Serum Levels of Prolactin, Luteinizing Hormone, and Follicle Stimulating Hormone During Pregnancy in the Rat1

Groups of mated and nonmated female rats were decapitated at 0800, 1200, 1600, and 2000 hours on the day of estrus. In the mated animals, these times corresponded to 4, 8, 12, and 16 hours respectively following coitus. Serum levels of prolactin, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were determined in both groups by specific and sensitive radioimmunoassays. Prolactin levels in mated rats were elevated (p < 0.05) during the entire 16- hour period following coitus, whereas increases in LH were noted for only the first 8 hours. Variation in the levels of FSH did not permit a meaningful conclusion regarding release of this hormone following mating. Additional female rats were mated, and day 1 of pregnancy was designated the day sperm were found in the vaginal lavage. Groups of animals were killed at daily intervals until term (day 23) and blood was collected for gonadotropin assays. Levels of gonadotropins in serum throughout gestation were generally less than those observed during the diestrous phase of the estrous cycle. Levels during diestrus were: prolactin, 12.4 ng/ ml; LH, 6.4 ng/ml; FSH, 0.52 μg/ml. Exceptions to this observation during pregnancy were as follows: 1) prolactin-elevated levels were noted at days 1 (95 ± 31 ng/ml) and 23 (50 ± 16 ng/ ml); 2) LH-levels tended to increase from day 18 (0.54 ng/ml) until day 23 (5.13 ng/ml); and 3) FSH-concentrations tended to increase during the interval between days 13-18 (0.41-0.92 M-g/ ml) and subsequently declined until parturition (0.92-0.33 μg/ml). (Endocrinology90: 632, 1972)