The Behavioural and Vaginal Response of the Spayed Ewe to Oestrogen Injected at Various Times Relative to the Injection of Progesterone

Abstract

Three trials, each using seventy-two ovariectomized Suffolk cross-bred ewes, were conducted to determine the interaction on the behavioural and vaginal response of varying the time of injection of oestradiol benzoate (ODB; dose levels 16, 20 and 25 \ g=m\ g in oil) relative to that of pro-

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

“…Moreover, it appeared that progesterone need not be present during the period just before oestradiol administration, as was suggested by Moore & Robinson (1957). The long-term nature of this effect is consistent with the hypothesis that progesterone removes the refractoriness by reversing the desensitization produced by oestrogen.…”

“…In experiment 3, we were able to take advantage of the rapid fall in progesterone concentrations that is effected by removal of an implant to show that both oestrus and the LH surge can be induced when oestradiol was given as early as the time of progesterone withdrawal. In other words, oestradiol began to exert its effects at least as soon as the progesterone had disappeared, that is much earlier than previously suspected for either behaviour (Moore & Robinson, 1957;Signoret, 1971 ;Smith & Kelly, 1980) or the LH surge (Goodman et al 1981;Karsch et al 1980). However, the expression of its effect, whatever the time and the duration of oestradiol treatment, was delayed for about 24 h. Therefore, one can discern two kinds of inhibition by progesterone.…”

“…Many aspects of these facilitatory actions of progesterone, such as the required dose, and the duration and timing of the treatment, have been described (Robinson, Moore & Binet, 1956;Land, Thompson & Baird, 1972;Scaramuzzi et al 1972;Lishman, 1975;Smith & Kelly, 1980). As in many other species (Signoret, 1971;Morin, 1977), progesterone, in the ewe, can also inhibit the action of oestrogen if it is present too close to the time of oestrogen administration (Moore & Robinson, 1957;Scaramuzzi, Tillson, Thorneycroft & Caldwell, 1971;Signoret, 1971;Smith & Kelly, 1980). The results of these experiments led to the conclusion that 24-48 h must be allowed for the progesterone to be cleared from the blood before full responses to oestro¬ gen can be expected.…”

Induction of Ovulation in Seasonally Anovular Ewes by the Introduction of Rams: Effects of Progesterone and Active Immunization Against Androstenedione

“…Both these workers gave the last injection, or removed the intravaginal sponges, at the same time as introducing the rams whereas we removed our implants 48 h earlier. 48 h is the optimal period between the end of progesterone treatment and an injection of oestrogen for the induction of oestrus in ovariectomized ewes (Moore and Robinson 1957). We assumed that the oestrogen levels would begin to rise as soon as the frequency of LH pulses rose, i.e.…”

Control of Oestrus in Sheep

“…As suggested by Moore and Robinson (1957) and Shelton (1963) the spayed ewe assay is an extremely useful tool in the identification of progestogens likely to be suitable for the control of cestrus and ovulation in the ewe.…”