Immunohistochemical localization of the ACTH (MC-2) receptor in the rat placenta and adrenal gland

Abstract

The adrenocorticotropic hormone (ACTH) acts on adrenocortical cells and promotes steroidogenesis by specific binding to the ACTH (MC-2) receptor (ACTHR). To gain an insight into ACTH action on local steroidogenic organs, we examined the immunohistochemical expression of ACTHR in rat adrenal glands and placentas during the mid-late gestation period. Antibodies against synthetic ACTHR peptides were raised in rabbits, and Western blot analysis showed that the antibody reacted with specific proteins in the rat adrenal glands and placentas. The peroxidase-labeled antibody method revealed that ACTHR was distributed in the plasma membrane and cytoplasm of the parenchymal cells of the adrenocortical zona fasciculata. In the placenta, ACTHR was distributed in the junctional spongiotrophoblasts at day13 of gestation - with a gradual decrease in the staining during the gestational period, whereas ACTHR appeared in the placental labyrinthine cells from days 15 to 19 of gestation. Immunoelectron microscopy revealed that ACTHR was also localized in the ribosomes of the fasciculata cells and the labyrinthine cells. Our findings suggest that ACTHR may play a physiological role in steroidogenesis in the adrenal cortical parenchymal cells as well as in the trophoblasts of rat placentas during mid-late gestation.