Abstracts

Rationale: A growing body of experimental evidence indicates that generalised absence seizures arise from a cortical focus in the somatosensory neocortex. Studies using the Genetic Absence Epilepsy Rats from Strasbourg (GAERS), have demonstrated that seizures arise from a physiological cortico-thalamic 5-9 Hz somatosensory rhythm. However the intra-cortical localisation of the seizure generator is still unknown. This study aimed to localise and characterise the site of seizure initiation within the somatosensory cortex at network and cellular level, and to investigate how seizure discharges are initiated and propagated from this focal site. Methods: EEG and microEEG recordings were performed in freely moving GAERS (n=12) and those under neurolept-anaesthesia (n=8). Extracellular field recordings and single-cell juxtacellular recordings of cortical region/ neurons were performed in vivo along with EEG recording of the sensorimotor cortex in GAERS (n=19) and non-epileptic control (NEC) rats (n=11). The location of the recorded regions/cells was confirmed by extracellular iontophoresis/juxtacellular labelling with Neurobiotin post-experiments. A commonly used anti-epileptic drug carbamazepine (CBZ) was focally injected into specific brain regions to investigate regional differences in its effect on seizure activity. Results: In GAERS with bilateral depth electrodes in the somatosensory and motor corticies (M1), seizures were consistently initiated with 5-9 Hz oscillations within the S2 region preceding that of S1 by up to 3 seconds before the recruitment of other regions. Microinjection of CBZ into the S2 suppressed seizures (n=4), with no obvious effect in the S1 (n=4) or the M1 (n=3) regions. MicroEEG recordings revealed high frequency oscillations (300-800 Hz) occurring with each spike-and-wave (SW) complex in the S2 region before that of S1 (median, S2=-62ms vs S1=-33ms, p<0.01). Juxtacellular recordings showed that cells from the S2 region fired earlier with each SW complex (median, S2=-39 vs. S1=-22ms, p<0.01). Layer V and VI cells fired earlier than the superficial layer cells (VI=-35, V=-29, II-IV=-21 ms, p<0.01). The ictal firing probability was less for the superficial layer cells than for Layer V or VI cells (II-IV=37%, V=85%, VI=90%, p=0.02). A population of cells were identified within both layers V and VI, focused around S2, which continued firing in a rhythmic manner as well as increased burst firing during interictal periods. Furthermore, cells within layer VI in GAERS showed increased firing rate and % burst firing compared to those in NECs. Conclusions: These studies identify the deep cortical layers, most likely layer VI, in the S2 as the region where the focal initiator cells for absence seizure in GAERS reside. A population of apparently inherently rhythmic cells were identified in the deep cortical layers focused around the S2 region in both GAERS and NEC rats which may be the source of the physiological somatosensory 5-9 Hz rhythm which subsequently triggers seizures in the epileptic animals.