Prolactin Stimulates Emission of Nipple Pheromone in Ovariectomized New Zealand White Rabbits1

Using ovariectomized New Zealand white rabbits and the nipple-search behavior of pups as a bioassay, we evaluated the capacity of prolactin to stimulate the emission of an odor signal that allows newborn pups to locate the mother's nipples and suckle. Whereas emission of this so-called nipple pheromone was minimal in untreated, ovariectomized does, emission was stimulated by the administration of progesterone (1 mg/day) to estrogen-treated does (0.5 microgram estradiol benzoate/day) and decreased after withdrawal of progesterone despite continuous administration of estrogen. However, substituting ovine prolactin (1.5 mg/day) for progesterone, under continuous estrogen administration, resulted in an increase in pheromone emission to near-maximal levels. This stimulatory effect of prolactin was not progesterone-dependent since it also occurred in does pretreated with estrogen alone and, to a lesser extent, even in females without estrogen priming. From these and previous data, we propose that emission of nipple pheromone in the rabbit is induced during pregnancy by the combined action of estrogen and progesterone, and maintained during lactation by the dual action of estrogen and prolactin.