The imprint provided by cervical stimulation for the initiation and maintenance of daily prolactin surges: modulation by the uterus and ovaries.

This study describes the influence of ovarian steroids and a putative PRL release inhibitory substance of uterine origin on the imprint provided by cervical stimulation (CS) for the maintenance of nocturnal and diurnal surges of PRL secretion. Toward the end of the pseudopregnancy (psp) resulting from a sterile mating or artificial stimulation of the uterine cervix, ovarian estradiol secretion is enhanced, and luteal progesterone (P) secretion wanes. This shifting ratio terminates the surges of PRL by day 13 of psp. Ovariectomy on day 8 of psp and simulation of this pattern of steroid secretion with sc Silastic implants also resulted in termination of surges by day 13. However, ovariectomy on days 2, 4, or 6, followed immediately by implantation of the inhibitory regimen of ovarian steroids, did not result in termination of the noctural surge by days 7, 9, or 11, respectively. The diurnal surge was absent in all cases. A third and fourth CS applied on days 7 and 8 of psp were not able to overcome the effects of the inhibitory steroid regimen begun on day 8. PRL surges induced in ovariectomized rats by one 30-sec CS persist for 6 days (suboptimal stimulation), while surges induced by two 30-sec CS persist for at least 10 days (optimal). However, implantation o P or hysterectomy permits suboptimally stimulated ovariectomized rats to secrete nocturnal surges through day 10. These data indicate that optimal CS programs the PRL-releasing apparatus to secrete surges for a preset interval, during which time the system remains unresponsive to physiological inhibitory and excitatory input. However, suboptimal CS can be reinforced by imposition of P or removal of PRL inhibitory activity of uterine origin.