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.

Ligand-gated mechanisms leading to ictogenesis in focal epileptic disorders.

We review here the neuronal mechanisms that cause seizures in focal epileptic disorders and, specifically, those involving limbic structures that are known to be implicated in human mesial temporal lobe epilepsy. In both epileptic patients and animal models, the initiation of focal seizures - which are most often characterized by a low-voltage fast onset EEG pattern - is presumably dependent on the synchronous firing of GABA-releasing interneurons that, by activating post-synaptic GABAA receptors, cause large increases in extracellular [K+] through the activation of the co-transporter KCC2. A similar mechanism may contribute to seizure maintenance; accordingly, inhibiting KCC2 activity transforms seizure activity into a continuous pattern of short-lasting epileptiform discharges. It has also been found that interactions between different areas of the limbic system modulate seizure occurrence by controlling extracellular [K+] homeostasis. In line with this view, low-frequency electrical or optogenetic activation of limbic networks restrain seizure generation, an effect that may also involve the activation of GABAB receptors and activity-dependent changes in epileptiform synchronization. Overall, these findings highlight the paradoxical role of GABAA signaling in both focal seizure generation and maintenance, emphasize the efficacy of low-frequency activation in abating seizures, and provide experimental evidence explaining the poor efficacy of antiepileptic drugs designed to augment GABAergic function in controlling seizures in focal epileptic disorders.

A clinical investigation of psychotic vulnerability in early-onset Obsessive-Compulsive Disorder through Cognitive-Perceptive basic symptoms.

Childhood-onset Obsessive-Compulsive Disorder (OCD) shows distinct comorbidity patterns and developmental pathways, as well as an increased risk of psychosis with respect to adult-onset forms. Nevertheless, little is known about the prodromal symptoms of psychosis in children and adolescents with a primary diagnosis of OCD. The present study was aimed at evaluating the occurrence of Cognitive-Perceptual basic symptoms (COPER) and high- risk criterion Cognitive Disturbances (COGDIS) in pediatric and adults OCD patients, verifying if they might vary according to the age of onset of OCD. The study included 90 outpatients with a primary diagnosis of obsessive-compulsive disorder. The study sample was collapsed into three groups according to the age at onset: 1) very early onset group (< 10 years); 2) early onset group (11-18 years); 3) adult-onset group (> 18 years). All patients were administered the Yale-Brown Obsessive- Compulsive Scale (Y-BOCS) and its Child version (CY-BOCS), the Schizophrenia Proneness Instrument-Adult (SPIA) and its Child and Adolescent version (SPI-CY) and the Social and Occupational Functioning Assessment Scale (SOFAS). COPER and COGDIS symptoms were positively associated with OCD severity and detectable, respectively, in 28.9 and 26.7% of our study sample. The very early onset group significantly had higher COPER and COGDIS symptoms than the adult-onset group. Our data suggest that COPER and COGDIS symptoms are frequent in obsessive patients, in particular in those with earlier onset; therefore, their detection in childhood-onset OCD may represent an early and specific indicator of psychotic vulnerability.

A hypothesis for the role of axon demyelination in seizure generation.

Loss of myelin and altered oligodendrocyte distribution in the cerebral cortex are commonly observed both in postsurgical tissue derived from different focal epilepsies (such as focal cortical dysplasias and tuberous sclerosis) and in animal models of focal epilepsy. Moreover, seizures are a frequent symptom in demyelinating diseases, such as multiple sclerosis, and in animal models of demyelination and oligodendrocyte dysfunction. Finally, the excessive activity reported in demyelinated axons may promote hyperexcitability. We hypothesize that the extracellular potassium rise generated during epileptiform activity may be amplified by the presence of axons without appropriate myelin coating and by alterations in oligodendrocyte function. This process could facilitate the triggering of recurrent spontaneous seizures in areas of altered myelination and could result in further demyelination, thus promoting epileptogenesis.

GABAA receptor-mediated networks during focal seizure onset and progression in vitro.

Focal seizures are triggered by the pathological synchronization of a functionally altered group of neurons. In vivo and in vitro results in rodents and single unit studies in humans suggest that seizure can be initiated by increased activity in interneuronal networks. We review here the data derived from in vitro perparations to describe the function of GABAergic network in different phases of focal seizures. The data demonstrate that GABA-mediated synchronization of interneuronal activity has an active role in shaping focal seizure dynamics.

A Novel Focal Seizure Pattern Generated in Superficial Layers of the Olfactory Cortex.

Seizure patterns identified in focal epilepsies caused by diverse etiologies are likely due to different pathogenic mechanisms. We describe here a novel, region-specific focal seizure pattern that mimics seizure activity observed in a subpopulation of patients submitted to presurgical monitoring with intracerebral electrodes. Distinctive seizure-like events (SLEs) are induced in the olfactory regions by acute treatment of both tangential brain slices and the isolated guinea pig brain with the potassium channel blocker 4-aminopyridine. Analysis of field potentials, intracellular activities, and extracellular potassium changes demonstrates that SLEs in the piriform cortex initiate in the superficial layer 1 lacking principal neurons with an activity-dependent increase of extracellular potassium. SLE progression (but not onset) does not require the participation of synaptic transmission and is mediated by diffusion of potassium to deep cortical layers. The novel seizure pattern here described is not observed in other cortical regions; it is proposed to rely on the peculiar organization of the superficial piriform cortex layers, which are characterized by unmyelinated axons and perisynaptic astroglial envelopes. This study reveals a sequence of ictogenic events in the olfactory cortex that were never described before in other cortical structures and supports the notion that altered potassium homeostasis and unmyelinated fibers may represent a potential vehicle for focal ictogenesis.SIGNIFICANCE STATEMENT We describe a novel seizure pattern peculiar of the olfactory cortex that resembles focal seizures with low-voltage fast activity at onset observed in humans. The findings suggest that network mechanisms responsible for seizure onset can be region specific.

Enhanced thalamo-hippocampal synchronization during focal limbic seizures.

OBJECTIVE: The key factors that promote the termination of focal seizures have not been fully clarified. The buildup of neuronal synchronization during seizures has been proposed as one of the possible activity-dependent, self-limiting mechanisms. We investigate if increased thalamo-cortical coupling contributes to enhance synchronization during the late phase of focal seizure-like events (SLEs) generated in limbic regions. METHODS: Recordings were simultaneously performed in the nucleus reuniens of the thalamus, in the hippocampus and in the entorhinal cortex of the isolated guinea pig brain during focal bicuculline-induced SLEs with low voltage fast activity at onset. RESULTS: Spectral coherence and cross-correlation analysis demonstrated a progressive thalamo-cortical entrainment and synchronization in the generation of bursting activity that characterizes the final part of SLEs. The hippocampus is the first activated structure at the beginning of SLE bursting phase and thalamo-hippocampal synchronization is progressively enhanced as SLE develops. The thalamus takes the lead in generating the bursting discharge as SLE end approaches. SIGNIFICANCE: As suggested by clinical studies performed during pre-surgical intracranial monitoring, our data confirm a role of the midline thalamus in leading the synchronous bursting activity at the end of focal seizures in the mesial temporal regions.

Synchronous inhibitory potentials precede seizure-like events in acute models of focal limbic seizures.

Interictal spikes in models of focal seizures and epilepsies are sustained by the synchronous activation of glutamatergic and GABAergic networks. The nature of population spikes associated with seizure initiation (pre-ictal spikes; PSs) is still undetermined. We analyzed the networks involved in the generation of both interictal and PSs in acute models of limbic cortex ictogenesis induced by pharmacological manipulations. Simultaneous extracellular and intracellular recordings from both principal cells and interneurons were performed in the medial entorhinal cortex of the in vitro isolated guinea pig brain during focal interictal and ictal discharges induced in the limbic network by intracortical and brief arterial infusions of either bicuculline methiodide (BMI) or 4-aminopyridine (4AP). Local application of BMI in the entorhinal cortex did not induce seizure-like events (SLEs), but did generate periodic interictal spikes sensitive to the glutamatergic non-NMDA receptor antagonist DNQX. Unlike local applications, arterial perfusion of either BMI or 4AP induced focal limbic SLEs. PSs just ahead of SLE were associated with hyperpolarizing potentials coupled with a complete blockade of firing in principal cells and burst discharges in putative interneurons. Interictal population spikes recorded from principal neurons between two SLEs correlated with a depolarizing potential. We demonstrate in two models of acute limbic SLE that PS events are different from interictal spikes and are sustained by synchronous activation of inhibitory networks. Our findings support a prominent role of synchronous network inhibition in the initiation of a focal seizure.

Simultaneous enhancement of excitation and postburst inhibition at the end of focal seizures.

OBJECTIVE: Comprehension of the events that lead to seizure termination contributes to the development of strategies to confine propagation of ictal discharges. It is commonly assumed that the inhibitory control fails during seizures and recovers after the end of the ictal event. We examine the possibility that a progressive increase of inhibition that counters an increase in the strength of excitation contributes to terminating a focal seizure. METHODS: We analyzed seizures acutely induced by pharmacological manipulations (bicuculline and 4-aminopyridine) in the entorhinal cortex and in the hippocampus of the in vitro isolated guinea pig brain. RESULTS: As seizures ended, extracellular and intracellular recordings showed periodic bursting that progressively decreased in frequency. During the late bursting phase, the duration, number, and rate of occurrence of spikes within single bursts remained constant, whereas cumulative spike amplitude (index of excitation during a burst) and interburst interval (index of inhibition between bursts) progressively increased. The increment of average/cumulative burst excitation and interburst interval toward seizure end was confirmed in human focal seizures recorded with intracerebral electrodes in patients with drug-resistant partial epilepsies. A postburst refractory period of circa 2 seconds that increases with time toward the end of the seizure was confirmed in the experimental model by probing interburst epochs in the CA1 region with local dentate gyrus stimulation just suprathreshold for burst generation. INTERPRETATION: Our findings support the concept that focal seizures are terminated by the simultaneous and opposing enhancement of excitation (burst activity) in addition to postburst inhibition. We hypothesize that a seizure stops when postburst inhibition becomes large enough to prevent reactivation of excitation.

Network dynamics during the progression of seizure-like events in the hippocampal-parahippocampal regions.

Seizure patterns in temporal lobe epilepsies have been described both in humans and in animal models. The involvement of specific hippocampal-parahippocampal subregions in the initiation and progression of temporal lobe seizures is not defined yet. We analyzed limbic network dynamics during seizures induced by 3-min arterial perfusion of 50 µM bicuculline in the in vitro isolated guinea pig brain preparation. As for human and animal temporal lobe epilepsies, 2 seizure types characterized at onset by either fast activity (FA) or hypersynchronous activity (HSA) were observed in our acute model. Simultaneous extracellular recordings were performed from ventral hippocampal-parahippocampal subregions with multichannel electrodes, and laminar analysis and propagation directions were computed to define reciprocal interactions during seizures. FA seizures started with fast oscillations generated in CA1-subiculum and entorhinal cortex, followed by irregular spikes and progressively regular bursts well defined in all subfields, with the exception of pre- and parasubiculum that do not participate in seizure activity. Dentate gyrus was not involved at FA seizure onset and became prominent during the transition to bursting in both FA and HSA patterns. HSA seizures were similar to FA events, but lacked initial FA. During seizures, reliable and steady propagation within the intra-hippocampal re-entrant loop was observed.

Activity-dependent extracellular potassium changes in unmyelinated versus myelinated areas in olfactory regions of the isolated female guinea-pig brain.

The potassium released in the extracellular space during neuronal activity is rapidly removed by glia and neurons to maintain tissue homeostasis. Oligodendrocyte-derived myelin axonal coating contributes to potassium buffering and is therefore crucial to control brain excitability. We studied activity-dependent extracellular potassium ([K+]o) changes in the piriform cortex (PC), a region that features highly segregated bundles of myelinated and unmyelinated fibers. Four-aminopyridine (4AP; 50 µM) treatment or patterned high-frequency stimulations (hfST) were utilized to generate [K+]o changes measured with potassium-sensitive electrodes in the myelinated lateral olfactory tract (LOT), in the unmyelinated PC layer I and in the myelinated deep PC layers in the ex vivo isolated guinea-pig brain. Seizure-like events induced by 4AP are initiated by the abrupt [K+]o rise in the layer I formed by unmyelinated fibers (Uva et al., 2017). Larger [K+]o shifts occurred in unmyelinated layers compared to the myelinated LOT. LOT hfST that mimicks pre-seizure discharges also generated higher [K+]o changes in unmyelinated PC layer I than in LOT and deep PC layers. The treatment with the Kir4.1 potassium channel blocker BaCl2 (100 µM) enhanced the [K+]o changes generated by hfST in myelinated structures. Our data show that activity-dependent [K+]o changes are intrinsically different in myelinated vs unmyelinated cortical regions. The larger [K+]o shifts generated in unmyelinated structures may represent a vehicle for seizure generation.

The Relationship between Childhood Trauma Experiences and Psychotic Vulnerability in Obsessive Compulsive Disorder: An Italian Cross-Sectional Study.

People with obsessive compulsive disorder (OCD) are at increased risk of developing psychotic disorders; yet little is known about specific clinical features which might hint at this vulnerability. The present study was aimed at elucidating the pathophysiological mechanism linking OCD to psychosis through the investigation of childhood trauma experiences in adolescents and adults with OCD. One hundred outpatients, aged between 12 and 65 years old, were administered the Yale-Brown Obsessive Compulsive Scale (Y-BOCS) and its Child version (CY-BOCS), as well as the Childhood Trauma Questionnaire (CTQ); Cognitive-Perceptual basic symptoms (COPER) and high-risk criterion Cognitive Disturbances (COGDIS) were assessed in the study sample. Greater childhood trauma experiences were found to predict psychotic vulnerability (p = 0.018), as well as more severe OCD symptoms (p = 0.010) and an earlier age of OCD onset (p = 0.050). Participants with psychotic vulnerability reported higher scores on childhood trauma experiences (p = 0.02), specifically in the emotional neglect domain (p = 0.01). In turn, emotional neglect and psychotic vulnerability were found higher in the pediatric group than in the adult group (p = 0.01). Our findings suggest that childhood trauma in people with OCD may represent an indicator of psychotic vulnerability, especially in those with an earlier OCD onset. Research on the pathogenic pathways linking trauma, OCD, and psychosis is needed.

Seizure-like discharges induced by 4-aminopyridine in the olfactory system of the in vitro isolated guinea pig brain.

PURPOSE: The study of the interactions leading to network- or region-specific propagation of seizures is crucial to understand ictogenesis. We have recently found that systemic (arterial) application of the potassium channel blocker, 4-aminopyridine (4AP), induces different and independent seizure activities in olfactory and in limbic structures. Here, we have characterized the network and cellular features that support 4AP-induced seizure-like events in the olfactory cortex. METHODS: Simultaneous extracellular recordings were performed from the piriform cortex, the entorhinal cortex, the olfactory tubercle, and the amygdala of the in vitro isolated guinea pig brain preparation. Intracellular, sharp electrode recordings were obtained from neurons of different layers of the region of ictal onset, the piriform cortex. Seizure-like discharges were induced by both arterial perfusion and local intracortical injections of 4AP. KEY FINDINGS: Arterial application of 4AP induces independent seizure activities in limbic and olfactory cortices. Both local applications of 4AP and cortico-cortical disconnections demonstrated that region-specific seizure-like events initiated in the primary olfactory cortex and propagate to anatomically related areas. Seizures induced by arterial administration of 4-AP are preceded by runs of fast activity at circa 30-40 Hz and are independently generated in the hemispheres. Simultaneous extracellular and intracellular recordings in the piriform cortex revealed that the onset of seizure correlates with (1) a gradual amplitude increase of fast activity runs, (2) a large intracellular depolarization with action potential firing of superficial layer neurons, and (3) no firing in a subpopulation of deep layers neurons. During the ictal event, neuronal firing was abolished for 10-30 s in all neurons and gradually restored and synchronized before seizure termination. SIGNIFICANCE: Our data show that olfactory neuronal networks sustain the generation of seizure-like activities that are independent from those observed in adjacent and connected limbic cortex regions. The data support the concept that functionally and anatomically hard-wired networks generate region-specific seizure patterns that could be substrates for system epilepsy.

An excitatory loop with astrocytes contributes to drive neurons to seizure threshold.

Seizures in focal epilepsies are sustained by a highly synchronous neuronal discharge that arises at restricted brain sites and subsequently spreads to large portions of the brain. Despite intense experimental research in this field, the earlier cellular events that initiate and sustain a focal seizure are still not well defined. Their identification is central to understand the pathophysiology of focal epilepsies and to develop new pharmacological therapies for drug-resistant forms of epilepsy. The prominent involvement of astrocytes in ictogenesis was recently proposed. We test here whether a cooperation between astrocytes and neurons is a prerequisite to support ictal (seizure-like) and interictal epileptiform events. Simultaneous patch-clamp recording and Ca2+ imaging techniques were performed in a new in vitro model of focal seizures induced by local applications of N-methyl-D-aspartic acid (NMDA) in rat entorhinal cortex slices. We found that a Ca2+ elevation in astrocytes correlates with both the initial development and the maintenance of a focal, seizure-like discharge. A delayed astrocyte activation during ictal discharges was also observed in other models (including the whole in vitro isolated guinea pig brain) in which the site of generation of seizure activity cannot be precisely monitored. In contrast, interictal discharges were not associated with Ca2+ changes in astrocytes. Selective inhibition or stimulation of astrocyte Ca2+ signalling blocked or enhanced, respectively, ictal discharges, but did not affect interictal discharge generation. Our data reveal that neurons engage astrocytes in a recurrent excitatory loop (possibly involving gliotransmission) that promotes seizure ignition and sustains the ictal discharge. This neuron-astrocyte interaction may represent a novel target to develop effective therapeutic strategies to control seizures.

The association between childhood trauma and motor structure in obsessive-compulsive rituals: An ethological and evolutionary approach.

OBJECTIVE: Ethological models have highlighted a specific motor structure of compulsions in obsessive-compulsive disorder (OCD), based on the rigid repetitions of acts, and with the adaptive significance of facing unpredictable conditions. Such an evolutionary mechanism might explain the robust association between childhood traumatic experiences (CTEs) and OCD. However, a relationship between CTEs and the motor structure of compulsions has not been investigated yet. The first objective of the study was to confirm a specific motor structure of OCD compulsions with respect to control behaviors; the second objective was to assess a possible association between the motor structure of compulsions and CTEs severity. METHOD: Thirty-two OCD outpatients (13 female, Mage = 44.50 years, SE = 19.71) and 27 healthy controls (10 female, Mage = 37.62 years, SE = 16.20), matched for sex and age, provided a videotape of their compulsions and corresponding ordinary acts, respectively. Behavior was scored with the software "Observer." Participants were administered the Yale-Brown Obsessive Compulsive Scale and the Childhood Trauma Questionnaire. A dependent t test was used to compare the motor structure of behavior between the groups; Pearson's correlations to investigate associations between motor parameters and CTEs. RESULTS: Compulsions showed a specific motor structure due to the repetition of functional and nonfunctional acts. CTEs severity was especially associated with the repetition of functional acts, independently from OCD severity. CONCLUSION: Our findings, in confirming a peculiar motor structure for OCD compulsions, hint for the first time at a link between CTEs and compulsive repetition of functional acts, which would represent a plastic developmental response to CTEs unpredictability. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

GABAA signaling, focal epileptiform synchronization and epileptogenesis.

Under physiological conditions, neuronal network synchronization leads to different oscillatory EEG patterns that are associated with specific behavioral and cognitive functions. Excessive synchronization can, however, lead to focal or generalized epileptiform activities. It is indeed well established that in both epileptic patients and animal models, focal epileptiform EEG patterns are characterized by interictal and ictal (seizure) discharges. Over the last three decades, employing in vitro and in vivo recording techniques, several experimental studies have firmly identified a paradoxical role of GABAA signaling in generating interictal discharges, and in initiating-and perhaps sustaining-focal seizures. Here, we will review these experiments and we will extend our appraisal to evidence suggesting that GABAA signaling may also contribute to epileptogenesis, i.e., the development of plastic changes in brain excitability that leads to the chronic epileptic condition. Overall, we anticipate that this information should provide the rationale for developing new specific pharmacological treatments for patients presenting with focal epileptic disorders such as mesial temporal lobe epilepsy (MTLE).

Ultrasounds induce blood-brain barrier opening across a sonolucent polyolefin plate in an in vitro isolated brain preparation.

The blood-brain barrier (BBB) represents a major obstacle to the delivery of drugs to the central nervous system. The combined use of low-intensity pulsed ultrasound waves and intravascular microbubbles (MB) represents a promising solution to this issue, allowing reversible disruption of the barrier. In this study, we evaluate the feasibility of BBB opening through a biocompatible, polyolefin-based plate in an in vitro whole brain model. Twelve in vitro guinea pig brains were employed; brains were insonated using a planar transducer with or without interposing the polyolefin plate during arterial infusion of MB. Circulating MBs were visualized with an ultrasonographic device with a linear probe. BBB permeabilization was assessed by quantifying at confocal microscopy the extravasation of FITC-albumin perfused after each treatment. US-treated brains displayed BBB permeabilization exclusively in the volume under the US beam; no significant differences were observed between brains insonated with or without the polyolefin plate. Control brains not perfused with MB did not show signs of FITC-albumin extravasation. Our preclinical study suggests that polyolefin cranial plate could be implanted as a skull replacement to maintain craniotomic windows and perform post-surgical repeated BBB opening with ultrasound guidance to deliver therapeutic agents to the central nervous system.

Mapping region-specific seizure-like patterns in the in vitro isolated guinea pig brain.

Specific neurophysiological seizure patterns in patients with focal epilepsy depend on cerebral location and the underlying neuropathology. Location-specific patterns have been also reported in experimental models. Two focal seizure patterns, named p-type and l-type, typical of neocortical and mesial temporal regions were identified in both patients explored with intracerebral EEG and in animal models. These two patterns were recorded in the olfactory regions and in the entorhinal cortex after either 4AP or BMI administration. Here we mapped epileptiform activities in other cortices to verify the existence of specific epileptiform patterns. Field potentials were simultaneously recorded at multiple locations in olfactory, limbic and neocortical regions of the isolated guinea pig brain after arterial administration of either 4AP or BMI. Most neocortical areas did not generate new distinctive focal seizure-like event (SLE), beside the p-type and l-type patterns. Spiking activity was typically recorded after BMI in all new analyzed regions, whereas SLEs were commonly observed during 4AP perfusion. We confirmed the presence of reproducible region-specific epileptiform patterns in all explored cortical areas and demonstrated that strongly inter-connected areas generate similar SLEs. Our study suggests that p- and l-type SLE represent the most common focal seizure patterns during acute manipulations with pro-epileptic compounds.

Independent epileptiform discharge patterns in the olfactory and limbic areas of the in vitro isolated Guinea pig brain during 4-aminopyridine treatment.

In vitro studies performed on brain slices demonstrate that the potassium channel blocker 4-aminopyridine (4AP, 50 microM) discloses electrographic seizure activity and interictal discharges. These epileptiform patterns have been further analyzed here in a isolated whole guinea pig brain in vitro by using field potential recordings in olfactory and limbic structures. In 8 of 13 experiments runs of fast oscillatory activity (fast runs, FRs) in the piriform cortex (PC) propagated to the lateral entorhinal cortex (EC), hippocampus and occasionally to the medial EC. Early and late FRs were asynchronous in the hemispheres showed different duration [1.78 +/- 0.51 and 27.95 +/- 4.55 (SD) s, respectively], frequency of occurrence (1.82 +/- 0.49 and 34.16 +/- 6.03 s) and frequency content (20-40 vs. 40-60 Hz). Preictal spikes independent from the FRs appeared in the hippocampus/EC and developed into ictal-like discharges that did not propagate to the PC. Ictal-like activity consisted of fast activity with onset either in the hippocampus (n = 6) or in the mEC (n = 2), followed by irregular spiking and sequences of diffusely synchronous bursts. Perfusion of the N-methyl-d-aspartate receptor antagonist 2-amino-5-phosphonopentanoic acid (100 microM) did not prevent FRs, increased the duration of limbic ictal-like discharges and favored their propagation to olfactory structures. The AMPA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (50 microM) blocked ictal-like events and reduced FRs. In conclusion, 4AP-induced epileptiform activities are asynchronous and independent in olfactory and hippocampal-entorhinal regions. Epileptiform discharges in the isolated guinea pig brain show different pharmacological properties compared with rodent in vitro slices.

Activity- and pH-dependent adenosine shifts at the end of a focal seizure in the entorhinal cortex.

Adenosine (ADO) is an endogenous modulator of neuronal excitability, with anticonvulsant and neuroprotective effects. It has been proposed that the activity-dependent release of ADO promoted by the extracellular acidification occurring during seizures contributes to seizure termination. To verify this hypothesis, we recorded field potentials, pH and ADO changes measured with enzymatic biosensors during acute focal seizures in the medial entorhinal cortex (mEC) of the isolated guinea-pig brain maintained in vitro. The effect of ADO on seizure-like events (SLEs) induced by GABAa receptor antagonism with bicuculline methiodide (BMI; 50 µM) was assessed by arterial applications of 1 mM ADO. ADO either reduced or prevented epileptiform activity. The A1 receptor antagonist DPCPX (100-500 µM) prolonged BMI-induced seizures and was able to precipitate SLEs in the absence of proconvulsant. Simultaneous recordings of brain activity, extracellular ADO and pH shifts demonstrated that ADO decreases at the onset and progressively rises toward the end of SLEs induced by either BMI or 4-aminopyridine (4AP; 50 µM), reaching maximal values 1-5 min after SLE termination. ADO changes were preceded by a SLE-dependent extracellular acid shift. Both pH acidification and ADO changes were abolished by 22 mM HEPES in the arterial perfusate. In these conditions, SLE duration was prolonged. Our data confirm that ADO plays a role in regulating brain excitability. Its increase depends on seizure-induced acid pH shift and it is maximal after the end of the SLE. These findings strongly suggest that ADO contributes to termination of focal seizures and to the establishment of the postictal depression.