Investigating The Parallel Pathways of Seizure Generalization in a Mouse Model of Temporal Lobe Epilepsy

Abstract

Introduction: Temporal lobe epilepsy (TLE) is the most common form of focal onset epilepsy, accounting for almost two-thirds of its cases. In mesial temporal lobe epilepsy (MTLE), the most common type of TLE, seizures arise in the hippocampus as a result of hippocampal sclerosis. However, it is still not well understood how seizures in MTLE spread from the ictal focus. Thus, this project aims at investigating the parallel pathways of seizure propagation, in an acute MTLE mouse model, by utilizing the neural activity reporter c-Fos, through immunohistochemical analysis. Memory consolidation and olfactory pathways are hypothesised to be involved in seizure propagation in KA models of MTLE.

Methods: CD-1 mice were anesthetized and injected unilaterally in the hippocampus with either kainic acid (KA) or saline, in controls. The KA infused animals were perfused 90 minutes after the first seizure, for optimum c-Fos expression, while the saline infused animals were perfused 90 minutes after regaining consciousness from anaesthesia. Diazepam was administrated in case of sever seizures. Brains were either sliced or cleared through a delipidation process. Tissues were stained for c-Fos and imaged using either a slide scanner, to visualize coronal slices, or a light -sheet microscope, in case of 3d imaging of whole brain. Images were modified using FIJI, then quantified with QuPath.

Results: Surprisingly, saline injected mice had similar if not higher c-Fos fluorescence, than the KA treated animals. This was observed in almost all regions of interest. Nevertheless, one distinct observation in KA treated mice, was the lack of c-Fos fluorescence in the ipsilateral dorsal hippocampal areas, close to the site of injection. Contralaterally however, there was a prominent c-Fos fluorescence in the dorsal hippocampus. Interestingly, c-Fos fluorescence was present in caudal ipsilateral areas. Several ipsilateral cortical and subcortical areas were fluorescent in KA injected mice, including, the piriform, the entorhinal area, ectorhinal area, perirhinal area, auditory area, temporal association areas, parietal association areas, somatosensory areas and lateral septal nucleus.

Conclusion: As expected, areas involved in memory consolidation and olfaction were active during the initial stages of status epilepticus. Nevertheless, the lack of c-Fos fluorescence in the ipsilateral hippocampus is puzzling. This could be a consequence of several mechanisms including, modification of c-Fos structure as a result of KA treatment, frequency dependent expression of c-Fos, depolarization block and necrosis. The activity in the controls, however, was most likely a result of KA contamination. However, further detailed investigation is needed to reach a cocreate conclusion on seizure propagation pathways and the lake of c-Fos in ipsilateral hippocampal areas.