Ultrastructural localization of prolactin in the rat anterior pituitary glands by preembedding peroxidase-labeled antibody method: observations in normal, castrated, or estrogen-stimulated specimen.

Ultrastructural localization of prolactin (PRL) was studied immunocytochemically (preembedding peroxidase-labeled antibody method) in a variety of pituitaries, including those from 1) normal, 2) castrated, and 3) castrated and estrogen-stimulated rats. In the normal rat, PRL was observed in cisternae of the rough endoplasmic reticulum (RER), perinuclear spaces, Golgi saccules, and secretory granules. In the castrated rats, PRL cells were rather atrophic and were filled with many small PRL-positive secretory granules. RER and Golgi saccules were rather inconspicuous and were almost devoid of PRL localization. The serum PRL level was markedly lowered. With estrogen stimulation after castration, the serum PRL level was markedly elevated and PRL cells showed a pronounced increase of PRL filled cisternae in the RER. From these observations, the role of secretory granules, Golgi apparatus, and RER in hormonal secretion was defined, and it was postulated that some peptide hormones would be secreted along two alternative pathways, i.e., either 1) a long (regulated) pathway or 2) a short (accelerated) pathway, in accordance with their secretory activities, which could be altered by various stimulations such as the use of estrogen.