The lateral spread of ictal discharges in neocortical brain slices

Ictal wavefront propagation in slices and simulations with conductance-based refractory density model

The mechanisms determining ictal discharge (ID) propagation are still not clear. In the present study, we aimed to examine these mechanisms in animal and mathematical models of epileptiform activity. Using double-patch and extracellular potassium ion concentration recordings in rat hippocampal-cortical slices, we observed that IDs moved at a speed of about 1 mm/s or less. The mechanisms of such slow propagation have been studied with a mathematical, conductance-based refractory density (CBRD) model that describes the GABA- and glutamatergic neuronal populations’ interactions and ion dynamics in brain tissue. The modeling study reveals two main factors triggerring IDs: (i) increased interneuronal activity leading to chloride ion accumulation and a consequent depolarizing GABAergic effect and (ii) the elevation of extracellular potassium ion concentration. The local synaptic transmission followed by local potassium ion extrusion and GABA receptor-mediated chloride ion accumulation underlies the ID wavefront’s propagation. In contrast, potassium ion diffusion in the extracellular space is slower and does not affect ID’s speed. The short discharges, constituting the ID, propagate much faster than the ID front. The accumulation of sodium ions inside neurons due to their hyperactivity and glutamatergic currents boosts the Na⁺/K⁺ pump, which terminates the ID. Knowledge of the mechanism of ID generation and propagation contributes to the development of new treatments against epilepsy.

During an epileptic seizure, neuronal excitation spreads across the brain tissue and is accompanied by significant changes in ionic concentrations. Ictal discharge front spreads at low speeds, less than 1 mm/s. Mechanisms underlying this phenomenon are not yet well understood. We study these mechanisms using electrophysiological recordings in brain slices and computer simulations. Our detailed biophysical model describing neuronal populations’ interaction, spatial propagation, and ionic dynamics reproduces the generation and propagation of spontaneously repeating ictal discharges. The simulations are consistent with our recordings of the electrical activity and the extracellular potassium ion concentration. We distinguished between the two alternative mechanisms of the ictal wavefront propagation: (i) the diffusion of potassium ions released from excited neurons, which depolarizes distant neurons and thus supports excitation, and (ii) the axonal spread of excitation followed by the local extracellular potassium ion accumulation that supports the excitation. Our simulations provide evidence in favor of the latter mechanism. Our experiment-based modeling contributes to a mathematical description of brain tissue functioning and potentially contributes to developing new treatments against epilepsy.

Evidence for spreading seizure as a cause of theta-alpha activity electrographic pattern in stereo-EEG seizure recordings

Intracranial electroencephalography is a standard tool in clinical evaluation of patients with focal epilepsy. Various early electrographic seizure patterns differing in frequency, amplitude, and waveform of the oscillations are observed. The pattern most common in the areas of seizure propagation is the so-called theta-alpha activity (TAA), whose defining features are oscillations in the θ - α range and gradually increasing amplitude. A deeper understanding of the mechanism underlying the generation of the TAA pattern is however lacking. In this work we evaluate the hypothesis that the TAA patterns are caused by seizures spreading across the cortex. To do so, we perform simulations of seizure dynamics on detailed patient-derived cortical surfaces using the spreading seizure model as well as reference models with one or two homogeneous sources. We then detect the occurrences of the TAA patterns both in the simulated stereo-electroencephalographic signals and in the signals of recorded epileptic seizures from a cohort of fifty patients, and we compare the features of the groups of detected TAA patterns to assess the plausibility of the different models. Our results show that spreading seizure hypothesis is qualitatively consistent with the evidence available in the seizure recordings, and it can explain the features of the detected TAA groups best among the examined models.

The Effect of Low Magnesium Concentration on Ictal Discharges In A Non-Synaptic Model

Magnesium (Mg[Formula: see text] is an essential mineral for several cellular functions. The concentration of this ion below the physiological concentration induces recurrent neuronal discharges both in slices of the hippocampus and in neuronal cultures. These epileptiform discharges are initially sensitive to the application of [Formula: see text]-methyl-D-aspartate (NMDA) receptor antagonists, but these antagonists may lose their effectiveness with prolonged exposure to low [Mg[Formula: see text]], when extracellular Ca[Formula: see text] reduction occurs, typical of ictal periods, indicating the absence of synaptic connections. The study herein presented aimed at investigating the effect of reducing the [Mg[Formula: see text]] during the induction of Nonsynaptic Epileptiform Activities (NSEA). As an experimental protocol, NSEA were induced in rat hippocampal dentate gyrus (DG), using a bath solution containing high-K[Formula: see text] and zero-added-Ca[Formula: see text]. Additionally, computer simulations were performed using a mathematical model that represents electrochemical characteristics of the tissue of the DG granular layer. The experimental results show that the reduction of [Mg[Formula: see text]] causes an increase in the duration of the ictal period and a reduction in the interictal period, intensifying epileptiform discharges. The computer simulations suggest that the reduction of the Mg[Formula: see text] level intensifies the epileptiform discharges by a joint effect of reducing the surface charge screening and reducing the activity of the Na/K pump.

A simple model of epileptic seizure propagation: Potassium diffusion versus axo-dendritic spread

The mechanisms of epileptic discharge generation and spread are not yet fully known. A recently proposed simple biophysical model of interictal and ictal discharges, Epileptor-2, reproduces well the main features of neuronal excitation and ionic dynamics during discharge generation. In order to distinguish between two hypothesized mechanisms of discharge propagation, we extend the model to the case of two-dimensional propagation along the cortical neural tissue. The first mechanism is based on extracellular potassium diffusion, and the second is the propagation of spikes and postsynaptic signals along axons and dendrites. Our simulations show that potassium diffusion is too slow to reproduce an experimentally observed speed of ictal wavefront propagation (tenths of mm/s). By contrast, the synaptic mechanism predicts well the speed and synchronization of the pre-ictal bursts before the ictal front and the afterdischarges in the ictal core. Though this fact diminishes the role of diffusion and electrodiffusion, the model nevertheless highlights the role of potassium extrusion during neuronal excitation, which provides a positive feedback that changes at the ictal wavefront the balance of excitation versus inhibition in favor of excitation. This finding may help to find a target for a treatment to prevent seizure propagation.

Ex vivo multi-electrode analysis reveals spatiotemporal dynamics of ictal behavior at the infiltrated margin of glioma

The purpose of this study is to develop a platform in which the cellular and molecular underpinnings of chronic focal neocortical lesional epilepsy can be explored and use it to characterize seizure-like events (SLEs) in an ex vivo model of infiltrating high-grade glioma. Microelectrode arrays were used to study electrophysiologic changes in ex vivo acute brain slices from a PTEN/p53 deleted, PDGF-B driven mouse model of high-grade glioma. Electrode locations were co-registered to the underlying histology to ascertain the influence of the varying histologic landscape on the observed electrophysiologic changes. Peritumoral, infiltrated, and tumor sites were sampled in tumor-bearing slices. Following the addition of zero Mg²⁺ solution, all three histologic regions in tumor-bearing slices showed significantly greater increases in firing rates when compared to the control sites. Tumor-bearing slices demonstrated increased proclivity for SLEs, with 40 events in tumor-bearing slices and 5 events in control slices (p-value = .0105). Observed SLEs were characterized by either low voltage fast (LVF) onset patterns or short bursts of repetitive widespread, high amplitude low frequency discharges. Seizure foci comprised areas from all three histologic regions. The onset electrode was found to be at the infiltrated margin in 50% of cases and in the peritumoral region in 36.9% of cases. These findings reveal a landscape of histopathologic and electrophysiologic alterations associated with ictogenesis and spread of tumor-associated seizures.

Acute Focal Seizures Start As Local Synchronizations of Neuronal Ensembles

Understanding seizure formation and spread remains a critical goal of epilepsy research. We used fast in vivo two-photon calcium imaging in male mouse neocortex to reconstruct, with single-cell resolution, the dynamics of acute (4-aminopyridine) focal cortical seizures as they originate within a spatially confined seizure initiation site (intrafocal region), and subsequently propagate into neighboring cortical areas (extrafocal region). We find that seizures originate as local neuronal ensembles within the initiation site. This abnormal hyperactivity engages increasingly larger areas in a saltatory fashion until it breaks into neighboring cortex, where it proceeds smoothly and is then detected electrophysiologically (LFP). Interestingly, PV inhibitory interneurons have spatially heterogeneous activity in intrafocal and extrafocal territories, ruling out a simple role of inhibition in seizure formation and spread. We propose a two-step model for the progression of focal seizures, where neuronal ensembles activate first, generating a microseizure, followed by widespread neural activation in a traveling wave through neighboring cortex during macroseizures.SIGNIFICANCE STATEMENT We have used calcium imaging in mouse sensory cortex in vivo to reconstruct the onset of focal seizures elicited by local injection of the chemoconvulsant 4-aminopyridine. We demonstrate at cellular resolution that acute focal seizures originate as increasingly synchronized local neuronal ensembles. Because of its spatial confinement, this process may at first be undetectable even by nearby LFP electrodes. Further, we establish spatial footprints of local neural subtype activity that correspond to consecutive steps of seizure microprogression. Such footprints could facilitate determining the recording location (e.g., inside/outside an epileptogenic focus) in high-resolution studies, even in the absence of a priori knowledge about where exactly a seizure started.

Divergent paths to seizure-like events

Much debate exists about how the brain transitions into an epileptic seizure. One source of confusion is that there are likely to be critical differences between experimental seizure models. To address this, we have compared the evolving activity patterns in two widely used in vitro models of epileptic discharges. Brain slices from young adult mice were prepared in the same way and bathed either in 0 Mg2+ or 100 µmol/L 4AP artificial cerebrospinal fluid. We have found that while local field potential recordings of epileptiform discharges in the two models appear broadly similar, patch-clamp analysis reveals an important difference in the relative degree of glutamatergic involvement. 4AP affects parvalbumin-expressing interneurons more than other cortical populations, destabilizing their resting state and inducing spontaneous bursting behavior. Consequently, the most prominent pattern of transient discharge ("interictal event") in this model is almost purely GABAergic, although the transition to seizure-like events (SLEs) involves pyramidal recruitment. In contrast, interictal discharges in 0 Mg2+ are only maintained by a very large glutamatergic component that also involves transient discharges of the interneurons. Seizure-like events in 0 Mg2+ have significantly higher power in the high gamma frequency band (60-120Hz) than these events do in 4AP, and are greatly delayed in onset by diazepam, unlike 4AP events. We, therefore, conclude that the 0 Mg2+ and 4AP models display fundamentally different levels of glutamatergic drive, demonstrating how ostensibly similar pathological discharges can arise from different sources. We contend that similar interpretative issues will also be relevant to clinical practice.

Role of paroxysmal depolarization in focal seizure activity

We analyze the role of inhibition in sustaining focal epileptic seizure activity. We review ongoing seizure activity at the mesoscopic scale that can be observed with microelectrode arrays as well as at the macroscale of standard clinical EEG. We provide clinical, experimental, and modeling data to support the hypothesis that paroxysmal depolarization (PD) is a critical component of the ictal machinery. We present dual-patch recordings in cortical cultures showing reduced synaptic transmission associated with presynaptic occurrence of PD, and we find that the PD threshold is cell size related. We further find evidence that optically evoked PD activity in parvalbumin neurons can promote propagation of neuronal excitation in neocortical networks in vitro. Spike sorting results from microelectrode array measurements around ictal wave propagation in human focal seizures demonstrate a strong increase in putative inhibitory firing with an approaching excitatory wave, followed by a sudden reduction of firing at passage. At the macroscopic level, we summarize evidence that this excitatory ictal wave activity is strongly correlated with oscillatory activity across a centimeter-sized cortical network. We summarize Wilson-Cowan-type modeling showing how inhibitory function is crucial for this behavior. Our findings motivated us to develop a network motif of neurons in silico, governed by a reduced version of the Hodgkin-Huxley formalism, to show how feedforward, feedback, PD, and local failure of inhibition contribute to observed dynamics across network scales. The presented multidisciplinary evidence suggests that the PD not only is a cellular marker or epiphenomenon but actively contributes to seizure activity.NEW & NOTEWORTHY We present mechanisms of ongoing focal seizures across meso- and macroscales of microelectrode array and standard clinical recordings, respectively. We find modeling, experimental, and clinical evidence for a dual role of inhibition across these scales: local failure of inhibition allows propagation of a mesoscopic ictal wave, whereas inhibition elsewhere remains intact and sustains macroscopic oscillatory activity. We present evidence for paroxysmal depolarization as a mechanism behind this dual role of inhibition in shaping ictal activity.

Feedforward inhibition ahead of ictal wavefronts is provided by both parvalbumin- and somatostatin-expressing interneurons

Key points

There is a rapid interneuronal response to focal activity in cortex, which restrains laterally propagating activity, including spreading epileptiform activity.

The interneuronal response involves intense activation of both parvalbumin‐ and somatostatin‐expressing interneurons.

Interneuronal bursting is time‐locked to glutamatergic barrages in the pre‐ictal period.

Ca²⁺ imaging using conditional expression of GCaMP6f provides an accurate readout of the evolving firing patterns in both types of interneuron.

The activation profiles of the two interneuronal classes are temporally offset, with the parvalbumin population being activated first, and typically, at higher rates.

Abstract

Previous work has described powerful restraints on laterally spreading activity in cortical networks, arising from a rapid feedforward interneuronal response to focal activity. This response is particularly prominent ahead of an ictal wavefront. Parvalbumin‐positive interneurons are considered to be critically involved in this feedforward inhibition, but it is not known what role, if any, is provided by somatostatin‐expressing interneurons, which target the distal dendrites of pyramidal cells. We used a combination of electrophysiology and cell class‐specific Ca²⁺ imaging in mouse brain slices bathed in 0 Mg²⁺ medium to characterize the activity profiles of pyramidal cells and parvalbumin‐ and somatostatin‐expressing interneurons during epileptiform activation. The GCaMP6f signal strongly correlates with the level of activity for both interneuronal classes. Both interneuronal classes participate in the feedfoward inhibition. This contrasts starkly with the pattern of pyramidal recruitment, which is greatly delayed. During these barrages, both sets of interneurons show intense bursting, at rates up to 300Hz, which is time‐locked to the glutamatergic barrages. The activity of parvalbumin‐expressing interneurons appears to peak early in the pre‐ictal period, and can display depolarizing block during the ictal event. In contrast, somatostatin‐expressing interneuronal activity peaks significantly later, and firing persists throughout the ictal events. Interictal events appear to be very similar to the pre‐ictal period, albeit with slightly lower firing rates. Thus, the inhibitory restraint arises from a coordinated pattern of activity in the two main classes of cortical interneurons.

There is a rapid interneuronal response to focal activity in cortex, which restrains laterally propagating activity, including spreading epileptiform activity.

The interneuronal response involves intense activation of both parvalbumin‐ and somatostatin‐expressing interneurons.

Interneuronal bursting is time‐locked to glutamatergic barrages in the pre‐ictal period.

Ca²⁺ imaging using conditional expression of GCaMP6f provides an accurate readout of the evolving firing patterns in both types of interneuron.

The activation profiles of the two interneuronal classes are temporally offset, with the parvalbumin population being activated first, and typically, at higher rates.

Multivariate regression methods for estimating velocity of ictal discharges from human microelectrode recordings

OBJECTIVE

Epileptiform discharges, an electrophysiological hallmark of seizures, can propagate across cortical tissue in a manner similar to traveling waves. Recent work has focused attention on the origination and propagation patterns of these discharges, yielding important clues to their source location and mechanism of travel. However, systematic studies of methods for measuring propagation are lacking.

APPROACH

We analyzed epileptiform discharges in microelectrode array recordings of human seizures. The array records multiunit activity and local field potentials at 400 micron spatial resolution, from a small cortical site free of obstructions. We evaluated several computationally efficient statistical methods for calculating traveling wave velocity, benchmarking them to analyses of associated neuronal burst firing.

MAIN RESULTS

Over 90% of discharges met statistical criteria for propagation across the sampled cortical territory. Detection rate, direction and speed estimates derived from a multiunit estimator were compared to four field potential-based estimators: negative peak, maximum descent, high gamma power, and cross-correlation. Interestingly, the methods that were computationally simplest and most efficient (negative peak and maximal descent) offer non-inferior results in predicting neuronal traveling wave velocities compared to the other two, more complex methods. Moreover, the negative peak and maximal descent methods proved to be more robust against reduced spatial sampling challenges. Using least absolute deviation in place of least squares error minimized the impact of outliers, and reduced the discrepancies between local field potential-based and multiunit estimators.

SIGNIFICANCE

Our findings suggest that ictal epileptiform discharges typically take the form of exceptionally strong, rapidly traveling waves, with propagation detectable across millimeter distances. The sequential activation of neurons in space can be inferred from clinically-observable EEG data, with a variety of straightforward computation methods available. This opens possibilities for systematic assessments of ictal discharge propagation in clinical and research settings.

Reliable and Elastic Propagation of Cortical Seizures In Vivo

Mapping the fine-scale neural activity that underlies epilepsy is key to identifying potential control targets of this frequently intractable disease. Yet, the detailed in vivo dynamics of seizure progression in cortical microcircuits remain poorly understood. We combine fast (30-Hz) two-photon calcium imaging with local field potential (LFP) recordings to map, cell by cell, the spread of locally induced (4-AP or picrotoxin) seizures in anesthetized and awake mice. Using single-layer and microprism-assisted multilayer imaging in different cortical areas, we uncover reliable recruitment of local neural populations within and across cortical layers, and we find layer-specific temporal delays, suggesting an initial supra-granular invasion followed by deep-layer recruitment during lateral seizure spread. Intriguingly, despite consistent progression pathways, successive seizures show pronounced temporal variability that critically depends on GABAergic inhibition. We propose an epilepsy circuit model resembling an elastic meshwork, wherein ictal progression faithfully follows preexistent pathways but varies flexibly in time, depending on the local inhibitory restraint.

Slow Spatial Recruitment of Neocortex during Secondarily Generalized Seizures and Its Relation to Surgical Outcome

Understanding the spatiotemporal dynamics of brain activity is crucial for inferring the underlying synaptic and nonsynaptic mechanisms of brain dysfunction. Focal seizures with secondary generalization are traditionally considered to begin in a limited spatial region and spread to connected areas, which can include both pathological and normal brain tissue. The mechanisms underlying this spread are important to our understanding of seizures and to improve therapies for surgical intervention. Here we study the properties of seizure recruitment-how electrical brain activity transitions to large voltage fluctuations characteristic of spike-and-wave seizures. We do so using invasive subdural electrode arrays from a population of 16 patients with pharmacoresistant epilepsy. We find an average delay of ∼30 s for a broad area of cortex (8 × 8 cm) to be recruited into the seizure, at an estimated speed of ∼4 mm/s. The spatiotemporal characteristics of recruitment reveal two categories of patients: one in which seizure recruitment of neighboring cortical regions follows a spatially organized pattern consistent from seizure to seizure, and a second group without consistent spatial organization of activity during recruitment. The consistent, organized recruitment correlates with a more regular, compared with small-world, connectivity pattern in simulation and successful surgical treatment of epilepsy. We propose that an improved understanding of how the seizure recruits brain regions into large amplitude voltage fluctuations provides novel information to improve surgical treatment of epilepsy and highlights the slow spread of massive local activity across a vast extent of cortex during seizure.

Single unit action potentials in humans and the effect of seizure activity

Spike-sorting algorithms have been used to identify the firing patterns of isolated neurons ('single units') from implanted electrode recordings in patients undergoing assessment for epilepsy surgery, but we do not know their potential for providing helpful clinical information. It is important therefore to characterize both the stability of these recordings and also their context. A critical consideration is where the units are located with respect to the focus of the pathology. Recent analyses of neuronal spiking activity, recorded over extended spatial areas using microelectrode arrays, have demonstrated the importance of considering seizure activity in terms of two distinct spatial territories: the ictal core and penumbral territories. The pathological information in these two areas, however, is likely to be very different. We investigated, therefore, whether units could be followed reliably over prolonged periods of times in these two areas, including during seizure epochs. We isolated unit recordings from several hundred neurons from four patients undergoing video-telemetry monitoring for surgical evaluation of focal neocortical epilepsies. Unit stability could last in excess of 40 h, and across multiple seizures. A key finding was that in the penumbra, spike stereotypy was maintained even during the seizure. There was a net tendency towards increased penumbral firing during the seizure, although only a minority of units (10-20%) showed significant changes over the baseline period, and notably, these also included neurons showing significant reductions in firing. In contrast, within the ictal core territories, regions characterized by intense hypersynchronous multi-unit firing, our spike sorting algorithms failed as the units were incorporated into the seizure activity. No spike sorting was possible from that moment until the end of the seizure, but recovery of the spike shape was rapid following seizure termination: some units reappeared within tens of seconds of the end of the seizure, and over 80% reappeared within 3 min (τrecov = 104 ± 22 s). The recovery of the mean firing rate was close to pre-ictal levels also within this time frame, suggesting that the more protracted post-ictal state cannot be explained by persistent cellular neurophysiological dysfunction in either the penumbral or the core territories. These studies lay the foundation for future investigations of how these recordings may inform clinical practice.See Kimchi and Cash (doi:10.1093/awv264) for a scientific commentary on this article.

Local changes in neocortical circuit dynamics coincide with the spread of seizures to thalamus in a model of epilepsy

During the generalization of epileptic seizures, pathological activity in one brain area recruits distant brain structures into joint synchronous discharges. However, it remains unknown whether specific changes in local circuit activity are related to the aberrant recruitment of anatomically distant structures into epileptiform discharges. Further, it is not known whether aberrant areas recruit or entrain healthy ones into pathological activity. Here we study the dynamics of local circuit activity during the spread of epileptiform discharges in the zero-magnesium in vitro model of epilepsy. We employ high-speed multi-photon imaging in combination with dual whole-cell recordings in acute thalamocortical (TC) slices of the juvenile mouse to characterize the generalization of epileptic activity between neocortex and thalamus. We find that, although both structures are exposed to zero-magnesium, the initial onset of focal epileptiform discharge occurs in cortex. This suggests that local recurrent connectivity that is particularly prevalent in cortex is important for the initiation of seizure activity. Subsequent recruitment of thalamus into joint, generalized discharges is coincident with an increase in the coherence of local cortical circuit activity that itself does not depend on thalamus. Finally, the intensity of population discharges is positively correlated between both brain areas. This suggests that during and after seizure generalization not only the timing but also the amplitude of epileptiform discharges in thalamus is entrained by cortex. Together these results suggest a central role of neocortical activity for the onset and the structure of pathological recruitment of thalamus into joint synchronous epileptiform discharges.

Evaluation of anticonvulsant actions of dibromophenyl enaminones using in vitro and in vivo seizure models

Epilepsy and other seizure disorders are not adequately managed with currently available drugs. We recently synthesized a series of dibromophenyl enaminones and demonstrated that AK6 and E249 were equipotent to previous analogs but more efficacious in suppressing neuronal excitation. Here we examined the actions of these lead compounds on in vitro and in vivo seizure models. In vitro seizures were induced in the hippocampal slice chemically (zero Mg2+ buffer and picrotoxin) and electrically using patterned high frequency stimulation (HFS) of afferents. In vivo seizures were induced in rats using the 6 Hz and the maximal electroshock models. AK6 (10 µM) and E249 (10 µM) depressed the amplitude of population spikes recorded in area CA1 of the hippocampus by -50.5±4.3% and -40.1±3.1% respectively, with partial recovery after washout. In the zero Mg2+ model, AK6 (10 µM) depressed multiple population spiking (mPS) by -59.3±6.9% and spontaneous bursts (SBs) by -65.9±7.2% and in the picrotoxin-model by -43.3±7.2% and -50.0±8.3%, respectively. Likewise, E249 (10 µM) depressed the zero-Mg2+-induced mPS by -48.8±9.5% and SBs by -55.8±15.5%, and in the picrotoxin model by -37.1±5.5% and -56.5±11.4%, respectively. They both suppressed post-HFS induced afterdischarges and SBs. AK6 and E249 dose-dependently protected rats in maximal electroshock and 6 Hz models of in vivo seizures after 30 min pretreatment. Their level of protection in both models was similar to that obtained with phenytoin Finally, while AK6 had no effect on locomotion in rats, phenytoin significantly decreased locomotion. AK6 and E249, suppressed in vitro and in vivo seizures to a similar extent. Their in vivo activities are comparable with but not superior to phenytoin. The most efficacious, AK6 produced no locomotor suppression while phenytoin did. Thus, AK6 and E249 may be excellent candidates for further investigation as potential agents for the treatment of epilepsy syndromes with possibly less CNS side effects.

Mechanisms of seizure propagation in 2-dimensional centre-surround recurrent networks

Understanding how seizures spread throughout the brain is an important problem in the treatment of epilepsy, especially for implantable devices that aim to avert focal seizures before they spread to, and overwhelm, the rest of the brain. This paper presents an analysis of the speed of propagation in a computational model of seizure-like activity in a 2-dimensional recurrent network of integrate-and-fire neurons containing both excitatory and inhibitory populations and having a difference of Gaussians connectivity structure, an approximation to that observed in cerebral cortex. In the same computational model network, alternative mechanisms are explored in order to simulate the range of seizure-like activity propagation speeds (0.1-100 mm/s) observed in two animal-slice-based models of epilepsy: (1) low extracellular [Formula: see text], which creates excess excitation and (2) introduction of gamma-aminobutyric acid (GABA) antagonists, which reduce inhibition. Moreover, two alternative connection topologies are considered: excitation broader than inhibition, and inhibition broader than excitation. It was found that the empirically observed range of propagation velocities can be obtained for both connection topologies. For the case of the GABA antagonist model simulation, consistent with other studies, it was found that there is an effective threshold in the degree of inhibition below which waves begin to propagate. For the case of the low extracellular [Formula: see text] model simulation, it was found that activity-dependent reductions in inhibition provide a potential explanation for the emergence of slowly propagating waves. This was simulated as a depression of inhibitory synapses, but it may also be achieved by other mechanisms. This work provides a localised network understanding of the propagation of seizures in 2-dimensional centre-surround networks that can be tested empirically.

Evidence of an inhibitory restraint of seizure activity in humans

The location and trajectory of seizure activity is of great importance, yet our ability to map such activity remains primitive. Recently, the development of multi-electrode arrays for use in humans has provided new levels of temporal and spatial resolution for recording seizures. Here, we show that there is a sharp delineation between areas showing intense, hypersynchronous firing indicative of recruitment to the seizure, and adjacent territories where there is only low-level, unstructured firing. Thus, there is a core territory of recruited neurons and a surrounding 'ictal penumbra'. The defining feature of the 'ictal penumbra' is the contrast between the large amplitude EEG signals and the low-level firing there. Our human recordings bear striking similarities with animal studies of an inhibitory restraint, indicating that they can be readily understood in terms of this mechanism. These findings have important implications for how we localize seizure activity and map its spread.

Seizure activity in the brain is characterized by the recruitment of cortical neuronal activity. Schevon and colleagues study seizure activity in human subjects and find that the recruitment of neurons is hypersynchronous and that there is an intrinsic restraint on the propagation of this activity.

Novel actions of oxazolidinones: in vitro screening of a triazolyloxazolidinone for anticonvulsant activity

OBJECTIVE

To test the hypothesis that a triazolyloxazolidinone (PH084) has anticonvulsant activity by examining its effects on in vitro seizure models in the rat hippocampus.

MATERIALS AND METHODS

Whole-cell synaptic currents, action potentials and extracellular population spikes (PS) were recorded in the cell body area of rat hippocampal CA1 region in acutely prepared slices. Chemical [picrotoxin (100 µM) and zero magnesium] and electrical seizures were induced and the effect of PH084 (10 µM) was tested on cellular responses, multiple spikes and spontaneous bursting frequencies.

RESULTS

PH084 depressed evoked excitatory postsynaptic currents, action potential firing frequency and PS amplitude. All of these responses did not recover to baseline after 15-20 min washout of PH084. Perfusion with zero magnesium ion (Mg(2+))-containing buffer converted a single PS to multiple PS (mPS) accompanied by spontaneous burst. PH084 suppressed the mPS and the spontaneous burst frequency and it also suppressed the picrotoxin-induced mPS number. However, it did not affect the frequency of stimulus train-induced after discharge or bursts. Furthermore, 8-10 min pretreatment with PH084 did not affect the ability of zero Mg(2+) buffer, picrotoxin or stimulus train to induce epileptiform activity.

CONCLUSIONS

Thus, while PH084 may have potential for anticonvulsant activity against chemically induced seizures, it has little or no potential against electrically induced seizures or in preventing epileptiform discharge.

How inhibition influences seizure propagation

Inhibitory neuron behaviour is of fundamental importance to epileptic pathophysiology. When inhibition is compromised, such as by GABAergic blockade (Curtis et al., 1970; Connors, 1984; Traub and Miles, 1991) or by shifts in GABAergic reversal potential (Huberfeld et al., 2007), epileptiform discharges occur far more readily. Other studies have shown enhanced inhibition in vivo in the surrounding cortical territories associated with both focal pathological and physiological activity (Prince and Wilder, 1967; Dichter and Spencer, 1969a,b; Goldensohn and Salazar, 1986; Traub and Miles, 1991; Liang and Jones, 1997; Liang et al., 1998; Schwartz and Bonhoeffer, 2001). This gave rise to the concept of an "inhibitory restraint". This concept can explain the often confusing anatomical reorganizations seen in chronically epileptic brains (Sloviter, 1987; Cossart et al., 2001), indicating which changes might be pro-epileptic, and which oppose the epileptic state. It also may explain key electrophysiological features of epileptic seizures. Here we describe current knowledge about the restraint, gleaned mainly from acute pharmacological experiments in animals, both in vivo and in vitro, and speculate how this may alter our understanding of human seizure activity in clinical practice. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

Dynamic neurovascular coupling and uncoupling during ictal onset, propagation, and termination revealed by simultaneous in vivo optical imaging of neural activity and local blood volume

Traditional models of ictal propagation involve the concept of an initiation site and a progressive outward march of activation. The process of neurovascular coupling, whereby the brain supplies oxygenated blood to metabolically active neurons presumably results in a similar outward cascade of hyperemia. However, ictal neurovascular coupling has never been assessed in vivo using simultaneous measurements of membrane potential change and hyperemia with wide spatial sampling. In an acute rat ictal model, using simultaneous intrinsic optical signal (IOS) and voltage-sensitive dye (VSD) imaging of cerebral blood volume and membrane potential changes, we demonstrate that seizures consist of multiple dynamic multidirectional waves of membrane potential change with variable onset sites that spread through a widespread network. Local blood volume evolves on a much slower spatiotemporal scale. At seizure onset, the VSD waves extend beyond the IOS signal. During evolution, spatial correlation with hemodynamic signal only exists briefly at the maximal spread of the VSD signal. At termination, the IOS signal extends spatially and temporally beyond the VSD waves. Hence, vascular reactivity evolves in a separate but parallel fashion to membrane potential changes resulting in a mechanism of neurovascular coupling and uncoupling, which is as dynamic as the seizure itself.

Low-magnesium medium induces epileptiform activity in mouse olfactory bulb slices

Magnesium-free medium can be used in brain slice studies to enhance glutamate receptor function, but this manipulation causes seizure-like activity in many cortical areas. The rodent olfactory bulb (OB) slice is a popular preparation, and potentially ictogenic ionic conditions have often been used to study odor processing. We studied low Mg(2+)-induced epileptiform discharges in mouse OB slices using extracellular and whole cell electrophysiological recordings. Low-Mg(2+) medium induced two distinct types of epileptiform activity: an intraglomerular delta-frequency oscillation resembling slow sniff-induced activity and minute-long seizure-like events (SLEs) consisting of large negative-going field potentials accompanied by sustained depolarization of output neurons. SLEs were dependent on N-methyl-D-aspartate receptors and sodium currents and were facilitated by α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors. The events were initiated in the glomerular layer and propagated laterally through the external plexiform layer at a slow time scale. Our findings confirm that low-Mg(2+) medium should be used with caution in OB slices. Furthermore, the SLEs resembled the so-called slow direct current (DC) shift of clinical and experimental seizures, which has recently been recognized as being of great clinical importance. The OB slice may therefore provide a robust and unique in vitro model of acute seizures in which mechanisms of epileptiform DC shifts can be studied in isolation from fast oscillations.

The march of epileptic activity across cortex is limited (for a while) by the powerful forces of surrounding inhibition

Modular Propagation of Epileptiform Activity: Evidence for an Inhibitory Veto in Neocortex. Trevelyan AJ, Sussillo D, Watson BO, Yuste R. J Neurosci 2006;26(48):12447–12455. What regulates the spread of activity through cortical circuits? We present here data indicating a pivotal role for a vetoing inhibition restraining modules of pyramidal neurons. We combined fast calcium imaging of network activity with whole-cell recordings to examine epileptiform propagation in mouse neocortical slices. Epileptiform activity was induced by washing Mg2+ ions out of the slice. Pyramidal cells receive barrages of inhibitory inputs in advance of the epileptiform wave. The inhibitory barrages are effectively nullified at low doses of picrotoxin (2.5–5 μM). When present, however, these inhibitory barrages occlude an intense excitatory synaptic drive that would normally exceed action potential threshold by approximately a factor of 10. Despite this level of excitation, the inhibitory barrages suppress firing, thereby limiting further neuronal recruitment to the ictal event. Pyramidal neurons are recruited to the epileptiform event once the inhibitory restraint fails and are recruited in spatially clustered populations (150–250 μm diameter). The recruitment of the cells within a given module is virtually simultaneous, and thus epileptiform events progress in intermittent (0.5–1 Hz) steps across the cortical network. We propose that the interneurons that supply the vetoing inhibition define these modular circuit territories.

Feedforward Inhibition Contributes to the Control of Epileptiform Propagation Speed Trevelyan AJ, Sussillo D, Yuste R. J Neurosci 2007;27(13):3383–3387. It is still poorly understood how epileptiform events can recruit cortical circuits. Moreover, the speed of propagation of epileptiform discharges in vivo and in vitro can vary over several orders of magnitude (0.1–100 mm/s), a range difficult to explain by a single mechanism. We previously showed how epileptiform spread in neocortical slices is opposed by a powerful feedforward inhibition ahead of the ictal wave. When this feedforward inhibition is intact, epileptiform spreads very slowly (100 μm/s). We now investigate whether changes in this inhibitory restraint can also explain much faster propagation velocities. We made use of a very characteristic pattern of evolution of ictal activity in the zero magnesium (0 Mg2+) model of epilepsy. With each successive ictal event, the number of preictal inhibitory barrages dropped, and in parallel with this change, the propagation velocity increased. There was a highly significant correlation ( p < 0.001) between the two measures over a 1,000-fold range of velocities, indicating that feedforward inhibition was the prime determinant of the speed of epileptiform propagation. We propose that the speed of propagation is set by the extent of the recruitment steps, which in turn is set by how successfully the feedforward inhibitory restraint contains the excitatory drive. Thus, a single mechanism could account for the wide range of propagation velocities of epileptiform events observed in vitro and in vivo.

The source of afterdischarge activity in neocortical tonic-clonic epilepsy

Tonic-clonic seizures represent a common pattern of epileptic discharges, yet the relationship between the various phases of the seizure remains obscure. Here we contrast propagation of the ictal wavefront with the propagation of individual discharges in the clonic phase of the event. In an in vitro model of tonic-clonic epilepsy, the afterdischarges (clonic phase) propagate with relative uniform speed and are independent of the speed of the ictal wavefront (tonic phase). For slowly propagating ictal wavefronts, the source of the afterdischarges, relative to a given recording electrode, switched as the wavefront passed by, indicating that afterdischarges are seeded from wavefront itself. In tissue that has experienced repeated ictal events, the wavefront generalizes rapidly, and the afterdischarges in this case show a different "flip-flop" pattern, with frequent switches in their direction of propagation. This same flip-flop pattern is also seen in subdural EEG recordings in patients suffering intractable focal seizures caused by cortical dysplasias. Thus, in both slowly and rapidly generalizing ictal events, there is not a single source of afterdischarge activity: rather, the source is continuously changing. Our data suggest a complex view of seizures in which the ictal event and its constituent discharges originate from distinct locations.

Glutamate and dopamine receptors contribute to the lateral spread of epileptiform discharges in rat neocortical slices

PURPOSE

The effects of AMPA-type glutamate receptor as well as dopamine D1 and D2 receptors on the lateral propagation of epileptiform field potentials (EFP) were studied across adjacent areas of rat neocortical tissues.

METHODS

Epileptiform burst discharges were induced by superfusion of Mg(2+)-free artificial cerebrospinal fluid. Simultaneous field potential recordings of EFP were obtained from four microelectrodes placed 2-3 mm apart across coronal slices in the third layer of the neocortex. The effects of AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), dopamine D1 receptor agonist SKF 81297, and dopamine D2 receptor agonist quinpirole on lateral propagation of burst discharges were investigated.

RESULTS

CNQX, applied focally between recording sites, blocked rapid propagation across treated areas and resulted in the emergence of spatially separate, independent pacemakers. Focal application of SKF 81297 between recording sites increased the repetition rate of EFP, but reduced the amplitude as well as the duration of epileptic discharges. However, addition of SKF 81297 to the bath medium abolished EFP. Both local and systemic applications of quinpirole irreversibly enhanced repetition rate of epileptiform burst discharges.

CONCLUSIONS

The results indicate the prerequisite of AMPA synaptic transmission for synchronized lateral propagation of Mg(2+)-free ACSF-induced epileptic activity and the modulatory effects of dopamine D1 and D2 receptors on both EFP initiation and propagation in epileptic tissues.

The effect of electrical stimulation and lesioning of the anterior thalamic nucleus on kainic acid-induced focal cortical seizure status in rats

PURPOSE

The present study aimed to clarify the effect of electrical stimulation and lesioning of the anterior nucleus of the thalamus (ANT) on kainic acid (KA)-induced focal cortical seizures in a rat model. To address the mechanism underlying these anticonvulsant actions, cerebral glucose metabolism after ANT electrical stimulation and lesioning was also examined.

METHODS

Wistar rats were divided into five major groups: control (n = 9), unilateral (n = 9), and bilateral (n = 9) ANT electrical stimulation, and unilateral (n = 9) and bilateral (n = 9) ANT lesioning. After KA injection, average clinical-seizure frequencies in each group were measured. Electrical stimulation of ANT was introduced after induction of seizure status epilepticus. Stimulation was on for 30 min and off for 30 min per 60-min cycle. Local cerebral glucose utilization (LCGU) was also measured by using [(14)C] 2-deoxyglucose autoradiography in three groups of rats: control (n = 7), bilateral ANT stimulation (n = 7), and bilateral ANT lesioning (n = 7).

RESULTS

Unilateral ANT electrical stimulation and lesioning significantly reduced clinical seizure frequency, compared with control animals. Strikingly, no animals treated with bilateral ANT procedures demonstrated any clinical seizure. LCGU was markedly increased in the sensorimotor cortex, striatum, thalamus, mammillary body, and midbrain tegmentum of control group rats after KA injection, but no increase in LCGU was noted in rats treated with bilateral ANT lesioning or stimulation.

CONCLUSIONS

The electrical stimulation and lesioning of ANT suppressed focal cortical clinical seizures induced by KA injection. Additionally, an analysis of cerebral metabolic changes indicated that these procedures might suppress the function as amplifier and synchronizer of seizure activity.

Feedforward inhibition contributes to the control of epileptiform propagation speed

It is still poorly understood how epileptiform events can recruit cortical circuits. Moreover, the speed of propagation of epileptiform discharges in vivo and in vitro can vary over several orders of magnitude (0.1-100 mm/s), a range difficult to explain by a single mechanism. We previously showed how epileptiform spread in neocortical slices is opposed by a powerful feedforward inhibition ahead of the ictal wave. When this feedforward inhibition is intact, epileptiform spreads very slowly (approximately 100 microm/s). We now investigate whether changes in this inhibitory restraint can also explain much faster propagation velocities. We made use of a very characteristic pattern of evolution of ictal activity in the zero magnesium (0 Mg2+) model of epilepsy. With each successive ictal event, the number of preictal inhibitory barrages dropped, and in parallel with this change, the propagation velocity increased. There was a highly significant correlation (p < 0.001) between the two measures over a 1000-fold range of velocities, indicating that feedforward inhibition was the prime determinant of the speed of epileptiform propagation. We propose that the speed of propagation is set by the extent of the recruitment steps, which in turn is set by how successfully the feedforward inhibitory restraint contains the excitatory drive. Thus, a single mechanism could account for the wide range of propagation velocities of epileptiform events observed in vitro and in vivo.

Blood-brain barrier disruption results in delayed functional and structural alterations in the rat neocortex

Disruption of the blood-brain barrier (BBB) is a characteristic finding in common neurological disorders. Human data suggest BBB disruption may underlie cerebral dysfunction. Animal experiments show the development of epileptiform activity following BBB breakdown. In the present study we investigated the neurophysiological, structural and functional consequences of BBB disruption. Adult rats underwent focal BBB disruption in the rat sensory-motor cortex using the bile salt sodium deoxycholate (DOC). Magnetic resonance imaging in-vivo showed an early BBB disruption with delayed reduction in cortical volume. This was associated with a reduced number of neurons and an increased number of astrocytes. In-vitro experiments showed that the threshold for spreading depression and the propagation velocity of the evoked epileptic potentials were increased 1 month after treatment. Furthermore, animals' motor functions deteriorated during the first few weeks following BBB disruption. Treatment with serum albumin resulted in a similar cell loss confirming that the effect of DOC was due to opening of the BBB. Our findings suggest that delayed neurodegeneration and functional impairment occur following the development of the epileptic focus in the BBB-permeable cerebral cortex.

Modular propagation of epileptiform activity: evidence for an inhibitory veto in neocortex

What regulates the spread of activity through cortical circuits? We present here data indicating a pivotal role for a vetoing inhibition restraining modules of pyramidal neurons. We combined fast calcium imaging of network activity with whole-cell recordings to examine epileptiform propagation in mouse neocortical slices. Epileptiform activity was induced by washing Mg2+ ions out of the slice. Pyramidal cells receive barrages of inhibitory inputs in advance of the epileptiform wave. The inhibitory barrages are effectively nullified at low doses of picrotoxin (2.5-5 microM). When present, however, these inhibitory barrages occlude an intense excitatory synaptic drive that would normally exceed action potential threshold by approximately a factor of 10. Despite this level of excitation, the inhibitory barrages suppress firing, thereby limiting further neuronal recruitment to the ictal event. Pyramidal neurons are recruited to the epileptiform event once the inhibitory restraint fails and are recruited in spatially clustered populations (150-250 microm diameter). The recruitment of the cells within a given module is virtually simultaneous, and thus epileptiform events progress in intermittent (0.5-1 Hz) steps across the cortical network. We propose that the interneurons that supply the vetoing inhibition define these modular circuit territories.

Long-range oscillatory Ca2+waves in rat spinal dorsal horn

Synchronous activity of large populations of neurons shapes neuronal networks during development. However, re-emergence of such activity at later stages of development could severely disrupt the orderly processing of sensory information, e.g. in the spinal dorsal horn. We used Ca2+ imaging in spinal cord slices of neonatal and young rats to assess under which conditions synchronous activity occurs in dorsal horn. No spontaneous synchronous Ca2+ transients were detected. However, increasing neuronal excitability by application of 4-aminopyridine after pretreatment of the slice with blockers of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate, gamma-aminobutyric acid (GABA)(A) and glycine receptors evoked repetitive Ca2+ waves in dorsal horn. These waves spread mediolaterally with a speed of 1.0 +/- 0.1 mm/s and affected virtually every dorsal horn neuron. The Ca2+ waves were associated with large depolarizing shifts of the membrane potential of participating neurons and were most likely synaptically mediated because they were abolished by blockade of action potentials or N-methyl-D-aspartate (NMDA) receptors. They were most pronounced in the superficial dorsal horn and absent from the ventral horn. A significant proportion of the Ca2+ waves spread to the contralateral dorsal horn. This seemed to be enabled by disinhibition as primary afferent-induced dorsal horn excitation crossed the midline only when GABA(A) and glycine receptors were blocked. Interestingly, the Ca2+ waves occurred under conditions where AMPA/kainate receptors were blocked. Thus, superficial dorsal horn NMDA receptors are able to sustain synchronous neuronal excitation in the absence of functional AMPA/kainate receptors.

Seizure spread through the life cycle: optical imaging in combined brain slices from immature, adult, and senile rats in vitro

The semiology of epileptic seizures changes during the lifetime. Hence, it can be assumed that age-related changes in brain plasticity influence the patterns of seizure onset, spread and propagation velocity. We employed the 4-aminopyridine model of epilepsy to study seizure-like events in vitro. Combined entorhinal cortex-hippocampus brain slices from juvenile (10-13 days), adult (2-3 months), and senile (24-27 months) rats were examined using electrophysiological recordings and imaging of intrinsic optical signals. In the juvenile group, seizure onset was multifocal in all slice regions including the hippocampus. Onset in adult animals was confined to the entorhinal cortex and to neocortical regions. In slices from senile animals, there was a preponderance of seizure onsets in the neocortex. Spread patterns were highly variable in the juvenile group and became gradually more monomorph with increasing age. Propagation velocities were highest in the adult group, with maximum values of 1.51 +/- 0.68 mm/s. In the juvenile group, they amounted to 0.97 +/- 0.39 mm/s, and to 1.18 +/- 0.42 mm/s in senile slices. The results of this study indicate that age-related changes in brain plasticity profoundly affect spread patterns, which may contribute to the clinically observed changes in seizure semiology during early childhood, adulthood and senescence.

Intracortical synchronization of epileptic discharges at different stages of ultrastructural rearrangements in a completely neuronally isolated area of rat neocortex

Cross-correlation functions were used to study the time delay (a measure of synchronization) in the appearance of epileptic discharges in distant areas of the cortex in the intact cortex and in neuronally isolated cortical strips in Wistar rats. Experiments were performed at different stages of axon sprouting 30 and 90 days after isolation of cortical areas and in intact cortex. Significant increases in the number of synapses in layer V of isolated cortical strips at 30 days correlated with significant decreases in the time delay, while decreases in the number of synapses at 90 days correlated with a significant increase in the time delay. This is evidence that newly formed synapses increase the extent of synchronization and thus affect epileptogenesis. The data obtained here suggest that large pyramidal cells in layer V of the rat neocortex form a neural network in pathological conditions, this supporting intracortical synchronization of epileptic discharges.

Initiation of spontaneous epileptiform events in the rat neocortex in vivo

We used voltage-sensitive dye imaging to visualize the distribution of initiation sites of the spontaneous interictal-like spikes (sISs) in rat neocortex, in vivo, induced by bicuculline or picrotoxin over the exposed cortex. The initiation site was small (approximately 200 microm diam). On average each initiation site initiated 2.0 +/- 0.8 sISs (9 animals, 499 sISs, 251 sites). This is significantly different from that in neocortical slices, where each initiation site initiated 30-100 sISs. The initiation sites were not randomly distributed. The distance between two consecutive sites tended to be either <800 or >1200 microm, suggesting a temporal "suppression annulus" surrounding each initiation site. Within the annulus, the likelihood for initiating the next sIS was reduced. Suppression annulus did not have a noticeable change in the presence of GABA(b) antagonist, suggesting it did not depend on the GABA(b) inhibition. We also applied bicuculline locally to a spot of 800 x 800 microm(2) for approximately 45 min. During this period approximately 1000 sISs occurred within the spot. Bicuculline or picrotoxin was then applied to the entire craniotomy window. The pretreatment created an obvious cluster of initiation sites. Around this cluster, the suppression annulus became obvious in individual animals. Our results suggest that, in disinhibited cortex, epileptiform events were initiated from small sites. The initiation sites may cluster in an area with increased local activity. Surrounding each initiation site there may be a temporal suppression annulus.

Cellular and network mechanisms underlying spontaneous sharp wave-ripple complexes in mouse hippocampal slices

The mammalian hippocampus displays a peculiar pattern of fast (approximately 200 Hz) network oscillations superimposed on slower sharp waves. Such sharp wave-ripple complexes (SPW-R) have been implicated in memory consolidation. We have recently described a novel and unique method for studying SPW-R in naive slices of murine hippocampus. Here, we used this model to analyse network and cellular mechanisms of this type of network activity. SPW-R are usually generated within area CA3 but can also originate within the isolated CA1 region. Cellular synchronisation during SPW-R requires both excitatory and inhibitory synaptic transmission as well as electrical coupling, the latter being particularly important for the high-frequency component. Extracellular and intracellular recordings revealed a surprisingly strong inhibition of most CA1 pyramidal cells during SPW-R. A minority of active cells, however, increases action potential frequency and fires in strict synchrony with the field ripples. This strong separation between members and non-members of the network may serve to ensure a high signal-to-noise ratio in information processing during sharp wave-ripple complexes.

Epileptogenic effect of cyclosporine in guinea-pig hippocampal slices

Cyclosporine A (CsA) neurotoxicity is a common cause of seizures in transplant patients and others receiving immunosuppressive therapy. CsA at concentrations higher than the levels estimated for cerebrospinal fluid of the patients suffering from seizure attacks was ineffective to induce epileptiform field potentials (EFP) in in vitro brain-slice preparation. The aim of this study was to test the effect of CsA at lower concentrations on neuronal activity. Guinea-pig hippocampal slices were exposed to artificial cerebrospinal fluid containing CsA (0.1-2 microM). Furthermore, the effects of CsA (0.25-10 microM) were tested on EFP elicited by omission of Mg2+ from superfusate. Low concentrations of CsA (0.1-0.25 microM) induced EFP while higher doses (0.5-2 microM) failed to decrease the seizure threshold. CsA at concentrations of 0.25 and 1 microM had no significant effect on the low Mg2+-induced EFP. Higher CsA concentration (10 microM) strongly suppressed EFP. The results indicate that CsA at doses that are probably clinically relevant increases the neuronal excitability.

Immature neocortical neurons exist as extensive syncitial networks linked by dendrodendritic electrical connections

The properties of immature cortex that may enable it to exhibit large-scale wavelike activity during a brief critical developmental period were investigated by imaging neuronal calcium signals in neonatal cortical slices under conditions of artificially enhanced excitability, conditions that produce a more frequent and robust version of the naturally occurring waves. Using pharmacological manipulation to probe the underlying mechanisms, I show that waves can propagate effectively when excitatory synaptic transmission is blocked. In contrast, propagation is very sensitive to reductions in gap junctional communication. In the barrel field cortex wave propagation is affected by the underlying cytoarchitecture in a way that is consistent with a role for dendrodendritic gap junctions. The ability of cortex to sustain wave activity ends around postnatal day 12, precisely when a major reduction in neuronal gap junctions takes place in cortex. These results suggest that in immature cortex gap junctions link neurons into extensive networks that may allow electrical activity to spread over long distances.

Optical imaging reveals characteristic seizure onsets, spread patterns, and propagation velocities in hippocampal-entorhinal cortex slices of juvenile rats

We have combined recordings with extracellular microelectrodes or ion-sensitive electrodes and imaging of intrinsic optical signal changes to study the spatiotemporal pattern of seizure onset and spread during development. We have employed the entorhinal cortex-hippocampus brain slice preparation of juvenile rats at different stages of postnatal maturation. Three age groups were analyzed: 4-6 days (age group I), 10-14 days (age group II), and 20-23 days (age group III). Seizure-like events were induced by perfusion of slices with Mg(2+)-free artificial cerebrospinal fluid thereby removing the Mg(2+) block of the N-methyl-d-aspartate receptor. Seizure susceptibility was highest in age groups II and III. In age group I seizure-like events originated mainly in the hippocampus proper. Seizure-like events in age group II originated mainly in the entorhinal cortex and this tendency was even more pronounced in age group III. Invasion of the hippocampal formation via the perforant path-dentate gyrus and via the subiculum was seen in age groups I and II. In contrast, in age group III the hippocampus was invaded exclusively via the subiculum pathway. The velocity of spread at which seizure-like events propagated within different regions of the slice increased with postnatal age. The characteristics of onset, spread patterns, and propagation velocities as revealed by this study allow insight into the evolving properties of the developing brain.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Activity-dependent intracellular acidification correlates with the duration of seizure activity

Synchronized neuronal activity (seizures) can appear in the presence or absence of synaptic transmission. Mechanisms of seizure initiation in each of these conditions have been studied, but relatively few studies have addressed seizure termination. In particular, how are seizures terminated in the absence of synaptic activity where there is no loss of excitatory drive or augmentation of inhibitory inputs? We have studied dynamic activity-dependent changes of intracellular pH in the absence of synaptic transmission using the fluorescent pH indicator carboxylseminaphthorhodafluo-1. During epileptiform activity we observed intracellular acidification, whereas between seizures the intracellular pH recovered. Experimental conditions that shortened the epileptiform discharge correlated with more rapid intracellular acidification. On the other hand, experimental manipulation of intracellular pH altered the duration of the seizure discharge, with acidification resulting in early termination of the epileptiform activity. These data show a direct relationship between the level of intracellular acidification and the duration of the seizures, suggesting that an intracellular pH-dependent process can terminate nonsynaptic neuronal synchronization.

Ictal patterns of neocortical seizures monitored with intracranial electrodes: correlation with surgical outcome

PURPOSE

Numerous factors have been analyzed in attempts to predict the outcome of surgical resections in patients with neocortical epilepsy. We examined the correlation between surgical outcome and electrocorticographic features of neocortical ictal patterns.

METHODS

Twenty six patients with neocortical epilepsy underwent monitoring with subdural grid electrodes before surgery. Ictal patterns were analyzed retrospectively and correlated with three types of outcome: seizure free, worthwhile improvement (>75% reduction of seizure frequency), and no worthwhile improvement. The duration of follow-up was 2-5 years.

RESULTS

Ictal patterns were divided according to the size of epileptogenic zone (focal, regional, multifocal); velocity and type of seizure propagation (fast contiguous, slow contiguous, noncontiguous); pattern of the onset of ictal activity; part of the cortex involved in the origin of the seizure (frontal, frontocentroparietal, etc.). Spread to medial temporal structures (as assessed by subtemporal strips) also was evaluated in selected cases. Statistically significant correlation with surgical outcome (p = 0.026) was shown for only one variable: type of spread. Patients with slow spread (n = 8) demonstrated the best outcomes (five are seizure free), whereas patients with noncontiguous spread (n = 5) demonstrated the worst outcomes (four did not improve significantly). Patients with fast contiguous spread (n = 13) showed intermediate outcomes.

CONCLUSIONS

Types of propagation of ictal neocortical activity correlate with surgical outcome. Analysis of ictal pattern during intracranial recordings may help to predict surgical outcome for neocortical epilepsy.

Focal epileptogenesis in a rat model of polymicrogyria

Polymicrogyria, a developmental cortical malformation associated with epilepsy, can be modeled in rats with a transcortical freeze lesion on the day of birth (P0) or P1. We have used field potential recordings to characterize the incidence, propagation patterns, and distribution of epileptiform activity in slices from rats with experimental microgyri. Interictal-like epileptiform activity was evoked in slices from 85% of freeze-lesioned rats aged P12-P118. These data show age-specific properties of epileptogenesis, including: a delay in onset, a decrease in the incidence of epileptiform activity in rats >P40 that was specific to those lesioned on P0 as opposed to P1, and a shift in the likely site of initiation to areas further from the microgyrus in mature animals. Several observations suggest that the area adjacent to the microgyrus, which appears histologically normal in Nissl stains, contains the necessary epileptogenic neuronal circuits: 1) in 78% of slices, epileptiform activity could be evoked only from a focal zone adjacent to the microgyrus (paramicrogyral zone) and not within the microgyrus proper; 2) epileptiform activity consistently originated from a particular site within this paramicrogyral zone, independent of the location of the stimulating electrode, suggesting that the generator is outside of the microgyrus; 3) evoked epileptiform activities in the paramicrogyral cortex were unaltered after separation of this zone from the microgyrus with a transcortical cut; and 4) the short-latency graded field potential evoked in the paramicrogyral zone contained an additional negativity not seen in control slices. The epileptiform activity was blocked reversibly by N-methyl--aspartate receptor antagonists in slices from mature as well as immature freeze-lesioned rats. These results suggest that aberrant synaptic connectivity develops in rat cortex surrounding the microgyrus and produces a focal epileptogenic zone whose capacity to generate epileptiform activities does not depend on connections with the malformation itself. We hypothesize that afferents, originating from cortical and extracortical sites, lose their targets in the region of the malformation and make appropriate laminar contacts in the cortex adjacent to the malformation, creating an overabundance of excitatory input to this cortical zone. Increased excitatory feedback onto specific cortical elements may be one factor involved in epileptogenesis in this model of a cortical malformation.

Pattern and pharmacology of propagating epileptiform activity in mouse cerebral cortex

Multiple extracellular recording electrodes were used to study the intra- and interhemispheric spread of stimulus-evoked epileptiform responses in adult mouse neocortical slices. Bath application of 20 microM bicuculline methiodide induced epileptiform activity that propagated at approximately 0.08 m/s over several millimeters in rostro-caudal and medio-lateral direction within the ipsilateral hemisphere and across the corpus callosum to the contralateral hemisphere. A vertical incision from layer II to subcortical regions did not prevent the spread to remote cortical regions, indicating that layer I plays a major role in the lateral propagation of epileptiform activity. The intra- and interhemispheric spread was not influenced by application of an N-methyl-d-aspartate (NMDA) receptor antagonist, but blocked by an antagonist acting at the (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptor. The potential role of potassium channel activation in controlling the generation or spread of epileptiform activity was tested by applying the potassium channel opener cromakalim and the serotonin type 1A (5-HT1A) receptor agonist (+/-)-8-hydroxydipropylaminotetralin (8-OH-DPAT) to the disinhibited slices. Whereas cromakalim reduced the neuronal excitability and blocked all epileptiform responses, 8-OH-DAPT did not affect the activity pattern. Our results suggest that propagating epileptiform activity in disinhibited neocortical structures is predominantly mediated by activation of AMPA receptors and controllable by activation of a voltage-dependent potassium current.

Functional anatomy of limbic epilepsy: a proposal for central synchronization of a diffusely hyperexcitable network

The limbic/mesial temporal lobe epilepsy syndrome has been defined as a focal epilepsy, with the implication that there is a well defined focus of onset, traditionally centered around the hippocampus. The pathology of the hippocampus in this syndrome has been well described and a number of physiological abnormalities have been defined in this structure in animal models and humans with epilepsy. However, anatomical and physiological abnormalities have also been described in other limbic sites in this form of epilepsy. Previous studies have shown broadly synchronized or multifocal seizure onset within the limbic system of the animal models and human patients. We hypothesized that the epileptogenic circuit for the initiation of seizures was distributed throughout the limbic system with a possible central synchronizing process. In vitro studies showed that multiple limbic sites in epileptic animals (hippocampus, entorhinal cortex, piriform cortex and amygdala) have epileptiform changes with prolonged depolarizations and multiple superimposed action potentials. In vivo studies revealed that thalamic stimulation yields short latency excitatory responses in the entorhinal cortex and hippocampus. In addition, in epileptic animals, thalamic stimulation caused epileptiform responses in the hippocampus. Based on the findings of this study and on previous anatomy and physiology reports, we hypothesize that the process of seizure initiation involves broad circuit interactions involving multiple independent limbic structures, and that the midline thalamus may act as a physiological synchronizer. We offer a new proposal for the functional anatomy of limbic epilepsy that takes widespread hyperexcitability in the limbic system and the potential for thalamic synchronization into consideration.

Initiation of spontaneous epileptiform activity in the neocortical slice

Cortical local circuitry is important in epileptogenesis. Voltage-sensitive dyes and fast imaging were used to visualize the initiation of spontaneous paroxysmal events in adult rat neocortical slices. Although spontaneous paroxysmal events could start from anywhere in the preparation, optical imaging revealed that all spontaneous events started at a few confined initiation foci and propagated to the whole preparation. Multielectrode recording over hundreds of spontaneous events revealed that often two or three initiation foci coexisted in each preparation (n = 10). These foci took turns being dominant; the dominant focus initiated the majority of the spontaneous paroxysmal events during that period. The dominant focus and dynamic rearrangement of foci suggest that the initiation of spontaneous epileptiform events involves a local multineuronal process, perhaps with potentiated synapses.

Imaging of delta- and mu-opioid receptors in temporal lobe epilepsy by positron emission tomography

The involvement of opioid neurotransmitter systems in seizure mechanisms is well documented. In previous positron emission tomography (PET) studies in patients with unilateral temporal lobe epilepsy, we have found evidence for differential regulation of the opioid-receptor subtypes. The present study extends our previous observations to delta-opioid receptors by using the delta-receptor-selective antagonist [11C]methylnaltrindole ([11C]MeNTI). Paired measurements of delta- and mu-opioid receptor binding and metabolic activity were performed with PET using [11C]MeNTI and [11C]carfentanil ([11C]CFN) and [18F]fluorodeoxyglucose ([18F]FDG), respectively. Binding of [11C]MeNTI and [11C]CFN increased and [18F]FDG uptake decreased in the temporal cortex (TC) ipsilateral to the focus. Decreases in [18F]FDG uptake were more widespread regionally than were increases in opioid receptors. Increases in the delta- and mu-receptor binding showed different regional patterns. Increases in mu-receptor binding were confined to the middle aspect of the inferior TC, whereas binding of delta receptors increased in the mid-inferior TC and anterior aspect of the middle and superior TC. The increase in delta receptors suggests their anticonvulsant action, as previously shown for the delta-receptor subtype, whereas the different regional pattern of receptor alterations suggest the distinct roles of different opioid-receptor subtypes in seizure phenomena.

Increased susceptibility of brain slices from carbonic anhydrase II-deficient mice to low [Mg2+]O-induced seizures

Brain pH is thought to be an influential factor in determining susceptibility to seizures. We compared the susceptibility of brain slices from carbonic anhydrase II (CA II)-deficient mice to epileptiform activity induced by low extracellular [Mg2+], with slices from normal littermates, both bathed in artificial cerebrospinal fluid at pH 7.3. In both entorhinal cortex and hippocampal field CA1, epileptiform activity started earlier in CA II-deficient slices. Raising extracellular [CO2] (20%; extracellular pH, 6.7) reversibly blocked the epileptiform activity in normal, but not in CA II-deficient, slices. The data, combined with previous in vivo findings showing an increased resistance of mutants to seizures, suggest the presence of in vivo anticonvulsant acidosis with long-term compensatory changes that lead to in vitro 'proconvulsant' behavior in CA II-deficient slices clamped at pH 7.3.