N. Le Belle

Dopaminergic inhibition of reproduction in teleost fishes: ecophysiological and evolutionary implications.

Abstract: In many teleosts, dopamine (DA) exerts direct inhibitory control on gonadotropes, counteracting the stimulatory effect of gonadotropin‐releasing hormone (GnRH) on gonadotropin release. This dual control by GnRH and DA has been demonstrated in various adult teleosts and has major implications for aquaculture. Because of its unique life cycle, the European eel has provided a powerful model for demonstrating the key role of DA in the control of puberty. Data from tetrapods suggest that the inhibitory role of DA on reproduction is not restricted to the teleosts. Thus, DA inhibitory control could represent an ancient evolutionary component in the neuroendocrine regulation of reproduction that may have been differentially maintained throughout vertebrate evolution. The intensity of DA inhibition, its main site of action, and its involvement in the control of puberty, seasonal reproduction, ovulation, spermiation, or even sex change may differ among classes of vertebrates, as well as within smaller phylogenetic units such as teleosts or mammals. An inhibitory role for DA has been reported also in some invertebrates, indicating that neuronal DA pathways may have been recruited in various groups of metazoa to participate in the control of reproduction. In addition to the incontestable GnRH neurons, the recruitment of DA neurons for the neuroendocrine control of reproduction provides an additional brain pathway for the integration of various species‐specific, internal, and environmental cues. In teleosts, the plasticity of the DA neuroendocrine role may have contributed to their large diversity of biological cycles and to their successful adaptation to various environments.

Stimulation of gonadotropin release and of ovarian development, by the administration of a gonadoliberin agonist and of dopamine antagonists, in female silver eel pretreated with estradiol

In freshwater or seawater female silver eel, the release of gonadotropin (GTH) accumulated in the pituitary under estradiol (E2) influence could be stimulated by a conjugated treatment with a mammalian gonadoliberin agonist (GnRH-A = des-Gly10, (D-Ala6)-LH-RH ethylamide) and a blocker of dopamine receptor (pimozide). Furthermore, despite the GTH release, no reduction or even a significant increase in pituitary GTH levels were noted, indicating a stimulation of GTH synthesis. In consequence of the endogenous GTH release, a stimulation of ovarian development was induced, as demonstrated by the gonadosomatic index and histological study. Similar results were obtained with a combined treatment with GnRH-A and an inhibitor of catecholamine synthesis (L-alpha-methyl-3,4-dihydroxyphenylalanine). In contrast, no effect was produced by GnRH-A, pimozide, or L-alpha-methyl-DOPA, given alone. The results suggest that a double neuroendocrine mechanism (a lack of GnRH production and a dopaminergic inhibition of GnRH action) is involved in the prepubertal blockage of eel gonadotropic function before the reproductive migration.

Androgens stimulate gonadotropin-II b-subunit in eel pituitary cells in vitro

Primary cultures of juvenile eel (Anguilla anguilla) pituitary cells were used to study the direct effects of sex steroids on gonadotropin (GtH-II) cell content and release (radioimmunoassay) as well as on mRNAs levels for α and GtH-II β-subunits (dot-blot). Testosterone stimulated GtH-II production in a dose- and time-dependent manner by selectively increasing mRNAs for GtH-II β-subunit but not α-subunit. This positive effect was also induced by non-aromatizable androgens (androstanediol and dihydrotestosterone) but not by estradiol, indicating an androgen-specific effect in the eel. The androgen-specific stimulation of eel GtH-II β appears closer to the regulation of mammalian follice stimulating hormone-β (FSHβ) than that of salmonid GtH-II β or mammalian luteinizing hormone-β (LHβ)-subunits. Comparison with previous in vivo experiments suggests multiple sites of action of sex steroids on the brain-pituitary gonadotropic axis for the positive feedback on GtH-II synthesis in this juvenile fish.

Opposite effects of insulin-like growth factors (IGFs) on gonadotropin (GtH-II) and growth hormone (GH) production by primary culture of European eel (Anguilla anguilla) pituitary cells

Abstract Insulin-like growth factor-I (IGF-I) is a growth factor mainly produced by the liver under the control of growth hormone (GH) and implicated in the mediation of body growth regulation in teleosts as in other vertebrates. Its possible role in the interaction between growth and reproduction was investigated, by comparing its effects on GH and gonadotropin (GtH-II) productions, in juvenile sexually immature eels. This study was performed in vitro on primary culture of pituitary cells; to avoid effects of other growth factors, cells were cultured with basic culture medium (M199) without addition of serum. Eel GH and GtH-II levels in cells and media were measured by specific RIAs. Control cells released a large amount of GH, and after 2 weeks levels were more than twofold the initial cell content indicating a high production of GH by cultured cells. IGF-I inhibited GH release and production in a time- and dose-dependent manner. A maximal effect (>80% inhibition) was reached at 10 −9 M. Initial GtH-II cell content was at least 1000 times lower than GH cell content, and the amount released in 2 weeks reached only a few percent of cell content. IGF-I stimulated GtH-II cell content and release in a time- and dose-dependent manner. A maximal stimulation of GtH-II production (up to ×6) was observed at 10 −8 M after 2 weeks of culture. IGF-II showed the same potency as IGF-I to reduce GH production and stimulate GtH-II production. These results indicate opposite effects of IGFs on GH and GtH-II productions in eel pituitary cells and suggest that IGF-I may play an interface between growth and reproduction (puberty) in juvenile teleosts.

Brain Expression of Tyrosine Hydroxylase and Its Regulation by Steroid Hormones in the European Eel Quantified by Real-Time PCR

Abstract: In the eel, dopamine inhibits pubertal development. To investigate the regulatory mechanisms involved, we developed a quantitative real‐time RT‐PCR assay for measurement of brain expression of tyrosine hydroxylase (TH), the rate‐limiting enzyme in the biosynthesis of dopamine. TH expression was highest in the olfactory bulb, followed by the di‐/mesencephalic areas and the telencephalon/preoptic area. TH expression in the optic lobes and hindbrain was low or below the detection limit. In vivo treatment with testosterone, but not estradiol, resulted in increased TH expression in the forebrain, except the optic tectum, but not in the hindbrain. The results were confirmed by in situ hybridization.