Network Properties of the Dentate Gyrus in Epileptic Rats With Hilar Neuron Loss and Granule Cell Axon Reorganization. Paul S. Buckmaster and F. Edward Dudek. Department of Anatomy and Neurobiology, Colorado State University, Fort Collins, Colorado 80523.
APStracts 4:0033N, 1997.
ABSTRACT
Neuron loss in the hilus of the dentate gyrus and granule cell axon reorganization have been proposed as etiological factors in human temporal lobe epilepsy. To explore these possible epileptogenic mechanisms, electrophysiological and anatomical methods were used to examine the dentate gyrus network in adult rats that had been treated systemically with kainic acid. All kainate-treated rats, but no age-matched vehicle-treated controls, were observed to have spontaneous recurrent motor seizures beginning weeks to months after exposure to kainate. Epileptic kainate-treated rats and control animals were anesthetized for field-potential recording from the dentate gyrus in vivo. Epileptic kainate-treated rats displayed spontaneous positivities ("dentate EEG spikes") with larger amplitude and higher frequency than control animals. After electrophysiological recording, rats were perfused and their hippocampi processed for Nissl- and Timm-staining. Epileptic kainate-treated rats displayed significant hilar neuron loss and granule cell axon reorganization. It has been hypothesized that hilar neuron loss reduces lateral inhibition in the dentate gyrus, thereby decreasing seizure threshold. To assess lateral inhibition, simultaneous recordings were obtained from the dentate gyrus in different hippocampal lamellae, separated by 1 mm. The perforant path was stimulated with paired-pulse paradigms, and population spike amplitudes were measured. Responses were obtained from one lamella while a recording electrode in a distant lamella leaked saline or the GABAA-receptor antagonist bicuculline. Epileptic kainate-treated and control rats both showed significantly more paired-pulse inhibition when a lateral lamella was hyperexcitable. To assess seizure threshold in the dentate gyrus, two techniques were used. Measurement of stimulus threshold for evoking maximal dentate activation revealed significantly higher thresholds in epileptic kainate-treated rats compared to controls. In contrast, epileptic kainate- treated rats were more likely than controls to discharge spontaneous bursts of population spikes and to display stimulus-triggered afterdischarges when a focal region of the dentate gyrus was disinhibited with bicuculline. These spontaneous bursts and afterdischarges were confined to the disinhibited region and did not spread to other septotemporal levels of the dentate gyrus. Epileptic kainate-treated rats that displayed spontaneous bursts and/or afterdischarges had significantly larger percentages of Timm-staining in the granule cell and molecular layers than epileptic kainate-treated rats that failed to show spontaneous bursts or afterdischarges. In summary, this study reveals functional abnormalities in the dentate gyrus of epileptic kainate- treated rats; however, lateral inhibition persists, suggesting that vulnerable hilar neurons are not necessary for generating lateral inhibition in the dentate gyrus. 

Received 17 September 1996; accepted in final form 14 January 1997.
APS Manuscript Number J750-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 5 February 1997