Steroid hormone modulation of hypothalamic-pituitary function and gonadotropin release in Bos taurus and Bos indicus females

• Ano de defesa: 2025
• Autor(a) principal: Silva, Lucas Oliveira e
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: Estradiol; FSH; Genetic group; GnRH; Grupo genético; LH; Progesterona; Progesterone
• Link de acesso: https://www.teses.usp.br/teses/disponiveis/11/11139/tde-05082025-103902/

Understanding the regulatory processes underlying the hypothalamic-pituitary-ovarian (HPO) axis is essential for controlling reproductive function in bovine females, enabling the development and improvement of biotechnologies for optimizing reproductive efficiency. The objective of this thesis was to determine the role of the ovarian steroid hormones, progesterone (P4) and estradiol (E2), in modulating HPO axis function and the secretion of gonadotropins (FSH and LH) in bovine females. Additionally, this study aimed to characterize differences in hypothalamic-pituitary function between Bos taurus and Bos indicus females. Study 1 evaluated the influence of follicular stage (pre- or post-deviation), as well as circulating P4 concentrations, on FSH and LH release profiles and ovarian dynamics in Bos indicus (Nelore) heifers submitted to an E2/P4-based synchronization protocol. The initial follicular stage did not affect gonadotropin release profile or subsequent follicular wave development. However, higher circulating P4 concentrations led to greater suppression of LH concentrations prior to follicular emergence, besides increased FSH concentrations and a lower number of subordinate follicles after emergence, causing a slight delay in the timing of follicular deviation. Moreover, although the early development of the future dominant follicle was not affected, its growth after deviation was negatively impacted by higher P4 concentrations, resulting in a smaller and less steroidogenic dominant follicle. In Study 2, Bos taurus (Hereford), Bos indicus (Brahman), and crossbred (F1) heifers, previously ovariectomized, were submitted to hormonal conditions of absence, decreasing, or high circulating P4 concentrations to determine the effects of these hormonal environments, as well as genetic group, on LH pulse and surge release profiles. High P4 reduced, while decreasing P4 concentrations increased LH pulse frequency compared to the absence of P4. However, neither the absence nor decreasing P4 concentrations resulted in a greater GnRH-induced LH surge than that observed under high P4 concentrations. Additionally, regardless of the P4 environment, Bos indicus heifers exhibited lower LH pulse amplitude and lower GnRH-induced LH surge, despite having a similar pulse frequency compared to Bos taurus heifers. In Study 3, the same Brahman and Hereford heifers, under elevated P4 concentrations, were submitted to a high GnRH pulse frequency, induced either endogenously or exogenously, to determine the effect of this stimulus, as well as genetic group, on LH pulse and surge release. Both kisspeptin and GnRH treatments effectively induced a high LH pulse frequency, despite elevated P4 concentrations. However, this stimulus did not increase pituitary responsiveness to a GnRH challenge. Additionally, while LH pulsatile secretion (endogenous or induced) was similar between genetic groups, the GnRH-induced LH surge was consistently lower in Bos indicus heifers. Collectively, these findings demonstrate that P4 plays a key regulatory role in the HPO axis, modulating FSH secretion and LH pulse frequency, thereby directly influencing follicular development and E2 production. However, neither elevated P4 concentrations nor increased GnRH pulse frequency appeared to exert a direct and independent effect on pituitary responsiveness to GnRH. Likely, this effect may be mediated by a rapid increase in circulating E2 concentrations under low P4 and consequent high GnRH/LH pulse frequency. Finally, although minor differences were observed in the pattern of LH pulsatile secretion, results from these studies suggest that distinct pituitary responsiveness to an ovulatory GnRH stimulus may contribute to differences in reproductive function between Bos indicus and Bos taurus females, regardless of P4 concentrations.