Interaction between superficial layers of the entorhinal cortex and the hippocampus in normal and epileptic temporal lobe

Magnetic resonance imaging fingerprints of status epilepticus: A case-control study

OBJECTIVE

Status epilepticus (SE) is frequently associated with peri-ictal magnetic resonance imaging (MRI) abnormalities (PMA). However, the anatomical distribution of these alterations has not been systematically studied. The aim of this study was to assess the localization patterns of PMA in patients with SE.

METHODS

In this prospective case-control study, we compared the distribution and combinations of diffusion-restricted PMA to diffusion-restricted lesions caused by other neurological conditions. All patients of the SE group and the control group underwent MRI including a diffusion-weighted imaging sequence. Patients with SE were imaged within 48 h after its onset.

RESULTS

We enrolled 201 patients (51 with SE and 150 controls). The most frequent locations of PMA in SE were cortex (25/51, 49%), followed by hippocampus (20/51, 39%) and pulvinar of thalamus (10/51, 20%). In the control group, the cortex was involved in 80 of 150 (53%), white matter in 53 of 150 (35%), and basal ganglia in 33 of 150 (22%). In the control group, the pulvinar of thalamus was never affected and hippocampal structures were rarely involved (7/150, 5%). Involvement of the pulvinar of thalamus and the hippocampus had high specificity for SE at 100% (95% confidence interval [CI] = 98-100) and 95% (95% CI = 91-98), respectively. The sensitivity, however, was low for both locations (pulvinar of thalamus: 20%, 95% CI = 10-33; hippocampus: 39%, 95% CI = 26-54).

SIGNIFICANCE

Diffusion-restricted MRI lesions observed in the pulvinar of thalamus and hippocampus are strongly associated with SE. These changes may help physicians in diagnosing SE-related changes on MRI in an acute setting, especially in cases of equivocal clinical and electroencephalographic manifestations of SE.

A deep network-based model of hippocampal memory functions under normal and Alzheimer's disease conditions

We present a deep network-based model of the associative memory functions of the hippocampus. The proposed network architecture has two key modules: (1) an autoencoder module which represents the forward and backward projections of the cortico-hippocampal projections and (2) a module that computes familiarity of the stimulus and implements hill-climbing over the familiarity which represents the dynamics of the loops within the hippocampus. The proposed network is used in two simulation studies. In the first part of the study, the network is used to simulate image pattern completion by autoassociation under normal conditions. In the second part of the study, the proposed network is extended to a heteroassociative memory and is used to simulate picture naming task in normal and Alzheimer's disease (AD) conditions. The network is trained on pictures and names of digits from 0 to 9. The encoder layer of the network is partly damaged to simulate AD conditions. As in case of AD patients, under moderate damage condition, the network recalls superordinate words ("odd" instead of "nine"). Under severe damage conditions, the network shows a null response ("I don't know"). Neurobiological plausibility of the model is extensively discussed.

Microcircuits for spatial coding in the medial entorhinal cortex

The hippocampal formation is critically involved in learning and memory and contains a large proportion of neurons encoding aspects of the organism's spatial surroundings. In the medial entorhinal cortex (MEC), this includes grid cells with their distinctive hexagonal firing fields as well as a host of other functionally defined cell types including head direction cells, speed cells, border cells, and object-vector cells. Such spatial coding emerges from the processing of external inputs by local microcircuits. However, it remains unclear exactly how local microcircuits and their dynamics within the MEC contribute to spatial discharge patterns. In this review we focus on recent investigations of intrinsic MEC connectivity, which have started to describe and quantify both excitatory and inhibitory wiring in the superficial layers of the MEC. Although the picture is far from complete, it appears that these layers contain robust recurrent connectivity that could sustain the attractor dynamics posited to underlie grid pattern formation. These findings pave the way to a deeper understanding of the mechanisms underlying spatial navigation and memory.

Gut metabolite S-equol ameliorates hyperexcitability in entorhinal cortex neurons following Theiler murine encephalomyelitis virus-induced acute seizures

Objective

A growing body of evidence indicates a potential role for the gut–brain axis as a novel therapeutic target in treating seizures. The present study sought to characterize the gut microbiome in Theiler murine encephalomyelitis virus (TMEV)‐induced seizures, and to evaluate the effect of microbial metabolite S‐equol on neuronal physiology as well as TMEV‐induced neuronal hyperexcitability ex vivo.

Methods

We infected C57BL/6J mice with TMEV and monitored the development of acute behavioral seizures 0–7 days postinfection (dpi). Fecal samples were collected at 5–7 dpi and processed for 16S sequencing, and bioinformatics were performed with QIIME2. Finally, we conducted whole‐cell patch‐clamp recordings in cortical neurons to investigate the effect of exogenous S‐equol on cell intrinsic properties and neuronal hyperexcitability.

Results

We demonstrated that gut microbiota diversity is significantly altered in TMEV‐infected mice at 5–7 dpi, exhibiting separation in beta diversity in TMEV‐infected mice dependent on seizure phenotype, and lower abundance of genus Allobaculum in TMEV‐infected mice regardless of seizure phenotype. In contrast, we identified specific loss of S‐equol‐producing genus Adlercreutzia as a microbial hallmark of seizure phenotype following TMEV infection. Electrophysiological recordings indicated that exogenous S‐equol alters cortical neuronal physiology. We found that entorhinal cortex neurons are hyperexcitable in TMEV‐infected mice, and exogenous application of microbial‐derived S‐equol ameliorated this TMEV‐induced hyperexcitability.

Significance

Our study presents the first evidence of microbial‐derived metabolite S‐equol as a potential mechanism for alteration of TMEV‐induced neuronal excitability. These findings provide new insight for the novel role of S‐equol and the gut–brain axis in epilepsy treatment.

The actin binding protein drebrin helps to protect against the development of seizure-like events in the entorhinal cortex

The actin binding protein drebrin plays a key role in dendritic spine formation and synaptic plasticity. Decreased drebrin protein levels have been observed in temporal lobe epilepsy, suggesting the involvement of drebrin in the disease. Here we investigated the effect of drebrin knockout on physiological and pathophysiological neuronal network activities in mice by inducing gamma oscillations, involved in higher cognitive functions, and by analyzing pathophysiological epileptiform activity. We found that loss of drebrin increased the emergence of spontaneous gamma oscillations suggesting an increase in neuronal excitability when drebrin is absent. Further analysis showed that although the kainate-induced hippocampal gamma oscillations were unchanged in drebrin deficient mice, seizure like events measured in the entorhinal cortex appeared earlier and more frequently. The results suggest that while drebrin is not essential for normal physiological network activity, it helps to protect against the formation of seizure like activities during pathological conditions. The data indicate that targeting drebrin function could potentially be a preventive or therapeutic strategy for epilepsy treatment.

Deficiency of anti-inflammatory cytokine IL-4 leads to neural hyperexcitability and aggravates cerebral ischemia-reperfusion injury

Systematic administration of anti-inflammatory cytokine interleukin 4 (IL-4) has been shown to improve recovery after cerebral ischemic stroke. However, whether IL-4 affects neuronal excitability and how IL-4 improves ischemic injury remain largely unknown. Here we report the neuroprotective role of endogenous IL-4 in focal cerebral ischemia–reperfusion (I/R) injury. In multi-electrode array (MEA) recordings, IL-4 reduces spontaneous firings and network activities of mouse primary cortical neurons. IL-4 mRNA and protein expressions are upregulated after I/R injury. Genetic deletion of Il-4 gene aggravates I/R injury in vivo and exacerbates oxygen-glucose deprivation (OGD) injury in cortical neurons. Conversely, supplemental IL-4 protects Il-4^−/− cortical neurons against OGD injury. Mechanistically, cortical pyramidal and stellate neurons common for ischemic penumbra after I/R injury exhibit intrinsic hyperexcitability and enhanced excitatory synaptic transmissions in Il-4^−/− mice. Furthermore, upregulation of Nav1.1 channel, and downregulations of KCa3.1 channel and α6 subunit of GABA_A receptors are detected in the cortical tissues and primary cortical neurons from Il-4^−/− mice. Taken together, our findings demonstrate that IL-4 deficiency results in neural hyperexcitability and aggravates I/R injury, thus activation of IL-4 signaling may protect the brain against the development of permanent damage and help recover from ischemic injury after stroke.

Anticonvulsant effect of dipropofol by enhancing native GABA currents in cortical neurons in mice

Temporal lobe epilepsy (TLE), the most common pharmacoresistant focal epilepsy disorder, remains a major unmet medical need. Propofol is used as a short-acting medication for general anesthesia and refractory status epilepticus with issues of decreased consciousness and memory loss. Dipropofol, a derivative of propofol, has been reported to exert antioxidative and antibacterial activities. Here we report that dipropofol exerted anticonvulsant activity in a mouse model of kainic acid-induced seizures. Whole cell patch-clamp recordings of brain slices from the medial entorhinal cortex (mEC) revealed that dipropofol hyperpolarized the resting membrane potential and reduced the number of action potential firings, resulting in suppression of cortical neuronal excitability. Furthermore, dipropofol activated native tonic GABAA currents of mEC layer II stellate neurons in a dose-dependent manner with an EC50 value of 9.3 ± 1.6 μM (mean ± SE). Taken together, our findings show that dipropofol activated GABAA currents and exerted anticonvulsant activities in mice, thus possessing developmental potential for new anticonvulsant therapy.

NEW & NOTEWORTHY The anticonvulsant effect of dipropofol was shown in a mouse model of kainic acid-induced seizures. Whole cell patch-clamp recordings of brain slices showed suppression of cortical neuronal excitability by dipropofol. Dipropofol activated the native tonic GABAA currents in a dose-dependent manner.

Imaging the Creative Unconscious: Reflexive Neural Responses to Objects in the Visual and Parahippocampal Region Predicts State and Trait Creativity

What does it take to have a creative mind? Theories of creative cognition assert that the quantity of automatic associations places fundamental constraints on the probability of reaching creative solutions. Due to the difficulties inherent in isolating automated associative responses from cognitive control, the neural basis underlying this faculty remains unknown. Here we acquired fMRI data in an incidental-viewing paradigm in which subjects performed an attention-demanding task whilst viewing task-irrelevant objects. By assigning a standard creativity task on the same objects out of the scanner, as well as a battery of psychometric creativity tests, we could assess whether stimulus-bound neural activity was predictive of state or trait variability in creativity. We found that stimulus-bound responses in superior occipital regions were linearly predictive of trial-by-trial variability in creative performance (state-creativity), and that in more creative individuals (trait-creativity) this response was more strongly expressed in entorhinal cortex. Additionally, the mean response to the onset of objects in parahippocampal gyrus was predictive of trait differences in creativity. This work suggests that, creative individuals are endowed with occipital and medial temporal reflexes that generate a greater fluency in associative representations, making them more accessible for ideation even when no ideation is explicitly called for.

Target-selectivity of parvalbumin-positive interneurons in layer II of medial entorhinal cortex in normal and epileptic animals

The medial entorhinal cortex layer II (MEClayerII ) is a brain region critical for spatial navigation and memory, and it also demonstrates a number of changes in patients with, and animal models of, temporal lobe epilepsy (TLE). Prior studies of GABAergic microcircuitry in MEClayerII revealed that cholecystokinin-containing basket cells (CCKBCs) select their targets on the basis of the long-range projection pattern of the postsynaptic principal cell. Specifically, CCKBCs largely avoid reelin-containing principal cells that form the perforant path to the ipsilateral dentate gyrus and preferentially innervate non-perforant path forming calbindin-containing principal cells. We investigated whether parvalbumin containing basket cells (PVBCs), the other major perisomatic targeting GABAergic cell population, demonstrate similar postsynaptic target selectivity as well. In addition, we tested the hypothesis that the functional or anatomic arrangement of circuit selectivity is disrupted in MEClayerII in chronic TLE, using the repeated low-dose kainate model in rats. In control animals, we found that PVBCs innervated both principal cell populations, but also had significant selectivity for calbindin-containing principal cells in MEClayerII . However, the magnitude of this preference was smaller than for CCKBCs. In addition, axonal tracing and paired recordings showed that individual PVBCs were capable of contacting both calbindin and reelin-containing principal cells. In chronically epileptic animals, we found that the intrinsic properties of the two principal cell populations, the GABAergic perisomatic bouton numbers, and selectivity of the CCKBCs and PVBCs remained remarkably constant in MEClayerII . However, miniature IPSC frequency was decreased in epilepsy, and paired recordings revealed the presence of direct excitatory connections between principal cells in the MEClayerII in epilepsy, which is unusual in normal adult MEClayerII . Taken together, these findings advance our knowledge about the organization of perisomatic inhibition both in control and in epileptic animals. © 2015 Wiley Periodicals, Inc.

Bombesin facilitates GABAergic transmission and depresses epileptiform activity in the entorhinal cortex

Bombesin and the bombesin-like peptides including neuromedin B (NMB) and gastrin-releasing peptide (GRP) are important neuromodulators in the brain. We studied their effects on GABAergic transmission and epileptiform activity in the entorhinal cortex (EC). Bath application of bombesin concentration-dependently increased both the frequency and amplitude of sIPSCs recorded from the principal neurons in the EC. Application of NMB and GRP exerted the same effects as bombesin. Bombesin had no effects on mIPSCs recorded in the presence of TTX but slightly depressed the evoked IPSCs. Omission of extracellular Ca(2+) or inclusion of voltage-gated Ca(2+) channel blockers, Cd(2+) and Ni(2+), blocked bombesin-induced increases in sIPSCs suggesting that bombesin increases GABA release via facilitating extracellular Ca(2+) influx. Bombesin induced membrane depolarization and slightly increased the input resistance of GABAergic interneurons recorded from layer III of the EC. The action potential firing frequency of the interneurons was also increased by bombesin. Bombesin-mediated depolarization of interneurons was unlikely to be mediated by the opening of a cationic conductance but due to the inhibition of inward rectifier K(+) channels. Bath application of bombesin, NMB and GRP depressed the frequency of the epileptiform activity elicited by deprivation of Mg(2+) from the extracellular solution suggesting that bombesin and the bombesin-like peptides have antiepileptic effects in the brain.

Depression, stress, epilepsy and adult neurogenesis

Epilepsy and depression share an unusually high coincidence suggestive of a common etiology. Disrupted production of adult-born hippocampal granule cells in both disorders may contribute to this high coincidence. Chronic stress and depression are associated with decreased granule cell neurogenesis. Epilepsy is associated with increased production - but aberrant integration - of new cells early in the disease and decreased production late in the disease. In both cases, the literature suggests these changes in neurogenesis play important roles in their respective diseases. Aberrant integration of adult-generated cells during the development of epilepsy may impair the ability of the dentate gyrus to prevent excess excitatory activity from reaching hippocampal pyramidal cells, thereby promoting seizures. Effective treatment of a subset of depressive symptoms, on the other hand, may require increased granule cell neurogenesis, indicating that adult-generated granule cells can modulate mood and affect. Given the robust changes in adult neurogenesis evident in both disorders, competing effects on brain structure are likely. Changes in relative risk, disease course or response to treatment seem probable, but complex and changing patterns of neurogenesis in both conditions will require sophisticated experimental designs to test these ideas. Despite the challenges, this area of research is critical for understanding and improving treatment for patients suffering from these disorders.

[The concept of an epileptogenic network in human partial epilepsies]

An anatomical and functional model of drug-resistant partial seizures is presented and discussed based on research conducted by our team over the last decade. This research is based on the study of intracerebral stereoelectroencephalography (SEEG) recordings in an attempt to identify the neural networks involved in generating paroxystic activities so as to understand their dynamics in space and time, and to propose targeted therapies that could "control" these networks. Today, the classical notion of epileptic focus should be replaced by a more complex model that takes into account the potential interactions within the neuronal networks involved in seizures. During partial epileptic seizures, the cerebral structures involved are the seat of characteristic oscillations that may be synchronized or, on the contrary, that can desynchronize in a transitory manner. These epileptic rhythms disturb the physiological rhythms that underlie the cognitive and emotional processes, which can thus be altered in partial epilepsy, even if located far from the original discharge site. We suggest that seizures originate in a group of structures that are highly epileptogenic (epileptogenic zone network, [EZN]) whose activity is synchronized before the appearance of fast oscillations that are transitorily desynchronized. Later, other cortical and subcortical structures are the seat of slower, synchronized rhythmic modifications (propagation network, [PN]). The emergence of clinical signs in the seizure depend on these phenomena, which in some cases can mimic a normal cognitive process or, on the contrary, lead to a deep rupture in normal cerebral functioning.

Voxel-based morphometry of temporal lobe epilepsy: an introduction and review of the literature

We review the applications and results of voxel-based morphometry (VBM) studies that have reported brain changes associated with temporal lobe epilepsy (TLE). A PubMed search yielded 18 applications of VBM to study brain abnormalities in patients with TLE up to May 2007. Across studies, 26 brain regions were found to be significantly reduced in volume relative to healthy controls. There was a strong asymmetrical distribution of temporal lobe abnormalities preferentially observed ipsilateral to the seizure focus, particularly of the hippocampus (82.35% of all studies), parahippocampal gyrus (47.06%), and entorhinal (23.52%) cortex. The contralateral hippocampus was reported as abnormal in 17.65% of studies. There was a much more bilateral distribution of extratemporal lobe atrophy, preferentially affecting the thalamus (ipsilateral = 61.11%, contralateral = 50%) and parietal lobe (ipsilateral = 47.06%, contralateral = 52.94%). VBM generally reveals a distribution of brain abnormalities in patients with TLE consistent with the region-of-interest neuroimaging and postmortem literature. It is unlikely that VBM has any clinical utility given the lack of robustness for individual comparisons. However, VBM may help elucidate some unresolved important research questions such as how recurrent temporal lobe seizures affect hippocampal and extrahippocampal morphology using serial imaging acquisitions.

Synaptic responses in superficial layers of medial entorhinal cortex from rats with kainate-induced epilepsy

Mesial temporal lobe epilepsy patients often display shrinkage of the entorhinal cortex, which has been attributed to neuronal loss in medial entorhinal cortex layer III (MEC-III). MEC-III neuronal loss is reproduced in chronic epileptic rats after kainate-induced (KA) status epilepticus. Here we examined, in vitro, functional changes in superficial entorhinal cortex layers. Alterations in superficial layer circuitry were suggested by showing that presubiculum, parasubiculum and deep MEC stimulation evoked 100-300 Hz field potential transients and prolonged EPSPs (superimposed on IPSPs) in superficial MEC which were partially blocked by APV (in contrast to control) and fully blocked by CNQX. Contrary to controls, bicuculline (5 and 30 microM) had minor effects on evoked field potentials in KA rats. GAD65/67 in situ hybridization revealed preserved interneurons in MEC-III. In conclusion, hyperexcitability in superficial MEC neurons is not due to loss of GABAergic interneurons and probably results from alterations in synaptic connectivity within superficial MEC.

Presubiculum stimulation in vivo evokes distinct oscillations in superficial and deep entorhinal cortex layers in chronic epileptic rats

The characteristic cell loss in layer III of the medial entorhinal area (MEA-III) in human mesial temporal lobe epilepsy is reproduced in the rat kainate model of the disease. To understand how this cell loss affects the functional properties of the MEA, we investigated whether projections from the presubiculum (prS), providing a main input to the MEA-III, are altered in this epileptic rat model. Injections of an anterograde tracer in the prS revealed bilateral projection fibers mainly to the MEA-III in both control and chronic epileptic rats. We further examined the prS-MEA circuitry using a 16-channel electrode probe covering the MEA in anesthetized control and chronic epileptic rats. With a second 16-channel probe, we recorded signals in the hippocampus. Current source density analysis indicated that, after prS double-pulse stimulation, afterdischarges in the form of oscillations (20-45 Hz) occurred that were confined to the superficial layers of the MEA in all epileptic rats displaying MEA-III neuronal loss. Slower oscillations (theta range) were occasionally observed in the deep MEA layers and the dentate gyrus. This kind of oscillation was never observed in control rats. We conclude that dynamical changes occur in an extensive network within the temporal lobe in epileptic rats, manifested as different kinds of oscillations, the characteristics of which depend on local properties of particular subareas. These findings emphasize the significance of the entorhinal cortex in temporal lobe epilepsy and suggest that the superficial cell layers could play an important role in distributing oscillatory activity.

Dynamique des réseaux neuraux dans les épilepsies partielles humaines

INTRODUCTION

The anatomo-functional organization of partial drug-resistant epilepsies is the subject of much current research aiming at better understanding these pathologies and improving their treatment. The work carried out by our team on the study of intracerebral recording falls within this category of research. The objectives are to identify the neural networks involved in the generation of paroxysmal activity and to understand their spatio-temporal dynamics, in order to be able in the long term to propose targeted therapeutic approaches likely to "control" these networks.

STATE OF ART

The traditional concept of epileptic "focus" must nowadays be replaced by a more complex model taking into account potential interactions within the neural networks involved in the seizure. Indeed, during partial seizures, involved cerebral structures are the site of characteristic oscillations which may be synchronized or on the contrary transiently desynchronized. These epileptic rhythms may disturb the physiological rhythms underlying normal cognitive processes; these cognitive processes may thus be impaired in partial epilepsy, even those remote from the site of origin of the discharge. In this article we describe a model of organization of human partial seizures, through characterization of the relationships ("synchrony") between intracerebral signals recorded in the involved structures. We propose that seizures are generated in an initial network of highly epileptogenic brain structures (epileptogenic zone network, EZN) whose activity is synchronized; this activity is then transiently desynchronized with the appearance of fast oscillations. During a second ictal phase, other cortical and subcortical structures are the seat of slower rhythmic modifications that are synchronized (propagation network, PN). The emergence of a particular clinical semiology in the course of the seizure depends on these phenomena which can in certain cases "mimic" a normal cerebral process or on the contrary provoke a major rupture in normal cerebral functioning.

CONCLUSIONS

These studies contribute to improvement in our knowledge of the neural networks involved in partial epilepsies. In the future, this type of research may contribute to the development of specific treatments that target certain pathophysiological mechanisms involved in seizure generation.

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.

Physiological changes in chronic epileptic rats are prominent in superficial layers of the medial entorhinal area

PURPOSE

We investigated whether the functional network properties of the medial entorhinal area (MEA) of the entorhinal cortex were altered in a rat model of chronic epilepsy that is characterized by extensive cell loss in MEA layer III.

METHODS

Responses were evoked in the entorhinal cortex by electrical stimulation of the subiculum in anesthetized chronic epileptic rats, 2-4 months after status epilepticus, induced by systemic kainate (KA) injections. Laminar field potentials were measured using a 16-channel silicon probe that covered all six layers of the MEA; an estimate of the local transmembrane currents was made using current source density analysis.

RESULTS

Double-pulse stimulation of the subiculum evoked responses in deep and superficial layers of the MEA in control and KA rats. A current sink in layer I and at the border of layer I and II that was induced by antidromic activation of MEA-II, was much more prominent in KA rats with extensive neuronal loss in MEA-III than in control rats or KA rats with minor MEA-III loss. Furthermore, KA rats that displayed MEA-III loss presented a series of oscillations induced by subicular stimulation in the beta/gamma-frequency range (20-100 Hz), which were confined to superficial layers of MEA. These oscillations were never observed in control rats or KA rats with minor MEA-III loss.

CONCLUSIONS

These results indicate that the observed alterations in the superficial MEA responses to subiculum stimulation and the occurrence of beta/gamma-oscillations are related phenomena, which are a consequence of altered and impaired inhibition within these MEA layers in chronic epileptic rats.

Maturation of cultured hippocampal slices results in increased excitability in granule cells

The preparation of hippocampal slices results in loss of input neurons to dentate granule cells, which leads to the reorganization of their axons, the mossy fibers, and alters their functional properties in long-term cultures, but its temporal aspects in the immature hippocampus are not known. In this study, we have focused on the early phase of this plastic reorganization process by analyzing granule cell function with field potential and whole cell recordings during the in vitro maturation of hippocampal slices (from 1 to 17 days in vitro, prepared from 6 to 7-day-old rats), and their morphology using extracellular biocytin labelling technique. Acute slices from postnatal 14-22-day-old rats were analyzed to detect any differences in the functional properties of granule cells in these two preparations. In field potential recordings, small synaptically-evoked responses were detected at 2 days in vitro, and their amplitude increased during the culture time. Whole cell voltage clamp recordings revealed intensive spontaneous excitatory postsynaptic currents, and the susceptibility to stimulus-evoked bursting increased with culture time. In acutely prepared slices, neither synaptically-evoked responses in field potential recordings nor any bursting in whole cell recordings were detected. The excitatory activity was under the inhibitory control of gamma-aminobutyric acid type A receptor. Extracellularily applied biocytin labelled dentate granule cells, and revealed sprouting and aberrant targeting of mossy fibers in cultured slices. Our results suggest that reorganization of granule cell axons takes place during the early in vitro maturation of hippocampal slices, and contributes to their increased excitatory activity resembling that in the epileptic hippocampus. Cultured immature hippocampal slices could thus serve as an additional in vitro model to elucidate mechanisms of synaptic plasticity and cellular reactivity in response to external damage in the developing hippocampus.

Entorhinal Cortex Involvement in Human Mesial Temporal Lobe Epilepsy: An Electrophysiologic and Volumetric Study

PURPOSE

Several studies have demonstrated diminution in the volume of entorhinal cortex (EC) ipsilateral to the pathologic side in patients with temporal lobe epilepsy (TLE). The relation between the degree of EC atrophy and the epileptogenicity of this structure has never been directly studied. The purpose of the study was to determine whether atrophy of the EC evaluated by the quantitative magnetic resonance imaging (MRI) method is correlated with the epileptogenicity of this structure in TLE.

METHODS

Intracerebral recordings (SEEG method) of seizures from 11 patients with mesial TLE were analyzed. Seizures were classified according to patterns of onset: pattern 1 was the emergence of a low-frequency, high-amplitude rhythmic spiking followed by a tonic discharge, and pattern 2 was the emergence of a tonic discharge in the mesial structures. A nonlinear measure of SEEG signal interdependencies was used to evaluate the functional couplings occurring between hippocampus (Hip) and EC at seizure onset. MRI volumetric analysis was performed by using a T(1)-weighted three-dimensional gradient-echo sequence in TLE patients and 12 healthy subjects.

RESULTS

Significant interactions between Hip and Ec were quantified at seizure onset. The EC was found to be the leader structure in most of the pattern 2 seizures. Volumetric measurements of EC demonstrated an atrophy in 63% of patients ipsilateral to the epileptic side. A significant correlation between the strength of EC-Hip coupling and the degree of atrophy was found. In addition, in those patients that had a normal EC volume, the EC was never the leader structure in Ec-Hip coupling.

CONCLUSIONS

These results validate the potential role of volumetry to predict the epileptogenesis of the EC in patients with hippocampal sclerosis and MTLE.

Protein kinase associated with gating and closing transmission mechanisms in temporoammonic pathway

The entorhinal cortex (EC) is a major source of afferent input to the hippocampus via the perforant and temporoammonic pathways; however, the detailed transmission mechanism in the temporoammonic pathway remains to be clarified. Thus, we determined interaction among GABA(A), AMPA/glutamate receptors and protein kinases (PKA and PKC) in the exocytosis of GABA and glutamate using multiprobe microdialysis, as well as propagation of neuronal excitability using optical recording in the EC-Hippocampal formation. Multiprobe microdialysis demonstrated that EC-evoked GABA release in ventral CA1 was predominantly regulated by the PKC-related rather than PKA-related exocytosis mechanism and was augmented by the activation of glutamatergic transmission. Contrary to GABA release, EC-evoked glutamate release was predominantly regulated by PKA-related rather than PKC-related mechanisms and was suppressed by activation of GABAergic transmission. Optical recording demonstrated that there are two sub-pathways in the temporoammonic pathway; direct projects from EC layers (II-IV) to dendrites on pyramidal cells and GABAergic interneurons in ventral hippocampal CA1. PKC activation enhanced trisynaptic transmission, whether the GABA(A) receptor was functional or blocked, whereas PKC activation enhanced and inhibited temporoammonic transmission when the GABA(A) receptor was functional and blocked, respectively. Thus, GABAergic inhibition, which is regulated by PKC activity, in the temporoammonic pathway is more significant than that in the trisynaptic pathway.

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.

Quantitative Neuropathology of the Entorhinal Cortex Region in Patients with Hippocampal Sclerosis and Temporal Lobe Epilepsy

PURPOSE

Clinical, radiologic, and experimental evidence indicates that the entorhinal cortex (EC) region may be linked to the pathophysiology of hippocampal sclerosis (HS) in patients with temporal lobe epilepsy. Few neuropathologic studies of this region have been undertaken in patients with HS undergoing surgery, some suggesting preferential loss of layer III neurones.

METHODS

We carried out a quantitative analysis in 26 patients with HS, nine patients with lesional temporal lobe epilepsy (LTLE), and eight postmortem controls. We measured neuronal densities in EC by using a three-dimensional cell-counting technique on NeuN immunostained and Nissl-stained sections. We also quantified the density of calretinin-positive interneurones in this region and the density of neurones in adjacent subiculum and CA1 subfields. We also assessed the patterns of gliosis in the EC in the patient groups and the presence of any neocortical neurone loss.

RESULTS

No significant difference was found in the mean neuronal densities in the EC region between HS and LTLE groups or postmortem controls. Laminar gliosis in midcortical layers was seen in a proportion of HS cases but also in the LTLE group. No significant difference was seen in the density of calretinin interneurones and no correlation between the presence of neocortical neuronal loss and EC neuronal densities.

CONCLUSIONS

A stereotypical pattern of neuronal loss and gliosis in the EC region in patients with HS is not confirmed that distinguishes this pathologic process from that in patients with lesional TLE.

Dendritic morphology, local circuitry, and intrinsic electrophysiology of principal neurons in the entorhinal cortex of macaque monkeys

Little is known about the neuroanatomical or electrophysiological properties of individual neurons in the primate entorhinal cortex. We have used intracellular recording and biocytin-labeling techniques in the entorhinal slice preparation from macaque monkeys to investigate the morphology and intrinsic electrophysiology of principal neurons. These neurons have previously been studied most extensively in rats. In monkeys, layer II neurons are usually stellate cells, as in rats, but they occasionally have a pyramidal shape. They tend to discharge trains, not bursts, of action potentials, and some display subthreshold membrane potential oscillations. Layer III neurons are pyramidal, and they do not appear to display membrane potential oscillations. The distribution of dendrites and of axon collaterals suggests that neurons in layers II and III are interconnected by a network of associational fibers. Layer V and VI neurons are pyramidal and tend to discharge trains of action potentials. The distribution of dendrites and axon collaterals suggests that there is an associative network of principal neurons in layers V and VI, and they also project axon collaterals toward superficial layers. Importantly, entorhinal cortical neurons in monkeys appear to exhibit significant differences from those in rats. Morphologically, neurons in monkey entorhinal layers II and III have more primary dendrites, more dendritic branches, and greater total dendritic length than in rats. Electrophysiologically, layer II neurons in monkeys exhibit less sag, and subthreshold oscillations are less robust and slower. Some monkey layer III neurons discharge bursts of action potentials that are not found in rats. The interspecies differences revealed by this study may influence information processing and pathophysiological processes in the primate entorhinal cortex. J. Comp. Neurol. 470:317-329, 2004.

Synaptic and non-synaptic mechanisms of amygdala recruitment into temporolimbic epileptiform activities

Lateral amygdala (LA) activity during synchronized-epileptiform discharges in temporolimbic circuits was investigated in rat horizontal slices containing the amygdala, hippocampus (Hip), perirhinal (Prh) and lateral entorhinal (LEnt) cortex, through multiple-site extra- and intracellular recording techniques and measurement of the extracellular K+ concentration. Application of 4-aminopyridine (50 microm) induced epileptiform discharges in all regions under study. Slow interictal-like burst discharges persisted in the Prh/LEnt/LA after disconnection of the Hip, seemed to originate in the Prh as shown from time delay analyses, and often preceded the onset of ictal-like activity. Disconnection of the amygdala resulted in de-synchronization of epileptiform discharges in the LA from those in the Prh/LEnt. Interictal-like activity was intracellularly reflected in LA projection neurons as gamma-aminobutyric acid (GABA)A/B receptor-mediated synaptic responses, and depolarizing electrogenic events (spikelets) residing on the initial phase of the GABA response. Spikelets were considered antidromically conducted ectopic action potentials generated at axon terminals, as they were graded in amplitude, were not abolished through hyperpolarizing membrane responses (which effectively blocked evoked orthodromic action potentials), lacked a clear prepotential or synaptic potential, were not affected through blockers of gap junctions, and were blocked through remote application of tetrodotoxin at putative target areas of LA projection neurons. Remote application of a GABAB receptor antagonist facilitated spikelet generation. A transient elevation in the extracellular K+ level averaging 3 mm above baseline occurred in conjunction with interictal-like activity in all areas under study. We conclude that interictal-like discharges in the LA/LEnt/Prh spread in a predictable manner through the synaptic network with the Prh playing a leading role. The rise in extracellular K+ may provide a depolarizing mechanism for recruitment of interneurons and generation of ectopic action potentials at axon terminals of LA projection neurons. Antidromically conducted ectopic action potentials may provide a spreading mechanism of seizure activity mediated by diffuse axonal projections of LA neurons.

Reciprocal connections of the hippocampal area CA1, the lateral nucleus of the amygdala and cortical areas in a combined horizontal slice preparation

The entorhinal and perirhinal cortices, the hippocampus and the amygdala are heavily interconnected limbic structures that are implicated in memory, and under pathological conditions, in seizure generation and propagation of temporal lobe epilepsy. In-vitro coronal preparations have been limited by the anatomical disposition of these structures. Here we describe a modified horizontal slice preparation that includes all these structures in the same plane. To evaluate whether axonal connectivities are preserved, fluorescent tracers were used. Most of the connections known from in-vivo studies within and between the entorhinal and perirhinal cortices, the amygdala (basolateral nucleus, lateral nucleus, and amygdalopiriform transition area) and the hippocampus were preserved in the 400 microm-thick horizontal slices employed.

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.

Subregional pathology of the amygdala complex and entorhinal region in surgical specimens from patients with pharmacoresistant temporal lobe epilepsy

The hippocampus, amygdala complex, and entorhinal region represent anatomically linked limbic structures of the mesiotemporal lobe. Chronic seizures and mnestic deficits in patients with pharmacoresistant mesial temporal lobe epilepsy (TLE) appear to correlate with distinct patterns of histopathological alterations in these areas. The complex anatomical organization of the amygdala and entorhinal region, however, render a detailed neuropathological evaluation of surgical specimens difficult. In this study, we present a combined cytoarchitectonical, pigmentarchitectonical, myelinarchitectonical, and immunohistochemical reconstruction of the amygdala, entorhinal region, and hippocampus from surgical TLE specimens (n = 20) in order to analyze their regional and cellular patterns of pathology. Anterior mesiotemporal lobes dissected in different spatial planes were obtained from 4 autopsy control patients and used for the characterization of neuroanatomical landmarks. Lateral, basal, and granular subnuclei of the amygdala were consistently identified in the surgical specimens. Major histopathological alterations included neuronal cell loss as revealed by extracellular lipofuscin accumulation, glial satellitosis, as well as cellular and fibrillary gliosis. The regional distribution of neuropathological changes varied considerably between different subnuclei but the lateral nucleus was more often involved than basal and granular nuclei. These amygdala nuclei appeared to be more severely affected compared to the adjacent entorhinal region. In addition, patients presenting with secondary generalized tonic-clonic seizures showed significantly more damage in mesiotemporal structures. Pathological alterations in the amygdala and entorhinal region were found to be associated with Ammon's horn sclerosis in most but not all cases. Our findings reveal the amygdala as a major target for epilepsy-associated neuronal cell damage. Significant variations in the lesional pattern among patients with chronic TLE would also be compatible with different spreading pathways of epileptogenic activity within the mesial temporal lobe.

Comparison of intrinsic optical signals associated with low Mg2+-and 4-aminopyridine-induced seizure-like events reveals characteristic features in adult rat limbic system

PURPOSE

To analyze the intrinsic optical signal change associated with seizure-like events in two frequently used in vitro models-the low-Mg2+ and the 4-aminopyridine (4-AP) models-and to monitor regions of onset and spread patterns of these discharges by using imaging of intrinsic optical signals (IOS).

METHODS

Combined hippocampal-entorhinal-cortex slices of adult rats were exposed to two different treatments: lowering extracellular Mg2+ concentrations or application of 100 microM 4-AP. The electrographic features of the discharges were monitored using extracellular microelectrodes. Optical imaging was achieved by infrared transillumination of the slice and analysis of changes in light transmission using a subtraction approach. The electrographic features were compared with the optical changes. Regions of onset and spread patterns were analyzed in relevant anatomic regions of the slice.

RESULTS

Both lowering extracellular Mg2+ concentrations and application of 4-AP induced seizure-like events. The relative duration of the intrinsic optical signal change associated with seizure-like events in the low-Mg2+ model was significantly longer compared with that seen with those occurring in the 4-AP model, although duration of field potentials did not differ significantly in the two models. Seizure-like events of the low-Mg2+ model originated predominantly in the entorhinal cortex, with subsequent propagation toward the subiculum and neocortical structures. In contrast, no consistent region of onset or spread patterns were seen in the 4-AP model, indicating that the seizure initiation is not confined to a particular region in this model.

CONCLUSIONS

We conclude that different forms of spontaneous epileptiform activity are associated with characteristic optical signal changes and that optical imaging represents an excellent method to assess regions of seizure onset and spread patterns.