Evoked epileptiform discharges in the rat anterior piriform cortex: generation and local propagation

Connectivity of the Piriform Cortex and its Implications in Temporal Lobe Epilepsy

Background

The piriform cortex has been implicated in the initiation, spread and termination of epileptic seizures. This understanding has extended to surgical management of epilepsy, where it has been shown that resection or ablation of the piriform cortex can result in better outcomes. How and why the piriform cortex may play such a crucial role in seizure networks is not well understood. To answer these questions, we investigated the functional and structural connectivity of the piriform cortex in both healthy controls and temporal lobe epilepsy (TLE) patients.

Methods

We studied a retrospective cohort of 55 drug-resistant unilateral TLE patients and 26 healthy controls who received structural and functional neuroimaging. Using seed-to-voxel connectivity we compared the normative whole-brain connectivity of the piriform to that of the hippocampus, a region commonly involved in epilepsy, to understand the differential contribution of the piriform to the epileptogenic network. We subsequently measured the inter-piriform coupling (IPC) to quantify similarities in the inter-hemispheric cortical functional connectivity profile between the two piriform cortices. We related differences in IPC in TLE back to aberrations in normative piriform connectivity, whole brain functional properties, and structural connectivity.

Results

We find that relative to the hippocampus, the piriform is functionally connected to the anterior insula and the rest of the salience ventral attention network (SAN). We also find that low IPC is a sensitive metric of poor surgical outcome (sensitivity: 85.71%, 95% CI: [19.12%, 99.64%]); and differences in IPC within TLE were related to disconnectivity and hyperconnectivity to the anterior insula and the SAN. More globally, we find that low IPC is associated with whole-brain functional and structural segregation, marked by decreased functional small-worldness and fractional anisotropy.

Conclusions

Our study presents novel insights into the functional and structural neural network alterations associated with this structure, laying the foundation for future work to carefully consider its connectivity during the presurgical management of epilepsy.

Perspective on mTOR-dependent Protection in Status Epilepticus

BACKGROUND

The piriform cortex, known as area tempestas, has a high propensity to trigger limbic epileptic seizures. Recent studies on human patients indicate that a resection containing the piriform cortex produces a marked improvement in patients suffering from intractable limbic seizures. This calls for looking back at the pharmacological and anatomical data on area tempestas. Within the piriform cortex, status epilepticus can be induced by impairing the desensitization of AMPA receptors. The mechanistic target of rapamycin complex1 (mTORC1) is a promising candidate.

OBJECTIVE

The present perspective aims to link the novel role of the piriform cortex with recent evidence on the modulation of AMPA receptors under the influence of mTORC1. This is based on recent evidence and preliminary data, leading to the formulation of interaction between mTORC1 and AMPA receptors to mitigate the onset of long-lasting, self-sustaining, neurotoxic status epilepticus.

METHODS

The perspective grounds its method on recent literature along with the actual experimental procedure to elicit status epilepticus from the piriform cortex and the method to administer the mTORC1 inhibitor rapamycin to mitigate seizure expression and brain damage.

RESULTS

The available and present perspectives converge to show that rapamycin may disrupt the seizure circuitry initiated in the piriform cortex to mitigate seizure duration, severity, and brain damage.

CONCLUSION

The perspective provides a novel scenario to understand refractory epilepsy and selfsustaining status epilepticus. It is expected to provide a beneficial outcome in patients suffering from temporal lobe epilepsy.

Cell assembly formation and structure in a piriform cortex model

The piriform cortex is rich in recurrent excitatory synaptic connections between pyramidal neurons. We asked how such connections could shape cortical responses to olfactory lateral olfactory tract (LOT) inputs. For this, we constructed a computational network model of anterior piriform cortex with 2000 multicompartment, multiconductance neurons (500 semilunar, 1000 layer 2 and 500 layer 3 pyramids; 200 superficial interneurons of two types; 500 deep interneurons of three types; 500 LOT afferents), incorporating published and unpublished data. With a given distribution of LOT firing patterns, and increasing the strength of recurrent excitation, a small number of firing patterns were observed in pyramidal cell networks: first, sparse firings; then temporally and spatially concentrated epochs of action potentials, wherein each neuron fires one or two spikes; then more synchronized events, associated with bursts of action potentials in some pyramidal neurons. We suggest that one function of anterior piriform cortex is to transform ongoing streams of input spikes into temporally focused spike patterns, called here "cell assemblies", that are salient for downstream projection areas.

Epileptogenesis-induced changes of hippocampal-piriform connectivity

OBJECTIVE

Tissue remodeling has been described in brain circuits that are involved in the generation and propagation of epileptic seizures. Human and animal studies suggest that the anterior piriform cortex (aPC) is crucial for seizure expression in focal epilepsies. Here, we investigate the effect of kainic-acid (KA)-induced seizures on the effective connectivity of the aPC with bilateral hippocampal CA3 regions using cerebro-cerebral evoked potentials (CCEPs).

METHODS

Adult male Sprague-Dawley rats were implanted with a tripolar electrode in the left aPC for stimulation and recording, and with unipolar recording electrodes in bilateral CA3 regions. Single pulse stimulations were given to the aPC and CCEPs were averaged before KA injections and after the emergence of spontaneous recurrent seizures (SRS). Similar recordings at equivalent time intervals were obtained from animals that received saline injections instead of KA (controls).

RESULTS

In the experimental group, the percentage change of increased amplitude of the contralateral (but not ipsilateral) CA3 CCEPs between pre-KA injection and after the emergence of SRS was significantly greater than in controls. No significant single-pulse-induced spectral change responses were observed in either epileptic or control rats when comparing pre- and post-stimulus time intervals. Also, we found no correlation between seizure frequency and the extent of amplitude changes in the CCEPs.

CONCLUSIONS

In the KA model, epileptogenesis results in plastic changes that manifest as an amplification of evoked potential amplitudes recorded in the contralateral hippocampus in response to single-pulse stimulation of the aPC. These results suggest epileptogenesis-induced facilitation of interhemispheric connectivity between the aPC and the hippocampus. Since the amplitude increase of the contralateral CCEP is a possible in vivo biomarker of epilepsy, any intervention (e.g. neuromodulatory) that can reverse this phenomenon may hold a potential antiepileptic efficacy.

Parallel pathways of seizure generalization

Generalized convulsive status epilepticus is a life-threatening emergency, because recurrent convulsions can cause death or injury. A common form of generalized convulsive status epilepticus is of focal onset. The neuronal circuits activated during seizure spread from the hippocampus, a frequent site of seizure origin, to the bilateral motor cortex, which mediates convulsive seizures, have not been delineated. Status epilepticus was initiated by electrical stimulation of the hippocampus. Neurons transiently activated during seizures were labelled with tdTomato and then imaged following brain slice clearing. Hippocampus was active throughout the episode of status epilepticus. Neuronal activation was observed in hippocampus parahippocampal structures: subiculum, entorhinal cortex and perirhinal cortex, septum, and olfactory system in the initial phase status epilepticus. The tdTomato-labelled neurons occupied larger volumes of the brain as seizures progressed and at the peak of status epilepticus, motor and somatosensory cortex, retrosplenial cortex, and insular cortex also contained tdTomato-labelled neurons. In addition, motor thalamic nuclei such as anterior and ventromedial, midline, reticular, and posterior thalamic nuclei were also activated. Furthermore, circuits proposed to be crucial for systems consolidation of memory: entorhinal cortex, retrosplenial cortex, cingulate gyrus, midline thalamic nuclei and prefrontal cortex were intensely active during periods of generalized tonic-clonic seizures. As the episode of status epilepticus waned, smaller volume of brain was activated. These studies suggested that seizure spread could have occurred via canonical thalamocortical pathway and many cortical structures involved in memory consolidation. These studies may help explain retrograde amnesia following seizures.

Indispensable Amino Acid-Deficient Diets Induce Seizures in Ketogenic Diet-Fed Rodents, Demonstrating a Role for Amino Acid Balance in Dietary Treatments for Epilepsy

Background

Low protein amounts are used in ketogenic diets (KDs), where an essential (indispensable) amino acid (IAA) can become limiting. Because the chemically sensitive, seizurogenic, anterior piriform cortex (APC) is excited by IAA limitation, an imbalanced KD could exacerbate seizure activity.

Objective

We questioned whether dietary IAA depletion worsens seizure activity in rodents fed KDs.

Methods

In a series of 6 trials, male rats or gerbils of both sexes (6-8/group) were given either control diets (CDs) appropriate for each trial, a KD, or a threonine-devoid (ThrDev) diet for ≥7 d, and tested for seizures using various stimuli. Microchip analysis of rat APCs was also used to determine if changes in transcripts for structures relevant to seizurogenesis are affected by a ThrDev diet. Glutamate release was measured in microdialysis samples from APCs during the first meal after 7 d on a CD or a ThrDev diet.

Results

Adult rats showed increased susceptibility to seizures in both chemical (58%) and electroshock (doubled) testing after 7 d on a ThrDev diet compared with CD (each trial, P ≤ 0.05). Seizure-prone Mongolian gerbils had fewer seizures after receiving a KD, but exacerbated seizures (68%) after 1 meal of KD minus Thr (KD-T compared with CD, P < 0.05). In kindled rats fed KD-T, both counts (19%) and severities (77%) of seizures were significantly elevated (KD-T compared with CD, P < 0.05). Gene transcript changes were consistent with enhanced seizure susceptibility (7-21 net-fold increases, P = 0.045-0.001) and glutamate release into the APC was increased acutely (4-fold at 20 min, 2.6-fold at 60 min, P < 0.05) after 7 d on a ThrDev diet.

Conclusion

Seizure severity in rats and gerbils was reduced after KDs and exacerbated by ThrDev, both in KD- and CD-fed animals, consistent with the mechanistic studies. We suggest that a complete protein profile in KDs may improve IAA balance in the APC, thereby lowering the risk of seizures.

Seizure development in the acute intrahippocampal epileptic focus

Currently, an epileptic seizure is considered to involve a temporary network that exists for a finite period of time. Formation of this network evolves through spread of epileptiform activity from a seizure onset zone (SOZ). Propagation of seizures evoked by kainic acid injection in hippocampus to different brain areas was analyzed at macro- and micro-intervals. The mean latency of seizure occurrence in different brain areas varied between 0.5 sec and 85 sec (mean 14.9 ± 14.5 (SD)), and it increased after each consecutive seizure in areas located contralateral to the area of injection, but not in the ipsilateral sites. We have shown that only 41% of epileptic individual events in target brain areas were driven by epileptic events generated in the SOZ once the seizure began. Fifty-nine percent of epileptiform events in target areas occurred one millisecond before or after events in the SOZ. These data illustrate that during seizure maintenance, only some individual epileptiform events in areas outside of SOZ could be consistently triggered by the SOZ; and the majority must be triggered by a driver located outside the SOZ or brain areas involved in ictal activity could be coupled to each other via an unknown mechanism such as stochastic resonance.

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.

Effects of essential amino acid deficiency: down-regulation of KCC2 and the GABAA receptor; disinhibition in the anterior piriform cortex

The anterior piriform cortex (APC) is activated by, and is the brain area most sensitive to, essential (indispensable) amino acid (IAA) deficiency. The APC is required for the rapid (20 min) behavioral rejection of IAA deficient diets and increased foraging, both crucial adaptive functions supporting IAA homeostasis in omnivores. The biochemical mechanisms signaling IAA deficiency in the APC block initiation of translation in protein synthesis via uncharged tRNA and the general amino acid control kinase, general control nonderepressing kinase 2. Yet, how inhibition of protein synthesis activates the APC is unknown. The neuronal K(+) Cl(-) cotransporter, neural potassium chloride co-transporter (KCC2), and GABAA receptors are essential inhibitory elements in the APC with short plasmalemmal half-lives that maintain control in this highly excitable circuitry. After a single IAA deficient meal both proteins were reduced (vs. basal diet controls) in western blots of APC (but not neocortex or cerebellum) and in immunohistochemistry of APC. Furthermore, electrophysiological analyses support loss of inhibitory elements such as the GABAA receptor in this model. As the crucial inhibitory function of the GABAA receptor depends on KCC2 and the Cl(-) transmembrane gradient it establishes, these results suggest that loss of such inhibitory elements contributes to disinhibition of the APC in IAA deficiency. The circuitry of the anterior piriform cortex (APC) is finely balanced between excitatory (glutamate, +) and inhibitory (GABA, -) transmission. GABAA receptors use Cl(-), requiring the neural potassium chloride co-transporter (KCC2). Both are rapidly turning-over proteins, dependent on protein synthesis for repletion. In IAA (indispensable amino acid) deficiency, within 20 min, blockade of protein synthesis prevents restoration of these inhibitors; they are diminished; disinhibition ensues. GCN2 = general control non-derepressing kinase 2, eIF2α = α-subunit of the eukaryotic initiation factor 2.

Converging PET and fMRI evidence for a common area involved in human focal epilepsies

OBJECTIVES

Experiments in animal models have identified specific subcortical anatomic circuits, which are critically involved in the pathogenesis and control of seizure activity. However, whether such anatomic substrates also exist in human epilepsy is not known.

METHODS

We studied 2 separate groups of patients with focal epilepsies arising from any cortical location using either simultaneous EEG-fMRI (n = 19 patients) or [¹¹C]flumazenil PET (n = 18).

RESULTS

Time-locked with the interictal epileptiform discharges, we found significant hemodynamic increases common to all patients near the frontal piriform cortex ipsilateral to the presumed cortical focus. GABA(A) receptor binding in the same area was reduced in patients with more frequent seizures.

CONCLUSIONS

Our findings of cerebral blood flow and GABAergic changes, irrespective of where interictal or ictal activity occurs in the cortex, suggest that this area of the human primary olfactory cortex may be an attractive new target for epilepsy therapy, including neurosurgery, electrical stimulation, and focal drug delivery.

Metabotropic glutamate receptors mGluR4 and mGluR8 regulate transmission in the lateral olfactory tract-piriform cortex synapse

The piriform cortex (PC) is the primary terminal zone of projections from the olfactory bulb, termed the lateral olfactory tract (LOT). The PC plays a critical role in processing of olfactory stimuli and is also a highly seizure prone area thought to be involved in some forms of temporal lobe epilepsy. Pharmacological and immunohistochemical studies provide evidence for the localization of various metabotropic glutamate receptors (GluRs) in the PC. We employed whole-cell patch clamp recordings from PC pyramidal cells to determine the roles of group III mGluRs in modulating synaptic transmission at the LOT-PC synapse. The group III mGluR agonist, L-AP4, induced a concentration-dependent inhibition of synaptic transmission at the LOT-PC synapse at concentrations that activate mGluR4 and mGluR8, but not mGluR7 or other mGluR subtypes (EC50=473nM). In addition, the selective mGluR8 agonist, DCPG (300nM), also suppressed synaptic transmission at the LOT synapse. Furthermore, the inhibitory actions of L-AP4 and Z-cyclopentyl-AP4, a selective mGluR4 agonist, were potentiated by the mGluR4 positive allosteric modulator, PHCCC (30microM). The high potency of L-AP4, combined with the observed effects of DCPG and PHCCC, suggests that both mGluR4 and mGluR8 play a role in the l-AP4-induced inhibition of synaptic transmission at the LOT-PC synapse.

Mechanisms of Food Intake Repression in Indispensable Amino Acid Deficiency

Animals reject diets that lead to indispensable amino acid (IAA) depletion or deficiency. This behavior is adaptive, as continued IAA depletion is incompatible with maintenance of protein synthesis and survival. Following rejection of the diet, animals begin foraging for a better IAA source and develop conditioned aversions to cues associated with the deficient diet. These responses require a sensory system to detect the IAA depletion and alert the appropriate neural circuitry for the behavior. The chemosensor for IAA deprivation is in the highly excitable anterior piriform cortex (APC) of the brain. Recently, the well-conserved general AA control non-derepressing system of yeast was discovered to be activated by IAA deprivation via uncharged tRNA in mammalian APC. This system provides the sensory limb of the mechanism for recognition of IAA depletion that leads to activation of the APC, diet rejection, and subsequent adaptive strategies.

Anticonvulsant effects of damage to structures involved in seizure induction in rats exposed to soman

In nerve agent research, it is assumed that the regions from which seizure activity is triggered may offer clues for the designing of effective anticonvulsive therapy. In the present study, selective brain lesions were made to identify critical cholinergic pathways and seizure controlling areas involved in the induction of epileptiform activity in rats challenged with soman. The results showed that rats with bilateral aspiration lesion of the seizure controlling substrate, area tempestas (AT) in the piriform cortex, displayed marked anticonvulsant effects, whereas such effects were not seen when substantia nigra was destroyed. Aspiration lesion of the medial septal area (MS) including the vertical limb of the diagonal band nucleus (DBN) caused increased latency to the onset of convulsions, whereas damage to the nucleus basalis magnocellularis (NBM), nucleus accumbens, or both MS and NBM did not cause anticonvulsant effects. Saporin lesion of MS, DBN (horizontal limb), or MS+DBN had no anticonvulsant effects, suggesting that aspiration lesion of MS disrupted pathways beyond cholinergic ones. Severe aphagia/adipsia and reduced body weight occurred in rats with lesions in the septal area. In separate sham operated rats, a strong positive correlation was found between body weight and latency to onset of convulsions in response to soman. Thus, weight loss and a relatively high dose of soman (1.6 x LD(50)) in this context may have masked potential anticonvulsant effects among some lesioned animals. It is inferred that MS and AT/piriform cortex occur as prime target areas for induction of seizures by soman.

Enhanced cyclooxygenase-2 expression in olfactory-limbic forebrain following kainate-induced seizures

Cyclooxygenase-2 is expressed at low levels in a subset of neurons in CNS and is rapidly induced by a multiplicity of factors including seizure activity. A putative relationship exists between cyclooxygenase-2 induction and glutamatergic neurotransmission. Cyclooxygenase-1 is constitutively expressed in glial cells and has been specifically linked to microglia. In this study we evaluated cyclooxygenase-2 protein immunocytochemically and found markedly enhanced immunostaining primarily in olfactory-limbic regions at 2, 6 and 24 h following kainate-induced status epilepticus. Impressive enhanced cyclooxygenase-2 immunoreactivity was localized in anterior olfactory nucleus, tenia tecta, nucleus of the lateral olfactory tract, piriform cortex, lateral and basolateral amygdala, orbital frontal cortex, nucleus accumbens (shell) and associated areas of ventral striatum, entorhinal cortex, dentate gyrus granule cells and hilar neurons, hippocampal CA subfields and subiculum. Alternate sections were processed for dual immunocytochemical analysis utilizing c-Fos and cyclooxygenase-2 antiserum to examine the possibility that the neuronal induction of cyclooxygenase-2 was associated with seizure activity. Neurons that showed a timeline of cyclooxygenase-2 upregulation were found to possess c-Fos immunopositive nuclei. Additional results from all seizure groups showed cyclooxygenase-1 induction in microglia, which was confirmed by Western blot analysis of hippocampus. Western blot and real-time quantitative RT-PCR analysis showed significant upregulation of cyclooxygenase-2 expression, confirming its induction in neurons. These data indicate that cyclooxygenase-2 induction in a neuronal network can be a useful marker for pathways associated with seizure activity.

Further characterization of muscarinic agonist-induced epileptiform bursting activity in immature rat piriform cortex, in vitro

The characteristics of muscarinic acetylcholine receptor agonist-induced epileptiform bursting seen in immature rat piriform cortex slices in vitro were further investigated using intracellular recording, with particular focus on its postnatal age-dependence (P+14-P+30), pharmacology, site(s) of origin and the likely contribution of the muscarinic acetylcholine receptor agonist-induced post-stimulus slow afterdepolarization and gap junction functionality toward its generation. The muscarinic agonist, oxotremorine-M (10 microM), induced rhythmic bursting only in immature piriform cortex slices; however, paroxysmal depolarizing shift amplitude, burst duration and burst incidence were inversely related to postnatal age. No significant age-dependent changes in neuronal membrane properties or postsynaptic muscarinic responsiveness accounted for this decline. Burst incidence was higher when recorded in anterior and posterior regions of the immature piriform cortex. In adult and immature neurones, oxotremorine-M effects were abolished by M1-, but not M2-muscarinic acetylcholine receptor-selective antagonists. Rostrocaudal lesions, between piriform cortex layers I and II, or layer III and endopiriform nucleus in adult or immature slices did not influence oxotremorine-M effects; however, the slow afterdepolarization in adult (but not immature) lesioned slices was abolished. Gap junction blockers (carbenoxolone or octanol) disrupted muscarinic bursting and diminished the slow afterdepolarization in immature slices, suggesting that gap junction connectivity was important for bursting. Our data show that neural networks within layers II-III function as primary oscillatory circuits for burst initiation in immature rat piriform cortex during persistent muscarinic receptor activation. Furthermore, we propose that muscarinic slow afterdepolarization induction and gap junction communication could contribute towards the increased epileptiform susceptibility of this brain area.

Detection of late epilepsy by the texture analysis of MR brain images in the lithium-pilocarpine rat model

The aim of the study was to detect by texture analysis non easily visible anomalies of magnetic resonance (MR) images of piriform and entorhinal cortices relevant to the lithium-pilocarpine (Li-Pilo) model of temporal lobe epilepsy in rats. Status epilepticus was induced by Li-Pilo in twenty male rats 21 day-old. T(2)-weighted MR images of their brain, were obtained before injection of Li-Pilo and one day after status epilepticus. An hyperintense signal was found in the piriform and entorhinal cortices of six rats, which developed chronic epilepsy after a latent period of one to three months. Among the 14 other rats which displayed images similar to those obtained before injection, four remained healthy but 10 rats developed late epileptic symptoms, raising the problem of hidden cortical damage which may be too subtle to be detected by classic MRI examination. A numeric treatment of digital images was then undertaken by texture analysis, to derive image information from a purely computational point of view. The combined texture and discriminant analyses based on pixels pattern anomalies, selected 3 texture parameters derived from co-occurrence matrix which characterized structural abnormalities relevant to the hyperintense signal, not only in the modified images of 6 rats but also in images of 10 rats with apparently non modified images. These three texture's parameters allowed to classify the twenty rats of our experiment as follows: sixteen epileptic rats were effectively classified with cortical lesions, two non epileptic were correctly classified with healthy cortex, but two healthy rats were not correctly classified. This misclassification is discussed on the basis of the time dependence of the onset of seizure in the Li-Pilo model. These promising results suggest to apply this method to MRI examinations for an improvement of the early diagnostic of human epilepsy.

Comparison of spontaneous and evoked epileptiform activity in three in vitro epilepsy models

Rat neocortical slices express spontaneous epileptiform activity after incubation with GABA(A) receptor blocker bicuculline (BIC, 20 microM), with potassium channel blocker 4-aminopyridine (4-AP, 50 microM) or in Mg(2+)-free medium (LMG). Various parameters of spontaneous and evoked epileptiform discharges and their pharmacological sensitivity were analysed using extracellular field potential recordings in this comparative in vitro study. All types of convulsant solution induced spontaneous epileptiform activity, however, the analysed parameters showed that characteristics of epileptiform discharges are rather different in the three models. The longest duration of discharges was recorded in LMG, while the highest frequency of spontaneous events was detected in 4-AP. The epileptiform field responses elicited by electrical stimulation appeared in an all-or-none manner in BIC. On the contrary, in 4-AP and in LMG the amplitude of the responses increased gradually with increasing stimulation intensities. The NMDA receptor antagonist D,L-2-amino-5-phosphonovaleric acid (APV, 25 microM) abolished the LMG induced spontaneous epileptiform activity and significantly reduced the frequency of the epileptiform discharges in BIC and 4-AP. Blocking the AMPA type of glutamate transmission with 1-(aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466, 40 microM) rapidly abolished BIC-induced spontaneous epileptiform activity and caused a significant decrease in the frequency of 4-AP induced spontaneous epileptiform discharges. However, it had only a weak effect on the LMG-induced epileptiform activity. We conclude that the contribution of NMDA and AMPA types of glutamate receptors to the development and maintenance of epileptiform activity in cortical cell assemblies is different in the three models. There are significant alterations in contribution of NMDA and AMPA types of glutamate receptors to the above-mentioned processes in the different convulsants. In BIC the synchronisation is mainly due to the altered network properties, namely inhibition is reduced in the local circuits. Although inhibition is reduced in the local circuits, the AMPA receptor antagonist relatively easily blocked the synchronised excitation. In 4-AP, and especially in LMG, changes in the membrane characteristics of neurones play a crucial role in the increased excitability. In this case the AMPA antagonist was less effective.