Effects of high hydrostatic pressure on the pituitary–gonad axis in the European eel, Anguilla anguilla (L.) Marie-Emilie Se ´bert a , Aline Ame ´rand b , Aure ´lie Vettier b , Finn-Arne Weltzien a,1 , Catherine Pasqualini c , Philippe Se ´bert b , Sylvie Dufour a, * a USM 0401, UMR 5178 CNRS/MNHN/UPMC Biologie des Organismes Marins et Ecosyste `mes, De ´partement des Milieux et Peuplements Aquatiques, Muse ´um National d’Histoire Naturelle, 75231 Paris Cedex 05, France b Unite ´ Haute Pression et Me ´tabolisme, UPCI-EA3879, UFR Me ´decine, CS 93837, 29238 Brest Cedex 3, France c De ´veloppement, Evolution et Plasticite ´ du Syste `me Nerveux, UPR CNRS 2197, Institut de Neurobiologie Alfred Fessard, CNRS, 91198 Gif-sur-Yvette Cedex, France Received 15 September 2006; revised 4 January 2007; accepted 19 January 2007 Available online 25 January 2007 Abstract European silver eels are thought to undergo sexual maturation during their oceanic reproductive migration from the European continent to their spawning area in the Sargasso Sea. Tracking data and various anatomical and physiological features suggest that silver eels migrate in deep sea, leading us to hypothesise that high hydrostatic pressure (HP) influences the induction of eel reproduction. We subjected female and male silver eels to 101ATA for 3 and 7 weeks, respectively, in a hyperbaric chamber equipped with a freshwater recirculation system. In comparison with control eels kept at 1 ATA, HP effects were tested against the messenger RNA levels of pituitary gonadotropins (LHb, FSHb) using quantitative real-time RT-PCR. The effects of HP on gonadal activity were estimated by measuring gonadosomatic index, oocyte diameter and plasma levels of vitellogenin (Vtg) and sex steroids (E 2 , 11-KT). At the pituitary level, LHb expression tended to increase while FSHb expression decreased in both sex, leading to an increase in the LHb/FSHb ratio. This suggests a differential effect of HP on the expression of the two gonadotropins. In females submitted to HP, we observed a significant increase in oocyte diameter and plasma levels of 11-KT and E 2 . A similar trend was observed for 11-KT plasma levels in males. In females, Vtg plasma levels also sig- nificantly increased, reflecting the stimulatory effect of sex steroids on hepatic vitellogenesis. Our results suggest that HP plays a specific and positive role in eel reproduction but additional environmental and internal factors are necessary to ensure complete sexual maturation. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Reproduction; Gonadotropins; Hydrostatic pressure; Teleost; Ecophysiology 1. Introduction The teleostean European eel, Anguilla anguilla (L.), exhibits a striking life cycle with oceanic and continental phases. They reproduce in the Atlantic Ocean, supposedly in the Sargasso Sea (Schmidt, 1923; Bertin, 1956; Fricke and Kaese, 1995; for review: McCleave, 2003) and leptocephalus larvae drift towards the European coast. Larvae metamorphose into glass eels that move into conti- nental waters and develop as yellow eels (Bertin, 1958; Tesch, 1977; Brusle ´, 1989). After several years, the growth phase ends with a second metamorphosis termed ‘‘silver- ing’’, corresponding to the transition from a sedentary yel- low eel to a migratory silver eel (Lecomte-Finiger, 1990; Fontaine, 1994). In the ‘‘silver’’ stage, eels stop feeding and start their downstream migration before leaving the continent to reach their breeding area some 6000 km away. The silvering process is not only a metamorphosis pre- paring the eel to the oceanic environment but also marks the onset of puberty (Aroua et al., 2005). Indeed, compared 0016-6480/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.ygcen.2007.01.009 * Corresponding author. Fax: +33 1 40 79 36 18. E-mail address: dufour@mnhn.fr (S. Dufour). 1 Present address: Department of Molecular Biosciences Department - Programme for Physiology, University of Oslo. PO BOX 1041 Blindern 0371 Oslo, Norway. www.elsevier.com/locate/ygcen General and Comparative Endocrinology 153 (2007) 289–298