Steroidogenic response of carp ovaries to piscine FSH and LH depends on the reproductive phase.

The gonadotropins (GTHs) follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are the key regulators of reproduction. We determined the competence of heterologous recombinant GTHs at eliciting steroid secretion from carp ovaries at different reproductive stages. We collected carp ovaries at: early, mid and end vitellogenesis, when most of the oocytes still contained a germinal vesicle (GV) at a central stage, and mature ovaries with a migrating GV. Plasma estradiol (E2) levels at early vitellogenesis were high and decreased thereafter. Basal secretion levels of E2 increased with oocyte diameter and GSI value, whereas 17α,20β-dihydroxy-4-pregnen-3-one (DHP) was detected only in females with mature follicles. Carp ovary fragments were exposed to recombinant fish GTHs belonging to different teleost orders: Japanese eel (Anguilla japonica, Anguilliformes), Manchurian trout (Brachymystax lenok, Salmoniformes), and Nile tilapia (Oreochromis niloticus); to mammalian GTHs (pFSH and hCG), or to carp and tilapia pituitary extract (CPE and TPE, respectively). All of the recombinant GTHs tested stimulated steroid secretion. However, the steroid secretion differed according to the type of GTH and the developmental state of the ovary. CPE increased the secretion of both E2 and DHP at almost all stages of ovarian maturity. In mature ovarian fragments, DHP secretion was higher in response to recombinant LHs (eel and tilapia) than to recombinant FSH. Early- and mid-vitellogenic ovaries showed no secretion of DHP and high secretion of E2 in response to all recombinant GTHs tested. This is in line with the hypothesis that LH regulates the final stages of maturation, when the involvement of FSH is marginal. These results may contribute to understanding the mechanisms that determine differential activation of steroid secretion and specificity in fish.