MICROANATOMY OF HUMAN EPILEPTIC HIPPOCAMPAL FORMATION

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Temporal lobe epilepsy (TLE) is the most frequent form of adult intractable seizures and is surgically treatable. This laboratory, studying the clinical pathology of human TLE in our NIH Program Project, has shown that: 1) Using quantified techniques, hippocampal damage, termed hippocampal sclerosis (HS), is the most frequent (65%) finding in standardized surgical specimens from patients with focal TLE. 2) HS comprises a specific amount and pattern of neuron loss that is relatively specific for epileptic hippocampi and is not duplicated in other diseases. 3) The areas of HS are associated with significant aberrant axon sprouting and neosynaptogenesis of the excitatory mossy fibers (MF) and of the inhibitory GABA system, especially in the fascia dentata (FD). 4) Despite the significant cell loss, these ares of HS and synaptic reorganizations are the regions of seizure onsets. These data have lead to the hypothesis that the reorganized axon circuits, especially in the FD, are mechanisms for hippocampal seizure onsets. These aberrant axon circuits are both feedback and feedforward and could explain how an area that is neuron poor can generate seizures. The presence of remodeled axon circuits does not indicate that there are concomitant changes in the postsynaptic receptors and their mRNAs in the new target neurons. In order for the FD to be hyperexcitable, there should be changes in the amount and type of excitatory and inhibitory receptors in the area of axon sprouting. In the FD, it has been assumed that the reorganized MFs in the innermost supragranular region are glutamatergic excitatory, and the sprouted GABA axons are inhibitory. Our research will use newly established molecular techniques of in situ hybridization (ISH) with oligonucleotide probes, in addition to already established immunocytochemical (ICC) and histochemical techniques, to test the following rejectable hypotheses: 1) In the epileptic supragranular layer of the FD, sprouted feedback glutamatergic MFs will relate to significant differential increases in excitatory receptor subtypes along with upregulation of their mRNAs. 2) In the same region of the FD, sprouting of the GABA terminals in the supragranular and granular layers will not significantly relate to increases in the inhibitory GABA receptor subtypes along with their mRNAs. 3) In granule cells, glutamate receptors will have genomic changes from FLOP to FLIP (longer depolarizations). Our methods will objectively measure and quantify the results with an image analysis computer. The results will be statistically compared to autopsies and hippocampi from surgical specimens with temporal tumors, which are the best available control comparisons for human experimentation. Just as importantly, as many ICC and ISH studies will be performed as possible on each human hippocampus to maximize comparisons between patients and biological variables, and compared to other subprojects of the program project. These studies will provide insight into the molecular pathophysiology of HS and suggest important pathogenic mechanisms of this form of intractable focal epilepsy.