Gonadotropin Levels in Lactating Rats: Effect of Ovariectomy12

The effect of suckling on synthesis and release of FSH and LH was investigated in intact and ovariectomized, lactating rats. Litter size was adjusted on Day 2 postpartum to 0, 3, 6 or 12 pups, and ovariectomies were performed at this time. Pituitary concentrations of FSH and LH were low on Day 2 and increased by Day 6. Pituitary FSH levels were greater in ovariectomized rats than in intact dams by Day 6 in rats with 0 and 3 pups and by Day 18 in dams nursing 6 pups. Pituitary LH concentrations were similar in all groups on Day 6, and the levels increased in ovariectomized rats with 0 and 3 pups by Day 12 and in dams nursing 6 pups by Day 18. Pituitary FSH and LH levels of intact and ovariectomized rats nursing 12 pups did not differ significantly during the first 18 days of lactation. Serum FSH and LH levels were elevated after ovariectomy; the rate and magnitude of these increases were diminished as litter size increased. Serum FSH and LH were elevated to detectable levels in all dams 12 hr after their pups were removed. These results demonstrate that synthesis and release of FSH and LH are progressively diminished as litter size increases in the lactating rat and that the secretion of these hormones increases within 12 hr after the suckling stimulus is removed. Pituitary prolactin levels were similar in all groups on Day 6 postpartum, but on Days 12 and 18, levels were higher in dams nursing 12 pups. Serum prolactin levels increased with litter size on Days 6 and 12; and on Day 18 the levels were similar in all suckled groups. (Endocrinology93: 540, 1973)