Steroidal modulation of the hypothalamic/pituitary control of gonadotropin release in heifers from distinct genotypic groups

• Grant number: 22/12348-0
• Support Opportunities: Scholarships abroad - Research Internship - Doctorate
• Start date: March 01, 2023
• End date: February 29, 2024
• Field of knowledge: Agronomical Sciences - Veterinary Medicine - Animal Reproduction
• Principal Investigator: Roberto Sartori Filho
• Grantee: Lucas Oliveira e Silva
• Supervisor: Rodolfo C. Cardoso
• Host Institution: Escola Superior de Agricultura Luiz de Queiroz (ESALQ). Universidade de São Paulo (USP). Piracicaba , SP, Brazil
• Institution abroad: Texas A&M University, United States
• Associated to the scholarship: 21/09924-7 - Effect of steroid hormones on the modulation of pituitary responsiveness and gonadotropin release in cattle, BP.DR

Abstract

The present project aims to evaluate the effect of circulating concentrations of estradiol (E2) and progesterone (P4), modulated exogenously, on the responsiveness of the pituitary and release of gonadotropins, as well as understanding the influence of GnRH pulse frequency in the regulatory pathways exerted by these steroids in heifers from distinct genotypic groups (Bos taurus vs. Bos indicus). In this sense, 2 experiments will be performed using 16 post-pubertal ovariectomized heifers (Hereford, n = 8; and Brahman, n =8) at Texas A&M University, College Station, TX, USA. In Experiment 1, heifers from both genetic groups will be submitted to experimental models of high or low P4 concentrations, by exogenous manipulation. On d -7, heifers will receive 2 intravaginal P4 devices (1.9 g). In heifers designated to High P4 groups, the devices will be kept for 7 d, whereas in heifers designated to Low P4 groups, one device will be removed 5 d after the insertion, followed by the second, 18 h later. In addition, heifers will be designed to receive or not hourly administrations of 2 µg GnRH pulses iv, during 8 h, starting on D -0.5. On D0, 4 h after the end of GnRH pulses treatment, all heifers will receive a challenge with 100 µg GnRH (gonadorelin) im. Therefore, 4 experimental groups will be created in a 2x2 factorial arrangement (High P4 vs. Low P4 and Control vs. p-GnRH). Blood samples will be collected each 12 h from D -3 to D0 (prior to GnRH challenge) to evaluate P4 concentrations. To evaluate LH and FSH profile, samples will be taken each 15 min over 8 h, starting on D -0.5, and hourly on D0, until 5 h after the GnRH challenge. After the end of the first replicate, heifers will be reassigned to a different experimental group over more three replicates. Similarly, in Experiment 2, the heifers will be submitted to the same experimental design described for High P4 group in Experiment 1. Moreover, they will be designated to receive or not a pre-treatment with E2 on D -3, and to receive or not the same GnRH pulses treatment described before, creating 4 experimental groups in a 2x2 factorial arrangement (High E2 vs. Low E2 and Control vs. p-GnRH). On D0, all heifers will receive a challenge with 100 µg GnRH im. Blood samples will be collected exactly as described for Experiment 1, including evaluation of E2 concentrations each 12 hours from D -3 to D0. Also, for Experiment 2, 4 replicates will be performed. Statistical analyses will be performed by GLIMMIX of SAS 9.4, and hormonal concentrations will be evaluated by radioimmunoassay (RIA). (AU)