Elsevier

Epilepsy Research

Volume 85, Issues 2–3, August 2009, Pages 199-205
Epilepsy Research

Different phases of afterdischarge during rapid kindling procedure in mice

https://doi.org/10.1016/j.eplepsyres.2009.02.020Get rights and content

Summary

The basic mechanisms of hippocampal networks in epileptogenesis are not entirely understood. To help achieve a better understanding of these mechanisms, we studied the extra-cellular electrically evoked responses in the hippocampi of mice during rapid kindling. Kindling protocol was achieved by stimulating the dorsal right hippocampus six times daily for four days using bipolar electrodes to produce sub-convulsive electrical discharges. Motor responses and analyzed electroencephalographic recordings showed progression from partial complex seizures to generalized seizures associated with different consecutive patterns within the afterdischarges. A spike-wave pattern appeared immediately after stimulation in combination with a poly-spike complex superimposed over the wave (AD1). AD1 was followed by a poly-spike complex (AD2), which was followed by a progressive modification of repetitive spikes (AD3). An ictal depression event was observed at the end of each AD3. Theta oscillations were observed at stage 1–2 of kindling, while beta/gamma oscillations appeared within AD2, associated with stage 4–5 from Racine's score. Benzodiazepine, a GABA (A) agonist (Diazepam) administered at non-sedative doses and only on days 3 and 4 of kindling, limited beta and gamma frequency bands and the progression of seizure severity, suggesting that the failure of GABA (A) agonism mediates the propagation or generalization of seizures. We conclude that different phases of afterdischarge occur during kindling and that high frequencies mediate generalization of seizures.

Introduction

The rapid kindling model in rodents represents an elegant experimental approach for dissecting and analyzing the oscillatory components of the evoked electrical discharge related with different types of seizures. This model allows for exploration of the neuronal network that contributes to the progression of partial seizures to generalized seizures (Delgado and Sevillano, 1961, Goddard et al., 1969, Racine, 1972, Leung, 1987), without producing excessive damage to the brain, as in other experimental models of epilepsy (Morimoto et al., 2004, Pitkanen et al., 2006). The rapid kindling procedure administers repetitive, sub-convulsive electrical stimulation to the hippocampus (Goddard et al., 1969, Lothman et al., 1991). As a consequence of these stimulations, the clinical manifestations and the evoked electrical activity (afterdischarge, AD) observed in electroencephalographic (EEG) recordings display progressively increased complexity and duration.

Different EEG patterns in epilepsy (Penfield and Jasper, 1954) may represent a dynamic process in vivo – occurring within only a few seconds – among the pyramidal-interneuron network (Buzsáki et al., 1983, Buzsáki et al., 1989), neuron-glia interactions (Somjen, 2001), biochemical reactions (Tu and Bazan, 2003), and gene expression (Musto and Bazan, 2006), which are all critical components in seizures. Unfortunately, the specific neural networks (associated with specific EEG patterns) that are activated have not yet been studied in detail; therefore, the relevance of EEG morphology may be underestimated. In this study, we describe three different dimensions of AD during hippocampal kindling. We observed frequencies in beta and gamma bands in kindling progression, which are related with seizure severity. Intraperitoneal administration of diazepam (the δ-amino-butyric-acid (GABA) (A) agonist) before the occurrence of the kindled state resulted in limited beta and gamma oscillations and, consequently, a reduction in severe seizures for at least one week after kindling acquisition. These observations suggest that a progressive failure of inhibition during kindling mediates seizure propagation and severity leading to a recurrent, hyperexcitable hippocampal network.

Section snippets

Materials and methods

Studies were performed on C57BL/6 adult male mice (20–25 g, Charles River Lab, Inc., MA) according to National Institutes of Health guidelines, in adherence to the recommendations from the Declaration of Helsinki, and in accordance with nationally accepted principles in the care and use of experimental animals. Protocols were approved by the Louisiana State University Health Sciences Center Institutional Animal Care and Use Committee. Fourteen animals were used for EEG pattern analysis during

Results

From a total of 45 EEG recordings from AD during kindling, 80% presented phases of afterdischarges as is described in Fig. 2B. The duration, power of the signal, and frequencies of the AD increased from stage 1–2 to stage 4–5 (Fig. 2A). In both stages, the ADs were immediately followed by a low-energy signal representing ictal-depression (ID) activity (Fig. 2B).

Also, each stage evoked different consecutive EEG patterns, denoted by AD1, AD2 and AD3 (Fig. 2B). At stage 1–2, which represented the

Discussion

According to these experiments, we propose that different phases of afterdischarge occur during kindling and that high frequencies mediate generalization of seizures. Hippocampal response to these stimulations, or AD, becomes more complex during kindling. Most experimental designs using the kindling model focus on the duration of the AD as the main dependent variable (Racine, 1972, Goddard et al., 1969, Morimoto et al., 2004). Here, however, we concentrated on other components of the AD, like

Acknowledgements

The authors would like to thank Dr. Nicolas G. Bazan, Louisiana State University Health Sciences Center, New Orleans, Neuroscience Center of Excellence, for his insightful advice and comments. This work was supported by NIH/NCRR grant P20 RR016816.

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