Abstract
Pituitary prolactin (PRL) is a protein hormone with a broad range of physiological targets, including the ovary. The pituitary is not the only origin of PRL-like activity, though. Other hormones, including human decidual-derived PRL (1, 2) and rat decidual luteotropin (3), are also able to bind to the PRL receptor with similar affinities. In the mouse, rat, hamster, cow, sheep, and human, placental-derived hormones are produced that bind to the PRL receptor (4). These placental lactogens (PL) are the predominant hormones with PRL-like activity in the circulation during mid- to late pregnancy. Two PLs are synthesized in rodents, and they are designated as placental lactogen I (PL-I) and placental lactogen II (PL-II). High levels of PL-I accumulate transiently at midgestation (5), and the appearance of PL-I in the maternal serum coincides with a cessation of the early gestational surges of PRL release from the pituitary (6). PL-II synthesis is first detected at midpregnancy, and unlike PL-I, PL-II levels increase until parturition (7).
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