Alterations in the Control of Luteinizing Hormone Pulse Frequency Underlie the Seasonal Variation in Estradiol Negative Feedback in the Ewe1

The results of this study are consistent with the conclusion that estradiol, acting on a pulse generating system whose frequency is partially damped by environmental effects, is responsible for the low frequency of LH pulses during seasonal anestrus.

Abstract

In this study, we have examined the factors responsible for the low frequency of pulsatile luteinizing hormone (LH) secretion observed in intact anestrous ewes. Because estradiol is a potent inhibitor of LH in anestrus, we first examined its effect on LH pulses. Treatment of ovariectomized ewes with implants that produced physiological estradiol concentrations ranging from I to 3 pg/mI either drastically decreased LH pulse frequency or completely abolished LH pulses in anestrus. This contrasts with the response in the breeding season, when estradiol exerts a weak inhibition of LH by decreasing the amplitude, but not the frequency, of LH pulses. These data, together with the observation that this estradiol treatment did not suppress the pituitary response to gonadotropin releasing hormone (GnRH) in anestrus, lead to the hypothesis that estradiol is a more potent inhibitor of LH secretion in anestrus because it gains the capacity to suppress the frequency of hypothalamic GnRH discharges. Seasonal changes in pulsatile LH secretion that are independent of steroid feedback may also contribute to the low LH pulse frequency of anestrous ewes, because LH pulses in untreated ovariectomized ewes were 40% less frequent in anestrus than in the breeding season. Interestingly, there was a corresponding increase in pulse amplitude in ovariectomized anestrous ewes, so that mean LH levels were identical at both times of the year. Finally, there may be a general increase in the sensitivity of the system that generates LH pulses to all inhibitory steroids in anestrus since the ability of progesterone to suppress LH pulse frequency was also enhanced at this time of year. The results of this series of experiments are consistent with the conclusion that estradiol, acting on a pulse generating system whose frequency is partially damped by environmental effects, is responsible for the low frequency of LH pulses during seasonal anestrus.