Abstracts

Rationale: Interictal spikes (IIS) are brief, high amplitude waveforms commonly observed in electroencephalography (EEG) recordings of patients with epilepsy, during periods between seizures. IIS correspond to a large intracellular depolarization with evoked action potentials emanating from affected brain areas and are considered to be a marker of epilepsy. While it is known that IIS can result in transient cognitive impairment, whether IIS have any impact on brain development is not known. In this study we assessed the effect of IIS without seizures in developing rats. Methods: 62 Sprague Dawley rat pups were divided into three groups in this study: rats had no exposure to isoflurane or flurothyl (Cont, n=15); rats exposed to isoflurane only (Iso, n=23); rats exposed to isoflurane and flurothyl with IIS (F+IIS, n=24). Half of the rats from each group had intracranial electrodes fixed in the right hippocampal area at age P9 for later EEG monitoring. Starting from P12, all rats in F+IIS group were exposed to flurothyl and isoflurane for four hours a day for ten days. In brief, liquid flurothyl was administered to maintain single recurrent spikes seen on the EEGs. The animals were closely monitored for behavioral and EEG indications of ictal behavior. Five pups in the F+IIS group were excluded for further study due to either death from complications of surgery or development of seizures during flurothyl exposure. Rats in the Iso group were also monitored for the same duration while receiving only isoflurane. We assessed the acute effects of IIS on apoptosis using TUNEL staining and neurogenesis using BrdU staining. The rats of other half of the groups, which did not had surgery, received the same treatments respectively. They were housed for another two months to assess the long term effects of IIS by comparing effects on: i) hippocampus cell counts; ii) memory in the Morris and Radial-Arm Water Maze tests; and iii) synaptic plasticity with long term potentiation and paired-pulse facilitation/inhibition. Results: IIS were observed in all rodent pups in F+IIS group and averaged approximately 18 spikes per minute of the total daily exposure time that ranged from 2-4 hours in the F+IIS group compared with 2 spikes per minute in the Iso group. Compared to the other groups F+IIS had reduced neurogenesis and increased apoptosis. No differences in hippocampal cell counts were found. In the two-trial Radial-Arm Water Maze test rats in F+IIS group had longer latencies and more total and working memory errors than the other groups. No differences were seen in the groups in the Morris water maze. Rats with F+IIS had impaired long-term potentiation compared to the other groups. Conclusions: The model successfully induces IIS without seizures and allows us to study the direct effect of IIS on long- term brain development. IIS induced during the neonatal period lead to statistically significant deficits on in spatial memory, synaptic plasticity, apoptosis, and neurogenesis. This study shows that IIS can impair brain development and result in long-standing cognitive deficits.