Reduced inhibition and increased output of layer II neurons in the medial entorhinal cortex in a model of temporal lobe epilepsy

In vitro ictogenesis and parahippocampal networks in a rodent model of temporal lobe epilepsy

Temporal lobe epilepsy (TLE) is a chronic epileptic disorder involving the hippocampal formation. Details on the interactions between the hippocampus proper and parahippocampal networks during ictogenesis remain, however, unclear. In addition, recent findings have shown that epileptic limbic networks maintained in vitro are paradoxically less responsive than non-epileptic control (NEC) tissue to application of the convulsant drug 4-aminopyridine (4AP). Field potential recordings allowed us to establish here the effects of 4AP in brain slices obtained from NEC and pilocarpine-treated epileptic rats; these slices included the hippocampus and parahippocampal areas such as entorhinal and perirhinal cortices and the amygdala. First, we found that both types of tissue generate epileptiform discharges with similar electrographic characteristics. Further investigation showed that generation of robust ictal-like discharges in the epileptic rat tissue is (i) favored by decreased hippocampal output (ii) reinforced by EC-subiculum interactions and (iii) predominantly driven by amygdala networks. We propose that a functional switch to alternative synaptic routes may promote network hyperexcitability in the epileptic limbic system.

Perirhinal cortex hyperexcitability in pilocarpine-treated epileptic rats

The perirhinal cortex (PC), which is heavily connected with several epileptogenic regions of the limbic system such as the entorhinal cortex and amygdala, is involved in the generation and spread of seizures. However, the functional alterations occurring within an epileptic PC network are unknown. Here, we analyzed this issue by using in vitro electrophysiology and immunohistochemistry in brain tissue obtained from pilocarpine-treated epileptic rats and age-matched, nonepileptic controls (NECs). Neurons recorded intracellularly from the PC deep layers in the two experimental groups had similar intrinsic and firing properties and generated spontaneous depolarizing and hyperpolarizing postsynaptic potentials with comparable duration and amplitude. However, spontaneous and stimulus-induced epileptiform discharges were seen with field potential recordings in over one-fifth of pilocarpine-treated slices but never in NEC tissue. These network events were reduced in duration by antagonizing NMDA receptors and abolished by NMDA + non-NMDA glutamatergic receptor antagonists. Pharmacologically isolated isolated inhibitory postsynaptic potentials had reversal potentials for the early GABA(A) receptor-mediated component that were significantly more depolarized in pilocarpine-treated cells. Experiments with a potassium-chloride cotransporter 2 antibody identified, in pilocarpine-treated PC, a significant immunostaining decrease that could not be explained by neuronal loss. However, interneurons expressing parvalbumin and neuropeptide Y were found to be decreased throughout the PC, whereas cholecystokinin-positive cells were diminished in superficial layers. These findings demonstrate synaptic hyperexcitability that is contributed by attenuated inhibition in the PC of pilocarpine-treated epileptic rats and underscore the role of PC networks in temporal lobe epilepsy.

Development of epileptiform excitability in the deep entorhinal cortex after status epilepticus

Epileptiform neuronal activity during seizures is observed in many brain areas, but its origins following status epilepticus (SE) are unclear. We have used the Li low-dose pilocarpine rat model of temporal lobe epilepsy to examine early development of epileptiform activity in the deep entorhinal cortex (EC). We show that during the 3-week latent period that follows SE, an increasing percentage of neurons in EC layer 5 respond to a single synaptic stimulus with polysynaptic burst depolarizations. This change is paralleled by a progressive depolarizing shift of the inhibitory postsynaptic potential reversal potential in layer 5 neurons, apparently caused by upregulation of the Cl(-) inward transporter NKCC1 and concurrent downregulation of the Cl(-) outward transporter KCC2, both changes favoring intracellular Cl(-) accumulation. Inhibiting Cl(-) uptake in the latent period restored more negative GABAergic reversal potentials and eliminated polysynaptic bursts. The changes in the Cl(-) transporters were highly specific to the deep EC. They did not occur in layers 1-3, perirhinal cortex, subiculum or dentate gyrus during this period. We propose that the changes in Cl(-) homeostasis facilitate hyperexcitability in the deep entorhinal cortex leading to epileptiform discharge there, which subsequently affects downstream cortical regions.

Spatiotemporal characteristics of astroglial death in the rat hippocampo-entorhinal complex following pilocarpine-induced status epilepticus

Recently we reported that astroglial loss and subsequent gliogenesis in the dentate gyrus play a role in epileptogenesis following pilocarpine-induced status epilepticus (SE). In the present study we investigated whether astroglial damages in the hippocampo-entorhinal complex following SE are relevant to pathological or electrophysiological properties of temporal lobe epilepsy. Astroglial loss/damage was observed in the entorhinal cortex and the CA1 region at 4 weeks and 8 weeks after SE, respectively. These astroglial responses in the hippocampo-entorhinal cortex were accompanied by hyperexcitability of the CA1 region (impairment of paired-pulse inhibition and increase in excitability ratio). Unlike the dentate gyrus and the entorhinal cortex, CA1 astroglial damage was protected by conventional anti-epileptic drugs. alpha-Aminoadipic acid (a specific astroglial toxin) infusion into the entorhinal cortex induced astroglial damage and changed the electrophysiological properties in the CA1 region. Astroglial regeneration in the dentate gyrus and the stratum oriens of the CA1 region was found to originate from gliogenesis, while that in the entorhinal cortex and stratum radiatum of the CA1 region originated from in situ proliferation. These findings suggest that regional specific astroglial death/regeneration patterns may play an important role in the pathogenesis of temporal lobe epilepsy.

Temporal lobe epilepsy: differential pattern of damage in temporopolar cortex and white matter

Our purpose was to quantify structural changes of the temporopolar cortex (TPC) and its white matter (TPWM) in temporal lobe epilepsy (TLE) using MRI volumetry and texture analysis. We studied 23 patients with hippocampal atrophy, and 20 healthy controls. Gradient magnitude and entropy were calculated to model signal intensity blurring on T1-MRI. Two observers assessed signal changes and atrophy visually. Compared to controls, TLE patients had a decrease in TPC and TPWM volume ipsilateral to the seizure focus. The gradient magnitude and entropy were decreased ipsilateral to the focus only in TPWM, indicating blurring of this compartment. Eighty-seven percent of TLE patients had at least one volumetric or textural abnormality. Although sensitivity of visual and quantitative assessment of TPC atrophy was comparable (43 and 39%), specificity was higher for volumetry (54% vs. 95%). Compared to visual analysis of signal changes in TPWM on T1-MRI, texture metrics had higher sensitivity (65% vs. 17%) and specificity (100% vs. 69%). The proportion of patients with blurring of TPWM as determined by texture analysis was higher than that seen on visual inspection of T2 images (78% vs. 43%). We found no clear association between volumetric or textural changes of TPC and TPWM and outcome after surgery. Structural changes of the anatomically distinct TPC and TPWM are found ipsilateral to the seizure focus in the majority of TLE patients with hippocampal sclerosis. MRI post-processing allows dissociating different pathological tissue characteristics and shows that atrophy involves gray and white matter, whereas blurring is confined to white matter.

Dynamics of evoked local field potentials in the hippocampus of epileptic rats with spontaneous seizures

A change in neuronal network excitability within the hippocampus is one of the hallmarks of temporal lobe epilepsy (TLE). In the dentate gyrus (DG), however, neuronal loss and mossy fiber sprouting are associated with enhanced inhibition rather than progressive hyperexcitability. The aim of this study was to investigate how alterations in excitability take place in association with spontaneous seizures expressed in the DG before, during, and after a seizure. For this purpose, we used freely moving rats that had developed spontaneous seizures after a kainate-induced status epilepticus (SE). Continuous EEG was recorded in the DG during several days along with local field potentials (LFPs) that were evoked every 15-30 s by applying paired-pulse stimuli to the angular bundle. Input-output relations showed increased paired pulse depression in epileptic compared with control rats, suggesting a rather strong inhibition in the DG during the interictal state. A characteristic pattern of changes in intrinsic excitability was observed during the ictal period: an increase in the population spike (PS) amplitude, mostly during the early phase of a seizure and often followed by a decrease of the main evoked potential amplitude. The paired-pulse extracellular postsynaptic potential (fEPSP) ratio increased during the seizure and did slowly recover to preictal levels after the seizure ended. Although clear changes in excitability occurred during and after seizure activity, changes of LFP parameters were more subtle before seizure onset; a significant reduction of LFP and PS amplitudes was observed that started 1-2 min in advance in approximately 33% of the cases; in approximately 18%, an increase of LFP/PS amplitude was observed; in the other cases, no significant change was observed. Taken together, these results provide evidence that, in this experimental model, DG physiology is more likely to follow the already ongoing seizure activity rather than to contribute to its generation.

The pilocarpine model of temporal lobe epilepsy

Understanding the pathophysiogenesis of temporal lobe epilepsy (TLE) largely rests on the use of models of status epilepticus (SE), as in the case of the pilocarpine model. The main features of TLE are: (i) epileptic foci in the limbic system; (ii) an “initial precipitating injury”; (iii) the so-called “latent period”; and (iv) the presence of hippocampal sclerosis leading to reorganization of neuronal networks. Many of these characteristics can be reproduced in rodents by systemic injection of pilocarpine; in this animal model, SE is followed by a latent period and later by the appearance of spontaneous recurrent seizures (SRSs). These processes are, however, influenced by experimental conditions such as rodent species, strain, gender, age, doses and routes of pilocarpine administration, as well as combinations with other drugs administered before and/or after SE. In the attempt to limit these sources of variability, we evaluated the methodological procedures used by several investigators in the pilocarpine model; in particular, we have focused on the behavioural, electrophysiological and histopathological findings obtained with different protocols. We addressed the various experimental approaches published to date, by comparing mortality rates, onset of SRSs, neuronal damage, and network reorganization. Based on the evidence reviewed here, we propose that the pilocarpine model can be a valuable tool to investigate the mechanisms involved in TLE, and even more so when standardized to reduce mortality at the time of pilocarpine injection, differences in latent period duration, variability in the lesion extent, and SRS frequency.

Degenerative alterations in the visual pathway after NMDA-induced retinal damage in mice

In the present study, intravitreal injection of N-methyl-d-aspartate (NMDA) into the left eye induced retinal damage (decreases in the number of retinal ganglion cells) at 1 day after the injection. At 7 days after the injection, atrophy of the optic tract was observed on the contralateral side, but not on the ipsilateral side. Number of neuronal nuclear specific protein (NeuN)-immunostained neurons were decreased in the contralateral dorsal LGN (dLGN) and contralateral ventral LGN-lateral (vLGN-l) at 90 and 180 days, respectively, after the injection. Furthermore, expressions of glial fibrillary acid protein (GFAP) were increased in the contralateral dLGN and contralateral vLGN-l at 7 and 30 days, respectively, and those of brain-derived neurotrophic factor (BDNF) were increased in the contralateral dLGN at 30 and 90 days and in the contralateral vLGN-l at 7 and 30 days. All NeuN-positive neuronal cells exhibited BDNF, whereas only some GFAP-positive astroglial cells exhibited BDNF. However, the contralateral ventral LGN-medial (vLGN-m) and ipsilateral LGN displayed no significant differences related to NeuN, GFAP, or BDNF immunohistochemistry. Taken together, these results indicate that time-dependent alterations occurred after the NMDA injection along the retinogeniculate pathway (from retina to LGN), and that the degree of damage in the LGN was region-dependent. In addition, the increased activated astroglial cells and expressions of BDNF in the damaged regions may play some roles in the cell-survival process of the LGN.

Network hyperexcitability within the deep layers of the pilocarpine-treated rat entorhinal cortex

In this study we report that in the presence of normal buffer, epileptiform discharges occur spontaneously (duration = 2.60 +/- 0.49 s) or can be induced by electrical stimuli (duration = 2.50 +/- 0.62 s) in the entorhinal cortex (EC) of brain slices obtained from pilocarpine-treated rats but not in those from age-matched, nonepileptic control (NEC) animals. These network-driven epileptiform events consist of field oscillatory sequences at frequencies greater than 200 Hz that most often initiate in the lateral EC and propagate to the medial EC with 4-63 ms delays. The NMDA receptor antagonist CPP depresses the rate of occurrence (P < 0.01) of these spontaneous epileptiform discharges but fails in blocking them. Paradoxically, stimulus-induced epileptiform responses are enhanced in duration during CPP application. However, concomitant application of NMDA and non-NMDA glutamatergic antagonists abolishes spontaneous and stimulus-induced epileptiform events. Intracellular recordings from lateral EC layer V cells indicate a lower frequency of spontaneous hyperpolarizing postsynaptic potentials in pilocarpine-treated tissue than in NEC (P < 0.002) both under control conditions and with glutamatergic receptor blockade; the reversal potential of pharmacologically isolated GABA(A) receptor-mediated inhibitory postsynaptic potentials has similar values in the two types of tissue. Finally, immunohistochemical analysis shows that parvalbumin-positive interneurons are selectively reduced in number in EC deep layers. Collectively, these results indicate that reduced inhibition within the pilocarpine-treated EC layer V may promote network epileptic hyperexcitability.

Neuropathogical features of a rat model for perinatal hypoxic-ischemic encephalopathy with associated epilepsy

Hypoxic-ischemic (HI) encephalopathy is an important neurological problem of the perinatal period. Little is known of the long-term progression of HI insults or the maladaptive changes that lead to epilepsy. Using rats with unilateral carotid occlusion followed by hypoxia at postnatal day 7, this study provides an initial analysis of the epilepsy caused by a perinatal HI insult with chronic and continuous behavioral monitoring. The histopathology was investigated at postnatal day 30 and later at > or =6 months of age using cresyl violet, Timm, and rapid Golgi staining and immunocytochemistry. The resultant epilepsy showed an increase in seizure frequency over time, with a preponderance for seizure clusters and behavioral features of an ipsilateral cerebral syndrome. In addition to parasagittal infarcts and porencephalic cysts in severe lesions, columnar neuronal death was found with cytomegaly in isolated groups of dysmorphic cortical neurons. Cortical dysgenesis was seen in the form of deep laminar cell loss, microgyri, white matter hypercellularity, and blurring of the white and gray matter junction. Mossy fiber sprouting was not only detected in the atrophied ipsilateral dorsal hippocampus of HI rats with chronic epilepsy, but was also found in comparable grades in spared ipsi- and contralateral ventral hippocampi. The cortical lesions in this animal model show histological similarities with those found in humans after perinatal HI. The occurrence of cortical abnormalities that are associated with epilepsy in humans correlates with the consequent detection of spontaneous recurrent seizures.

Hyperexcitability of the CA1 hippocampal region during epileptogenesis

Temporal Lobe Epilepsy (TLE) is often preceded by a latent (seizure-free) period during which complex network reorganizations occur. In experimental epilepsy, network hyperexcitability is already present during the latent period, suggesting a modification of information processing. The purpose of this study was to assess the input/output relationship in the hippocampal CA1 region during epileptogenesis. Field recordings in strata pyramidale and radiatum were used to measure the output of CA1 pyramidal cells as a function of the synaptic inputs they receive following the stimulation of Shaffer collaterals in slices obtained from sham and pilocarpine-treated animals during the latent and chronic periods. We show that there is a transient increase of the input and output field responses during the latent period as compared to sham and epileptic animals. The coupling between excitatory inputs and cell firing was also increased during the latent period. This increase persisted in epileptic animals, although to a lesser extent. We also confirm that paired-pulse facilitation occurs before the chronic phase. The present data further support the view that hyperexcitability is present at an early stage of epileptogenesis. Network output is more facilitated during the latent than during the chronic period. Hyperexcitability may participate to epileptogenesis, but it is not sufficient in itself to produce seizures.

Axo-somatic inhibition of projection neurons in the lateral nucleus of amygdala in human temporal lobe epilepsy: an ultrastructural study

Here, we report ultrastructural alterations in the synaptic circuitry of the human amygdala related to neuronal cell densities in surgical specimens of patients suffering from temporal lobe epilepsy (TLE). The neuronal cell densities quantified in the basolateral complex of amygdala were significantly reduced only in the lateral nucleus (LA) of TLE patients as compared to autopsy or non-Ammon's horn sclerosis (AHS) controls (Nissl staining, immunostaining against the neuronal marker NeuN). For this reason, we focussed on the LA to perform a more detailed quantitative ultrastructural analysis, which revealed an inverse correlation between the number of axo-somatic inhibitory synaptic profiles at the somata of glutamic acid decarboxylase (GAD)-negative projection neurons and the extent of perisomatic fibrillary gliosis. In contrast, the density of GAD-immunoreactive interneurons positively correlated with the number of axo-somatic inhibitory synaptic profiles. The fibrillary material in perisomatic glial cell processes was preferentially labeled by the astroglial marker S100B. In addition, a qualitative study of the dendrites of GAD- and parvalbumin (PARV)-containing interneurons showed that they were often contacted by asymmetrical excitatory synapses. Our results are in line with anatomical data from rodents and cats, which show that amygdalar interneurons form axo-somatic inhibitory synapses on GAD-negative projection neurons, whereas the interneurons themselves receive excitatory input from recurrent collaterals of projection neurons and from cortico- and thalamo-amygdalar afferents. The structural reorganization patterns observed in the GABAergic circuitry are compatible with a reduced feedback or feed forward inhibition of amygdalar projection neurons in human TLE.

Electrocorticography discharge patterns in patients with a cavernous hemangioma and pharmacoresistent epilepsy

OBJECT

Neurodevelopmental lesions (NDLs) such as glioneuronal tumors and cortical dysplasia produce characteristic electrocorticography (ECoG) discharge patterns. Because cavernomas, another congenital abnormality, are also associated with pharmacoresistant epilepsy, the authors wondered whether they exhibit discharge patterns similar to those occurring in NDLs.

METHODS

Intraoperative ECoG recordings from 19 patients with cavernomas and 54 with NDLs were reviewed for continuous spikes, bursts, or recruiting discharges and to determine whether these patterns were spatially coincident with the lesion. Relative densities of microglia and the intensity of Fe3+ staining in surgical samples were evaluated. Seizure outcome was assessed 1 year after surgery.

RESULTS

The mean ages at seizure onset and surgery were higher in patients in the cavernoma group than in the NDL group (22.5 and 36.4 years compared with 10.0 and 25.2 years, respectively). Neocortical discharge patterns occurred equally in patients with either cavernomas (53%) or NDLs (41%). In the mesiotemporal area coincident bursts occurred more often in patients with cavernomas than patients with NDLs (55% compared with 10%, respectively). Coincident continuous spiking was associated with a longer duration of epilepsy in patients with cavernomas (23.5 years compared with 11.4 years for those without coincident continuous spiking) and with a lower age at seizure onset in those with NDLs (4.1 years compared with 11.8 years for those without coincident continuous spiking). In the cavernoma group the absence of coincident bursts was associated with high microglia density. There were no associations between the intensity of Fe3+ staining and discharge patterns, although the discharge patterns were associated with a worse outcome in patients with NDLs.

CONCLUSIONS

In patients with NDLs, continuous spiking patterns may be markers of a widespread epileptogenic zone due to an early insult to the developing brain; in patients with cavernomas, such patterns may indicate secondary epileptogenesis. Microglia may inhibit discharge patterns in patients with cavernomas.

Differential contribution of kainate receptors to excitatory postsynaptic currents in superficial layer neurons of the rat medial entorhinal cortex

Although in situ hybridization studies have revealed the presence of kainate receptor (KAR) mRNA in neurons of the rat medial entorhinal cortex (mEC), the functional presence and roles of these receptors are only beginning to be examined. To address this deficiency, whole cell voltage clamp recordings of locally evoked excitatory postsynaptic currents (EPSCs) were made from mEC layer II and III neurons in combined entorhinal cortex-hippocampal brain slices. Three types of neurons were identified by their electroresponsive membrane properties, locations, and morphologies: stellate-like "Sag" neurons in layer II (S), pyramidal-like "No Sag" neurons in layer III (NS), and "Intermediate Sag" neurons with varied morphologies and locations (IS). Non-NMDA EPSCs in these neurons were composed of two components, and the slow decay component in NS neurons had larger amplitudes and contributed more to the combined EPSC than did those observed in S and IS neurons. This slow component was mediated by KARs and was characterized by its resistance to either 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466, 100 microM) or 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[lsqb]f[rsqb]quinoxaline-7-sulfonamide (NBQX, 1 microM), relatively slow decay kinetics, and sensitivity to 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10-50 microM). KAR-mediated EPSCs in pyramidal-like NS neurons contributed significantly more to the combined non-NMDA EPSC than did those from S and IS neurons. Layer III neurons of the mEC are selectively susceptible to degeneration in human temporal lobe epilepsy (TLE) and animal models of TLE such as kainate-induced status epilepticus. Characterizing differences in the complement of postsynaptic receptors expressed in injury prone versus injury resistant mEC neurons represents an important step toward understanding the vulnerability of layer III neurons seen in TLE.

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.

Persistently decreased basal synaptic inhibition of hippocampal CA1 pyramidal neurons after neonatal hypoxia-induced seizures

Hypoxia is the most common cause of neonatal seizures and can lead to epilepsy, but the epileptogenic mechanisms are not yet understood. We have previously shown that hypoxia-induced seizures in the neonatal rat result in acutely decreased amplitudes and frequency of spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) in hippocampal CA1 pyramidal neurons. In the current study, we asked whether such changes persist for several days following hypoxia-induced seizures. Similar to the acute findings, we observed decreased frequency and amplitudes of sIPSCs and decreased mIPSC amplitudes in CA1 pyramidal neurons at 3-5 days after hypoxia. However, in contrast to the acute findings, we observed no differences between hypoxia-treated and control groups in mIPSC frequency. Additionally, by 7 days after hypoxia, sIPSC amplitudes in the hypoxia group had recovered to control levels, but sIPSC frequency remained decreased. These data indicate that the persistently decreased sIPSC frequency result from decreased firing of presynaptic inhibitory interneurons, with only transient possible changes in postsynaptic responses to GABA release.

Lacosamide: a review of preclinical properties

Lacosamide (LCM), (SPM 927, (R)-2-acetamido-N-benzyl-3-methoxypropionamide, previously referred to as harkoseride or ADD 234037) is a member of a series of functionalized amino acids that were specifically synthesized as anticonvulsive drug candidates. LCM has demonstrated antiepileptic effectiveness in different rodent seizure models and antinociceptive potential in experimental animal models that reflect distinct types and symptoms of neuropathic as well as chronic inflammatory pain. Recent results suggest that LCM has a dual mode of action underlying its anticonvulsant and analgesic activity. It was found that LCM selectively enhances slow inactivation of voltage-gated sodium channels without affecting fast inactivation. Furthermore, employing proteomic affinity-labeling techniques, collapsin-response mediator protein 2 (CRMP-2 alias DRP-2) was identified as a binding partner. Follow-up experiments confirmed a functional interaction of LCM with CRMP-2 in vitro. LCM did not inhibit or induce a wide variety of cytochrome P450 enzymes at therapeutic concentrations. In safety pharmacology and toxicology studies conducted in mice, rats, rabbits, and dogs, LCM was well tolerated. Either none or only minor side effects were observed in safety studies involving the central nervous, respiratory, gastrointestinal, and renal systems and there is no indication of abuse liability. Repeated dose toxicity studies demonstrated that after either intravenous or oral administration of LCM the adverse events were reversible and consisted mostly of exaggerated pharmacodynamic effects on the CNS. No genotoxic or carcinogenic effects were observed in vivo, and LCM showed a favorable profile in reproductive and developmental animal studies. Currently, LCM is in a late stage of clinical development as an adjunctive treatment for patients with uncontrolled partial-onset seizures, and it is being assessed as monotherapy in patients with painful diabetic neuropathy. Further trials to identify LCM's potential in pain and for other indications have been initiated.

Loss of synapses in the entorhinal-dentate gyrus pathway following repeated induction of electroshock seizures in the rat

The goal of this study was to answer the question of whether repeated administration of electroconvulsive shock (ECS) seizures causes structural changes in the entorhinal-dentate projection system, whose neurons are known to be particularly vulnerable to seizure activity. Adult rats were administered six ECS seizures, the first five of which were spaced by 24-hr intervals, whereas the last two were only 2 hr apart. Stereological approaches were employed to compare the total neuronal and synaptic numbers in sham- and ECS-treated rats. Golgi-stained material was used to analyze dendritic arborizations of the dentate gyrus granule cells. Treatment with ECS produced loss of neurons in the entorhinal layer III and in the hilus of the dentate gyrus. The number of neurons in the entorhinal layer II, which provides the major source of dentate afferents, and in the granular layer of the dentate gyrus, known to receive entorhinal projections, remained unchanged. Despite this, the number of synapses established between the entorhinal layer II neurons and their targets, dentate granule cells, was reduced in ECS-treated rats. In addition, administration of ECS seizures produced atrophic changes in the dendritic arbors of dentate granule cells. The total volumes of entorhinal layers II, III, and V-VI were also found to be reduced in ECS-treated rats. By showing that treatment with ECS leads to partial disconnection of the entorhinal cortex and dentate gyrus, these findings shed new light on cellular processes that may underlie structural and functional brain changes induced by brief, generalized seizures.

Epileptogenic roles of astroglial death and regeneration in the dentate gyrus of experimental temporal lobe epilepsy

Recent studies have demonstrated that blockade of neuronal death in the hippocampus cannot prevent epileptogenesis in various epileptic models. These reports indicate that neurodegeneration alone is insufficient to cause epilepsy, and that the role of astrocytes in epileptogenesis should be reconsidered. Therefore, the present study was designed to elucidate whether altered morphological organization or the functionalities of astrocytes induced by status epilepticus (SE) is responsible for epileptogenesis. Glial responses (reactive microgliosis followed by astroglial death) in the dentate gyrus induced by pilocarpine-induced SE were found to precede neuronal damage and these alterations were closely related to abnormal neurotransmission related to altered vesicular glutamate and GABA transporter expressions, and mossy fiber sprouting in the dentate gyrus. In addition, newly generated astrocytes showed down-regulated expressions of glutamine synthase, glutamate dehydrogenase, and glial GABA transporter. Taken together, our findings suggest that glial responses after SE may contribute to epileptogenesis and the acquisition of the properties of the epileptic hippocampus. Thus, we believe that it is worth considering new therapeutic approaches to epileptogenesis involving targeting the inactivation of microglia and protecting against astroglial loss.

Cell domain-dependent changes in the glutamatergic and GABAergic drives during epileptogenesis in the rat CA1 region

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 post pilocarpine). It is, however, increased during the late phase of the latent period (7-10 days post pilocarpine), 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.

Downregulation of tonic GABA currents following epileptogenic stimulation of rat hippocampal cultures

Deficits in GABAergic inhibitory transmission are a hallmark of temporal lobe epilepsy and have been replicated in animal and tissue culture models of epilepsy. GABAergic inhibition comprises phasic and tonic inhibition that is mediated by synaptic and extrasynaptic GABAA receptors, respectively. We have recently demonstrated that chronic stimulation with cyclothiazide (CTZ) or kainic acid (KA) induces robust epileptiform activity in hippocampal neurons both in vitro and in vivo. Here, we report a downregulation of tonic GABA inhibition after chronic epileptogenic stimulation of rat hippocampal cultures. Chronic pretreatment of hippocampal neurons with CTZ or KA resulted in a marked reduction in GABAergic inhibition, as shown by a significant decrease in whole-cell GABA currents and in the frequency of miniature inhibitory postsynaptic currents (mIPSCs). Interestingly, synaptically localized GABAA receptors remained relatively stable, as evidenced by the unaltered amplitude of mIPSCs, as well as the unchanged punctate immunoreactivity of gamma2 subunit-containing postsynaptic GABAA receptors. In contrast, tonic GABA currents, assessed either by a GABAA receptor antagonist bicuculline or a selective extrasynaptic GABAA receptor agonist THIP, were significantly reduced following epileptogenic stimulation. These results reveal a novel form of neural plasticity, that epileptogenic stimulation can selectively downregulate extrasynaptic GABAA receptors while leaving synaptic GABAA receptors unchanged. Thus, in addition to synaptic alteration of GABAergic transmission, regulation of tonic inhibition may also play an important role during epileptogenesis.

Reorganization of CA3 area of the mouse hippocampus after pilocarpine induced temporal lobe epilepsy with special reference to the CA3-septum pathway

We showed that when CA3 pyramidal neurons in the caudal 80% of the dorsal hippocampus had almost disappeared completely, the efferent pathway of CA3 was rarely detectable. We used the mouse pilocarpine model of temporal lobe epilepsy (TLE), and injected iontophoretically the anterograde tracer phaseolus vulgaris leucoagglutinin (PHA-L) into gliotic CA3, medial septum and the nucleus of diagonal band of Broca, median raphe, and lateral supramammillary nuclei, or the retrograde tracer cholera toxin B subunit (CTB) into gliotic CA3 area of hippocampus. In the afferent pathway, the number of neurons projecting to CA3 from medial septum and the nucleus of diagonal band of Broca, median raphe, and lateral supramammillary nuclei increased significantly. In the hippocampus, where CA3 pyramidal neurons were partially lost, calbindin, calretinin, parvalbumin immunopositive back-projection neurons from CA1-CA3 area were observed. Sprouting of Schaffer collaterals with increased number of large boutons in both sides of CA1 area, particularly in the stratum pyramidale, was found. When CA3 pyramidal neurons in caudal 80% of the dorsal hippocampus have almost disappeared completely, surviving CA3 neurons in the rostral 20% of the dorsal hippocampus may play an important role in transmitting hyperactivity of granule cells to surviving CA1 neurons or to dorsal part of the lateral septum. We concluded that reorganization of CA3 area with its downstream or upstream nuclei may be involved in the occurrence of epilepsy.

Subiculum network excitability is increased in a rodent model of temporal lobe epilepsy

In this study, we used in vitro electrophysiology along with immunohistochemistry and molecular techniques to study the subiculum--a limbic structure that gates the information flow from and to the hippocampus--in pilocarpine-treated epileptic rats. Comparative data were obtained from age-matched nonepileptic controls (NEC). Subicular neurons in hippocampal-entorhinal cortex (EC) slices of epileptic rats were: (i) hyperexcitable when activated by CA1 or EC inputs; and (ii) generated spontaneous postsynaptic potentials at higher frequencies than NEC cells. Analysis of pharmacologically isolated, GABA(A) receptor-mediated inhibitory postsynaptic potentials revealed more positive reversal potentials in epileptic tissue (-67.8 +/- 6.3 mV, n = 16 vs. -74.8 +/- 3.6 mV in NEC, n = 13; P < 0.001) combined with a reduction in peak conductance (17.6 +/- 11.3 nS vs. 41.1 +/- 26.7 nS in NEC; P < 0.003). These electrophysiological data correlated in the epileptic subiculum with (i) reduced levels of mRNA expression and immunoreactivity of the neuron-specific potassium-chloride cotransporter 2; (ii) decreased number of parvalbumin-positive cells; and (iii) increased synaptophysin (a putative marker of sprouting) immunoreactivity. These findings identify an increase in network excitability within the subiculum of pilocarpine-treated, epileptic rats and point at a reduction in inhibition as an underlying mechanism.

Increased expression of ferritin, an iron-storage protein, in specific regions of the parahippocampal cortex of epileptic rats

PURPOSE

Iron accumulation in the brain has been associated with neurodegenerative disorders, including epilepsy. In our previous SAGE study, we showed that ferritin, an iron-storage protein, was one of the genes (Ferritin-H) that showed overexpression before the chronic epileptic phase. In this study we used ferritin as indicator for disturbed iron homeostasis to acquire insight into whether this could play a role in the pathogenesis of temporal lobe epilepsy.

METHODS

With immunocytochemistry, we studied the regional and cellular distribution of ferritin protein in an animal model for temporal lobe epilepsy in which spontaneous seizures develop a few weeks after electrically induced status epilepticus (SE).

RESULTS

Increased ferritin expression was observed in regions known to be vulnerable to cell death, mainly in reactive microglial cells of epileptic rats. Ferritin expression after SE was initially high, especially throughout the hippocampus, but decreased over time. In the chronic epileptic phase, it was still upregulated in regions where extensive cell loss occurs during the early acute and latent period. Within the parahippocampal region, the most persistent ferritin overexpression was present in microglial cells in layer III of the medial entorhinal area. The upregulation was most extensive in rats that had developed a progressive form of epilepsy with frequent seizures (approximately five to 10 seizures per day).

CONCLUSIONS

The fact that ferritin upregulation is still present in specific limbic regions in chronic epileptic rats, when neuronal loss is absent or minimal, suggests a role of iron in the pathogenesis and progression of epilepsy.

Entorhinal cortex entrains epileptiform activity in CA1 in pilocarpine-treated rats

Layer III neurons of the medial entorhinal cortex (mEC) project to CA1 via the temporoammonic pathway and exert a powerful feed-forward inhibition of CA1 pyramidal neurons. The present study evaluates the hypothesis that disrupted inhibition of CA1 pyramidal neurons causes an eased propagation of entorhinal seizures to the hippocampus via the temporoammonic pathway. Using a method to induce a confined epileptic focus in brain slices, we investigated the spread of epileptiform activity from the disinhibited mEC to CA1 in control and pilocarpine-treated rats that had displayed status epilepticus and spontaneous recurrent seizures. In pilocarpine-treated rats, the mEC showed a moderate layer III cell loss and an enhanced susceptibility to epileptiform discharges compared to control animals. Entorhinal discharges propagated to CA1 in pilocarpine-treated rats but not in controls. Disconnecting CA3 from CA1 did not affect the spread of epileptiform activity to CA1 excluding its propagation via the trisynaptic hippocampal loop. Mimicking the invasion of epileptiform discharges by repetitive stimulation of the temporoammonic pathway caused a facilitation of field potentials in CA1 that were contaminated by population spikes and afterdischarges in pilocarpine-treated but not control rats. Single cell recordings of CA1 pyramidal neurons revealed a dramatic loss of feed-forward inhibition and the occurrence of strong postsynaptic excitatory potentials in pilocarpine-treated rats. Excitatory responses in CA1 were characterized by multiple NMDA receptor-mediated afterdischarges and a strong paired-pulse facilitation in response to activation of the temporoammonic pathway. Our results suggest that, irrespective of the enhanced seizure-susceptibility of the mEC in epileptic rats, the loss of feed-forward inhibition and the enhanced NMDA receptor-mediated excitability CA1 pyramidal cells ease the spread of epileptiform activity from the mEC to CA1 via the temporoammonic pathway bypassing the classical trisynaptic hippocampal loop.

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.

Seizure-induced plasticity of h channels in entorhinal cortical layer III pyramidal neurons

The entorhinal cortex (EC) provides the predominant excitatory drive to the hippocampal CA1 and subicular neurons in chronic epilepsy. Discerning the mechanisms underlying signal integration within EC neurons is essential for understanding network excitability alterations involving the hippocampus during epilepsy. Twenty-four hours following a single seizure episode when there were no behavioral or electrographic seizures, we found enhanced spontaneous activity still present in the rat EC in vivo and in vitro. The increased excitability was accompanied by a profound reduction in I(h) in EC layer III neurons and a significant decline in HCN1 and HCN2 subunits that encode for h channels. Consequently, dendritic excitability was enhanced, resulting in increased neuronal firing despite hyperpolarized membrane potentials. The loss of I(h) and the increased neuronal excitability persisted for 1 week following seizures. Our results suggest that dendritic I(h) plays an important role in determining the excitability of EC layer III neurons and their associated neural networks.

Medial perforant path inhibition mediated by mGluR7 is reduced after status epilepticus

Metabotropic glutamate receptor (mGluR)-mediated inhibition within the dentate gyrus is altered after epilepsy. Whether these changes occur during the developmental period of the disease (i.e., the latent period) has not yet been investigated. Field excitatory postsynaptic potentials (fEPSPs) were recorded in the lateral (LPP) and medial perforant path (MPP) simultaneously in adult mouse hippocampal slices 3-9 days after pilocarpine (PILO)-induced status epilepticus. Genetically manipulated mice (mGluR8 knockout and mGluR4/8 double knockout) and pharmacologically selective agonists were used to identify specific mGluR subtypes affected after PILO. Pharmacological activation of mGluR7 by L-AP4 in both wild-type and mGluR4/8 double knockout mice selectively reduced fEPSPs in the MPP, but not LPP, and this level of inhibition was significantly reduced 3-9 days after PILO-induced SE. Activation of mGluR2/3 reversibly depressed the fEPSP slopes in both the MPP and LPP, but no alterations were noted after PILO. mGluR8 activation selectively inhibited evoked responses in the LPP, but not in the MPP, and this level of inhibition did not change after PILO treatment. These data suggest that reduced presynaptic inhibition mediated by mGluR7, but not mGluR2/3 or mGluR8, may play a role during the latent period in generating hyperexcitability in the dentate and thereby contribute to epileptogenesis.

Neuronal density, size and shape in the human anterior cingulate cortex: a comparison of Nissl and NeuN staining

There are an increasing number of quantitative morphometric studies of the human cerebral cortex, especially as part of comparative investigations of major psychiatric disorders. In this context, the present study had two aims. First, to provide quantitative data regarding key neuronal morphometric parameters in the anterior cingulate cortex. Second, to compare the results of conventional Nissl staining with those observed after immunostaining with NeuN, an antibody becoming widely used as a selective neuronal marker. We stained adjacent sections of area 24b from 16 adult brains with cresyl violet or NeuN. We measured the density of pyramidal and non-pyramidal neurons, and the size and shape of pyramidal neurons, in laminae II, III, Va, Vb and VI, using two-dimensional counting methods. Strong correlations between the two modes of staining were seen for all variables. However, NeuN gave slightly higher estimates of neuronal density and size, and a more circular perikaryal shape. Brain pH was correlated with neuronal size, measured with both methods, and with neuronal shape. Age and post-mortem interval showed no correlations with any parameter. These data confirm the value of NeuN as a tool for quantitative neuronal morphometric studies in routinely processed human brain tissue. Absolute values are highly correlated between NeuN and cresyl violet stains, but cannot be interchanged. NeuN may be particularly useful when it is important to distinguish small neurons from glia, such as in cytoarchitectural studies of the cerebral cortex in depression and schizophrenia.