A Tale of Two Rhythms: The Emerging Roles of Oxytocin in Rhythmic Prolactin Release
Richard Bertram, Cleyde V. Helena, Arturo Gonzalez-Iglesias, Joel Tabak, Marc E. Freeman
Hormone secretion often occurs in a pulsatile manner. In this article we discuss two rhythms of prolactin release in female rats and the ongoing research that we and others have performed to understand the mechanisms underlying them. The peptide hormone oxytocin appears to play an important role in both rhythms. One rhythm occurs during the first half of pregnancy, but can also be induced in ovariectomized rats. This is characterized by a circadian pattern with two prolactin surges per day. Two methods for triggering this rhythm are discussed, each utilizing a unique physiological pathway that includes oxytocin action on pituitary lactotrophs. The second rhythm occurs during the estrous cycle and is characterized by a surge of prolactin on the afternoon of proestrus. We discuss recent findings that oxytocin is more effective at stimulating prolactin release from lactotrophs taken from animals on the afternoon of proestrus than from those of animals on the morning of diestrus 1, raising the possibility that this hormone plays a physiological role in the regulation of prolactin secretion during the estrous cycle.