Seasonal changes in LH and prolactin concentrations in ewes of two breeds.

The current experiment was designed to investigate whether changes in prolactin concentrations might be involved in the seasonal change in responsiveness of the hypothalamic-pituitary axis to the negative feedback effects of oestradiol in the ewe. Twelve Dorset Horn ewes (long breeding season) and 12 Welsh Mountain ewes (short breeding season) were ovariectomized after insertion of empty subcutaneous implants (N = 2 per breed), or ones containing oestradiol-17 beta (N = 10 per breed). Intact ewes, 10 of each breed, were used to monitor breeding activity. Mean weekly plasma LH concentrations in oestrogen-treated ovariectomized ewes of both breeds were elevated over the period corresponding to their respective breeding seasons, but were basal during anoestrus. The changes in prolactin concentrations, however, followed the same temporal pattern in both breeds and were approximately parallel to changes in daylength. These data support the suggestion of a seasonal change in negative feedback responses to oestradiol which corresponds to changes in breeding activity, but any involvement of prolactin in this response remains questionable.