Differential pituitary response to GnRH in pregnant Booroola-Assaf and Assaf ewes.

Pituitary response to exogenous gonadotrophin-releasing hormone (GnRH) was examined at a late stage of pregnancy in 10 Booroola-Assaf ewes heterozygous at the FecB locus (FecBFec+) and in 11 Assaf ewes that were non-carriers (Fec+Fec+). Basal plasma luteinizing hormone (LH) and follicle stimulating hormone (FSH) concentrations were similar in the two genotypes. Administration of 100 micrograms of GnRH resulted in a significant increase (P < 0.05) in plasma LH and FSH concentrations in most of the ewes treated. Maximal responses were observed 105-135 min after GnRH treatment. Pituitary responses to GnRH were significantly lower (P < 0.05) in Booroola-Assaf than in Assaf ewes. The decreased pituitary responsiveness observed in FecB gene carriers compared with Fec+Fec+ ewes might be due to differences in the concentrations of ovarian or uterine hormones modulating the release of gonadotrophin. The results suggest that FecB-specific differences can be observed at the pituitary level.