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Key points In focal epilepsy the propagation of seizure discharges arising at restricted brain sites is opposed by feedforward inhibition. Failure of this inhibition marks focal seizure propagation to distant neurons., The cellular source of inhibition and the mechanism of inhibition failure are, however, undefined., Here we reveal that a subclass of GABAergic interneurons, i.e. the parvalbumin-expressing, fast-spiking interneurons, are a main source of the inhibitory signal that locally restrains seizures. Furthermore, a firing impairment in these interneurons, probably due to a drastic membrane depolarization, is an important event that by reducing the overall strength of local inhibition allows seizures to propagate across the cortex., Our data suggest that modulation of fast-spiking interneuron activity may represent a new therapeutic strategy to prevent generalization of focal epilepsies., Abstract In different animal models of focal epilepsy, seizure-like ictal discharge propagation is transiently opposed by feedforward inhibition. The specific cellular source of this signal and the mechanism by which inhibition ultimately becomes ineffective are, however, undefined. We used a brain slice model to study how focal ictal discharges that were repetitively evoked from the same site, and at precise times, propagate across the cortex. We used Ca2+ imaging and simultaneous single/dual cell recordings from pyramidal neurons (PyNs) and different classes of interneurons in rodents, including G42 and GIN transgenic mice expressing the green fluorescence protein in parvalbumin (Pv)-fast spiking (FS) and somatostatin (Som) interneurons, respectively. We found that these two classes of interneurons fired intensively shortly after ictal discharge generation at the focus. The inhibitory barrages that were recorded in PyNs occurred in coincidence with Pv-FS, but not with Som interneuron burst discharges. Furthermore, the strength of inhibitory barrages increased or decreased in parallel with increased or decreased firing in Pv-FS interneurons but not in Som interneurons. A firing impairment of Pv-FS interneurons caused by a membrane depolarization was found to precede ictal discharge onset in neighbouring pyramidal neurons. This event may account for the collapse of local inhibition that allows spatially defined clusters of PyNs to be recruited into propagating ictal discharges. Our study demonstrates that Pv-FS interneurons are a major source of the inhibitory barrages that oppose ictal discharge propagation and raises the possibility that targeting Pv-FS interneurons represents a new therapeutic strategy to prevent the generalization of human focal seizures. [ABSTRACT FROM AUTHOR]

Despite intensive studies, our understanding of the cellular and molecular mechanisms underlying epileptogenesis remains largely unsatisfactory. Our defective knowledge derives in part from the lack of adequate experimental models of the distinct phases that characterize the epileptic event, that is, initiation, propagation, and cessation. The aim of our study is the development of a new brain slice model in which a focal seizure can be repetitively evoked at a precise and predictable site. Epileptiform activities were studied by fast Ca2+ imaging coupled with simultaneous single and double patch-clamp or extracellular recordings from neurons of entorhinal cortex (EC) slices from Wistar rats and C57BL6J mice at postnatal days 13–17. In the presence of 4-aminopyridine (4-AP) and low Mg2+, activation of layer V–VI neurons by local N-methyl-d-aspartate (NMDA) applications evolved into an ictal discharge (ID) that propagated to the entire EC. NMDA-evoked IDs were similar to spontaneous events. IDs with similar pattern and duration could be repetitively triggered from the same site by successive NMDA stimulations. The high ID reproducibility is an important feature of the model that allowed testing of the effects of currently used antiepileptic drugs (AEDs) on initiation, propagation, and cessation of focal ictal events. By offering the unique opportunity to repetitively evoke an ID from the same restricted site, this model represents a powerful approach to study the cellular and molecular events at the basis of initiation, propagation, and cessation of focal seizures. [ABSTRACT FROM AUTHOR]

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 patchclamp 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. [ABSTRACT FROM AUTHOR]

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 patchclamp 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. [ABSTRACT FROM AUTHOR]