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

[Mechanisms of action of various neuropeptides on anterior pituitary cells. Electrophysiologic and microspectrofluorometric approach].

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The gonadotropin-releasing hormone associated peptide reduces calcium entry in prolactin-secreting cells

The precursor molecule to the GnRH contains a peptide named GnRH-associated peptide (GAP) with PRL-inhibiting properties. In this work, we have studied the electrophysiological properties and responses to GAP of three different types of PRL-secreting cells: 1) the rat tumor cell line GH3, 2) normal rat pituitary cells in primary culture, and 3) human PRL-secreting adenoma cells. Using different but complementary techniques we show that GAP reduces intracellular Ca++ levels, [Ca++]i, and inhibits Ca++ transients in these cells. This reduction of [Ca++]i results from coordinate actions of GAP on K+ and Ca++ conductances and may explain the inhibitory effect of GAP on hormonal secretion by PRL-secreting cells