Evidence that the onset of the breeding season in the ewe may be independent of decreasing plasma prolactin concentrations

Abstract

Ten ewes of each of two breeds, Dorset Horn (long breeding season) and Welsh Mountain (short breeding season), were given subcutaneous oestradiol-17 beta implants and then ovariectomized. Another 10 ewes of each breed were left intact. On 3 May 1982, all the ewes were housed in an artificial photoperiod of 16L:8D. After 4 weeks, half of the ewes of each breed and physiological state were abruptly exposed to a short-day (8L:16D) photoperiod while the others remained in long days (16L:8D). The time of onset of the breeding season was significantly (P less than 0.05) advanced in ewes switched to short days (12 August +/- 10 days) compared to those maintained in long days (4 September +/- 14 days). Dorset Horn ewes began to cycle (20 July +/- 7 days) significantly (P less than 0.001) earlier than Welsh Mountain ewes (19 September +/- 6 days). Disparities in the time of onset of cyclic activity in ewes of different breeds and daylength groups were echoed in disparities in the time at which plasma LH and FSH concentrations rose in oestrogen-implanted, ovariectomized ewes of the same light treatment group. Prolactin concentrations showed an immediate decrease in ewes switched to short days, but remained elevated in long-day ewes. Since the breeding season started in the presence of high prolactin concentrations in long-day ewes, it seems unlikely that prolactin is an important factor determining the timing of the onset of cyclic activity.

Seasonal changes in ovulatory activity, plasma prolactin, and melatonin concentrations, in Mouflon (Ovis gmelini musimon) and Manchega (Ovis aries) ewes

“…Also, melatonin can suppress prolactin secretion [27,41] and Manchega ewes had higher melatonin concentrations during the summer solstice. Low prolactin concentrations that coincide with the onset of ovarian activity have been demonstrated in many studies [18,37], but there are reports of ovarian activity in the presence of high prolactin values induced by photoperiodic manipulation [46]. Our results show that while prolactin concentrations were high when Manchega ewes started to cycle, they were at low concentrations when Mouflons started cycling, and support the idea that fluctuations in circulating prolactin are not implicated in the onset of cyclic ovulatory activity.…”

“…Our results show that while prolactin concentrations were high when Manchega ewes started to cycle, they were at low concentrations when Mouflons started cycling, and support the idea that fluctuations in circulating prolactin are not implicated in the onset of cyclic ovulatory activity. Whether prolactin is related to cyclic activity has been quite controversial, and although most investigations have not found a clear relationship [43,46], the timing of prolactin secretion manipulation may be crucial for an effect on reproductive activity. Indeed, when prolactin was suppressed towards the end of the estrous season, the season was found to be extended [6], whereas suppression of prolactin during the anestrous period did not affect the timing of onset of cyclic activity [19,33].…”

Photoperiodic requirements for timing onset and duration of the breeding season of the ewe: Synchronization of an endogenous rhythm of reproduction

“…1973;Worthy and Haresign 1983;Robinson and Karsch 1984;Robinson et al 1985a;Worthy et al 1985;Karsch et al 1989). This is suggested by our finding that pinealectomy at various times of the year had different effects on both onset and duration of the subsequent breeding season.…”

Reproductive refractoriness of the ewe to inductive photoperiod is not caused by inappropriate secretion of melatonin

“…This condition is thought to play a critical role in determining the annual reproductive cycle of the ewe; specifically, photorefractoriness leads t o the natural transitions both into and out of the sexually active season in certain breeds of sheep (Worthy and Haresign, 1983;Robinson and Karsch, 1984;Robinson et al, 1985;Worthy et al, 1985). Photorefractoriness can also be produced in artificial photoperiods by maintaining sheep for extended periods in constant long or short day lengths (Thwaites, 1965 ;WodzickaTomaszewska et al, 1967;Ducker et al, 1973;Karsch et al, 1986).…”