EFFECTS OF ADMINISTRATION OF N-METHYL-D,L-ASPARTATE (NMA) ON GONADOTROPIN-SECRETION IN UNTREATED AND STEROID-TREATED OVARIECTOMIZED MARES DURING THE BREEDING-SEASON AND IN INTACT AND OVARIECTOMIZED MARES DURING ANESTRUS

The purpose of this experiment was to investigate whether N-methyl-D,L -aspartate stimulated gonadotropin secretion in mares and to determine the response in two experimental paradigms where gonadotropin secreti on is low or elevated. In Experiment 1, conducted during the breeding season (summer), eight long-term ovariectomized mares were treated dai ly for 21 d with progesterone plus estradiol (n = 4) or oil vehicle. B eginning on Day 14, each mare received, in a randomized design on alte rnate days, an intravenous injection of either 0, 0.5, 1.0, or 5.0 mg/ kg NMA. Treatment with NMA was not accompanied by a change in gonadotr opin release in vehicle-treated ovariectomized mares. In steroid-treat ed mares, gonadotropin values were suppressed compared with vehicle co ntrols, but NMA only induced a significant increase in follicle-stimul ating hormone (FSH) and only at the highest dose. In Experiment 2, the effect of the administration of NMA was compared in intact anestrous mares (n = 4) and long-term ovariectomized mares (n = 4) during the no nbreeding season. At the highest doses (1.0 and 5.0 mg/kg), NMA induce d a significant increase in luteinizing hormone (LH) and FSH in intact mares. In contrast, pretreatment concentrations of LH and FSH were hi gher in long-term ovariectomized mares and NMA only increased LH, but not FSH, at the highest dose. The results demonstrate that the adminis tration of NMA stimulates the release of LH and FSH in mares. During a nestrus, ovarian intact mares appear more responsive than long-term ov ariectomized mares. At high doses, NMA may induce differential release of the gonadotropins in long-term ovariectomized mares.