Light and electron microscopic studies on the influence of stress on prolactin-immunoreactivity in rat anterior pituitary lobe.

An increasing number of evidence suggests an important role of prolactin in the modulation of stress response. However, the mechanisms of its action on the HPA axis are not yet understood. Glucocorticoids, liberated from adrenal cortex due to hormonal signals from pituitary corticotrophs are known to play a key role in systemic stress response. Previously we found evidence that corticosteroid-binding globulin (CBG) is involved in rapid, membrane-mediated actions of adrenal steroids. Here we studied qualitatively immunostainings for prolactin and CBG in pituitaries of male rats that had been subjected to osmotic challenge. We also examined late pregnant, parturient and early lactating rats, assuming that parturition represents a strong physiological stress. We employed double immunofluorescencent staining of semithin sections and immunoelectron microscopy. In stressed males we found increased prolactin immunofluorescence associated with membranes while in controls this staining was predominantly cytoplasmatic. CBG immunofluorescence was found in almost all prolactin cells of stressed males while such double staining was only occasionally observed in controls. Similar observations were made in females: While parturient rats showed intense membrane associated double staining for both antigens, late pregnant and early lactating animals showed patterns similar to that of male controls. Immunoelectron microscopy revealed increased exocytosis of prolactin containing vesicles in lactating rats. CBG was localized on cell membranes and additionally within prolactin vesicles. Our observations suggest prolactin liberation from pituitary lactotrophs along with CBG upon systemic stress response. Membrane effects of glucocorticoids mediated by CBG may be linked to stimulus secretion of prolactin.