Alterations in Endogenous Gonadotropin Secretion and Pituitary Responsiveness to Gonadotropin-Releasing Hormone in Adult Ewes, Following Indirect Selection in

Indirect selection to improve reproductive performance in the female, through use of a physiologically related trait in pre-pubertal males, the LH response to a GnRH challenge, has created two lines of sheep with altered pituitary gland sensitivity to both physiological and pharmacological doses of GnRH. These lines also exhibit correlated between-line differences in female reproductive performance during the first, and at the beginning of the second, breeding season. This study was designed to determine whether the observed between-line differences in female reproductive performance were related to between-line differences in endogenous gonadotropin secretion during the luteal and follicular phases of the estrous cycle and whether the between-line difference in the LH response to a GnRH challenge was maintained in adult females. The results indicated that, despite similar mean gonadotropin and steroid concentrations during the luteal and follicular phases of the estrous cycle, sig-nificant between-line differences in LH pulse amplitude were present during the follicular phase (high line> low line). In addition, the results indicated that the between-line difference in the LH response to a GnRH challenge was maintained in adult ewes, being most apparent during the follicular phase, when the magnitude of the difference was similar to that in prepubertal female lambs. Therefore, in addition to showing maintenance of the between-line difference in the selected characteristic in postpubertal females, the results demonstrate that adult ewes from the two lines differ in additional, unselected but physiologically related traits and provide a model system with which to study steroidal regulation of gonadotropin secretion in the ewe