Date of Award

Spring 1986

Degree Type

Thesis - Restricted

Degree Name

Master of Science (MS)




Early pregnancy in nocturnal rodents is maintained by pituitary prolactin secreted in two daily peaks: a nocturnal surge and a diurnal surge. Pseudopregnancy induced by artificial stimulation of the cervix has been used here as a model to define the timing necessities of these two surges. High doses of CB-154 (bromocryptine) were used to block both endogenous prolactin surges. Daily replacement injections of prolactin at various points of the light,dark cycle were made in an attempt to mimic each of the hormonal surges of pseudopregnancy. Prolactin, in doses of 1 ug/g body weight at the time of the diurnal surge did not maintain pseudopregnancy. In contrast, prolactin at times corresponding to the nocturnal surge did maintain pseudopregnancy normally, as evidenced by l lack of vaginal cyclicity. Positive decidual responses and elevated levels of progesterone were also observed in rats receiving nocturnal injections. These results prove that the nocturnal surge is essential, and also sufficient alone to maintain pseudopregnancy. This suggests that the relavant tar get tissue( s ) undergo a circadian variation in prolactin sensitivity that then defines the phase during which prolactin is most effective.