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Role of an Equine Homologue of Gonadotropin-Inhibiting Hormone in Controlling Sectretion of Luteinizing Hormone in the MarePrezotto, Ligia Dias 2012 May 1900 (has links)
Four experiments were conducted to test the hypothesis that RF-amide related peptide 3 (RFRP3) negatively regulate the secretion of LH in mares. In Exp. 1, mares received native gonadotropin-releasing hormone (GnRH) continuously at a rate of 20 microgram/h, delivered subcutaneously using Alzet osmotic pumps during the luteal phase of the estrous cycle. Mares were treated with i.v. bolus injections of 0, 500 and 1,000 microgram eRFRP3 on days 4, 6 and 8 of cycle. Mean concentrations of LH in the peripheral circulation averaged 1.2 +/- 0.2 ng/mL and did not differ among groups before or following RFRP3 treatment. In Exp. 2, pituitary venous effluent was sampled for characterization of episodic release of LH. Mares received either saline or eRFRP3 (250 microgram) i.v. every 10 min for 6 h beginning 2 h after onset of sampling. At hour 6, each mare was challenged with 1 mg GnRH. Neither mean ICS concentrations of LH (1.3 +/- 0.2 ng/ml), nor frequency (3.6 +/- 0.55 episodes/h), amplitude (0.2 +/- 0.03 ng/ml), or duration (36.3 +/- 3.5 min) of individual secretory episodes, differed between groups before or after eRFRP3 treatment. Area under the GnRH-induced LH curve (arbitrary units) also did not differ between control and RFRP3 treated mares (175.9 +/- 11.4 vs. 192.6 +/- 10.6). In Exp. 3, winter anovulatory mares (n=6) were treated continuously for 7 d with GnRH (100 microgram/h) to stimulate synthesis of LH and increase circulating concentrations of LH to values similar to the breeding season. The ICS was catheterized for blood sampling and mares were treated with saline or RFRP3 (5 mg) in a replicated Latin square design. Treatment with RFRP3 failed to alter ICS mean concentration of LH (0.95 +/- .03 ng/ml). Finally in Exp. 4, mares in the follicular phase of the estrous cycle were assigned randomly to receive either saline (n=3) or 10 microgram/kg BW of oRFRP3 (n=3) in a single injection. No effect on mean concentration of LH was observed. In contrast to observations in birds and other mammals, results of the current experiments fail to provide evidence for functional activity of eRFRP3 or oRFRP3 in regulating LH release in the mare.
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Interactive Roles of Gonadotropin-Releasing Hormone and RF-Amide Related Peptide 3 in Adenohypophyseal Physiology and Reporduction in the MareThorson, Jennifer Frances 02 October 2013 (has links)
The seasonal termination of ovarian cycles in mares initiated near the time of the autumnal equinox is a significant managerial issue for horse breeders world-wide. Studies presented herein had two over-arching aims. In Aim I, objectives were to develop the principals needed to apply gonadotropin-releasing hormone (GnRH) therapeutics for routinely establishing pregnancies in the winter anovulatory mare. We first tested the hypothesis that continuous administration of native GnRH, beginning in either early February or March, would induce ovulation without reversion to an anovulatory state following treatment withdrawal. Continuous 28-d treatments elevated circulating luteinizing hormone (LH) and stimulated spontaneous ovulation much earlier than controls. However, mares treated only in February ceased ovarian cycles at termination of treatment. In contrast, mares administered GnRH in March continued to exhibit estrous cycles. Thus, we concluded that GnRH treatment must continue through March to ensure continued escape from winter anovulation. We then tested the hypothesis that the Julian day of conception could be accelerated in winter anovulatory mares treated continuously with native GnRH for 56 d beginning on February 1. Indeed, GnRH treatment caused a marked increase in the frequency of pregnancy compared to controls. Data illustrated that continuous administration of native GnRH is a practical and highly efficient option for managing seasonal anovulation. In Aim II, we examined hypothalamic distribution, adenohyphyseal receptor gene expression, and ligand functionality of RFRP3 in the mare during the breeding and non-breeding seasons.
Hypothalamic RFRP3 mRNA was detected in the mare; however, neither hypothalamic expression of RFRP3 nor its anterior pituitary receptor differed between reproductive states. We then used equine adenohypophyseal cell culture to test the hypothesis that RFRP3 reduces the responsiveness of the equine gonadotrope to GnRH. Addition of RFRP3 to cell culture failed to counter the effects of GnRH. Finally, the effects of a RFRP3 receptor-signaling antagonist (RF9) were examined in winter anovulatory mares. A robust increase in circulating concentrations of LH relative to controls was observed in response to RF9 treatments, but treatments had no effect on adenohypophyseal responsiveness to GnRH. Data provide indirect evidence that antagonism of the RFRP3 system by RF9 may be at the GnRH neuronal level.
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