Demonstration of nerve fibers containing the C-terminal flanking peptide of neuropeptide Y (CPON) in the pig pineal gland (Sus domesticus): an immunocytochemical study by light and electron microscopy.

Abstract

The pineal gland is richly innervated by the neuropeptide Y (NPY)-positive nerves that mostly exibit immunoreactivity for the enzyme tyrosine hydroxylase, a marker of sympathetic nerve fibers. NPY is synthesized as a part of larger prepromolecule. The present study was undertaken to demonstrate that the posttranslational processing of preproNPY resulted in the presence of C-terminal flanking peptide of NPY (CPON) in nerve fibers of the pig pineal gland. An immunohistochemical anti-CPON technique was done over mounted sections of perfusion-paraformaldehyde-fixed material. An immunocytochemical preembedding technique was done to study the CPON-positive nerve terminals under electron microscopy. The pig pineal gland is densely innervated by CPON-immunoreactive nerve fibers. These nerve fibers follow from the pineal capsule into the connective tissue septa and farther into the pineal parenchyma, where the varicose branches terminate between the pinealocytes. The fiber density was the highest in the peripheral and ventral parts of the gland. At the ultrastructural level, the CPON-immunoreactive nerve terminals were found in the perivascular spaces and in the parenchyma. The terminals contained small vesicles (30-40 nm in diameter), some of which showed an eccentrically located dense core. In addition, large clear vesicles (80-100 nm in diameter) were present. Some of the CPON-immunoreactive nerve terminals were found in close apposition to the pinealocyte cell membrane, making a synapticlike contact with the pinealocytes. Our results show the presence of dense CPON-IR innervation in the pig pineal gland. The ultrastructural localization of CPON-IR nerve terminals shows that the peptide can be released to both perivascular and intercellular spaces. The functional role for this peptide in pig pineal gland is still an open question.