Electrophysiologic abnormalities of the hippocampus in the pilocarpine/cycloheximide model of chronic spontaneous seizures

Purpose: Mossy fiber sprouting (MFS) and synaptic reorganization in the dentate gyrus (1)(3) is considered one of the physiopathologic mechanisms in temporal lobe epilepsy. Supragranular MFS can be blocked by cycloheximide (CHX) without interfering with the genesis of spontaneous recurrent seizures. The aim of this study was to investigate electrophysiologic properties of the hippocampus in the CHX/pilocarpine (CHX/PILO) model as compared with the conventional PILO model. Methods: In vitro electrophysiology was performed 2 months after status epilepticus (SE) induction using extracellular recordings in hippocampal slices from PILO (n = 8) and CHX/PILO animals (n = 10). Field potential responses were evoked in the CA1 and DG regions during perfusion with normal artificial cerebrospinal fluid (aCSF) and aCSF containing 3.5, 5, or 8 mM K+ without or with bicuculline added. Neo-Timm staining was used for the assessment of supragranular MFS. Results: Evoked potentials in PILO- and CHX/PILO-treated rats displayed small-amplitude polyspiking activity (epileptiform responses) in CAI and an apparently normal isolated population spike in DG. More important, PILO and CHX/PILO animals did not differ regarding electrophysiologic abnormalities, even under high K+ or high K+/bicuculline. Analysis of the neo-Timm staining revealed strong supragranular MFS in PILO-injected rats and significantly less staining in CHX/PILO rats. Thus, occurrence of abnormal stimulus responses and high K+- or high K+/bicuculline-induced epileptiform activities did not depend on the degree of MFS. Conclusions: We therefore suggest that other mechanisms such as anomalous intrinsic bursting and disinhibition rather than MFS might account for the increased hippocampal hyperexcitability in this model.