Abstracts

RATIONALE: Malformations of cortical development are a heterogenous group of disorders often found in patients with drug-resistant epilepsy. Electrophysiological studies of human dysplastic cortex and rodent models revealed intrinsic epileptogenicity in the malformation itself as well as in its vicinity and widespread alterations in distribution and function of inhibitory and excitatory neurotransmitters. Little is known about the exact site of initiation of epileptic activity and about its temporal and spatial propagation. We addressed this questions in an animal model of cortical dysplasias in rats.
METHODS: [italic]Lesion induction[/italic]: At day of birth (P0) a copper probe cooled with liquid nitrogen was placed on the exposed skull resulting in a longitudinal deep microsulcus in adult rats. Sham-operated animals were treated similarly with an uncooled copper probe.
[italic]Optical imaging[/italic]: 500[mu]m thick coronal brain slices were incubated with the voltage sensitive dye RH 795 for at least 1 h before recordings. Field potentials were recorded with standard glass electrodes filled with KCl. Voltage related fluorescence changes were recorded with a 464-element photodiode array (fields of view: 3.50mm or 1.75mm diam, 800 frames/s) and calculated as fluorescence change relative to resting light intensity (dI/I) using NeuroPlex software.
[italic]Induction of epileptiform activity[/italic]: Epileptiform activity was induced by immersion in Mg-free bath solution. To trigger epileptiform events short electrical stimuli were applied to the junction of deep layer VI and white matter in remote brain regions.
[italic]Histology[/italic]: After recording the slices were postfixed, cut, and processed for either cresyl-violet staining or immunohistochemistry using NeuN as a neuronal marker.
RESULTS: All freeze-lesioned animals (n=12) showed the formation of a typical deep microgyrus which consisted of cells originally committed to superficial cortical layers and a loss of deep cortical layers. Mostly the dysplasia was located in the hindlimb representation cortex HL or in the frontal motor cortex Fr. After 20-40 min of immersion in Mg-free bath solution epileptiform activity emerged in slices from freeze-lesioned as well as sham-operated animals. In [italic]sham-operated controls[/italic] epileptiform activity was initiated in different cortical areas which remained stable initiation sites even with electrical stimulation in remote brain regions. Spontaneous and evoked epileptiform activity arouse predominantly from superficial cortical layers in the medial somatosensory cortex Par1 and in HL. Spread of epileptic activity to adjacent areas was concentrated on superficial cortical layers. In slices with [italic]freeze-lesions[/italic] spontaneous epileptic activity always initiated in the dysplastic cortex. This was also the case for evoked activity which only rarely originated in Par1 (as in controls). Similar to controls the epileptic activity predominantly spread through superficial cortical layers to adjacent areas.
CONCLUSIONS: This study demonstrates that the initiation site of epileptic activity in an animal model of cortical dysplasia lies within the dysplastic cortex itself and shows spread of this activity into the surrounding cortical areas predominantly through superficial cortical laminae.
[Supported by: grants from HHU and DFG (SFB 194, SP108/16-2).]