Optical monitoring of neuronal activity during spontaneous sharp waves in chronically epileptic human neocortical tissue

Reactive astrocytes contribute to epileptogenesis in patients with cavernous angioma

OBJECTIVE

Patients with cavernous angioma (CA) often suffer from severe epilepsy, and surgical resection is often performed to attenuate these epileptic seizures. Several studies have suggested that surgical removal of the surrounding hemosiderin-pigmented tissues adjacent to CA achieves better seizure control than restricted lesionectomy. Pathological examination of the resected foci reveals not only hemosiderin pigmentation but also various degrees of inflammatory change, such as hemosiderin-laden macrophages, gliosis and fibrosis. However, there is some controversy regarding the epileptogenic potential of these regions due to the uncertain nature of the mechanisms contributing to these histopathological changes.

METHODS

To investigate the correlations between neuron hyperexcitability and evident pathological changes, we performed ex vivo flavoprotein fluorescence imaging using surgically resected epileptogenic foci surrounding CA. The mirror surfaces of the tissues used for the physiological experiment were also subjected to morphological examination.

RESULTS

Hemosiderin-laden macrophages and many gemistocytic astrocytes were observed in the area adjacent to CA, where horizontal spreading excitations were detected significantly more frequently. Outside these areas, we found fine granular iron deposits and only a few fibrillary astrocytes, and weakly propagating excitations were detected. Furthermore, areas of enhanced activation were more clearly correlated with the glial proliferation index than with iron deposition.

CONCLUSION

These results suggest that the epileptogenesis in patients with CA may be based on a biological process, such as alteration of glial function, rather than direct chemical reactions involving iron deposition.

Topiramate Decelerates Bicarbonate-Driven Acid-Elimination of Human Neocortical Neurons: Strategic Significance for its Antiepileptic, Antimigraine and Neuroprotective Properties

BACKGROUND

Mammalian central neurons regulate their intracellular pH (pHi) strongly and even slight pHi-fluctuations can influence inter-/intracellular signaling, synaptic plasticity and excitability.

OBJECTIVE

For the first time, we investigated topiramate´s (TPM) influence on pHi-behavior of human central neurons representing a promising target for anticonvulsants and antimigraine drugs.

METHODS

In slice-preparations of tissue resected from the middle temporal gyrus of five adults with intractable temporal lobe epilepsy, BCECF-AM-loaded neocortical pyramidal-cells were investigated by fluorometry. The pHi-regulation was estimated by using the recovery-slope from intracellular acidification after an Ammonium-Prepulse (APP).

RESULTS

Among 17 pyramidal neurons exposed to 50 μM TPM, seven (41.24%) responded with an altered resting-pHi (7.02±0.12), i.e., acidification of 0.01-0.03 pH-units. The more alkaline the neurons, the greater the TPM-related acidifications (r=0.7, p=0.001, n=17). The recovery from APPacidification was significantly slowed under TPM (p<0.001, n=5). Further experiments using nominal bicarbonate-free (n=2) and chloride-free (n=2) conditions pointed to a modulation of the HCO3 -- driven pHi-regulation by TPM, favoring a stimulation of the passive Cl-/HCO3 --antiporter (CBT) - an acid-loader predominantly in more alkaline neurons.

CONCLUSION

TPM modulated the bicarbonate-driven pHi-regulation, just as previously described in adult guinea-pig hippocampal neurons. We discussed the significance of the resulting subtle acidifications for beneficial antiepileptic, antimigraine and neuroprotective effects as well as for unwanted cognitive deficits.

Reproducibility of EEG-MEG fusion source analysis of interictal spikes: Relevance in presurgical evaluation of epilepsy

Fusion of electroencephalography (EEG) and magnetoencephalography (MEG) data using maximum entropy on the mean method (MEM-fusion) takes advantage of the complementarities between EEG and MEG to improve localization accuracy. Simulation studies demonstrated MEM-fusion to be robust especially in noisy conditions such as single spike source localizations (SSSL). Our objective was to assess the reliability of SSSL using MEM-fusion on clinical data. We proposed to cluster SSSL results to find the most reliable and consistent source map from the reconstructed sources, the so-called consensus map. Thirty-four types of interictal epileptic discharges (IEDs) were analyzed from 26 patients with well-defined epileptogenic focus. SSSLs were performed on EEG, MEG, and fusion data and consensus maps were estimated using hierarchical clustering. Qualitative (spike-to-spike reproducibility rate, SSR) and quantitative (localization error and spatial dispersion) assessments were performed using the epileptogenic focus as clinical reference. Fusion SSSL provided significantly better results than EEG or MEG alone. Fusion found at least one cluster concordant with the clinical reference in all cases. This concordant cluster was always the one involving the highest number of spikes. Fusion yielded highest reproducibility (SSR EEG = 55%, MEG = 71%, fusion = 90%) and lowest localization error. Also, using only few channels from either modality (21EEG + 272MEG or 54EEG + 25MEG) was sufficient to reach accurate fusion. MEM-fusion with consensus map approach provides an objective way of finding the most reliable and concordant generators of IEDs. We, therefore, suggest the pertinence of SSSL using MEM-fusion as a valuable clinical tool for presurgical evaluation of epilepsy.

Directional spread of activity in synaptic networks of the human lateral amygdala

Spontaneous epileptiform activity has previously been observed in lateral amygdala (LA) slices derived from patients with intractable-temporal lobe epilepsy. The present study aimed to characterize intranuclear LA synaptic connectivity and to test the hypothesis that differences in the spread of flow of neuronal activity may relate to spontaneous epileptiform activity occurrence. Electrical activity was evoked through electrical microstimulation in acute human brain slices containing the LA, signals were recorded as local field potentials combined with fast optical imaging of voltage-sensitive dye fluorescence. Sites of stimulation and recording were systematically varied. Following recordings, slices were anatomically reconstructed using two-dimensional unitary slices as a reference for coronal and parasagittal planes. Local spatial patterns and spread of activity were assessed by incorporating the coordinates of electrical and optical recording sites into the respective unitary slice. A preferential directional spread of evoked electrical signals was observed from ventral to dorsal, rostral to caudal and medial to lateral regions in the LA. No differences in spread of evoked activity were observed between spontaneously and non-spontaneously active LA slices, i.e. basic properties of evoked synaptic responses were similar in the two functional types of LA slices, including input-output relationship, and paired-pulse depression. These results indicate a directed propagation of synaptic signals within the human LA in spontaneously active epileptic slices. We suggest that the lack of differences in local and in systemic information processing has to be found in confined epileptiform circuits within the amygdala likely involving well-known "epileptic neurons".

Subiculum-entorhinal cortex interactions during in vitro ictogenesis

PURPOSE

Our aim was to establish the contribution of neuronal networks located in the entorhinal cortex (EC) and subiculum to the generation of interictal and ictal onset patterns recorded in vitro.

METHODS

We employed field potential recordings of epileptiform activity in rat brain slices induced with the application of the K(+) channel blocker 4-aminopyridine. Local connections between the EC and subiculum were severed to understand how EC-subicular circuits contribute to patterns of epileptiform synchronization.

RESULTS

First, we found that ictal discharges occurred synchronously in these two structures, initiating from either the EC or subiculum, and were characterized by low voltage fast (LVF) or sudden onsets. Second, sudden onset ictal events initiated more frequently in the EC, whereas LVF onset ictal discharges appeared more likely to initiate in the subiculum (P<0.001). In both structures, polyspike interictal discharges occurred in brain slices generating sudden onset ictal events while isolated slow interictal discharges were recorded in experiments characterized by LVF onset ictal activity. Third, severing the connections between subiculum and EC desynchronized both interictal and ictal discharges occurring in these two regions, leading to a significant decrease in ictal duration (regardless of the onset type) along with blockade of polyspike interictal activity in subiculum.

CONCLUSIONS

These findings highlight the contribution of EC-subicular interactions to epileptiform synchronization and, specifically, to ictogenesis in this in vitro model.

Combined EEG/MEG can outperform single modality EEG or MEG source reconstruction in presurgical epilepsy diagnosis

We investigated two important means for improving source reconstruction in presurgical epilepsy diagnosis. The first investigation is about the optimal choice of the number of epileptic spikes in averaging to (1) sufficiently reduce the noise bias for an accurate determination of the center of gravity of the epileptic activity and (2) still get an estimation of the extent of the irritative zone. The second study focuses on the differences in single modality EEG (80-electrodes) or MEG (275-gradiometers) and especially on the benefits of combined EEG/MEG (EMEG) source analysis. Both investigations were validated with simultaneous stereo-EEG (sEEG) (167-contacts) and low-density EEG (ldEEG) (21-electrodes). To account for the different sensitivity profiles of EEG and MEG, we constructed a six-compartment finite element head model with anisotropic white matter conductivity, and calibrated the skull conductivity via somatosensory evoked responses. Our results show that, unlike single modality EEG or MEG, combined EMEG uses the complementary information of both modalities and thereby allows accurate source reconstructions also at early instants in time (epileptic spike onset), i.e., time points with low SNR, which are not yet subject to propagation and thus supposed to be closer to the origin of the epileptic activity. EMEG is furthermore able to reveal the propagation pathway at later time points in agreement with sEEG, while EEG or MEG alone reconstructed only parts of it. Subaveraging provides important and accurate information about both the center of gravity and the extent of the epileptogenic tissue that neither single nor grand-averaged spike localizations can supply.

Coalescence of deep and superficial epileptic foci into larger discharge units in adult rat neocortex

Epilepsy is a disease of neuronal hyper-synchrony that can involve both neocortical and hippocampal brain regions. While much is known about the network properties of the hippocampus little is known of how epileptic neocortical hyper-synchrony develops. We aimed at characterizing the properties of epileptic discharges of a neocortical epileptic focus. We established a multi-electrode-array method to record the spatial patterns of epileptiform potentials in acute adult rat brain slices evoked by 4-Aminopyridine in the absence of magnesium. Locations of discharges mapped to two anatomical regions over the somatosensory cortex and over the lateral convexity separated by a gap at a location matching the dysgranular zone. Focal epileptiform discharges were recorded in superficial and deep neocortical layers but over superficial layers, they exhibited larger surface areas. They were often independent even when closely spaced to one another but they became progressively coupled resulting in larger zones of coherent discharge. The gradual coupling of multiple, independent, closely spaced, spatially restricted, focal discharges between deep and superficial neocortical layers represents a possible mechanism of the development of an epileptogenic zone.

Combining EEG and MEG for the reconstruction of epileptic activity using a calibrated realistic volume conductor model

To increase the reliability for the non-invasive determination of the irritative zone in presurgical epilepsy diagnosis, we introduce here a new experimental and methodological source analysis pipeline that combines the complementary information in EEG and MEG, and apply it to data from a patient, suffering from refractory focal epilepsy. Skull conductivity parameters in a six compartment finite element head model with brain anisotropy, constructed from individual MRI data, are estimated in a calibration procedure using somatosensory evoked potential (SEP) and field (SEF) data. These data are measured in a single run before acquisition of further runs of spontaneous epileptic activity. Our results show that even for single interictal spikes, volume conduction effects dominate over noise and need to be taken into account for accurate source analysis. While cerebrospinal fluid and brain anisotropy influence both modalities, only EEG is sensitive to skull conductivity and conductivity calibration significantly reduces the difference in especially depth localization of both modalities, emphasizing its importance for combining EEG and MEG source analysis. On the other hand, localization differences which are due to the distinct sensitivity profiles of EEG and MEG persist. In case of a moderate error in skull conductivity, combined source analysis results can still profit from the different sensitivity profiles of EEG and MEG to accurately determine location, orientation and strength of the underlying sources. On the other side, significant errors in skull modeling are reflected in EEG reconstruction errors and could reduce the goodness of fit to combined datasets. For combined EEG and MEG source analysis, we therefore recommend calibrating skull conductivity using additionally acquired SEP/SEF data.

Interictal-like network activity and receptor expression in the epileptic human lateral amygdala

While the amygdala is considered to play a critical role in temporal lobe epilepsy, conclusions on underlying pathophysiological mechanisms have been derived largely from experimental animal studies. Therefore, the present study aimed to characterize synaptic network interactions, focusing on spontaneous interictal-like activity, and the expression profile of transmitter receptors in the human lateral amygdala in relation to temporal lobe epilepsy. Electrophysiological recordings, obtained intra-operatively in vivo in patients with medically intractable temporal lobe epilepsy, revealed the existence of interictal activity in amygdala and hippocampus. For in vitro analyses, slices were prepared from surgically resected specimens, and sections from individual specimens were used for electrophysiological recordings, receptor autoradiographic analyses and histological visualization of major amygdaloid nuclei for verification of recording sites. In the lateral amygdala, interictal-like activity appeared as spontaneous slow rhythmic field potentials at an average frequency of 0.39 Hz, which occurred at different sites with various degrees of synchronization in 33.3% of the tested slices. Pharmacological blockade of glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, but not N-methyl-D-aspartate receptors, abolished interictal-like activity, while the γ-aminobutyric acid A-type receptor antagonist bicuculline resulted in a dampening of activity, followed by highly synchronous patterns of slow rhythmic activity during washout. Receptor autoradiographic analysis revealed significantly higher α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, kainate, metabotropic glutamate type 2/3, muscarinic type 2 and adrenoceptor α(1) densities, whereas muscarinergic type 3 and serotonergic type 1A receptor densities were lower in the lateral amygdala from epileptic patients in comparison to autopsy controls. Concerning γ-aminobutyric acid A-type receptors, agonist binding was unaltered whereas antagonist binding sites were downregulated in the epileptic lateral amygdala, suggesting an altered high/low-affinity state ratio and concomitant reduced pool of total γ-aminobutyric acid A-type receptors. Together these data indicate an abnormal pattern of receptor densities and synaptic function in the lateral nucleus of the amygdala in epileptic patients, involving critical alterations in glutamate and γ-aminobutyric acid receptors, which may give rise to domains of spontaneous interictal discharges contributing to seizure activity in the amygdala.

Spatiotemporal dynamics of epileptiform propagations: imaging of human brain slices

Seizure activities often originate from a localized region of the cerebral cortex and spread across large areas of the brain. The properties of these spreading abnormal discharges may account for clinical phenotypes in epilepsy patients, although the manner of their propagation and the underlying mechanisms are not well understood. In the present study we performed flavoprotein fluorescence imaging of cortical brain slices surgically resected from patients with partial epilepsy caused by various symptomatic lesions. Elicited neural activities in the epileptogenic tissue spread horizontally over the cortex momentarily, but those in control tissue taken from patients with brain tumors who had no history of epilepsy demonstrated only localized responses. Characteristically, the epileptiform propagation comprised early and late phases. When the stimulus intensity was changed gradually, the early phase showed an all-or-none behavior, whereas the late phase showed a gradual increase in the response. Moreover, the two phases were propagated through different cortical layers, suggesting that they are derived from distinct neural circuits. Morphological investigation revealed the presence of hypertrophic neurons and loss of dendritic spines, which might participate in the aberrant activities observed by flavoprotein fluorescence imaging. These findings indicate that synchronized activities of the early phase may play a key role in spreading abnormal discharges in human cortical epilepsies.

L-Type calcium channel blockade reduces network activity in human epileptic hypothalamic hamartoma tissue

Purpose

Human hypothalamic hamartomas (HHs) are associated with gelastic seizures, intrinsically epileptogenic, and notoriously refractory to medical therapy. We previously reported that the L-type calcium channel antagonist nifedipine blocks spontaneous firing and γ-aminobutyric acid (GABA)_A–induced depolarization of single cells in HH tissue slices. In this study, we examined whether blocking L-type calcium channels attenuates emergent activity of HH neuronal networks.

Methods

A high-density multielectrode array was used to record extracellular signals from surgically resected HH tissue slices. High-frequency oscillations (HFOs, ripples and fast ripples), field potentials, and multiunit activity (MUA) were studied (1) under normal and provoked [4-aminopyridine (4-AP)] conditions; and (2) following nifedipine treatment.

Key Findings

Spontaneous activity occurred during normal artificial cerebrospinal fluid (aCSF) conditions. Nifedipine reduced the total number and duration of HFOs, abolished the association of HFOs with field potentials, and increased the inter-HFO burst intervals. Notably, the number of active regions was decreased by 45 ± 9% (mean ± SEM) after nifedipine treatment. When considering electrodes that detected activity, nifedipine increased MUA in 58% of electrodes and reduced the number of field potentials in 67% of electrodes. Provocation with 4-AP increased the number of events and, as the number of electrodes that detected activity increased 248 ± 62%, promoted tissue-wide propagation of activity. During provocation with 4-AP, nifedipine effectively reduced HFOs, the association of HFOs with field potentials, field potentials, MUA, and the number of active regions, and limited propagation.

Significance

This is the first study to report (1) the presence of HFOs in human subcortical epileptic brain tissue in vitro; (2) the modulation of “pathologic” high-frequency oscillations (i.e., fast ripples) in human epileptic tissue by L-type calcium channel blockers; and (3) the modulation of network physiology and synchrony of emergent activity in human epileptic tissue following blockade of L-type calcium channels. Attenuation of activity in HH tissue during normal and provoked conditions supports a potential therapeutic usefulness of L-type calcium channel blockers in epileptic patients with HH.

Thalamic afferent activation of supragranular layers in auditory cortex in vitro: a voltage sensitive dye study

We studied auditory thalamocortical interactions in vitro, using an auditory thalamocortical brain slice preparation. Cortical activity evoked by electrical stimulation of the medial geniculate nucleus (MGN) was investigated through field potential recordings and voltage sensitive dyes. Experiments were performed in slices obtained from adult mice (9-14 weeks). Stimulus evoked activity was detected in the granular and supragranular layers after a short latency (5-6 ms). In 9-14 weeks old mice infragranular activity was detected in 10 of 24 preparations and was found to be increased in younger mice (p 31-64). In 14 of 24 slices a prominent horizontal spread was observed, which extended into cortical areas lateral to A1. In these experiments, the shortest onset latencies and largest signal amplitudes were located in the supragranular layers of A1. In areas lateral to A1, shortest onset latencies were located in the granular layer, while largest signal amplitudes were found in the supragranular layers. Evoked cortical activity was sensitive to removal of extracellular Ca(2+) or application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM). Short repetitive stimulation, resembling thalamic burst activity (three pulses at 100 Hz), resulted in an increase of signal amplitude and excited area by approximately 25%, without changing the overall spatiotemporal activity profile. Blockade of N-methyl-D-aspartate receptors by 2-amino-5-phosphonopentanoate (AP5, 50 microM) reduced amplitudes and excited area by approximately 15-30%, irrespective of stimulation frequency. Application of bicuculline (10 microM) greatly increased cortical responses to thalamic stimulation. Under these conditions, evoked activity displayed a pronounced horizontal spread in combination with a 2-3-fold increase in amplitude. In conclusion, afferent thalamic inputs primarily activate supragranular and granular layers in the auditory cortex of adult mice. This activation is predominantly mediated by non-NMDA receptors, while GABA(A) receptor-mediated inhibition limits the horizontal and vertical spread of activity.

Different regional neuroinhibitory effects of adenosine on stimulus-induced patterns of bioelectric activity of rat hippocampal and neocortical tissues

Adenosine is an inhibitory modulator of brain activity with neuroprotective and anticonvulsant properties. To investigate the distribution of bioelectric activities under application of adenosine, rat hippocampal and neocortical slices were incubated with the voltage-sensitive dye RH795 and neuronal activity was monitored using a fast-imaging photodiode array combined with standard field potential recordings. The effects of adenosine (1-50 micromol/l) on the spatial distribution of stimulus-induced activities were studied in non-epileptiform as well as epileptiform conditions. Epileptiform activity was induced by omission of Mg(2+) from the bath medium. The adenosine's inhibitory effects on the amplitude and spatial extent of stimulus-induced bioelectric activity in the hippocampus were most prominent in strata radiatum and pyramidale in both control and epileptic mediums. Adenosine's inhibitory actions were different on various layers of neocortical tissues in non-epileptiform and epileptiform conditions. Layers II and III showed the most inhibition by application of adenosine in control slices. In epileptiform medium, however, adenosine exerts significant suppressive effects only in layer I of neocortical slices. The data demonstrate a region-specific modulatory potential of adenosine on neuronal network excitability in the hippocampus and neocortex. This may be important in local adenosine therapy in epilepsy.

Hypothalamic hamartoma: basic mechanisms of intrinsic epileptogenesis

The hypothalamic hamartoma (HH) is a rare developmental malformation commonly associated with gelastic seizures that are notoriously refractory to medical therapy. Recent evidence supports the intrinsic seizure propensity of HH. Despite increasing clinical recognition of this condition, the mechanisms of seizure genesis in HH tissue remain unclear. This review summarizes the histochemical and electrophysiological properties of HH neurons, and relates these findings to those characteristics identified in other types of epileptic tissue. Initial studies have revealed two distinct populations of neurons in surgically resected HH tissue. One group consisted of small gamma-aminobutyric acid (GABA)-expressing neurons that occurred principally in clusters and displayed spontaneous rhythmic firing. The second group was composed of large, quiescent, pyramidal-like neurons with more extensive dendritic and axonal arborization. We propose that the small, spontaneously firing GABAergic neurons send inhibitory projections to and drive the synchrony of large output neurons. These observations constitute the basis for future investigations aimed at elucidating the mechanisms of subcortical epileptogenesis.

Methodological approaches to exploring epileptic disorders in the human brain in vitro

Brain surgery, and in particular epilepsy surgery, offers the unique opportunity to study viable human central nervous tissue in vitro. This does not only open a window to address the basic mechanisms underlying human disease, such as epilepsy, but it allows to venture into investigating neurophysiological functions per se. In the present paper, we describe the most commonly used methods in the electrophysiological (and, at least to some extent, also histochemical and molecular) analysis of human tissue in vitro. In addition, we consider the pitfalls and limitations of such studies, in particular regarding the issue of tissue sampling procedures and control experiments.

Cellular and molecular mechanisms of epilepsy in the human brain

Animal models have provided invaluable data for identifying the pathogenesis of epileptic disorders. Clearly, the relevance of these experimental findings would be strengthened by the demonstration that similar fundamental mechanisms are at work in the human epileptic brain. Epilepsy surgery has indeed opened the possibility to directly study the functional properties of human brain tissue in vitro, and to analyze the mechanisms underlying seizures and epileptogenesis. Here, we summarize the findings obtained over the last 40 years from electrophysiological, histochemical and molecular experiments made with the human brain tissue. In particular, this review will focus on (i) the synaptic and non-synaptic properties of neocortical neurons along with their ability to produce synchronous activity; (ii) the anatomical and functional alterations that characterize limbic structures in patients presenting with mesial temporal lobe epilepsy; (iii) the issue of antiepileptic drug action and resistance; and (iv) the pathophysiology of seizure genesis in Taylor's type focal cortical dysplasia. Finally, we will address some of the problems that are inherent to this type of experimental approach, in particular the lack of proper controls and possible strategies to obviate this limitation.

Epilepsy surgery: perioperative investigations of intractable epilepsy

Recent advances in our understanding of the basic mechanisms of epilepsy have derived, to a large extent, from increasing ability to carry out detailed studies on patients surgically treated for intractable epilepsy. Clinical and experimental perioperative studies divide into three different phases: before the surgical intervention (preoperative studies), on the intervention itself (intraoperative studies), and on the period when the part of the brain that has to be removed is available for further investigations (postoperative studies). Before surgery, both structural and functional neuroimaging techniques, in addition to their diagnostic roles, could be used to investigate the pathophysiological mechanisms of seizure attacks in epileptic patients. During epilepsy surgery, it is possible to insert microdialysis catheters and electroencephalogram electrodes into the brain tissues in order to measure constituents of extracellular fluid and record the bioelectrical activity. Subsequent surgical resection provides tissue that can be used for electrophysiological, morphological, and molecular biological investigations. To take full advantage of these opportunities, carefully designed experimental protocols are necessary to compare the data from different phases and characterize abnormalities in the human epileptic brain.

Intrinsic excitability, synaptic potentials, and short-term plasticity in human epileptic neocortex

Although studies of epileptic human hippocampus suggest changes of synaptic and intrinsic excitability, few changes, save the appearance of spontaneous field/synaptic potentials, are known in epileptic neocortical tissue. However, invasive EEG and histological studies suggest that neocortical tissue, even in mesial temporal lobe epilepsy, can play an important role as an irritative zone or extrahippocampal focus. We hypothesized that intrinsic neuronal and synaptic excitability, as well as short-term plasticity, are altered in neocortical areas, particularly with elevated K+ levels as occur during seizures. We analyzed neuronal firing properties, synaptic responses, and paired-pulse plasticity in human neocortical slices from tissue resected during epilepsy surgery, both under normal and under pathological conditions, i.e., after elevating K+ (4/8 mM), with rat neocortical slices as controls. Neuronal firing properties were not different. We did find, however, alterations of synaptic responsiveness in epileptic tissue, i.e., an elevated network excitability with K+ elevations, and reduction of paired-pulse depression.

Optical imaging of epileptiform activity in experimentally induced cortical malformations

Electrophysiological studies of human cortical dysplasia and rodent models revealed widespread hyperexcitability in the malformation itself as well as in its vicinity. We here analyzed the initiation of paroxysmal epileptiform activity using optical imaging of neuronal activity in rats with cortical malformations induced by neonatal freeze lesions. Brain slice preparations were incubated with the voltage-sensitive dye RH795 and neuronal activity was monitored using a fast-imaging photodiode array combined with standard field potential recordings. Spontaneous paroxysmal epileptiform activity emerged in all slices from animals with cortical malformations and sham-operated controls 20-40 min after omission of extracellular Mg(2+). Following electrophysiological and optical recordings, slices were histochemically processed. Using this approach, the present study demonstrated that in animals with freeze-lesion-induced focal cortical malformations, paroxysmal epileptiform activity always emerged from the dysplastic cortex and then spread to adjacent areas through superficial layers. This distribution of initiation sites was significantly different to sham-operated controls in which epileptogenic foci were located in various cytoarchitectonic areas. The present study indicates that following global changes in excitability, the dysplastic cortex itself is the main initiation site of paroxysmal epileptiform activity in animals with focal cortical malformations.

Heat Shock Protein-27 Is Upregulated in the Temporal Cortex of Patients with Epilepsy

PURPOSE

Heat shock protein-27 (HSP-27) belongs to the group of small heat shock proteins that become induced in response to various pathologic conditions. HSP-27 has been shown to protect cells and subcellular structures, particularly mitochondria, and serves as a carrier for estradiol. It is a reliable marker for tissues affected by oxidative stress. Oxidative stress and related cellular defence mechanisms are currently thought to play a major role during experimentally induced epileptic neuropathology. We addressed the question whether HSP-27 becomes induced in the neocortex resected from patients with pharmacoresistant epilepsy.

METHODS

Human epileptic temporal neocortex was obtained during neurosurgery, and control tissue was obtained at autopsy from subjects without known neurologic diseases. The tissues were either frozen for Western blot analysis or fixed in Zamboni's fixative for the topographic detection of HSP-27 at the cellular level by means of immunohistochemistry.

RESULTS

HSP-27 was highly expressed in all epilepsy specimens and in the cortex of a patient who died in the final stage of multiple sclerosis (positive control), whereas only low amounts of HSP-27 were detectable in control brains. In epilepsy patients, HSP-27 was present in astrocytes and in the walls of blood vessels. The intracortical distribution patterns varied strongly among the epilepsy specimens.

CONCLUSIONS

These results demonstrate that HSP-27 becomes induced in response to epileptic pathology. Although the functional aspects of HSP-27 induction during human epilepsy have yet to be elucidated, it can be concluded that HSP-27 is a marker for cortical regions in which a stress response has been caused by seizures.

Spreading depression enhances the spontaneous epileptiform activity in human neocortical tissues

Spreading depression (SD) is a well-known phenomenon in animal models of experimental epilepsy. However, the interaction of SD with epileptiform activity in human neuronal tissues is not clear. The aim of the present study was to investigate the effect of SD on synchronous rhythmic sharp field potentials in human neocortical slices. Spreading depression was elicited in human neocortical slices that exhibited sharp potentials. Extracellular field potentials were recorded from the third and fifth layers. SD significantly enhanced the repetition rate and amplitude of spontaneous rhythmic potentials in all tested slices. The results indicate that SD may facilitate the synchronization of different foci of rhythmic sharp field potentials and increase the excitability in human brain tissue.

Physiology of human cortical neurons adjacent to cavernous malformations and tumors

PURPOSE

Focal neocortical seizures can be associated with a number of specific pathologies including supratentorial tumors and cavernous malformations (CMs), both of which are highly epileptogenic.

METHODS

To begin to address the question of whether these lesions have different mechanisms of epileptogenesis, we used intracellular recordings from neurons adjacent to intracerebral neoplasms and cerebral CMs. Developmental anomalies were not included in this study.

RESULTS

Neurons adjacent to CMs had a greater propensity to show large (>5 mV), complex spontaneous synaptic events than did neurons neighboring neoplastic substrates (50 vs. 4.7% of cells and 75 and 8% of patients, respectively; p < 0.004; p < 0.05). Both spontaneous excitatory and inhibitory events were noted. In contrast, in tissue adjacent to tumors, low-amplitude (<3 mV) spontaneous excitatory activity predominated. Neurons neighboring CMs also exhibited more excitable responses to synaptic stimulation, with multiple action potentials riding on prolonged excitatory postsynaptic potentials (EPSPs) being evoked in 71% of these cells, versus 32% of cells from the tumor group; p < 0.05. In studies using hippocampal tissue, we noted a similar pattern of spontaneous activity in tissue adjacent to CMs.

CONCLUSIONS

These data suggest that CMs may induce seizure activity via a different pathophysiologic mechanism(s) than glial tumors.

Stimulus-induced patterns of bioelectric activity in human neocortical tissue recorded by a voltage sensitive dye

Stimulus-induced pattern of bioelectric activity in human neocortical tissue was investigated by use of the voltage sensitive dye RH795 and a fast optical recording system. During control conditions stimulation of layer I evoked activity predominantly in supragranular layers showing a spatial extent of up to 3000 microm along layer III. Stimulation in white matter evoked distinct activity in infragranular layers with a spatial extent of up to 3000 microm measured along layer V. The mean amplitude of optical signals close to the stimulated sites in layer I and white matter determined 25 ms following the stimulus, decreased by 50% at a lateral distance of approximately 900 microm and 1200 microm, respectively. Velocity of spread along the vertical stimulation axis reached 0.24 m/s in the supragranular layers (layers I to III) and then decreased to 0.09 m/s following layer I activation; stimulation of white matter induced a velocity of spread in layer V of 0.38 m/s, which slowed down to 0.12 m/s when passing the lower border of lamina IV. The horizontal velocities of spread determined from the stimulation site to a lateral distance of 500 microm reached 0.26-0.28 m/s and 0.28-0.35 m/s for layer I and white matter stimulation, respectively. At larger distances velocity of spread decreased. Increased excitability (Mg(2+)-free solution) had no significant effect on the spatio-temporal distribution of evoked activity as compared with control conditions. There were also no obvious differences between the results obtained in slices, which generated spontaneously sharp waves and those which were not spontaneously active. About 30% of the slices (n=7) displayed a greatly different response pattern, which seemed not to be related in a simple way to the stimulation as was the case in the majority of the investigated slices. The activity pattern of those slices appeared atypical in regard to their deviations of the vertical and horizontal extent of activity, to their reduced spatial extent of activity during increased excitability, to their layer-related distribution of activity, and to the appearance of afterdischarges.Concluding, in 30% of the human temporal lobe slices atypical activity pattern occurred which obviously reflect intrinsic epileptiform properties of the resected tissue. The majority of slices showed stereotyped activity pattern without evidence for increased excitability.

Blockade of astrocyte metabolism causes delayed excitation as revealed by voltage-sensitive dyes in mouse brainstem slices

Fluoroacetate is known to block cell metabolism and to change potassium conductances selectively in astrocytes. In a functional neuronal network with ongoing activity, we investigated the effects of such a blockade of the astrocytic metabolism by fluoroacetate on neuronal signal propagation. Transverse 400- microm slices were prepared from the caudal medulla of mice of postnatal day 3-8, which contained the hypoglossal nucleus receiving excitatory synaptic input from the ventral respiratory group. Propagation of excitation within this network was measured by optical imaging using the voltage-sensitive dye RH 795. A 464-element photodiode array allowed fast recordings of voltage changes within a small population of cells. The spatial and temporal resolution was advanced to 32 microm and 1.27 ms, respectively. Changes of cellular membrane potential levels were expressed as relative changes of fluorescence (DeltaI/I). Stimulus-evoked excitation of neurons propagating from the ventral respiratory group to the hypoglossal nucleus peaked after 7.2+/-0.6 ms ( n=6). The latency of this early excitatory response is consistent with the time course of stimulus-evoked EPSPs in whole-cell recordings. Mean changes of fluorescence in the hypoglossal nucleus were -2.1+/-0.5 x 10(-3) (DeltaI/I). After incubation in 1 mM fluoroacetate, the early depolarization was reduced to 69.1+/-9.8% of control ( n=6, p=0.034). Additionally, fluoroacetate induced a delayed excitatory response, such that fluorescence intensity did not return to baseline within 1s. Propagation velocity and spatial distribution of the voltage signal were not affected by fluoroacetate. Our results suggest that blockade of astrocyte metabolism impairs fast synaptic transmission and induces a delayed excitation, probably resulting from the combination of reduced repolarization of neurons and persistent depolarization of astrocytes.

Optical analysis of acute spontaneous epileptiform discharges in the in vivo rat cerebral cortex

We examined the spatiotemporal patterns of spontaneous epileptiform activity observed in the in vivo rat cerebral cortex using an optical recording technique of detecting transmembrane voltage changes. The surface of the cerebral cortex was exposed under anesthesia and stained with a fluorescent voltage-sensitive dye, RH414. Acute spontaneous epileptiform discharges were induced by application of a GABA(A) receptor antagonist, bicuculline. Changes in the intensity of fluorescence were recorded from the cerebral cortex using a 464-channel optic fiber photodiode system. We succeeded in recording spontaneous epileptiform discharges, and constructed their initiation-site maps. We found that the initiation site was neither unique nor randomly located, but exhibited a multimodal distribution pattern. The incidence of epileptiform discharges was different between the initiation sites, and some sites showed dominance in the induction of spontaneous epileptiform discharges.

Spatio-temporal patterns of neuronal activity: analysis of optical imaging data using geometric shape matching

Optical imaging of neuronal network activity yields information of spatial dynamics which generally is analyzed visually. The transient appearance of spatial activity patterns is difficult to gauge in a quantifiable manner, or may even altogether escape detection. Here, we employ geometric shape matching using Fréchet distances or straight skeletons to search for pre-selected patterns in optical imaging data with adjustable degrees of tolerance. Data were sampled from fluorescence changes of a voltage-sensitive dye recorded with a 464-photodiode array. Fluorescence was monitored in a neuronal network in vitro. Neuronal activity prompting fluorescence fluctuations consisted of spontaneous epileptiform discharges in neocortical slices from patients undergoing epilepsy surgery. The experiments show that: (a) spatial activity patterns can be detected in optical imaging data; (b) shapes such as "mini-foci" appear in close correlation to bioelectric discharges monitored with field potential electrodes in a reproducible manner; (c) Fréchet distances yield more conservative matches regarding rectangular, and less conservative hits with respect to radially symmetric shapes than the straight skeleton approach; and (d) tolerances of 0.03-0.1 are suited to detect faithful images of pre-selected shapes, whereas values >0.8 will report matches with any polygonal pattern. In conclusion, the methods reported here are suited to detect and analyze spatial, geometric dynamics in optical imaging data.

Effects of retigabine on rhythmic synchronous activity of human neocortical slices

The antiepileptic effects of the novel antiepileptic drug retigabine (D-23129) [N-(2-amino-4-(4-flurobenzylamino)phenyl) carbamid acid ethyl ester] were tested in neocortical slice preparations (n=23) from 17 patients (age, 3-42 years) who underwent surgery for the treatment of intractable epilepsy. Epileptiform events consisted of spontaneously occurring rhythmic sharp waves, as well as of epileptiform field potentials (EFP) elicited by superfusion with Mg(2+)-free solution without or with addition of 10 micromol/l bicuculline. (1) Spontaneous rhythmic sharp waves (n=6), with retigabine application, the repetition rate was decreased down to 12-47% of initial value (10 micromol/l, n=3) after 180 min or suppressed completely within 12 min (50 micromol/l, n=3). (2) Low Mg(2+) EFP (n=9), with retigabine application, the repetition rate was decreased down to 50 and 65% of initial value (10 micromol/l; n=2) after 180 min or suppressed completely after 9-55 min (10, 50 and 100 micromol/l; n=2 in each case). In one slice only a transient reduction of the repetition rate was seen with 10 micromol/l retigabine. (3) Low Mg(2+) EFP with addition of bicuculline (n=8), with retigabine application, the repetition rate was decreased down to 12-55% of initial value (10 micromol/l; n=4) after 180 min or suppressed completely after 6-30 min (50 and 100 micromol/l; n=2 in each case). The depressive effect of retigabine was reversible in all but one slice. The results show a clear antiepileptic effect of retigabine in human neocortical slices on spontaneously occurring rhythmic sharp waves and different types of induced seizure activity.

[Neurofunctional tests in presurgical strategies for partial epilepsies]

The presurgical evaluation of drug-resistant partial epilepsies primarily relies on two major investigations, including a long term video-EEG monitoring which aimed at recording the patient's typical seizures, and a specifically designed high quality magnetic resonance imaging (MRI). The latter demonstrates an abnormality within the epileptogenic lobe in the majority of cases, which might not, however, necessarily match the epileptogenic zone. Numerous functional neuro-imaging techniques have been progressively added to the pre-surgical evaluation of refractory partial epilepsies, such as the study of cerebral glucose metabolism, benzodiazepine receptor availability, and methionine incorporation using positron emission tomography (PET), the evaluation of ictal cerebral blood flow changes using single photon emission computerized tomography (SPECT), the measurement of N-acetyl-aspartate concentration with magnetic resonance spectroscopy, and the mapping of eloquent areas using functional MRI. These investigations can help to confirm the origin of seizure onset previously suggested by MRI and electro-clinical data, and provide independent prognostic information regarding the chance of a successful surgical treatment. Moreover, functional neuro-imaging data can have a critical diagnostic value when MRI is strictly normal or shows multifocal abnormalities. However, the variety and rapid evolution of functional neuro-imaging techniques makes it difficult to propose a standard protocol. Finally, it remains mandatory to proceed to an intracranial EEG investigation in a substantial number of patients, including the majority of those suffering from an extra-temporal epilepsy.