Electrical stimulation of the epileptic focus in absence epileptic WAG/Rij rats: assessment of local and network excitability

OBJECTIVE The study aims to investigate whether there is a higher excitability in the deep cortical layers of the peri-oral region of the somatosensory cortex as compared to other cortical regions in absence epileptic WAG/Rij rats and whether this is unique for this type of epileptic rats, as would be predicted by the cortical focus theory of absence epilepsy. METHODS Excitability of cortical structures was assessed in a double pulse paradigm (inter-pulse interval 400 ms, 400 μs pulse duration, varying stimulation intensities (20-100 μA)). Electrical stimulation was applied to the subgranular layers of the somatosensory and motor cortex of freely moving WAG/Rij and control Wistar rats. Electrical evoked potentials (EEPs) and afterdischarges (ADs) were recorded during wakefulness, drowsiness and non-REM sleep. RESULTS WAG/Rij rats, stimulated in the somatosensory cortex, showed higher amplitudes for the N1 and N3 components of the EEPs as compared to WAG/Rij rats stimulated in the motor cortex. This effect was present in all states of alertness and at all tested intensities. In addition, this effect was not (N1) or to much less extent (N3) present in nonepileptic control rats. Stimulation-induced 8 Hz ADs were predominantly found in WAG/Rij rats. ADs were longer after stimulation in the somatosensory than in the motor cortex and preferentially occurred during drowsiness. CONCLUSION There is a heightened excitability in the deep layer neurons of the perioral region of somatosensory cortex, which is unique for WAG/Rij rats. Moreover, the presence of 8 Hz ADs might point toward additional changes in the cortico-thalamo-cortical network. Drowsiness is an excellent state for 8 Hz ADs, mimicking spike and wave discharges (SWDs). The results are in good agreement with the cortical-focus theory of absence epilepsy.

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