NEUROLOGICAL DEFICITS IN A PERI-ADOLESCENT RAT MODEL OF NON-CONVULSIVE STATUS EPILEPTICUS: AN ELECTROPHYSIOLOGICAL, BEHAVIORAL, AND HISTOLOGICAL APPROACH

Abstract

Background: Non-convulsive seizures are prolonged abnormal outbursts of synchronous neuronal activity that induce variable levels of impaired consciousness and/or abnormal behavior without convulsions (muscle tonic stiffening or rhythmic clonic activity). In current clinical practice, such seizures are often missed and therefore remain untreated due to their indistinct clinical presentation, and the need for a confirmatory inter-ictal electroencephalogram (EEG) to diagnose them. Although such seizures were reported decades ago, it is unknown whether cerebral insults may occur if such seizures remain uncontrolled and potentially lead to neuronal death. There is substantial debate among clinicians on whether to treat NCSE as aggressively as convulsive status epilepticus, and there are no randomized studies upon which to base treatment decisions.

Methods: Male postnatal day 41 (P41) peri-adolescent rats received one, or two 24 hours apart intra-hippocampal 0.00625μg injections of kainic acid to induce EEG-confirmed prolonged non-convulsive seizures (NCSE). Controls were sham treated with normal saline. Animals were then monitored for 30 days via video-EEG to count for abnormal electrical activity (P45), then subjected to a behavioral panel behavioral panel which included tests for anxiety-like behaviors (light- dark box (LD) and open field (OF) tests), depressive-like behaviors (forced swim test (FST)), and hippocampal-dependent visuospatial navigation (Morris water maze (MWM)), followed by the MAAV test (P65). Rats were sacrificed 30-60 minutes after testing. Immunohistochemistry for neuronal nuclear antigen (NeuN) was then performed for histopathological analysis.

Results: All rats developed one (LSKA) or two (LRKA) episodes of hippocampal NCSE following KA injections, with electrographic patterns of evolving rhythmic fast spikes and polyspikes, accompanied by behavioral arrest, staring, and oromotor automatisms. Average post-KA latencies to NCSE were comparable between LSKA and LRKA on the first day of induction, as were the average seizure durations. On the second day of induction, LRKA rats had similar seizure latencies and durations to the first day of induction. Continuous EEG recordings revealed no seizure recurrence. One-month post-NCSE, the LRKA rats had significant deficits in visuospatial navigation in the MWM and in auditory and contextual learning in the MAAV test compared to both LSKA and control rats, which were comparable. The LD and OF tests indicated hyperactivity and anxiety-like behaviors with decreased exploratory tendencies in the LRKA group. All groups showed comparable immobility to controls in the FST. No overt cell loss was observed in hippocampal NeuN stained sections.

Conclusion: Our pilot data points to the persistence of neurodevelopmental deficits long with anxiety-like behaviors one month following two but not one episode of NCSE. Preliminary data shows that this was not accompanied by overt hippocampal neuronal loss. Ongoing studies in our laboratory aim at confirming the persistence of these detrimental behavioral deficits and their underlying mechanisms. These harmful consequences, and the impact of seizure burden, if confirmed, call for a more urgent diagnosis and treatment of NCSE.