Abrupt decrease in daylength and short-term changes in the plasma concentrations of FSH, LH and prolactin in anoestrous ewes

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Abstract
The plasma concentrations of FSH [follicle stimulating hormone, follitropin], LH [lutropin] and prolactin in ewes were measured at frequent intervals during 24 h periods in anestrus at different times from the onset of the breeding season. Ewes kept under natural daylength conditions were compared with ewes in which the onset of the breeding season was advanced by exposure to constant short days (8L h light:16 D h dark). The FSH concentrations during mid-anestrus did not vary during the day and there was no effect of short days or any changes which could be associated with the onset of ovulation. In all of the ewes pulsatile LH releases were observed on each sampling occasion. During mid-anestrus the occurrence of LH pulses varied between 1 and 3/day and 1 of the pulses appeared to be synchronized with dawn. Exposure to short days did not affect the frequency of LH release. In both groups of ewes an increased frequency of LH pulses was observed in the period 12-14 days before the 1st ovulation but this was associated with a decrease in the magnitude of each pulse. Prolactin concentrations were raised during anestrus and tended to be higher during the hours of darkness and in the early morning. Exposure to short days for 3 wk abolished these diurnal changes and reduced the concentrations to non-detectable amounts.

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