Effect of steroid hormones on the modulation of pituitary responsiveness and gonadotropin release in cattle

• Grant number: 21/09924-7
• Support Opportunities: Scholarships in Brazil - Doctorate
• Start date: April 01, 2022
• End date: May 01, 2025
• Field of knowledge: Agronomical Sciences - Veterinary Medicine - Animal Reproduction
• Principal Investigator: Roberto Sartori Filho
• Grantee: Lucas Oliveira e Silva
• Host Institution: Escola Superior de Agricultura Luiz de Queiroz (ESALQ). Universidade de São Paulo (USP). Piracicaba , SP, Brazil
• Associated research grant: 18/03798-7 - Studies on ovarian and uterine physiology and improvement of Assisted Reproduction Technologies to increase reproductive efficiency in cattle, AP.TEM
• Associated scholarship(s): 22/12348-0 - Steroidal modulation of the hypothalamic/pituitary control of gonadotropin release in heifers from distinct genotypic groups, BE.EP.DR

Abstract

The present project proposes the conduction of five experiments aiming to understand how the circulating concentrations of estradiol (E2) and progesterone (P4), modulated exogenously or endogenously, affect the responsiveness of the pituitary and the release of LH and FSH, as well as understanding what is the influence of GnRH pulsatility in the regulatory pathways exerted by these steroids in cows. Exp. 1 will evaluate the influence of circulating P4 concentrations, promoted by the insertion of an intravaginal device, and the follicle development stage (pre or post-dominance), on the FSH release profile and ovarian dynamics in Nellore heifers (n = 40) submitted to an E2/P4-based synchronization protocol. In Exp. 2, 16 Holstein cows pre-synchronized to be on the seventh day of the estrous cycle (with presence of a dominant follicle and a functional corpus luteum) will be pre-stimulated with estradiol cypionate (EC) and/or equine chorionic gonadotropin (eCG) 2 days before a GnRH-induced ovulation, to assess the effects of exogenous or endogenous elevation of E2 concentrations on LH and FSH release. In Exp. 3, Holstein cows (n = 16) will be enrolled to experimental models of high or low P4 concentration, by exogenous manipulation, to determine the effect of P4 concentrations associated or not with serial administration of GnRH pulses i.v. on LH and FSH release, after a challenge with GnRH. Similarly, in Exp. 4, cows (n = 16) will be enrolled only to the high P4 concentration model and will be previously treated with E2 and/or GnRH pulses i.v. to determine the interaction of these factors in the pituitary sensitization to GnRH. Finally, based on the results of previous experiments, Exp. 5 proposes to use sheep (n = 25) as an experimental model to evaluate the effect of hormonal protocols suggested in previous experiments at the pituitary level, evaluating expression of GnRH receptors (R-GnRH) and synthesis of gonadotropins.