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

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Abstract
Nine ewes of each of 2 breeds, dorset Horn (long breeding season) and Welsh Mountain (short breeding season), were ovariectomized after insertion of s.c. implants containing estradiol-17.beta.. A further 9 ewes of each breed were left entire. All of the ewes were placed in an artificial photoperiod of 8L:16D [light/dark cycle] on 12 December 1980. After 5 weeks half of the ewes of each breed and physiological state were abruptly changed into a long-day photoperiod (16L:8D) while the rest remained in short days. The time of onset of seasonal anestrus in entire animals was significantly advanced in ewes changed to long days, with the end of the breeding season coming at the normal time of year in ewes maintained in short days. These differences in estrous cycle activity were reflected by differences in the time at which LH [luteinizing hormone] concentrations in ovariectomized, estrogen-treated ewes on the 2 light treatments fell to basal values. Prolactin concentrations showed an immediate and sustained rise in ewes changed to long days, but remained low in ewes maintained in short days. Since the onset of seasonal anestrus occurred in the absence of high levels of prolactin (in short-day ewes), prolactin is not the major vehicle by which seasonal changes in hypothalamic responsiveness to the negative feedback effects of estradiol are produced. Apparently anoestrus may be due to photorefractoriness.

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