Replacement of progesterone restores the nocturnal surge of prolactin following suppression with a gonadotropin-releasing hormone agonist during early pregnancy in the rat

Continuous administration of a GnRH agonist (GnRH-Ag) at a dose of 5 micrograms/day, commencing on day 7 of pregnancy resulted in the suppression of daily nocturnal surges of prolactin (PRL) on day 8, and serum progesterone (P4) levels with subsequent termination of pregnancy. Replacement with dydrogesterone, a synthetic analog of P4 at a dose of 4 mg/day s.c. restored the magnitude of nocturnal PRL surges. These data suggest that GnRH-Ag may act either at the level of the brain to suppress the nocturnal PRL surge, resulting in a fall in serum P4 levels or at the level of the corpus luteum itself or at both sites simultaneously to terminate pregnancy.