Suckling stimulus, lactation, and suppression of ovulation in the rat.

Lactation pseudopregnancy in rats suckling a 5-pup litter lasted 22.0 +/- 0.4 days (mean +/- SEM; n = 11). By day 13 of lactation (day 1 of lactation = day of parturition), the continuation of lactation pseudopregnancy was dependent on the suckling stimulus, as litter removal on day 13 resulted consistently in ovulation on day 16. Measurements of various hormones before and after litter removal revealed high concentrations of progesterone and PRL during lactation and a rapid drop of both hormone concentrations after litter removal. Lactation pseudopregnancy in rats suckling a 10-pup litter lasted 26.1 +/- 0.9 days (n = 16). After litter removal on day 13 of lactation, the lactation pseudopregnancy continued for a further 7- to 11-day period, as evidenced by daily vaginal smears which remained mucified during that period. Measurements of hormone concentrations revealed continuously high concentrations of PRL before litter removal and a pattern of PRL secretion characterized by at least two diurnal peaks during the first days after litter removal. Progesterone concentrations decreased by 50% after litter removal, but the levels then remained constant and well above those found after the removal of 5-pup litters. It is argued that the different response to litter removal on day 13 of lactation between rats suckling 5 or 10 pups is due to the initiation of PRL peaks in rats with 10-pup litters: these PRL peaks are able to maintain luteal function for some period. It is further argued that the initiation of PRL peaks in rats with 10-pup litters is due to the high blood concentrations of progesterone at the time of litter removal compared to those of rats with a 5-pup litter.