Abstracts

Rationale: Brain malformations lead to cognitive disabilities and seizures in both human and animal models. Complex layered structures like the neocortex and cerebellum are particularly susceptible to these malformations, which can disrupt their lamination and neuronal connectivity, and result in heterotopia. Heterotopia is defined as breaches in the pia of the cerebellum between lobules and accumulation of neurons and glia in layer 1 in the neocortex. Such alteration in neuronal migration in connectivity discloses susceptibility to hyperexcitability and concurrently, spontaneous seizure in animal models. This study investigated whether sporadic neocortical and/or cerebellar malformations in C57BL/6J mice are associated with a lowered seizure threshold.

Methods: To induce generalized tonic-clonic seizures in male and female C57BL/6J adult mice, the inhaled chemo-convulsant flurothyl was administered using an inhalation chamber and a flow-controlled syringe at 30µl/min. Time to seizure onset was recorded as a measure of brain excitability changes. After the seizures, the mice were euthanized, and their brains were post-fixed before being collected for histological analysis. Cryosections of the neocortex and cerebellar vermis from C57BL/6J mice were Nissl stained and examined for molecular layer heterotopia using brightfield microscopy.

Results: Over 60% of the mice exhibited neocortical and/or cerebellar heterotopia, with no observed sex differences in the prevalence of these malformations. Heterotopia was found in both the cortical midline and the dorsal neocortex and between the lobules VIII and IX of the cerebellum as expected. Chi-squared analysis revealed that the proportion with cerebellar malformation was at a significantly higher prevalence compared to neocortical alone of malformations in both regions. Seizure onset time was decreased specifically in C57BL/6J females when both cerebellar and neocortical heterotopia were observed compared to normal brain lacking malformation. No significant effects of seizure latency onset were seen among male C57BL/6J mice.


Conclusions: These findings highlight the need to consider the presence of malformations in C57BL/6J mice when studying brain development, epilepsy, and other nervous system diseases. This study also provides insight into the differences that can exist in the onset of seizures and the correlation with a malformation that impacts a common research mouse strain widely used with genetic mutations.

Funding: Funding for this work was supported by an NIH Grant R15S088776