The effects of febrile status epilepticus on hippocampal circuit function

Abstract

Rationale . Febrile status epilepticus (FSE) in children has been linked to cognitive deficits and epileptogenesis later in life. Recent evidence in the experimental hyperthermia animal model of FSE (eFSE) has shown that spatial memory deficits may be a result of structural and functional changes to specific cells types in the hippocampal circuit. Experimentally, the effects of FSE on cognitive deficits are mitigated by the blockade of neuron-restrictive silencing factor (NRSF) binding to its target chromatin. The goal of the current study is to understand the relationship between cognitive outcomes and the efficacy of hippocampal circuit throughput and whether this relationship is altered by NRSF treatment post FSE. Methods . Postnatal day 10–11 male Sprague-Dawley rat pups (N = 13) underwent procedures for eFSE hyperthermia induction. Pups were placed, two at a time, inside a 3 L flask, the bottom of which was lined with absorbent paper. Prior to hyperthermia, a glycerin-based hydrating ointment was applied to the paws, ears, and tail of the pups to mitigate potential hyperthermic skin injury. Pups were subjected to a continuous stream of warm air until hyperkinesis and chewing automatisms were identified. Seizure behaviors typically progressed to clonic movements and eventual tonic extension. After approximately 60 min of hyperthermia, eFSE pups were immersed in cool water (−23.0 °C) to aid in the return of core temperature to normothermia and to promote seizure cessation. Pups were then placed on a euthermic pad maintained at 37 °C for 30 min, and then were returned to their home cage and dam. An intact NRSE or a scrambled (SCR) NRSE sequence oligodeoxynucleotide (ODN) was infused ICV 3 h post eFSE (Sigma-Aldrich). Four experimental groups were used in these experiments. The first was a non-eFSE control animal that received a scrambled sequence of NRSE (CTL-SCR). The second was a non-eFSE control who received the NRSE sequenced ODN (CTL-NRSE). The third were eFSE animals infused with scrambled ODN (eFSE-SCR). Finally, there was a group of eFSE animals infused with NRSE ODNs (eFSE-NRSE). 2.5 nmol of either NRSE or SCR ODN were infused bilaterally into the ventricles at a rate of 0.5 l/min and a volume of 2.5 l/hemispheres. ICV infusions were repeated 24 h after eFSE. While blind to treatment group, treated and untreated rats were trained on an active avoidance spatial task on a rotating arena in order to measure cognitive outcomes in relation to treatment condition. To measure circuit throughput in the dorsal hippocampus across experimental conditions, we utilized 64-channel silicon probes to measure EEGs and neuronal action potentials across synaptic input regions along the apical and basal dendrites of CA1 pyramidal cells and dentate gyrus granular cells under urethane-anesthesia. Following acute probe recording experiments, animals were perfused with fixative and brain tissue was harvested for immunohistochemical procedures. Results . Preliminary data suggests group differences in the ability to learn the active avoidance spatial task while analysis of cell spiking data suggests interhemispheric differences in neuronal activity across all groups. Acute recordings also revealed that there were notable hallmarks during EEG recordings across animals and groups in theta-, beta-, and gamma- range oscillations in deep CA1; dentate gyrus EEGs; and also the response to mechanical perturbation when the probe was lowered.