Hypothalamic-Pituitary Function in Normal Ewes and Ewes which Grazed Oestrogenic Subterranean Clover for Several Years

Abstract

Plasma luteinizing hormone (LH) concentrations were measured in normal Merino ewes and in Merino ewes with lowered fertility which had resulted from prolonged grazing of Trifolium subterraneum L. cv. Dinninup. During the anoestrous season, LH was measured at frequent intervals before and following administration of oestradiol-17 P or gonadotrophin releasing hormone (GnRH), and around the time of spontaneous oestrus.All ewes responded to GnRH treatment and there were no differences between the two groups in terms of the amount of LH released or the time to reach maximal plasma hormone concentration.Comparable proportions of ewes from both groups responded to oestradiol-17 p treatment with LH levels showing initially a negative feedback response (i.e. suppression to approximately 80 % of pretreatment levels) followed by positive feedback response. During the positive feedback phase, the quantity of LH which was released was comparable in both groups; however, the time interval over which this hormone was secreted was significantly shorter for clover-infertile ewes (P < O' 05).During the anoestrous season the mean LH level was lower in normal ewes than in clover-infertile ewes [0·45±0·41 (n = 31) versus 0·82±0·73 (n = 48) ng/ml, P < 0'01]. In the oestrous season the mean LH level for normal ewes rose to 1·23 ± O· 65 ng/ml (n = 76) but there was no change for the clover-infertile ewes (0·77 ± O· 59 ng/ml, n = 73). These experiments indicate that in the clover-infertile ewe LH concentration remains static throughout the breeding and non-breeding seasons but the results suggest that this altered endocrine status is not due to any differing level of sensitivity to negative feedback effects of oestradiol-17 p as compared with normal ewes.

Effects of Oestradiol on Plasma Concentrations of Luteinizing Hormone in Ovariectomized Ewes with Clover Disease

“…This confirms the findings of Findlay et al (1973), and suggests that Rodgers et al (1980) and Charnley et al (1981) did not find this effect because they used entire ewes. Basal levels of LH are higher in entire affected ewes than in controls during anoestrus (Rodgers et al 1980;Charnley et al 1981), but not in ovariectomized ewes. The higher basal values indicate an altered physiological state in the entire affected ewes, which may explain why they give a greater LH response to oestradiol than occurs when both groups have a similar basal level of LH.…”

“…Failure of positive feedback on LH is the primary cause of the permanent infertility in the 'androgenized' rat. In contrast, the entire clover-affected ewe can fully compensate for the defects in both behavioural responsiveness and the sensitivity of positive feedback, so that the duration of oestrus is normal in affected ewes (Lightfoot et al 1974) and the surge of LH occurs normally (Rodgers et al 1980;Charnley et al 1981). As discussed above, the method by which the entire ewe compensates is unclear.…”

“…The initial report by Findlay et al in 1973 suggested that affected ewes were less able to produce a surge of LH, but in subsequent studies (Rodgers et al 1980;Charnley et al 1981) this difference did not occur.…”

Effects of intracerebroventricular infusion of genistein on gonadotrophin subunit mRNA and immunoreactivity of gonadotrophins and oestrogen receptor-α in the pituitary cells of the anoestrous ewe

“…Anderson et al (2002) have demonstrated that the seasonal decline in LH pulse frequency depends on the presence of thyroid hormones occurring in the absence of steroids during spring/ summer in ewes. Chamley et al (1981Chamley et al ( , 1985 reported that during the anoestrous season both the overall mean LH concentration and the mean basal LH concentration were significantly higher in clover-infertile ewes than in control ones. These authors concluded that the increase in LH pulse frequency after ingestion of phytoestrogens might occur due to changes in the neuroendocrine regulation of LH secretion.…”

Effects of prolonged exposure of ewes to oestrogenic pasture 2. Occurrence of abnormalities of the external genitalia and altered mating performance

“…It is generally accepted that plant oestrogens interfere with the hypothalamic-pituitary-ovarian axis in sheep grazing oestrogenic pasture (e.g., Charnley et al 1981). In a review of clover disease, Marshall (1973) listed a number of physiological conditions in sheep associated with pasture oestrogens.…”