Effect of gonadotrophin-releasing hormone on circulating levels of immunoreactive luteinizing hormone in fetal rats

Concentrations of luteinizing hormone (LH) were measured in plasma samples obtained from rat fetuses after injecting them with synthetic gonadotrophin-releasing hormone (GnRH) in utero. On days 17 1/2 , 18 1/2 or 19 1/2 post-conception (p.c.), fetuses in one uterine horn were injected subcutaneously with 100 ng GnRH in 2 μl of saline. Fetuses in the other uterine horn were injected with saline (four litters at each gestational age). Plasma samples were collected beginning 30 min after injection of GnRH. The concentration of LH in 20 μl of fetal plasma was measured by a microvolume radioimmunoassay. On day 17 1/2 p.c., LH was detectable only in plasma samples of GnRH-injected fetuses. In 18 1/2 - and 19 1/2 -day-old fetuses LH reached detectable levels (>6 ng/ml) in most saline-injected fetuses. In 18 1/2 - and 19 1/2 -day-old fetuses of both sexes, plasma LH titers of GnRH-injected fetuses were significantly higher than in saline-injected littermates. On day 19 1/2 GnRH-treated females had higher levels of plasma LH than similarly treated males. Thus fetal rat pituitaries were shown to have the capacity to release LH in vivo response to circulating GnRH as early as 17 1/2 days p.c.