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New onset epilepsy and seizures are common neurological disorders in aged people, second only to stroke and dementia. They are frequently related to other pathological conditions including stroke, trauma, tumors and neurological diseases whereas in about one-third of cases the origin is unknown. Besides the origin, the cellular and molecular events that suddenly trigger seizures are poorly defined. Using an acute model of seizure generation that better resembles new onset seizures, we studied GABAergic interneurons and astrocytes during seizure generation. We found that seizures are preceded by a GABAergic rhythmic hyperactivity that synchronizes pyramidal neurons by inducing a rebound spiking that favors seizures' onset. Furthermore, the intense activity in GABAergic interneurons evokes Ca 2+ elevations in astrocytes that, by releasing glutamate, further excite neuronal network. Elucidating the cellular and molecular events that generate seizures may reveal new targets for treatment of new onset seizures and epilepsy.

The release of glutamate from astrocytes activates synchronous slow inward currents (SICs) in hippocampal pyramidal neurons, which are mediated by the NMDA receptor and represent a nonsynaptic mechanism to promote the synchronization of neuronal activity. Two recent studies demonstrate that SICs generate neuronal paroxysmal depolarizations resembling those typical of interictal epileptiform activity and proposed that there could be an astrocytic basis of epilepsy (Kang et al., 2005; Tian et al., 2005). We tested this hypothesis using two in vitro models of epileptiform activity in hippocampal slices. Removal of extracellular Mg2+ and application of picrotoxin or perfusion with 0.5 mM Mg2+ and 8.5 mM K+-containing saline result mainly in neuronal ictal- and interictal-like epileptiform activity, respectively. Although both models trigger epileptiform activity, astrocytic Ca2+ oscillations were increased only after slice perfusion with 0 mM Mg2+ and picrotoxin. The activation of astrocytic Ca2+ signaling correlates with an increased frequency of SICs, and, when paired neurons were within 100 microm of one another with synchronous neuronal Ca2+ elevations, the generation of synchronous neuronal depolarizations and action potential discharges. TTX blocked both ictal- and interictal-like epileptiform activity without affecting SICs or SIC-mediated neuronal synchronization. In contrast, NMDA receptor antagonists, which block SICs, did not prevent the generation of either ictal- or interictal-like events. Based on this clear-cut pharmacology, our data demonstrate that nonsynaptic glutamate release from astrocytes is not necessary for the generation of epileptiform activity in vitro, although we cannot exclude the possibility that it may modulate the strength of the ictal (seizure)-like event.

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]