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

Abstract

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 steroidogenesis in the adult sea lamprey for the purpose of identifying putative potent agonists and antagonists. In the in vivo studies, the effect on steroid production was examined by injecting males with lamprey GnRH-I, -III and analogs at 8°C and 16°C. The following peptides and analogs were tested: lamprey GnRH-I and -III, [D-Glu 6] lamprey GnRH-I, cyclo [Glu 6-Trp 7-Lys 8] lamprey GnRH-I, cyclo [D-Glu 6-Trp 7-Lys 8] lamprey 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, effectively stimulated plasma oestradiol after 4 h in lampreys held at 8°C or 16°C. In the in vitro studies, lamprey GnRH-I and -III significantly stimulated the pituitary to release a putative gonadotropin capable of stimulating the ovaries to release oestradiol when incubated at 18°C. [D-Glu 6] lamprey GnRH-I at all doses suppressed the putative pituitary response on the testis at 14°C, whereas cyclo [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 analogs are dependent on temperature and/or stage of reproduction likely reflecting differences in metabolic turnover or degradation rates of GnRH, GTH, and/or their receptors. From these studies, proposed putative agonists/antagonists have been identified that may be used to enhance reproduction in lampreys. Agonists/antagonists will be tested further to determine their ability to inhibit spermatogenesis without destroying the mating competitiveness of males. This would be a valuable tool in a sterile-male release program in the Great Lakes.