Abstracts

Recently we have described the presence of pathological high frequency oscillations in the kainic acid rat model of epilepsy. These oscillations, which may be a marker of progressive epileptogenesis, appear to reflect changes in neuronal network excitability associated with an initial precipitating event that may lead to chronic epilepsy. The goal of these studies was to investigate (1) the existence of similar patterns in mice after pilocarpine (PILO) induced status epilepticus (SE) and (2) the neuronal substrates of such oscillations., C57B6/J adult mice (15-25g) were subjected to electrophysiological recordings under urethane anesthesia (1.5g/kg) between 1 and 3 weeks after PILO (320mg/kg, s.c.) induced SE. Simultaneously, extracellular unit activity was recorded by a glass pipette (15-25M[Omega]) and local field potentials were recorded by a 20-[micro]m tungsten microelectrodes placed [le]500[mu]m from the tip of the glass microelectrode. Juxtacellular labeling was performed according to the protocol described by Pinault (1996). After completion of electrophysiological experiments, brains were sliced and sections processed for peroxidase-based neurobiotin visualization used for morphological reconstructions. Cross-correlation analysis between unit activity and local field potentials and power spectral analysis were performed using Datapac (Mission Viejo, CA) software., Similar to electrographic patterns observed in the kainic acid-treated rats, PILO-treated mice had interictal epileptiform events (IEE) with and without high frequency signals (100-500 Hz) and electrographic (EEG) seizures. Power spectral analysis of interictal local field potentials indicated the existence of activity with peaks at 2.0, 5.0-7.0 and 10-40 Hz. Two interneurons, one pyramidal and two granular cells were identified using juxtacellular labeling. One granular cell had no spontaneous activity, but discharged exclusively during epileptiform activity. During seizures, the discharge rate of the hilar interneuron increased to 0.59/sec from a baseline of 0.26/sec. The frequency of discharges of remaining cells (one CA1 interneuron, one pyramidal and one granular cells) was not affected either by the IEE or EEG seizures., During the early stage of epileptogenesis, the electrographic EEG patterns and spectral frequency characteristics of interictal and ictal discharges are similar between kainic-treated rats and PILO-treated mice. Simultaneous extracellular unit and field potential recording, combined with juxtacellular labeling, may identify neuronal mechanisms of progressive epileptogenesis in a mouse model of chronic epilepsy., (Supported by NIH grant NS 02808.)