CHARACTERISTICS OF GNRH BINDING IN THE GONADS AND EFFECTS OF LAMPREY GNRH-I AND GNRH-III ON REPRODUCTION IN THE ADULT SEA LAMPREY

In the present study, both lamprey GnRH-I and -III stimulated steroido genesis and induced ovulation in adult female sea lampreys during thei r final reproductive stage. One injection of lamprey GnRH-III at 0.1 o r 0.2 mu g/g lamprey stimulated plasma estradiol levels in lampreys he ld at each of three water temperatures, 13 degrees, 17 degrees, and 19 degrees, corresponding to increasing stages of maturation. Four succe ssive injections, 3 to 4 days apart, of lamprey GnRH-III at 0.1 or 0.2 mu g/g body weight induced ovulation in 100 or 88% of lampreys, respe ctively, compared to 21% in controls by Day 31. Lamprey GnRH-III also had a direct stimulatory effect on estradiol production in the sea lam prey gonads in vitro. Lamprey GnRH-III at 100 or 1000 ng/ml stimulated estradiol levels in media incubated with either lamprey ovaries or te stes. In contrast to a previous finding in which lamprey GnRH-III was more potent than lamprey GnRH-I in inducing spermiation in adult male sea lampreys (Deragon and Sower, 1994), the results from the present s tudy indicate that lamprey GnRH-I and -III are equally potent in induc ing ovulation and stimulating steroidogenesis in female sea lampreys. In addition, GnRH binding sites have been demonstrated for the first t ime in both the testis and the ovary of the adult sea lamprey using an analog of mammalian GnRH ([D-Lys(6)] mammalian GnRH) as a labeled lig and. Scatchard analysis suggested the presence of a high affinity bind ing site in both the testis and the ovary. In summary, lamprey GnRH-II I is biologically active in stimulating the pituitary-gonadal axis in adult female sea lampreys. This is the first report demonstrating the presence of a GnRH binding site in the gonads of an Agnathan. The evid ence for a direct stimulatory effect of lamprey GnRH in the gonads, th e presence of GnRH binding site, and the absence of GnRH in the plasma suggest that, like other vertebrates including rat, rabbit, teleost f ish, and human, there may be a GnRH-like factor produced in the gonads of the lamprey and it may act as a paracrine/autocrine modulator of g onadal function. This study further strengthens the paracrine regulato ry role of GnRH peptides in the gonads of vertebrates, which appear to be evolutionarily conserved. (C) 1997 Academic Press.