Compelling evidence suggests that the hippocampus contains neurons critically involved in the genesis and propagation of the epileptic discharge in human temporal lobe epilepsy (TLE). The broad long-term goal of this research is to identify, and electrophysiologically and pharmacologically characterize hippocampal neurons that initiate the electrical seizure discharge in TLE. Studies in kindled rat and human epileptic dentate reveal that mossy fibers, the axons of dentate granule (DG) cells, reorganize by sprouting and projecting back into the DG cell and inner molecular layers of the dentate gyrus. Severity of sprouting varies, ranging from severe, as seen in dentate from patients with hippocampal onset seizures, to mild, as observed in those patients with neocortical onset seizures, or seizures associated with tumor. However, the physiologic significance of this reorganization remains unclear. One manifestation of reorganization may be an alteration in synaptic inhibition. Establishing the chemical and electrical nature of these altered synaptic connections, and discovering other related morphologic alterations will lead to a greater understanding of the mechanisms of epileptogenesis. Adenosine is a potent anticonvulsant in many seizure models, increases dramatically in human temporal lobes during seizures, and is critically important in inhibition and physiologic seizure termination. An abnormal response to adenosine may lead to increased excitation and seizure initiation. These experiments are designed to test the hypothesis that reorganized, or sprouted neurons in the epileptic human dentate gyrus are less responsive to adenosine than those neurons which have not undergone reorganization. Slices of human epileptic dentate gyrus, surgically removed from patients with medically intractable partial epilepsy, will be studied with whole-cell voltage clamp recording, and intracellular dye injection. Separate slices will be processed for cell density measurements and Timm stain density and distribution. DG and hilar cells will be morphologically analyzed following electrophysiological characterization before and during application of adenosine analogs. Axonal and dendritic branch patterns, projections, and spine density will be quantitated using a computer assisted 3-dimensional reconstruction technique.
Aim 1 will demonstrate differences in adenosine mediated K+ currents between adult and neonatal rats. These experiments are necessary mainly to gain skill using the patch clamp technique in acutely dissociated dentate cells, and to demonstrate that whole-cell slice recordings are similar to those obtained from cells.
Aim 2 serves to relate electrical properties of human DG and hilar neurons to cell morphology.
Aim 3 will correlate the effect of adenosine on DG cell extracellular responses to reorganization and cell loss, while Aim 4 will correlate the effects of adenosine on DG cell whole-cell recorded properties to cell morphology. These studies will expand our knowledge of morphologic changes in epilepsy, relating these changes to altered physiology. In addition, if reorganized DG cells respond differently to adenosine than """"""""normal"""""""" DG cells, drugs which target the adenosine system may be useful in epilepsy.
| Swanson, T H; Drazba, J A; Rivkees, S A (1995) Adenosine A1 receptors are located predominantly on axons in the rat hippocampal formation. J Comp Neurol 363:517-531 |
