Abstracts

Brief interictal discharges precede the onset of seizures during epileptogenesis due to high-frequency activation and consequent long-term potentiation of the strength of recurrent excitatory synapses. Partial blockade of NMDA receptors during high-frequency activation of glutamatergic synapses produces long-term decreases in synaptic strength. NMDA antagonists applied during synchronous network activation reduce the strength of recurrent excitatory synapses, and decrease the probability of future synchronous network activation, including seizures.
We used the CA3 region of the hippocampus as a model of network synchronization to test the effects of NMDA antagonists on burst propagation. Whole-cell and extracellular recordings of spontaneous bursts were analyzed to compare the effects of SDZ-220-581, an orally active competitive NMDA antagonist (20-80 mM), to D-APV (2.5-10 mM) which we used in previous studies. In CA3 mini-slices, surface expression assays were performed to determine the amount of NMDA receptors located in the intracellular pool compared to the cellular membrane.
In extracellular recordings, sequentially decreasing application of SDZ significantly increased the interburst interval by 139% compared to control (n=5), and significantly decreased burst duration by 24%. We were not able to wash SDZ due to its lipid solubility. Similarly, sequentially decreasing application of D-APV significantly increased the interburst interval by 101% (after wash) compared to control and significantly decreased burst duration by 17% (n=15, P[lt]0.05). In CA3 mini-slices, long-term potentiation caused a significant decrease number of NR1 and NR2A subunits in the intracellular pool (20%, n=5). This was reversed following long-term depression produced by application of D-APV. Finally, we used whole-cell recordings to evaluate changes in after-hyperpolarization (AHP) amplitude that is modulated by Ca2+-dependent K+ channels. AHP amplitude decreased in response to D-APV, indicating a reduction of calcium influx into CA3 pyramidal cells due to a decreased number of available NMDA receptors.
1. Decreasing doses of D-APV significantly increased interburst intervals that persisted for [gt]30-60 min after removal of the drug.
2. A persistant reduction of interburst interval occurred along with a 20% increase in intracellular pool of NMDA receptors, following application and wash of D-APV. This may be due to the decrease of available extracellular NMDA receptors on CA3 pyramidal cells.
3. These data suggest that transient application of competitive NMDA antagonist decreases burst probability of CA3 network activity and that the persistence of this effect is due to metaplasticity induced by endocytosis of membrane-bound NMDA receptors. This strategy may serve as a novel treatment for epilepsy.
[Supported by: NIH.]