The Effects of Various Methods of Cervical Stimulation on Continuation of Prolactin Surges in Rats

Abstract

In rats, the induction of pregnancy or pseudopregnancy is dependent upon the number of penile intromissions as well as the interintromission interval. The mating-induced stimulation of the uterine cervix is “stored” within the brain and repetitively expressed as two daily surges of prolactin for at least 10 days. It has been reported that the prolactin surges persist for only 6 days if the uterine cervices of ovariectomized (OVX) rats are stimulated artificially (CS). The present study demonstrates the effects of different methods of CS on prolongation of nocturnal (N) and diurnal (D) surges of prolactin secretion through Day 10 in OVX rats. Female rats were permitted to mate with males from proestrus through estrus, or received a mechanical (glass rod) or electrical stimulus of the uterine cervix at 1700 hr proestrus. All animals were OVX on the morning of estrus and blood was sampled by decapitation 10 days later. Both N and D surges of prolactin were present on Day 10 in mated rats or rats receiving a mechanical CS on proestrus. However, rats stimulated electrically on proestrus had neither N nor D surges of prolactin on Day 10. On the other hand, a second electrical stimulus on the morning of estrus prior to OVX induced prolactin surges which persisted through Day 10. Similar results were obtained in rats whose uterine cervices were artificially stimulated 14 days after OVX. However, N surges, when significantly elevated, were smaller than those of acutely OVX rats and D surges were present in only a small proportion of the animals sampled on Day 10. These results suggest that the stimulus provided by penile intromission can account for the induction and persistence of daily prolactin surges independently of ovarian support and that this physiological stimulus can be simulated by electrical or mechanical stimulation of the uterine cervix at appropriate intervals.