Kisspeptin and the preovulatory Gonadotrophin-Releasing Hormone/Luteinising Hormone surge in the ewe: basic aspects and potential applications in the control of ovulation
Résumé
The identification of the neural mechanisms controlling ovulation in mammals has long been a 'holy grail' over recent decades, although the recent discovery of the kisspeptin systems has totally changed our views on this subject. Kisspeptin cells are the major link between gonadal steroids and gonadotrophin-releasing hormone (GnRH) neurones. In the female rodent, kisspeptin cells of the preoptic area are involved in the positive-feedback action of oestrogen on GnRH secretion, although the picture appears more complicated in the ewe. As in rodents, activation of preoptic kisspeptin neurones accompanies the GnRH surge in the ewe but an active role for arcuate kisspeptin neurones has also been proposed. Experimentally, kisspeptin is able to restore reproductive function when the hypothalamic-hypophyseal ovarian axis is quiescent. For example, i.v. infusion of a low dose of peptide in anoestrous ewes induces an immediate and sustained release of gonadotrophin, which subsides and then provokes a luteinising hormone (LH) surge a few hours later. This pharmacological intervention induces the same hormonal changes normally observed during the follicular phase of the oestrous cycle, including the secretion of oestrogen and its negative- and positive-feedback actions on the secretion of LH and follicle-stimulating hormone. Accordingly, a high percentage of kisspeptin-infused animals ovulated. Although the multiple facets of how the kisspeptin systems modulate GnRH secretion are not totally understood, the demonstration that exogenous kisspeptin administration can induce ovulation in anovulatory animals paves the way for future therapeutic applications aiming to control reproduction.