Faculty Bibliography

PURPOSE/OBJECTIVE:The neocortex can generate various forms of epileptiform activity, including one that depends on N-methyl-D-aspartate (NMDA)-type glutamate receptors (NMDARs), and another dependent on non-NMDA-type (AMPA) glutamate receptors (AMPARs). Previous work in vitro suggests that both forms of activity are initiated by neurons of layer 5, but the spatial patterns of horizontal propagation have been studied only for the AMPAR form. We have tested the hypothesis that both types of epileptiform activity spread via common pathways in one cortical layer, suggesting that lamina-specific intervention might selectively interrupt both. METHODS:Slices of rat somatosensory cortex were maintained in vitro and treated with the gamma-aminobutyric acid type A (GABA(A))-receptor antagonist picrotoxin. Single all-or-none epileptiform discharges were evoked with an electrical stimulus, and extracellular microelectrodes were used to track the vertical and lateral spread of the discharges. RESULTS:In both high and low concentrations of picrotoxin, the non-NMDAR antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) completely blocked propagation, whereas the NMDAR antagonist D-2-amino-5-phosphonovaleric acid (DAPV) only shortened the duration of discharges. When extracellular [Mg2+] was reduced in the presence of picrotoxin and CNQX, NMDAR-dependent epileptiform discharges could be initiated. NMDAR-dependent discharges spread at about one fifth the conduction velocity of AMPAR-dependent events. Analysis of spatiotemporal field-potential patterns suggested that both NMDAR- and AMPAR-mediated propagation involved early activity in layers 5 and 6, followed by larger-amplitude activity in upper cortical layers along the path of propagation. CONCLUSIONS:Our results imply that a common pathway mediates the propagation of these two forms of epileptiform activity, and suggests that lamina-specific surgical intervention might maximize anticonvulsant effect while minimally disrupting cortical function.

PURPOSE/OBJECTIVE:Epileptiform discharges that resemble interictal spikes can be generated by slices of neocortex treated with antagonists of gamma-aminobutyric acid A (GABA(A)) receptors. These discharges can propagate horizontally for long distances. We tested the hypothesis that propagation occurs through preferred horizontal pathways that lie in a particular cortical layer. METHODS:Slices were prepared from the primary somatosensory cortex of rats, maintained in vitro, and bathed with the GABA(A) receptor antagonist picrotoxin. Electrical stimuli were used to evoke single all-or-none paroxysmal field potentials (PFP) that were recorded with pairs or arrays of field potential electrodes. RESULTS:To test which laminae are necessary for propagation, vertical cuts were made to force the PFP to spread horizontally through particular layers. If slices were bathed in a high dose of picrotoxin (35 microM), a bridge of cortex 350 microm thick placed at any lamina was sufficient to support PFP propagation. However, in low picrotoxin doses (2.5 microM), similarly sized bridges had to include tissue from layers 4/5 or 5/6 to support propagation. When slices were cut horizontally (i.e., parallel to the pia) in strips. either upper-, middle-, or lower-layer strips were sufficient to support PFP propagation if the picrotoxin concentration was high; however, in low picrotoxin doses, only horizontal strips that included layer 5 could support propagation. Finally, in intact picrotoxin-treated slices, focal applications of GABA were systematically applied to different laminae as the PFP propagated past; GABA was most effective at blocking or delaying propagation when it was applied to layer 5b. CONCLUSIONS:We conclude that epileptiform propagation can occur through a variety of horizontal pathways when cortical inhibition is strongly impaired. However, when inhibition is reduced only moderately, axonal pathways in layer 5 are critical for seizure spread.

1. Medial temporal lobe sclerosis is a common pathological finding in patients with medically intractable temporal lobe epilepsy. This disease is characterized by extensive cell loss in the hilus and the hippocampal CA1 and CA3 cell fields in addition to synaptic reorganization throughout the dentate gyrus. 2. The dentate granule cells from hippocampal slices of patients diagnosed with medial temporal lobe sclerosis exhibit reduced synaptic inhibition with concommitant hyperexcitability. These physiological changes were studied relative to the hippocampi of patients with temporal lobe tumors in which the cell loss and synaptic reorganization are not seen. 3. We attempted to determine if this disinhibition was because of changes in the postsynaptic sensitivity to the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) by studying the responses to exogenously applied transmitter. As in rodents, the GABA responses in human dentate granule cells studied at the resting membrane potential were depolarizing and were mediated primarily by GABAA receptors. In many cases, these depolarizing GABA responses could trigger action potentials. Thus in some situations, GABA could act as an excitatory neurotransmitter. 4. We found that GABAA receptor-mediated responses in the sclerotic hippocampi were approximately 80% longer than in the comparison population. This difference was not because of changes in either the GABA reversal potential or the GABA-induced conductance change. The data support the hypothesis that the GABA transport system is impaired in sclerotic tissue: application of the GABA uptake inhibitor NNC711 (a tiagibine derivative) greatly prolonged the GABA responses in the tumor-related temporal lobe epilepsy tissue, but had little effect on the sclerotic tissue.