ROLES OF PROGESTERONE AND ESTRADIOL IN DETERMINING THE TEMPORAL SEQUENCE AND QUANTITATIVE EXPRESSION OF SEXUAL RECEPTIVITY AND THE PREOVULATORY LH SURGE IN THE EWE

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 Progesterone was able to facilitate the expression of oestrus even when it was withdrawn 8 days before oestrogen treatment, suggesting that it exerts its effect by restoring the sensitivity of the oestrogen-refractory animal to oestrogen.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progesterone inhibited the stimulatory effect of oestrogen, but this effect disappeared as soon as the progesterone was withdrawn.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.