Changes in gonadotrophins and prolactin levels in isolated (seasonally or lactationally) anovular ewes associated with ovulation caused by the introduction of rams

Effect of the introduction of rams during the anoestrous season on the pulsatile secretion of LH in ovariectomized ewes

“…However, the introduction of rams will induce ovulation in these ewes provided they have been conditioned by a period of isolation from rams (Underwood, Shier & Davenport, 1944;Schinckel, 1954;Oldham, Martin & Knight, 1978). It has been proposed that the ram stimulus accomplishes this by overcoming the inhibitory actions of oestradiol, since the initial response of seasonally anovulatory ewes to the introduction of rams is an immediate increase in the frequency of the LH pulses (Martin, Oldham & Lindsay, 1980a;Martin et al, 1980b;Poindron et al, 1980). It is unlikely that negative feedback by progesterone is an important component in the effect, because the levels of this hormone are low in anovulatory ewes due to the absence of corpora lutea.…”

“…In addition to an increase in the basal levels of LH, entire anoestrous ewes experience a preovulatory surge 6-52 h after the introduction of rams (Oldham et al, 1978;Knight, Peterson & Payne, 1979;Martin et al, 1980b;Poindron et al, 1980). How this is brought about is not known but it has been suggested that the surge may be induced by a direct action of the ram stimulus on the hypothalamic mechanisms controlling LH secretion, so bypassing the normal system of positive feedback by oestrogen (Oldham et al, 1978;Knight et al, 1979 Statistical analysis.…”

Female effect in sheep. II. Role of volatile substances from the sexually receptive female; implication of the sense of smell

“…In sheep, the male's presence induces rapid changes in the pattern of pituitary hormone secretion in the female: the presentation of the ram to anestrous ewes induces an LH pulse in a matter of minutes (Martin et al, 1978;Poindron et al, 1980 [34]. All the reported significant differences are one-tailed.…”

Ram-induced oestrus and ovulation in lactating and weaned Corriedale ewes

“…Suckling has been suggested to delay the onset of cyclicity after parturition (Maulé on and Dauzier, 1965;Restall, 1971;Shevah et al, 1974) by promoting the release of brain opioids (Malven, 1986) which, in turn, affect LH secretion via the inhibitory effect of oestradiol (McNeilly, 1994). Removing the suckling stimulus in Experiment 1 may have facilitated the rapid increase in LH frequency required for a ram-induced ovulation to occur (Poindron et al, 1980). The overall significance of the effect of the time ewes spent away from their lambs while exposed to rams in Experiment 2 also points towards lamb interference, possibly through suckling, acting upon LH pulsatility.…”

“…Lactating anovular ewes exhibited similar LH and ovulatory responses as did seasonally anovular ewes when exposed to rams under a favourable photoperiod (Poindron et al, 1980), and the ram effect has been successfully applied for rebreeding autumn lambing ewes (Geytenbeek et al, 1984;Wright et al, 1989).…”