Pulsed GnRH secretion and the FSH secretory peaks that initiate ovarian antral follicular wave emergence in anestrous ewes

Abstract

In ewes, immunization against GnRH blocks LH pulses but mean serum FSH concentrations are only partly reduced; the fate of the FSH peaks that precede ovarian follicular waves has not been studied. In this study, we used immunization against GnRH to examine the need for pulsed GnRH secretion in the genesis of FSH peaks in the anestrous ewe. Six anestrous ewes were given a GnRH immunogen on Day 0 and a booster injection on Day 28. Control ewes ( n = 6) received adjuvant only. Transrectal ultrasonography was performed daily for 2 days prior to and 10 days following both the primary (Days −2 to 10) and booster (Days 26–38) injections and for a 13-day period beginning 26 days after booster injection (Days 54–66). Blood samples were collected daily. Intensive bleeding (every 12 min for 7 h) was performed on Days 9, 37, and 65 of the experimental period to characterize the pulsatile pattern of LH secretion. GnRH antibody titers were increased and LH pulses were abolished immediately after booster immunization ( P < 0.05). The number of FSH peaks, FSH peak concentration and amplitude and basal FSH concentrations were only decreased in immunized ewes in the period of observations starting 26 days after booster immunization ( P < 0.05); however, some peaks were still seen. The number of follicular waves was decreased in the period around booster immunization and no follicular waves were seen during the period starting 26 days after booster immunization in immunized ewes ( P < 0.05). In summary, in anestrous ewes, when pulsed LH secretion was abolished by immunization against GnRH, the peaks in serum concentrations of FSH that trigger ovarian follicular waves continued for a period of time. We concluded that although blocking the effects of GnRH gradually causes a diminution of FSH secretion, there is no acute requirement for GnRH in the regulation of FSH peaks. The existence of FSH peaks in the absence of follicular waves, and pulsed LH secretion, suggests that some endogenous rhythm may drive the occurrence of FSH peaks, independent of both changes in negative feedback by secretory products from ovarian antral follicles and GnRH.