Differential Regulation of the Two Forms of Gonadotropin-Releasing Hormone (mGnRH and cGnRH-II) by Sex Steroids in the European Female Silver Eel (Anguilla anguilla)

Abstract

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 specific radioimmunoassays. Female silver eels received chronic treatments with various steroids (estradiol, testosterone, androstenedione, 5α-androstane-3β,17β-diol). Estradiol or the combination of estradiol and androgens induced increases in brain and pituitary mGnRH levels and pituitary GTH level, whereas androgens given alone had no significant effect. In contrast, androgens or their combination with estradiol 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 differential regulation by steroids, with a positive estrogen-dependent feedback on mGnRH (as well as on GTH) and a negative androgen-dependent feedback on cGnRH-II. These data are in agreement with previous results obtained in experimentally matured female eels (induced by a gonadotropic treatment which stimulates the production of both estrogens and androgens) showing increases in mGnRH and GTH levels, as well as a decrease 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. This positive feedback would play an important role, amplifying pubertal stimulation of the gonadotropic axis, in this fish species.

Morphological changes of Sertoli cells during the male reproductive cycle of the teleost Piaractus mesopotamicus (Holmberg, 1887)

“…In vivo or in vitro studies show that 11-KT is most effective as a direct stimulator for spermatogenesis (Cavaco et al, 1998), while T is most effective as a stimulator of hypothalamic (Amano et al, 1994) and pituitary (Montero et al, 1995) activity, leading to further activation of the testis. The sex steroid levels in fish are also influenced by behavior, e.g., social modulation (Oliveira et al, 2002).…”

In vitroInduction of Luteinizing Hormone Synthesis by Estrogen in Organ-Cultured Pituitary Glands of the Japanese Eel, Anguilla japonica

“…In the same way as in other teleosts, chronic treatment of female silver European eels Anguilla anguilla with E2 and testosterone induces increases in pituitary GTH (Dufour et al , 1983;Montero et al , 1995;Quérat et al ., 1991). In accordance with these in vivo data, there have been reports that testosterone stimulates LH production in a dose-and time-dependent manner in primary culture of dispersed pituitary cells isolated from juvenile eels (Huang et al ., 1997).…”

Effect of thermal regime on the expression of key reproductive genes during hormonally-induced vitellogenesis in female European eels

“…At the same time, a general increase in aromatase gene expression was recorded, suggesting that there is an intense E2 synthesis which could have a positive effect on eel maturation and on aagnrh1 gene expression (Montero et al, 1995).…”

“…In female European eel, it was demonstrated that E2 has a major effect in vivo on LH content (Dufour et al, 1983;Montero et al, 1995). Seoka et al, 2003;Pedersen, 2004; Kagawa et al, 2005;Palstra et al, 2006; Horie et al, 2008).…”

“…In female European eel, it was demonstrated that E2 has a major effect in vivo on LH content (Dufour et al, 1983;Montero et al, 1995). Conversely, in vitro treatment with E2 was not effective, suggesting that the positive feedback found in vivo is mediated by indirect actions (Huang et al, 1997;Dufour et al, 2003).…”