EFFECTS OF DIFFERENTIAL MATING STIMULATION ON THE ONSET OF PROLACTIN SURGES IN PSEUDOPREGNANT RATS

In the female rat, stimulation of the uterine cervix (CS) during matin g or by artificial means induces daily diurnal and nocturnal surges in prolactin (PRL) secretion which, in the absence of fertilization, res ult in an 12-day anestrous period called pseudopregnancy (PSP). The am ount or type of mating stimulation received by the female during matin g determines whether or not PSP occurs, but it has not yet been determ ined whether different amounts of mating stimulation can alter the tim e of onset of PRL surges. The present studies examined the latency in days to the first nocturnal PRL surge following mating. Plasma PRL was measured in samples obtained via intra-atrial catheters at 0200-0500h on the day of estrus (Day 0) and on the subsequent 3 days (Days 1-3). In Experiment 1, proestrus females received mating stimulation which was more than (15 intromissions, 15I) or less than (mounts-without-int romission only, MO) sufficient to induce PSP. Surges were absent in 15 I females until Days 1-2 and in MO females on ah days. In Experiment 2 , females received five intromissions (5I) in paced and nonpaced matin g tests, types of mating treatments which were expected to induce PSP in some but not all females. PRL surges were not evident at any sampli ng time in females that continued to cycle, while PRL surges occurred consistently on Day 2 among PSP females. Among PSP females, those rece iving 5I showed significantly higher PRL on Day 0 than did 15I females . In both experiments, plasma progesterone concentrations were not hig her in PSP than in non-PSP animals until Day 3. In Experiment 3, PRL l evels in single samples obtained by cardiac puncture on Day 0 were sim ilar to those seen in the first two experiments. Thus, PRL secretion a t the time of the first postmating nocturnal surge is influenced by th e type of CS received some 8-10 h earlier. However, if sufficient CS i s received to induce the neural changes of PSP, the nocturnal PRL surg es are expressed in an all-or-none fashion by 1-2 days after mating.