In the present study, both
lamprey GnRH-I and -III stimulated steroidogenesis and induced ovulation in adult female sea lampreys during their final reproductive stage. One injection of
lamprey GnRH-III at 0.1 or 0.2 microg/g lamprey stimulated plasma
estradiol levels in lampreys held at each of three water temperatures, 13 degrees , 17 degrees , and 19 degrees , corresponding to increasing stages of maturation. Four successive
injections, 3 to 4 days apart, of
lamprey GnRH-III at 0.1 or 0.2 microg/g
body weight induced ovulation in 100 or 88% of lampreys, respectively, compared to 21% in controls by Day 31.
Lamprey GnRH-III also had a direct stimulatory effect on
estradiol production in the sea lamprey gonads in vitro.
Lamprey GnRH-III at 100 or 1000 ng/ml stimulated
estradiol levels in media incubated with either lamprey ovaries or testes. 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 study indicate that
lamprey GnRH-I and -III are equally potent in inducing ovulation and stimulating steroidogenesis in female sea lampreys. In addition,
GnRH binding sites have been demonstrated for the first time in both the testis and the ovary of the adult sea lamprey using an analog of mammalian
GnRH ([D-Lys6] mammalian
GnRH) as a labeled
ligand. Scatchard analysis suggested the presence of a high affinity binding site in both the testis and the ovary. In summary,
lamprey GnRH-III 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 evidence for a direct stimulatory effect of lamprey
GnRH in the gonads, the presence of
GnRH binding site, and the absence of
GnRH in the plasma suggest that, like other vertebrates including rat, rabbit, teleost fish, 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 gonadal function. This study further strengthens the paracrine regulatory role of
GnRH peptides in the gonads of vertebrates, which appear to be evolutionarily conserved.