The Temporal Relationship between Implantation, Termination of Prolactin Surges, and Increased Testosterone Secretion in the Pregnant Mouse*

PRL and testosterone (T) levels were measured in mice mated during the postpartum estrus and permitted to nurse 10 pups. Various delays in implantation were induced by separating females from their pups either on day 0 (the day of vaginal plug detection) or on day 4, 6, or 8, which resulted in implantation on day 4, 6, 8, or 10, respectively. Trunk blood was collected after decapitation during the times diurnal PRL surges were expected to occur (3 days after implantation) or to have disappeared (5 days after implantation). When implantation occurred on day 4 (pups removed on day 0 of lactational pregnancy), the diurnal PRL surge was present on day 7 but absent on day 9 as predicted. When implantation was delayed, diurnal PRL surges were expressed on the third day after implantation but, without exception, had terminated by the fifth day after nidation. Regardless of the day of implantation (day 4, 6,8, or 10 after mating), increased T secretion was always associated with the cessation of the diurnal PRL surge. A second experiment examined the effects of Silastic implants of T on PRL surges. Elevated blood T levels did not disrupt the biphasic pattern of PRL release unless a pharmacological dose of T was given. Thus, the simultaneous occurrence of increased T production and the disappearance of PRL surges is a reflection of the stage of placental-fetal maturation rather than a direct effect of T on PRL secretion. Association of elevated blood T levels with the degree of placental-fetal development suggests placental regulation of androgen production at midgestation. The disappearance of PRL surges on tjie same day T secretion increases supports the idea that product(s) of the conceptus simultaneously control pituitary and ovarian function during the period that pituitary stimulation of luteal activity is relinquished to the placenta.