In European eels, the hormone receptor that responds to GnRH (gnrhr2) is turned up by estrogen and by testosterone only when it is converted to estrogen. Other steroids like progesterone or cortisol don’t change it, and activin peptides actually lower its levels. This shows a feedback loop where estrogen boosts the pituitary’s sensitivity to GnRH, similar to what happens in mammals during ovulation.

Eel species are basal teleosts with a unique life cycle including an arrest of sexual maturation before the reproductive oceanic migration. Our early studies showed that this blockade results from a deficient production of pituitary gonadotropins, due in part to a low responsiveness to gonadotropin-releasing hormone (GnRH). Three GnRH receptors have been identified in the eel, among them <i>gnrhr2</i> is the main pituitary receptor whose expression increases during the sexual maturation induced by gonadotropic treatments. We investigated the role of gonadal hormones in the feedback regulation of <i>gnrhr2</i> expression in the eel. The effects of steroids and activins were tested <i>in vitro</i> on primary cultures of eel pituitary cells and <i>gnrhr2</i> transcripts measured by qPCR. <i>In silico</i> analysis of eel <i>gnrhr2</i> promoter was performed to predict transcription factor binding sites and comparisons were made with <i>gnrhr</i> promoters from other teleosts and mammals. Estradiol and testosterone strongly and dose-dependently increased <i>gnrhr2</i> transcript levels as measured by qPCR. This stimulatory regulation was not observed with a non-aromatizable androgen, 11 keto-testosterone, and the effect of testosterone was abolished in the presence of an aromatase inhibitor, fadrozole, indicating an estrogen-specific positive control of eel <i>gnrhr2</i> expression. Other steroids, progesterone and cortisol, had no effect on <i>gnrhr2</i> expression. Gonadal peptides, activins A and B, were also tested, and showed an inhibitory effect on <i>gnrhr2</i> expression. Our results show that gonadal steroids exert a positive feedback, mediated by estradiol, on pituitary sensitivity to GnRH in the eel, in line with the regulatory mechanisms of the ovulatory luteinizing hormone (LH) surge in mammals. While investigation on <i>gnrhr</i> promoters is significantly lacking outside mammals, <i>in silico</i> analysis of the eel <i>gnrhr2</i> promoter allowed us to infer transcription factor binding sites potentially involved in the regulation of <i>gnrhr2</i> expression. Comparison was made with <i>gnrhr</i> promoters from other teleosts and mammals to discuss their evolutionary conservation. This study in the eel, a basal teleost representative, contributes to our understanding of the regulatory mechanisms of the complex eel life cycle and to raise basic knowledge on the regulation and evolution of pituitary GnRH receptivity in vertebrates.