Control of gonadotrophin release in Scottish Blackface and Finnish Landrace ewes during seasonal anoestrus

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Abstract
The patterns of LH [luteinzing hormone] and FSH secretion were measured in 4 experimental groups of Finnish Landrace and Scottish Blackface ewes: long-term (18 mo.) ovariectomized ewes (Group 1), long-term ovariectomized ewes with an estradiol implant, which has been shown to produce peripheral levels of .apprx. 5 pg/ml (Group 2), long-term ovariectomized ewes with an estradiol implant for 18 mo. which was subsequently removed (surgery on Day 0) (Group 3) and short-term ovariectomized ewes (surgery on Day 0) (Group 4). LH and FSH concentrations were monitored in all groups at approximately weekly intervals, before and after Day 0. Finnish Landrace ewes in Groups 1, 2 and 3 had significantly higher mean FSH concentrations than did Scottish Blackface ewes (P < 0.01). FSH and LH concentrations increased significantly in Groups 3 and 4, but values in Group 4 were significantly lower (P < 0.01) than those in Group 1 ewes even up to 30 days after ovariectomy. In Group 3, LH concentrations increased to levels similar to those in Group 1. The pattern of LH release was, however, significantly different, with a lower LH pulse frequency (P < 0.05), but higher pulse amplitude (P < 0.05). This difference was maintained at least until 28 days after implant removal. It is suggested that removal of negative feedback by ovariectomy demonstrates an underlying breed difference in the pattern of FSH secretion and that ovarian factors other than estradiol are also involved in the negative-feedback control of hypothalamic/pituitary gland function. Negative-feedback effects can be maintained for long periods, at least 28 days, after ovariectomy or estradiol implant removal.

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