Anterior Pituitary Responsiveness of the Cyclic and Seasonally Anovulatory Mare to Continuous Infusions of Gonadotropin-Releasing Hormone

In Experiment 1, 12 cyclic mares were assigned randomly to one of two groups (n = 6/group): 1) Control, saline; and 2) GnRH, 100 mu g/h. Between 3 and 6 d after ovulation (Day 0), Alzet osmotic minipumps (Model 2ML1) containing saline or GnRH were placed subcutaneous and connected to a jugular infusion catheter. Five-min samples were collected from the intercavernous sinus (ICS) of 10/12 mares (5/group) during 8 h on Day 4, followed by an additional 6-h intensive sampling period 36 h after induced luteal regression (Day 6). Treatment with GnRH markedly increased (P < 0.01) secretion of LH during both luteal and follicular phases. During the luteal phase, treatment with GnRH eliminated the very large, intermittent secretory episodes of LH characteristic of controls and produced frequent episodes of LH release of short duration. In Experiment 2, 12 anovulatory mares and 3 mares with some residual follicular activity (n = 15) were used during the fall (December 5 to 20) and winter (February 15 to 29) seasons. Mares were assigned randomly to: 1) Control, 2) GnRH-20; continuous infusion of GnRH at 20 mu g/h, or 3) GnRH-100; continuous infusion of GnRH at 100 mu g/h. Treatments were administered subcutaneously for 14 d using Alzet minipumps. Both the 20- and 100-mu g/h treatments increased (P less than 0.01) mean circulating concentrations of LH compared to controls before the winter solstice, but mares did not respond to the GnRH- 20 dose after the winter solstice. GnRH-100 caused a seasonally-independent increase (P less than 0.0001) in follicle size and ovulation frequency compared to controls The equine gonadotrope responded to continuous administration of high-dose GnRH during both ovulatory and anovulatory seasons, but was less responsive late compared to early in the anovulatory season.