Changes in the LH response to GnRH in the ovine fetus and neonate.

The in vivo responsiveness of the ovine fetal and neonatal pituitary was investigated using acute and chronic preparations. Several studies were performed on six fetuses and six lambs from 112 days' gestation through 5 weeks after birth. Ten micrograms of gonadotropin-releasing hormone (GnRH) was injected intravenously into each fetus and lamb, and serial blood samples were collected from the fetus, ewe and lamb. The luteinizing hormone (LH) content of each sample was determined by species-specific radioimmunoassay. Both the fetus and the neonate showed a response to GnRH. A significant elevation of LH was found in all animals by 15 minutes. There was no response in the ewe when GnRH was injected into its fetus. Great variability in response was noted among various age groups and between animals, with the maximum individual LH response reaching 33.7 ng/ml. The maximum LH response of the neonate during the first week of life was significantly depressed (p < 0.05) as compared to that of the fetus and the older neonate.