Rhythmic neuronal activity in S2 somatosensory and insular cortices contribute to the initiation of absence-related spike-and-wave discharges.
OBJECTIVE
The origin of bilateral synchronous spike-and-wave discharges (SWDs) that underlie absence seizures has been widely debated. Studies in genetic rodent models suggest that SWDs originate from a restricted region in the somatosensory cortex. The properties of this initiation site remain unknown. Our goal was to characterize the interictal, preictal and ictal neuronal activity in the primary and secondary cortical regions (S1, S2) and in the adjacent insular cortex (IC) in Genetic Absence Epilepsy Rats from Strasbourg (GAERS).


METHODS
We performed electroencephalography (EEG) recordings in combination with multisite local field potential (LFP) and single cell juxtacellular recordings, and cortical electrical stimulations, in freely moving rats and those under neurolept-anesthesia.


RESULTS
The onset of the SWDs was preceded by 5-9 Hz field potential oscillations, which were detected earlier in S2 and IC than in S1. Sustained SWDs could be triggered by a 2-s train of 7-Hz electrical stimuli at a lower current intensity in S2 than in S1. In S2 and IC, subsets of neurons displayed rhythmic firing (5-9 Hz) in between seizures. S2 and IC layers V and VI neurons fired during the same time window, whereas in S1 layer VI, neurons fired before layer V neurons. Just before the spike component of each SW complex, short-lasting high-frequency oscillations consistently occurred in IC ∼20 msec before S1.


CONCLUSIONS
Our findings demonstrate that the S2/IC cortical areas are a critical component of the macro-network that is responsible for the generation of absence-related SWDs.
Departments of Medicine, Surgery and Neurology, The Royal Melbourne Hospital, The University of Melbourne, Parkville, Victoria 3050, Australia.
Thomas W
TW
Zheng
Terence J
TJ
O'Brien
Margaret J
MJ
Morris
Christopher A
CA
Reid
Valentina
V
Jovanovska
Patrick
P
O'Brien
Leena
L
van Raay
Arun K
AK
Gandrathi
Didier
D
Pinault
D000200:Action Potentials / Q000502:physiology*; D000818:Animals; D002540:Cerebral Cortex / Q000502:physiology; D004569:Electroencephalography / Q000379:methods; D004832:Epilepsy, Absence / Q000503:physiopathology*; D008297:Male; D009474:Neurons / Q000502:physiology*; D010507:Periodicity; D051381:Rats; D017208:Rats, Wistar; D013003:Somatosensory Cortex / Q000502:physiology*