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Pituitary function during pseudopregnancy and lactation in the rat

Summary. Plasma concentrations of FSH on Days 1, 2, 3, 8 and 9 of lactation (day of parturition = Day 0) were significantly higher than on the corresponding day of pseudopregnancy (day of vaginal plug = Day 1). Pituitary FSH content on Days 4–12 of lactation was also significantly higher than in pseudopregnant rats. However, pituitary LH content on Days 2–12 of lactation was significantly lower than in pseudopregnancy, whereas plasma concentrations of LH were almost similar. Two consecutive injections of LH-RH (100 ng/rat, i.v.), 60 min apart, resulted in a significant release of FSH in lactating rats (Days 3 and 15) in response to the second injection of LH-RH, but there was no increase in pseudopregnant rats on Day 3. The first and second injections of LH-RH produced significantly less release of LH on Days 3 and 15 of lactation than on Day 3 of pseudopregnancy. These results indicate that FSH secretion is not suppressed by suckling whereas the suckling stimulus is a potent inhibitor of LH.