Roles of progesterone and oestradiol in determining the temporal sequence and quantitative expression of sexual receptivity and the preovulatory LH surge in the ewe

Abstract

In a series of experiments using a quantitative method for measuring receptivity and implants that allowed rapid and controlled changes in the blood concentrations of oestradiol-17 beta and progesterone, we have re-examined the roles of these steroids in the induction of sexual behaviour and the LH surge in ovariectomized ewes. Progesterone priming was found to increase the proportion of ewes showing oestrus, reduce the latency to the onset of oestrus, and increase the 'intensity' of the behaviour as measured by the receptivity index, but it did not affect the size of the LH surge. Progesterone was able to facilitate the expression of oestrus even when it was withdrawn 8 days before oestrogen treatment, suggesting that it exerts its effect by restoring the sensitivity of the oestrogen-refractory animal to oestrogen. When it was present at the time oestrogen was administered, progesterone inhibited the stimulatory effect of oestrogen, but this effect disappeared as soon as the progesterone was withdrawn. Thereafter, expression of both the behavioural and endocrine responses was delayed by 24-30 h. These data show that the timing of the preovulatory behavioural and endocrine events is determined primarily by the time of progesterone withdrawal. The amount of oestradiol and the timing of any rise in concentration serve only to modulate these effects.

Male effect associated with suckling interruption on the reproductive performance of Santa Inês ewes

“…In addition to stress, several experimental evidence point to a correlation between final P4 production and fertility (oLdHaM and pEarCE, 1988;FabrE-nys and Martin, 1991;Caraty and sKinnEr, 1999), for example, reduced fertility after first ovulation of ewes exposed to rams, as reported by sKinnEr et al (2000). These authors mentioned that P4 provoked a delay in the preovulatory LH peak and affected final oocyte maturation or further follicle luteinization.…”

Progesterone Treatment That either Blocks or Augments the Estradiol-Induced Gonadotropin-Releasing Hormone Surge Is Associated with Different Patterns of Hypothalamic Neural Activation

“…It is possible that these cells are involved in functions other than the modulation of GnRH secretion. One crucial action of progesterone during the luteal phase is the stimulation of estrous behavior in ewes at the time of the preovulatory surge [67]. It is, therefore, tempting to suggest that the high level of non-GnRH cellular activation in the progesterone-pretreated animals that surged might reflect the activation of neuronal substrates that subserve estrous behavior [21].…”

Long-term suppression of reproductive function by a single dose of gonadotropin-releasing hormone antagonists in a sheep model

“…However, estrus was synchronous with the second ovulation. This phenomenon of ovulation in the absence of overt estrous behavior has been documented extensively in the sheep and occurs after a period of anestrus, because of the lack of P priming (17).…”