The effects of lamprey GnRH-I, -III and analogs on steroidogenesis in the sea lamprey (Petromyzon marinus)

The objective of this study was to examine the in vivo and in vitro effects of lamprey gonadotropin-releasing hormone (GnRH)-I, -III or analogs on ste roidogenesis in the adult sea lamprey for the purpose of identifying putati ve potent agonists and antagonists. In the in vivo studies, the effect on s teroid production was examined by injecting males with lamprey GnRH-I, -III and analogs at 8 degrees C and 16 degrees C. The following peptides and an alogs were tested: lamprey GnRH-I and -III, [D-Glu(6)] lamprey GnRH-I, cycl e [Glu(6)-Trp(7)-Lys(8)] lamprey GnRH-I, cycle [D-Glu(6)-Trp(7)-Lys(8)] lam prey GnRH-I, [Gly(6)] lamprey GnRH-I, [D-Phe(2.6), Pro(3)] lamprey GnRH-I, [Phe(2)] lamprey GnRH-I, [Trp(3)] lamprey GnRH-I, and [Gly(6)] lamprey GnRH -III. All peptides tested in vivo, except [Trp(3)] lamprey GnRH-I, effectiv ely stimulated plasma oestradiol after 4 h in lampreys held at 8 degrees C or 16 degrees C. In the in vitro studies, lamprey GnRH-I and -III significa ntly stimulated the pituitary to release a putative gonadotropin capable of stimulating the ovaries to release oestradiol when incubated at 18 degrees C, [D-Glu(6)] lamprey GnRH-I at all doses suppressed the putative pituitar y response on the testis at 14 degrees C, whereas cycle [Glu(6)-Trp(7)-Lys( 8)] lamprey GnRH-I only suppressed the pituitary at a dose of 100 and 1000 ng/ml. it is suggested from these studies that the actions and differences between the in vivo and in vitro studies on lamprey GnRH-I and -III and ana logs are dependent on temperature and/or stage of reproduction likely refle cting differences in metabolic turnover or degradation rates of GnRH, GTH, and/or their receptors. From these studies, proposed putative agonists/anta gonists have been identified that may be used to enhance reproduction in la mpreys. Agonists/antagonists will be tested further to determine their abil ity to inhibit spermatogenesis without destroying the mating competitivenes s of males. This would be a valuable tool in a sterile-male release program in the Gnat Lakes. (C) 2000 Elsevier Science B.V. All rights reserved.