Recurrent mossy fiber sprouting in the hippocampal dentate gyrus is a unique feature of temporal lobe epilepsy. The formation of new mossy fiber-granule cell synapses creates complex circuitry that is presumably capable of supporting reverberating excitation and thereby diminishing the normally high resistance of the dentate gyrus to seizure propagation. The overall objective of this project is to characterize properties of mossy fiber-granule cell synapses that regulate circuit function. Pilocarpine-treated rats, which become epileptic and also develop a consistently dense recurrent mossy fiber pathway, will be used to investigate three unusual features of recurrent mossy fiber circuitry about which little information is currently available: namely, hilar ectopic granule cells, expression of GABA and neuropeptide Y, and release of zinc. Pilocarpine- induced status epilepticus increases the production of new dentate granule cells, some of which are found in the hilus. Hilar ectopic granule cells possess several unique features that suggest they play a key role in triggering synchronous granule cell activity. These features include reciprocal connections with other granule cells, abnormal excitatory innervation, apparent paucity of inhibitory innervation, and spontaneous bursting in association with CA3 pyramidal cells. Whole cell patch clamp recording and electron microscopy will be used to characterize the innervation of these cells and test the hypothesis that differences between the innervation of hilar ectopic and normally-situated granule cells accounts in large part for their differences in excitability. GABA and NPY are inhibitory transmitters that are strongly expressed in mossy fibers after a seizure; NPY continues to be strongly expressed during the interictal period. Studies will test the hypothesis that these transmitters are released at mossy fiber-granule cell synapses and serve mainly to reduce the further release of glutamate. In this way, they are proposed to limit the ability of the recurrent mossy fiber pathway to enhance seizure propagation. Release of zinc from recurrent mossy fiber boutons has been proposed to increase granule cell excitability. However, recent data suggest that it may instead act as a brake on reverberating excitation by opposing the activation of postsynaptic NMDA receptors. To investigate this issue, the effects of zinc chelators will be tested on granule cell epileptiform activity that depends, at least in part, on NMDA receptor activation.
