Postictal single-cell firing patterns in the hippocampus

Patients with epilepsy have varying degrees of postictal impairment including confusion and amnesia. This impairment adds substantially to the disease burden of epilepsy. However, the mechanism responsible for postictal cognitive impairment is unclear. The purpose of this study was to study single-cell firing patterns in hippocampal cells after spontaneous seizures in rats previously subjected to status epilepticus.

In this study, we monitored place cells and interneurons in the CA1 region of the hippocampus before and after spontaneous seizures in six epileptic rats with a history of status epilepticus. Place cells fire action potentials when the animal is in a specific location in space, the so-called place field. Place cell function correlates well with performance in tasks of visual-spatial memory and appears to be an excellent surrogate measure of spatial memory.

Twelve spontaneous seizures were recorded. After the seizures, a marked decrease in firing rate of action potentials from place cells was noted, whereas interneuron firing was unchanged. In addition, when place cell firing fields persisted or returned, they had aberrant firing fields with reduced coherence and information content. In addition to postictal suppression of firing patterns, seizures led to the emergence of firing fields in previously silent cells, demonstrating a postictal remapping of the hippocampus.

These findings demonstrate that postictal alterations in behavior are not due solely to reduced neuronal firing. Rather, the postictal period is characterized by robust and dynamic changes in cell-firing patterns resulting in remapping of the hippocampal map.