Effects of gonadorelin on gonadotropin expression, plasma sex steroid concentrations and ovarian follicle dynamics in mature tambaqui (Colossoma macropomum)

Abstract

In tambaqui (Colossoma macropomum), a rheophilic Amazonian fish of significant aquaculture importance that typically fails to spawn under captive conditions, gonadorelin has demonstrated potential in inducing ovulation and enabling strip spawning. However, the molecular mechanisms underlying gonadorelin action, particularly on the transcriptional regulation of follicle-stimulating hormone (fshβ) and luteinizing hormone (lhβ), remain poorly understood and warrant further investigation. The present study aimed to investigate the efficacy of this synthetic drug in modulating the expression of gonadotropins and in enhancing circulating concentrations of 17β-estradiol (E2), 17α-hydroxyprogesterone (17α-OHP), and testosterone (T) in tambaqui; as well as its effects on the developmental dynamics and maturation stages of ovarian follicles. Gonadorelin at 60 μg kg⁻¹ body weight had no statistical effect on lhβ expression, but decreased fshβ expression in females (p = 0.021). E₂, 17α-OHP and T increased in all treated fish. In the treated group, the appearance of post-ovulatory complexes and the presence of eggs in the coelomic cavity indicated that 6 of 9 (67 %) of the females ovulated, while none of the control group did. The findings indicate that gonadorelin suppresses fshβ transcription in mature female tambaqui, and this downregulation may promote final oocyte maturation and ovulation by favoring a hormonal environment with increased progesterone influence. Hence, our data provides the first detailed analysis of the effects of a drug on tambaqui gonadotropins and sex steroids and sets a foundation for refining dose strategies to improve large-scale controlled breeding practices in tambaqui aquaculture.