PRIOR PARITY REDUCES POST-COITAL DIURNAL AND NOCTURNAL PROLACTIN SURGES IN RATS

Mating stimuli received by female rats activate a neuroendocrine mnemo nic system which produces daily diurnal and nocturnal prolactin (PRL) surges for the first half of gestation, surges which help maintain cor pora lutea function and a viable pregnancy. Since these PRL surges may be regulated in part by endogenous opioids and opioid sensitivity dec lines as a function of multiple births, we decided to investigate the possibility that prior parity might affect the post-coital diurnal and nocturnal PRL surges, reducing their magnitude and/or occurrence. Age -matched, nulliparous and primiparous rats were mated to males from ou r colony. On days 5 or 10 of pregnancy females received jugular cathet ers. Blood samples were collected at regular intervals from 1000 h on day 7 to 1000 h on day 8, and from 1000 h on day 12 to 1000 h on day 1 3 of gestation in separate sets of multigravid and primigravid rats. M easurement of plasma PRL by radioimmunoassay revealed that prior repro ductive experience altered the patterns and levels of plasma PRL. Plas ma PRL levels were significantly reduced during both the diurnal and n octurnal surges on days 7-8 in multigravid rats when compared with lev els in primigravid rats. No differences in PRL levels were found betwe en primigravid and multigravid groups on days 12 to 13 of gestation. T he changes in diurnal and nocturnal PRL surges during early pregnancy indicate that prior parity reduces the subsequent secretion of PRL, po ssibly by altering the neuroendocrine regulation of this hormone.