Conditions sufficient for nonsynaptic epileptogenesis in the CA1 region of hippocampal slices

Mecamylamine inhibits seizure-like activity in CA1-CA3 hippocampus through antagonism to nicotinic receptors

Cholinergic modulation of hippocampal network function is implicated in multiple behavioral and cognitive states. Activation of nicotinic and muscarinic acetylcholine receptors affects neuronal excitability, synaptic transmission and rhythmic oscillations in the hippocampus. In this work, we studied the ability of the cholinergic system to sustain hippocampal epileptiform activity independently from glutamate and GABA transmission. Simultaneous CA3 and CA1 field potential recordings were obtained during the perfusion of hippocampal slices with the aCSF containing AMPA, NMDA and GABA receptor antagonists. Under these conditions, spontaneous epileptiform discharges synchronous between CA3 and CA1 were recorded. Epileptiform discharges were blocked by addition of the calcium-channel blocker Cd²⁺ and disappeared in CA1 after a surgical cut between CA3 and CA1. Cholinergic antagonist mecamylamine abolished CA3-CA1 synchronous epileptiform discharges, while antagonists of α7 and α4β2 nAChRs, MLA and DhβE, had no effect. Our results suggest that activation of nicotinic acetylcholine receptors can sustain CA3-CA1 synchronous epileptiform activity independently from AMPA, NMDA and GABA transmission. In addition, mecamylamine, but not α7 and α4β2 nAChRs antagonists, reduced bicuculline-induced seizure-like activity. The ability of mecamylamine to decrease hippocampal network synchronization might be associated with its therapeutic effects in a wide variety of CNS disorders including addiction, depression and anxiety.

Antiepileptic effects of cobalt, manganese and magnesium on bicuculline-induced epileptiform activity in hippocampal neurons

BACKGROUND

Calcium signaling is described as a relevant factor in synchronization of neurons and increased excitability in epileptogenesis. Aim of the present investigations was to test the antiepileptic effect of the classical inorganic calcium channel blockers cobalt (Co²⁺), manganese (Mn²⁺) and magnesium (Mg²⁺).

METHODS

Experiments were carried out on hippocampal slices of guinea pigs. Epileptiform field potentials (EFP) were elicited by adding bicuculline (10 µmol/l) to the artificial cerebrospinal fluid (CSF). Kalium was elevated from normal (4 mmol/l) to 8 mmol/l. Co²⁺ (CoCl₂; 2, 1, 0.5 and 0.1 mmol/l), Mn²⁺ (MnCl₂; 2, 1, 0.5 and 0.1 mmol/l) and Mg²⁺ (MgCl₂; 8, 6, 5, 4 and 2 mmol/l) were added to the superfusate.

RESULTS

Concentrations of 2, 1 and 0.5 mmol/l Co²⁺, 2 and 1 mmol/l Mn²⁺ and 8 respectively 6 mmol/l Mg²⁺ were able to suppress EFP sufficient in a dose dependent manner. In concentrations of 0.1 mmol/l Co²⁺, 0.5 mmol/l and 0.1 mmol/l Mn²⁺ and 5 respectively 4 and 2 mmol/l Mg²⁺ suppression was incomplete. With washout of the inorganic calcium channel blockers the EFP reappeared.

DISCUSSION

All tree inorganic calcium channel blockers were able to suppress EFP in a dosage dependent and reversible manner. Weak reappearance of EFP after washout of Co²⁺ might be due to additional cytotoxic effects. The following mechanisms may contribute: i) blockade of voltage-activated calcium channels in the postsynaptic membrane, ii) changes in the activation of voltage-dependent sodium channels, iii) blockade of synaptic transmission.

Reducing Extracellular Ca2+ Induces Adenosine Release via Equilibrative Nucleoside Transporters to Provide Negative Feedback Control of Activity in the Hippocampus

Neural circuit activity increases the release of the purine neuromodulator adenosine into the extracellular space leading to A₁ receptor activation and negative feedback via membrane hyperpolarization and inhibition of transmitter release. Adenosine can be released by a number of different mechanisms that include Ca²⁺ dependent processes such as the exocytosis of ATP. During sustained pathological network activity, ischemia and hypoxia the extracellular concentration of calcium ions (Ca²⁺) markedly falls, inhibiting exocytosis and potentially reducing adenosine release. However it has been observed that reducing extracellular Ca²⁺ can induce paradoxical neural activity and can also increase adenosine release. Here we have investigated adenosine signaling and release mechanisms that occur when extracellular Ca²⁺ is removed. Using electrophysiology and microelectrode biosensor measurements we have found that adenosine is directly released into the extracellular space by the removal of extracellular Ca²⁺ and controls the induced neural activity via A₁ receptor-mediated membrane potential hyperpolarization. Following Ca²⁺ removal, adenosine is released via equilibrative nucleoside transporters (ENTs), which when blocked leads to hyper-excitation. We propose that sustained action potential firing following Ca²⁺ removal leads to hydrolysis of ATP and a build-up of intracellular adenosine which then effluxes into the extracellular space via ENTs.

Structural Diversity and Biological Activities of Indole Diketopiperazine Alkaloids from Fungi

Indole diketopiperazine alkaloids are secondary metabolites of microorganisms that are widely distributed in filamentous fungi, especially in the genera Aspergillus and Penicillium of the phylum Ascomycota or sac fungi. These alkaloids represent a group of natural products characterized by diversity in both chemical structures and biological activities. This review aims to summarize 166 indole diketopiperazine alkaloids from fungi published from 1944 to mid-2015. The emphasis is on diverse chemical structures within these alkaloids and their relevant biological activities. The aim is to assess which of these compounds merit further study for purposes of drug development.

Hypocalcemia-induced seizure: demystifying the calcium paradox

Calcium is essential for both neurotransmitter release and muscle contraction. Given these important physiological processes, it seems reasonable to assume that hypocalcemia may lead to reduced neuromuscular excitability. Counterintuitively, however, clinical observation has frequently documented hypocalcemia’s role in induction of seizures and general excitability processes such as tetany, Chvostek’s sign, and bronchospasm. The mechanism of this calcium paradox remains elusive, and very few pathophysiological studies have addressed this conundrum. Nevertheless, several studies primarily addressing other biophysical issues have provided some clues. In this review, we analyze the data of these studies and propose an integrative model to explain this hypocalcemic paradox.

The role of local field potential coupling in epileptic synchronization

THIS REVIEW HOPES TO CLEARLY EXPLAIN THE FOLLOWING VIEWPOINTS

(1) Neuronal synchronization underlies brain functioning, and it seems possible that blocking excessive synchronization in an epileptic neural network could reduce or even control seizures. (2) Local field potential coupling is a very common phenomenon during synchronization in networks. Removal of neurons or neuronal networks that are coupled can significantly alter the extracellular field potential. Interventions of coupling mediated by local field potentials could result in desynchronization of epileptic seizures. (3) The synchronized electrical activity generated by neurons is sensitive to changes in the size of the extracellular space, which affects the efficiency of field potential transmission and the threshold of cell excitability. (4) Manipulations of the field potential fluctuations could help block synchronization at seizure onset.

Heterogeneous firing behavior during ictal-like epileptiform activity in vitro

Seizure activity in vivo is caused by populations of neurons displaying a high degree of variability in activity pattern during the attack. The reason for this variability is not well understood. Here we show in an in vitro preparation that hippocampal CA1 pyramidal cells display four types of afterdischarge behavior during stimulus-induced ictal-like events in the presence of Cs(+) (5 mM): type I (43.7%) consisting of high-frequency firing riding on a plateau potential; type II (28.2%) consisting of low-frequency firing with no plateau potential; type III (18.3%) consisting of high-frequency firing with each action potential preceded by a transient hyperpolarization and time-locked to population activity, no plateau potential; "passive" (9.9%) typified by no afterdischarge. Type I behavior was blocked by TTX (0.2 μM) and intracellular injection of QX314 (12.5-25 mM). TTX (0.2 μM) or phenytoin (50 μM) terminated ictal-like events, suggesting that the persistent Na(+) current (I(NaP)) is pivotal for type I behavior. Type I behavior was not correlated to intrinsic bursting capability. Blockade of the M current (I(M)) with linopirdine (10 μM) increased the ratio of type I neurons to 100%, whereas enhancing I(M) with retigabine (50-100 μM) greatly reduced the epileptiform activity. These results suggest an important role of I(M) in determining afterdischarge behavior through control of I(NaP) expression. We propose that type I neurons act as pacemakers, which, through synchronization, leads to recruitment of type III neurons. Together, they provide the "critical mass" necessary for ictogenesis to become regenerative.

Orthogonal wave propagation of epileptiform activity in the planar mouse hippocampus in vitro

PURPOSE

In vitro brain preparations have been used extensively to study the generation and propagation of epileptiform activity. Transverse and longitudinal slices of the rodent hippocampus have revealed various patterns of propagation. Yet intact connections between the transverse and longitudinal pathways should generate orthogonal (both transverse and longitudinal) propagation of seizures involving the entire hippocampus. This study utilizes the planar unfolded mouse hippocampus preparation to reveal simultaneous orthogonal epileptiform propagation and to test a method of arresting propagation.

METHODS

This study utilized an unfolded mouse hippocampus preparation. It was chosen due to its preservation of longitudinal neuronal processes, which are thought to play an important role in epileptiform hyperexcitability. 4-Aminopyridine (4-AP), microelectrodes, and voltage-sensitive dye imaging were employed to investigate tissue excitability.

KEY FINDINGS

In 50-μm 4-AP, stimulation of the stratum radiatum induced transverse activation of CA3 cells but also induced a longitudinal wave of activity propagating along the CA3 region at a speed of 0.09 m/s. Without stimulation, a wave originated at the temporal CA3 and propagated in a temporal-septal direction could be suppressed with glutamatergic receptor antagonists. Orthogonal propagation traveled longitudinally along the CA3 pathway, secondarily invading the CA1 region at a velocity of 0.22 ± 0.024 m/s. Moreover, a local lesion restricted to the CA3 region could arrest wave propagation.

SIGNIFICANCE

These results reveal a complex two-dimensional epileptiform wave propagation pattern in the hippocampus that is generated by a combination of synaptic transmission and axonal propagation in the CA3 recurrent network. Epileptiform propagation block via a transverse selective CA3 lesion suggests a potential surgical technique for the treatment of temporal lobe epilepsy.

Potassium diffusive coupling in neural networks

Conventional neural networks are characterized by many neurons coupled together through synapses. The activity, synchronization, plasticity and excitability of the network are then controlled by its synaptic connectivity. Neurons are surrounded by an extracellular space whereby fluctuations in specific ionic concentration can modulate neuronal excitability. Extracellular concentrations of potassium ([K(+)](o)) can generate neuronal hyperexcitability. Yet, after many years of research, it is still unknown whether an elevation of potassium is the cause or the result of the generation, propagation and synchronization of epileptiform activity. An elevation of potassium in neural tissue can be characterized by dispersion (global elevation of potassium) and lateral diffusion (local spatial gradients). Both experimental and computational studies have shown that lateral diffusion is involved in the generation and the propagation of neural activity in diffusively coupled networks. Therefore, diffusion-based coupling by potassium can play an important role in neural networks and it is reviewed in four sections. Section 2 shows that potassium diffusion is responsible for the synchronization of activity across a mechanical cut in the tissue. A computer model of diffusive coupling shows that potassium diffusion can mediate communication between cells and generate abnormal and/or periodic activity in small (section sign 3) and in large networks of cells (section sign 4). Finally, in section sign 5, a study of the role of extracellular potassium in the propagation of axonal signals shows that elevated potassium concentration can block the propagation of neural activity in axonal pathways. Taken together, these results indicate that potassium accumulation and diffusion can interfere with normal activity and generate abnormal activity in neural networks.

Differential influence of non-synaptic mechanisms in two in vitro models of epileptic field bursts

Non-synaptic interactions are known to promote epileptiform activity through mechanisms that have primarily been studied in one particular in vitro model (low Ca(2+) model). Here we characterize another non-synaptic model, where ictal-like field bursts are induced in the CA1 area of rat hippocampal slices by exposure to Cs(+) (4-5mM) together with blockers of fast chemical synaptic transmission, and compare it with the low Ca(2+) model. The Cs-induced field bursts were blocked by 1 microM tetrodotoxin, but persisted in the presence of 200 microM Cd(2+) or 300 microM Ni(2+). Hyperosmotic condition (addition of 30 mM sucrose), reduced burst amplitude, but, unlike field bursts induced by 0mM Ca(2+)/5.25 mM K(+), sucrose had no effect on frequency or duration. Intracellular alkalinization-acidification sequence induced by NH(4)Cl potentiated and blocked, respectively, the field bursts. Octanol (100-250 microM) blocked all activity in most experiments. A quantitative comparison of three gap junction antagonists (carbenoxolone (100 microM), quinidine (100-250 microM), and endothelin-3 (1-2 microM)) indicated that gap junction communication is implicated in both models. However, endothelin-3 had selective effect on the low Ca(2+)-induced field burst. The data suggest that extracellular space-dependent processes, including field effects, significantly contribute to ongoing field burst activity, whereas initiation of a field burst can occur with or without the aid of such interactions, depending on the level of neuronal excitability. Gap junctions seem to have a general role in initiating field bursts. However, the contribution to this effect from neuronal versus glial connexin types differs in the two epileptic models studied.

Firing patterns and synchronization in nonsynaptic epileptiform activity: the effect of gap junctions modulated by potassium accumulation

Several lines of evidence point to the modification of firing patterns and of synchronization due to gap junctions (GJs) as having a role in the establishment of epileptiform activity (EA). However, previous studies consider GJs as ohmic resistors, ignoring the effects of intense variations in ionic concentration known to occur during seizures. In addition to GJs, extracellular potassium is regarded as a further important factor involved in seizure initiation and sustainment. To analyze how these two mechanisms act together to shape firing and synchronization, we use a detailed computational model for in vitro high-K(+) and low-Ca(2+) nonsynaptic EA. The model permits us to explore the modulation of electrotonic interactions under ionic concentration changes caused by electrodiffusion in the extracellular space, altered by tortuosity. In addition, we investigate the special case of null GJ current. Increased electrotonic interaction alters bursts and action potential frequencies, favoring synchronization. The particularities of pattern changes depend on the tortuosity and array size. Extracellular potassium accumulation alone modifies firing and synchronization when the GJ coupling is null.

Chemistry and biological activity of azoprenylated secondary metabolites

N-Prenyl secondary metabolites (isopentenylazo-, geranylazo-, farnesylazo- and their biosynthetic derivatives) represent a family of extremely rare natural products. Only in recent years have these alkaloids been recognized as interesting and valuable biologically active secondary metabolites. To date about 35 alkaloids have been isolated from plants mainly belonging to the Rutaceae family, and from fungi, bacteria, and/or obtained by chemical synthesis. These metabolites comprise anthranilic acid derivatives, diazepinones, and indole, and xanthine alkaloids. Many of the isolated prenylazo secondary metabolites and their semisynthetic derivatives are shown to exert valuable in vitro and in vivo anti-cancer, anti-inflammatory, anti-bacterial, anti-viral, and anti-fungal effects. The aim of this comprehensive review is to examine the different types of prenylazo natural products from a chemical, phytochemical and biological perspective.

Organic and inorganic calcium antagonists inhibit veratridine-induced epileptiform activity in CA3 neurons of the guinea pig

Veratridine is believed to cause epileptiform discharges via its effects on sodium channels. We addressed the question whether calcium currents, known to contribute to the generation of paroxysmal depolarization shifts (PDS) in most models of epilepsies, also contribute to veratridine-induced epileptiform activity. Therefore, we recorded from CA3 neurons (n=50) of veratridine-treated hippocampal slices and analyzed the effects of two calcium antagonists. Veratridine (0.5-1.0 microM) elicited spontaneous epileptiform bursts, paroxysmal depolarization shifts (PDS) lasting 100-300 ms, and depolarizations (LD) lasting up to several minutes. Most often PDS directly preceded LD which resulted in typical composite depolarizations termed veratridine-induced complexes (VC). VC persisted even in the presence of CNQX and APV (25 micromol/l, both), or in nominally calcium-free saline, revealing the non-synaptic nature of these potentials. Cobalt (1-2mM) abolished VC within minutes, but allowed LD type-like potentials to be elicited by depolarizing current pulses. Verapamil (50 microM) also diminished or abolished amplitudes of VC. All inhibitory effects of cobalt and verapamil were at least partly reversible. Due to the effects of both calcium antagonists we conclude that veratridine-induced epileptiform activity depends not only on sodium, but also on calcium currents.

Cellular and network mechanisms of electrographic seizures

Epileptic seizures constitute a complex multiscale phenomenon that is characterized by synchronized hyperexcitation of neurons in neuronal networks. Recent progress in understanding pathological seizure dynamics provides crucial insights into underlying mechanisms and possible new avenues for the development of novel treatment modalities. Here we review some recent work that combines in vivo experiments and computational modeling to unravel the pathophysiology of seizures of cortical origin. We particularly focus on how activity-dependent changes in extracellular potassium concentration affects the intrinsic dynamics of neurons involved in cortical seizures characterized by spike/wave complexes and fast runs.

Periodicity in Na+ channel properties alters excitability of a model neuron

The voltage gated Na channels play vital role in action potential waveform shaping and propagation. We have shown earlier that the duration and amplitude of a prolonged depolarization alter all the steady state and kinetic parameters of rNa(v)1.2a voltage gated Na channel in a pseudo-oscillatory fashion. In the present study, we show that the Hodgkin-Huxley voltage and time dependent rate constants of activation (alpha(m) and beta(m)) and fast inactivation (alpha(h) and beta(h)), obtained from the analyses of Na currents and steady state activation and inactivation plots, following application of prepulses in both slow (1-100s) and fast (100-1000ms) ranges, vary with the duration of a prepulse in a pseudo-oscillatory manner. Using these Hodgkin-Huxley kinetic parameters in simulation, the excitability and firing pattern of a model neuron are shown to vary in a history dependent periodic fashion.

Preseizure increased gamma electroencephalographic activity has no effect on extracellular potassium or calcium

Extracellular ion concentrations change during seizures in seizure models. [K(+)](o) increases and [Ca(2+)](o) decreases, resulting from population discharges, enhanced neuronal excitability, though not obviously before seizure onset. In acute pharmacological epilepsy models, there are striking increases in preictal high-frequency (gamma) electroencephalographic (EEG) activity. It is not known whether enhanced gamma EEG results in ionic changes, because gamma and ions have not been measured simultaneously. In this study, unanesthetized, paralyzed rats were given intravenous injections of kainic acid or picrotoxin to induce EEG discharges. Changes in EEG, [K(+)](o), and [Ca(2+)](o) in cortex and hippocampus were recorded. Kainic acid caused small [K(+)](o) fluctuations, without a temporal relationship of these with increased gamma EEG or with onset of discharges. Gamma EEG increases after picrotoxin also failed to affect [K(+)](o) and [Ca(2+)](o). Picrotoxin-induced electrical discharges led to [K(+)](o) rises of >9 mM and [Ca(2+)](o) falls of 0.1-0.2 mM. Kainic acid-induced discharges generated only moderate (2-3 mM) rises in [K(+)](o) and no changes in [Ca(2+)](o). In both models, there were large potassium rises (15-80 mM) and calcium falls (>0.5 mM), suggesting spreading depressions. Small [K(+)](o) fluctuations after kainic acid are consistent with disruption in potassium homeostasis, possibly because of depolarization of astrocytes. To reveal possible latent [K(+)](o) or [Ca(2+)](o) changes, we injected fluorocitrate intracortically to impair astrocytic function, before administering picrotoxin. Even fluorocitrate did not cause gamma-related ion changes but did cause low-magnitude, transient, potassium increases and slower potassium homeostasis during discharges, minor changes consistent with involvement of both astrocytes and neurons in [K(+)](o) regulation. (c) 2007 Wiley-Liss, Inc.

Verruculogen associated with Aspergillus fumigatus hyphae and conidia modifies the electrophysiological properties of human nasal epithelial cells

Background

The role of Aspergillus fumigatus mycotoxins in the colonization of the respiratory tract by conidia has not been studied extensively, even though patients at risk from invasive aspergillosis frequently exhibit respiratory epithelium damage. In a previous study, we found that filtrates of A. fumigatus cultures can specifically alter the electrophysiological properties of human nasal epithelial cells (HNEC) compared to those of non pathogenic moulds.

Results

We fractionated the organic phase of filtrate from 3-day old A. fumigatus cultures using high-performance liquid chromatography. The different fractions were tested for their ability to modify the electrophysiological properties of HNEC in an in vitro primary culture model.

The fraction collected between 20 and 30 min mimicked the effects of the whole filtrate, i.e. decrease of transepithelial resistance and increase of potential differences, and contained secondary metabolites such as helvolic acid, fumagillin, and verruculogen. Only verruculogen (10^-8 M) had effects similar to the whole filtrate. We verified that verruculogen was produced by a collection of 67 human, animal, plant and environmental A. fumigatus isolates. Using MS-MS analysis, we found that verruculogen was associated with both mycelium and conidia extracts.

Conclusion

Verruculogen is a secondary metabolite that modifies the electrophysiological properties of HNEC. The role of these modifications in the colonization and invasion of the respiratory epithelium by A. fumigatus on first contact with the epithelium remains to be determined.

The effect of neuronal population size on the development of epileptiform discharges in the low calcium model of epilepsy

The CA1 region of the rat hippocampal slice generates spontaneous electrographic seizures (field bursts) when exposed to ACSF containing < or = 0.2 mM calcium. It has been proposed that, particularly during the early part of a field burst, synchronised activity in small independent aggregates of neurons results in low amplitude irregular population spikes and subsequent fusion of aggregates generates high amplitude, regular discharging spikes. In the present experiments, we have tested the hypothesis that progression from aggregate formation to aggregate fusion requires a critical mass of participating neurons. We found that isolated CA1 segments >2 mm are still able to generate high amplitude, regular discharging population spikes, but when segment length is reduced to 1-2 mm, only 29% generate spikes with these characteristics; in the remainder, the field burst shows a DC shift+/-low amplitude irregular population spikes. No field bursts were seen in segments < 0.7 mm or in 50% of those 0.7-1 mm in length (in the remaining 50%, only the DC component of the field burst was present). Exposing 1-2 mm segments to hypo-osmolar perfusate induced a return of high amplitude rhythmic discharging population spikes in the field burst. We interpret these observations by indicating that progression from aggregate formation to aggregate fusion requires a critical neuronal mass and can be enhanced by reducing osmolarity of the perfusate.

Effects of Potassium Concentration on Firing Patterns of Low-Calcium Epileptiform Activity in Anesthetized Rat Hippocampus: Inducing of Persistent Spike Activity

PURPOSE

It has been shown that a low-calcium high-potassium solution can generate ictal-like epileptiform activity in vitro and in vivo. Moreover, during status epileptiform activity, the concentration of [K+]o increases, and the concentration of [Ca2+]o decreases in brain tissue. Therefore we tested the hypothesis that long-lasting persistent spike activity, similar to one of the patterns of status epilepticus, could be generated by a high-potassium, low-calcium solution in the hippocampus in vivo.

METHODS

Artificial cerebrospinal fluid was perfused over the surface of the exposed left dorsal hippocampus of anesthetized rats. A stimulating electrode and a recording probe were placed in the CA1 region.

RESULTS

By elevating K+ concentration from 6 to 12 mM in the perfusate solution, the typical firing pattern of low-calcium ictal bursts was transformed into persistent spike activity in the CA1 region with synaptic transmission being suppressed by calcium chelator EGTA. The activity was characterized by double spikes repeated at a frequency approximately 4 Hz that could last for >1 h. The analysis of multiple unit activity showed that both elevating [K+]o and lowering [Ca2+]o decreased the inhibition period after the response of paired-pulse stimulation, indicating a suppression of the after-hyperpolarization (AHP) activity.

CONCLUSIONS

These results suggest that persistent status epilepticus-like spike activity can be induced by nonsynaptic mechanisms when synaptic transmission is blocked. The unique double-spike pattern of this activity is presumably caused by higher K+ concentration augmenting the frequency of typical low-calcium nonsynaptic burst activity.

Fast Pseudo-Periodic Oscillation in the Rat Brain Voltage-gated Sodium Channel α Subunit

In the work reported here, we have investigated the changes in the activation and fast inactivation properties of the rat brain voltage-gated sodium channel (rNa(v) 1.2a) alpha subunit, expressed heterologously in the Chinese Hamster Ovary (CHO) cells, by short depolarizing prepulses (10-1000 ms). The time constant of recovery from fast inactivation (tau(fast)) and steady-state parameters for activation and inactivation varied in a pseudo-oscillatory fashion with the duration and amplitude of a sustained prepulse. A consistent oscillation was observed in most of the steady-state and non-inactivating current parameters with a time period close to 225 ms, although a faster oscillation of time period 125 ms was observed in the tau(fast). The studies on the non-inactivating current and steady-state activation indicate that the phase of oscillation varies from cell to cell. Co-expression of the beta1 subunit with the alpha subunit channel suppressed the oscillation in the charge movement per single channel and free energy of steady-state inactivation, although the oscillation in the half steady-state inactivation potential remained unaltered. Incidentally, the frequencies of oscillation in the sodium channel parameters (4-8 Hz) correspond to the theta component of network oscillation. This fast pseudo-oscillatory mechanism, together with the slow pseudo-oscillatory mechanism found in these channels earlier, may contribute to the oscillations in the firing properties observed in various neuronal subtypes and many pathological conditions.

Decrease in synaptic transmission can reverse the propagation direction of epileptiform activity in hippocampus in vivo

Most types of epileptiform activity with synaptic transmission have been shown to propagate from the CA3 to CA1 region in hippocampus. However, nonsynaptic epileptiform activity induced in vitro is known to propagate slowly from the caudal end of CA1 toward CA2/CA3. Understanding the propagation modes of epileptiform activity, and their causality is important to revealing the underlying mechanisms of epilepsy and developing new treatments. In this paper, the effect of the synaptic transmission suppression on the propagation of epilepsy in vivo was investigated by using multiple-channel recording probes in CA1. Nonsynaptic epileptiform activity was induced by calcium chelator EGTA with varied concentrations of potassium. For comparison, disinhibition synaptic epileptiform activity was induced by picrotoxin (PTX) with or without partial suppression of excitatory synaptic transmission. The propagation velocity was calculated by measuring the time delay between two electrodes separated by a known distance. The results show that in vivo nonsynaptic epileptiform activity propagates with a direction and velocity comparable to those observed in in vitro preparations. The direction of propagation for nonsynaptic activity is reversed from the PTX-induced synaptic activity. A reversal in propagation direction and change in velocity were also observed dynamically during the process of synaptic transmission suppression. Even a partial suppression of synaptic transmission was sufficient to significantly change the propagation direction and velocity of epileptiform activity. These results suggest the possibility that the measurement of propagation can provide important information about the synaptic mechanism underlying epileptic activity.

Potassium model for slow (2-3 Hz) in vivo neocortical paroxysmal oscillations

In slow neocortical paroxysmal oscillations, the de- and hyperpolarizing envelopes in neocortical neurons are large compared with slow sleep oscillations. Increased local synchrony of membrane potential oscillations during seizure is reflected in larger electroencephalographic oscillations and the appearance of spike- or polyspike-wave complex recruitment at 2- to 3-Hz frequencies. The oscillatory mechanisms underlying this paroxysmal activity were investigated in computational models of cortical networks. The extracellular K(+) concentration ([K(+)](o)) was continuously computed based on neuronal K(+) currents and K(+) pumps as well as glial buffering. An increase of [K(+)](o) triggered a transition from normal awake-like oscillations to 2- to 3-Hz seizure-like activity. In this mode, the cells fired periodic bursts and nearby neurons oscillated highly synchronously; in some cells depolarization led to spike inactivation lasting 50-100 ms. A [K(+)](o) increase, sufficient to produce oscillations could result from excessive firing (e.g., induced by external stimulation) or inability of K(+) regulatory system (e.g., when glial buffering was blocked). A combination of currents including high-threshold Ca(2+), persistent Na(+) and hyperpolarization-activated depolarizing (I(h)) currents was sufficient to maintain 2- to 3-Hz activity. In a network model that included lateral K(+) diffusion between cells, increase of [K(+)](o) in a small region was generally sufficient to maintain paroxysmal oscillations in the whole network. Slow changes of [K(+)](o) modulated the frequency of bursting and, in some case, led to fast oscillations in the 10- to 15-Hz frequency range, similar to the fast runs observed during seizures in vivo. These results suggest that modifications of the intrinsic currents mediated by increase of [K(+)](o) can explain the range of neocortical paroxysmal oscillations in vivo.

Models and mechanisms of experimental epilepsies

PURPOSE

To review the value of experimental models of epilepsy to help us understand the underlying basic neuronal mechanisms.

METHODS

The methods addressed here include a variety of acute and chronic experimental epilepsies and their investigation in vitro, in vivo, and in computer simulations.

RESULTS

Epileptic discharges are emergent properties of neuronal networks, depending crucially on intrinsic neuronal properties, and on the structural and functional organisation of the synaptic networks that interconnect them. Fast oscillations are another emergent property of these networks; while they are involved in normal function, they can play a crucial role in the initiation of at least some kinds of focal seizure.

DISCUSSION

Brief "interictal" epileptic events are now relatively well understood. Rather less so are the mechanisms of chronic epileptic foci, and particularly of the prolonged hypersynchronous hyperexcitability characteristic of focal seizures, but here too progress is being made, offering the prospect of better targeted treatments.

Low-calcium epileptiform activity in the hippocampus in vivo

It has been clearly established that nonsynaptic interactions are sufficient for generating epileptiform activity in brain slices. However, it is not known whether this type of epilepsy model can be generated in vivo. In this paper we investigate low-calcium nonsynaptic epileptiform activity in an intact hippocampus. The calcium chelator EGTA was used to lower [Ca2+]o in the hippocampus of urethane anesthetized rats. Spontaneous and evoked field potentials in CA1 pyramidal stratum and in CA1 stratum radiatum were recorded using four-channel silicon recording probes. Three different types of epileptic activity were observed while synaptic transmission was gradually blocked by a decline in hippocampal [Ca2+]o. A short latency burst, named early-burst, occurred during the early period of EGTA application. Periodic slow-waves and a long latency high-frequency burst, named late-burst, were seen after synaptic transmission was mostly blocked. Therefore these activities appear to be associated with nonsynaptic mechanisms. Moreover, the slow-waves were similar in appearance to the depolarization potential shifts in vitro with low calcium. In addition, excitatory postsynaptic amino acid antagonists could not eliminate the development of slow-waves and late-bursts. The slow-waves and late-bursts were morphologically similar to electrographic seizure activity seen in patients with temporal lobe epilepsy. These results clearly show that epileptic activity can be generated in vivo in the absence of synaptic transmission. This type of low-calcium nonsynaptic epilepsy model in an intact hippocampus could play an important role in revealing additional mechanisms of epilepsy disorders and in developing novel anti-convulsant drugs.

Depolarization block of neurons during maintenance of electrographic seizures

Epileptic seizures are associated with neuronal hyperactivity. Here, however, we investigated whether continuous neuronal firing is necessary to maintain electrographic seizures. We studied a class of "low-Ca2+" ictal epileptiform bursts, induced in rat hippocampal slices, that are characterized by prolonged (2-15 s) interruptions in population spike generation. We found that, during these interruptions, neuronal firing was suppressed rather than desynchronized. Intracellular current injection, application of extracellular uniform electric fields, and antidromic stimulation showed that the source of action potential disruption was depolarization block. The duration of the extracellular potassium transients associated with each ictal burst was not affected by disruptions in neuronal firing. Application of phenytoin or veratridine indicated a critical role for the persistent sodium current in maintaining depolarization block. Our results show that continuous neuronal firing is not necessary for the maintenance of experimental electrographic seizures.

Synchrony levels during evoked seizure-like bursts in mouse neocortical slices

Slices (n = 45) from the somatosensory cortex of mouse (P8-13) generated spontaneous bursts of activity (0.10 +/- 0.05 Hz) that were recorded extracellularly. Multiunit action potential (AP) activity was integrated and used as an index of population activity. In this experimental model, seizure-like activity (SLA) was evoked with bicuculline (5-10 microM) or N-methyl-d-aspartate (NMDA, 5 microM). SLA was an episode with repetitive bursting at a frequency of 0.50 +/- 0.06 Hz. To evaluate whether SLA was associated with a change in synchrony, we obtained simultaneous intracellular and extracellular recordings (n = 40) and quantified the relationship between individual cells and the surrounding population of neurons. During the SLA there was an increase in population activity and bursting activity was observed in neurons and areas that were previously silent. We defined synchrony as cellular activity that is consistently locked with the population bursts. Signal-averaging techniques were used to determine this component. To quantitatively assess change in synchronous activity at SLA onset, we estimated the entropy of the single cell's spike trains and subdivided this measure into network burst-related information and noise-related entropy. The burst-related information was not significantly altered at the onset of NMDA-evoked SLA and slightly increased when evoked with bicuculline. The signal-to-noise ratio determined from the entropy estimates showed a significant decrease (instead of an expected increase) during SLA. We conclude that the increased population activity during the SLA is attributed to recruitment of neurons rather than to increased synchrony of each of the individual elements.