Interaction of sex steroids and cortisol on the control of reproductive function and demineralization from the vertebral skeleton in a migratory teleost fish, the eel (Anguilla anguilla).

• Sbaihi M., Kacem A., Aroua S., Le Belle N., Lopez E., Meunier F., Dufour S.
• ¹ MNHN, Départementdes Milieux et Peuolements Aquatiques, USM 0401, UMR,CNRS.5178, 7 rue Cuvier, 75231 Paris Cedex 05, France.
• ² MNHN, UMS 0403, 43 rue Cuvier, 75231 Paris Cedex 05, France.
• The Regional Workshop on Marine Sciences & Natural Resources 25-26 May 2004.

Abstract

Reproductive function in teleosts, as in other verterbrates, is controlled by the neuroendocrine-gonadotropin axis. Other neuroendocrine axes may also be involved, particulary for mobilisation of metabolic, energetic and calcium reserves necessary for reproduction.

The European eel (Anguilla anguilla) provides a model life cycle to study these endocrine interactions as vitellogenesis occurs during a long fasting and migration period: all metabolic, energetic and calcium needs essential for gonadal development and migration are provided by mobilising reserves accumulated during the juvenile growth phase.

We have studied the role and interaction of the sex steroids E2 and testosterone (T), and cortisol (F) in the direct control of reproductive function, and indirect control (metabolic) by studying the effects of hormonal treatment on the following parameters: Vg synthesis in vivo and in vitro ; LH synthesis in vivo and in vitro; digestive tract regression for metabolic reserve mobilisation; resorption of vertebrae for calcium reserve mobilisation. We have provided evidence for three mechanisms of skeletal vertebrae demineralisation (diffuse demineralisation, perilacunar osteocytic and osteoclastic resorption).

The results demonstrate that sex steroids and cortisol have differential and synergystic effects on the control of reproduction and its associated metabolisms: E2 plays a major role in direct hepatic control of Vg synthesis and induces skeletal vertebrae resorption with mobilisation of phospho-calcium reserves necessary for Vg synthesis. T alone is not involved in Vg synthesis, but stimulates LH synthesis by acting directly on LH ? mRNA levels in the pituitary gland and also induces the mobilisation of metabolic reserves (regression of the digestive tract) but does not act on calcium reserves. F stimulates LH synthesis by a direct pituitary action on LH ? mRNA levels and is important in mobilising calcium and metabolic reserves necessary for Vg synthesis; these three actions are enhanced in the presence of T. Thyroxin, a thyroid hormone, also strongly induces skeletal vertebrae resorption. Finally, combining all three steroids, (E2, T, F) produces a maximal effect on the majority of parameters studied, demonstrating a synergystic effect of sex steroids and cortisol on the control of reproduction and calcium metabolism.