Immunocytochemical characterization of Delta-opioid and Mu-opioid receptor protein in the bovine pineal gland

Abstract

Opioidergic innervation has been identified in the mammalian pineal gland. Recently, opioid receptors in bovine pineal glands have been characterized; the activation of these receptors leads to the stimulation of melatonin synthesis. In this study, the precise localization of opioid receptors in bovine pineal glands was determined by an immunohistochemical technique using antibodies raised against delta-opioid and mu-opioid receptors. Immunoreactivity of these two receptors was present at a moderate level in pinealocytes. A double-labeling study has shown that delta-opioid receptors are localized predominantly with mu-opioid receptors in the same pinealocytes. These immunopositive pinealocytes are often located in a group; however, some of them are dispersed individually. In addition, both types of receptors were found in glial cells and processes. A small number of delta-receptor-immunoreactive nerve fibers were observed in the perivascular space and intraparenchyma of the pineal gland. Mu-opioid receptor immunoreactivity was found in a number of nerve fibers throughout the gland, and in terminal-like dots on pinealocytes. There was immunocolocalization between delta-opioid receptors or mu-opioid receptors and leu-enkephalin in some nerve fibers. The results of this study indicate that the modulatory effect of the opioid system on melatonin secretion in pineal glands might act via opioid receptors on pinealocytes, whereas receptors located on nerve fibers might modulate the release of opioid peptides.