Plasma and ovarian oestradiol and the variability in the LH surge induced in ewes by the ram effect

Abstract

The proportion of anoestrous ewes ovulating after exposure to a sexually active ram is variable mainly due to whether an LH surge is induced. The aim of this study was to determine the role of oestradiol (E 2 ) in the ram-induced LH surge. In one study, we measured the plasma concentrations of E 2 in ewes of different breeds before and after the 'ram effect' and related these patterns to the presence and latency of the LH surge, while another compared ovarian responses with the 'ram effect' following exposure to rams for 2 or 12 h. In all ewes, the concentration of E 2 increased 2-4 h after rams were introduced and remained elevated for 14.5G0.86 h. The quantity of E 2 secreted before the LH surge varied among breeds as did the mean concentration of E 2 . The granulosa cells of IF ewes collected after 12 h exposure to rams secreted more E2 and progesterone and had higher levels of StAR than the 2 h group but in MV ewes there was no differences between these groups for any of these parameters. These results demonstrate that the LH surge induced by the rams is a result of increased E 2 secretion associated with increased levels of STAR in granulosa cells and that these responses varied among breeds. The results suggest that the variable occurrence of a LH surge and ovulation may be the result of variable ovarian responses to the 'ram effect' and insensitivity of the hypothalamus to the E 2 -positive feedback signal. Free French abstract: A French translation of this abstract is freely available at

The comparison of reproductive hormone receptor expressions of the sheep ovary and hormone concentrations in two Chinese breeds

“…The normal LH surge is essential for ovulation. Without this surge, sheep will not spontaneously ovulate, enter oestrus or generate a corpus luteum (Fabre-Nys et al, 2015;Muñoz et al, 2012). Inhibited LH signalling has been shown to suppress early luteal angiogenesis which is essential for normal luteal development and function (Brown, Fabre, Cognie, & Scaramuzzi, 2014).…”

The “Ram Effect”: A “Non-Classical” Mechanism for Inducing LH Surges in Sheep

“…This socio-sexual stimulation is often referred to as the “ram effect”. In most ewes this male-induced LH surge is preceded by a sustained increase in the plasma concentration of oestradiol lasting between 8 and 56 hours [ 29 ] and is similar to the reproductive neuroendocrine events seen in cyclic ewes during the breeding season [ 3 , 4 ].…”

“…The causes of these early LH surges have not been studied. In our laboratory precocious LH surges were seen in about 15% Ile-de-France and Mérinos d’Arles ewes [ 26 , 29 ] and as happens with “normal” LH surges, they induced ovulation [ 26 ]. The time interval 0–8 hours is much shorter than that observed when the LH surge is induced with exogenous oestradiol [ 6 , 9 ]; these LH surges occur too soon to be explained by the normal oestradiol-induced positive feedback mechanism.…”

The “ram effect”: new insights into neural modulation of the gonadotropic axis by male odors and socio-sexual interactions

“…However, partly because of the difficulty in measuring the very low concentrations of circulating estradiol present during anestrus there has been very little experimental support for this hypothesis (Knight et al, 1978 ; Johnson et al, 2011 ). In a recent study we showed that, in all ewes the introduction of rams is followed by an increase in the circulating concentration of estradiol (Figure 2A , Fabre-Nys et al, 2015 ). In most anestrus ewes, the LH surge induced by the “ram effect” was preceded by increases in the circulating concentration of estradiol at least three-fold above the basal concentration for 14.5 ± 0.86 h (min 6 h; max 36 h, Figure 2B ).…”

“…This variability in a highly seasonal breed, the Mouton Vendéen, is due to the low sensitivity of the ovary that releases very little amount of steroids in response to stimulation by a ram. The granulosa cells of these ewes in culture, also have a low response to in vitro stimulation by IGF-I and FSH and reduced expression of StAR (Fabre-Nys et al, 2015 ). In other breeds such as the Romane the frequency and latency of the LH surge is variable, although the quantities of estradiol secreted after the “ram effect” do not differ from those in breeds such as the Mérinos d'Arles and Ile de France that respond well to it.…”