In vitro bioactivities of various forms of GnRH in relation to their susceptibility to degradation at the pituitary level in the rainbow trout, Oncorhynchus mykiss

Effects of Secretoneurin and Gonadotropin-Releasing Hormone Agonist on the Spawning of Captive Greater Amberjack (Seriola dumerili)

The greater amberjack (Seriola dumerili), a pelagic marine species with a global distribution, has considerable worldwide potential as an aquaculture species. However, difficulties have been encountered in inducing spontaneous spawning in cultured fish stocks. In this study, we analysed the key regulatory factors, secretoneurin (SN) and gonadotropin-releasing hormone (GnRH), in greater amberjack. Active peptides of SN and GnRH, SdSNa, and SdGnRH, respectively, were obtained by comparative analysis of homologous proteins from different species. Amino acid substitutions of the SdGnRH decapeptide at position 6 with a dextrorotatory (D) amino acid and at position 10 with an ethylamide group yielded a super-active agonist (SdGnRHa). The injection of SdSNa and SdGnRHa elevated luteinizing hormone, thyroid-stimulating hormone, and oxytocin levels in the sera of sexually mature fish, whereas it reduced the level of follicle-stimulating hormone. Furthermore, in response to the SdSNa and SdGnRHa injections, we detected an increase in the expression of genes associated with oocyte development and spermatogenesis. We established that the greater amberjack cultured along the southern coast of China reached sexual maturity at three years of age, and its reproductive season extended from February to April. Spawning of the cultured greater amberjack was successfully induced with a single injection of SdGnRHa/SdSN/DOM/HCG. Our findings indicate that similar to GnRHa, SNa is a potential stimulator of reproduction that can be used to artificially induce spawning in marine fish.

Fish reproductive biology - Reflecting on five decades of fundamental and translational research

Driven by the broad diversity of species and physiologies and by reproduction-related bottlenecks in aquaculture, the field of fish reproductive biology has rapidly grown over the last five decades. This review provides my perspective on the field during this period, integrating fundamental and applied developments and milestones. Our basic understanding of the brain-pituitary-gonadal axis led to overcoming the failure of farmed fish to ovulate and spawn in captivity, allowing us to close the fish life cycle and establish a predictable, year-round production of eggs. Dissecting the molecular and hormonal mechanisms associated with sex determination and differentiation drove technologies for producing better performing mono-sex and reproductively-sterile fish. The growing contingent of passionate fish biologists, together with the availability of innovative platforms such as transgenesis and gene editing, as well as new models such as the zebrafish and medaka, have generated many discoveries, also leading to new insights of reproductive biology in higher vertebrates including humans. Consequently, fish have now been widely accepted as vertebrate reproductive models. Perhaps the best testament of the progress in our discipline is demonstrated at the International Symposia on Reproductive Physiology of Fish (ISRPF), at which our scientific family has convened every four years since the grandfather of the field, the late Ronald Billard, organized the inaugural 1977 meeting in Paimpont, France. As the one person who has been fortunate enough to attend all of these meetings since their inception, I have witnessed first-hand the astounding evolution of our field as we capitalized on the molecular and biotechnological revolutions in the life sciences, which enabled us to provide a higher resolution of fish reproductive and endocrine processes, answer more questions, and dive into deeper comprehension. Undoubtedly, the next (five) decades will be similarly exciting as we continue to integrate physiology with genomics, basic and translational research, and the small fish models with the aquacultured species.

Temperature effects along the reproductive axis during spawning induction of grass carp (Ctenopharyngodon idella)

For grass carp (Ctenopharyngodon idella) raised in the Ivory Coast (with water temperatures of 26-31 degrees C), induced spawning is obligatory for fry production. However, ovulation rates following hormonal treatment are often low. We hypothesized that high temperatures are an inhibiting factor for the reproductive axis (brain-pituitary-gonad) in these conditions. By in vivo and in vitro experiments, we tried to determine the thermosensitive steps during spawning induction. We compared gonadotropin and maturation-inducing steroid (MIS) profiles during a spawning induction at controlled temperatures of 24 and 28 degrees C in relation to ovulation success. We performed pituitary cell cultures and ovarian fragment incubations at controlled temperatures. The ovulation rate was lower at 28 degrees C (10%) than at 24 degrees C (36%). At the pituitary level, we found only minor thermal impacts on GnRH-stimulated LH release, but our data suggest an increase of the dopaminergic inhibition by high temperatures. The main effects were found at the ovary level, where ovary responsiveness to gonadotropin by MIS synthesis was disturbed, as well as oocyte responsiveness to MIS triggering final maturation, and probably ovulation. These results show the importance of regulating temperature during spawning induction to ensure a high rate of ovulation.