Spontaneous rhythmic synchronous activity in epileptic human and normal monkey temporal lobe

Modulation of large rhythmic depolarizations in human large basket cells by norepinephrine and acetylcholine

Rhythmic brain activity is critical to many brain functions and is sensitive to neuromodulation, but so far very few studies have investigated this activity on the cellular level in vitro in human brain tissue samples. This study reveals and characterizes a novel rhythmic network activity in the human neocortex. Using intracellular patch-clamp recordings of human cortical neurons, we identify large rhythmic depolarizations (LRDs) driven by glutamate release but not by GABA. These LRDs are intricate events made up of multiple depolarizing phases, occurring at ~0.3 Hz, have large amplitudes and long decay times. Unlike human tissue, rat neocortex layers 2/3 exhibit no such activity under identical conditions. LRDs are mainly observed in a subset of L2/3 interneurons that receive substantial excitatory inputs and are likely large basket cells based on their morphology. LRDs are highly sensitive to norepinephrine (NE) and acetylcholine (ACh), two neuromodulators that affect network dynamics. NE increases LRD frequency through β-adrenergic receptor activity while ACh decreases it via M4 muscarinic receptor activation. Multi-electrode array recordings show that NE enhances and synchronizes oscillatory network activity, whereas ACh causes desynchronization. Thus, NE and ACh distinctly modulate LRDs, exerting specific control over human neocortical activity.

Distinctive biophysical features of human cell-types: insights from studies of neurosurgically resected brain tissue

Electrophysiological characterization of live human tissue from epilepsy patients has been performed for many decades. Although initially these studies sought to understand the biophysical and synaptic changes associated with human epilepsy, recently, it has become the mainstay for exploring the distinctive biophysical and synaptic features of human cell-types. Both epochs of these human cellular electrophysiological explorations have faced criticism. Early studies revealed that cortical pyramidal neurons obtained from individuals with epilepsy appeared to function "normally" in comparison to neurons from non-epilepsy controls or neurons from other species and thus there was little to gain from the study of human neurons from epilepsy patients. On the other hand, contemporary studies are often questioned for the "normalcy" of the recorded neurons since they are derived from epilepsy patients. In this review, we discuss our current understanding of the distinct biophysical features of human cortical neurons and glia obtained from tissue removed from patients with epilepsy and tumors. We then explore the concept of within cell-type diversity and its loss (i.e., "neural homogenization"). We introduce neural homogenization to help reconcile the epileptogenicity of seemingly "normal" human cortical cells and circuits. We propose that there should be continued efforts to study cortical tissue from epilepsy patients in the quest to understand what makes human cell-types "human".

Can in vitro studies aid in the development and use of antiseizure therapies? A report of the ILAE/AES Joint Translational Task Force

In vitro preparations (defined here as cultured cells, brain slices, and isolated whole brains) offer a variety of approaches to modeling various aspects of seizures and epilepsy. Such models are particularly amenable to the application of anti-seizure compounds, and consequently are a valuable tool to screen the mechanisms of epileptiform activity, mode of action of known anti-seizure medications (ASMs), and the potential efficacy of putative new anti-seizure compounds. Despite these applications, all disease models are a simplification of reality and are therefore subject to limitations. In this review, we summarize the main types of in vitro models that can be used in epilepsy research, describing key methodologies as well as notable advantages and disadvantages of each. We argue that a well-designed battery of in vitro models can form an effective and potentially high-throughput screening platform to predict the clinical usefulness of ASMs, and that in vitro models are particularly useful for interrogating mechanisms of ASMs. To conclude, we offer several key recommendations that maximize the potential value of in vitro models in ASM screening. This includes the use of multiple in vitro tests that can complement each other, carefully combined with in vivo studies, the use of tissues from chronically epileptic (rather than naïve wild-type) animals, and the integration of human cell/tissue-derived preparations.

Synaptic alterations and neuronal firing in human epileptic neocortical excitatory networks

Epilepsy is a prevalent neurological condition, with underlying neuronal mechanisms involving hyperexcitability and hypersynchrony. Imbalance between excitatory and inhibitory circuits, as well as histological reorganization are relatively well-documented in animal models or even in the human hippocampus, but less is known about human neocortical epileptic activity. Our knowledge about changes in the excitatory signaling is especially scarce, compared to that about the inhibitory cell population. This study investigated the firing properties of single neurons in the human neocortex in vitro, during pharmacological blockade of glutamate receptors, and additionally evaluated anatomical changes in the excitatory circuit in tissue samples from epileptic and non-epileptic patients. Both epileptic and non-epileptic tissues exhibited spontaneous population activity (SPA), NMDA receptor antagonization reduced SPA recurrence only in epileptic tissue, whereas further blockade of AMPA/kainate receptors reversibly abolished SPA emergence regardless of epilepsy. Firing rates did not significantly change in excitatory principal cells and inhibitory interneurons during pharmacological experiments. Granular layer (L4) neurons showed an increased firing rate in epileptic compared to non-epileptic tissue. The burstiness of neurons remained unchanged, except for that of inhibitory cells in epileptic recordings, which decreased during blockade of glutamate receptors. Crosscorrelograms computed from single neuron discharge revealed both mono- and polysynaptic connections, particularly involving intrinsically bursting principal cells. Histological investigations found similar densities of SMI-32-immunopositive long-range projecting pyramidal cells in both groups, and shorter excitatory synaptic active zones with a higher proportion of perforated synapses in the epileptic group. These findings provide insights into epileptic modifications from the perspective of the excitatory system and highlight discrete alterations in firing patterns and synaptic structure. Our data suggest that NMDA-dependent glutamatergic signaling, as well as the excitatory synaptic machinery are perturbed in epilepsy, which might contribute to epileptic activity in the human neocortex.

Diverse nature of interictal oscillations: EEG-based biomarkers in epilepsy

Interictal electroencephalogram (EEG) patterns, including high-frequency oscillations (HFOs), interictal spikes (ISs), and slow wave activities (SWAs), are defined as specific oscillations between seizure events. These interictal oscillations reflect specific dynamic changes in network excitability and play various roles in epilepsy. In this review, we briefly describe the electrographic characteristics of HFOs, ISs, and SWAs in the interictal state, and discuss the underlying cellular and network mechanisms. We also summarize representative evidence from experimental and clinical epilepsy to address their critical roles in ictogenesis and epileptogenesis, indicating their potential as electrophysiological biomarkers of epilepsy. Importantly, we put forwards some perspectives for further research in the field.

Bursting of excitatory cells is linked to interictal epileptic discharge generation in humans

Knowledge about the activity of single neurons is essential in understanding the mechanisms of synchrony generation, and particularly interesting if related to pathological conditions. The generation of interictal spikes—the hypersynchronous events between seizures—is linked to hyperexcitability and to bursting behaviour of neurons in animal models. To explore its cellular mechanisms in humans we investigated the activity of clustered single neurons in a human in vitro model generating both physiological and epileptiform synchronous events. We show that non-epileptic synchronous events resulted from the finely balanced firing of excitatory and inhibitory cells, which was shifted towards an enhanced excitability in epileptic tissue. In contrast, interictal-like spikes were characterised by an asymmetric overall neuronal discharge initiated by excitatory neurons with the presumptive leading role of bursting pyramidal cells, and possibly terminated by inhibitory interneurons. We found that the overall burstiness of human neocortical neurons is not necessarily related to epilepsy, but the bursting behaviour of excitatory cells comprising both intrinsic and synaptically driven bursting is clearly linked to the generation of epileptiform synchrony.

The subiculum and its role in focal epileptic disorders

The subicular complex (hereafter referred as subiculum), which is reciprocally connected with the hippocampus and rhinal cortices, exerts a major control on hippocampal outputs. Over the last three decades, several studies have revealed that the subiculum plays a pivotal role in learning and memory but also in pathological conditions such as mesial temporal lobe epilepsy (MTLE). Indeed, subicular networks actively contribute to seizure generation and this structure is relatively spared from the cell loss encountered in this focal epileptic disorder. In this review, we will address: (i) the functional properties of subicular principal cells under normal and pathological conditions; (ii) the subiculum role in sustaining seizures in in vivo models of MTLE and in in vitro models of epileptiform synchronization; (iii) its presumptive role in human MTLE; and (iv) evidence underscoring the relationship between subiculum and antiepileptic drug effects. The studies reviewed here reinforce the view that the subiculum represents a limbic area with relevant, as yet unexplored, roles in focal epilepsy.

Presence of synchrony-generating hubs in the human epileptic neocortex

KEY POINTS

•Initiation of pathological synchronous events such as epileptic spikes and seizures is linked to the hyperexcitability of the neuronal network in both humans and animals. •In the present study, we show that epileptiform interictal-like spikes and seizures emerged in human neocortical slices by blocking GABA_A receptors, following the disappearance of the spontaneously occurring synchronous population activity. •Large variability of temporally and spatially simple and complex spikes was generated by tissue from epileptic patients, whereas only simple events appeared in samples from non-epileptic patients. •Physiological population activity was associated with a moderate level of principal cell and interneuron firing, with a slight dominance of excitatory neuronal activity, whereas epileptiform events were mainly initiated by the synchronous and intense discharge of inhibitory cells. •These results help us to understand the role of excitatory and inhibitory neurons in synchrony-generating mechanisms, in both epileptic and non-epileptic conditions.

ABSTRACT

Understanding the role of different neuron types in synchrony generation is crucial for developing new therapies aiming to prevent hypersynchronous events such as epileptic seizures. Paroxysmal activity was linked to hyperexcitability and to bursting behaviour of pyramidal cells in animals. Human data suggested a leading role of either principal cells or interneurons, depending on the seizure morphology. In the present study, we aimed to uncover the role of excitatory and inhibitory processes in synchrony generation by analysing the activity of clustered single neurons during physiological and epileptiform synchronies in human neocortical slices. Spontaneous population activity was detected with a 24-channel laminar microelectrode in tissue derived from patients with or without preoperative clinical manifestations of epilepsy. This population activity disappeared by blocking GABA_A receptors, and several variations of spatially and temporally simple or complex interictal-like spikes emerged in epileptic tissue, whereas peritumoural slices generated only simple spikes. Around one-half of the clustered neurons participated with an elevated firing rate in physiological synchronies with a slight dominance of excitatory cells. By contrast, more than 90% of the neurons contributed to interictal-like spikes and seizures, and an intense and synchronous discharge of inhibitory neurons was associated with the start of these events. Intrinsically bursting principal cells fired later than other neurons. Our data suggest that a balanced excitation and inhibition characterized physiological synchronies, whereas disinhibition-induced epileptiform events were initiated mainly by non-synaptically synchronized inhibitory neurons. Our results further highlight the differences between humans and animal models, and between in vivo and (pharmacologically manipulated) in vitro conditions.

KCC2 antagonism increases neuronal network excitability but disrupts ictogenesis in vitro

The potassium-chloride cotransporter 2 (KCC2) plays a role in epileptiform synchronization, but it remains unclear how it influences such a process. Here, we used tetrode recordings in the in vitro rat entorhinal cortex (EC) to analyze the effects of the KCC2 antagonist VU0463271 on 4-aminopyridine (4AP)-induced ictal and interictal activity. During 4AP application, ictal events were associated with significant increases in interneurons and principal cells activities. VU0463271 application transformed ictal discharges to shorter ictal-like events that were not accompanied by significant increases in interneuron or principal cell firing. Interictal events persisted during VU0463271 application at an accelerated frequency of occurrence with significant increases in interneuron and principal cell activity. Further analysis revealed that interneuron and principal cell firing rate during 4AP-induced interictal events were increased after VU0463271 application without changes in synchronicity. Overall, our results demonstrate that in the EC, KCC2 antagonism enhances both interneuron and principal cell excitability, while paradoxically decreasing the ability of neuronal networks to generate structured ictal events.NEW & NOTEWORTHY We are the first to use tetrode recordings in the entorhinal cortex to demonstrate that antagonizing potassium-chloride cotransporter 2 (KCC2) function abolishes ictal discharges and the associated, dynamic changes in single-unit firing in the in vitro 4-aminopyrine model of epileptiform synchronization. Interictal discharges were, however, shorter and more frequent during KCC2 antagonism, while the associated single-unit activity increased, suggesting augmented neuronal excitability. Our findings highlight the complex role of KCC2 in disease pathology.

Inhibition, oscillations and focal seizures: An overview inspired by some historical notes

GABA (i.e., γ-amino-butyric acid) is the main inhibitory neurotransmitter in the adult mammalian brain. Once released from inhibitory cells, it activates pre- and post-synaptic GABA receptors that have been categorized into type A and type B. GABA_A receptors open ionotropic anionic channels while GABA_B receptors are metabotropic, acting through second messengers. In the 1980s, decreased GABA receptor signaling was considered an appealing factor in making cortical neurons generate synchronous epileptiform oscillations and thus a good, perhaps obvious, candidate for causing focal epileptic disorders. However, studies published during the last four decades have demonstrated that interneuron firing - which causes GABA release and thus GABA_A receptor activation - can lead to the generation of both physiological (e.g., theta and gamma oscillations or sharp wave-ripples) and pathological oscillations including focal interictal spikes, high frequency oscillations and seizures. Taken together, the reviews published in this special issue of Neurobiology of Disease highlight the key role of inhibition, and in particular of GABA_A receptor signaling, in neuronal network functions under physiological and pathological conditions that include epilepsy and Alzheimer's disease.

From adagio to allegretto: The changing tempo of theta frequencies in epilepsy and its relation to interneuron function

Despite decades of research, our understanding of epilepsy, including how seizures are generated and propagate, is incomplete. However, there is growing recognition that epilepsy is more than just the occurrence of seizures, with patients often experiencing comorbid deficits in cognition that are poorly understood. In addition, the available therapies for treatment of epilepsy, from pharmaceutical treatment to surgical resection and seizure prevention devices, often exacerbate deficits in cognitive function. In this review, we discuss the hypothesis that seizure generation and cognitive deficits have a similar pathological source characterized by, but not limited to, deficits in theta oscillations and their influence on interneurons. We present a new framework that describes oscillatory states in epilepsy as alternating between hyper- and hypo-synchrony rather than solely the spontaneous transition to hyper-excitability characterized by the seizures. This framework suggests that as neural oscillations, specifically in the theta range, vary their tempo from a slowed almost adagio tempo during interictal periods to faster, more rhythmic allegretto tempo preictally, they impact the function of interneurons, modulating their ability to control seizures and their role in cognitive processing. This slow wave oscillatory framework may help explain why current therapies that work to reduce hyper-excitability do not completely eliminate seizures and often lead to exacerbated cognitive deficits.

Hyperexcitability of the network contributes to synchronization processes in the human epileptic neocortex

KEY POINTS

Hyperexcitability and hypersynchrony of neuronal networks are thought to be linked to the generation of epileptic activity in both humans and animal models. Here we show that human epileptic postoperative neocortical tissue is able to generate two different types of synchronies in vitro. Epileptiform bursts occurred only in slices derived from epileptic patients and were hypersynchronous events characterized by high levels of excitability. Spontaneous population activity emerged in both epileptic and non-epileptic tissue, with a significantly lower degree of excitability and synchrony, and could not be linked to epilepsy. These results help us to understand better the role of excitatory and inhibitory neuronal circuits in the generation of population events, and to define the subtle border between physiological and pathological synchronies.

ABSTRACT

Interictal activity is a hallmark of epilepsy diagnostics and is linked to neuronal hypersynchrony. Little is known about perturbations in human epileptic neocortical microcircuits, and their role in generating pathological synchronies. To explore hyperexcitability of the human epileptic network, and its contribution to convulsive activity, we investigated an in vitro model of synchronous burst activity spontaneously occurring in postoperative tissue slices derived from patients with or without preoperative clinical and electrographic manifestations of epileptic activity. Human neocortical slices generated two types of synchronies. Interictal-like discharges (classified as epileptiform events) emerged only in epileptic samples, and were hypersynchronous bursts characterized by considerably elevated levels of excitation. Synchronous population activity was initiated in both epileptic and non-epileptic tissue, with a significantly lower degree of excitability and synchrony, and could not be linked to epilepsy. However, in pharmacoresistant epileptic tissue, a higher percentage of slices exhibited population activity, with higher local field potential gradient amplitudes. More intracellularly recorded neurons received depolarizing synaptic potentials, discharging more reliably during the events. Light and electron microscopic examinations showed slightly lower neuron densities and higher densities of excitatory synapses in the human epileptic neocortex. Our data suggest that human neocortical microcircuits retain their functionality and plasticity in vitro, and can generate two significantly different synchronies. We propose that population bursts might not be pathological events while interictal-like discharges may reflect the epileptogenicity of the human cortex. Our results show that hyperexcitability characterizes the human epileptic neocortical network, and that it is closely related to the emergence of synchronies.

Hippocampal sharp wave-ripple: A cognitive biomarker for episodic memory and planning

Sharp wave ripples (SPW-Rs) represent the most synchronous population pattern in the mammalian brain. Their excitatory output affects a wide area of the cortex and several subcortical nuclei. SPW-Rs occur during "off-line" states of the brain, associated with consummatory behaviors and non-REM sleep, and are influenced by numerous neurotransmitters and neuromodulators. They arise from the excitatory recurrent system of the CA3 region and the SPW-induced excitation brings about a fast network oscillation (ripple) in CA1. The spike content of SPW-Rs is temporally and spatially coordinated by a consortium of interneurons to replay fragments of waking neuronal sequences in a compressed format. SPW-Rs assist in transferring this compressed hippocampal representation to distributed circuits to support memory consolidation; selective disruption of SPW-Rs interferes with memory. Recently acquired and pre-existing information are combined during SPW-R replay to influence decisions, plan actions and, potentially, allow for creative thoughts. In addition to the widely studied contribution to memory, SPW-Rs may also affect endocrine function via activation of hypothalamic circuits. Alteration of the physiological mechanisms supporting SPW-Rs leads to their pathological conversion, "p-ripples," which are a marker of epileptogenic tissue and can be observed in rodent models of schizophrenia and Alzheimer's Disease. Mechanisms for SPW-R genesis and function are discussed in this review.

Hippocampus and epilepsy: Findings from human tissues

Surgical removal of the epileptogenic zone provides an effective therapy for several focal epileptic syndromes. This surgery offers the opportunity to study pathological activity in living human tissue for pharmacoresistant partial epilepsy syndromes including temporal lobe epilepsies with hippocampal sclerosis, cortical dysplasias, epilepsies associated with tumors and developmental malformations. Slices of tissue from patients with these syndromes retain functional neuronal networks and may generate epileptic activities. The properties of cells in this tissue may not be greatly changed, but excitatory synaptic transmission is often enhanced and GABAergic inhibition is preserved. Typically epileptic activity is not generated spontaneously by the neocortex, whether dysplastic or not, but can be induced by convulsants. The initiation of ictal discharges in the neocortex depends on both GABAergic signaling and increased extracellular potassium. In contrast, a spontaneous interictal-like activity is generated by tissues from patients with temporal lobe epilepsies associated with hippocampal sclerosis. This activity is initiated, not in the hippocampus but in the subiculum, an output region, which projects to the entorhinal cortex. Interictal events seem to be triggered by GABAergic cells, which paradoxically excite about 20% of subicular pyramidal cells while simultaneously inhibiting the majority. Interictal discharges thus depend on both GABAergic and glutamatergic signaling. The depolarizing effects of GABA depend on a pathological elevation in levels of chloride in some subicular cells, similar to those of developmentally immature cells. Such defect is caused by a perturbed expression of the cotransporters regulating intracellular chloride concentration, the importer NKCC1 and the extruder KCC2. Blockade of NKCC1 actions by the diuretic bumetanide restores intracellular chloride and thus hyperpolarizing GABAergic actions and consequently suppressing interictal activity.

Different mechanisms of ripple-like oscillations in the human epileptic subiculum

OBJECTIVE

Transient high-frequency oscillations (HFOs; 150-600Hz) in local field potentials generated by human hippocampal and parahippocampal areas have been related to both physiological and pathological processes. The cellular basis and effects of normal and abnormal forms of HFOs have been controversial. This lack of agreement is clinically significant, because HFOs may be good markers of epileptogenic areas. Better defining the neuronal correlate of specific HFO frequency bands could improve electroencephalographic analyses made before epilepsy surgery.

METHODS

Here, we recorded HFOs in slices of the subiculum prepared from human hippocampal tissue resected for treatment of pharmacoresistant epilepsy. With combined intra- or juxtacellular and extracellular recordings, we examined the cellular correlates of interictal and ictal HFO events.

RESULTS

HFOs occurred spontaneously in extracellular field potentials during interictal discharges (IIDs) and also during pharmacologically induced preictal discharges (PIDs) preceding ictal-like events. Many of these events included frequencies >250Hz and so might be considered pathological, but a significant proportion were spectrally similar to physiological ripples (150-250Hz). We found that IID ripples were associated with rhythmic γ-aminobutyric acidergic and glutamatergic synaptic potentials with moderate neuronal firing. In contrast, PID ripples were associated with depolarizing synaptic inputs frequently reaching the threshold for bursting in most pyramidal cells.

INTERPRETATION

Our data suggest that IID and PID ripple-like oscillations (150-250Hz) in human epileptic hippocampus are associated with 2 distinct population activities that rely on different cellular and synaptic mechanisms. Thus, the ripple band could not help to disambiguate the underlying cellular processes.

Cortical inhibition, pH and cell excitability in epilepsy: what are optimal targets for antiepileptic interventions?

Epilepsy is characterised by the propensity of the brain to generate spontaneous recurrent bursts of excessive neuronal activity, seizures. GABA-mediated inhibition is critical for restraining neuronal excitation in the brain, and therefore potentiation of GABAergic neurotransmission is commonly used to prevent seizures. However, data obtained in animal models of epilepsy and from human epileptic tissue suggest that GABA-mediated signalling contributes to interictal and ictal activity. Prolonged activation of GABA(A) receptors during epileptiform bursts may even initiate a shift in GABAergic neurotransmission from inhibitory to excitatory and so have a proconvulsant action. Direct targeting of the membrane mechanisms that reduce spiking in glutamatergic neurons may better control neuronal excitability in epileptic tissue. Manipulation of brain pH may be a promising approach and recent advances in gene therapy and optogenetics seem likely to provide further routes to effective therapeutic intervention.

Inhibitory networks of fast-spiking interneurons generate slow population activities due to excitatory fluctuations and network multistability

Slow population activities (SPAs) exist in the brain and have frequencies below ~5 Hz. Despite SPAs being prominent in several cortical areas and serving many putative functions, their mechanisms are not well understood. We studied a specific type of in vitro GABAergic, inhibition-based SPA exhibited by C57BL/6 murine hippocampus. We used a multipronged approach consisting of experiment, simulation, and mathematical analyses to uncover mechanisms responsible for hippocampal SPAs. Our results show that hippocampal SPAs are an emergent phenomenon in which the "slowness" of the network is due to interactions between synaptic and cellular characteristics of individual fast-spiking, inhibitory interneurons. Our simulations quantify characteristics underlying hippocampal SPAs. In particular, for hippocampal SPAs to occur, we predict that individual fast-spiking interneurons should have frequency-current (f-I) curves that exhibit a suitably sized kink where the slope of the curve decreases more abruptly in the gamma frequency range with increasing current. We also predict that these interneurons should be well connected with one another. Our mathematical analyses show that the combination of synaptic and intrinsic conditions, as predicted by our simulations, promotes network multistability. Population slow timescales occur when excitatory fluctuations drive the network between different stable network firing states. Since many of the parameters we use are extracted from experiments and subsequent measurements of experimental f-I curves of fast-spiking interneurons exhibit characteristics as predicted, we propose that our network models capture a fundamental operating mechanism in biological hippocampal networks.

High-frequency oscillations and other electrophysiological biomarkers of epilepsy: underlying mechanisms

In the normal mammalian brain, neuronal synchrony occurs on a spatial scale of submillimeters to centimeters and temporal scale of submilliseconds to seconds that is reflected in the occurrence of high-frequency oscillations, physiological sharp waves and slow wave sleep oscillations referred to as Up-Down states. In the epileptic brain, the well-studied pathologic counterparts to these physiological events are pathological high-frequency oscillations and interictal spikes that could be electrophysiological biomarkers of epilepsy. Establishing these abnormal events as biomarkers of epilepsy will largely depend on a better understanding of the mechanisms underlying their generation, which will not only help distinguish pathological from physiological events, but will also determine what roles these pathological events play in epileptogenesis and epileptogenicity. This article focuses on the properties and neuronal mechanisms supporting the generation of high-frequency oscillations and interictal spikes, and introduces a new phenomenon called Up-spikes.

Risperidone and divalproex differentially engage the fronto-striato-temporal circuitry in pediatric mania: a pharmacological functional magnetic resonance imaging study

OBJECTIVE

The current study examined the impact of risperidone and divalproex on affective and working memory circuitry in patients with pediatric bipolar disorder (PBD).

METHOD

This was a six-week, double-blind, randomized trial of risperidone plus placebo versus divalproex plus placebo for patients with mania (n = 21; 13.6 ± 2.5 years of age). Functional magnetic resonance imaging (fMRI) outcomes were measured using a block design, affective, N-back task with angry, happy, and neutral face stimuli at baseline and at 6-week follow-up. Matched healthy controls (HC; n = 15, 14.5 ± 2.8 years) were also scanned twice.

RESULTS

In post hoc analyses on the significant interaction in a 3×2×2 analysis of variance (ANOVA) that included patient groups and HC, the risperidone group showed greater activation after treatment in response to the angry face condition in the left subgenual anterior cingulate cortex (ACC) and striatum relative to the divalproex group. The divalproex group showed greater activation relative to the risperidone group in the left inferior frontal gyrus and right middle temporal gyrus. Over the treatment course, the risperidone group showed greater change in activation in the left ventral striatum than the divalproex group, and the divalproex group showed greater activation change in left inferior frontal gyrus and right middle temporal gyrus than the risperidone group. Furthermore, each patient group showed increased activation relative to HC in fronto-striato-temporal regions over time. The happy face condition was potentially less emotionally challenging in this study and did not elicit notable findings.

CONCLUSIONS

When patients performed a working memory task under emotional duress inherent in the paradigm, divalproex enhanced activation in a fronto-temporal circuit whereas risperidone increased activation in the dopamine (D₂) receptor-rich ventral striatum. Clinical trial registration information-Risperidone and Divalproex Sodium With MRI Assessment in Pediatric Bipolar; http://www.clinicaltrials.gov; NCT00176202.

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.

Distinct modes of modulation of GABAergic transmission by Group I metabotropic glutamate receptors in rat entorhinal cortex

Activation of metabotropic glutamate receptors (mGluRs) modulates synaptic transmission, whereas the roles of mGluRs in GABAergic transmission in the entorhinal cortex (EC) are elusive. Here, we examined the effects of mGluRs on GABAergic transmission onto the principal neurons in the superficial layers of the EC. Bath application of DHPG, a selective Group I mGluR agonist, increased the frequency and amplitude of spontaneous IPSCs (sIPSCs) whereas application of DCG-IV, an agonist for Group II mGluRs or L-AP4, an agonist for Group III mGluRs failed to change significantly sIPSC frequency and amplitude. Bath application of DHPG failed to change significantly the frequency and amplitude of miniature IPSCs (mIPSCs) recorded in the presence of tetradotoxin but significantly reduced the amplitude of IPSCs evoked by extracellular field stimulation or in synaptically connected interneuron-pyramidal neuron pairs in layer III of the EC. DHPG increased the frequency but reduced the amplitude of APs recorded from entorhinal interneurons. Bath application of DHPG generated membrane depolarization and increased the input resistance of GABAergic interneurons. DHPG-mediated depolarization of GABAergic interneurons was mediated by inhibition of background K(+) channels which are insensitive to extracellular Cs(+), TEA, 4-AP, and Ba(2+). DHPG-induced facilitation of sIPSCs was mediated by mGluR(5) and required the function of Galphaq but was independent of phospholipase C activity. Elevation of synaptic glutamate concentration by bath application of glutamate transporter inhibitors significantly increased sIPSC frequency and amplitude demonstrating a physiological role of mGluRs in GABAergic transmission. Our results provide a cellular and molecular mechanism to explain the physiological and pathological roles of mGluRs in the EC.

Inhibitory actions of the gamma-aminobutyric acid in pediatric Sturge-Weber syndrome

OBJECTIVE

The mechanisms of epileptogenesis in Sturge-Weber syndrome (SWS) are unknown. We explored the properties of neurons from human pediatric SWS cortex in vitro and tested in particular whether gamma-aminobutyric acid (GABA) excites neurons in SWS cortex, as has been suggested for various types of epilepsies.

METHODS

Patch-clamp and field potential recordings and dynamic biphoton imaging were used to analyze cortical tissue samples obtained from four 6- to 14-month-old pediatric SWS patients during surgery.

RESULTS

Neurons in SWS cortex were characterized by a relatively depolarized resting membrane potential, as was estimated from cell-attached recordings of N-methyl-D-aspartate channels. Many cells spontaneously fired action potentials at a rate proportional to the level of neuronal depolarization. The reversal potential for GABA-activated currents, assessed by cell-attached single channel recordings, was close to the resting membrane potential. All spontaneously firing neurons recorded in cell-attached mode or imaged with biphoton microscopy were inhibited by GABA. Spontaneous epileptiform activity in the form of recurrent population bursts was suppressed by glutamate receptor antagonists, the GABA(A) receptor agonist isoguvacine, and the positive allosteric GABA(A) modulator diazepam. Blockade of GABA(A) receptors aggravated spontaneous epileptiform activity. The NKCC1 antagonist bumetanide had little effect on epileptiform activity.

INTERPRETATION

SWS cortical neurons have a relatively depolarized resting membrane potential and spontaneously fire action potentials that may contribute to increased network excitability. In contrast to previous data depicting excitatory and proconvulsive actions of GABA in certain pediatric and adult epilepsies, GABA plays mainly an inhibitory and anticonvulsive role in SWS pediatric cortex.

Modulation of GABAergic transmission by muscarinic receptors in the entorhinal cortex of juvenile rats

Whereas the entorhinal cortex (EC) receives profuse cholinergic innervations from the basal forebrain and activation of cholinergic receptors has been shown to modulate the activities of the principal neurons and promote the intrinsic oscillations in the EC, the effects of cholinergic receptor activation on GABAergic transmission in this brain region have not been determined. We examined the effects of muscarinic receptor activation on GABA(A) receptor-mediated synaptic transmission in the superficial layers of the EC. Application of muscarine dose-dependently increased the frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) recorded from the principal neurons in layer II/III via activation of M(3) muscarinic receptors. Muscarine slightly reduced the frequency but had no effects on the amplitude of miniature IPSCs recorded in the presence of tetrodotoxin. Muscarine reduced the amplitude of IPSCs evoked by extracellular field stimulation and by depolarization of GABAergic interneurons in synaptically connected interneuron and pyramidal neuron pairs. Application of muscarine generated membrane depolarization and increased action potential firing frequency but reduced the amplitude of action potentials in GABAergic interneurons. Muscarine-induced depolarization of GABAergic interneurons was mediated by inhibition of background K(+) channels and independent of phospholipase C, intracellular Ca(2+) release, and protein kinase C. Our results demonstrate that activation of muscarinic receptors exerts diverse effects on GABAergic transmission in the EC.

Inhibition dominates in shaping spontaneous CA3 hippocampal network activities in vitro

We have assessed the balance of excitation and inhibition in in vitro rodent hippocampal slices exhibiting spontaneous, basal sharp waves (bSPWs). A defining signature of a network exhibiting bSPWs is the rise and fall in local field activities with frequencies ranging from 0.5 to 4.5 Hz. This variation of extracellular local field activities manifests at the intracellular level as postsynaptic potentials (PSPs). In correspondence with the local field bSPWs, we consider "sparse" and "synchronous" parts of bSPWs at the intracellular level. We have used intracellular data of bSPW-associated PSPs together with mathematical extraction techniques to quantify the mean and variance of synaptic conductances that a neuron experiences during bSPW episodes. We find that inhibitory conductances dominate in pyramidal cells and in a putative interneuron, and that inhibitory variances are much greater than excitatory ones during synchronous parts of bSPWs. Specifically, we find that there is at least a twofold increase in inhibitory conductance dominance from "sparse" to "synchronous" bSPW states and that this transition is associated with inhibitory fluctuations of greater than 10% of the change in mean inhibitory conductance. On the basis of our findings, we suggest that such inhibitory fluctuations during transition may be a physiological feature of systems expressing such population activities. In summary, our results provide a quantified basis for understanding the interaction of excitatory and inhibitory neuronal subpopulations in bSPW activities.

[Epileptiform activities generated in vitro by human temporal lobe tissue]

Drug-resistant partial epilepsies, including temporal lobe epilepsies with hippocampal sclerosis and cortical dysplasias, offer the opportunity to study human epileptic activity in vitro since the preferred therapy often consists of the surgical removal of the epileptogenic zone. Slices of this tissue retain functional neuronal networks and may generate epileptic activity. The properties of cells in this tissue do not seem to be significantly changed, but excitatory synaptic characteristics are enhanced and GABAergic inhibition is preserved. Typically, epileptic activity is not generated spontaneously by the neocortex, whether dysplastic or not, but can be induced by convulsants. The initiation of ictal discharges in neocortex depends on both GABAergic signaling and increased extracellular potassium. In contrast, a spontaneous interictal-like activity is generated by tissues from patients with temporal lobe epilepsies associated with hippocampal sclerosis. This activity is initiated not in the hippocampus but in the subiculum, an output region that projects to the entorhinal cortex. Interictal events seem to be triggered by GABAergic cells, which paradoxically excite approximately 20% of subicular pyramidal cells, while simultaneously inhibiting the majority. Interictal discharges are therefore sustained by both GABAergic and glutamatergic signaling. The atypical depolarizing effects of GABA depend on a pathological elevation in the basal levels of chloride in some subicular cells, similar to those of developmentally immature cells. This defect is caused by the perturbation of the expression of the cotransporters regulating the intracellular chloride concentration, the importer NKCC1, and the extruder KCC2. Blockade of excessive NKCC1 by the diuretic bumetanide restores intracellular chloride and thus hyperpolarizing GABAergic actions, suppressing interictal activity.

The MeCP2-null mouse hippocampus displays altered basal inhibitory rhythms and is prone to hyperexcitability

Rett syndrome is an autism-spectrum disorder caused by loss of function mutations within the gene encoding methyl CpG-binding protein 2 (MeCP2). While subtle decreases in synaptic plasticity have been detected within cortical and hippocampal neurons of Mecp2-null mice, only minimal information exists regarding how the loss of MeCP2 affects network activity in the brain. To address this issue, we compared the intrinsic network activities of Mecp2-null hippocampal slices derived from symptomatic mice to wild-type slices. Extracellular and whole-cell patch recordings revealed that although spontaneous, IPSP-based rhythmic activity is present in Mecp2-null slices; its frequency is significantly reduced from wild-type. This reduction was not associated with alterations in the gross electrophysiological properties of hippocampal neurons, but was associated with a decreased level of spontaneous glutamate receptor-mediated synaptic currents in hippocampal CA3 neurons. Paradoxically, however, repetitive sharp wave-like discharges were readily induced in the Mecp2-null hippocampal slices by a brief train of high-frequency stimulation commonly used to establish long-term potentiation at wild-type slices. Taken together, our data indicate that the Mecp2-null hippocampal CA3 circuit has diminished basal inhibitory rhythmic activity, which in turn renders the circuitry prone to hyperexcitability.

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.

Dysfunction of GABAA receptor glycolysis-dependent modulation in human partial epilepsy

A reduction in GABAergic neurotransmission has been put forward as a pathophysiological mechanism for human epilepsy. However, in slices of human epileptogenic neocortex, GABAergic inhibition can be clearly demonstrated. In this article we present data showing an increase in the functional lability of GABAergic inhibition in epileptogenic tissue compared with nonepileptogenic human tissue. We have previously shown that the glycolytic enzyme GAPDH is the kinase involved in the glycolysis-dependent endogenous phosphorylation of the alpha1-subunit of GABA(A) receptor, a mechanism necessary for maintaining GABA(A) function. In human epileptogenic cortex obtained during curative surgery of patients with partial seizures, we demonstrate an intrinsic deficiency of GABA(A) receptor endogenous phosphorylation resulting in an increased lability of GABAergic currents in neurons isolated from this tissue when compared with neurons from nonepileptogenic human tissue. This feature was not related to a reduction in the number of GABA(A) receptor alpha1-subunits in the epileptogenic tissue as measured by [(3)H]flunitrazepam photoaffinity labeling. Maintaining the receptor in a phosphorylated state either by favoring the endogenous phosphorylation or by inhibiting a membrane-associated phosphatase is needed to sustain GABA(A) receptor responses in epileptogenic cortex. The increased functional lability induced by the deficiency in phosphorylation can account for transient GABAergic disinhibition favoring seizure initiation and propagation. These findings imply new therapeutic approaches and suggest a functional link to the regional cerebral glucose hypometabolism observed in patients with partial epilepsy, because the dysfunctional GABAergic mechanism depends on the locally produced glycolytic ATP.

Spontaneous rhythmic field potentials of isolated mouse hippocampal-subicular-entorhinal cortices in vitro

The rodent hippocampal circuit is capable of exhibiting in vitro spontaneous rhythmic field potentials (SRFPs) of 1-4 Hz that originate from the CA3 area and spread to the CA1 area. These SRFPs are largely correlated with GABA-A IPSPs in pyramidal neurons and repetitive discharges in inhibitory interneurons. As such, their generation is thought to result from cooperative network activities involving both pyramidal neurons and GABAergic interneurons. Considering that the hippocampus, subiculum and entorhinal cortex function as an integrated system crucial for memory and cognition, it is of interest to know whether similar SRFPs occur in hippocampal output structures (that is, the subiculum and entorhinal cortex), and if so, to understand the cellular basis of these subicular and entorhinal SRFPs as well as their temporal relation to hippocampal SRFPs. We explored these issues in the present study using thick hippocampal-subicular-entorhinal cortical slices prepared from adult mice. SRFPs were found to spread from the CA1 area to the subicular and entorhinal cortical areas. Subicular and entorhinal cortical SRFPs were correlated with mixed IPSPs/EPSPs in local pyramidal neurons, and their generation was dependent upon the activities of GABA-A and AMPA glutamate receptors. In addition, the isolated subicular circuit could elicit SRFPs independent of CA3 inputs. We hypothesize that the SRFPs represent a basal oscillatory activity of the hippocampal-subicular-entorhinal cortices and that the subiculum functions as both a relay and an amplifier, spreading the SRFPs from the hippocampus to the entorhinal cortex.

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.

Postnatal development of intrinsic GABAergic rhythms in mouse hippocampus

The local circuitry of the mammalian limbic cortices, including the hippocampus, is capable of generating spontaneous rhythmic activities of 0.5-4 Hz when isolated in vitro. These rhythmic activities are mediated by synchronous inhibitory postsynaptic potentials in pyramidal neurons as the result of repeated discharges of inhibitory interneurons. As such, they are thought to represent an intrinsic inhibitory rhythm. It is unknown at present whether such a rhythm occurs in the immature rodent hippocampus and, if so, the postnatal time window in which it develops. We explored these issues using our recently developed whole mouse hippocampal isolate preparation in vitro. We found that spontaneous rhythmic field potentials started to emerge in mouse hippocampal isolates around postnatal day 10, stabilized after postnatal day 15 and persisted into adulthood. In postnatal days 11-14 mouse hippocampi, the properties of these rhythmic potentials were in keeping with a CA3-driven, IPSP-based intrinsic network activity. The lack of spontaneous field rhythm in neonatal (postnatal days 2-7) hippocampi cannot be attributed to the excitatory activities mediated by gamma-aminobutyric acid type A (GABA-A) receptors, as chloride-dependent hyperpolarizing inhibitory postsynaptic potentials were detectable in neonatal pyramidal neurons at voltages near resting potentials and pharmacological antagonisms of GABA-A receptors produced robust epileptiform discharges in neonatal hippocampi. High frequency afferent stimulation or applications of 4-aminopyridine at low micromolar concentrations failed to induce persistent field rhythm in neonatal hippocampi, suggesting that an overall weak glutamatergic drive is not the sole causing factor. We suggest that the inhibitory postsynaptic potential-based spontaneous rhythmic field potentials develop in a discrete time window during the second postnatal week in the mouse hippocampus due to a fine-tuning in the structure and function of CA3 recurrent circuitry and associated GABAergic inhibitory interneurons.

Antiepileptic effects of single and repeated oral administrations of S-312-d, a novel calcium channel antagonist, on tonic convulsions in spontaneously epileptic rats

We investigated the effects of single and repeated administrations of S-312-d (methyl-4,7-dihydro-3-isobutyl-6-methyl-4-(3-nitrophenyl)-thieno-[2,3-b]pyridine-5-carboxylate), a newly synthesized L-type Ca(2+)-channel blocker, on tonic convulsions and absence-like seizures in the spontaneously epileptic rat (SER: zi/zi, tm/tm), a genetically based animal model of human epilepsy. Single oral administrations of S-312-d dose-dependently inhibited tonic convulsions and the effects lasted for more than 2 h, although they did not attenuate the absence-like seizures. We also examined the effects of repeated administrations of S-312-d at 1 mg/kg once a day for 4 days on SER. A significant decrease in the number and total duration of tonic convulsions was observed 45 and 75 min after the first administration of the drug, respectively. The effects lasted for 24 h without changes in the background EEG or blood pressure. This inhibitory effect on the tonic convulsions was gradually strengthened by subsequent daily administrations of S-312-d and lasted for 3 days after the cessation of drug treatment. In contrast, the repeated treatment with S-312-d did not influence absence-like seizures of SER. These results suggest that S-312-d is a candidate drug that has antiepileptic effects against the convulsive seizures in human epilepsy.

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.

Periodic oscillatory activity in parahippocampal slices maintained in vitro

Brain slices maintained in vitro have been extensively used for studying neuronal synchronization. However, the validity of this approach may be questioned since pharmacological procedures are usually required to elicit spontaneous events similar to the EEG activity recorded in vivo. Here, we report that when superfused with control medium, rat brain slices comprising the entorhinal and perirhinal cortices along with a portion of the basolateral/lateral nuclei of the amygdala can synchronously generate periodic oscillatory activity at 5-11 Hz every 5-30 s. The periodic events: (i) correspond intracellularly to synaptic depolarizations in regularly firing neurons analyzed in the three areas; (ii) have no fixed site of onset; (iii) spread with time lags of 8-20 ms; and (iv) continue to occur asynchronously after their surgical isolation. NMDA receptor antagonism reduced the duration of the oscillatory events, while glutamatergic non-NMDA receptor antagonism abolished them. Activation of mu-opioid receptors, a procedure that hyperpolarizes interneurons thus decreasing GABA release, reversibly decreased the rate of occurrence of periodic oscillatory activity (POA). However, periodic events continued to occur during application of GABA(A) or GABA(B) receptor antagonists as well as in the presence of the cholinergic agent carbachol. We also found that POA was abolished by baclofen and irreversibly reduced by the gap junction decoupler carbenoxolone. These findings demonstrate that parahippocampal networks in a brain slice preparation can generate periodic, synchronous activity under quasi-physiological conditions. These network oscillations (i) reflect the activation of ionotropic glutamatergic and GABAergic receptors, (ii) are contributed by gap-junction interactions, and (iii) are controlled by GABA(B) receptors that are presumably located presynaptically.

Size does matter: generation of intrinsic network rhythms in thick mouse hippocampal slices

Rodent hippocampal slices of < or = 0.5 mm thickness have been widely used as a convenient in vitro model since the 1970s. However, spontaneous population rhythmic activities do not consistently occur in this preparation due to limited network connectivity. To overcome this limitation, we develop a novel slice preparation of 1 mm thickness from adult mouse hippocampus by separating dentate gyrus from CA3/CA1 areas but preserving dentate-CA3-CA1 connectivity. While superfused in vitro at 32 or 37 degrees C, the thick slice exhibits robust spontaneous network rhythms of 1-4 Hz that originate from the CA3 area. Via assessing tissue O2, K+, pH, synaptic, and single-cell activities of superfused thick slices, we verify that these spontaneous rhythms are not a consequence of hypoxia and nonspecific experimental artifacts. We suggest that the thick slice contains a unitary circuitry sufficient to generate intrinsic hippocampal network rhythms and this preparation is suitable for exploring the fundamental properties and plasticity of a functionally defined hippocampal "lamella" in vitro.

Cell type-specific synaptic dynamics of synchronized bursting in the juvenile CA3 rat hippocampus

Spontaneous synchronous bursting of the CA3 hippocampus in vitro is a widely studied model of physiological and pathological network synchronization. The role of inhibitory conductances during network bursting is not understood in detail, despite the fact that several antiepileptic drugs target GABA(A) receptors. Here, we show that the first manifestation of a burst event is a cell type-specific flurry of GABA(A) receptor-mediated inhibitory input to pyramidal cells, but not to stratum oriens horizontal interneurons. Moreover, GABA(A) receptor-mediated synaptic input is proportionally smaller in these interneurons compared with pyramidal cells. Computational models and dynamic-clamp studies using experimentally derived conductance waveforms indicate that both these factors modulate spike timing during synchronized activity. In particular, the different kinetics and the larger strength of GABAergic input to pyramidal cells defer action potential initiation and contribute to the observed delay of firing, so that the interneuronal activity leads the burst cycle. In contrast, excitatory inputs to both neuronal populations during a burst are kinetically similar, as required to maintain synchronicity. We also show that the natural pattern of activation of inhibitory and excitatory conductances during a synchronized burst cycle is different within the same neuronal population. In particular, GABA(A) receptor-mediated currents activate earlier and outlast the excitatory components driving the bursts. Thus, cell type-specific balance and timing of GABA(A) receptor-mediated input are critical to set the appropriate spike timing in pyramidal cells and interneurons and coordinate additional neurotransmitter release modulating burst strength and network frequency.

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.

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.

Control of bursting by local inhibition in the rat subiculum in vitro

The subiculum, which provides the major hippocampal output, contains different cell types including weak/strong bursting and regular-spiking cells, and fast-spiking interneurons. These cellular populations play different roles in the generation of physiological rhythms and epileptiform activity. However, their intrinsic connectivity and the synaptic regulation of their discharge patterns remain unknown. In the present study, the local synaptic responses of subicular cell types were examined in vitro. To this purpose, slices were prepared at a specific orientation that permitted the antidromic activation of projection cells as a tool to examine local circuits. Patch recordings in cell-attached and whole-cell configurations were combined with neurobiotin labelling to classify cell types. Strong (approximately 75 %), but not weak (approximately 22 %), bursting cells typically fired bursts in response to local synaptic excitation, whereas the majority of regular-spiking cells (approximately 87 %) remained silent. Local excitation evoked single spikes in more than 70 % of fast-spiking interneurons. This different responsiveness was determined by intrinsic membrane properties and not by the amplitude and pharmacology of synaptic currents. Inhibitory GABAergic responses were also detected in some cells, typically as a component of an excitatory/inhibitory sequence. A positive correlation between the latency of the excitatory and inhibitory responses, together with the glutamatergic control (via non-NMDA receptors) of inhibition, suggested a local mechanism. The effect of local inhibition on synaptically activated firing of different cell types was evaluated. It is shown that projection bursting cells of the subiculum are strongly controlled by local inhibitory circuits.

Effect of levetiracetam on epileptiform discharges in human neocortical slices

PURPOSE

The anticonvulsant effects of the novel antiepileptic drug (AED) levetiracetam (LEV) were tested in neocortical slice preparations from 23 patients who underwent surgery for the treatment of refractory epilepsy.

METHODS

Slices were used to evaluate the effects of LEV on two different models of epilepsy: low-Mg2+-induced untriggered and bicuculline-evoked stimulus-triggered epileptiform burst discharges and spontaneously appearing rhythmic sharp waves.

RESULTS

LEV (0.1-1 mM) did not influence spontaneously appearing rhythmic sharp waves or Mg2+-free aCSF-induced epileptiform field potentials. LEV affected neither the amplitudes or duration nor the repetition rates of burst discharges in these epilepsy models. However, LEV (100-500 microM) significantly suppressed the ictal-like discharges elicited by the gamma-aminobutyric acid subtype A (GABAA)-receptor antagonist bicuculline. A marked reduction of the amplitude and duration of bicuculline-evoked field response in the presence of LEV was observed.

CONCLUSIONS

The results indicate the potential for LEV to inhibit epileptiform burst discharges in human neocortical tissue, which is consistent with its effects in animal models of epilepsy. These results also support the seizure reduction observed in clinical trials and support that this may, in part, be related to the ability of LEV to inhibit epileptiform discharges.

On the origin of interictal activity in human temporal lobe epilepsy in vitro

The origin and mechanisms of human interictal epileptic discharges remain unclear. Here, we describe a spontaneous, rhythmic activity initiated in the subiculum of slices from patients with temporal lobe epilepsy. Synchronous events were similar to interictal discharges of patient electroencephalograms. They were suppressed by antagonists of either glutamatergic or gamma-aminobutyric acid (GABA)-ergic signaling. The network of neurons discharging during population events comprises both subicular interneurons and a subgroup of pyramidal cells. In these pyramidal cells, GABAergic synaptic events reversed at depolarized potentials. Depolarizing GABAergic responses in neurons downstream to the sclerotic CA1 region contribute to human interictal activity.

Electrophysiological properties of interneurons from intraoperative spiking areas of epileptic human temporal neocortex

We combined visual-assisted whole-cell recordings with Lucifer Yellow labelling to record in vitro from interneuron-like cells from human epileptic neocortical tissue that exhibited spiking activity in situ. Information from intraoperative electrocorticography, which was performed to tailor resection, were used to select those areas that participate in interictal spiking. Whole-cell recordings from slices prepared from these areas showed that both pyramidal and interneuron-like cells were present and retained their main intrinsic firing patterns, i.e. regular spiking and fast spiking, respectively. Moreover, non-pyramidal interneuron-like cells remained innervated by excitatory inputs as both spontaneous and evoked non-NMDA mediated excitatory postsynaptic potentials were recorded. We conclude that putative inhibitory GABAergic cells are present and functional in the human epileptic tissue.