Hyperexcitability in the dentate gyrus of the epileptic Mongolian gerbil.

There are anatomical differences in the hippocampi of seizure-resistant (SR) vs. seizure-sensitive (SS) Mongolian gerbils. SS gerbils have more GABAergic neurons and more GABAergic axon terminals contacting inhibitory basket cells in the dentate gyrus. Hence, it has been hypothesized that inhibition of basket cells causes disinhibition of granule cells and results in hyperexcitability in SS gerbils. To test this hypothesis we measured the level of excitability in the dentate gyrus of anesthetized adult SS and SR gerbils, using a paired-pulse stimulus paradigm to evoke population spikes. Population spikes of SS and SR gerbils both showed strong paired-pulse inhibition at short interstimulus intervals and paired-pulse facilitation at intermediate interstimulus intervals. However, compared to SR gerbils, SS gerbils had significantly stronger paired-pulse facilitation which persisted at longer interstimulus intervals. These results show that SS gerbils have hyperexcitable dentate gyri and support the view that less effective inhibition is an underlying basis for this hyperexcitability.