Effects of lamprey gonadotropin-releasing hormone and analogs on steroidogenesis and spermiation in male sea lampreys.

The biological activities of lamprey GnRH and analogs were determined in the adult male lamprey,Petromyzon marinus. Two successive injections of lamprey GnRH at 0.005, 0.065 or 0.163 μg/g body weight or [D-Ala(6),Pro(9)-NHEt] mammal GnRH at 0.05 μg/g stimulated plasma estradiol and progesterone levels in adult male lampreys undergoing final maturation. In these experiments, a lamprey GnRH putative antagonist, [D-Phe(2,6),Pro(3)] lamprey GnRH, at 0.075, 0.150 or 0.3 μg/g stimulated plasma estradiol but not progesterone levels. Four successive injections of lamprey GnRH at 0.05 or 0.1 μg/g stimulated plasma progesterone and spermiation. [D-Phe(2,6),Pro(3)] lamprey GnRH at 0.05 or 0.1 μg/g depressed plasma progesterone levels and inhibited spermiation. In contrast, lamprey GnRH analog, [D-Ala(6),Pro(9)-OH free carboxylic acid] lamprey GnRH, at 0.05 or 0.1 μg/g stimulated plasma progesterone levels and inhibited spermiation. In summary, lamprey GnRH is biologically active in stimulating the pituitary-gonadal axis in adult male lampreys as determined by steroidogenesis and spermiation.