Seasonal Variation in the Oestrogen Induced Lh Discharge of Ovariectomized Finnish Landrace and Scottish Blackface Ewes

Abstract

The discharge of LH following the injection of 50 j i g oestradiol benzoate was studied in i2 ovariectomized Blackface and 12 ovariectomized Finn ewes on 6 occasions (November, January, March, May, July and November). Both breeds were more sensitive to the oestradiol during the breeding season than during anoestrus. The Finn ewes were less sensitive than the Blackfaces as judged by the proportion of ewes discharging LH, the maximum plasma concentration of LH attained, the interval to the discharge of LH, and the absolute reduction in LH concentration immediately following injection. The failure of oestrogen to trigger the preovulatory discharge of LH may be one of the components of seasonal anoestrus and it is suggested that some of the variation in the ovulation rate of sheep may arise in part from differences in sensitivity to feedback from developing follicles.

Seasonal variation in oestrus, the secretion of oestrogen and progesterone and LH levels before ovulation in the ewe

“…In castrated ewes, the interval between injection of oestradiol-17ß benzoate and the beginning of the LH discharge is independent of the time of year (Land et al, 1976), and the earlier discharge of LH in November may therefore simply be related to the earlier rise in oestrogen secretion. The lack of a preovulatory rise of oestradiol-17ß and LH in June presumably represents a failure of the hypothalamic-pituitary system to respond to withdrawal of luteal progesterone by secretion of FSH and LH in amounts sufficient to support the final matura¬ tion of the Graafian follicle.…”

“…It has been suggested that the higher ovulation rates in certain breeds of sheep, e.g. Finnish Landrace, are associated with a relative insensitivity of the hypothalamic-pituitary system to the negative feedback effects of oestradiol (Land, Wheeler & Carr, 1976).…”

Plasma FSH, LH, the positive feedback of oestrogen, ovulation and luteal function in the ewe given bromocriptine to suppress prolactin during seasonal anoestrus

“…As shown in (Land et al, 1976) and negative feedback may be increased (Legan & Karsch, 1979) (Roberts, Carlson & McCracken, 1976), and prolactin concentrations rise spectacularly during anoestrus (Ravault, 1976). Both of these responses may reflect a general change in spontaneous hypothalamic activity rather than a specific effect dependent upon steroids as discussed by Land & Carr (1979).…”

“…Seasonal anoestrus in sheep has been postulated to arise in part from a reduced sensitivity of the positive feedback response to oestrogen (Land, Wheeler & Carr, 1976). Since hyperprolactinaemia has also been reported to block the oestrogen-induced release of LH (Kann, Martinet & Schirar, 1978), and hyperprolactinaemia is a characteristic of anoestrous sheep (Walton, McNeilly, McNeilly & Cunningham, 1977), we have investigated the effects of reducing the prolactin concentrations in sheep during anoestrus with the dopamine agonist, bromocriptine.…”

Improvement in gonadal function through modification of negative feed-back

“…The observation that LH release in ewes of high prolificacy was less sensitive to the beed-back effects of oestradiol (Land, Wheeler and Carr, 1976) (Findlay and Cumming, 1977 …”