Propagation dynamics of epileptiform activity acutely induced by bicuculline in the hippocampal-parahippocampal region of the isolated Guinea pig brain

PURPOSE: Aim of the study is to investigate the involvement of parahippocampal subregions in the generation and in the propagation of focal epileptiform discharges in an acute model of seizure generation in the temporal lobe induced by arterial application of bicuculline in the in vitro isolated guinea pig brain preparation. METHODS: Electrophysiological recordings were simultaneously performed with single electrodes and multichannel silicon probes in the entorhinal, perirhinal, and piriform cortices and in the area CA1 of the hippocampus of the in vitro isolated guinea pig brain. Interictal and ictal epileptiform discharges restricted to the temporal region were induced by a brief (3-5 min) arterial perfusion of the GABA(A) receptor antagonist, bicuculline methiodide (50 microM). Current source density analysis of laminar field profiles performed with the silicon probes was carried out at different sites to establish network interactions responsible for the generation of epileptiform potentials. Nonlinear regression analysis was conducted on extracellular recordings during ictal onset in order to quantify the degree of interaction between fast activities generated at different sites, as well as time delays. RESULTS: Experiments were performed in 31 isolated guinea pig brains. Bicuculline-induced interictal and ictal epileptiform activities that showed variability of spatial propagation and time course in the olfactory-temporal region. The most commonly observed pattern (n = 23) was characterized by the initial appearance of interictal spikes (ISs) in the piriform cortex (PC), which propagated to the lateral entorhinal region. Independent and asynchronous preictal spikes originated in the entorhinal cortex (EC)/hippocampus and progressed into ictal fast discharges (around 25 Hz) restricted to the entorhinal/hippocampal region. The local generation of fast activity was verified and confirmed both by CSD and phase shift analysis performed on laminar profiles. Fast activity was followed by synchronous afterdischarges that propagated to the perirhinal cortex (PRC) (but not to the PC). Within 1-9 min, the ictal discharge ceased and a postictal period of depression occurred, after which periodic ISs in the PC resumed. Unlike preictal ISs, postictal ISs propagated to the PRC. CONCLUSIONS: Several studies proposed that reciprocal connections between the entorhinal and the PRC are under a very efficient inhibitory control (1). We report that ISs determined by acute bicuculline treatment in the isolated guinea pig brain progress from the PC to the hippocampus/EC just before ictal onset. Ictal discharges are characterized by a peculiar pattern of fast activity that originates from the entorhinal/hippocampal region and only secondarily propagates to the PRC. Postictal propagation of ISs to the PRC occurred exclusively when an ictal discharge was generated in the hippocampal/entorhinal region. The results suggest that reiteration of ictal events may promote changes in propagation pattern of epileptiform discharges that could act as trigger elements in the development of temporal lobe epilepsy.     

Discharge threshold is enhanced for several seconds after a single interictal spike in a model of focal epileptogenesis

Interictal spikes (ISs) are typically observed between seizures in focal epilepsies. Whether ISs are causally involved or represent protective elements in the transition toward an ictal discharge is an open question. Previous studies suggested that inhibition or disfacilitation occurs during the period elapsing between two ISs induced by local application of either bicuculline or penicillin in the piriform cortex of the in vitro isolated guinea pig brain preparation. We further investigated this issue by studying responses to afferent stimulation during the interspike period (6.3 +/- 2.5 s; mean +/- SD). Properly set stimulation intensity of the lateral olfactory tract resets ISs exclusively (and not before) 4-10 s (5.6 +/- 2.0 s; mean +/- SD) after a preceding spontaneous spike. This finding demonstrates the existence of a period of enhanced threshold to stimulus-evoked activation that coincides with the interspike interval in the absence of stimulation. Current source density analysis of depth laminar profiles demonstrated that both stimulus-evoked and spontaneous ISs were generated by the activation of an identical cortical circuit. Our study suggests that interictal spiking could play a protective role or at least provide an effective restraint against the onset of a focal ictal discharge.     

Expression of adhesion factors induced by epileptiform activity in the endothelium of the isolated guinea pig brain in vitro

PURPOSE: Brain inflammation has been recently considered in the pathogenesis of focal epilepsies. Synthesis of pro-inflammatory mediators in the brain was described both in experimental models of seizures and in human postsurgical tissue. Inflammatory mediators may up-regulate endothelial adhesion molecules, therefore promoting adhesion and homing of leucocytes into the brain. In the present study, expression of inducible adhesion factors in brain endothelium was verified after pharmacological induction of seizure-like activity in specific brain areas of the in vitro isolated guinea pig brain. METHODS: Experiments were performed in isolated guinea-pig brains maintained in vitro by arterial perfusion. In this preparation, brief application of the GABAa receptor-antagonist, bicuculline, consistently induced focal ictal discharges in the limbic region that secondarily diffuse to the neocortex, as verified by simultaneous electrophysiological recording of extracellular activity. At the end of the electrophysiological experiment (after 5 h in vitro), brains were fixed and immunostaining for adhesion molecules P-selectin and ICAM-1 and for Fos protein was evaluated. RESULTS: Immunohistochemical analysis of isolated brains in which seizure-like activity was induced revealed expression of inducible adhesion factors P-selectin and ICAM-1 in the endothelium of small-medium size brain vessels. In particular, the expression of these molecules was consistently observed in all areas involved in epileptic seizure-like ictal activity (limbic cortices and neocortex), and was infrequently found in regions that generated interictal spiking (piriform cortex), suggesting a trigger role played by seizures for endothelial activation. An increase in Fos protein expression was evident in all analyzed limbic areas and in the neocortex, indicating a correlation between the areas of neuronal and endothelial activation. In control brains maintained in vitro for comparable times without induction of epileptiform activity, no immunoreactivity for Fos and adhesion molecules was observed. CONCLUSIONS: Seizure-like activity in an in vitro isolated brain preparation induces the expression of adhesion molecules in the cerebral endothelium. These observations indicate that local endothelial activation may represent a crucial step for the development of an inflammatory response induced by seizures, and suggest a possible novel pathogenic mechanism during the process of epileptogenesis.     

The relationship between interictal and ictal paroxysms in an in vitro model of focal hippocampal epilepsy

In studies of focal epilepsy it is frequently assumed that the interictal spike is the elementary form of epileptic activity and that seizure, or ictal, episodes evolve by temporal summation and spatial expansion of interictal paroxysms. We examined this hypothesis in an in vitro model of acute focal epilepsy produced by perfusing rat hippocampal slices with solutions containing moderately elevated concentrations of K+. Some of the preparations treated in this way displayed recurring electrical seizures in the CA1 field. Each seizure episode typically evolved by a seemingly smooth progression of brief interictal bursts into sustained ictal discharge. However, exposure of preparations showing electrical seizures to blockers of synaptic transmission or to cholinergic agonists abolished interictal spiking in all hippocampal fields but did not impede the initiation of ictal episodes in area CA1. Likewise, severing the connections between areas CA3 and CA1 abolished interictal spiking in area CA1 without disrupting the initiation of seizures in this region. These data clearly show that in this model, focal seizures arise independent of interictal spikes and through different cellular mechanisms. While interictal electrogenesis requires chemical synaptic excitation, ictal episodes can be initiated and maintained by nonsynaptic neuronal interactions.     

Increased discharge threshold after an interictal spike in human focal epilepsy

It is commonly assumed that interictal spikes (ISs) in focal epilepsies set off a period of inhibition that transiently reduces tissue excitability. Post-spike inhibition was described in experimental models but was never demonstrated in the human epileptic cortex. In the present study post-spike excitability was retrospectively evaluated on intracerebral stereo-electroencephalographic recordings performed in the epileptogenic cortex of five patients suffering from drug-resistant focal epilepsy secondary to Taylor-type neocortical dysplasias. Patients typically presented with highly periodic interictal spiking activity at 2.33 +/- 0.87 Hz (mean +/- SD) in the dysplastic region. During the stereo-electroencephalographic procedure, low-frequency stimulation at 1 Hz was systematically performed for diagnostic purposes to identify the epileptogenic zone. The probability of evoking an IS during the interspike period in response to 1-Hz stimuli delivered close to the ictal-onset zone was examined. Stimuli that occurred early after a spontaneous IS (within 70% of the inter-IS period) had a very low probability of generating a further IS. On the contrary, stimuli delivered during the late inter-IS period had the highest probability of evoking a further IS. The generation of stimulus-evoked ISs is occluded for several hundred milliseconds after the occurrence of a preceding spike discharge. As previously shown in animal models, these findings suggest that, during focal, periodic interictal spiking, human neocortical excitability is phasically controlled by post-spike inhibition.     

The "epileptogenic zone" in humans: representation of intercritical events by spatio-temporal maps

In the preoperative investigation of partial epilepsies, electrical characteristics of the so-called epileptogenic area are of critical interest to localize this area. We have always emphasized that the analysis of ictal events themselves, is more valuable than that of inter-ictal anomalis. One reason for this emphasis has been the absence of a reliable method to evaluate the relationship between the topography of ictal and interictal events. An technique for imaging an intracerebral potential field is reported. It allows to analyse the spatio-temporal distribution of interictal events recorded during the SEEG exploration. This technique applied to an illustrative patient shows that the temporo-spatial distribution of interictal spikes is related to that of the ictal discharge itself. A new hypothesis concerning the build up of an epileptogenic area in man is presented.     

Do Interictal Discharges Promote or Control Seizures? Experimental Evidence from an In Vitro Model of Epileptiform Discharge

Summary: Interictal and ictal discharges are recorded from limbic structures in temporal lobe epilepsy patients. In clinical practice, interictal spikes are used to localize the epileptogenic area, but they also are assumed to promote ictal events. Here I review data obtained from combined slices of mouse hippocampus–entorhinal cortex that indicate an inverse relation between interictal and ictal events. In this preparation, application of 4-aminopyridine or Mg²⁺-free medium induce (a) interictal discharges that originated from CA3 and propagate (via the Schaffer collaterals) to CA1 and entorhinal cortex, to return to the hippocampus through the dentate area; and (b) ictal discharges that initiate in the entorhinal cortex and propagate to the hippocampus via the dentate gyrus. Interictal activity occurs throughout the experiment (up to 6 h), whereas ictal discharges disappear after 1–2 h. Schaffer collateral cut abolishes interictal discharges in CA1, entorhinal cortex, and dentate and reestablishes entorhinal ictal discharges. Moreover, ictal discharge generation in the entorhinal cortex after Schaffer collateral cut is prevented by mimicking CA3 activity with rhythmic electrical stimulation of CA1 outputs. Thus hippocampal interictal activity controls the ability of the entorhinal cortex to generate seizures. It also may be proposed that Schaffer collateral cut may model the epileptic condition in which CA3 damage results in loss of hippocampal control over the entorhinal cortex. In conclusion, these experiments demonstrate that interictal activity controls rather than promotes ictal events, and functional integrity of CA3 constitutes a critical control mechanism in temporal lobe epilepsy.     

Interical spiking increases 2-deoxy[14C]glucose uptake and c-fos—like reacitivity

Although interical spikes are thought to share pathophysiological mechanisms with partial-onset seizure discharges, positron emission tomographic studies of the interictal stare have paradoxically shown focal hypometabolism whereas seizures produce hypermetabolism. To address this question, we performed functional mapping studies in an interictal spiking model in the rat. Recording screw electrodes were inserted through the skull bone so as to depress underlying cortex. Interictal spiking was subsequently induced by systemic administration of bicuculline methiodide. 2-deoxy[¹⁴C]glucose studies revealed increased glucose utilization in superficial and middle cortical layers at spiking screw sites. Nonspiking screw sites in the same animals and in controls did not show increased uptake. Convulsive seizures caused additional 2-deoxy[¹⁴C]glucose uptake at screw sites and in widespread forebrain areas. c-fos immunoreactivity occurred in superficial cortex at interictal spiking, but not nonspiking, sites. Convulsive seizures induced widespread forebrain c-fos immunoreactivity. These data suggest interictal epileptiform activity occurs in cells adjacent to cortical injury; these activate deeper layers via local connections. Interictal and ictal epileptiform states share common mechanisms, as both induce glucose hypermetabolism and immediate-early gene product activation. Possible reasons for failure to detect hypermetabolism in interictal human subjects are discussed.     

Factors in the interictal record predicting an ictal episode: a case study.

This report deals with a patient with intractable seizures, who had 64 subdural electrodes implanted onto the left frontal and temporal cerebral cortex in anticipation of probable seizure surgery. One specific region on the left frontal lobe proved to be the focus for both the interictal spikes and also for the ictal activity. Our goal was to determine what electrical characteristics in the interictal record predicted the ictal episodes. The number of interictal spikes and also the amount of fast activity (15-128/sec) progressively increased before each of the 5 ictal episodes; an analysis showed that the number of spikes and their frequency proved the more significant variable. One other characteristic that changed before the ictal episodes was a great decrease in the interval between spikes as an example of a diminishing variability or increasing periodicity. Thus, the important factors in the interictal record that lead to an ictal event are the number of spikes, the amount of fast activity and the decreasing variability between the spikes.     

Ictal magnetoencephalographic discharges from elementary visual hallucinations of status epilepticus

PURPOSE: To report the rare opportunity to study ictal magnetoencephalography (MEG) in a 26 year old man with simple partial status epilepticus that presented as elementary visual hallucinations (EVHs) in the right upper visual field. METHODS: The patient described his EVHs as "snowing on TV," "flickering lights," and "rotating coloured balls" that continued for several days. MEG and simultaneous EEG were recorded twice: during an episode of EVHs (ictal recordings) and after EVHs were controlled by medications (interictal recordings). RESULTS: During EVHs, MEG showed continuous periodic epileptiform discharges over the left posterior superior temporal region, while simultaneous EEG showed rhythmic theta waves and sporadic spikes over the left temporal region. The MEG discharge consisted of a three phase spike complex. Equivalent current dipoles (ECDs), modelled from spike complexes, localised in the left superior temporal area. After drug treatment controlled the EVHs, interictal MEG and EEG showed rare spikes over the same left temporal region. The average ictal ECD moment (mean (SD)) (128.7 (32.8 nAm)) was significantly weaker than the average interictal ECD moment (233.0 (63.9) nAm) (p<0.05). CONCLUSIONS: The continuous, periodic, and clustered discharges seen on ictal MEG were the sources of EVH. The weaker ictal ECD sources were frequently not detected by scalp EEG, while the stronger interictal sources, presumably originating from an extensive interictal zone, were sufficiently large to be seen as EEG spikes.     

Refractory periods following interictal spikes in acute experimentally induced epileptic foci

Under epileptic conditions, interictal epileptic events are followed by large inhibitions which prevent the transition to ictal discharges. In the present experiments the refractory period following interictal epileptic spikes was investigated in animal experiments. Interictal epileptic activity was elicited by application of penicillin onto the motor cortex of anesthetized rats.Interictal epileptic discharges were followed by an absolute refractory phase lasting 200–300 msec, in which no epileptic event could be elicited by epicortical stimulation. This was followed by a relative refractory period up to 900 msec after onset of the conditioning spike; spikes elicited with intervals between 300 and 900 msec were smaller than those with greater intervals and required higher stimulation intensities. This period ends by a sharp drop of threshold. In two-thirds of the experiments, spikes were favoured in intervals of 300–500 msec due to a sag of the threshold, which possibly indicates recurrent neuronal excitations. Stimulations with frequencies of about 1 /sec favoured a transition from a pattern with spikes appearing in an irregular sequence every 2–3 sec, to a discharge pattern with spikes appearing with regular intervals of about I sec. This change of firing pattern was associated with a drop of the spike threshold.It is concluded that interictal epileptic events are followed by a refractory period comprising different components. Alterations of the neuronal inhibitions responsible for these refractory phases may be critical for the activity of the focus and may determine the transition from interictal to ictal discharges.     

Analysis of the dynamics and origin of epileptic activity in patients with tuberous sclerosis evaluated for surgery of epilepsy

OBJECTIVE: The epilepsies associated with the tuberous sclerosis complex (TSC) are very often refractory to medical therapy. Surgery for epilepsy is an effective alternative when the critical link between the localization of seizure onset in the scalp and a particular cortical tuber can be established. In this study we perform analysis of ictal and interictal EEG to improve such link. METHODS: The ictal and interictal recordings of four patients with TSC undergoing surgery for epilepsy were submitted to independent component analysis (ICA), followed by source analysis, using the sLORETA algorithm. The localizations obtained for the ictal EEG and for the average interictal spikes were compared. RESULTS: The ICA of ictal EEG produced consistent results in different events, and there was good agreement with the tubers that were successfully removed in three of the four patients (one patient refused surgery). In some patients there was a large discrepancy between the localization of ictal and interictal sources. The interictal activity produced more widespread source localizations. CONCLUSIONS: The use of ICA of ictal EEG followed by the use of source analysis methods in four cases of epilepsy and TSC was able to localize the epileptic generators very near the lesions successfully removed in surgery for epilepsy. SIGNIFICANCE: The ICA of ictal EEG events may be a useful add-on to the tools used to establish the connection between epileptic scalp activity and the cortical tubers originating it, in patients with TSC considered for surgery of epilepsy.     

On the ictogenic properties of the piriform cortex in vitro

Purpose: The piriform cortex (PC) is known to be epileptic‐prone and it may be involved in the manifestation of limbic seizures. Herein, we have characterized some electrophysiologic and pharmacologic properties of the spontaneous epileptiform activity generated by PC networks maintained in vitro. Methods: We performed field potential recordings from the PC in coronal or sagittal rat brain slices along with pharmacologic manipulations of γ‐aminobutyric acid (GABA)ergic and glutamatergic signaling during application of the convulsant drug 4‐aminopyridine (4AP, 50 μm ). Key Findings: Coronal and sagittal preparations generated interictal‐like and ictal‐like epileptiform discharges with similar duration and frequency. Ictal‐like discharges in sagittal slices were initiated mostly in the PC anterior subregion, whereas interictal activity did not have any preferential site of origin. In sagittal slices, high frequency oscillations (HFOs) at 80–200 Hz were detected mainly at the beginning of the ictal discharge in both posterior and anterior subregions. N‐Methyl‐d ‐aspartate (NMDA) receptor antagonism abolished ictal discharges, but failed to influence interictal activity. In the absence of ionotropic glutamatergic transmission, PC networks generated slow, GABA receptor–dependent events. Finally, GABA_A receptor antagonism during application of 4AP only, abolished ictal discharges and disclosed recurrent interictal activity. Significance: Our findings demonstrate that PC networks can sustain in vitro epileptiform activity induced by 4AP. HFOs, which emerge at the onset of ictal activity, may be involved in PC ictogenesis. As reported in several cortical structures, ionotropic glutamatergic neurotransmission is necessary but not sufficient for ictal discharge generation, a process that also requires operative GABA_A receptor–mediated signaling.     

Synchronization of GABAergic interneuronal networks during seizure-like activity in the rat horizontal hippocampal slice

We studied the contribution of GABAergic (gamma-aminobutyric acid) neurotransmission to epileptiform activity using the horizontal hippocampal rat brain slice. Seizure-like (ictal) activity was evoked in the CA1 area by applying high-frequency trains (80 Hz for 2 s) to the Schaffer collaterals. Whole-cell recordings from stratum oriens-alveus interneurons revealed burst firing with superimposed high-frequency spiking which was synchronous with field events and pyramidal cell firing during ictal activity. On the other hand, interictal interneuronal bursts were synchronous with large-amplitude inhibitory postsynaptic potentials (IPSPs) in pyramidal cells. Excitatory and inhibitory postsynaptic potentials were simultaneously received by pyramidal neurons during the ictal afterdischarge, and were synchronous with interneuronal bursting and field potential ictal events. The GABAA receptor antagonist bicuculline greatly reduced the duration of the ictal activity in the CA1 layer, and evoked rhythmic interictal synchronous bursting of interneurons and pyramidal cells. With intact GABAergic transmission, interictal field potential events were synchronous with large amplitude IPSPs (9.8 +/- 2.4 mV) in CA1 pyramidal cells, and with interneuronal bursting. Simultaneous dual recordings revealed synchronous IPSPs received by widely separated pyramidal neurons during ictal and interictal periods, indicative of widespread interneuronal firing synchrony throughout the hippocampus. CA3 pyramidal neurons fired in synchrony with interictal field potential events recorded in the CA1 layer, and glutamate receptor antagonists abolished interictal interneuronal firing and synchronous large amplitude IPSPs received by CA1 pyramidal cells. These observations provide evidence that the interneuronal network may be entrained in hyperexcitable states by GABAergic and glutamatergic mechanisms.     

A Stereoelectroencephalographic (SEEG) Study of Light-Induced Mesiotemporal Epileptic Seizures

Summary: Purpose: This study explored the mechanism of light-induced complex partial seizures by using ictal intracerebral recordings in a patient with refractory epilepsy of the right temporal lobe.
Methods: Presurgical evaluation of this patient was realized by means of video-EEG recordings, WADA test, magnetic resonance imaging (MRI), F¹⁸-deoxyglucose and C¹¹-flumazenil positron emission tomography (PET) interictal neuroimaging data, and stereoelectroencephalographic (SEEG) ictal recordings.
Results: SEEG investigations demonstrated the right mesiotemporal origin of all the patient's seizures. This result was confirmed by a successful right temporal lobectomy. Moreover, SEEG recordings revealed a frequent interictal spiking activity in the right occipital visual cortex that was undetectable on scalp recordings. However, the occipital cortex was not involved at the onset of mesiotemporal ictal discharges and was not hyperresponsive to focal electrical stimulation.
Conclusions: This study shows that, in spite of interictal occipital spiking, the photosensitivity of mesiotemporal seizures can be observed in both the absence of occipital cortex involvement during ictal discharges and demonstrable hyper-excitability of the occipital cortex to light or direct electrical stimulation.     

The functional role of metabotropic glutamate receptors in epileptiform activity induced by 4-aminopyridine in the rat amygdala slice

The metabotropic glutamate receptor (mGluR) antagonist, (RS)-α-methyl-4-carboxyphenylglycine (MCPG; 500 μM), was tested on intracellularly recorded epileptiform activity induced by 4-aminopyridine (4-AP) in amygdala neurons. Superfusing 4-AP (1 mM) produced interictal spiking followed by ictal bursting. MCPG prevented the progressive transition from interictal spiking to ictal bursting but affected neither induction of interictal spiking nor maintenance of ongoing ictal bursting. These data suggest that mGluRs may be involved in the induction of ictal seizure events.     

Interictal-ictal interactions and limbic seizure generation.

Interictal discharges are used in clinical practice to localize the epileptogenic focus in patients with partial epilepsy. However, the interaction between interictal and ictal discharges remains debatable. For instance, interictal events may lead to seizure onset in some models of epileptiform discharge. By contrast, in other models, disappearance of interictal activity (for example by activation of GABAB receptors) induces or potentiates ictal events. We have recently obtained new evidence for a control exerted by interictal discharges on ictal activity in rodent combined slices of hippocampus-entorhinal cortex. In this preparation continuous application of 4-aminopyridine induces: (i) interictal activity which initiates in CA3 and propagates via CA1 and subiculum to the entorhinal cortex, and return to the hippocampus through the dentate gyrus; and (ii) ictal discharges, which originate in the entorhinal cortex and propagate via the dentate gyrus to the hippocampus. Ictal discharges disappear over time, while synchronous interictal discharges continue to occur. Lesioning the Schaffer collaterals abolishes interictal discharges in CA1, entorhinal cortex and dentate gyrus and discloses entorhinal ictal discharges that propagate, via the dentate gyrus, to the CA3 subfield. Interictal activity of CA3 origin also prevents the occurrence of ictal events recorded in the entorhinal cortex in Mg(2+)-free medium. Moreover, in both models, ictal discharge generation in the entorhinal cortex after Schaffer collateral cut is prevented by mimicking CA3 activity through rhythmic electrical stimulation of CA1 hippocampal outputs. Hence, our data demonstrate that hippocampus interictal discharges control the expression of electrographic seizures in entorhinal cortex. Sectioning the Schaffer collaterals may model the epileptic condition in which cell damage in the CA3 subfield results in loss of CA3 control over the entorhinal cortex. Hence, the functional integrity of hippocampal CA3 neurons may represent a critical control point in temporal lobe epilepsy.     

Associative Synaptic Potentials in the Piriform Cortex of the Isolated Guinea-pig Brain In Vitro

The involvement of local and remote associative fibres in the generation of piriform cortex synaptic potentials was investigated in the isolated guinea-pig brain maintained in vitro by arterial perfusion by implementing current source density analysis (CSD) on cortical field potential profiles. Previous hypotheses were verified using acute surgical isolation of piriform cortical areas to study different synaptic events separately. Stimulation of the lateral olfactory tract activated associative potentials throughout the piriform cortex. In the anterior piriform cortex, the current sinks responsible for the generation of associative potentials were located in the superficial portion of layer lb and in layer III. In the posterior piriform cortex, two associative events were observed: an early sink located in the superficial part of layer Ib, followed by a sink in the deep part of the same layer. In the anterior piriform cortex, local associative synaptic potentials were separated from the component carried by long projective fibres by surgically isolating a small area of cortex monosynaptically activated by lateral olfactory tract stimulation. In this patch of lateral olfactory tract-connected anterior piriform cortex, local associative sinks were observed in the superficial lb layer and in layer III. Monosynaptic activation of the isolated patch of anterior piriform cortex induced purely associative potentials throughout the piriform cortex. These potentials were mediated by the synaptic activation of apical dendrites in the superficial lb layer and selectively abolished by severing the long associative fibres. The anterior piriform cortex layer III sink and the posterior piriform cortex deep lb associative component were evoked by the activation of large population spikes in the monosynaptic anterior piriform cortex and the disynaptic posterior piriform cortex response respectively. These two sinks are presumably generated locally through a polysynaptic circuit, whose activation depends on the degree of cortical excitation. Olfactory signal processing in the guinea-pig piriform cortex during states of normal excitability is supported by the interactions between associative inputs impinging on the synapses located separately on the dendrites of pyramidal neurons. An increase in the synchronization of piriform cortex neuron discharge activates usually silent local circuit synapses.     

Masking synchronous GABA-mediated potentials controls limbic seizures

PURPOSE: We determined how CA3-driven interictal discharges block ictal activity generated in the entorhinal cortex during bath application of 4-aminopyridine (4AP, 50 microM). METHODS: Field potential and [K+]o recordings were obtained from mouse combined hippocampus-entorhinal cortex slices maintained in vitro. RESULTS: 4AP induced N-methyl-d-aspartate (NMDA) receptor-dependent ictal discharges that originated in the entorhinal cortex, disappeared over time, but were reestablished by cutting the Schaffer collateral (n = 20) or by depressing CA3 network excitability with local application of glutamatergic receptor antagonists (n = 5). In addition, two types of interictal activity occurred throughout the experiment. The first type was CA3 driven and was abolished by a non-NMDA glutamatergic receptor antagonist. The second type was largely contributed by gamma-aminobutyric acid type A (GABAA) receptor-mediated conductances and persisted during blockade of glutamatergic transmission. The absence of CA3-driven interictal discharges in the entorhinal cortex after Schaffer collateral cut facilitated the GABA-mediated interictal potentials that corresponded to large [K+]o elevations and played a role in ictal discharge initiation. Accordingly, ictal discharges along with GABA-mediated interictal potentials disappeared during GABAA-receptor blockade (n = 7) or activation of mu-opioid receptors that inhibit GABA release (n = 4). CONCLUSIONS: Our findings suggest that CA3-driven interictal events restrain ictal discharge generation in the entorhinal cortex by modulating the size of interictal GABA-mediated potentials that lead to large [K+]o elevations capable of initiating ictal discharges in this structure.     

Ictal epileptiform events induced by removal of extracellular magnesium in slices of entorhinal cortex are blocked by baclofen

Most manipulations employed to elicit epileptiform activity in brain slices result in brief, interictal-like events. In slices of the rat entorhinal cortex perfusion with a magnesium-depleted medium elicits prolonged seizure events which are more reminiscent of ictal activity. The interictal events in brain slice models are blocked by baclofen, but there is an indication from one study (H. S. Swartzwelder, D. V. Lewis, W. W. Anderson, and W. A. Wilson, 1987, Brain Res. 410: 362-366) that ictal events may not be affected by this drug. The present study examined the effects of baclofen on ictal events recorded in entorhinal cortex slices in magnesium-free medium. Simultaneous studies of effects of baclofen on interictal responses in the hippocampal region of the same slices were made on many occasions. The results show that both types of epileptiform activity are suppressed with a similar facility by baclofen.