Abstracts

Interictal spikes (IS) are morphologically defined, episodic, transient EEG discharges that occur as a result of synchronous, paroxysmal depolarization of neurons resulting in a series of action potentials. The localization of focal IS correspond closely to the site of the seizure focus and IS remain one of the most important diagnostic features on the EEG.
In humans there is a clear relationship between state and IS. Inactivity, drowsiness, and sleep are potent activators of IS. The relationship between IS and state in rodents is less clear. In this experiment, we investigated the relationship between theta and IS in the rat pilocarpine model., Fifteen 60 day old rats 15 P60 rats were treated by lithium/pilocarpine and underwent status epilepticus. Among these animals, five showed spontaneous seizures and interictal spikes two months after status epilepticus induction. They were implanted with a multielectrode array driving electrodes in the CA1 region of the dorsal hippocampus so that bipolar differential recordings above and bellow the cell layer could be performed.
Rats were trained to perform two types of behavioral task: 1) Pellet chasing task: rats were trained to forage for pellets randomly scattered in a circular open field arena 2) Linear track shuttle: rats were trained to shuttle between the two ends of a linear track.
Hippocampal EEG theta/delta ratios, the ratio of the sum of the power-spectrum in the theta band (6-12Hz) by the sum of the power-spectrum in the delta band (1-5Hz), were performed in both tasks., Strikingly, we found that more IS were observed in sessions with a low hippocampal theta/delta ratio than during periods with high ratios. Within the same session, IS appeared during low theta behaviors (either standing or walking at a speed of [lt]10 cm/sec, ratios ranging from 0 to 4) and disappeared in high locomotion, high theta periods (speed [gt]10cm/sec, ratio [gt]4). Pepe add a figure showing this or the EEG versus spikes., These results show that there is a strong correlation between theta and IS. During theta activity IS are suppressed; during periods of decreased theta IS are activated. The relationship between theta and IS demonstrate that as in humans, there is a role of general arousal systems in IS modulation. IS modulation in rodents can be used to model the human condition.[figure1], (Supported by Western Massachusetts Epilepsy Awareness Fund, Friends of Shannon McDermott, Sara Fund, and NINDS (Grants: NS27984 and NS44295).)