Noradrenaline concentrations in the hypothalamus of anoestrus ewes following the ram-induced luteinizing hormone release

The Two Populations of Kisspeptin Neurons Are Involved in the Ram-Induced LH Pulsatile Secretion and LH Surge in Anestrous Ewes

Exposure to a ram during spring stimulates luteinizing hormone (LH) secretion and can induce ovulation in sexually quiescent ewes ("ram effect"). Kisspeptin (Kiss) present in the arcuate nucleus (ARC) and the preoptic area (POA) is a potent stimulators of LH secretion. Our aim was to investigate whether Kiss neurons mediate the increase in LH secretion during the ram effect. With double immunofluorescent detection, we identified Kiss neurons (Kiss IR) activated (Fos IR) by exposure to a ram for 2 hours (M2) or 12 hours (M12) or to ewes for 2 hours (C). The density of cells Kiss + Fos IR and the proportion of Kiss IR cells that were also Fos IR cells were higher in M2 and M12 than in C in ARC (P < 0.002) and POA (P < 0.02). In ARC, these parameters were also higher in M12 than in M2 (P < 0.02 and P < 0.05). Kiss antagonist (P234 10-6M) administered by retrodialysis in POA for 3 hours at the time of introduction of the ram reduced the amplitude of the male-induced increase in LH concentration compared with solvent (P < 0.02). In ARC, P234 had a more limited effect (P < 0.038 1 hour after P234) but pulse frequency increased less than after solvent (P = 0.07). In contrast, Kiss antagonist (P271 10-4M) infused in ARC but not POA 6 to 18 hours after introduction of the ram prevented the LH surge in the ewe (0/6 vs 4/5 and 4/6 in C). These results suggest that both populations of Kiss neurons are involved in the ram-induced pulsatile LH secretion and in the LH surge.

A high level of male sexual activity is necessary for the activation of the medial preoptic area and the arcuate nucleus during the 'male effect' in anestrous goats

In small ungulates such as sheep or goats, the introduction of a male among a group of anovulatory females during the anestrus season leads to the reactivation of the gonadotrope axis and ovulation, a phenomenon known as the 'male effect'. In goats, our previous studies have demonstrated the importance of male sexual activity for an efficient reactivation of the gonadotrope axis assessed through ovulation and blood LH pulsatility. In the present experiment, we assessed whether the level of male sexual activity would also induce differential activation of two brain regions of key importance for the reactivation of GnRH activity, namely the medial preoptic area and the hypothalamic arcuate nucleus. In both structures, we observed a differential activation of Fos in females, depending on the level of buck sexual activity. Indeed, goats unexposed to males showed low levels of expression of Fos while those exposed to sexually inactive bucks showed an intermediate level of Fos expression. Finally, the highest level of Fos expression was found in females exposed to sexually active males. However, and contrary to our initial hypothesis, we were not able to find any specific activation of kisspeptin cells in the arcuate nucleus following the introduction of highly sexually active males. As a whole, these results demonstrate that the level of male sexual activity is a key factor to stimulate brain regions involved in the control of the gonadotrope axis in the context of the male effect in goats.

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

During spring sheep do not normally ovulate but exposure to a ram can induce ovulation. In some ewes an LH surge is induced immediately after exposure to a ram thus raising questions about the control of this precocious LH surge. Our first aim was to determine the plasma concentrations of oestradiol (E2) E2 in anoestrous ewes before and after the “ram effect” in ewes that had a “precocious” LH surge (starting within 6 hours), a “normal” surge (between 6 and 28h) and “late» surge (not detected by 56h). In another experiment we tested if a small increase in circulating E2 could induce an LH surge in anoestrus ewes. The concentration of E2 significantly was not different at the time of ram introduction among ewes with the three types of LH surge. “Precocious” LH surges were not preceded by a large increase in E2 unlike “normal” surges and small elevations of circulating E2 alone were unable to induce LH surges. These results show that the “precocious” LH surge was not the result of E2 positive feedback. Our second aim was to test if noradrenaline (NA) is involved in the LH response to the “ram effect”. Using double labelling for Fos and tyrosine hydroxylase (TH) we showed that exposure of anoestrous ewes to a ram induced a higher density of cells positive for both in the A1 nucleus and the Locus Coeruleus complex compared to unstimulated controls. Finally, the administration by retrodialysis into the preoptic area, of NA increased the proportion of ewes with an LH response to ram odor whereas treatment with the α1 antagonist Prazosin decreased the LH pulse frequency and amplitude induced by a sexually active ram. Collectively these results suggest that in anoestrous ewes NA is involved in ram-induced LH secretion as observed in other induced ovulators.