Neuroendocrine Regulation of the Estrous Cycle and Seasonal Breeding in the Ewe

Legan, Sandra J.; Karsch, Fred J.

doi:10.1093/biolreprod/20.1.74

Cited by 125 publications

(61 citation statements)

References 34 publications

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“…There appears to be a transition period before the first full-length cycle, lasting 1-4 weeks in most ewes, and during which the oestradiol negative feedback mechanism can change from a closed-loop to an open-loop mode of operation (Legan & Karsch, 1979). The foregoing data also suggest that the decrease in response to oestradiol negative feedback is not always sustained, but rather may fluctuate between a closed-and open-loop mode one or more times before remaining 'open' throughout the breeding season.…”

Section: Methodsmentioning

confidence: 99%

Changes in LH pulse frequency and serum progesterone concentrations during the transition to breeding season in ewes

I’Anson

Legan²

1988

Reproduction

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Summary. To characterize the changes in LH pulse frequency during the transition to breeding season, LH pulse patterns and serum progesterone profiles were determined in 8 intact ewes from mid-anoestrus to the early breeding season. Overall, 8 increases in LH pulse frequency were observed and these were restricted to 5 ewes. Of the 8 increases, 7 occurred during the 4 weeks before the first cycle, 5 of them within 1 week after a pulse frequency typical of anoestrus (0\p=n-\2per 8 h). Six of them occurred less than 1 week before either a full-length luteal phase (n = 2) or a 1\p=n-\3-day increment in progesterone (n = 4). Seven of these brief progesterone increases were observed in 6 ewes, 5 of them immediately preceding the first full-length luteal phase. These results are consistent with the hypothesis that the seasonal decrease in response to oestradiol negative feedback at the beginning of the breeding season causes an increase in GnRH, and thereby LH pulse frequency. In addition, they demonstrate that the first increase in tonic LH secretion occurs in < 1 week and, in most ewes, initiates either the first fulllength cycle or a transient increase in progesterone, the latter occurring more often.

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Section: Methodsmentioning

confidence: 99%

Changes in LH pulse frequency and serum progesterone concentrations during the transition to breeding season in ewes

I’Anson

Legan²

1988

Reproduction

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“…It has been established that in the ewe, the breeding season is associated with enhanced pituitary activity as reflected by frequent and pulsatile LH release and an overall increase in the concentration of LH in the peripheral circulation (Baird et al 1976;Legan et al 1977). With the onset of the anoestrous season, this frequency with which LH pulses are secreted from the pituitary decreases and there is a concomitant fall in the peripheral plasma LH concentration (Scaramuzzi and Baird 1977;Legan and Karsch 1979). This change in the pattern of UI secretion has been attributed to an increase in sensitivity of the hypothalamicpituitary axis to negative feedback effects of E z (Legan and Karsch 1979).…”

Section: Series IImentioning

confidence: 99%

“…With the onset of the anoestrous season, this frequency with which LH pulses are secreted from the pituitary decreases and there is a concomitant fall in the peripheral plasma LH concentration (Scaramuzzi and Baird 1977;Legan and Karsch 1979). This change in the pattern of UI secretion has been attributed to an increase in sensitivity of the hypothalamicpituitary axis to negative feedback effects of E z (Legan and Karsch 1979).…”

Section: Series IImentioning

confidence: 99%

Hypothalamic-Pituitary Function in Normal Ewes and Ewes which Grazed Oestrogenic Subterranean Clover for Several Years

Wa¹,

Adams²,

Rd³

et al. 1981

Aust. Jnl. Of Bio. Sci.

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Plasma luteinizing hormone (LH) concentrations were measured in normal Merino ewes and in Merino ewes with lowered fertility which had resulted from prolonged grazing of Trifolium subterraneum L. cv. Dinninup. During the anoestrous season, LH was measured at frequent intervals before and following administration of oestradiol-17 P or gonadotrophin releasing hormone (GnRH), and around the time of spontaneous oestrus.All ewes responded to GnRH treatment and there were no differences between the two groups in terms of the amount of LH released or the time to reach maximal plasma hormone concentration.Comparable proportions of ewes from both groups responded to oestradiol-17 p treatment with LH levels showing initially a negative feedback response (i.e. suppression to approximately 80 % of pretreatment levels) followed by positive feedback response. During the positive feedback phase, the quantity of LH which was released was comparable in both groups; however, the time interval over which this hormone was secreted was significantly shorter for clover-infertile ewes (P < O' 05).During the anoestrous season the mean LH level was lower in normal ewes than in clover-infertile ewes [0·45±0·41 (n = 31) versus 0·82±0·73 (n = 48) ng/ml, P < 0'01]. In the oestrous season the mean LH level for normal ewes rose to 1·23 ± O· 65 ng/ml (n = 76) but there was no change for the clover-infertile ewes (0·77 ± O· 59 ng/ml, n = 73). These experiments indicate that in the clover-infertile ewe LH concentration remains static throughout the breeding and non-breeding seasons but the results suggest that this altered endocrine status is not due to any differing level of sensitivity to negative feedback effects of oestradiol-17 p as compared with normal ewes.

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“…The seasonality of reproductive activity in the ewe is controlled by environmental factors [53], mainly photoperiod, which organize the annual sexual cycle [50]. Ewes [38,44]. It should also be mentioned that a direct, non-oestradiol-dependent effect of photoperiod on LHRH/LH pulsatility has been clearly demonstrated, but up to now, it appears quantitatively minor [58,92].…”

Section: Introductionmentioning

confidence: 98%

“…(3, has been recently identified using molecular biological methods [40]. Using an in situ hybridization probe, Shughrue et al [70] 44 (1991) 476-482. …”

Section: Introductionmentioning

confidence: 99%