glutamate receptor subunit epsilon-2 (Grin2b/Nr2b) and brain-derived neurotrophic factor (Bdnf), that is thought to occur in temporal lobe epilepsy (TLE). We examined the underlying DNA methylation mechanisms and investigated whether these mechanisms contribute to the expression of these gene targets in the epileptic hippocampus. Experimental TLE was provoked by kainic acid-induced SE. Bisulfite sequencing analysis revealed increased Grin2b/Nr2b and decreased Bdnf DNA methylation levels that corresponded to decreased Grin2b/Nr2b and increased Bdnf mRNA and protein expression in the epileptic hippocampus. Blockade of DNA methyltransferase (DNMT) activity with zebularine decreased global DNA methylation levels and reduced Grin2b/Nr2b, but not Bdnf, DNA methylation levels. Interestingly, we found that DNMT blockade further decreased Grin2b/Nr2b mRNA expression whereas GRIN2B protein expression increased in the epileptic hippocampus, suggesting that a posttranscriptional mechanism may be involved. Using chromatin immunoprecipitation analysis we found that DNMT inhibition restored the decreases in AP2alpha transcription factor levels at the Grin2b/Nr2b promoter in the epileptic hippocampus. DNMT inhibition increased field excitatory postsynaptic potential in hippocampal slices isolated from epileptic rats. Electroencephalography (EEG) monitoring confirmed that DNMT inhibition did not significantly alter the disease course, but promoted the latency to seizure onset or SE. Thus, DNA methylation may be an early event triggered by SE that persists late into the epileptic hippocampus to contribute to gene expression changes in TLE.

Mechanisms of physiological and epileptic HFO generation

High frequency oscillations (HFO) have a variety of characteristics: band-limited or broad-band, transient burst-like phenomenon or steady-state. HFOs may be encountered under physiological or under pathological conditions (pHFO). Here we review the underlying mechanisms of oscillations, at the level of cells and networks, investigated in a variety of experimental in vitro and in vivo models. Diverse mechanisms are described, from intrinsic membrane oscillations to network processes involving different types of synaptic interactions, gap junctions and ephaptic coupling. HFOs with similar frequency ranges can differ considerably in their physiological mechanisms. The fact that in most cases the combination of intrinsic neuronal membrane oscillations and synaptic circuits are necessary to sustain network oscillations is emphasized. Evidence for pathological HFOs, particularly fast ripples, in experimental models of epilepsy and in human epileptic patients is scrutinized. The underlying mechanisms of fast ripples are examined both in the light of animal observations, in vivo and in vitro, and in epileptic patients, with emphasis on single cell dynamics. Experimental observations and computational modeling have led to hypotheses for these mechanisms, several of which are considered here, namely the role of out-of-phase firing in neuronal clusters, the importance of strong excitatory AMPA-synaptic currents and recurrent inhibitory connectivity in combination with the fast time scales of IPSPs, ephaptic coupling and the contribution of interneuronal coupling through gap junctions. The statistical behaviour of fast ripple events can provide useful information on the underlying mechanism and can help to further improve classification of the diverse forms of HFOs.

The activity within the CA3 excitatory network during Theiler's virus encephalitis is distinct from that observed during chronic epilepsy

Viral infections of the central nervous system (CNS) are associated with an increased risk for seizures during the acute infection period and the subsequent development of chronic epilepsy that is often difficult to treat. In previous work, we have shown that mice of the C57BL/6 strain infected with Theiler's murine encephalomyelitis virus (TMEV) exhibit a similar sequence, thereby providing a potential useful model of virus-induced epilepsy. The present study examines spontaneous and miniature excitatory postsynaptic currents in CA3 pyramidal cells recorded from brain slices prepared during both the acute phase during encephalitis and 2 months following TMEV infection. Animals that develop chronic epilepsy following TMEV infection exhibit considerable hippocampal sclerosis, directly implicating this brain region in the process of epileptogenesis. There are significant increases in amplitude and frequency of spontaneous and miniature excitatory currents in CA3 cells recorded in brain slices prepared during the acute infection period and 2 months after infection. However, the patterns of changes observed are markedly different during these two periods, suggesting that there are underlying changes in the network over time. These differences have implications for the treatment used during the acute infection and after chronic seizures develop.

Primary and secondary mechanisms of epileptogenesis in the temporal lobe: there is a before and an after

Extensive data involving several animal models of temporal lobe epilepsy highlight synaptic alterations that likely act synergistically during acquired epileptogenesis. Most of this research has utilized experimental models in which intense electrical activity in adult animals, primarily involving status epilepticus, causes variable neuronal death in the hippocampus and other temporal lobe structures. Neuronal death, including principal cells and specific interneurons, likely has several roles in epileptogenesis after brain injury. Both reduction of GABA-mediated inhibition from selective interneuron loss and the progressive formation of new recurrent excitatory circuits after death of principal neurons enhance excitability and promote seizures during the development of epilepsy. These epileptogenic circuits hypothetically continue to undergo secondary epileptogenesis, which involves further modifications that contribute to a progressive, albeit variable, increase in the frequency and severity of spontaneous recurrent seizures.

Transection of CA3 does not affect memory performance in rats

Longitudinal hippocampal pathways are needed for seizure synchronization, and there is evidence that their transection may abolish seizures. However, the effect of such transection on memory is unknown. In this study, we investigated the effect of transverse CA3 transections on memory function in Sprague-Dawley rats. With a stereotactic knife, a single CA3 transection was made unilaterally (n=5) or bilaterally (n=5). Sham surgery was done in another group (n=4). Morris water maze and novel object recognition tests were started 18 days later and revealed no significant differences between transected animals and controls. Cresyl-violet brain staining confirmed the locations of transections in the CA3 region. We conclude that normal performances in Morris water maze and novel object recognition tests do not appear to require intact transmission throughout the whole length of CA3, supporting the hypothesis that CA3 transections may be used in temporal lobe epilepsy to interrupt seizure circuitry while preserving memory.

Cortical up state activity is enhanced after seizures: a quantitative analysis

In the neocortex, neurons participate in epochs of elevated activity, or Up states, during periods of quiescent wakefulness, slow-wave sleep, and general anesthesia. The regulation of firing during and between Up states is of great interest because it can reflect the underlying connectivity and excitability of neurons within the network. Automated analysis of the onset and characteristics of Up state firing across different experiments and conditions requires a robust and accurate method for Up state detection. Using measurements of membrane potential mean and variance calculated from whole-cell recordings of neurons from control and postseizure tissue, the authors have developed such a method. This quantitative and automated method is independent of cell- or condition-dependent variability in underlying noise or tonic firing activity. Using this approach, the authors show that Up state frequency and firing rates are significantly increased in layer 2/3 neocortical neurons 24 hours after chemoconvulsant-induced seizure. Down states in postseizure tissue show greater membrane-potential variance characterized by increased synaptic activity. Previously, the authors have found that postseizure increase in excitability is linked to a gain-of-function in BK channels, and blocking BK channels in vitro and in vivo can decrease excitability and eliminate seizures. Thus, the authors also assessed the effect of BK-channel antagonists on Up state properties in control and postseizure neurons. These data establish a robust and broadly applicable algorithm for Up state detection and analysis, provide a quantitative description of how prior seizures increase spontaneous firing activity in cortical networks, and show how BK-channel antagonists reduce this abnormal activity.

Emergent dynamics of fast ripples in the epileptic hippocampus

Fast ripples are a type of transient high-frequency oscillations recorded from the epileptogenic regions of the hippocampus and the temporal cortex of epileptic humans and rodents. These events presumably reflect hypersynchronous bursting of pyramidal cells. However, the oscillatory spectral content of fast ripples varies from 250 to 800 Hz, well above the maximal firing frequency of most hippocampal pyramidal neurons. How such high-frequency oscillations are generated is therefore unclear. Here, we combine computational simulations of fast ripples with multisite and juxtacellular recordings in vivo to examine the underlying mechanisms in the hippocampus of epileptic rats. We show that populations of bursting cells firing individually at 100-400 Hz can create fast ripples according to two main firing regimes: (1) in-phase synchronous firing resulting in "pure" fast ripples characterized by single spectral peaks that reflect single-cell behavior and (2) out-of-phase firing that results in "emergent" fast ripples. Using simulations, we found that fast ripples generated under these two different regimes can be quantitatively separated by their spectral characteristics, and we took advantage of this separability to examine their dynamics in vivo. We found that in-phase firing can reach frequencies up to 300 Hz in the CA1 and up to 400 Hz in the dentate gyrus. The organization of out-of-phase firing is determined by firing delays between cells discharging at low frequencies. The two firing regimes compete dynamically, alternating randomly from one fast ripple event to the next, and they reflect the functional dynamic organization of the different regions of the hippocampus.

Repetitive perforant-path stimulation induces epileptiform bursts in minislices of dentate gyrus from rats with kainate-induced epilepsy

The epileptic hippocampus has an enhanced propensity for seizure generation, but how spontaneous seizures start is poorly understood. Using whole cell and field-potential recordings, this study explored whether repetitive perforant-path stimulation at physiological frequencies could induce epileptiform bursts in dentate gyrus minislices from rats with kainate-induced epilepsy. Control slices from saline-treated rats responded to single perforant-path stimulation with an excitatory postsynaptic potential (EPSP) and a single population spike in normal medium, and repetitive stimulation at different frequencies (0.1, 1, 2, 5, 10 Hz) did not cause significant increases in the responses. Most minislices (82%) from rats with kainate-induced epilepsy also responded to single perforant-path stimulation with an EPSP and a single population spike/action potential, but some slices (18%) had a more robust response with a prolonged duration and negative DC shift or responses with two to three population spikes. Repetitive perforant-path stimulation at 5-10 Hz, however, transformed the single-spike responses into epileptiform bursts with multiple spikes in half (52%) of the slices, while lower frequency (e.g., ≤ 1 Hz) stimulation failed to produce these changes. The emergence of epileptiform bursts was consistently associated with a negative field-potential DC shift and membrane depolarization. The results suggest that compared with the controls, the "gate" function of the dentate gyrus is compromised in rats with kainate-induced epilepsy, and epileptiform bursts (but not full-length seizure events) can be induced in minislices by repetitive synaptic stimulation at physiological frequencies in the range of hippocampal theta rhythm (i.e., 5-10 Hz).

Basement membrane protein nidogen-1 shapes hippocampal synaptic plasticity and excitability

The basement membrane (BM) is a specialized form of extracellular matrix (ECM) underlying epithelia and endothelia and surrounding many types of mesenchymal cells. Nidogen, along with collagen IV and laminin, is a major component of BMs. Although certain ECM proteins such as laminin or reelin influence neuronal function via interactions with cell-surface receptors such as integrins, behavioral neurological impairments due to deficits of BM components have been recognized only recently. Here, alterations in neuronal network function underlying these behavioral changes are revealed. Using nidogen-1 knockout mice, with or without additional heterozygous nidogen-2 knockout (NID1(-/-)/NID2(+/+) or NID1(-/-)/NID2(+/-)), we demonstrate that nidogen is essential for normal neuronal network excitability and plasticity. In nidogen-1 knockouts, seizurelike behavior occurs, and epileptiform spiking was seen in hippocampal in vivo EEG recordings. In vitro, hippocampal field potential recordings revealed that lack of nidogen-1, while not causing conspicuous morphological changes, led to the appearance of spontaneous and evoked epileptiform activity, significant increase of the input/output ratio of synaptically evoked responses in CA1 and dentate gyrus, as well as of paired pulse accentuation, and loss of perforant-path long-term synaptic potentiation. Nidogen-1 is thus essential for normal network excitability and plasticity.

How does the balance of excitation and inhibition shift during epileptogenesis?

Cell Domain-Dependent Changes in the Glutamatergic and GABAergic Drives during Epileptogenesis in the Rat CA1 Region El-Hassar L, Milh M, Wendling F, Ferrand N, Esclapez M, Bernard C. J Physiol 2007;578(Pt 1):193–211. An increased ratio of the glutamatergic drive to the overall glutamatergic/GABAergic drive characterizes the chronic stage of temporal lobe epilepsy (TLE), but it is unclear whether this modification is present during the latent period that often precedes the epileptic stage. Using the pilocarpine model of TLE in rats, we report that this ratio is decreased in hippocampal CA1 pyramidal cells during the early phase of the latent period (3–5 days postpilocarpine). It is, however, increased during the late phase of the latent period (7–10 days postpilocarpine), via cell domain-dependent alterations in synaptic current properties, concomitant with the occurrence of interictal-like activity in vivo. During the late latent period, the glutamatergic drive was increased in somata via an enhancement in EPSC decay time constant and in dendrites via an increase in EPSC frequency and amplitude. The GABAergic drive remained unchanged in the soma but was decreased in dendrites, since the drop off in IPSC frequency was more marked than the increase in IPSC kinetics. Theoretical considerations suggest that these modifications are sufficient to produce interictal-like activity. In epileptic animals, the ratio of the glutamatergic drive to the overall synaptic drive was not further modified, despite additional changes in synaptic current frequency and kinetics. These results show that the global changes to more glutamatergic and less GABAergic activities in the CA1 region precede the chronic stage of epilepsy, possibly facilitating the occurrence and/or the propagation of interictal activity.

Building and manipulating neural pathways with microfluidics

Communication between different brain regions, and between local circuits in the same brain region, is an important area of study for basic and translational neuroscience research. Selective and chronic manipulation of one of the components in a given neural pathway is frequently required for development and plasticity studies. We designed an in vitro platform that captures some of the complexity of mammalian brain pathways but permits easy experimental manipulation of their constituent parts. Organotypic cultures of brain slices were carried out in compartments interconnected by microchannels. We show that co-cultures from cortex and hippocampus formed functional connections by extending axons through the microchannels. We report synchronization of neural activity in co-cultures, and demonstrate selective pharmacological manipulation of activity in the constituent slices. Our platform enables chronic, spatially-restricted experimental manipulation of pre- and post-synaptic neurons in organotypic cultures, and will be useful to investigators seeking to understand development, plasticity, and pathologies of neural pathways.

Seizures, enhanced excitation, and increased vesicle number in Lis1 mutant mice

OBJECTIVE

In humans, abnormal neuronal migration and severe neuronal disorganization resulting from Lis1 (lissencephaly) haploinsufficiency contributes to cognitive impairment and seizures early in life. In Lis1 heterozygotic mice, severe hippocampal disorganization and cognitive impairment have also been reported. Using this mouse model, we examined the functional impact of LIS1 deficiency with particular focus on excitatory glutamate-mediated synaptic transmission.

METHODS

We used visualized patch-clamp recordings in acute hippocampal slices. We recorded spontaneous, miniature and stimulation-evoked excitatory postsynaptic current (EPSC). Additional mice were processed for immunohistochemistry, electron microscopy (EM), or video-electroencephalographic (EEG) monitoring.

RESULTS

Video-EEG confirmed the presence of spontaneous electrographic seizures in Lis1 mutant mice. In disorganized hippocampal slices from Lis1(+/-) mice, we noted a nearly two-fold significant increase in the frequency of spontaneous and miniature EPSC; no significant change in amplitude or decay was noted. Synaptic function assessed using brief repetitive or paired-pulse stimulation protocols, also revealed significant enhancement of glutamate-mediated excitation. Low concentrations of cadmium, a nonspecific blocker of voltage-dependent calcium channels mediating vesicle release, effectively restored paired-pulse facilitation deficits back to control levels. Analysis of synapse ultrastructure at the EM level identified a large increase in synaptic vesicle number.

INTERPRETATION

Seizure activity, possibly associated with increased glutamate-mediated excitation and an increased pool of vesicles at the presynaptic site, was demonstrated in a mouse model of type I lissencephaly.

Astrocytic dysfunction in epileptogenesis: consequence of altered potassium and glutamate homeostasis?

Focal epilepsy often develops following traumatic, ischemic, or infectious brain injury. While the electrical activity of the epileptic brain is well characterized, the mechanisms underlying epileptogenesis are poorly understood. We have recently shown that in the rat neocortex, long-lasting breakdown of the blood-brain barrier (BBB) or direct exposure of the neocortex to serum-derived albumin leads to rapid upregulation of the astrocytic marker GFAP (glial fibrillary acidic protein), followed by delayed (within 4-7 d) development of an epileptic focus. We investigated the role of astrocytes in epileptogenesis in the BBB-breakdown and albumin models of epileptogenesis. We found similar, robust changes in astrocytic gene expression in the neocortex within hours following treatment with deoxycholic acid (BBB breakdown) or albumin. These changes predict reduced clearance capacity for both extracellular glutamate and potassium. Electrophysiological recordings in vitro confirmed the reduced clearance of activity-dependent accumulation of both potassium and glutamate 24 h following exposure to albumin. We used a NEURON model to simulate the consequences of reduced astrocytic uptake of potassium and glutamate on EPSPs. The model predicted that the accumulation of glutamate is associated with frequency-dependent (>100 Hz) decreased facilitation of EPSPs, while potassium accumulation leads to frequency-dependent (10-50 Hz) and NMDA-dependent synaptic facilitation. In vitro electrophysiological recordings during epileptogenesis confirmed frequency-dependent synaptic facilitation leading to seizure-like activity. Our data indicate a transcription-mediated astrocytic transformation early during epileptogenesis. We suggest that the resulting reduction in the clearance of extracellular potassium underlies frequency-dependent neuronal hyperexcitability and network synchronization.

Active paradigms of seizure anticipation: computer model evidence for necessity of stimulation

It has been shown that the analysis of electroencephalographic (EEG) signals submitted to an appropriate external stimulation (active paradigm) is efficient with respect to anticipating epileptic seizures [S. Kalitzin, Clin. Neurophysiol. 116, 718 (2005)]. To better understand how an active paradigm is able to detect properties of EEG signals by means of which proictal states can be identified, we performed a simulation study using a computational model of seizure generation of a hippocampal network. Applying the active stimulation methodology, we investigated (i) how changes in model parameters that lead to a transition from the normal ongoing EEG to an ictal pattern are reflected in the properties of the simulated EEG output signals and (ii) how the evolution of neuronal excitability towards seizures can be reconstructed from EEG data using an active paradigm, rather than passively, using only ongoing EEG signals. The simulations indicate that a stimulation paradigm combined with appropriate analytical tools, as proposed here, may yield information about the change in excitability that precedes the transition to a seizure. Such information is apparently not fully reflected in the ongoing EEG activity. These findings give strong support to the development and application of active paradigms with the aim of predicting the occurrence of a transition to an epileptic seizure.

Impaired spatial representation in CA1 after lesion of direct input from entorhinal cortex

Place-specific firing in the hippocampus is determined by path integration-based spatial representations in the grid-cell network of the medial entorhinal cortex. Output from this network is conveyed directly to CA1 of the hippocampus by projections from principal neurons in layer III, but also indirectly by axons from layer II to the dentate gyrus and CA3. The direct pathway is sufficient for spatial firing in CA1, but it is not known whether similar firing can also be supported by the input from CA3. To test this possibility, we made selective lesions in layer III of medial entorhinal cortex by local infusion of the neurotoxin gamma-acetylenic GABA. Firing fields in CA1 became larger and more dispersed after cell loss in layer III, whereas CA3 cells, which receive layer II input, still had sharp firing fields. Thus, the direct projection is necessary for precise spatial firing in the CA1 place cell population.

Prosthetic systems for therapeutic optical activation and silencing of genetically-targeted neurons

Many neural disorders are associated with aberrant activity in specific cell types or neural projection pathways embedded within the densely-wired, heterogeneous matter of the brain. An ideal therapy would permit correction of activity just in specific target neurons, while leaving other neurons unaltered. Recently our lab revealed that the naturally-occurring light-activated proteins channelrhodopsin-2 (ChR2) and halorhodopsin (Halo/NpHR) can, when genetically expressed in neurons, enable them to be safely, precisely, and reversibly activated and silenced by pulses of blue and yellow light, respectively. We here describe the ability to make specific neurons in the brain light-sensitive, using a viral approach. We also reveal the design and construction of a scalable, fully-implantable optical prosthetic capable of delivering light of appropriate intensity and wavelength to targeted neurons at arbitrary 3-D locations within the brain, enabling activation and silencing of specific neuron types at multiple locations. Finally, we demonstrate control of neural activity in the cortex of the non-human primate, a key step in the translation of such technology for human clinical use. Systems for optical targeting of specific neural circuit elements may enable a new generation of high-precision therapies for brain disorders.

A chronic histopathological and electrophysiological analysis of a rodent hypoxic-ischemic brain injury model and its use as a model of epilepsy

Ischemic brain injury is one of the leading causes of epilepsy in the elderly, and there are currently no adult rodent models of global ischemia, unilateral hemispheric ischemia, or focal ischemia that report the occurrence of spontaneous motor seizures following ischemic brain injury. The rodent hypoxic-ischemic (H-I) model of brain injury in adult rats is a model of unilateral hemispheric ischemic injury. Recent studies have shown that an H-I injury in perinatal rats causes hippocampal mossy fiber sprouting and epilepsy. These experiments aimed to test the hypothesis that a unilateral H-I injury leading to severe neuronal loss in young-adult rats also causes mossy fiber sprouting and spontaneous motor seizures many months after the injury, and that the mossy fiber sprouting induced by the H-I injury forms new functional recurrent excitatory synapses. The right common carotid artery of 30-day old rats was permanently ligated, and the rats were placed into a chamber with 8% oxygen for 30 min. A quantitative stereologic analysis revealed that the ipsilateral hippocampus had significant hilar and CA1 pyramidal neuronal loss compared with the contralateral and sham-control hippocampi. The septal region from the ipsilateral and contralateral hippocampus had small but significantly increased amounts of Timm staining in the inner molecular layer compared with the sham-control hippocampi. Three of 20 lesioned animals (15%) were observed to have at least one spontaneous motor seizure 6-12 months after treatment. Approximately 50% of the ipsilateral and contralateral hippocampal slices displayed abnormal electrophysiological responses in the dentate gyrus, manifest as all-or-none bursts to hilar stimulation. This study suggests that H-I injury is associated with synaptic reorganization in the lesioned region of the hippocampus, and that new recurrent excitatory circuits can predispose the hippocampus to abnormal electrophysiological activity and spontaneous motor seizures.

Development of spontaneous seizures after experimental status epilepticus: implications for understanding epileptogenesis

This report examines several concepts concerning the latent period to the first convulsive seizure, subsequent increases in seizure frequency, and possible mechanisms of epileptogenesis after kainate-induced status epilepticus. Previous data concerning the latent period and seizure progression from intermittent and continuous behavioral monitoring are compared, and hypothetical mechanisms of acquired epilepsy are discussed. Data involving electrographic recordings with tethered animals or with radiotelemetry are assessed in terms of their potential for addressing different hypotheses concerning the latent period and progressive changes in seizure frequency. Experimental analyses of the time course of occurrence of spontaneous seizures are interpreted in terms of possible cellular mechanisms underlying epileptogenesis.

Activity-Dependent Depression of Local Excitatory Connections in the CA1 Region of Mouse Hippocampus

The existence of recurrent excitatory synapses between pyramidal cells in the hippocampal CA1 region has been known for some time yet little is known about activity-dependent forms of plasticity at these synapses. Here we demonstrate that under certain experimental conditions, Schaffer collateral/commissural fiber stimulation can elicit robust polysynaptic excitatory postsynaptic potentials due to recurrent synaptic inputs onto CA1 pyramidal cells. In contrast to CA3 pyramidal cell inputs, recurrent synapses onto CA1 pyramidal cells exhibited robust paired-pulse depression and a sustained, but rapidly reversible, depression in response to low-frequency trains of Schaffer collateral fiber stimulation. Blocking GABAB receptors abolished paired-pulse depression but had little effect on low-frequency stimulation (LFS)-induced depression. Instead, LFS-induced depression was significantly attenuated by an inhibitor of A1 type adenosine receptors. Blocking the postsynaptic effects of GABAB and A1 receptor activation on CA1 pyramidal cell excitability with an inhibitor of G-protein-activated inwardly rectifying potassium channels had no effect on either paired-pulse depression or LFS-induced depression. Thus activation of presynaptic GABAB and adenosine receptors appears to have an important role in activity-dependent depression at recurrent synapses. Together, our results indicate that CA3-CA1 and CA1-CA1 synapses exhibit strikingly different forms of short-term synaptic plasticity and suggest that activity-dependent changes in recurrent synaptic transmission can transform the CA1 region from a sparsely connected recurrent network into a predominantly feedforward circuit.

Recurrent circuits in layer II of medial entorhinal cortex in a model of temporal lobe epilepsy

Patients and laboratory animal models of temporal lobe epilepsy display loss of layer III pyramidal neurons in medial entorhinal cortex and hyperexcitability and hypersynchrony of less vulnerable layer II stellate cells. We sought to test the hypothesis that loss of layer III pyramidal neurons triggers synaptic reorganization and formation of recurrent, excitatory synapses among layer II stellate cells in epileptic pilocarpine-treated rats. Laser-scanning photo-uncaging of glutamate focally activated neurons in layer II while excitatory synaptic responses were recorded in stellate cells. Photostimulation revealed previously unidentified, functional, recurrent, excitatory synapses between layer II stellate cells in control animals. Contrary to the hypothesis, however, control and epileptic rats displayed similar levels of recurrent excitation. Recently, hyperexcitability of layer II stellate cells has been attributed, at least in part, to loss of GABAergic interneurons and inhibitory synaptic input. To evaluate recurrent inhibitory circuits in layer II, we focally photostimulated interneurons while recording inhibitory synaptic responses in stellate cells. IPSCs were evoked more than five times more frequently in slices from control versus epileptic animals. These findings suggest that in this model of temporal lobe epilepsy, reduced recurrent inhibition contributes to layer II stellate cell hyperexcitability and hypersynchrony, but increased recurrent excitation does not.

Epileptogenesis in the dentate gyrus: a critical perspective

The dentate gyrus has long been a focal point for studies on the molecular, cellular, and network mechanisms responsible for epileptogenesis in temporal lobe epilepsy (TLE). Although several hypothetical mechanisms are considered in this chapter, two that have garnered particular interest and experimental support are: (1) the selective loss of vulnerable interneurons in the region of the hilus and (2) the formation of new recurrent excitatory circuits after mossy fiber sprouting. Histopathological data show that specific GABAergic interneurons in the hilus are lost in animal models of TLE, and several lines of electrophysiological evidence, including intracellular analyses of postsynaptic currents, support this hypothesis. In particular, whole-cell recordings have demonstrated a reduction in the frequency of miniature inhibitory postsynaptic currents in the dentate gyrus and other areas (e.g., CA1 pyramidal cells), which provides relatively specific evidence for a reduction in GABAergic input to granule cells. These studies support the viewpoint that modest alterations in GABAergic inhibition can have significant functional impact in the dentate gyrus, and suggest that dynamic activity-dependent mechanisms of GABAergic regulation add complexity to this local synaptic circuitry and to analyses of epileptogenesis. In regard to mossy fiber sprouting, a wide variety of experiments involving intracellular or whole-cell recordings during electrical stimulation of the hilus, glutamate microstimulation, and dual recordings from granule cells support the hypothesis that mossy fiber sprouting forms new recurrent excitatory circuits in the dentate gyrus in animal models of TLE. Similar to previous studies on recurrent excitation in the CA3 area, GABA-mediated inhibition and the intrinsic high threshold of granule cells in the dentate gyrus tends to mask the presence of the new recurrent excitatory circuits and reduce the likelihood that reorganized circuits will generate seizure-like activity. How cellular alterations such as neuron loss in the hilus and mossy fiber sprouting influence functional properties is potentially important for understanding fundamental aspects of epileptogenesis, such as the consequences of primary initial injuries, mechanisms underlying network synchronization, and progression of intractability. The continuous nature of the axonal sprouting and formation of recurrent excitation could account for aspects of the latent period and the progressive nature of the epileptogenesis. Future studies will need to identify precisely how these hypothetical mechanisms and others contribute to the process whereby epileptic seizures are initiated or propagated through an area such as the dentate gyrus. Finally, in addition to its unique features and potential importance in epileptogenesis, the dentate gyrus may also serve as a model for other cortical structures in acquired epilepsy.

Synaptic reorganization in subiculum and CA3 after early-life status epilepticus in the kainic acid rat model

PURPOSE

The immature rat brain is highly susceptible to seizures, but has a resistance to pathological changes induced by seizures as compared to adult rats. However, prolonged seizures during early-life enhance cellular injury and hyperexcitability induced by convulsive insults later in adulthood. The mechanisms underlying these phenomena are not understood. In adult models, the CA1 axons reorganize their projections to subiculum. Seizure induced plasticity in this pathway has not been investigated in immature seizure models, and may contribute to the vulnerability to later seizures.

METHODS

On postnatal day 15, rats experienced convulsive status epilepticus with kainic acid (KA). Seizure induced plasticity was examined with Timm histochemistry and iontophoretic injections of sodium selenite, a retrograde tracer. Cellular injury was evaluated with Fluoro-Jade B histochemistry.

RESULTS

Retrograde tracing experiments determined a 67% larger dorsoventral extent of retrograde labeling in the CA1 pyramidal region after tracer injections in subiculum. The synaptic reorganization of the CA1 projection to subiculum was noted in the absence of overt neuronal injury in subiculum or CA1. In contrast, mossy fiber sprouting was detected into the stratum oriens of CA3 with limited neuronal injury to CA3 pyramidal neurons. No mossy fiber sprouting into the inner molecular layer of the dentate gyrus, or CA1 sprouting into the stratum moleculare of CA1 were noted.

CONCLUSIONS

The results indicate that the developing brain has distinct mechanisms of seizure induced reorganization as compared to the adult brain. Our experiments show that the concept of "resistance of the immature brain to excitotoxicity" is considerably more complicated than generally believed. Morphological plasticity in the immature brain appears more extensive in distal, but not proximal, projections of hippocampal pathways, and across hippocampal lamellae. The abnormal connectivity between hippocampal lamellae might play a role in the increased susceptibility to injury and hyperexcitability associated with later convulsive insults.

Enhanced excitatory synaptic connectivity in layer v pyramidal neurons of chronically injured epileptogenic neocortex in rats

Formation of new recurrent excitatory circuits after brain injuries has been hypothesized as a major factor contributing to epileptogenesis. Increases in total axonal length and the density of synaptic boutons are present in layer V pyramidal neurons of chronic partial isolations of rat neocortex, a model of posttraumatic epileptogenesis. To explore the functional consequences of these changes, we used laser-scanning photostimulation combined with whole-cell patch-clamp recording from neurons in layer V of somatosensory cortex to map changes in excitatory synaptic connectivity after injury. Coronal slices were submerged in artificial CSF (23 degrees C) containing 100 microM caged glutamate, APV (2-amino-5-phosphonovaleric acid), and high divalent cation concentration to block polysynaptic responses. Focal uncaging of glutamate, accomplished by switching a pulsed UV laser to give a 200-400 micros light stimulus, evoked single- or multiple-component composite EPSCs. In neurons of the partially isolated cortex, there were significant increases in the fraction of uncaging sites from which EPSCs could be evoked ("hot spots") and a decrease in the mean amplitude of individual elements in the composite EPSC. When plotted along the cortical depth, the changes in EPSCs took place mainly between 150 and 200 microm above and below the somata, suggesting a specific enhancement of recurrent excitatory connectivity among layer V pyramidal neurons of the undercut neocortex. These changes may shift the balance within cortical circuits toward increased synaptic excitation and contribute to epileptogenesis.

The role of synaptic reorganization in mesial temporal lobe epilepsy

The mechanisms underlying mesial temporal lobe epilepsy (MTLE) remain uncertain. Putative mechanisms should account for several features characteristic of the clinical presentation and the neurophysiological and neuropathological abnormalities observed in patients with intractable MTLE. Synaptic reorganization of the mossy fiber pathway has received considerable attention over the past two decades as a potential mechanism that increases the excitability of the hippocampal network through the formation of new recurrent excitatory collaterals. Morphological plasticity beyond the mossy fiber pathway has not been as thoroughly investigated. Recently, plasticity of the CA1 pyramidal axons has been demonstrated in acute and chronic experimental models of MTLE. As the hippocampal formation is topographically organized in stacks of slices (lamellae), synaptic reorganization of CA1 axons projecting to subiculum appears to increase the connectivity between lamellae, providing a mechanism for translamellar synchronization of cellular hyperexcitability, leading to pharmacologically intractable seizures.

Cell type-specific changes in spontaneous and minimally evoked excitatory synaptic activity in hippocampal CA1 interneurons of kainate-treated rats

The epileptiform activity in the kainic acid (KA) model of epilepsy arises from complex changes in excitation and inhibition. To assess the involvement of excitatory drive onto inhibitory interneurons in this epileptiform activity, we examined changes in spontaneous and minimally evoked excitatory post-synaptic currents (sEPSCs and eEPSCs) in CA1 interneurons in stratum oriens/alveus (O/A) and stratum radiatum (RAD) in rat hippocampal slices after KA treatment. The frequency and amplitude of sEPSCs and the amplitude of eEPSCs were unchanged in O/A interneurons, but the EPSC kinetics were significantly slower. These changes appear to be due to altered kinetics and voltage-dependent properties of the NMDA component of EPSCs in O/A interneurons. In contrast, sEPSCs and eEPSCs in RAD interneurons did not change after KA treatment. The distinct changes in excitatory synaptic activity in interneurons differentially involved in feedback (O/A) versus feedforward (RAD) inhibition suggest a cell type-specific reorganization of excitatory synapses after KA treatment. These modifications in excitatory input to interneurons could contribute to the maintenance of inhibition of CA1 pyramidal cells after KA treatment, or may also create network conditions favourable to epileptiform activity.

Axonal sprouting of GABAergic interneurons in temporal lobe epilepsy

Temporal lobe epilepsy is one of the most common forms of epilepsy. Numerous contributing factors and compensatory mechanisms have been associated with temporal lobe epilepsy. One feature found in both humans and animal models is sprouting of hippocampal principal cell axons, which suggests that axonal sprouting may be a general phenomenon associated with temporal lobe epilepsy. This article highlights the evidence showing that hippocampal GABAergic interneurons also undergo axonal sprouting in temporal lobe epilepsy. The caveats and unanswered questions associated with the current data and the potential physiological consequences of reorganizations in GABAergic circuits are discussed.

Do interictal spikes drive epileptogenesis?

Interictal spikes are periodic, very brief bursts of neuronal activity that are observed in the electroencephalogram of patients with chronic epilepsy. These spikes are useful diagnostically, but we do not know why they are so strongly associated with the spontaneous seizures that characterize chronic epilepsy. Interictal spikes appear before the first spontaneous seizures in animal models of acquired epilepsy, and spikes are sufficient to induce long-term changes in synaptic connections between neurons. Thus, spikes may guide the development of the neuronal circuits that initiate spontaneous seizures. If so, then attempts to prevent or cure epilepsy may best be directed at spikes rather than seizures.

Are altered excitatory synapses found in neuronal migration disorders?

Physiological and Morphological Characterization of Dentate Granule Cells in the p35 Knock-out Mouse Hippocampus: Evidence for an Epileptic Circuit

Patel LS, Wenzel HJ, Schwartzkroin PA

J Neurosci 2004;24:9005–9014

There is a high correlation between pediatric epilepsies and neuronal migration disorders. What remains unclear is whether intrinsic features of the individual dysplastic cells give rise to heightened seizure susceptibility, or whether these dysplastic cells contribute to seizure activity by establishing abnormal circuits that alter the balance of inhibition and excitation. Mice lacking a functional p35 gene provide an ideal model in which to address these questions, because these knockout animals not only exhibit aberrant neuronal migration but also demonstrate spontaneous seizures.

Extracellular field recordings from hippocampal slices, characterizing the input–output relation in the dentate, revealed little difference between wild-type and knockout mice under both normal and elevated extracellular potassium conditions. However, in the presence of the GABAA antagonist bicuculline, p35 knockout slices, but not wild-type slices, exhibited prolonged depolarizations in response to stimulation of the perforant path. No significant differences were found in the intrinsic properties of dentate granule cells (i.e., input resistance, time constant, action-potential generation) from wild-type versus knockout mice. However, antidromic activation (mossy fiber stimulation) evoked an excitatory synaptic response in more than 65% of granule cells from p35 knockout slices that was never observed in wild-type slices. Ultrastructural analyses identified morphological substrates for this aberrant excitation: recurrent axon collaterals, abnormal basal dendrites, and mossy fiber terminals forming synapses onto the spines of neighboring granule cells. These studies suggest that granule cells in p35 knockout mice contribute to seizure activity by forming an abnormal excitatory feedback circuit.

Prolonged NMDA-mediated Synaptic Response, Altered Ifenprodil Sensitivity, and Generation of Epileptiform-like Events in a Malformed Hippocampus of Rats Exposed to Methylazoxymethanol in Utero

Calcagnotto ME, Baraban SC

J Neurophysiol 2005 [Epub ahead of print]

Cortical malformations are often associated with refractory epilepsy and cognitive deficit. Clinical and experimental studies have demonstrated an important role for glutamate-mediated synaptic transmission in these conditions. With whole-cell voltage-clamp techniques, we examined evoked glutamate-mediated excitatory postsynaptic currents (eEPSCs) and responses to exogenously applied glutamate on hippocampal heterotopic cells in an animal model of malformation (i.e., rats exposed to methylazoxymethanol [MAM] in utero). Analysis of eEPSCs revealed that the late N-methyl-D-aspartate (NMDA) receptor–mediated eEPSC component was significantly increased on heterotopic cells compared with age-matched normotopic pyramidal cells. At a holding potential of +40 mV, heterotopic cells also exhibited eEPSCs with a slower decay-time constant. No differences in the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) component of eEPSCs were detected. In 23% of heterotopic pyramidal cells, electrical stimulation evoked prolonged burstlike responses. Focal application of glutamate (10 m M) targeted to different sites near the heterotopia also evoked epileptiform-like bursts on heterotopic cells. Ifenprodil (10 μM), an NR2B subunit antagonist, only slightly reduced the NMDA receptor–mediated component and amplitude of eEPSCs on heterotopic cells (MAM) but significantly decreased the late component and peak amplitude of eEPSCs in normotopic cells (Control). Our data demonstrate a functional alteration in the NMDA-mediated component of excitatory synaptic transmission in heterotopic cells and suggest that this alteration may be attributable, at least in part, to changes in composition and function of the NMDAR subunit. Changes in NMDA-receptor function may directly contribute to the hyperexcitability and cognitive deficits reported in animal models and patients with brain malformations.

Detection of increased local excitatory circuits in the hippocampus during epileptogenesis using focal flash photolysis of caged glutamate

PURPOSE

Local synaptic circuits, particularly recurrent excitation, are hypothesized to contribute to the generation and synchronization of epileptiform activity. The present study tested whether local excitatory circuits in the hippocampus are increased in an animal model of temporal lobe epilepsy, and thus may contribute to epileptic seizures.

METHODS

Rats were given hourly injections of kainic acid to induce status epilepticus, which led to chronic epilepsy with spontaneous recurrent seizures. Whole-cell recording was performed in hippocampal slices, and focal flash photolysis of caged glutamate was used to detect local excitatory circuits.

RESULTS

In the dentate gyrus of rats with kainate-induced epilepsy and mossy fiber sprouting, focal stimulations with caged glutamate at many different sites in the granule cell layer consistently evoked repetitive excitatory postsynaptic currents (EPSCs) in normal medium and prolonged bursts of action potentials in bicuculline; these responses were not observed in similarly treated slices from control rats. In CA1, focal flash photolysis of caged glutamate in stratum pyramidale revealed significantly more excitatory connections between CA1 pyramidal cells in rats with kainate-induced epilepsy than saline-treated control animals.

CONCLUSION

Focal flash photolysis of caged glutamate revealed that new local excitatory circuits are formed in both the dentate gyrus and CA1 area of rats with kainate-induced epilepsy, which supports the hypothesis that the progressive formation of new local excitatory circuits occurs in many locations during epileptogenesis.

Prolonged NMDA-mediated responses, altered ifenprodil sensitivity, and epileptiform-like events in the malformed hippocampus of methylazoxymethanol exposed rats

Cortical malformations are often associated with refractory epilepsy and cognitive deficit. Clinical and experimental studies have demonstrated an important role for glutamate-mediated synaptic transmission in these conditions. Using whole cell voltage-clamp techniques, we examined evoked glutamate-mediated excitatory postsynaptic currents (eEPSCs) and responses to exogenously applied glutamate on hippocampal heterotopic cells in an animal model of malformation i.e., rats exposed to methylazoxymethanol (MAM) in utero. Analysis revealed that the late N-methyl-D-aspartate (NMDA) receptor-mediated eEPSC component was significantly increased on heterotopic cells compared with age-matched normotopic pyramidal cells. At a holding potential of +40 mV, heterotopic cells also exhibited eEPSCs with a slower decay-time constant. No differences in the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) component of eEPSCs were detected. In 23% of heterotopic pyramidal cells, electrical stimulation evoked prolonged burst-like responses. Focal application of glutamate (10 mM) targeted to different sites near the heterotopia also evoked epileptiform-like bursts on heterotopic cells. Ifenprodil (10 microM), an NR2B subunit antagonist, only slightly reduced the NMDA receptor (NMDAR)-mediated component and amplitude of eEPSCs on heterotopic cells (MAM) but significantly decreased the late component and peak amplitude of eEPSCs in normotopic cells (control). Our data demonstrate a functional alteration in the NMDA-mediated component of excitatory synaptic transmission in heterotopic cells and suggest that this alteration may be attributable, at least in part, to changes in composition and function of the NMDAR subunit. Changes in NMDAR function may directly contribute to the hyperexcitability and cognitive deficits reported in animal models and patients with brain malformations.

Physiological and morphological characterization of dentate granule cells in the p35 knock-out mouse hippocampus: evidence for an epileptic circuit

There is a high correlation between pediatric epilepsies and neuronal migration disorders. What remains unclear is whether there are intrinsic features of the individual dysplastic cells that give rise to heightened seizure susceptibility, or whether these dysplastic cells contribute to seizure activity by establishing abnormal circuits that alter the balance of inhibition and excitation. Mice lacking a functional p35 gene provide an ideal model in which to address these questions, because these knock-out animals not only exhibit aberrant neuronal migration but also demonstrate spontaneous seizures. Extracellular field recordings from hippocampal slices, characterizing the input-output relationship in the dentate, revealed little difference between wild-type and knock-out mice under both normal and elevated extracellular potassium conditions. However, in the presence of the GABA(A) antagonist bicuculline, p35 knock-out slices, but not wild-type slices, exhibited prolonged depolarizations in response to stimulation of the perforant path. There were no significant differences in the intrinsic properties of dentate granule cells (i.e., input resistance, time constant, action potential generation) from wild-type versus knock-out mice. However, antidromic activation (mossy fiber stimulation) evoked an excitatory synaptic response in over 65% of granule cells from p35 knock-out slices that was never observed in wild-type slices. Ultrastructural analyses identified morphological substrates for this aberrant excitation: recurrent axon collaterals, abnormal basal dendrites, and mossy fiber terminals forming synapses onto the spines of neighboring granule cells. These studies suggest that granule cells in p35 knock-out mice contribute to seizure activity by forming an abnormal excitatory feedback circuit.

Evidence that injury-induced changes in hippocampal neuronal calcium dynamics during epileptogenesis cause acquired epilepsy

Alterations in hippocampal neuronal Ca(2+) and Ca(2+)-dependent systems have been implicated in mediating some of the long-term neuroplasticity changes associated with acquired epilepsy (AE). However, there are no studies in an animal model of AE that directly evaluate alterations in intracellular calcium concentration ([Ca(2+)](i)) and Ca(2+) homeostatic mechanisms (Ca(2+) dynamics) during the development of AE. In this study, Ca(2+) dynamics were evaluated in acutely isolated rat CA1 hippocampal, frontal, and occipital neurons in the pilocarpine model by using [Ca(2+)](i) imaging fluorescence microscopy during the injury (acute), epileptogenesis (latency), and chronic-epilepsy phases of the development of AE. Immediately after status epilepticus (SE), hippocampal neurons, but not frontal and occipital neurons, had significantly elevated [Ca(2+)](i) compared with saline-injected control animals. Hippocampal neuronal [Ca(2+)](i) remained markedly elevated during epileptogenesis and was still elevated indefinitely in the chronic-epilepsy phase but was not elevated in SE animals that did not develop AE. Inhibiting the increase in [Ca(2+)](i) during SE with the NMDA channel inhibitor MK801 was associated in all three phases of AE with inhibition of the changes in Ca(2+) dynamics and the development of AE. Ca(2+) homeostatic mechanisms in hippocampal neurons also were altered in the brain-injury, epileptogenesis, and chronic-epilepsy phases of AE. These results provide evidence that [Ca(2+)](i) and Ca(2+)-homeostatic mechanisms are significantly altered during the development of AE and suggest that altered Ca(2+) dynamics may play a role in the induction and maintenance of AE and underlie some of the neuroplasticity changes associated with the epileptic phenotype.

Layer-specific pyramidal cell oscillations evoked by tetanic stimulation in the rat hippocampal area CA1 in vitro and in vivo

Tetanic stimulation of axons terminating in the CA1 region of the hippocampus induces oscillations in the gamma-to-beta frequency band (13-100 Hz) and can induce long-term potentiation (LTP). The rapid pyramidal cell discharge is driven by a mainly GABA(A)-receptor-mediated slow depolarization and entrained mainly through ephaptic interactions. This study tests whether cellular compartmentalization can explain how cells, despite severely reduced input resistance, can still fire briskly and have IPSPs superimposed on the slow GABAergic depolarization, and whether this behaviour occurs in vivo. Oscillations induced in CA1 in vitro by tetanic stimulation of the stratum radiatum or oriens were analysed using intracellular and multichannel field potentials along the cell axis. Layer-specific effects of focal application of bicuculline indicate that the GABAergic depolarization is concentrated on tetanized dendrites. Current-source density analysis and characteristics of partial spikes indicate that early action potentials are initiated in the proximal nontetanized dendrite but cannot invade the tetanized dendrite, where recurrent EPSPs and evoked IPSPs were largely suppressed. As the oscillation progresses, IPSPs recover and slow the neuronal firing to beta frequencies, with a small subpopulation of neurons continuing to fire at gamma frequency. Carbonic anhydrase dependence, threshold intensity, frequency, field strength and spike initiation/propagation of tetanus-evoked oscillations in urethane-anaesthetized rats, validate our observations in vitro, and show that these mechanisms operate in healthy tissue. However, the disrupted electrophysiology of the tetanized dendrites will disable normal information processing, has implications for LTP induction and is likely to play a role in pathological synchronization as found during epileptic discharges.