Hippocampal epilepsy is characterized by synchronized hyperexcitable neurons. Many of the neurons in the hippocampus release the excitatory transmitter glutamate which contributes to the excitation. The general hypothesis that endogenous neuromodulators may influence levels of excitability in the hippocampus will be examined. Specifically, we will focus on neuropeptide Y (NPY) inhibition of excitatory neurons in the epileptic human hippocampus, and in a rat model of epilepsy. NPY is found throughout the hippocampus in both neurons and in presynaptic axons. It has been suggested as one of the brain's natural anti-seizure transmitters, and its expression and distribution appears to change with epilepsy. The presynaptic role of NPY in reducing glutamate actions will be studied with whole cell recordings in slices of the human hippocampus. Using a simplified model of hyperexcitability consisting of a single self-innervating rat hippocampal neuron, we will examine the effect of NPY and specific receptor agonists to test the hypothesis that NPY acts by a presynaptic mechanism via Y2 and Y5 receptors to reduce glutamate release in neurons showing epileptiform activity. In parallel we will directly test the hypothesis that NPY blocks glutamate release presynaptically by using the dye FM1-43 to study transmitter vesicle exocytosis in glutamatergic neurons. The hypothesis that NPY is found in GABAergic neurons will be tested with dual ultrastructural immunocytochemistry. The hypothesis that changes in neuronal activity mediated by glutamate will alter levels of expression of NPY and NPY receptor Y1-Y5 mRNA will be tested with cDNA-PCR and Northern blot analyses in parallel studies of different regions of the epileptic human and rat hippocampus and in a tissue culture model of hyperexcitable rat hippocampal neurons. NPY is potentially of great interest because its primary action in the normal hippocampus appears to be one of depressing hyperexcited neurons, without a substantial effect on normal neurotransmission.
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