Gonadotropin response to GnRH during sexual ontogeny in the common carp, Cyprinus carpio

This study was designed to reveal whether gonadotropic response to GnRH in the common carp (Cyprinus carpio) changes during sexual ontogeny and whether the response of FSHbeta and LHbeta subunits is uniform or differential. The study comprised fish at the following stages: juveniles (4-month-old females with primary oocytes and early spermatogenic males); maturing (9-month-old previtellogenic females and advanced spermatogenic males); and mature (16-month-old postvitellogenic females and spermiating males). Fish were injected with superactive salmon GnRH analogue (sGnRHa; 25 microg/kg) and blood was sampled 6, 12 and 24 h later for cGtH (LH) and sex steroid levels. Pituitaries were taken for determination of FSHbeta and LHbeta mRNA levels by slot-blot hybridization and for cGTH content in the same glands by radioimmunoassay (RIA). Values were compared with the levels prior to sGnRHa administration and with control fish sampled at the same intervals. Juvenile fish did not respond at all to sGnRHa. In maturing females, FSHbeta mRNA increased by >300%, while that of LHbeta increased by 200%. In maturing males, FSHbeta mRNA did not change and only a slight increase occurred in that of LHbeta. In 16-month-old postvitellogenic females, there was no response of FSHbeta mRNA, while that of LHbeta dramatically increased. In spermiating males of the same age, mRNA of both FSHbeta and LHbeta increased following sGnRHa injection. Immunoreactive cGtH was present in the pituitary and plasma of all fish examined, but in juveniles it did not change following sGnRHa injection. In maturing and mature fish of both genders, sGnRHa administration was followed by a marked increase in circulating cGtH, concomitant with a decrease in its pituitary content, indicating the limited amount of the hormone stored in the gland. In conclusion, the response of the gonadotropin subunit mRNAs in the common carp was found to be differential and dependent on the gender and the phase of sexual ontogeny.