Light and electron microscopic immunocytochemical study on the innervationof the pineal gland of the tree shrew (Tupaia glis), with special reference to peptidergic synaptic junctions with pinealocytes

Conventional and immunocytochemical, light- and electron-microscopic studie s on the innervation of the pineal gland of the tree shrew (Tupaia glis) we re made. Neuropeptide Y (NPY)-immunoreactive fibers, which were abundantly distributed in the gland, disappeared almost completely after superior cerv ical ganglionectomy, suggesting that these fibers are mostly postganglionic sympathetic fibers. By contrast, tyrosine hydroxylase (TH)-immunoreactive fibers, which were less numerous than NPY-fibers, remained in considerable numbers in ganglionectomized animals, indicating the innervation of TH-posi tive fibers from extrasympathetic sources. Bundles of substance P (SP)- or calcitonin gene-related peptide (CGRP)-immunoreactive fibers, entering the gland at its distal end, were left intact after ganglionectomy. SP-fibers w ere numerous, but CGRP-fibers were scarce in the gland. SP-immunoreactive f ibers were myelinated and nonmyelinated, and were regarded as peripheral fi bers because of the presence of a Schwann cell sheath. NPY- and SP-immunore active fibers and endings were mainly localized in the pineal parenchyma. N PY-immunoreactive endings synapsed frequently, and SP-positive ones did les s frequently, with the cell bodies of pinealocytes. The results suggest tha t NPY and SP directly control the activity of pinealocytes. Sections staine d for myelin showed that thick and less thick bundles of myelinated fibers entered the gland by way of the habenular and posterior commissures, respec tively. Under the electron microscope, the bundles were found to contain al so unmyelinated fibers. A considerable number of nerve endings synapsing wi th the cell bodies of pinealocytes remained in ganglionectomized animals; t hese endings were not immunoreactive for TH or SP. Such synaptic endings ma y be the terminals of commissural fibers. (C) 1999 Published by Elsevier Sc ience B.V. All rights reserved.