ELECTROPHYSIOLOGICAL CORRELATES OF SEIZURE SENSITIVITY IN THE DENTATEGYRUS OF EPILEPTIC JUVENILE AND ADULT GERBILS

1. Naturally occurring inherited epilepsy is common among Mongolian ge rbils, providing an opportunity to identify neurological factors that correlate with seizure behavior. In the present study we examine the o ntogeny of seizure behavior and compare the electrophysiology and anat omy of the dentate gyrus in epileptic and nonepileptic gerbils. 2. Beh avioral seizure testing revealed that young gerbils do not begin havin g seizures until they are 2 mo of age, at which time seizure incidence across animals is at its highest level. Most seizure-positive juvenil e gerbils became epileptic adults, but 30% outgrew their epileptic con dition. 3. The number of somatostatin- and parvalbumin-immunoreactive neurons in the dentate gyrus and Ammon's horn was counted, with the us e of quantitative stereological techniques, in juvenile and adult gerb ils. No significant differences were detected between epileptic and no nepileptic groups. 4. In dentate gyrus field potential responses to pe rforant path stimulation, adult epileptic gerbils showed enhanced pair ed-pulse inhibition at short (30 ms) interstimulus intervals and enhan ced facilitation at intermediate (70 ms) intervals compared with nonep ileptic controls. These differences were most pronounced when stimuli were delivered at faster (1.0 Hz) rather than slower (0.1 Hz) rates. 5 . Compared with seizure-negative juveniles, seizure-positive juveniles showed the same pattern of paired-pulse response abnormalities as epi leptic adults. However, seizure-positive juveniles had a lower thresho ld for maximal dentate activation than epileptic adults. 6. These resu lts demonstrate similar functional abnormalities in the dentate gyri o f epileptic adult gerbils and in juvenile gerbils before they experien ce multiple seizures. Such findings suggest that abnormalities in func tional inhibition of the dentate gyrus network precede and therefore m ight contribute to overt seizure activity.