The Horse: An Unexpected Animal Model for (Unexpected) Neuroendocrinology

Neuroendocrine research on the horse has mostly focused on the neuroendocrine control of reproduction and more specifically on the control of ovulation, which represents an economically important matter in horse breeding. Knowing the timing of ovulation is economically important in the equine industry, as it can be used to improve reproductive success during management of brood mares (i.e. insemination close to the time of ovulation) particularly when semen is valuable. Mares display a seasonal (long days) mono‐ovulatory polyestrus cycle. During the reproductive season in spring and summer, the estrus cycle length is about 22 days, with 5‐7 days of estrus. There are significant differences in estrus length between individuals and also within a given animal, according to the timing within the reproductive season. Ovarian cyclicity is controlled by the pituitary gonadotrophins: Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH). LH and FSH are themselves under the control of the hypothalamic neurohormone: Gonadotropin Releasing Hormone (GnRH). One of the most successful research approaches in this field relies on a non‐invasive methodology allowing the collection of blood from the pituitary efferent vein and the measurement the pulsatile release GnRH, LH and FSH. During the luteal phase, when the progesterone concentration is high, GnRH pulse frequency is low (mean inter‐pulse interval (IPI) 120min). Whereas during the pre‐ovulatory period, when estradiol concentration is high, GnRH pulses are high (IPI 30 min). In contrast to other mammals studied, there is no obvious GnRH surge preceding ovulation but an increase in pulse frequency, and the LH peak lasts for approximately 6 days, with the concentration peak occurring approximately 24h post‐ovulation. In sheep, primates and rodents, GnRH secretion is stimulated by a neuropeptide, Kisspeptin, which is also responsible for triggering the GnRH and LH preovulatory peaks. In the mare, despite stimulating LH and FSH secretions, Kisspeptin does not induce ovulation.