Neuroendocrine effects of progesterone

Progesterone (P) is secreted by the corpus luteum under the control of gona dotropin releasing hormone (GnRH)/luteinizing hormone (LH). Progesterone (P ) is essential for reproduction because: (1) it induces in the endometrium the transcription of specific genes involved in the implantation of the bla stocyst, (2) it modulates GnRH/LH secretion by decreasing GnRH pulse freque ncy, which in turn enriches the gonadotroph cells in FSH and avoids a secon d LH surge. Using the ewe as a model, we investigated the immediate GnRH an d LH responses to acute changes of circulating P levels. Our results show t hat P changes cause dramatic modifications in GnRH pulse frequency: P remov al induces an acceleration of the pulse generator, while P administration s lows the pulse frequency. LH secretion was modified in parallel to the chan ges in GnRH. Other experiments proved that these neuroendocrine effects of P an mediated by P itself, not by its hydroxylated metabolites, and occur a t the level of P receptors. Finally, these effects require priming by estra diol. Additionally, in the final stage of the follicular phase, P plays a r ole in the triggering of the LH surge. This has been shown in rodents, non- human primates, and in women. Such a phenomenon is not observed in ewes, al though in these species luteal P modulates the amplitude of the estradiol-i nduced LH surge. (C) 2000 Elsevier Science Inc. All rights reserved.