The onset of oestrus and ovulation in lactating rats.

Lactation delays the re-initiation of oestrous cyclicity in rats, resulting in physiological sterility for the duration of suckling. During this phase, the secretion of pituitary gonadotrophins is suppressed by an unknown mechanism. Continued application of the suckling stimulus by litter replacement (Bruce, 1958; Nicoll & Meites, 1959), or injections of prolactin (Meites & Nicoll, 1959), have been shown to prolong lactation considerably beyond the usual period. The present study aimed to demonstrate the role of prolactin in inhibiting the gonadotrophin secretion necessary for the re-establishment of oestrous cyclicity during lactation. Pregnant rats weighing approximately 300 g were obtained from the Institute colony and housed in individual cages. At parturition, the number of young in the litter was adjusted to eight, two or one as required. The day following the post-partum oestrus was regarded as Day 1 of lactation. The onset of oestrus was confirmed by the presence of cornified epithelial cells in the vaginal smear, and ovulation by the presence of ova in the oviducts at 10.00 hours on the day of oestrus. Oestrus and ovulation occur on Day 22 of lactation in rats suckling eight young each and on Day 16 in those suckling either two or one young each. The data presented in Table 1 show that withdrawal of the young on Days 10 or 14 resulted in the appearance ofoestrus and ovulation 2 days later. Increasing the number of young from two to eight on Day 15 caused a significant post¬ ponement of oestrus and ovulation, but reduction of the size of the litter to two again on Day 16 resulted in postponing oestrus and ovulation only by 1 day. Thus, it is clear that the greater the intensity of suckling, the longer is the dura¬ tion of sterility. Intensity of the suckling stimulus can be directly correlated with an increase in prolactin levels and therefore the effect of giving prolactin to lactating rats on the recurrence of oestrus was studied. Administration of 100 ^g prolactin (ovine prolactin, NIH-P-S9) as a single subcutaneous injection on Day 14 to rats suckling two young each led to a significant delay (from Day 16 to Day 21) in the onset of oestrus and ovulation (Table 2). Injections of prolactin from Days 14 to 16 resulted in a further delay in the re-establishment of cyclicity. A single prolactin injection on Day 20 to lactating rats suckling eight young each postponed oestrus and ovulation from the normal Day 22 to Day 27.