Daily prolactin pulse inhibits the corpus luteum during lactational quiescence in the marsupial, Macropus eugenii

Abstract

The corpus luteum (CL) of the tammar wallaby is inhibited by prolactin during lactation and seasonal quiescence. In seasonal quiescence a daily transient pulse of prolactin (PRL) of less than 2 h duration is sufficient to maintain inhibition. We investigated whether the same inhibition applies in lactation and, if so, how. Our results show that inhibition of the CL during lactation is maintained by a transient pulse of prolactin once a day. They also show that the minimum time without a PRL pulse for the CL to escape inhibition is more than 48 h and less than 72 h. Nevertheless, some animals had a longer refractory period than 72 h, which was reflected in a longer interval to the progesterone peak and birth. These results support the previous conclusion that PRL exercises its effect on a rate-limiting step in progesterone synthesis and secretion rate from the CL, which precedes any increase in its mass. Therefore, we conclude that the role of PRL is to act as a luteostatic agent, an effect that is in marked contrast to its luteotrophic effect in many eutherian species, including rodents.