Changes in FSH, LH and prolactin secretion and ovarian follicular development during lactation in the rat.

To elucidate the factors regulating ovarian follicular development during lactation in the rat, plasma and pituitary levels of FSH, LH and prolactin (PRL) were determined for each day of lactation along with the histological examination of size-distribution of ovarian follicles.Post-partum surges in FSH and LH were observed at 1700 hr on Day 0 of lactation (Day 0=day of parturition). Plasma FSH values were within the range of diestrous levels during Days 1 to 21 whereas pituitary content of FSH exceeded the values of 1400hr of proestrus (the highest level during the estrous cycle) from Day 3 onward. On the other hand, concentrations of plasma LH were generally below the range of diestrous values througout lactation. From Day 1 onward pituitary content of LH increased slowly but did not attain proestrous levels until Day 14. High concentrations of plasma PRL were observed from Day 4 to 10, although the time of suckling stimulus was not controlled. Pituitary content of PRL had abruptly increased by Day 3. Thereafter, high levels were maintained until Day 17, followed by a sudden decline on day 18.After post-partum ovulation, the number of healthy follicles larger than 201 μm in diameter abruptly decreased until Day 6. From Day 3 to 7, a few healthy follicles of 201-400 μm were present, but larger follicles were absent. After Day 8, follicles larger than 401 μm were always present.On the basis of these results it is suggested that during the early stages of lactation, the strong suckling stimulus and the consequent high levels of PRL result in a decreased plasma LH level which is insufficient to cause maturation of the follicles. Ovaries during early stages of lactation, however, were still responsive to exogenous gonadotropins initiating follicular maturation under the influence of normal suckling stimulus and high levels of PRL.