Differential Regulation of Luteinizing Hormone and Follicle-Stimulating Hormone Expression during Ovarian Development and under Sexual Steroid Feedback in the European Eel

Abstract

Pituitary gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are, in teleosts as in mammals, under the control of hypothalamic factors and steroid feedbacks. In teleosts, feedback regulations largely vary depending on species and physiological stage. In the present study the regulation of FSH and LH expression was investigated in the European eel, a fish of biological and phylogenetical interest as a representative of an early group of teleosts. The eel FSHβ subunit was cloned, sequenced and together with earlier isolated eel LHβ and glycoprotein hormone α (GPα) subunits used to study the differential regulation of LH and FSH. In situ hybridization indicated that FSHβ and LHβ are expressed by separate cells of the proximal pars distalis of the adenohypophysis, differently from the situation in mammals. The profiles of LHβ and FSHβ subunit expression were compared during experimental ovarian maturation, using dot-blot assays. Expression levels for LHβ and GPα increased throughout ovarian development with a positive correlation between these two subunits. Conversely, FSHβ mRNA levels decreased. To understand the role of sex steroids in these opposite variations, immature eels were treated with estradiol (E₂)and testosterone (T), both steroids being produced in eel ovaries during gonadal development. E₂ treatment induced increases in both LHβ and GPα mRNA levels, without any significant effect on FSHβ. In contrast, T treatment induced a decrease in FSHβ mRNA levels, without any significant effect on the other subunits. These data demonstrate that steroids exert a differential feedback on eel gonadotropin expression, with an E₂-specific positive feedback on LH and a T-specific negative feedback on FSH, leading to an opposite regulation of LH and FSH during ovarian development.

Molecular approaches related to the European eel (Anguilla anguilla) reproductive process

“…However, in European sea bass (Dicentrarchus labrax), glycoprotein-, FSH-, LH- mRNA increased simultaneously with the gonadosomatic index (GSI) during spermatogenesis (Mateos et al, 2003). In eel, FSH seems to mediate gonadotropin stimulation in the early stages of the gametogenesis, while LH seems to be involved in the end of gametogenesis (European eel: Aroua et al, 2005;Schmitz et al, 2005;Japanese eel: Jeng et al, 2007;Yoshiura et al, 1999).…”

Study of male reproductive physiology and sperm conservation in the european EEL ( Anguilla anguilla, L.)

“…All of them have been sequenced in European eel (Querat et al, 1990a,b;Degani et al, 2003;Schmitz et al, 2005). In most teleosts, FSH regulates the first steps of the gonad maturation, while LH is involved in the final gonad development.…”

“…However, some researchers indicated that both gonadotropins are present in immature previtellogenic eels (Schmitz et al, 2005;Aroua et al, 2005).…”

“…The gonadotropin expression has been deeply analyzed in European eel females Degani et al, 2003;Schmitz et al, 2005;Sébert et al, 2007), but in male European eel it has never been studied during male gonad development. In any case, most of the studies have been performed in female Japanese eel Jeng et al, 2002Jeng et al, , 2007Kamei et al, 2005Kamei et al, , 2006Han et al, 2003;Ozaki et al, 2007).…”

“…In any case, most of the studies have been performed in female Japanese eel Jeng et al, 2002Jeng et al, , 2007Kamei et al, 2005Kamei et al, , 2006Han et al, 2003;Ozaki et al, 2007). The repeated injections of exogenous sGTH in the Japanese and European eel female gave as results an abnormal LHβ and FSHβ expression, with an over-expression of LHβ during ovarian maturation and a hard decrease of FSHβ after the first injections Schmitz et al, 2005).…”