In vivo activity of native GnRHs and their analogues on ovulation in the African catfish, Clarias gariepinus (Burchell)

Four distinct forms of native gonadotropin-releasing hormone (GnRH) and two newly designed analogues were tested for their in vivo activity to induce ovulation in African catfish. The effects of these peptides on ovulatory parameters were compared with those of carp pituitary and [ d-Ala⁶, Pro⁹-NEt]-mammalian GnRH analogue (mGnRHa), two tested ovulation-inducing agents in African catfish. Assessment of ovulation was carried out by determining the ovulation ratio and the relative quantity of egg produced. From the results of the experiments, the order of potency of the native GnRH peptides is summarized as chicken GnRH-II (cGnRH-II) >salmon GnRH (sGnRH) >mammalian GnRH >chicken GnRH-I (cGnRH-I). Chicken GnRH-II was as potent as mGnRHa while cGnRH-I was totally ineffective. The new d-Orn⁶-cGnRH-II and d-Orn⁶-sGnRH with a substitution at position 6 with d-isomer residue were as potent as the most extensively used mGnRHa, indicating that the position 6 modification might be more crucial than the substitution at the C-terminal. On the basis of our results, the potential use and incorporation of cGnRH-II and sGnRH for the development of more generic spawning induction therapies are suggested.