Effects of steroid administration on pituitary luteinizing hormone and follicle-stimulating hormone in ovariectomized pony mares in the early spring: pituitary responsiveness to gonadotropin-releasing hormone and pituitary gonadotropin content.

These experiments tested the hypothesis that administration of steroid hormones to ovariectomized (OVX) mares during the vernal transition to the breeding season would influence LH and FSH secretion. Circulating gonadotropin concentrations, response to exogenous GnRH, and pituitary gonadotropin content were monitored. Experiments 1 and 2 were conducted, beginning 10 March, and 3 February, respectively, utilizing a total of 30 long-term OVX pony mares. In experiment 1, mares were administered vehicle (n = 5) or estradiol-17 beta (E2, n = 5, 5 mg/3 ml sesame oil), twice daily for 16 days. Blood samples were collected daily for assessment of circulating LH and FSH concentrations. On Day 10 of treatment, 400 micrograms GnRH were administered to all mares. LH increased significantly over days of treatment in the estradiol-treated group, but pituitary response to GnRH tended to be less than in control mares. Circulating FSH tended to decline over days of treatment in estradiol-treated mares, and the pituitary response to GnRH was significantly reduced. Pituitary LH, but not FSH, was increased on Day 16 of treatment with estradiol. In experiment 2, 20 OVX mares received, twice daily, vehicle (n = 5), E2, n = 5; 5 mg), progesterone (P4, n = 5; 100 mg), or progesterone plus estradiol (P4/E2, n = 5; 100 + 5 mg). Treatment continued for 14 days. GnRH (100 micrograms) challenges were administered on Days 6 and 13 of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of constant light exposure on circulating gonadotrophin levels and hypothalamic gonadotrophin-releasing hormone (GnRH) content in the ovariectomized pony mare.

Melatonin is thought to play a role in relaying photic information to the central nervous system as part of the seasonal reproductive cycle of the mare. However, the mechanisms by which melatonin may act are unknown. Therefore, this study was designed to determine whether exposure to constant light would, by reducing circulating melatonin concentrations, have any effect on hypothalamic gonadotrophin-releasing hormone (GnRH) content and circulating levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Blood samples were collected for 12 h at 15-min intervals from 8 ovariectomized (OVX) pony mares under ambient light conditions (12 h light, 12 h dark, October 6). Animals were then placed under representative ambient light (12 h light, 12 h dark; control, n = 4) or constant light (24 h light, 0 h dark; treatment, n = 4) in light-controlled rooms. Blood samples were collected daily and on experimental Days 14, 21 and 28 samples were collected at 15-min intervals for 12 h for analysis of circulating LH and melatonin (Bleed 2, 3 and 4, respectively). All animals were killed on Day 28 (following Bleed 4) and the hypothalami were collected. Mares exposed to constant light had significantly higher (P less than 0.05) LH concentrations in daily blood samples and showed significantly (P less than 0.05) higher LH concentrations during frequent sampling periods on Days 14 and 21 (Bleeds 2 and 3) compared with control mares. FSH did not differ significantly among groups in the daily samples. GnRH content was 1.5994 +/- 0.325 and 0.9457 +/- 0.193 pg/mg protein (treatment and control respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Restricted conceptus mobility results in failure of pregnancy maintenance in mares.

Cycling pony mares were bred and used to test the effect of restricted conceptus mobility on luteal maintenance (i.e. maternal recognition of pregnancy). In Experiment 1, uterine horns were ligated to restrict conceptus mobility to one uterine horn, Group 1; one horn plus the uterine body, Group 2; or one horn, the body and approximately 80% of the second horn, Group 3. Pregnancies were monitored with real-time ultrasonography. Four of five mares in Group 1 and two of four mares in Group 2 returned to estrus (Day 16.0 +/- 1.9 and 14.5 +/- 0.7, respectively) and subsequently lost the embryonic vesicles (Day 17.2 +/- 1.2 and 15.7 +/- 0.7, respectively). None of the four mares in Group 3 lost the vesicles. There was a significant effect of the interaction of treatment (amount of uterus available to the conceptus) and day on plasma progesterone (P) concentration (p less than 0.005). In Experiment 2, conceptus mobility was restricted to one uterine horn in two groups of mares, of which the second was treated with the synthetic progestin, Regu-Mate (allyl trenbolone). In the first group, each of three mares lost the vesicle (Day 17.3 +/- 4.3). In the second group, four of five mares maintained the pregnancies, indicating that pregnancy failure was due to the effects of declining P. These data indicate that restricted conceptus mobility results in luteolysis in the mare, and that the subsequent decline in P leads to embryonic death.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of progesterone and progesterone + oestrogen on total and specific uterine proteins in pony mares.

Eight ovariectomized pony mares were used to test the effect of various doses of progesterone (0, 50, 150, 450 mg/day, in oil, i.m., for 10 days) on progesterone and LH in the peripheral circulation, and on total protein and uteroferrin in uterine secretions. Progesterone increased uteroferrin, but there were no differences amongst doses of progesterone. Progesterone treatment decreased LH, and tended to increase total protein. Eighteen ovariectomized mares were given vehicle, oestradiol (10 mg/day, in oil, i.m.), progesterone or progesterone + oestradiol for 28 days. Both the last two steroid treatments significantly increased total protein and uteroferrin in the uterine secretions, compared to vehicle or oestradiol alone. Progesterone + oestradiol increased uteroferrin, but not total protein compared to progesterone. Nine ovariectomized progesterone-primed mares were used to compare systemic and intraluminal administration of oestradiol. There were no differences between routes of administration of oestradiol. In conclusion, administration of progesterone increased total protein and uteroferrin in uterine secretions, and progesterone + oestradiol increased them further.

Effects of pituitary stalk transection on endocrine function in Pony mares.

The pituitary stalk was transected in 10 Pony mares by a surgical approach that involved dorsal reflection of the brain and micro-dissection from the ventro-lateral aspect of the pituitary. Diabetes insipidus was the most immediate and marked result, requiring extensive electrolyte and antidiuretic therapy for approximately 48 h after operation. Fluid stasis then developed and no further supportive measures were necessary. Endocrine challenge tests with GnRH and TRH before and after stalk transection indicated a loss of responsiveness (GnRH) or suppressed responsiveness (TRH) after the operation. This technique permits isolation of the pituitary from its hypothalmic releasing and/or inhibiting hormones and therefore permits more refined study of the hypothalamic-pituitary axis.