Abstracts

Children with intractable focal cortical epilepsies are monitored with subdural grid arrays that record focal seizure onsets from areas that must be surgically resected and other areas that have minimal epileptic spiking at the margin of the resection. In children, early onset epilepsies may be associated with severe and diverse types of congenital neuropathologies. In vivo single and paired pulse evoked cortical potentials were used to compare latencies and amplitudes to glutamate and GABA receptors in membrane isolates resected beneath recording electrodes.
Subjects were a part of the Children[apos]s Hospital epilepsy program, and all procedures were approved by the IRB. Each patient gave prior consent to the standard protocols of stimulation for averaged evoked potentials. The first stimulus (S1) was followed by a second stimulus (S2), at intervals of 1000 msec., 500 msec., and 200 msec. Each paired pulse sequence occured ten times. Ten seconds elapsed between intervals. This protocol was given in the area of seizure onset and the control comparison cortex. Each stimulus was a biphasic constant current pulse of 300 microseconds duration per phase, currents at 6.5 mA., capacity-coupled, and the charge transfer per phase was less than 6 microcoulombs per phase. Resected cortical samples were frozen, white matter and pial vessels were removed before homogenization. The membrane isolate procedure was standardized and run simultaneously for both epileptic and control cortex in each patient. Immunoblots were done using commercially available antibodies for AMPA subunits (GluR1, GluR2), NMDA subunits (NR1, NR2A/B), GAD, and GABA-A. All immunoblots were developed on film for quantitative densitometry. Actin was used as a standard for all immunblot procedures to test for consistency in protein loading.
Three cases with different pathologies had the same stimulus protocols and immunoblots as described in the methods. (DNET), had good glutamate responses and blots, but GABA responses were decreased (GABA blot - low). (polymicrogyria), had good glutamate responses and blots, but GABA responses were decreased (GABA blot - low). (Tuberous Sclerosis), had good glutamate reponses and blots, but GABA responses were decreased (GABA blot - low).
Single and paired pulse stimulation showed preoperative axonal connections for local areas in focal epileptic and in the remote cortical regions that had minimal spiking. Following resections, quantitative studies of cortical receptor populations contributing to cortical evoked potentials demonstrated support for the upregulation of NMDA receptors or presence of disinhibiton. These studies have provided a systematic comparison of response thresholds and components of cortical potentials with in vivo amino acid receptors in human cortex.
[Supported by: NIH Grant NS38150.]