DIFFERENTIAL REGULATION OF THE 2 FORMS OF GONADOTROPIN-RELEASING-HORMONE (MGNRH AND CGNRH-II) BY SEX STEROIDS IN THE EUROPEAN FEMALE SILVEREEL (ANGUILLA-ANGUILLA)

The effect of steroids on the two gonadotropin-releasing hormone (GnRH ) forms present in the eel (mammalian GnRH, mGnRH and chicken GnRH-II, cGnRH-II), as well as on gonadotropin (GTH), was studied using specif ic radioimmunoassays. Female silver eels received chronic treatments w ith various steroids (estradiol, testosterone, androstenedione, 5 alph a-androstane-3 beta, 17 beta-diol). Estradiol or the combination of es tradiol and androgens induced increases in brain and pituitary mGnRH l evels and pituitary GTH level, whereas androgens given alone had no si gnificant effect. In contrast, androgens or their combination with est radiol reduced brain cGnRH-II levels (this form remaining undetectable in the pituitary), estradiol given alone having no significant effect . This work demonstrates that the two forms of GnRH undergo a differen tial regulation by steroids, with a positive estrogen-dependent feedba ck on mGnRH (as well as on GTH) and a negative androgen-dependent feed back on cGnRH-II. These data are in agreement with previous results ob tained in experimentally matured female eels (induced by a gonadotropi c treatment which stimulates the production of both estrogens and andr ogens) showing increases in mGnRH and GTH levels, as well as a decreas e in cGnRH-II [1]. The positive feedback of steroids on the mGnRH-GTH axis adds credence to the hypothesis according to which mGnRH would be the main form involved in the control of the gonadotropic function. T his positive feedback would play an important role, amplifying puberta l stimulation of the gonadotropic axis, in this fish species.