Inhibition of gonadotropin secretion during lactation in the rat: relative contribution of suckling and ovarian steroids

The relative effects of the suckling stimulus and ovarian steroids on gonadotropin inhibition during lactation were studied in intact and ovariectomized lactating rats. On Day 2 postpartum, litter sizes were adjusted to 0 or 8 pups and ovariectomy was performed. Serum LH concentrations in the intact lactators were barely detectable and approximately five-fold lower than basal cycling levels reported previously by this laboratory. Serum and pituitary LH concentrations gradually increased in ovariectomized lactators but did not differ markedly from intact lactators until Days 15 to 20 postpartum. At all times studied, the postcastration rise in serum and pituitary LH was clearly inhibited in ovariectomized lactators when compared to nonlactating, ovariectomized females. Serum FSH concentrations in the intact lactating animals remained at values similar to basal cycling levels, while pituitary FSH concentrations gradually increased. Serum and pituitary FSH concentrations in the ovariectomized lactators increased dramatically throughout lactation but did not equal those observed in nonlactating, ovariectomized females until Days 15 to 20. Serum progesterone concentrations increased during lactation, reaching maximal values near Day 10 and declining to baseline values by Day 20. Estradiol concentrations increased from very low levels on Day 2 to reach values similar to basal cycling levels by Day 10 and diestrus-2 levels by Day 20. These results demonstrate that in the absence of ovarian steroids, the suckling stimulus: 1) can effectively inhibit the postcastration rise in LH and FSH secretion for at least 15 days postpartum; 2) is a less potent inhibitor of FSH than LH secretion; 3) can not maintain serum LH and FSH below basal cycling values. The increase in estradiol concentration during lactation in the intact animals was unexpected since it occurred in the presence of unchanging, suppressed basal levels of LH.