Evidence that the onset of seasonal anoestrus in the ewe may be independent of increasing prolactin concentrations and daylength

Worthy, K.; Haresign, W.

doi:10.1530/jrf.0.0690041

Cited by 69 publications

(40 citation statements)

References 23 publications

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“…These responses are interpreted as the development of refractoriness to the stimula¬ tory photoperiod (melatonin). Indeed, it is postulated that the normal transition from the breeding to anoestrous seasons in the sheep is associated with refractoriness to the stimulatory ambient daylength rather than a direct inhibitory effect of increasing daylength in the early spring (Worthy & Haresign, 1983; Robinson & Karsch, 1984 ('extra-long' days) in the winter was sufficient to induce premature anoestrus in the present trial, and this was associated with a slight (though not statistically significant) rise in plasma prolactin concentrations.…”

Section: Discussionmentioning

confidence: 56%

Melatonin can induce year-round ovarian cyclicity in red deer (Cervus elaphus)

Adam¹,

Moir²,

Shiach³

1989

Reproduction

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). Plasma prolactin concentrations remained suppressed ( < 20 ng/ml) for the duration of melatonin-feeding (Groups CL/M and NL/M) whereas control values (Group C) were elevated (20\p=n-\120ng/ml) between April and August (P < 0\m=.\05).The ovarian response by hinds to melatonin therefore depends on initial reproductive status and recent photoperiodic history, and continued administration to cyclic hinds stimulates prolonged ovarian cyclicity irrespective of the time of year.

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Section: Discussionmentioning

confidence: 56%

Melatonin can induce year-round ovarian cyclicity in red deer (Cervus elaphus)

Adam¹,

Moir²,

Shiach³

1989

Reproduction

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“…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.…”

Section: Discussionmentioning

confidence: 99%

“…This has been taken as evidence that shortening days of late summer and autumn stimulate the onset of reproductive activity under natural environmental conditions. Several studies in the ewe, however, have shown that the onset of the natural breeding season may not require a decrease in day length; moreover, termination of the breeding season may not require an increase in day length (Worthy and Haresign 1983;Robinson and Karsch 1984;Robinson etal. 1985a;Worthy etal.…”

Section: Introductionmentioning

confidence: 99%

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

1990

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A study was conducted to test the hypothesis that different portions of the annual photoperiodic cycle play different roles in timing the breeding season of the ewe, Ovis aries, an animal in which an endogenous rhythm generates the seasonal reproductive transitions. Adult female sheep were pinealectomized to disrupt transduction of photoperiodic cues at 4 times of the year (summer and winter solstices, vernal and autumnal equinoxes), and the effects on seasonal reproductive neuroendocrine activity were evaluated. Time of pinealectomy greatly influenced the subsequent seasonal reproductive cycle such that the following inferences are possible. Lengthening days between the winter and summer solstices synchronize reproductive onset to the appropriate time of year. The relatively long days around the summer solstice act to suppress reproductive activity and forestall the start of the breeding season until late summer/early autumn. The shortening days between the summer solstice and autumnal equinox maintain a normal intensity and duration of reproductive neuroendocrine induction during the impending breeding season. However, the shortening days between the autumnal equinox and winter solstice (i.e., after breeding season onset) do not appear to play a critical role in maintaining the breeding season of that year, but may provide important cues for timing the breeding season of the following year.

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“…On the other hand, Worthy & Haresign (1983) reported that seasonal anoestrus occurred in the absence of high levels of prolactin in ewes on a short-day lighting regimen. It is unclear whether the decline in testosterone in the treated males in this study reflects a prolactin-mediated change in sensitivity of the hypothalamic-pituitary system to the negative feedback effects of testosterone.…”

Section: Discussionmentioning

confidence: 99%

Effects of melatonin implants on plasma concentrations of testosterone, thyroxine and prolactin in the male silver fox (Vulpes vulpes)

Forsberg

Madej²

1990

Reproduction

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Summary. Melatonin administration in the form of slow-release implants advanced breeding activity in silver fox males when treatment began in June. Plasma testosterone concentrations were significantly higher in treated animals than in controls from September to November, whereas in February and March they were significantly lower. Plasma prolactin concentrations were significantly reduced immediately following melatonin treatment in June but increased to greater levels than control values and 'peaked' after 7 months. This 'peak' was associated with a rapid decrease in testosterone secretion. The normal seasonal spring rise in prolactin secretion was prevented by melatonin administration. Thyroxine values decreased and were significantly lower after 2 months of melatonin treatment.

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Evidence that the onset of seasonal anoestrus in the ewe may be independent of increasing prolactin concentrations and daylength

Cited by 69 publications

References 23 publications

Melatonin can induce year-round ovarian cyclicity in red deer (Cervus elaphus)

Melatonin can induce year-round ovarian cyclicity in red deer (Cervus elaphus)

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

Effects of melatonin implants on plasma concentrations of testosterone, thyroxine and prolactin in the male silver fox (Vulpes vulpes)

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