Effects of met-enkephalin on the adrenocorticotropic cells of the rat pituitary

The effects of chronic administration of met-enkephalin (40 micrograms/d during 20 days) on ACTH producing cells of the rat adenohypophysis have been studied both in light (PAP-immunocytochemistry for ACTH) and electron microscope. In addition a morphometric analysis and a percent distribution of secretory granules were performed. The ACTH cells of treated animals showed ultrastructural signs of hyperactivity, and some of them vacuolization (small vacuoles or a large central vacuole). The secretory granules of the experimental animals are larger and more spherical than the ones in the untreated and the control animals. We discuss the possible mechanism of action whereby met-enkephalin influences ACTH cells.