Effects of NMDA- and AMPA-receptor antagonists on different forms of epileptiform activity in rat temporal cortex slices

Enhancing GABAergic Tonic Inhibition Reduces Seizure-Like Activity in the Neonatal Mouse Hippocampus and Neocortex

Approximately one-third of neonatal seizures do not respond to first-line anticonvulsants, including phenobarbital, which enhances phasic inhibition. Whether enhancing tonic inhibition decreases seizure-like activity in the neonate when GABA is mainly depolarizing at this age is unknown. We evaluated if increasing tonic inhibition using THIP [4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol, gaboxadol], a δ-subunit-selective GABAA receptor agonist, decreases seizure-like activity in neonatal C57BL/6J mice (postnatal day P5-8, both sexes) using acute brain slices. Whole-cell patch-clamp recordings showed that THIP enhanced GABAergic tonic inhibitory conductances in layer V neocortical and CA1 pyramidal neurons and increased their rheobase without altering sEPSC characteristics. Two-photon calcium imaging demonstrated that enhancing the activity of extrasynaptic GABAARs decreased neuronal firing in both brain regions. In the 4-aminopyridine and the low-Mg2+ model of pharmacoresistant seizures, THIP reduced epileptiform activity in the neocortex and CA1 hippocampal region of neonatal and adult brain slices in a dose-dependent manner. We conclude that neocortical layer V and CA1 pyramidal neurons have tonic inhibitory conductances, and when enhanced, they reduce neuronal firing and decrease seizure-like activity. Therefore, augmenting tonic inhibition could be a viable approach for treating neonatal seizures.

Limbic and olfactory cortical circuits in focal seizures

Epilepsies affecting the limbic regions are common and generate seizures often resistant to pharmacological treatment. Clinical evidence demonstrates that diverse regions of the mesial portion of the temporal lobe participate in limbic seizures; these include the hippocampus, the entorhinal, perirhinal and parahippocampal regions and the piriform cortex. The network mechanisms involved in the generation of olfactory-limbic epileptiform patterns will be here examined, with particular emphasis on acute interictal and ictal epileptiform discharges obtained by treatment with pro-convulsive drugs and by high-frequency stimulations on in vitro preparations, such as brain slices and the isolated guinea pig brain. The interactions within olfactory-limbic circuits can be summarized as follows: independent, region-specific seizure-like events (SLE) are generated in the olfactory and in the limbic cortex; SLEs generated in the hippocampal-parahippocampal regions tend to remain within these areas; the perirhinal region controls the neocortical propagation and the generalization of limbic seizures; interictal spiking in the olfactory regions prevents the invasion by SLEs generated in limbic regions. The potential relevance of these observations for human focal epilepsy is discussed.

Recurrent epileptiform discharges in the medial entorhinal cortex of kainate-treated rats are differentially sensitive to antiseizure drugs

OBJECTIVE

Approximately 30% of patients with epilepsy are refractory to existing antiseizure drugs (ASDs). Given that the properties of the central nervous systems of these patients are likely to be altered due to their epilepsy, tissues from rodents that have undergone epileptogenesis might provide a therapeutically relevant disease substrate for identifying compounds capable of attenuating pharmacoresistant seizures. To facilitate the development of such a model, this study describes the effects of classical glutamate receptor antagonists and 20 ASDs on recurrent epileptiform discharges (REDs) in brain slices derived from the kainate-induced status epilepticus model of temporal lobe epilepsy (KA-rats).

METHODS

Horizontal brain slices containing the medial entorhinal cortex (mEC) were prepared from KA-rats, and REDs were recorded from the superficial layers. 6-cyano-7-nitroquinoxaline-2,3-dione, (2R)-amino-5-phosphonovaleric acid, tetrodotoxin, or ASDs were bath applied for 20 minutes. Concentration-dependent effects and half maximal effective concentration values were determined for RED duration, frequency, and amplitude.

RESULTS

ASDs targeting sodium and potassium channels (carbamazepine, eslicarbazepine, ezogabine, lamotrigine, lacosamide, phenytoin, and rufinamide) attenuated REDs at concentrations near their average therapeutic plasma concentrations. γ-aminobutyric acid (GABA)ergic synaptic transmission-modulating ASDs (clobazam, midazolam, phenobarbital, stiripentol, tiagabine, and vigabatrin) attenuated REDs only at higher concentrations and, in some cases, prolonged RED durations. ASDs with other/mixed mechanisms of action (bumetanide, ethosuximide, felbamate, gabapentin, levetiracetam, topiramate, and valproate) and glutamate receptor antagonists weakly or incompletely inhibited RED frequency, increased RED duration, or had no significant effects.

SIGNIFICANCE

Taken together, these data suggest that epileptiform activity recorded from the superficial layers of the mEC in slices obtained from KA-rats is differentially sensitive to existing ASDs. The different sensitivities of REDs to these ASDs may reflect persistent molecular, cellular, and/or network-level changes resulting from disease. These data are expected to serve as a foundation upon which future therapeutics may be differentiated and assessed for potentially translatable efficacy in patients with refractory epilepsy.

The piriform, perirhinal, and entorhinal cortex in seizure generation

Understanding neural network behavior is essential to shed light on epileptogenesis and seizure propagation. The interconnectivity and plasticity of mammalian limbic and neocortical brain regions provide the substrate for the hypersynchrony and hyperexcitability associated with seizure activity. Recurrent unprovoked seizures are the hallmark of epilepsy, and limbic epilepsy is the most common type of medically-intractable focal epilepsy in adolescents and adults that necessitates surgical evaluation. In this review, we describe the role and relationships among the piriform (PIRC), perirhinal (PRC), and entorhinal cortex (ERC) in seizure-generation and epilepsy. The inherent function, anatomy, and histological composition of these cortical regions are discussed. In addition, the neurotransmitters, intrinsic and extrinsic connections, and the interaction of these regions are described. Furthermore, we provide evidence based on clinical research and animal models that suggest that these cortical regions may act as key seizure-trigger zones and, even, epileptogenesis.

The NMDA receptor complex as a therapeutic target in epilepsy: a review

A substantial amount of research has shown that N-methyl-D-aspartate receptors (NMDARs) may play a key role in the pathophysiology of several neurological diseases, including epilepsy. Animal models of epilepsy and clinical studies demonstrate that NMDAR activity and expression can be altered in association with epilepsy and particularly in some specific seizure types. NMDAR antagonists have been shown to have antiepileptic effects in both clinical and preclinical studies. There is some evidence that conventional antiepileptic drugs may also affect NMDAR function. In this review, we describe the evidence for the involvement of NMDARs in the pathophysiology of epilepsy and provide an overview of NMDAR antagonists that have been investigated in clinical trials and animal models of epilepsy.

Recovery of network-driven glutamatergic activity in rat hippocampal neurons during chronic glutamate receptor blockade

Previous studies indicated that a long-term decrease in the activity of ionotropic glutamate receptors induces cholinergic activity in rat and mouse hypothalamic neuronal cultures. Here we studied whether a prolonged inactivation of ionotropic glutamate receptors also induces cholinergic activity in hippocampal neurons. Receptor activity was chronically suppressed in rat hippocampal primary neuronal cultures with two proportionally increasing sets of concentrations of NMDA plus non-NMDA receptor antagonists: 100 microM/10 microM AP5/CNQX (1X cultures) and 200 microM/20 microM AP5/CNQX (2X cultures). Using calcium imaging we demonstrate that cholinergic activity does not develop in these cultures. Instead, network-driven glutamate-dependent activity, that normally is detected in hyper-excitable conditions, reappears in each culture group in the presence of these antagonists and can be reversibly suppressed by higher concentrations of AP5/CNQX. This activity is mediated by non-NMDA receptors and is modulated by NMDA receptors. Further, non-NMDA receptors, the general level of glutamate receptor activity and CaMK-dependent signaling are critical for development of this network-driven glutamatergic activity in the presence of receptor antagonists. Using electrophysiology, western blotting and calcium imaging we show that some neuronal parameters are either reduced or not affected by chronic glutamate receptor blockade. However, other parameters (including neuronal excitability, mEPSC frequency, and expression of GluR1, NR1 and betaCaMKII) become up-regulated and, in some cases, proportionally between the non-treated, 1X and 2X cultures. Our data suggest recovery of the network-driven glutamatergic activity after chronic glutamate receptor blockade. This recovery may represent a form of neuronal plasticity that compensates for the prolonged suppression of the activity of glutamate receptors.

Model-specific effects of bumetanide on epileptiform activity in the in-vitro intact hippocampus of the newborn mouse

The immature brain has a higher susceptibility to develop seizures, which often respond poorly to classical pharmacological treatment. It has been recently suggested that bumetanide, which blocks Na(+)-dependent K(+)-Cl(-)-cotransporter isoform 1 (NKCC1) and thus attenuates depolarizing GABAergic responses, could soothe epileptiform activity in immature nervous systems. To evaluate whether bumetanide consistently attenuates epileptiform activity, we investigated the effect of 10 microM bumetanide in five different in-vitro epilepsy models using field potential recordings in the CA3 region of intact mouse hippocampal preparations at postnatal day 4-7. Bumetanide reduced amplitude and frequency of ictal-like events (ILE) induced by 8.5 mM K(+), but it increased the frequency of ILE induced by 1 microM kainate. Inhibition of ligand-gated Cl(-) channels by 10 microM gabazine and 30 microM strychnine induced interictal activity (IA) that was only marginally affected by bumetanide. Removal of extracellular Mg(2+) induced both ILE and IA. Bumetanide had no effect on these ILE but enhanced the IA. Low-Mg(2+) solution containing 20 microM 4-AP induced late-recurrent discharges, which were slightly attenuated by bumetanide. In summary, our results demonstrate that bumetanide exerts diverse effects in different in-vitro epilepsy models.

Early developmental alterations of low-Mg2+ -induced epileptiform activity in the intact corticohippocampal formation of the newborn mouse in vitro

The generation, propagation and pharmacological properties of low-Mg2+ -induced epileptiform activity were examined in the intact corticohippocampal formation (CHF) of the newborn (P0-4) mouse in vitro. Multi-site field potential recordings in dentate gyrus (DG), CA3, CA1, entorhinal cortex (EC) and temporal cortex (TC) revealed in 0.2 mM Mg2+ -containing ACSF a stable pattern of spontaneous epileptiform activity consisting of recurrent ictal-like events (ILEs) and interictal events (IEs). Although this activity could be consistently observed as early as P0, ILEs were smaller in amplitude, less frequent and showed a slower onset in P0-2 as compared to P3-4 animals. In all age groups, epileptiform events were largest in CA3 and smallest in EC and TC. A specific pacemaker region could not be identified since ILEs appeared simultaneously at all recording sites. Reducing the extracellular Mg2+ concentration to 0.1 mM or nominally zero caused an increase in ILE frequency. Pharmacological studies in the P3-4 age group with 0.2 mM Mg2+ revealed a complete blockade of the ILEs by an NMDA receptor antagonist and a pronounced suppression of epileptiform activity by an AMPA/kainate antagonist. Application of a GABA-A receptor antagonist induced repetitive bursts of interictal discharges, which persisted for at least 1.5 h after washout of the antagonist. Our data demonstrate that the intact CHF in vitro preparation of the newborn mouse offers a most valuable model to study epileptiform activity in the immature limbic system.

Suppression of neuronal network excitability and seizure-like events by 2-methyl-4-oxo-3H-quinazoline-3-acetyl piperidine in juvenile rat hippocampus: involvement of a metabotropic glutamate receptor

We present data on the antiepileptic potency of 2-methyl-4-oxo-3H-quinazoline-3-acetyl piperidine (Q5) in juvenile (P9-13) rat hippocampal slices and in particular Q5's action mechanism and target. Q5 (200-500 microM), but not alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/Kainate receptor antagonists blocked low-[Mg2+]-induced seizure-like events (SLE) in the CA3 region. Q5 (100 microM) decreased Glu-induced [35S]guanosine 5'-O-(3-thiotriphosphate) binding enhancement in brain homogenates, without interaction with ionotropic Glu receptor sites and Glu transport. In voltage-clamped CA3 pyramidal cells, Q5 (500 microM) depressed activities of spontaneous excitatory and inhibitory postsynaptic currents without affecting miniature inhibitory currents. Metabotropic Glu receptor (mGluR) subtype antagonists affected network excitability dissimilarly. Intracellular Ca2+ ion transients induced by the mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) were suppressed by Q5. Agreeing predictions obtained by modelling Q5 binding to different experimental conformations of mGlu1, Q5 was bound partially to an mGluR binding site in the presence of 1mM ACPD. Findings suggest the apparent involvement of a novel phenotype of action or a new mGluR subtype in the specific suppression of epileptiform activity by Q5 through the depression of network excitability.

Model of frequent, recurrent, and spontaneous seizures in the intact mouse hippocampus

This study presents a model of chronic, recurrent, spontaneous seizures in the intact isolated hippocampal preparation from mice aged P8-P25. Field activity from the CA1 pyramidal cell layer was recorded and recurrent, spontaneous seizure-like events (SLEs) were observed in the presence of low Mg2+ (0.25 mM) artificial cerebrospinal fluid (ACSF). Hippocampi also showed interictal epileptiform discharges (IEDs) of 0.9-4.2 Hz occurring between seizures. No age-specific differences were found in SLE occurrence (2 SLEs per 10 min, on average), duration, and corresponding frequencies. After long exposure to low Mg2+ ACSF (>3 h), SLEs were completely reversible within minutes with the application of normal (2 mM Mg2+) ACSF. The AMPA antagonist, CNQX, blocked all epileptiform activity, whereas the NMDA antagonist, APV, did not. The gamma-aminobutyric acid (GABA)A antagonist, bicuculline, attenuated and fragmented SLEs, implicating interneurons in SLE generation. The L-type Ca2+ blocker, nifedipine, enhanced epileptiform activity. Analysis of dual site recordings along the septotemporal hippocampus demonstrated that epileptiform activity began first in the temporal pole of the hippocampus, as illustrated by disconnection experiments. Once an SLE had been established, however, the septal hippocampus was sometimes seen to lead the epileptiform activity. The whole hippocampus with intact local circuitry, treated with low Mg2+, provides a realistic model of recurrent spontaneous seizures, which may be used, in normal and genetically modified mice, to study the dynamics of seizures and seizure evolution, as well as the mechanisms of action of anti-epileptic drugs and other therapeutic modalities.

Persistent epileptiform activity induced by low Mg2+ in intact immature brain structures

We have determined the properties of seizures induced in vitro during the first postnatal days using intact rat cortico-hippocampal formations (CHFs) and extracellular recordings. Two main patterns of activity were generated by nominally Mg2+-free ACSF in hippocampal and cortical regions: ictal-like events (ILEs) and late recurrent interictal discharges (LRDs). They were elicited at distinct developmental periods and displayed different pharmacological properties. ILEs were first observed in P1 CHFs 52 +/- 7 min after application of low-Mg2+ ACSF (frequency 1.5 +/- 0.3 h-1, duration 86 +/- 3 s). There is a progressive age-dependent maturation of ILEs characterized by a decrease in their onset and an increase in their frequency and duration. ILEs were abolished by d-APV and Mg2+ ions. From P7, ILEs were followed by LRDs that appeared 89 +/- 8 min after application of low-Mg2+ ACSF (frequency approximately 1 Hz, duration 0.66 s, amplitude 0.31 +/- 0.03 mV). LRDs were no longer sensitive to d-APV or Mg2+ ions and persisted for at least 24 h in low-Mg2+ or in normal ACSF. ILEs and LRDs were synchronized in limbic and cortical regions with 10-40 ms latency between the onsets of seizures. Using a double chamber that enables independent superfusion of two interconnected CHFs, we report that ILEs and LRDs generated in one CHF propagated readily to the other one that was being kept in ACSF. Therefore, at a critical period of brain development, recurrent seizures induce a permanent form of hyperactivity in intact brain structures and this preparation provides a unique opportunity to study the consequences of seizures at early developmental stages.

The antiepileptic drug losigamone decreases the persistent Na+ current in rat hippocampal neurons

The tetronic acid derivative losigamone is a new anticonvulsant drug with a mechanism of action that was previously unknown. The drug decreases the frequency of spontaneous action potentials and suppresses repetitive firing of neurons. Here we tested the hypothesis that losigamone suppresses the persistent Na+ current (I(NaP)) in hippocampal neurons of rat brain slices and in cultured hippocampal neurons. Whole-cell voltage clamp recordings from neurons of juvenile rats (P15-P25) were performed with pipettes filled with Cs-gluconate or CsF. After pharmacological block of K+ and Ca2+ currents I(NaP) was revealed by applying slow depolarizing voltage ramps from -70 to 0 mV. Losigamone (100-200 microM) was dissolved in DMSO (0.1%) and was applied by bath application or local pressure application. Losigamone induced a decrease in amplitude of I(NaP) at depolarized membrane potentials which was reversible in cultured neurons. When tetrodotoxin (TTX) was added to the bath, I(NaP) was blocked and only a residual non-specific outward cation current (I(cat)) remained. Losigamone had no obvious effect on responses to voltage ramps under these conditions. Thus, losigamone did not affect I(cat) or induce any additional currents. The data suggest that losigamone decreases neuronal excitability via a decrease in I(NaP).

The effects of different 4-aminopyridine and morphine combinations on the intensity of morphine abstinence

The glutamatergic system is deeply involved in the development of opiate dependence and in the manifestation of opiate abstinence syndrome. In this study the effect of the increase in the endogenous glutamate (GLU) release due to 4-aminopyridine (4-AP), a potassium channel blocker, during the development of morphine (M) physical dependence and during the naloxone (NL)-precipitated abstinence syndrome was investigated. For the development of physical dependence M was intraperitoneally (i.p.) injected for 9 days 105 min following i.p. saline administration to a group of rats. In the first 3 days the dose of M was 10 mg x kg(-1). In the second 3 days the initial dose was doubled (20 mg x kg(-1)) and in the last 3 days the dose of M was raised to 40 mg x kg(-1). On day 10, the rats were divided into three groups at random and these three groups were i.p. given saline 105 min before 80 mg x kg(-1)M, 2 mg x kg(-1) 4-AP 105 min after 80 mg x kg(-1) M, and 80 mg x kg(-1) M 105 min before 2 mg x kg(-1) 4-AP, respectively. In a second group of rats, the rats were i.p. given 2 mg x kg(-1) 4-AP 105 min prior to M administration, which was increased every 3 days (10 mg x kg(-1), 20 mg x kg(-1), 40 mg x kg(-1)). On day 10, the rats were divided into two groups whose first injection was saline and 2 mg x kg(-1) 4-AP, respectively. The second injections of both groups after an interval of 105 min following the first one contained 80 mg x kg(-1) M. In contrast, one group of rats received only i.p. saline at every other injection time (the control group). Furthermore, another group of rats was i.p. administered 2 mg x kg(-1) 4-AP once a day, as the first injection. At the second injection time they were i.p. given saline. After a period of 15 min following the last administration on day 10, the rats belonging to all groups were i.p. injected with 2 mg x kg(-1) NL and immediately placed in a metal cage. Body weight loss (g), teeth chattering, rearing, wet-dog shaking, grooming, and jumping were determined or counted for 15 min. Penile erection, defecation, and diarrhoea were separately scored with one point for every individual occurrence, and the total score was named 'total number of others'. The administration of 4-AP before M appeared to intensify the development of dependence, and was most probably due to the Ca2+-induced inactivation of NMDA receptors as a result of excess release of GLU when the 4-AP took effect. The inactivation of NMDA receptors should have acted as a transient blockade of the receptors during the chronic administration period, and as well as after a single administration on day 10 before M injection and before abstinence. The intensification of the abstinence syndrome may be dependent on the excessive GLU released by 4-AP.

The anticonvulsant retigabine potently suppresses epileptiform discharges in the low Ca ++ and low Mg++ model in the hippocampal slice preparation

Retigabine (N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester, D-23129) is a broad acting anticonvulsant currently undergoing phase II clinical trials. An opening effect on leakage conductance K+ channels, potentiation of GABA induced currents and a weak blocking effect on Na+ and Ca++ channels were previously reported. The goal of this study was to investigate whether retigabine is capable of blocking epileptiform discharges in the low Ca++ and low Mg++ model in the hippocampal slice preparations and whether the anti-burst activity can be related to the K+ channel opening effect. In the low Ca++ model, synaptic transmission is blocked and discharges evolve from ephaptically-coupled neurons. Compounds which directly interfere with the threshold for action potential induction via alteration of ion channel function (i.e. Na+ channel blocker) may alter the discharges, while compounds interfering with synaptic transmission are not active. Retigabine suppressed the discharges in a concentration-dependent manner. A significant reduction in frequency without effect on amplitude was observed after application of 1 microM, and a full block of all discharges after application of 25 microM. The opener of the ATP sensitive K+ channels cromakalim was also active. Application of 300 microM cromakalim yielded to a lower frequency with no effects on the amplitude of discharges. Treatment with phenytoin and carbamazepine resulted in a marked reduction in amplitude accompanied by a rise in frequency; only at higher concentrations was a full block observed. The effect of retigabine therefore differs from sodium channel blockers and can be related to the K+ channel opening effect. In the low Mg++ model, excitatory neurotransmission is augmented by reducing the Mg++ block of NMDA channels. This results in development of interictal-like epileptiform activity in area CA1 in isolated hippocampal slices. Treatment with retigabine 10 microM resulted in a significant reduction of the discharges, and discharges were fully blocked after application of 25 microM. Qualitatively similar effects were observed with cromakalim and valproate, albeit at higher concentrations. The data indicate that retigabine exerts potent broad spectrum activity making it an interesting candidate for treatment of drug resistant patients.

Effect of a glutamate receptor antagonist (GYKI 52466) on 4-aminopyridine-induced seizure activity developed in rat cortical slices

In the present experiments we have tested the effect of the noncompetitive AMPA antagonist GYKI 52466 (20-80 microM) on spontaneous epileptic discharges developed as the consequence of 4-aminopyridine application in neocortex slices of adult rats. Parallel to the changes of spontaneous activity, the field potentials, evoked by electrical stimulation of the corpus callosum, were also analyzed. Glass microcapillary extracellular recording electrode was positioned in the third layer of the somatosensory cortex slice, while the stimulating electrode was placed at the border of the white and gray matter. 4-aminopyridine and GYKI 52466 were bath-applied. The application of 40 microM GYKI 52466 caused about 40% decrease in the frequency and the amplitude of spontaneous seizures as well as the duration of each discharges developed in 4-amino-pyridine. Pre-incubation with the AMPA antagonist effectively inhibited both the development of seizure activity and the maintenance of the discharges. GYKI 52466 also decreased the duration and amplitude of field responses evoked by stimulation of the corpus callosum. This inhibitory effect was dose-dependent. Our data in the in vitro cortex slice epilepsy model suggest that the non-competitive AMPA antagonist GYKI 52466 is a potent anticonvulsant and neuroprotective compound because it reduced the fully developed epileptic discharges or prevented their development.

Epileptiform discharges induced by combined application of bicuculline and 4-aminopyridine are resistant to standard anticonvulsants in slices of rats

Application of 4-aminopyridine (4AP) has previously been reported to produce different patterns of epileptiform discharges in entorhinal cortex-hippocampal-slices. Here we describe that 4-AP induced epileptiform activity in the EC becomes insensitive to anticonvulsant drugs (phenytoin, carbamazepine, valproic acid, phenobarbital) when GABAergic transmission is blocked by bicuculline. We propose that the activities induced by 4-aminopyridine and bicuculline may provide an in vitro model for the development of new drugs against difficult-to-treat focal epilepsy.

Interaction between superficial layers of the entorhinal cortex and the hippocampus in normal and epileptic temporal lobe

The entorhinal cortex (EC) is a major gateway for sensory information into the hippocampal formation. The information flow from layer II and III of the medial EC to the hippocampus is regulated in a frequency dependent manner. Spread of low Mg2+-induced epileptiform activity from EC to hippocampus differs in slices obtained from normal and kindled rats, and in adult versus juvenile rats. In slices from normal rats, low Mg2+-induced epileptiform activity in the EC had only moderate effects on the areas CA3 and CA1, apparently gated by powerful inhibition in the dentate gyrus. In slices from kindled rats, and from juvenile rats, there is facilitated propagation of the seizure-like events and late recurrent discharges through the EC-hippocampal slice. Temporal lobe epilepsy is associated with selective lesions in layer III of the medial EC. Such loss of layer III cells of the medial EC during epilepsy may contribute to the disturbance of frequency dependent information flow from the EC to the hippocampus, and, therefore, to the cognitive impairments associated with these disorders.

Epileptiform activity and changes in field potential responses induced by low [Mg2+]0 in a genetic rat model of absence epilepsy

The genetic absence epilepsy rats of Strasbourg (GAERS) display alterations in cortical synaptic transmission possibly facilitating the generation of ictaform activity and the late development into convulsive epilepsy. We studied low Mg2+-induced epileptiform activities and their long term effects on field potentials (fp) evoked by paired pulse stimulation in hippocampal area CA1 (CA1), medial entorhinal cortex (EC) and frontal cortex (FC) in in-vitro-slice preparations from GAERS and control (NE) adult rats (6 months). Omitting Mg2+-ions from artificial cerebrospinal fluid (ACSF) caused recurrent short discharges (in CA1) and seizure-like events (in EC) in both GAERS and NE rats. Latency to onset of activity as well as discharge pattern, frequency and amplitude of such events did not differ between the two strains, neither in CA1 nor in EC. In the FC, however, epileptiform events occurred in NE rats, but not in GAERS. Field potentials in normal ACSF were similar in both strains in CA1 and FC, while they were smaller in the EC of GAERS. Low [Mg2+]0 caused long-term changes of fp only in area CA1 where the population spikes were depressed in GAERS and increased in NE rats. We concluded that susceptibility to low [Mg2+]0-induced epileptic activity in EC and hippocampal area CA1 is not higher in GAERS than in NE adult rats. However, some properties like synaptic coupling in EC and long-term changes in synaptic efficacy induced by epileptiform activity in CA1 differ from that in NE rats. Whether the particularities in GAERS may be related to kindling by absence epileptic activities will be studied in further experiments.

Comparison of effects of valproate and trans-2-en-valproate on different forms of epileptiform activity in rat hippocampal and temporal cortex slices

PURPOSE

Reducing the extracellular magnesium or calcium or increasing the extracellular potassium induces different patterns of epileptiform activity in the hippocampus and the entorhinal cortex. Although in the low Ca2+ and K+ models, seizure-like events (SLEs) develop in area CA1 of the hippocampus, only short recurrent discharges develop in the low Mg2+ model. In contrast, in low Mg2+, SLEs and late recurrent discharges (LRDs) are observed in the entorhinal cortex.

METHODS

We compared the effects of valproate (VPA) and its major metabolite, trans-2-en-VPA (TVPA), on all these different model activities using extracellular field potential measurements. We also investigated the equilibration time course of VPA in the slice by using VPA-sensitive microelectrodes.

RESULTS

Both drugs reversibly blocked most forms of epileptiform activity. The only exception was the LRDs in the entorhinal cortex. In paired experiments, TVPA appeared to be more effective than VPA bath applied with the same concentration to the same slice. With our measurements of the VPA concentrations in slices, we showed that the concentrations used were close to therapeutic drug levels.

CONCLUSIONS

If TVPA stands the toxicological tests, it might be a useful alternative in the treatment of seizures.

Comparison of the effects of serotonin in the hippocampus and the entorhinal cortex

Among the molecular, cellular, and systemic events that have been proposed to modulate the function of the hippocampus and the entorhinal cortex (EC), one of the most frequently cited possibilities is the activation of the serotonergic system. Neurons in the hippocampus and in the EC receive a strong serotonergic projection from the raphe nuclei and express serotonin (5-HT) receptors at high density. Here we review the various effects of 5-HT on intrinsic and synaptic properties of neurons in the hippocampus and the EC. Although similar membrane-potential changes following 5-HT application have been reported for neurons of the entorhinal cortex and the hippocampus, the effects of serotonin on synaptic transmission are contrary in both areas. Serotonin mainly depresses fast and slow inhibition of the principal output cells of the hippocampus, whereas it selectively suppresses the excitation in the entorhinal cortex. On the basis of these data, we discuss the possible role of serotonin under physiological and pathophysiological circumstances.

Serotonin blocks different patterns of low Mg2+-induced epileptiform activity in rat entorhinal cortex, but not hippocampus

Low Mg2+-induced epileptiform activity in the entorhinal cortex is characterized by an initial expression of seizure-like events followed by late recurrent discharges. Both these forms of activity as well as the transition between them were blocked by serotonin. In contrast, serotonin had little effect upon the epileptiform activity in areas CA3 and CA1 of the hippocampus. Both forms of epileptiform activity in the entorhinal cortex are sensitive to N-methyl-D-aspartate receptor antagonists and it is shown here that serotonin blocked both types of epileptiform activity through an effective concentration-dependent reduction of N-methyl-D-aspartate receptor-mediated excitatory postsynaptic potentials in deep layer entorhinal cortex cells. Serotonin also prolonged or even prevented the transition between the two types of epileptiform activity and we suggest that this may be through activation of the Na+/K+-ATPase. The resistance of epileptiform activity in CA1 and CA3 to serotonin was most likely related to the inability of serotonin to reduce Schaffer collateral-evoked excitatory postsynaptic potentials. Given the strong serotonergic inputs to both the hippocampus and entorhinal cortex, the differential sensitivity of the two regions to serotonin suggests functional differences. In addition since the late recurrent discharges in the entorhinal cortex are resistant to all clinically used anticonvulsants, serotonin may open new avenues for the development of novel anticonvulsant compounds.

Effects of gamma-aminobutyric acid (GABA) agonists and GABA uptake inhibitors on pharmacosensitive and pharmacoresistant epileptiform activity in vitro

1. Lowering of the extracellular Mg(2+)-concentration induces various patterns of epileptiform activity in combined rat entorhinal cortex-hippocampal brain slices. After a prolonged period of exposure to Mg(2+)-free medium seizure-like events in the entorhinal cortex change to a state of late recurrent discharges which cannot be blocked by clinically available antiepileptic drugs. This late epileptiform activity thus represents a useful model to test the effects of new anticonvulsant substances. 2. A mechanism possibly underlying the development of sustained seizure-like activity is the loss of synaptically released gamma-aminobutyric acid (GABA). Drugs which increase the amount of GABA available in presynaptic endings might thus be useful in the treatment of these therapeutically complicated forms of epilepsy. 3. Therefore, we studied the effects of various substances increasing GABA-mediated inhibition on early and late forms of epileptiform activity. GABA and the GABAA receptor agonist muscimol blocked both the pharmacosensitive discharges in the hippocampus and entorhinal cortex as well as the late recurrent discharges in the medial entorhinal cortex. The GABAB receptor agonist baclofen blocked the recurrent short discharges very potently, but did not consistently block seizure-like events and late recurrent discharges in the entorhinal cortex. 4. GABA uptake blockers showed a differential potency to block the various discharge patterns. Whereas nipecotic acid and beta-alanine suppressed all forms of epileptiform activity albeit at high concentrations (1-5 mM), tiagabine was much more potent in blocking the hippocampal recurrent short discharges and the seizure-like events in the medial entorhinal cortex, but could not block the late recurrent discharges. 5. Our data support the idea that prolonged neuronal overactivity might result in a loss of synaptically available GABA. Selective block of uptake into glia cells or substitution of the transmitter may therefore be an efficient strategy for the treatment of severe prolonged epileptic discharges whereas block of neuronal GABA uptake fails to counteract synchronized discharges in this situation.

Anticonvulsant drug effects on spontaneous thalamocortical rhythms in vitro: ethosuximide, trimethadione, and dimethadione

Spontaneous generalized epileptiform discharges were elicited in rodent thalamocortical slices by perfusion with a medium containing no added Mg2+. In multiple-channel extracellular field potential recordings in thalamus and cortex, several distinct types of discharges were recorded, with two principal variants bearing marked similarity to spike-wave and generalized tonic-clonic seizure discharges recorded in patients with generalized seizure disorders. These discharges were termed sTBCs and cTBCs, respectively, for simple and complex thalamocortical burst complexes. The sensitivity of these discharges to the generalized absence anticonvulsants ethosuximide, trimethadione and dimethadione (the active metabolite of trimethadione) was studied. sTBCs were reduced or blocked by ethosuximide and dimethadione, when these drugs were applied in clinically relevant concentrations. The order of effectiveness of these agents was dimethadione > or = ethosuximide >> trimethadione. This paralleled the relative efficacy of these drugs in blocking T current in thalamic neurons. cTBCs were unaffected or exacerbated by these drugs. Structural control drugs including succinimide, the behaviorally inactive ring base of ethosuximide, and alpha, alpha-dimethyl-beta-methylsuccinimide, a convulsant succinimide, were inactive or exacerbated either sTBCs or cTBCs, respectively. These spontaneous generalized thalamocortical discharges in rodent thalamocortical slices may represent a potentially valuable in vitro model of generalized seizure discharges, with marked pharmacological and physiological similarities to various forms of clinical epileptic seizure activity.

Effects of losigamone on synaptic potentials and spike frequency habituation in rat entorhinal cortex and hippocampal CA1 neurones

Losigamone is an anticonvulsant both in vivo and in vitro. We here studied possible mechanisms for such effects with conventional intracellular recordings from pyramidal cells of area CA1 and entorhinal cortex in combined hippocampal-entorhinal cortex slices. Losigamone reversibly reduced the number of action potentials elicited by 1 s long depolarising current injections. In addition, the drug moderately reduced EPSP amplitudes while monosynaptic fast and slow IPSPs were unaffected.