Intracellular prolactin in rat corpus luteum and adrenal cortex

This study was done to examine whether PRL, which appears in both active milk secretory cells (MSC) (1) and milk (1,2) during normal lactation, could also be detected in PRL target cells which do not transfer this hormone into an exocrine secretory product. Ovaries, adrenals, and mammary and pituitary glands were collected from actively nursing rats decapitated on day 15 post-partum. All four tissues were processed for light microscopic immunohistochemical identification of PRL. Immunoreactive PRL was again found in pituitary PRL cells and in MSC. It was also detected within both lutein and adrenal cortical cells (zona fasciculata). In all three target tissues, PRL was present in target cell cytoplasm, but, in MSC and adrenal cells, it was also found occasionally in nuclei. These findings, which seem to indicate that transfer into an exocrine secretory product cannot be the only possible explanation for the appearance of this protein hormone inside its target cells, extend the evidence suggesting that PRL may have intracellular sites of action.