Reproduction is a biological process of great interest for aquaculture since its control is a limiting factor for production of fish in captivity. The reproductive success of a species under confinement depends on a deep understanding of the basic processes that govern the initiation of puberty and maintenance of reproductive cyclicity. In fish, as in other vertebrates, reproduction is regulated by a hormonal cascade that occurs over a neuro-endocrine network composed of the brain, the pituitary and the gonads (BPG axis). The main hormones involved include the gonadotropin-releasing hormone (GnRHs), gonadotropins (GTHs) and sex steroids. In this context, GTHs - follicle stimulating hormone (FSH) and luteinizing hormone (LH) - have a central role. Knowledge of its structure and mode of action is essential to understand and control the reproductive cycle. Studies in recent years have allowed elucidating biochemical and physiological aspects of these hormones in fish. However, the functional duality of GTHs is not entirely clear in many teleost species due to the lack of appropriate tools for their study, primarily assays to determine their levels at critical times of the reproductive cycle.
To address this problem in the sea bass (Dicentrarchus labrax), a marine Perciform of great interest for aquaculture, native FSH has been purified and different recombinant GTHs have been produced. This has allowed to characterize, biochemically and functionally, the sea bass GTHs and to develop specific assays (biological and immunological) to measure FSH, which represents a valuable tool for studying the reproductive endocrinology of this species. In addition, molecular tools have also been developed to study gene expression profiles of GnRHs, GnRHR and GTHs. These tools, along with others already available in our laboratory, have allowed investigating the role of GTHs during sexual differentiation and gametogenesis in the sea bass.
The results show that both GTHs specifically activate their respective receptors, showing no cross-reactivity in a wide range of concentrations, and that are able to induce the synthesis of sex steroids in vitro. During sexual differentiation, hormonal profiles and gene expression levels suggest a key role for FSH. Furthermore, the synthesis and biopotency of this hormone presents a sexual dimorphism and appears to be regulated primarily by sbGnRH. In addition, immunohistochemical analysis indicates that in the pituitary, FSH and LH are synthesized by different adenohypophysis cells. Finally, hormonal profiles obtained from prepubertal and adult sea bass suggest that FSH is involved in the initiation and maintenance of spermatogenesis and vitellogenesis. On the contrary, the LH appears to be involved in the final stages of gametogenesis such as spermiogenesis, spermiation, maturation and ovulation.