Abstracts

RATIONALE: The purpose of this study is to electrophysiologically characterize cortical neurons in brain slices taken from pediatric patients with focal epilepsy. Our intracellular characterization aims at identifying membrane properties that contribute to epileptic seizures pharmacologically and electrophysiologically characterized prior to the resection.
METHODS: Human neocortical tissue was removed from patients with medically intractable epilepsy. The sites selected for the slice studies were determined by source localization techniques and clinical relevance. Upon resection, the tissue was placed into artificial CSF (aCSF : 118 NaCl, 3 KCl, 1.5 CaCl[sub]2[/sub], 1 MgCl[sub]2[/sub], 25 NaHCO[sub]3[/sub], 1 NaH[sub]2[/sub]PO[sub]4[/sub], and 30 D-glucose, pH of 7.4 ) bubbled with carbogen (95% oxygen and 5 % CO2). Slices (500 [mu]M) were sectioned perpendicular to the gyri of the seizure focus with 6-layers of the cortex identifiable. The slices were immediately transferred to aCSF bubbled with carbogen and stored at room temperature. Experiments were conducted in a recording chamber at 29[degree]C. After 30 minutes the K⁺ concentration was raised from 3 to 5 mM. Population recordings were obtained with suction electrodes positioned on the surface of the cortical layers. Intracellular whole cell patch-clamp recordings were obtained from cortical neurons with the blind patch technique. To identify the cell layer and the cell type cells were stained with biocytin.
RESULTS: Intra- and extracellular recordings were simultaneously obtained from cortical human slices. Slow oscillatory population activity was recorded in slices from several patients. In one of the slices obtained from the epileptic focus of a patient that was refractory to several anticonvulsants, recurrent epileptiform population activity was induced by application of NMDA. In the slice obtained from this patient, epileptiform activity was not blocked by any of the examined antiepileptic substances (Lamotrigine, Topiramate, Zonisamide). At the cellular level we identified several different types of spiking neurons: low threshold spiking, regular spiking and fast spiking, which had similar discharge properties as neurons previously described by Foehring et al. ([italic]J Neurophysiol[/italic] 1991; 66: 1825-37). Unlike Foehring and colleagues, we were able to identify an intrinsically bursting neuron type. Depolarizing current injection increased the frequency of bursting and bursting activity was abolished by hyperpolarizing current injections.
CONCLUSIONS: The present study begins to characterize the activity of epileptic foci both in the pediatric patient and subsequently under in vitro conditions in a slice preparation. This approach enables us to directly compare data obtained at the cellular and network level in acute slice preparations with those obtained from clinical EEG. These observations may provide the basis for future investigations of oscillatory network behavior and mechanism specific pharmacotherapy involved in epilepsy.
[Supported by: Falk Foundation (WvD, KEH, CJM)
PEW Fellowship (FP)
Rett Syndrome Research Foundation (JMR)
NIH HL 60120 (JMR)]