Mammalian reproduction is dependent on both a successful pregnancy and on the subsequent period of lactation. In the rat, ovulation occurs shortly after parturition making it possible for a dam to be simultaneously pregnant and lactating. The present studies investigate the effect of
placental hormones on suckling-induced
prolactin (PRL) release and the contribution of
placental hormones to milk synthesis and secretion. A rat
choriocarcinoma cell line, Rcho-1, which secretes placental lactogens (PLs) following
transplantation in vivo, attenuated suckling-induced PRL release on both d 9 and d 14 of lactation by 43 and 58%, respectively. When PRL secretion was completely inhibited by
bromocriptine, a
dopamine agonist, Rcho-1-bearing dams still maintained a normal litters
weight gain, demonstrating that placental lactogens can continue an established lactation. The Rcho-1
tumors also initiated milk synthesis and secretion in nulliparous rats continuously exposed to pups. Whereas none of the 11 control virgins began lactating and had an average pup
weight loss of 2.07 g, the Rcho-1-bearing rats began lactating, as evidenced by a significant reduction in pup
weight loss. Thirty percent of these rats became fully lactationally competent. Northern blot analysis showed that the Rcho-1
tumors expressed both
PL-I and
PL-II mRNA in all experimental groups. These
tumors also secreted
PL-I into the circulation, as shown by radioimmunoassay.