Abstracts

Loss of activity following brain insult may contribute to subsequent expression of seizures. We showed previously that blockade of activity in hippocampal slice cultures with TTX or APV exacerbated spontaneous electrographic dentate granule cell seizures and increased supragranular Timm stain (Bausch and McNamara 2001). These data suggested a potential involvement of mossy fiber sprouting in seizure exacerbation. However, previous reports suggest that activity blockade also can alter intrinsic membrane properties, synapse formation and glutamate receptor function. All could contribute to seizure exacerbation, thus each possibility was investigated. Organotypic hippocampal slice cultures were treated with vehicle, APV or TTX for 17-21 days. Changes in function were examined using electrophysiology. Alterations in axonal morphology and synaptogenesis were documented using single cell fills with neurobiotin and synaptophysin immunohistochemistry, respectively. No changes were seen in intrinsic granule cell properties that could account for exacerbation of seizures in either APV- or TTX-treated cultures. In contrast, antagonists of glutamatergic transmission eliminated seizures in both APV- and TTX-treated cultures, implicating plasticity in excitatory glutamatergic circuits or inhibitory control of these circuits as potential mechanisms contributing to seizure exacerbation. Candidates include mossy fiber and non-mossy fiber pathways. For mossy fibers, single granule cell fills showed that neither TTX nor APV induced mossy fiber sprouting, despite increased supragranular Timm stain. Synaptophysin labeling also suggested that increased number of synapses do not account for seizure exacerbation. Granule cell dendrites received similar numbers of synaptophysin-immunoreactive (-IR) contacts in vehicle- and APV-treated cultures; synaptophysin-IR contacts were decreased in TTX-treated cultures. However, these data did not distinguish between excitatory and inhibitory synapses and did not document functional changes. To examine function, mEPSCs and mIPSCs were recorded in granule cells. Compared to vehicle, both APV- and TTX-treated cultures showed significant increases in mEPSC frequency, amplitude, area, rise time and decay time; increases were significantly greater in TTX- than APV-treated cultures. Data for mIPSCs are under analyses. These data suggest that the exacerbation of seizures in APV- and TTX- treated slice cultures is associated with an enhancement of excitatory synaptic transmission. (Supported by The American Epilepsy Society, NIH grant NS045964, and CDMRP grant PR030035.)