Excitatory amplification through divergent-convergent circuits: the role of the midline thalamus in limbic seizures

Thalamic neuromodulation in epilepsy: A primer for emerging circuit-based therapies

INTRODUCTION

Epilepsy is a common, often debilitating disease of hyperexcitable neural networks. While medically intractable cases may benefit from surgery, there may be no single, well-localized focus for resection or ablation. In such cases, approaching the disease from a network-based perspective may be beneficial.

AREAS COVERED

Herein, the authors provide a narrative review of normal thalamic anatomy and physiology and propose general strategies for preventing and/or aborting seizures by modulating this structure. Additionally, they make specific recommendations for targeting the thalamus within different contexts, motivated by a more detailed discussion of its distinct nuclei and their respective connectivity. By describing important principles governing thalamic function and its involvement in seizure networks, the authors aim to provide a primer for those now entering this fast-growing field of thalamic neuromodulation for epilepsy.

EXPERT OPINION

The thalamus is critically involved with the function of many cortical and subcortical areas, suggesting it may serve as a compelling node for preventing or aborting seizures, and so it has increasingly been targeted for the surgical treatment of epilepsy. As various thalamic neuromodulation strategies for seizure control are developed, there is a need to ground such interventions in a mechanistic, circuit-based framework.

Stress Affects Central Compensation of Neural Responses to Cochlear Synaptopathy in a cGMP-Dependent Way

In light of the increasing evidence supporting a link between hearing loss and dementia, it is critical to gain a better understanding of the nature of this relationship. We have previously observed that following cochlear synaptopathy, the temporal auditory processing (e.g., auditory steady state responses, ASSRs), is sustained when reduced auditory input is centrally compensated. This central compensation process was linked to elevated hippocampal long-term potentiation (LTP). We further observed that, independently of age, central responsiveness to cochlear synaptopathy can differ, resulting in either a low or high capacity to compensate for the reduced auditory input. Lower central compensation resulted in poorer temporal auditory processing, reduced hippocampal LTP, and decreased recruitment of activity-dependent brain-derived neurotrophic factor (BDNF) expression in hippocampal regions (low compensators). Higher central compensation capacity resulted in better temporal auditory processing, higher LTP responses, and increased activity-dependent BDNF expression in hippocampal regions. Here, we aimed to identify modifying factors that are potentially responsible for these different central responses. Strikingly, a poorer central compensation capacity was linked to lower corticosterone levels in comparison to those of high compensators. High compensators responded to repeated placebo injections with elevated blood corticosterone levels, reduced auditory brainstem response (ABR) wave I amplitude, reduced inner hair cell (IHC) ribbon number, diminished temporal processing, reduced LTP responses, and decreased activity-dependent hippocampal BDNF expression. In contrast, the same stress exposure through injection did not elevate blood corticosterone levels in low compensators, nor did it reduce IHC ribbons, ABR wave I amplitude, ASSR, LTP, or BDNF expression as seen in high compensators. Interestingly, in high compensators, the stress-induced responses, such as a decline in ABR wave I amplitude, ASSR, LTP, and BDNF could be restored through the "memory-enhancing" drug phosphodiesterase 9A inhibitor (PDE9i). In contrast, the same treatment did not improve these aspects in low compensators. Thus, central compensation of age-dependent cochlear synaptopathy is a glucocorticoid and cyclic guanosine-monophosphate (cGMP)-dependent neuronal mechanism that fails upon a blunted stress response.

Optogenetic activation of the reticular nucleus of the thalamus attenuates limbic seizures via inhibition of the midline thalamus

OBJECTIVE

The nucleus reticularis of the thalamus (nRT) is most studied in epilepsy for its role in the genesis of absence seizures; much less is known regarding its role in other seizure types, including those originating in limbic structures and the temporal lobe. As it is a major source of inhibitory input to higher order thalamic nuclei, stimulation of the nRT may be an effective strategy to disrupt seizure activity that requires thalamic engagement.

METHODS

We recorded single unit activity from the nRT prior to and after infusion of bicuculline into the area tempestas. We monitored single unit activity time-locked with interictal spikes. We optogenetically activated the nRT in both the area tempestas and amygdala kindling models. We tested a role for projections from the nRT to higher order midline thalamic nuclei through the use of retrogradely trafficked viral vector.

RESULTS

Mean firing rate in the nRT was decreased after infusion of bicuculline into the area tempestas as compared to the preinfusion baseline. nRT unit firing in response to interictal spikes was heterogeneous, with an approximately equal proportion of neurons displaying (1) no change in firing, (2) increased firing, and (3) decreasing firing. Optogenetic activation of the nRT significantly suppressed seizure activity in both the area tempestas and amygdala kindling models. Optogenetic activation of contralaterally targeting projections but not ipsilaterally targeting projections from the nRT to the midline thalamus significantly suppressed seizures in the kindling model.

SIGNIFICANCE

Although the nRT is typically thought of in the context of absence seizures, our data show that it may be a viable target for other seizure types. In two models that recapitulate the seizure types seen in temporal lobe epilepsy, nRT activation suppressed both electrographic and behavioral seizures. These data suggest that the nRT should be considered more broadly in the context of epilepsy.

Voxel-based morphometry analysis and machine learning based classification in pediatric mesial temporal lobe epilepsy with hippocampal sclerosis

Mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS) is a common type of pediatric epilepsy. We sought to evaluate whether the combination of voxel-based morphometry (VBM) and support vector machine (SVM), a machine learning method, was feasible for the classification of MTLE-HS. Three-dimensional T1-weighted MRI was acquired in 37 participants including 22 with MTLE-HS (16 left, 6 right) and 15 healthy controls (HCs). VBM was used to detect the regions of gray matter volume (GMV) abnormalities. The volumes of these regions were then calculated for each participant and used as the features in SVM. The SVM model was trained and tested with leave-one-out cross validation (LOOCV). We performed VBM-based comparison and SVM-based classification between left HS (LHS) and HC as well as between right HS (RHS) and HC. Both GMV increase and reduction were found in the group comparisons with VBM. Using SVM, we reached an area under the receiver operating characteristic curve (AUC) of 0.870, 0.976 and 0.902 for the classification between LHS and HC, between RHS and HC and between HS and HC respectively. The VBM findings were concordant with the clinical findings. Thus, our proposed method combining VBM findings with SVM, were applicable in the classification of padiatric MTLE-HS with high accuracy.

Electrographic Features of Spontaneous Recurrent Seizures in a Mouse Model of Extended Hippocampal Kindling

Epilepsy is a chronic neurological disorder characterized by spontaneous recurrent seizures (SRS) and comorbidities. Kindling through repetitive brief stimulation of a limbic structure is a commonly used model of temporal lobe epilepsy. Particularly, extended kindling over a period up to a few months can induce SRS, which may simulate slowly evolving epileptogenesis of temporal lobe epilepsy. Currently, electroencephalographic (EEG) features of SRS in rodent models of extended kindling remain to be detailed. We explored this using a mouse model of extended hippocampal kindling. Intracranial EEG recordings were made from the kindled hippocampus and unstimulated hippocampal, neocortical, piriform, entorhinal, or thalamic area in individual mice. Spontaneous EEG discharges with concurrent low-voltage fast onsets were observed from the two corresponding areas in nearly all SRS detected, irrespective of associated motor seizures. Examined in brain slices, epileptiform discharges were induced by alkaline artificial cerebrospinal fluid in the hippocampal CA3, piriform and entorhinal cortical areas of extended kindled mice but not control mice. Together, these in vivo and in vitro observations suggest that the epileptic activity involving a macroscopic network may generate concurrent discharges in forebrain areas and initiate SRS in hippocampally kindled mice.

Early ictal recruitment of midline thalamus in mesial temporal lobe epilepsy

The causal role of midline thalamus in the initiation and early organization of mesial temporal lobe seizures is studied. Three patients undergoing stereoelectroencephalography were enrolled for the placement of an additional depth electrode targeting the midline thalamus. The midline thalamus was recruited in all three patients at varying points of seizure initiation (0–13 sec) and propagation (9–60 sec). Stimulation of either thalamus or hippocampus induced similar habitual seizures. Seizure‐induced in the hippocampus rapidly recruited the thalamus. Evoked potentials demonstrated stronger connectivity from the hippocampus to the thalamus than in the opposite direction. The midline thalamus can be within the seizure initiation and symptomatogenic circuits.

Input Convergence, Synaptic Plasticity and Functional Coupling Across Hippocampal-Prefrontal-Thalamic Circuits

Executive functions and working memory are long known to involve the prefrontal cortex (PFC), and two PFC-projecting areas: midline/paramidline thalamus (MLT) and cornus ammonis 1 (CA1)/subiculum of the hippocampal formation (HF). An increasing number of rodent electrophysiology studies are examining these substrates together, thus providing circuit-level perspectives on input convergence, synaptic plasticity and functional coupling, as well as insights into cognition mechanisms and brain disorders. Our review article puts this literature into a method-oriented narrative. As revisited throughout the text, limbic thalamic and hippocampal afferents to the PFC gate one another’s inputs, which in turn are modulated by PFC interneurons and ascending monoaminergic projections. In addition, long-term synaptic plasticity, paired-pulse facilitation (PPF), and event-related potentials (ERP) dynamically vary across PFC-related circuits during learning paradigms and drug effects. Finally, thalamic-prefrontal loops, which have been shown to amplify both cognitive processes and limbic seizures, are also being implicated as relays in the prefrontal-hippocampal feedback, contributing to spatial navigation and decision making. Based on these issues, we conclude the review with a critical synthesis and some research directions.

Interaction between hippocampal-prefrontal plasticity and thalamic-prefrontal activity

The prefrontal cortex integrates a variety of cognition-related inputs, either unidirectional, e.g., from the hippocampal formation, or bidirectional, e.g., with the limbic thalamus. While the former is usually implicated in synaptic plasticity, the latter is better known for regulating ongoing activity. Interactions between these processes via prefrontal neurons are possibly important for linking mnemonic and executive functions. Our work further elucidates such dynamics using in vivo electrophysiology in rats. First, we report that electrical pulses into CA1/subiculum trigger late-onset (>400 ms) firing responses in the medial prefrontal cortex, which are increased after induction of long-term potentiation. Then, we show these responses to be attenuated by optogenetic control of the paraventricular/mediodorsal thalamic area. This suggests that recruitment and plasticity of the hippocampal-prefrontal pathway is partially related to the thalamic-prefrontal loop. When dysfunctional, this interaction may contribute to cognitive deficits, psychotic symptoms, and seizure generalization, which should motivate future studies combining behavioural paradigms and long-range circuit assessment.

Neurochemical Changes and c-Fos Mapping in the Brain after Carisbamate Treatment of Rats Subjected to Lithium-Pilocarpine-Induced Status Epilepticus

The administration of lithium–pilocarpine (LiPilo) in adult rats is a validated model reproducing the main clinical and neuropathological features of temporal lobe epilepsy (TLE). Previous studies have shown that carisbamate (CRS) has the property of modifying epileptogenesis in this model. When treated with CRS, about 50% of rats undergoing LiPilo status epilepticus (SE) develop non-convulsive seizures (NCS) instead of convulsive ones (commonly observed in TLE). The goal of this work was to determine some of the early changes that occur after CRS administration, as they could be involved in the insult- and epileptogenesis-modifying effects of CRS. Thus, we performed high-performance liquid chromatography (HPLC) to quantify levels of amino acids and monoamines, and c-Fos immunohistochemical labeling to map cerebral activation during seizures. Comparing rats treated one hour after SE onset with saline (CT), CRS, or diazepam (DZP), HPLC showed that 4 h after SE onset, dopamine (DA), norepinephrine (NE), and GABA levels were normal, whereas serotonin levels were increased. Using c-Fos labeling, we demonstrated increased activity in thalamic mediodorsal (MD) and laterodorsal (LD) nuclei in rats treated with CRS. In summary, at early times, CRS seems to modulate excitability by acting on some monoamine levels and increasing activity of MD and LD thalamic nuclei, suggesting a possible involvement of these nuclei in insult- and/or epileptogenesis-modifying effects of CRS.

Localized shape abnormalities in the thalamus and pallidum are associated with secondarily generalized seizures in mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (mTLE) is a common type of drug-resistant epilepsy and secondarily generalized tonic-clonic seizures (sGTCS) have devastating consequences for patients' safety and quality of life. To probe the mechanism underlying the genesis of sGTCS, we investigated the structural differences between patients with and without sGTCS in a cohort of mTLE with radiologically defined unilateral hippocampal sclerosis. We performed voxel-based morphometric analysis of cortex and vertex-wise shape analysis of subcortical structures (the basal ganglia and thalamus) on MRI of 39 patients (21 with and 18 without sGTCS). Comparisons were initially made between sGTCS and non-sGTCS groups, and subsequently made between uncontrolled-sGTCS and controlled-sGTCS subgroups. Regional atrophy of the ipsilateral ventral pallidum (cluster size=450 voxels, corrected p=0.047, Max voxel coordinate=107, 120, 65), medial thalamus (cluster size=1128 voxels, corrected p=0.049, Max voxel coordinate=107, 93, 67), middle frontal gyrus (cluster size=60 voxels, corrected p<0.05, Max voxel coordinate=-30, 49.5, 6), and contralateral posterior cingulate cortex (cluster size=130 voxels, corrected p<0.05, Max voxel coordinate=16.5, -57, 27) was found in the sGTCS group relative to the non-sGTCS group. Furthermore, the uncontrolled-sGTCS subgroup showed more pronounced atrophy of the ipsilateral medial thalamus (cluster size=1240 voxels, corrected p=0.014, Max voxel coordinate=107, 93, 67) than the controlled-sGTCS subgroup. These findings indicate a central role of thalamus and pallidum in the pathophysiology of sGTCS in mTLE.

Interaction between Thalamus and Hippocampus in Termination of Amygdala-Kindled Seizures in Mice

The thalamus and hippocampus have been found both involved in the initiation, propagation, and termination of temporal lobe epilepsy. However, the interaction of these regions during seizures is not clear. The present study is to explore whether some regular patterns exist in their interaction during the termination of seizures. Multichannel in vivo recording techniques were used to record the neural activities from the cornu ammonis 1 (CA1) of hippocampus and mediodorsal thalamus (MDT) in mice. The mice were kindled by electrically stimulating basolateral amygdala neurons, and Racine's rank standard was employed to classify the stage of behavioral responses (stage 1~5). The coupling index and directionality index were used to investigate the synchronization and information flow direction between CA1 and MDT. Two main results were found in this study. (1) High levels of synchronization between the thalamus and hippocampus were observed before the termination of seizures at stage 4~5 but after the termination of seizures at stage 1~2. (2) In the end of seizures at stage 4~5, the information tended to flow from MDT to CA1. Those results indicate that the synchronization and information flow direction between the thalamus and the hippocampus may participate in the termination of seizures.

Progressive neuronal activation accompanies epileptogenesis caused by hippocampal glutamine synthetase inhibition

Loss of glutamine synthetase (GS) in hippocampal astrocytes has been implicated in the causation of human mesial temporal lobe epilepsy (MTLE). However, the mechanism by which the deficiency in GS leads to epilepsy is incompletely understood. Here we ask how hippocampal GS inhibition affects seizure phenotype and neuronal activation during epilepsy development (epileptogenesis). Epileptogenesis was induced by infusing the irreversible GS blocker methionine sulfoximine (MSO) unilaterally into the hippocampal formation of rats. We then used continuous video-intracranial electroencephalogram (EEG) monitoring and c-Fos immunohistochemistry to determine the type of seizures and spatial distribution of neuronal activation early (1-5days postinfusion) and late (16-43days postinfusion) in epileptogenesis. Early in epileptogenesis, seizures were preferentially mild (stage 1-2), activating neurons in the entorhinal-hippocampal area, the basolateral amygdala, the piriform cortex, the midline thalamus, and the anterior olfactory area. Late in epileptogenesis, the seizures were generally more severe (stages 4-5) with neuronal activation extending to the neocortex, the bed nucleus of the stria terminalis, the mediodorsal thalamu\s, and the central nucleus of the amygdala. Our findings demonstrate that inhibition of GS focally in the hippocampal formation triggers a process of epileptogenesis characterized by gradual worsening of seizure severity and involvement of progressively larger neuronal populations over a period of several weeks. Knowledge about the underlying mechanism of epileptogenesis is important because such knowledge may result in more specific and efficacious treatments of MTLE by moving away from large and poorly specific surgical resections to highly targeted surgical or pharmacological interventions of the epileptogenic process.

The Spatiotemporal Dynamics of Phase Synchronization during Epileptogenesis in Amygdala-Kindling Mice

The synchronization among the activities of neural populations in functional regions is one of the most important electrophysiological phenomena in epileptic brains. The spatiotemporal dynamics of phase synchronization was investigated to reveal the reciprocal interaction between different functional regions during epileptogenesis. Local field potentials (LFPs) were recorded simultaneously from the basolateral amygdala (BLA), the cornu ammonis 1 of hippocampus (CA1) and the mediodorsal nucleus of thalamus (MDT) in the mouse amygdala-kindling models during the development of epileptic seizures. The synchronization of LFPs was quantified between BLA, CA1 and MDT using phase-locking value (PLV). During amygdala kindling, behavioral changes (from stage 0 to stage 5) of mice were accompanied by after-discharges (ADs) of similar waveforms appearing almost simultaneously in CA1, MDT, as well as BLA. AD durations were positively related to the intensity of seizures. During seizures at stages 1~2, PLVs remained relatively low and increased dramatically shortly after the termination of the seizures; by contrast, for stages 3~5, PLVs remained a relatively low level during the initial period but increased dramatically before the seizure termination. And in the theta band, the degree of PLV enhancement was positively associated with seizure intensity. The results suggested that during epileptogenesis, the functional regions were kept desynchronized rather than hyper-synchronized during either the initial or the entire period of the seizures; so different dynamic patterns of phase synchronization may be involved in different periods of the epileptogenesis, and this might also reflect that during seizures at different stages, the mechanisms underlying the dynamics of phase synchronization were different.

The frequency of spontaneous seizures in rats correlates with alterations in sensorimotor gating, spatial working memory, and parvalbumin expression throughout limbic regions

Cognitive deficits and psychotic symptoms are highly prevalent in patients with temporal lobe epilepsy (TLE). Imaging studies in humans have suggested that these comorbidities are associated with atrophy in temporal lobe structures and other limbic regions. It remains to be clarified whether TLE comorbidities are due to the frequency of spontaneous seizures or to limbic structural damage per se. Here, we used the pilocarpine model of chronic spontaneous seizures to evaluate the possible association of seizure frequency with sensorimotor gating, spatial working memory, and neuropathology throughout limbic regions. For TLE modeling, we induced a 2-h status epilepticus by the systemic administration of lithium-pilocarpine. Once spontaneous seizures were established, we tested the locomotor activity (open field), spatial working memory (eight-arm radial maze), and sensorimotor gating (prepulse inhibition of acoustic startle). After behavioral testing, the brains were sectioned for hematoxylin-eosin staining (cell density) and parvalbumin immunohistochemistry (GABAergic neuropil) in the prefrontal cortex, nucleus accumbens, thalamus, amygdala, hippocampus, and entorhinal cortex. The animal groups analyzed included chronic epileptic rats, their controls, and rats that received lithium-pilocarpine but eventually failed to express status epilepticus or spontaneous seizures. Epileptic rats showed deficits in sensorimotor gating that negatively correlated with the radial maze performance, and impairments in both behavioral tests correlated with seizure frequency. In addition to neuronal loss at several sites, we found increased parvalbumin immunostaining in the prefrontal cortex (infralimbic area), thalamus (midline and reticular nuclei), amygdala, Ammon's horn, dentate gyrus, and entorhinal cortex. These tissue changes correlated with seizure frequency and impairments in sensorimotor gating. Our work indicates that chronic seizures might impact the inhibitory-excitatory balance in the temporal lobe and its interconnected limbic regions, which could increase the likelihood of cognitive deficits and interictal psychiatric disorders.

Hippocampal-thalamic wiring in medial temporal lobe epilepsy: Enhanced connectivity per hippocampal voxel

OBJECTIVE

Medial temporal lobe epilepsy (TLE) with hippocampal sclerosis is often accompanied by widespread changes in ipsilateral and contralateral white matter connectivity. Recent studies have proposed that patients may show pathologically enhanced wiring of the limbic circuits. To better address this issue, we specifically probed connection patterns between hippocampus and thalamus and examined their impact on cognitive function.

METHODS

A group of 44 patients with TLE (22 with right and 22 with left hippocampal sclerosis) and 24 healthy control participants were examined with high-resolution T1 imaging, memory functional magnetic resonance imaging (fMRI) and probabilistic diffusion tractography. Thirty-four patients had further extensive neuropsychological testing. After whole brain segmentation with FreeSurfer, tractography streamline samples were drawn with hippocampus as the seed and thalamus as the target region. Two tractography strategies were applied: The first targeted the anatomic thalamic volume segmented in FreeSurfer and the second a functional region of interest in the mediodorsal thalamus derived from the activation during delayed recognition memory.

RESULTS

We found a pronounced enhancement of connectivity between the sclerotic hippocampus and the ipsilateral thalamus both in the right and left TLE as compared to healthy control participants. This finding held for both the anatomically and the functionally defined thalamic target. Although differences were apparent in the number of absolute fibers, they were most pronounced when correcting for hippocampal volume. In terms of cognitive function, the number of hippocampal-thalamic connections was negatively correlated with performance in a variety of executive tasks, notably in the Trail Making Test, thus suggesting that the pathologic wiring did not compensate cognitive curtailing.

SIGNIFICANCE

We suggest that TLE is accompanied by an abnormal and dysfunctional enhancement of connectivity between the hippocampus and the thalamus, which is maximal on the side of the sclerosis. This pathologic pattern of limbic wiring might reflect structural remodeling along common pathways of seizure propagation.

Suppressing limbic seizures by stimulating medial dorsal thalamic nucleus: factors for efficacy

OBJECTIVE

The optimal sites and stimulation protocols for brain stimulation in epilepsy have not been found. Clinical trials, which have shown modest benefit in seizure reduction, have involved patients with poorly localized intractable focal epilepsy and stimulation sites without clear relations to specific underlying seizure circuits. The medial dorsal thalamic nucleus is a key node in limbic seizure circuits, and we wished to know what stimulation parameters might control seizures in a kindling model of limbic epilepsy.

METHODS

In urethane-anesthetized rats, we induced limbic seizures by stimulation of the piriform cortex or CA3 of the hippocampus while recording in the entorhinal cortex or CA1 of the contralateral hippocampus to determine the effect of specific stimulation parameters on seizure duration.

RESULTS

Stimulation consistently suppressed seizure duration from baseline by over 80% (p < 0.001), frequently completely preventing the seizures. Position of the thalamic electrode, stimulus intensity and frequency had a significant influence, with higher stimulus intensities (40 V vs. 20 V) and frequencies (20 Hz vs. 7 Hz) significantly suppressing seizures. The most effective position was the lateral dorsal area of the medial dorsal nucleus (MD), which corresponded to the region of axon entry. Stimulation in the MD center was not effective. An anterior-posterior relationship of the stimulating electrode pair was effective, whereas a medial lateral orientation was not. Successful stimulation suppressed the evoked responses in the entorhinal cortex or CA1.

SIGNIFICANCE

Position and orientation of the stimulating electrode has to be precise, which suggests that the placement of the electrodes must be tailored to the individual's own seizure circuit. The data also indicate that successful deep brain stimulation induces a fundamental change in system physiology, which could be a marker to guide the development of stimulation parameters for each patient.

Extratemporal lobe circuits in temporal lobe epilepsy

There is increasing interest in the functional anatomy of epilepsy with the goal to identify the critical nodes in the seizure circuits so that therapy can be directed at them. This goal is especially important because direct delivery of therapy, either through electrical stimulation, drug infusion, or molecular therapies such as optogenetics, has become increasingly possible. In this article, we will review the basic functional anatomy of mesial temporal lobe epilepsy and its primary subcortical connection, the medial dorsal nucleus of the thalamus. Based on its anatomical connections and known physiological interactions, we propose a key role for this thalamic nucleus that is essential for the development of seizures, and this role suggests that this region is a potential therapeutic target.

Neuronal networks and energy bursts in epilepsy

Epilepsy can be defined as the abnormal activities of neurons. The occurrence, propagation and termination of epileptic seizures rely on the networks of neuronal cells that are connected through both synaptic- and non-synaptic interactions. These complicated interactions contain the modified functions of normal neurons and glias as well as the mediation of excitatory and inhibitory mechanisms with feedback homeostasis. Numerous spread patterns are detected in disparate networks of ictal activities. The cortical-thalamic-cortical loop is present during a general spike wave seizure. The thalamic reticular nucleus (nRT) is the major inhibitory input traversing the region, and the dentate gyrus (DG) controls CA3 excitability. The imbalance between γ-aminobutyric acid (GABA)-ergic inhibition and glutamatergic excitation is the main disorder in epilepsy. Adjustable negative feedback that mediates both inhibitory and excitatory components affects neuronal networks through neurotransmission fluctuation, receptor and transmitter signaling, and through concomitant influences on ion concentrations and field effects. Within a limited dynamic range, neurons slowly adapt to input levels and have a high sensitivity to synaptic changes. The stability of the adapting network depends on the ratio of the adaptation rates of both the excitatory and inhibitory populations. Thus, therapeutic strategies with multiple effects on seizures are required for the treatment of epilepsy, and the therapeutic functions on networks are reviewed here. Based on the high-energy burst theory of epileptic activity, we propose a potential antiepileptic therapeutic strategy to transfer the high energy and extra electricity out of the foci.

Electrophysiology in epilepsy surgery: Roles and limitations

Successful epilepsy surgery depends on the localization of the seizure onset zone in an area of the brain that can be safely resected. Defining these zones uses multiple diagnostic approaches, which include different types of electroencephalography (EEG) and imaging, and the results are best when all of the tests point to the same region. Although EEG obtained with scalp recordings is often sufficient for the purposes of localization, there are times when intracranial recordings directly from the brain are needed; but the planning, use, value, and interpretation of the these recordings are not standardized, in part because the questions that are to be answered vary considerably across many patients and their heterogenous types of epilepsy that are investigated. Furthermore, there is a desire to use the opportunity of direct brain recordings to understand the pathophysiology of epilepsy, as these recordings are viewed as an opportunity to answer questions that cannot be otherwise answered. In this review, we examine the situations that may require intracranial electrodes and discuss the broad issues that this powerful diagnostic tool can help address, for identifying the seizure focus and for understanding the large scale circuits of the seizures.

Involvement of thalamus in initiation of epileptic seizures induced by pilocarpine in mice

Studies have suggested that thalamus is involved in temporal lobe epilepsy, but the role of thalamus is still unclear. We obtained local filed potentials (LFPs) and single-unit activities from CA1 of hippocampus and parafascicular nucleus of thalamus during the development of epileptic seizures induced by pilocarpine in mice. Two measures, redundancy and directionality index, were used to analyze the electrophysiological characters of neuronal activities and the information flow between thalamus and hippocampus. We found that LFPs became more regular during the seizure in both hippocampus and thalamus, and in some cases LFPs showed a transient disorder at seizure onset. The variation tendency of the peak values of cross-correlation function between neurons matched the variation tendency of the redundancy of LFPs. The information tended to flow from thalamus to hippocampus during seizure initiation period no matter what the information flow direction was before the seizure. In some cases the information flow was symmetrically bidirectional, but none was found in which the information flowed from hippocampus to thalamus during the seizure initiation period. In addition, inactivation of thalamus by tetrodotoxin (TTX) resulted in a suppression of seizures. These results suggest that thalamus may play an important role in the initiation of epileptic seizures.

Spike-wave discharges in adult Sprague-Dawley rats and their implications for animal models of temporal lobe epilepsy

Spike-wave discharges (SWDs) are thalamocortical oscillations that are often considered to be the EEG correlate of absence seizures. Genetic absence epilepsy rats of Strasbourg (GAERS) and Wistar Albino Glaxo rats from Rijswijk (WAG/Rij) exhibit SWDs and are considered to be genetic animal models of absence epilepsy. However, it has been reported that other rat strains have SWDs, suggesting that SWDs may vary in their prevalence, but all rats have a predisposition for them. This is important because many of these rat strains are used to study temporal lobe epilepsy (TLE), where it is assumed that there is no seizure-like activity in controls. In the course of other studies using the Sprague-Dawley rat, a common rat strain for animal models of TLE, we found that approximately 19% of 2- to 3-month-old naive female Sprague-Dawley rats exhibited SWDs spontaneously during periods of behavioral arrest, which continued for months. Males exhibited SWDs only after 3 months of age, consistent with previous reports (Buzsáki et al., 1990). Housing in atypical lighting during early life appeared to facilitate the incidence of SWDs. Spike-wave discharges were often accompanied by behaviors similar to stage 1-2 limbic seizures. Therefore, additional analyses were made to address the similarity. We observed that the frequency of SWDs was similar to that of hippocampal theta rhythm during exploration for a given animal, typically 7-8 Hz. Therefore, activity in the frequency of theta rhythm that occurs during frozen behavior may not reflect seizures necessarily. Hippocampal recordings exhibited high frequency oscillations (>250 Hz) during SWDs, suggesting that neuronal activity in the hippocampus occurs during SWDs, i.e., it is not a passive structure. The data also suggest that high frequency oscillations, if rhythmic, may reflect SWDs. We also confirmed that SWDs were present in a common animal model of TLE, the pilocarpine model, using female Sprague-Dawley rats. Therefore, damage and associated changes to thalamic, hippocampal, and cortical neurons do not prevent SWDs, at least in this animal model. The results suggest that it is possible that SWDs occur in rodent models of TLE and that investigators mistakenly assume that they are stage 1-2 limbic seizures. We discuss the implications of the results and ways to avoid the potential problems associated with SWDs in animal models of TLE.

Regional thalamic neuropathology in patients with hippocampal sclerosis and epilepsy: a postmortem study

Purpose

Clinical, experimental, and neuroimaging data all indicate that the thalamus is involved in the network of changes associated with temporal lobe epilepsy (TLE), particularly in association with hippocampal sclerosis (HS), with potential roles in seizure initiation and propagation. Pathologic changes in the thalamus may be a result of an initial insult, ongoing seizures, or retrograde degeneration through reciprocal connections between thalamic and limbic regions. Our aim was to carry out a neuropathologic analysis of the thalamus in a postmortem (PM) epilepsy series, to assess the distribution, severity, and nature of pathologic changes and its association with HS.

Methods

Twenty-four epilepsy PM cases (age range 25–87 years) and eight controls (age range 38–85 years) were studied. HS was classified as unilateral (UHS, 11 cases), bilateral (BHS, 4 cases) or absent (No-HS, 9 cases). Samples from the left and right sides of the thalamus were stained with cresyl violet (CV), and for glial firbillary acidic protein (GFAP) and synaptophysin. Using image analysis, neuronal densities (NDs) or field fraction staining values (GFAP, synaptophysin) were measured in four thalamic nuclei: anteroventral nucleus (AV), lateral dorsal nucleus (LD), mediodorsal nucleus (MD), and ventrolateral nucleus (VL). The results were compared within and between cases.

Key Findings

The severity, nature, and distribution of thalamic pathology varied between cases. A pattern that emerged was a preferential involvement of the MD in UHS cases with a reduction in mean ND ipsilateral to the side of HS (p = 0.05). In UHS cases, greater field fraction values for GFAP and lower values for synaptophysin and ND were seen in the majority of cases in the MD ipsilateral to the side of sclerosis compared to other thalamic nuclei. In addition, differences in the mean ND between classical HS, atypical HS, and No-HS cases were noted in the ipsilateral MD (p < 0.05), with lower values observed in HS.

Significance

Our study demonstrates that stereotypical pathologic changes, as seen in HS, are not clearly defined in the thalamus. This may be partly explained by the heterogeneity of our PM study group. With quantitation, there is some evidence for preferential involvement of the MD, suggesting a potential role in TLE, which requires further investigation.

Muscarinic and nicotinic modulation of thalamo-prefrontal cortex synaptic plasticity [corrected] in vivo

The mediodorsal nucleus of the thalamus (MD) is a rich source of afferents to the medial prefrontal cortex (mPFC). Dysfunctions in the thalamo-prefrontal connections can impair networks implicated in working memory, some of which are affected in Alzheimer disease and schizophrenia. Considering the importance of the cholinergic system to cortical functioning, our study aimed to investigate the effects of global cholinergic activation of the brain on MD-mPFC synaptic plasticity by measuring the dynamics of long-term potentiation (LTP) and depression (LTD) in vivo. Therefore, rats received intraventricular injections either of the muscarinic agonist pilocarpine (PILO; 40 nmol/µL), the nicotinic agonist nicotine (NIC; 320 nmol/µL), or vehicle. The injections were administered prior to either thalamic high-frequency (HFS) or low-frequency stimulation (LFS). Test pulses were applied to MD for 30 min during baseline and 240 min after HFS or LFS, while field postsynaptic potentials were recorded in the mPFC. The transient oscillatory effects of PILO and NIC were monitored through recording of thalamic and cortical local field potentials. Our results show that HFS did not affect mPFC responses in vehicle-injected rats, but induced a delayed-onset LTP with distinct effects when applied following PILO or NIC. Conversely, LFS induced a stable LTD in control subjects, but was unable to induce LTD when applied after PILO or NIC. Taken together, our findings show distinct modulatory effects of each cholinergic brain activation on MD-mPFC plasticity following HFS and LFS. The LTP-inducing action and long-lasting suppression of cortical LTD induced by PILO and NIC might implicate differential modulation of thalamo-prefrontal functions under low and high input drive.

Kv1.1 and Kv1.2: similar channels, different seizure models

Voltage-gated K(+) channels (Kv) represent the largest family of genes in the K(+) channel family. The Kv1 subfamily plays an essential role in the initiation and shaping of action potentials, influencing action potential firing patterns and controlling neuronal excitability. Overlapping patterns with differential expression and precise localization of Kv1.1 and Kv1.2 channels targeted to specialized subcellular compartments contribute to distinctive patterns of neuronal excitability. Dynamic regulation of the components in these subcellular domains help to finely tune the cellular and regional networks. Disruption of the expression, distribution, and density of these channels through deletion or mutation of the genes encoding these channels, Kcna1 and Kcna2, is associated with neurologic pathologies including epilepsy and ataxia in humans and in rodent models. Kv1.1 and Kv1.2 knockout mice both have seizures beginning early in development; however, each express a different seizure type (pathway), although the channels are from the same subfamily and are abundantly coexpressed. Voltage-gated ion channels clustered in specific locations may present a novel therapeutic target for influencing excitability in neurologic disorders associated with some channelopathies.

Neuronal circuits in epilepsy: do they matter?

Seizures occur in groups of neurons and involve complex interactions across several regions. The focus of much epilepsy research has been on changes in single neuronal populations but the interpretation of the implications of these changes is often limited by not being able to place those observed changes appropriately in the overall function of the brain. Understanding regional interactions at the beginning and during the evolution of a seizure may help place the changes in the appropriate context of the pathophysiology of epilepsy and guide us in identifying more effective therapies. In this paper we will focus on the circuits that support the different stages of seizures. Although we are far from knowing how the system works to initiate and spread seizures, we hope to provide a framework upon which we can place cellular changes. The concepts of seizure focus, initiating seizure circuits, paths of spread and neuromodulatory centers will be used to develop a system's view of epilepsy.