Localization of immunoreactive prolactin in ependyma and circumventricular organs of rat brain

Immunoreactive prolactin (IMP) has been localized in the male rat brain using the soluble peroxidase-anti-peroxidase (PAP) technique. In normal untreated animals, reaction product was seen in choroid plexus (CP) and in ependymal cells of the ventricular lining with heaviest concentrations of positively staining cells in the 3rd ventricle near the subcommissural organ (SCO), in the lateral ventricles near the subfornical organ (SFO), and in the 4th ventricle near the area postrema (AP). IMP was also present in numerous ependymal cells resembling tanycytes in the cerebral aqueduct, central canal of the spinal cord at the level of the AP, the organum vasculosum of the lamina terminalis (OVLT) and the floor of the infundibular recess. Immunoreactive cells resembling neurons were localized within the substance of the AP, SCO, and OVLT. IMP was also present in fibers of the zona externa of the median eminence and infundibular stalk; a few cells of the pars tuberalis contained reaction product. Hypophysectomized rats and bromocriptine-treated rats exhibited a similar staining pattern except that bromocriptine treatment eliminated IMP from most CP cells. Hypophysectomy, bromocriptine or estrogen treatment enhanced staining for IMP in cells of the pars tuberalis; estrogen treatment or hypophysectomy produced an increase in the number and distribution of immunoreactive cells as well as increased density of reaction product in cells of the medial habenular nucleus. The functional relevance of prolactin in these locations in the brain, the possible routes of transport of prolactin from the pituitary gland to the central nervous system, and the strong suggestion of extra-pituitary sites of synthesis of a prolactin-like hormone are discussed.