Inactivation of the Somatosensory Cortex Prevents Paroxysmal Oscillations in Cortical and Related Thalamic Neurons in a Genetic Model of Absence Epilepsy

Absence seizures consist of bilateral spike-and-wave discharges (SWDs) occurring over widespread cortical and thalamic regions. In genetic models of absence epilepsy, recent in vivo investigations indicate that SWDs emerge first in the facial somatosensory cortex and then propagate via the corticothalamocortical loop. The specific involvement of this cortical region in ictogenic processes remained to be established and the participation of its related thalamocortical system in seizure initiation remained unclear. Here, using electrocorticographic (ECoG) and intracellular recordings in vivo from cortex and thalamus in the Genetic Absence Epilepsy Rat from Strasbourg (GAERS), we obtained novel evidence for the cortical focus theory of absence epilepsy. We report that blockade of action potential discharge and synaptic activities in facial somatosensory cortical neurons, by topical application of tetrodotoxin, prevents the occurrence of paroxysmal activities in local and distant cortical neurons and ECoGs, as well as in thalamocortical neurons in register with the somatosensory cortex. In contrast, pharmacological inhibition of a remote motor cortical region or of the related thalamic nuclei did not suppress ictal activities in the somatosensory cortex. This study demonstrates that SWDs in GAERS have a focal origin within the facial somatosensory cortex, which is sufficient and necessary to generate ictal activities.