Abstracts

The neocortex and hippocampus are two brain regions prone to epileptiform activity. However, it remains unclear whether a single area or multiple areas initiate seizure-like events in these brain regions and whether developmental age influences this, given that neurons vary in their response to GABA based on their stage of development. Therefore, we evaluated changes in the spatial dynamics of seizure-like events between the neonatal and adolescent brain.

We evaluated the localization of interictal and ictal-like (events longer than 10 sec) foci in the neocortex and hippocampus of neonatal (postnatal day, P8-10) and adolescent (P25-30) mice in vitro. Seizure-like activity was induced with 4-aminopyridine (4-AP) in both coronal and horizontal acute brain slices from transgenic mice expressing a genetically encoded calcium indicator, GCaMP6s, under the Thy1 promoter. The activity was recorded using a mesoscope to image the entire brain slice. We also studied the effect of bicuculline, a GABA_A receptor antagonist, on the initiation of seizures.

Mesoscope recordings revealed that during 4-AP perfusion, both coronal and horizontal neonatal slices exhibited a greater number of interictal foci than adolescents, while the number of ictal foci remained similar. We found that the locations of interictal and ictal foci were comparable between adolescent and neonate slices. Coronal slices tended to initiate activity in the somatosensory area and the lateral areas adjacent to it. Horizontal slices were more inclined to show foci in areas medial and lateral to the somatosensory cortex, along with the hippocampus. Neonatal slices displayed a decrease in interictal events with bicuculline in both coronal and horizontal slices, but only a reduction of ictal foci was seen in horizontal ones. The adolescent group exhibited no changes in the number of ictal or interictal foci following bicuculline treatment. In some slices, we observed spreading depolarization (SD). During 4-AP, the majority of SDs occurred in the adolescent group and were more prevalent in coronal slices compared to horizontal ones. We found that SDs initiated either randomly or after seizure-like events. After SD, all slices continued to show some form of activity; however, in most slices, the activity failed to propagate through the depolarized area. 

Our data reveal that neonates have a greater number of interictal foci. The number of foci decreased in neonates when inhibition was blocked. This data suggests that the neonatal brain is more prone to develop multifocal activity than the adolescent brain.