Biosynthesis and metabolism of native and oxidized neuropeptide Y in the hippocampal mossy fiber system.

Neuropeptide Y (NPY) gene expression is known to be modulated in the mossy fiber projection of hippocampal granule cells following seizure. We investigated NPY biosynthesis and metabolism in an attempt to characterize NPY biochemically as a neurotransmitter in the granule cell mossy fiber projection. NPY biosynthesis was compared in normal control animals and in animals that had experienced a single pentylenetetrazole-induced seizure. In situ hybridization analysis established the postseizure time course of preproNPY mRNA expression in the hippocampal formation, localizing the majority of increased preproNPY mRNA content to the hilus of the dentate gyrus. Radioimmunoassay analysis of the CA3/mossy fiber terminal subfield confirmed a subsequent increase in NPY peptide content. Biosynthesis of NPY peptide by granule cells and transport to the CA3/mossy fiber subfield was demonstrated by in vivo radiolabel infusion to the dentate gyrus/hilus followed by sequential HPLC purification of identified radiolabeled peptide from the CA3/mossy fiber terminal subfield. Additional in vivo radiolabeling studies revealed a postseizure increase in an unidentified NPY-like immunoreactive (NPY-LI) species. HPLC/radioimmunoassay analyses of CA3 subfield tissue extracts comparing normal control animals and pentylenetetrazole-treated animals confirmed the increased total NPY-LI, and demonstrated that the increased NPY-LI was comprised of a minor increase in native NPY and a major increase in the unknown NPY-LI. Data from subsequent and separate analyses incorporating immunoprecipitation with anti-C-terminal flanking peptide of NPY, further HPLC purification, and matrix-assisted laser desorption/ionization mass spectrometry support the conclusion that the unknown NPY-LI is methionine sulfoxide NPY. NPY and NPY-sulfoxide displayed differential calcium sensitivity for release from mossy fiber synaptosomes. Similar to NPY, NPY sulfoxide displayed high-affinity binding to each of the cloned Y1, Y2, Y4, and Y5 receptor subtypes. Postrelease inactivation of NPY was demonstrated in a mossy fiber synaptosomal preparation. Thus, the present study in combination with previously reported electrophysiological activity of NPY in the CA3 subfield demonstrates that NPY fulfills the classical criteria for a neurotransmitter in the hippocampal granule cell mossy fiber projection, and reveals the presence of two molecular forms of NPY that display differential mechanisms of release while maintaining similar receptor potencies.