Endocrine control of reproduction, fish

Fish are the largest and most divergent vertebrate class and display high variability in reproductive strategies. However, the endocrine regulation of fish reproduction is evolutionary conserved and similar to that of other vertebrates. The fish brain perceives sensory input regarding the season, internal physiological state and possible mates. These signals are then processed by the hypothalamus and induce the release of several neuropeptides and monoamines, which regulate the activity of gonadotrope cells. In turn, gonadotropes release follicle stimulating hormone (FSH) and luteinizing hormone (LH) to regulate the production of gonadal steroids from endocrine cells in the gonad. These steroids regulate gonadal development and germ cell maturation, as well as, convey gonadal feedback on the hypothalamus and pituitary. In females, FSH through the secretion of E2, stimulates the synthesis and secretion of vitellogenin that is transferred to the follicle by the blood. LH, that in some fish play a yet-unknown role in vitellogenesis, plays a role in the induction of final oocyte maturation, and through the secretion of DHP, in ovulation. Spermatogonial renewal is controlled by E2, while proliferation of spermatogonia toward meiosis is initiated by 11-KT - both are produced in response to FSH stimulation. LH initiates meiosis through DHP that also regulates sperm maturation through the regulation of seminal plasma pH. The identification of novel neuropeptides that regulate reproduction and recent development of reverse genetics techniques has challenged the dogma of endocrine regulation of fish reproduction. In this article, we review the central strategies of fish reproduction, the anatomy and endocrine components of the piscine hypothalamus-pituitary-gonadal axis and a few recently discovered neuropeptides that regulate fish reproduction. Finally, we briefly review the application of recently developed genome editing techniques in fish and show how these methods affect reproductive endocrinology research in fish. The data presented in this article demonstrate the importance and relevance of fish as models for endocrine regulation of reproduction research.