Location: Reproduction Research2013 Annual Report
1a. Objectives (from AD-416):
1. To determine the mechanism through which secretion of luteinizing hormone is suppressed in the prepubertal gilt. 2. Determine the mechanism of inhibition of luteinizing hormone release directly from the anterior pituitary gland of the gilt. 3. Determine the mechanism through which nutrition regulates gonadotropic output of the hypothalamic-pituitary axis of the gilt.
1b. Approach (from AD-416):
On average, 20% of gilts fail to reach puberty and become pregnant. Lack of sufficient secretion of gonadotropin hormones (e.g. Luteinizing hormone; LH) from the pituitary gland is a prominent reason for delayed puberty of pigs. Mechanisms in the hypothalamus that control LH secretion in the gilt are not well understood, but nutrition is an important component. The project goal is to minimize reproductive failure of replacement gilts. The objective will be to establish the function of RFamide-related and kisspeptin peptides in the control of LH secretion of the pig and identify their role in integrating nutrition with the gonadotropic axis of the gilt. Our approach, using intracerebroventricular cannulation, will be to establish the central effects of RFamide-related peptides on secretion of LH in gilts (Aim I). We will determine the direct effect of RFamide-related peptides on LH release from the pituitary gland of the pig (Aim II), and identify the relationship between energy balance and kisspeptin neurons in the hypothalamus of the gilt (Aim III). The rationale is that this work will advance our understanding of the basic biological mechanisms that control gonadotropin secretion in the gilt. The proposed research is significant, therefore, because application of this new fundamental knowledge is expected to lead to development of new strategies to minimize reproductive failure and maximize fertility of replacement gilts. This in turn will increase reproductive efficiency and decrease the expense of pork production.
3. Progress Report:
Luteinizing hormone (LH) secretion in the prepubertal ovariectomized (OVX) gilt was not affected by RFamide-related peptide-3 (RFRP3) when administered by central injection. Additional laboratory analysis, however, revealed that growth hormone (GH) secretion was stimulated (P < 0.05) by intracerebroventricular injection of RFRP3. This would be consistent with our results localizing expression of RFamide-related peptide neurons in areas of the porcine hypothalamus involved in GH secretion. Administering RFRP3 into the peripheral circulation had inconsistent effects on the pulsatile release of LH from OVX gilts. In the OVX gilt model, removal of the gonads allows strong stimulators of LH secretion, such as kisspeptin, to predominate and may make inhibition of LH secretion by RFRP3 difficult to reliably detect. To address this, we employed two alternative approaches. In the first alternative approach, we administered the putative RFRP receptor antagonist 1-adamantanecarbonyl-RF-NH2 (RF9) to prepubertal ovary-intact gilts to test the hypothesis that antagonizing the action of RFamide-related peptides would stimulate LH secretion. A single intravenous injection of RF9 (0.5 mg/kg BW) stimulated a release of LH in prepubertal gilts when compared with saline-treated control gilts. The release of LH stimulated by RF9 was similar to that of an endogenous pulse of LH rather than a surge-like release. Mean concentrations of LH in serum after treatment were greater (P < 0.01) for RF9 gilts than saline-treated gilts. The RF9 treatment did not alter gonadotropin-releasing hormone (GnRH)-induced secretion of LH, suggesting that the effect of RF9 on LH secretion is mediated at the level of the hypothalamus rather than by altering the sensitivity of the pituitary gland to GnRH. In the second alternative approach, we administered RFRP3 to sexually mature gonad-intact Chinese Meishan boars. Meishan boars are unique in the fact that they have relatively high serum concentrations of LH compared to other gonad-intact pig models. The RFRP was administered intravenously as a 5-mg loading dose followed by 0.5 mg RFRP3 every 15 min for a total dose of 8.5 mg. The LH secretory response followed one of two patterns; either there was a gradual but measurable decline in basal LH secretion occurring over several hours, or there was an immediate reduction in LH pulse amplitude that lasted for the duration of treatment, such that mean concentrations of LH in serum of RFRP3-treated boars were less (P < 0.05) than for saline-treated boars. In contrast to our observation in gilts, RFRP3 suppressed (P < 0.05) secretion of GH in the boar. Cumulatively, these data support the hypothesis that RFRP3 can regulate LH secretion in the pig and meets Specific Objective 1A. Reviews of the submitted manuscript were favorable, but reviewers asked for additional studies. These include 1) immunostaining of the pig hypothalamus to determine potential RFRP to GnRH neural contacts and 2) determining GH concentrations in gilts treated i.v. with RFRP3. We have collected hypothalamic tissue from ovary-intact gilts and will begin sectioning tissue to send to UC Davis to perform the required immunostaining. GH assays will be conducted as requested. To meet Specific Objective 2A, we will evaluate the effects of centrally and peripherally injected RFRP2 on LH secretion in the gilt. To meet Specific Objective 3, OVX gilts will be placed on nutritional diets in this coming year. Blood samples will be collected and analyzed at the U.S. Meat Animal Research Center, Clay Center, Nebraska to quantify hormonal differences. Hypothalami will be collected and sent to Washington State University to quantify expression of RFRP and Kisspeptin in the brain.