The parahippocampal cortices and kindling

Recent advances in epilepsy surgery

PURPOSE OF REVIEW

Technological innovations in the preoperative evaluation, surgical techniques and outcome prediction in epilepsy surgery have grown exponentially over the last decade. This review highlights and emphasizes relevant updates in techniques and diagnostic tools, discussing their context within standard practice at comprehensive epilepsy centres.

RECENT FINDINGS

High-resolution structural imaging has set an unprecedented opportunity to detect previously unrecognized subtle abnormalities. Machine learning and computer science are impacting the methodologies to analyse presurgical and surgical outcome data, building more accurate prediction models to tailor treatment strategies. Robotic-assisted placement of depth electrodes has increased the safety and ability to sample epileptogenic nodes within deep structures, improving our understanding of the seizure networks in drug-resistant epilepsy. The current available minimally invasive techniques are reasonable surgical alternatives to ablate or disrupt epileptogenic regions, although their sustained efficacy is still an active area of research.

SUMMARY

Epilepsy surgery is still underutilized worldwide. Every patient who continues with seizures despite adequate trials of two well selected and tolerated antiseizure medications should be evaluated for surgical candidacy. Collaboration between academic epilepsy centres is of paramount importance to answer long-standing questions in epilepsy surgery regarding the understanding of spatio-temporal dynamics in epileptogenic networks and its impact on surgical outcomes.

Social Relationship as a Factor for the Development of Stress Incubation in Adult Mice

While stress reactions can emerge long after the triggering event, it remains elusive how they emerge after a protracted, seemingly stress-free period during which stress incubates. Here, we study the behavioral development in mice isolated after observing an aggressive encounter inflicted upon their pair-housed partners. We developed a spatially resolved fine-scale behavioral analysis and applied it to standard behavioral tests. It reveals that the seemingly sudden behavioral changes developed gradually. These behavioral changes were not observed if the aggressive encounter happened to a stranger mouse, suggesting that social bonding is a prerequisite for stress incubation in this paradigm. This finding was corroborated by hemisphere-specific morphological changes in cortex regions centering at the anterior cingulate cortex, a cognitive and emotional center. Our non-invasive analytical methods to capture informative behavioral details may have applications beyond laboratory animals.

Association Between Anatomical Location of Surgically Induced Lesions and Postoperative Seizure Outcome in Temporal Lobe Epilepsy

BACKGROUND AND OBJECTIVES

To determine the association between surgical lesions of distinct gray and white structures and connections with favorable postoperative seizure outcomes.

METHODS

Patients with drug-resistant temporal lobe epilepsy (TLE) from 3 epilepsy centers were included. We employed a voxel-based and connectome-based mapping approach to determine the association between favorable outcomes and surgery-induced temporal lesions. Analyses were conducted controlling for multiple confounders, including total surgical resection/ablation volume, hippocampal volumes, side of surgery, and site where the patient was treated.

RESULTS

The cohort included 113 patients with TLE (54 women; 86 right-handed; mean age at seizure onset 16.5 years [SD 11.9]; 54.9% left) who were 61.1% free of disabling seizures (Engel Class 1) at follow-up. Postoperative seizure freedom in TLE was associated with (1) surgical lesions that targeted the hippocampus as well as the amygdala-piriform cortex complex and entorhinal cortices; (2) disconnection of temporal, frontal, and limbic regions through loss of white matter tracts within the uncinate fasciculus, anterior commissure, and fornix; and (3) functional disconnection of the frontal (superior and middle frontal gyri, orbitofrontal region) and temporal (superior and middle pole) lobes.

DISCUSSION

Better postoperative seizure freedom is associated with surgical lesions of specific structures and connections throughout the temporal lobes. These findings shed light on the key components of epileptogenic networks in TLE and constitute a promising source of new evidence for future improvements in surgical interventions.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that for patients with TLE, postoperative seizure freedom is associated with surgical lesions of specific temporal lobe structures and connections.

Pentylenetetrazol kindling induces cortical astrocytosis and increased expression of extracellular matrix molecules in mice

Although epilepsy is one of the most common chronic neurological disorders with a prevalence of approximately 1.0 %, the underlying pathophysiology remains to be elucidated. Understanding the molecular and cellular mechanisms involved in the development of epilepsy is important for the development of appropriate therapeutic strategy. In this study, we investigated the effects of status epilepticus on astrocytes, microglia, and extracellular matrix (ECM) molecules in the somatosensory cortex and piriform cortex of mice. Activation of astrocytes was observed in many cortices except the retrosplenial granular cortex after pentylenetetrazol (PTZ)-induced kindling in mice. Activated astrocytes in the cortex were found in layers 1-3 but not in layers 4-6. In the somatosensory and piriform cortices, no change was observed in the number of parvalbumin (PV)-positive neurons and PV-positive neurons covered with perineuronal nets. However, the amount of ECM in the extracellular space increased. The expression of VGLUT1- and GAD67-positive synapses also increased. Thus, in the PTZ-kindling epilepsy mice model, an increase in the number of ECM molecules and activation of astrocytes were observed in the somatosensory cortex and piriform cortex. These results indicate that PTZ-induced seizures affect not only the hippocampus but also other cortical areas. Our study findings may help to develop new therapeutic approaches to prevent seizures or their sequelae.

Association of Piriform Cortex Resection With Surgical Outcomes in Patients With Temporal Lobe Epilepsy

Importance

A functional area associated with the piriform cortex, termed area tempestas, has been implicated in animal studies as having a crucial role in modulating seizures, but similar evidence is limited in humans.

Objective

To assess whether removal of the piriform cortex is associated with postoperative seizure freedom in patients with temporal lobe epilepsy (TLE) as a proof-of-concept for the relevance of this area in human TLE.

Design, Setting, and Participants

This cohort study used voxel-based morphometry and volumetry to assess differences in structural magnetic resonance imaging (MRI) scans in consecutive patients with TLE who underwent epilepsy surgery in a single center from January 1, 2005, through December 31, 2013. Participants underwent presurgical and postsurgical structural MRI and had at least 2 years of postoperative follow-up (median, 5 years; range, 2-11 years). Patients with MRI of insufficient quality were excluded. Findings were validated in 2 independent cohorts from tertiary epilepsy surgery centers. Study follow-up was completed on September 23, 2016, and data were analyzed from September 24, 2016, through April 24, 2018.

Exposures

Standard anterior temporal lobe resection.

Main Outcomes and Measures

Long-term postoperative seizure freedom.

Results

In total, 107 patients with unilateral TLE (left-sided in 68; 63.6% women; median age, 37 years [interquartile range {IQR}, 30-45 years]) were included in the derivation cohort. Reduced postsurgical gray matter volumes were found in the ipsilateral piriform cortex in the postoperative seizure-free group (n = 46) compared with the non-seizure-free group (n = 61). A larger proportion of the piriform cortex was resected in the seizure-free compared with the non-seizure-free groups (median, 83% [IQR, 64%-91%] vs 52% [IQR, 32%-70%]; P < .001). The results were seen in left- and right-sided TLE and after adjusting for clinical variables, presurgical gray matter alterations, presurgical hippocampal volumes, and the proportion of white matter tract disconnection. Findings were externally validated in 2 independent cohorts (31 patients; left-sided TLE in 14; 54.8% women; median age, 41 years [IQR, 31-46 years]). The resected proportion of the piriform cortex was individually associated with seizure outcome after surgery (derivation cohort area under the curve, 0.80 [P < .001]; external validation cohorts area under the curve, 0.89 [P < .001]). Removal of at least half of the piriform cortex increased the odds of becoming seizure free by a factor of 16 (95% CI, 5-47; P < .001). Other mesiotemporal structures (ie, hippocampus, amygdala, and entorhinal cortex) and the overall resection volume were not associated with outcomes.

Conclusions and Relevance

These results support the importance of resecting the piriform cortex in neurosurgical treatment of TLE and suggest that this area has a key role in seizure generation.

Ketogenic diet protects against epileptogenesis as well as neuronal loss in amygdaloid-kindling seizures

Ketogenic diets (KD) have shown beneficial effects in terms of anticonvulsant and anti-epileptogenic properties in several experimental models. However, few studies have investigated the consequences of KD with regards to the anti-epileptogenic and neuroprotective effects in kindling-induced seizures. Here, postnatal day 28 male Sprague-Dawley rats received one of two experimental diets for 4 weeks: (a) a 'classic' 4:1 KD; and (b) a normal regular rodent chow diet (ND). Fully-kindled seizures were achieved by daily electrical stimulation in the amygdala. Seizure stage and after-discharge duration (ADD) were assessed daily. The after-discharge threshold (ADT) was measured every 5 days. The effects of the two diets on neuronal loss were observed before kindling and 20 days after stimulation by Nissl staining. We found that the progression of seizure stage and ADD was delayed by KD. KD prevented the ADT decrease on day 5. The incidence of generalized seizures was lower in the KD group compared to the ND group. The neuronal density was decreased in the ipsilateral hilus of the dentate gyrus (DG) and CA1 area, as well as the contralateral CA1 area before kindling in the KD group. However, KD prevented neuronal loss in the ipsilateral CA1 area 20 days after stimulation. Our data suggest that KD can protect against epileptogenesis by preventing both after-discharge generation and propagation in kindling seizures. In addition, KD also possesses a neuroprotective function during kindling although it changes hippocampal development in early life.

The roles of P2X7 receptor in regional-specific microglial responses in the rat brain following status epilepticus

Recently, we have reported that astroglial activations in response to status epilepticus (SE) show regional-specific manners in the rat hippocampus. However, it is unknown that microglial responses to SE would show regional-specific patterns. Therefore, the present study was designed to elucidate the regional-specific microglial activation and relationship between P2X7 receptor functions and SE-induced microglial responses in the rat brain. Following SE, microglia appeared amoeboid or phagocytic in the dentate gyrus and the piriform cortex. In contrast, elongated microglia were observed in the CA1 hippocampal regions and the frontoparietal cortex. In the dentate gyrus, the CA1 hippocampal regions, and the frontoparietal cortex, these microglial activation accelerated by BzATP (a P2X7 receptor agonist)-infusion, but inhibited by OxATP (a P2X7 receptor antagonist). However, SE-induced microglial activation in the piriform cortex was not affected by BzATP or OxATP-infusion. Therefore, our findings indicate that SE-induced microglial activation may show regional-specific manners, and suggest that P2X7 receptor function differently modulates SE-induced microglial responses in distinct brain regions.

Astroglial loss and edema formation in the rat piriform cortex and hippocampus following pilocarpine-induced status epilepticus

In the present study we analyzed aquaporin-4 (AQP4) immunoreactivity in the piriform cortex (PC) and the hippocampus of pilocarpine-induced rat epilepsy model to elucidate the roles of AQP4 in brain edema following status epilepticus (SE). In non-SE-induced animals, AQP4 immunoreactivity was diffusely detected in the PC and the hippocampus. AQP4 immunoreactivity was mainly observed in the endfeet of astrocytes. Following SE the AQP4-deleted area was clearly detected in the PC, not in the hippocampus. Decreases in dystrophin and α-syntrophin immunoreactivities were followed by reduction in AQP4 immunoreactivity. These alterations were accompanied by the development of vasogenic edema and the astroglial loss in the PC. In addition, acetazolamide (an AQP4 inhibitor) treatment exacerbated vasogenic edema and astroglial loss both in the PC and in the hippocampus. These findings suggest that SE may induce impairments of astroglial AQP4 functions via disruption of the dystrophin/α-syntrophin complex that worsen vasogenic edema. Subsequently, vasogenic edema results in extensive astroglial loss that may aggravate vasogenic edema.

Decrease in dystrophin expression prior to disruption of brain-blood barrier within the rat piriform cortex following status epilepticus

Increased permeability of the brain-blood barrier (BBB) in the piriform cortex (PC) has been reported in various animal models of temporal lobe epilepsy. Since BBB disruption induced by epileptogenic insult has not fully clarified, we attempted to determine whether changes in BBB-related molecules are associated with vasogenic edema in the PC. One day after status epilepticus (SE), PC neurons and astrocytes showed a pyknotic nucleus and shrunken cytoplasm accompanied by vasogenic edema. At this time point, SMI-71 (an endothelial barrier antigen) immunoreactivity had decreased in the PC. Prior to vasogenic edema formation (12 h after SE), dystrophin immunoreactivity disappeared within astrocytes, while the change in glial fibrillary acidic protein immunoreactivity was negligible. However, glucose transporter-1 (an endothelial cell marker) had increased at 12 h after SE. These findings indicate that dysfunction of dystrophin induced by SE may result in endothelial and astroglial damage with BBB breakdown and increase vascular permeability, leading to vasogenic edema that is involved in pathogenesis of epileptogenesis.

Extrafocal threshold reductions in amygdala-kindled rats

PURPOSE

Racine's classic study suggested that after discharge thresholds were reduced in the primary stimulation site (amygdala) of kindled rats, but that that they were not reduced in secondary (nonstimulated) sites. However, recent reports of neurochemical changes related to excitation and inhibition in nonstimulated sites in kindled brains would be expected to cause reductions in afterdischarge thresholds in these sites. More recently Sanei et al. have reported a significant threshold reduction in the piriform cortex of amygdala- and hippocampus-kindled cats, but not in the entorhinal cortex. The present study was designed to determine whether the results of Sanei et al. in cats could be replicated in rats kindled in the amygdala-a model commonly used in studies of seizure mechanisms and anticonvulsant drug development.

METHODS

Adult, male Long-Evans rats were kindled in the amygdala or given matched handling. Beginning 48 h following the last stimulation, afterdischarge thresholds were determined in the ipsilateral piriform and entorhinal cortices. Amygdala thresholds were determined 24 h later.

RESULTS

Afterdischarge thresholds were significantly reduced in both the amygdala and the ipsilateral entorhinal cortex of amygdala-kindled rats. Afterdischarge thresholds in the piriform cortex did not differ significantly between kindled and control subjects.

DISCUSSION

These data suggest that threshold reduction occurs outside the primary kindling site in rats as well as in cats. Extrafocal changes in afterdischarge threshold may be functionally important, and might possibly relate to extrafocal neurochemical changes and progressive generalization of seizure discharge from discrete focal sites.

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.

Cortical efferents of the perirhinal, postrhinal, and entorhinal cortices of the rat

We investigated the cortical efferents of the parahippocampal region by placing injections of the anterograde tracers, Phaseolus vulgaris-leuccoagglutinin, and biotinylated dextran amine, throughout the perirhinal (PER), postrhinal (POR), and entorhinal cortices of the rat brain. The resulting density of labeled fibers was evaluated in 25 subregions of the piriform, frontal, insular, temporal, cingulate, parietal, and occipital areas. The locations of labeled terminal fibers differed substantially depending on whether the location of the injection site was in PER area 35, PER area 36, POR, or the lateral or the medial entorhinal (LEA and MEA). The differences were greater for sensory regions. For example, the POR efferents preferentially target visual and spatial regions, whereas the PER efferents target all sensory modalities. The cortical efferents of each region largely reciprocate the cortical afferents, though the degree of reciprocity varied across originating and target regions. The laminar pattern of terminal fibers was consistent with the notion that the efferents are feedback projections. The density and amount of labeled fibers also differed substantially depending on the regional location of injection sites. PER area 36 and POR give rise to a greater number of heavy projections, followed by PER area 35. LEA also gives rise to widespread cortical efferents, arising mainly from a narrow band of cortex adjacent to the PER. In contrast, the remainder of the LEA and the MEA provides only weak efferents to cortical regions. Prior work has shown that nonspatial and spatial information is transmitted to the hippocampus via the PER-LEA and POR-MEA pathways, respectively. Our findings suggest that the return projections follow the same pathways, though perhaps with less segregration.

Anticonvulsant impact of lesions in the ventrolateral forebrain of rats challenged with soman

Mapping of trigger sites and/or propagation pathways for soman-induced seizures may provide clues for the designing of anticonvulsant drugs. In the present study, anticonvulsant efficacy against soman intoxication (1.3 x LD50) was examined in rats with either lesion of the perirhinal cortex, posterior piriform cortex, entorhinal cortex, hippocampal region, or amygdala. The results showed that prevention of convulsions or increased latency to onset of convulsions was ensured in rats with perirhinal or piriform cortical lesions, whereas anticonvulsant effects were not achieved in rats with damage to the entorhinal cortex, hippocampal region, or amygdala. The results from the present study suggest that critical structures for induction of seizures after soman exposure are located in the ventrolateral aspect of the forebrain. This suggestion is in compliance with convulsant reactions to microinfusions of soman or VX into ventrolateral brain structures and increased neuronal activity in corresponding structures revealed by c-fos staining in response to soman. Furthermore, results from studies of kindling, lesions, and microinfusion of chemoconvulsants in experimental epilepsy also imply that the perirhinal and piriform cortices are critically involved in seizure control.

Seizure activity in the rat hippocampus, perirhinal and prefrontal cortex associated with transient global cerebral ischemia

Epileptiform EEG activity associated with ischemia can contribute to early damage of hippocampal neurons, and seizure activity may also lead to dysfunction in extrahippocampal regions. In this study, seizure activity associated with the four-vessel occlusion model of cerebral ischemia was monitored using chronically implanted electrodes in the CA1/subicular region, the perirhinal cortex, and the prefrontal cortex of the rat. Background EEG amplitude was reduced in all recording sites during occlusion, but spiking and bursting activity was also observed. Seizure activity occurred in most animals during the first several hours of reperfusion, but was not observed on subsequent days. Epileptiform spikes and bursts were often synchronous between two or three recording sites, and spikes in the CA1 region also often occurred just prior to spikes in other sites. These results demonstrate that the four-vessel occlusion model can lead to patterns of seizure activity in the hippocampus, prefrontal and perirhinal cortices.

Epileptiform synchronization in the rat insular and perirhinal cortices in vitro

The hippocampus plays a primary role in temporal lobe epilepsy, a common form of partial epilepsy in adults. Recent studies, however, indicate that extrahippocampal areas such as the perirhinal and insular cortices represent important participants in this epileptic disorder. By employing field potential recordings in the in vitro 4-aminopyridine model of temporal lobe epilepsy, we have investigated here the contribution of glutamatergic and GABAergic signaling to epileptiform activity in these structures. First, we provide evidence of epileptiform synchronicity between the perirhinal and insular cortices, and resolve some pharmacological and network mechanisms involved in sustaining the interictal- and ictal-like discharges recorded there. Second, we report that in the absence of ionotropic glutamatergic transmission, GABAergic networks produce synchronous potentials that spread between the perirhinal and insular cortices. Finally, we have established that such activity is modulated by activating micro-opioid receptors. Our findings support clinical and experimental evidence concerning the involvement of the perirhinal and insular cortex networks in temporal lobe epilepsy, and provide observations that may impact research focussing on the role of the insular cortex in nociception.

Is exposure to enriched environment beneficial for functional post-lesional recovery in temporal lobe epilepsy?

Exposure to enriched environment has been shown to induce robust neuronal plasticity in both intact and injured adult central nervous system, including up-regulation of multiple neurotrophic factors, enhanced neurogenesis in the dentate gyrus of the hippocampus, and improved spatial learning and memory function. Neuronal plasticity, though mostly adaptive and abnormal, also occurs during certain neurodegenerative conditions such as the temporal lobe epilepsy (TLE). The TLE is characterized by hippocampal neurodegeneration, aberrant mossy fiber sprouting, spontaneous recurrent motor seizures, cognitive deficits, and abnormally enhanced neurogenesis during the early phase and dramatically declined neurogenesis during the chronic phase of the disease. As environmental enrichment has been found to be beneficial for treating animal models of Alzheimer's, Parkinson's, and Huntington's diseases, there is considerable interest in determining the efficacy of this strategy for preventing or treating chronic TLE after the initial precipitating brain injury. This review first discusses the proof of principle behind the potential application of the environmental enrichment strategy for preventing or treating TLE after brain injury. The subsequent chapters confer the portrayed beneficial effects of enrichment for functional post-lesional recovery in TLE and the possible complications which may arise from housing epilepsy-prone or epileptic rats in enriched environmental conditions. The final segment discusses studies that are essential for further understanding the efficacy of this approach for preventing or treating TLE.

Neuronal structures involved in the induction and propagation of seizures caused by nerve agents: Implications for medical treatment

In epilepsy research, studies have been made to identify brain areas critical for triggering and/or controlling propagated seizure activity. The purpose of the present study was to focus on a similar approach in nerve agent research by reviewing relevant literature to map potential trigger sites and propagation pathways for seizures. The piriform cortex and medial septal area emerge as prime target areas for soman-induced seizures. The cholinergic hyperactivation in the latter structures seems to induce increased glutamatergic activity in the piriform, entorhinal, and perirhinal cortices along with the hippocampal region. For prophylactic or early treatment, mapping of muscarinic subreceptors in the piriform cortex and medial septum would be guiding for designing anticholinergic drugs with optimal properties. Sustained seizures governed by glutamatergic over-activity may primarily be terminated by drugs with optimal glutamatergic antagonism primarily in the piriform, entorhinal, and perirhinal cortices. Studies of radiolabeled ligands to map subreceptors may provide specification of wanted drug properties to guide the choice among existing agents or to synthesize novel ones.

The relevance of kindling for human epilepsy

Kindling is one of the most widely used models of seizures and epilepsy, and it has been used in its more than three decade history to provide many key insights into seizures and epilepsy. It remains a mainstay of epilepsy related research, but the question remains how the results from kindling experiments further our understanding of the underlying neurobiology of human epilepsy. In this article we compare the basic features of kindling and human epilepsy, especially human limbic or temporal lobe epilepsy. In this review we focus on a limited number of topics that may show areas in which kindling has been often cited as a tool for better understanding of human epilepsy. These areas include the underlying circuits, the importance of seizure spontaneity, the associated neuropathology, the contribution of genetics, seizure susceptibility, and the underlying pathophysiology of epilepsy. In the course of this article we will show that there are many features that kindling can teach us by direct comparison or implication about human temporal epilepsy. We will also see that not all findings associated with kindling may be applicable to the human condition. Ultimately we wish to encourage critical thinking about kindling and the similarities that it shares and does not share with the human epilepsy so the results from studies using this model are applied rationally to further our insights the mechanisms of human epilepsy.

Status epilepticus induces region-specific changes in dendritic spines, dendritic length and TrkB protein content of rat brain cortex

Induction of status epilepticus (SE) with kainic acid results in a large reorganization of neuronal brain circuits, a phenomenon that has been studied primarily in the hippocampus. The neurotrophin BDNF, by acting through its receptor TrkB, has been implicated in such reorganization. In the present work we investigated, by Western blot and immunohistochemistry, whether regional changes of TrkB expression within the rat brain cortex are correlated with altered neuronal morphology and/or with apoptotic cell death. We found that the full-length TrkB protein decreased within the cortex when measured 24 h to 1 week after induction of SE. Analysis by immunohistochemistry revealed that TrkB staining diminished within layer V of the retrosplenial granular b (RSGb) and motor cortices, but not within the auditory cortex. In layer II/III, differential changes were also observed: TrkB decreased in the motor cortex, did not change within the RSGb but increased within the auditory cortex. Reduced TrkB was associated with dendritic atrophy and decreased spine density in pyramidal neurons within layer V of the RSGb. No correlation was observed between regional and cellular changes of TrkB protein and apoptosis, measured by the TdT-mediated dUTP nick end labeling (TUNEL) method. The global decrease of TrkB within the neocortex and the associated dendritic atrophy may counteract seizure propagation in the epileptic brain but may also underlie cognitive impairment after seizures.

Effect of focal low-frequency stimulation on amygdala-kindled afterdischarge thresholds and seizure profiles in fast- and slow-kindling rat strains

PURPOSE

To determine whether low-frequency, 1-Hz sine-wave stimulation (LFS) applied to a fully kindled amygdala focus would show antiepileptic properties in rats that were either naturally seizure prone (Fast) or seizure resistant (Slow).

METHODS

Normal twisted and/or "spanning" bipolar electrode configurations were implanted in the amygdalae of adult male Fast and Slow rats. In experiment one, rats were kindled daily to stage-5 levels through one electrode type until stable afterdischarge thresholds (ADTs) were obtained. Next, LFS was applied through the kindled electrode, and ADTs were redetermined 1 min later, and daily for a week, without reapplying the LFS. In experiment two, a single, normal bipolar kindling electrode was implanted in the amygdala and centered between two poles of a spanning electrode. After stable kindled ADTs were obtained, LFS was applied to the amygdala "area" through the spanning electrode. ADTs were redetermined at the kindled electrode as earlier.

RESULTS

LFS through the kindling electrode had no effect on ADTs 1 min later, but the ADTs increased dramatically 24 h later and then slowly returned to baseline over days. In experiment two, LFS applied through the nonkindled spanning electrode also showed a small but significant threshold elevation at the interposing kindled electrode. Importantly, no obvious neuropathology was associated with these LFS treatments.

CONCLUSIONS

LFS applied directly to the kindled network has significant threshold-elevating properties that are less evident when applied to the "general area"; here LFS must be delivered through a larger surface area and/or at higher intensity.

Acute changes in the neuronal expression of GABA and glutamate decarboxylase isoforms in the rat piriform cortex following status epilepticus

The piriform cortex (PC) is the largest region of the mammalian olfactory cortex with strong connections to other limbic structures, including the amygdala, hippocampus, and entorhinal cortex. In addition to its functional importance in the classification of olfactory stimuli, the PC has been implicated in the study of memory processing, spread of excitatory information, and the facilitation and propagation of seizures within the limbic system. Previous data from the kindling model of epilepsy indicated that alterations in GABAergic inhibition in the transition zone between the anterior and posterior PC, termed here central PC, are particularly involved in the processes underlying seizure propagation. In the present study we studied alterations in GABAergic neurons in different parts of the PC following seizures induced by kainate or pilocarpine in rats. GABA neurons were labeled either immunohistochemically for GABA or its synthesizing enzyme glutamate decarboxylase (GAD) or by in situ hybridization using antisense probes for GAD65 and GAD67 mRNAs. For comparison with the PC, labeled neurons were examined in the basolateral amygdala, substantia nigra pars reticulata, and the hippocampal formation. In the PC of controls, immunohistochemical labeling for GABA and GAD yielded consistently higher neuronal densities in most cell layers than labeling for GAD65 or GAD67 mRNAs, indicating a low basal activity of these neurons. Eight hours following kainate- or pilocarpine-induced seizures, severe neuronal damage was observed in the PC. Counting of GABA neurons in the PC demonstrated significant decreases in densities of neurons labeled for GABA or GAD proteins. However, a significantly increased density of neurons labeled for GAD65 and GAD67 mRNAs was determined in layer II of the central PC, indicating that a subpopulation of remaining neurons up-regulated the mRNAs for the GAD isoenzymes. One likely explanation for this finding is that remaining GABA neurons in layer II of the central PC maintain high levels of activity to control the increased excitability of the region. In line with previous studies, an up-regulation of GAD67 mRNA, but not GAD65 mRNA, was observed in dentate granule cells following seizures, whereas no indication of such up-regulation was determined for the other brain regions examined. The data substantiate the particular susceptibility of the central PC to seizure-induced plasticity and indicate that this brain region provides an interesting tool to study the regulation of GAD isoenzymes.

Anterior perirhinal cortex kindling produces long-lasting effects on anxiety and object recognition memory

Temporal lobe epilepsy (TLE) is frequently accompanied by memory impairments and, although their bases are unknown, most research has focused on the hippocampus. The present study investigated the importance of another medial temporal lobe structure, the perirhinal cortex (Prh), in changes in memory in TLE using kindling as a model. Rats were kindled twice daily with anterior Prh stimulation until three fully generalized seizures were evoked. Beginning 7 days later and on successive days, rats were tested in an elevated plus maze, a large circular open field, an open field object exploration task and a delayed-match-to-place task in a water maze in order to assess anxiety-related and exploratory behaviour, object recognition memory and spatial cognition. Kindling increased anxiety-related behaviour in both the elevated plus and open field mazes and disrupted spontaneous object recognition but spared all other behaviours tested. These results are consistent with other findings indicating a greater role for the Prh in object memory and emotional behaviour than in spatial memory and contrast with the selective disruption of spatial memory produced by dorsal hippocampal kindling. The site-selectivity of the behavioural disruptions produced by kindling indicates that such effects are probably mediated by changes particular to the site of seizure initiation rather than to changes in the characteristic circuitry activated by limbic seizure generalization. Further investigation of the behavioural effects of Prh kindling may be useful for studying the mechanisms of mnemonic and affective dysfunction associated with TLE and offer insights into bases for variability in such dysfunction across patients.

Distribution of galanin and galanin transcript in the brain of a galanin-overexpressing transgenic mouse

The distribution of galanin mRNA-expressing cells and galanin-immunoreactive (IR) cell bodies and processes was studied in the brain of mice overexpressing galanin under the PDGF-B promoter (GalOE mice) and of wild type (WT) mice, both in colchicine-treated and non-treated animals. In this abstract, we only describe the results in GalOE mouse. A widespread ectopic expression of galanin (both mRNA and peptide) was found, that is a situation when neither transcript nor peptide could be seen in WT mice, not even after colchicine treatment. However, in some regions, such as claustrum, basolateral amygdala, thalamus, CA1 pyramidal cells, and Purkinje cells only galanin mRNA could be detected. In the forebrain galanin was seen in the mitral cells of the olfactory bulb, throughout the cortex, in the basolateral amygdaloid nucleus, claustrum, granular and pyramidal cell layers of the hippocampus, subiculum and presubiculum. In the thalamus, the anterodorsal, mediodorsal, intermediodorsal and mediodorsal lateral nuclei, the reuniens and reticular nuclei showed ectopic expression of galanin. Within the hypothalamus, neurons of the suprachiasmatic nucleus contained galanin. In the mesencephalon, the geniculate nucleus, nucleus ruber, the mesencephalic trigeminal and reticulotegmental nuclei ectopically expressed galanin. In the cerebellum, galanin was observed in the Purkinje cells and in the lateral and interposed cerebellar nuclei. In the pons, sensory and motor nuclei of the trigeminal nerve, the laterodorsal and dorsal tegmental nuclei, the pontine, reticulotegmental and gigantocellular reticular nuclei expressed galanin. Within the medulla oblongata, labeled cells were detected in the facial, ambiguus, prepositus, lateral paragigantocellular and lateral reticular nuclei, and spinal trigeminal nucleus. High densities of galanin-IR fibers were found in the axonal terminals of the lateral olfactory tract, the hippocampal and presumably the cerebellar mossy fibers system, in several thalamic and hypothalamic regions and the lower brain stem. Possible functional consequences of galanin overexpression are discussed.

Time-dependent changes in CRH concentrations and release in discrete brain regions following global ischemia: effects of MK-801 pretreatment

The excitatory actions of corticotropin-releasing hormone (CRH) in the brain and the neuroprotective effects of CRH antagonists in models of ischemia suggest a role for this peptide in the cascade of events leading to cellular damage. The present study aimed to characterize endogenous activation of CRH in discrete brain regions following global ischemia. Time-dependent changes in CRH concentrations were assessed in 10 brain regions including hippocampal, parahippocampal, and hypothalamic regions as well as the amygdala and the frontal cortex at three post-ischemic intervals: 4, 24, and 72 h (Experiment 1). The impact of pretreatment with a neuroprotective dose of the NMDA antagonist (5R,10S)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801; hydrogen maleate) on 24-h ischemia-induced CRH concentrations in the 10 brain regions was also determined (Experiment 2). In vivo microdialysis was used to assess dynamic fluctuations in CRH release at the dorsal hippocampus (CA1 pyramidal layer) and central nucleus of the amygdala (CeA; Experiment 3). Our findings revealed a rapid elevation of CRH concentrations at the piriform cortex (Pir) and hypothalamic nuclei following global ischemia. This was followed by decreased CRH concentrations at the amygdala, the frontal cortex (FC), the CA3, and the hypothalamus 24-h post-ischemia. MK-801 reversed the decreases in the hypothalamic nuclei but not in the other brain regions. Seventy-two hours post-ischemia, CRH levels returned to control values in all regions except the dentate gyrus (DG) where elevated CRH levels were observed. In vivo, a significant increase in CRH release in response to global ischemia was found at the CeA with no alterations at the CA1. These findings support brain region-specific ischemia-induced CRH alterations and suggest that CRH actions to mediate neuronal damage at the hippocampal CA1 layer may be indirect.

Kindling and status epilepticus models of epilepsy: rewiring the brain

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

Bilateral microinjections of vigabatrin in the central piriform cortex retard AMYGDALA kindling in rats

The piriform cortex (PC) is the largest region of the mammalian olfactory cortex with strong connections to limbic structures, including the amygdala, hippocampus, and entorhinal cortex. Various previous studies in rodents suggest that the PC might be very important in the development and maintenance of limbic kindling, i.e. a widely used model of temporal lobe epilepsy. GABAergic inhibition in the transition zone between the anterior and posterior PC, termed here central PC, seems to be particularly involved in the processes leading to progression of kindled seizures. This prompted us to study whether elevation of GABA levels in this subregion of the PC by bilateral microinjection of vigabatrin is capable of suppressing amygdala kindling. Rats were stimulated once daily until fully kindled (stage 5) seizures had developed. Vigabatrin (10 microg) was injected 24 h before the first stimulation as well as 6 h before the 5th and 10th stimulation, which approximately doubled the number of stimulations required for kindling development compared with controls. This marked retardation of kindling acquisition was predominantly due to a significant inhibition of the progression from stage 1 to stage 2 and stage 3 to stage 4 seizures, demonstrating that microinjection of vigabatrin into the central PC markedly inhibits the progression and secondary generalization of focal seizures emanating from the amygdala.

Epileptiform burst discharges in hippocampal CA3 neurons of young but not mature Noda epileptic rats (NER)

Noda epileptic rat (NER), originally found in a colony of Crj; Wistar rats, shows spontaneous tonic-clonic convulsion characterized by the appearance of high voltage polyspikes in cortical and hippocampal EEG once every 2-3 days after 2-4 months of age. Electrophysiological studies using hippocampal slice preparations of NER were performed to determine whether hippocampal neurons have abnormal excitability. When a single stimulus (1-25 V) was delivered to the mossy fibers of NER at 4-6 weeks old before they showed any seizures, a long-lasting depolarization shift (DS) accompanied by repetitive firings and after-hyperpolarization following the abnormal firing was observed in seven of 14 hippocampal CA3 neurons. A lower stimulation intensity evoked DS and abnormal firing in three of nine CA3 neurons of NER at 10-15 weeks old which had already showed seizures at 10-15 weeks of age. However, the abnormal firing was not observed in any 10 neurons of the animals at more than 20 weeks old nor in Wistar rats. The input impedances of CA3 neurons in NER with abnormal firing were lower than those without abnormal firing and those in Wistar rats. The abnormal excitability obtained in NER at an age when it did not display any seizures suggests that the hippocampus may play a role in epileptogenicity in NER.

Sex differences in models of temporal lobe epilepsy: role of testosterone

Kainic acid and pilocarpine were used to assess sex differences in temporal lobe seizures. Adult Sprague-Dawley rats were injected with kainic acid (10-12 mg/kg) or with pilocarpine (380 mg/kg) and behavior was recorded for the next 3 h. Trunk blood was collected for hormonal measurements. Our data indicate that the male is more susceptible to the convulsant effects of agents that produce temporal lobe-like seizures. Males presented a higher amount of full limbic convulsions than females. To assess the role of plasma testosterone levels in kainate-induced seizures, a group of males was gonadectomized and half received testosterone replacement. The presence of testosterone, in intact and in gonadectomized males with testosterone replacement, increased the susceptibility to seizure. Seizures were either stronger (full limbic) or more frequent in animals with testosterone compared to animals devoid of testosterone. These results suggest that differences in plasma levels of testosterone may be partially responsible for the observed gender differences in seizure susceptibility. Our data reveal a reciprocal relationship between kainic acid-induced temporal lobe seizures and plasma testosterone. Testosterone enhances the occurrence and the severity of seizures. Conversely, kainic-acid-induced seizures decrease plasma testosterone. The higher plasma corticosterone levels found in these males suggest that kainic acid-induced seizures activate the hypothalamic-pituitary-adrenal axis which may induce alterations in plasma levels of male reproductive hormones.

Changes in GABA(B) receptor immunoreactivity after recurrent seizures in rats

GABA(B) receptors play an important role in the excitability of neuronal networks and can influence seizure activity. Here we demonstrate for the first time that kindling, an animal model for human temporal lobe epilepsy, leads to both early and delayed changes of GABA(B) receptor immunoreactivity in hippocampal and cortical areas. We propose that the altered GABA(B) receptor levels might be a compensatory mechanism to reduce excitability induced by recurrent kindled seizures, or alternatively, may promote the development of kindled epilepsy.

Suppressed kindling epileptogenesis in mice with ectopic overexpression of galanin

The neuropeptide galanin has been shown to suppress epileptic seizures. In cortical and hippocampal areas, galanin is normally mainly expressed in noradrenergic afferents. We have generated a mouse overexpressing galanin in neurons under the platelet-derived growth factor B promoter. RIA and HPLC analysis revealed up to 8-fold higher levels of galanin in transgenic as compared with wild-type mice. Ectopic galanin overexpression was detected especially in dentate granule cells and hippocampal and cortical pyramidal neurons. Galanin-overexpressing mice showed retardation of seizure generalization during hippocampal kindling, a model for human complex partial epilepsy. The high levels of galanin in mossy fibers found in the transgenic mice were further increased after seizures. Frequency facilitation of field excitatory postsynaptic potentials, a form of short-term synaptic plasticity assessed in hippocampal slices, was reduced in mossy fiber-CA3 cell synapses of galanin-overexpressing mice, indicating suppressed glutamate release. This effect was reversed by application of the putative galanin receptor antagonist M35. These data provide evidence that ectopically overexpressed galanin can be released and dampen the development of epilepsy by means of receptor-mediated action, at least partly by reducing glutamate release from mossy fibers.