Gonadal Steroidogenesis in Response to Estradiol-17  Administration in the Sea Bream ( Sparus aurata L.) Joa ˜o B. Condec ¸a and Adelino V. M. Canario Centro de Cie ˆncias do Mar, Universidade do Algarve, Campus de Gambelas, 8000 Faro, Portugal Accepted June 13, 2001 The sea bream ( Sparus aurata) is a protandrous hermaph- rodite teleost fish in which estrogen administration in- duces testicular regression without influencing ovarian development. To analyze the changes in steroidogenesis of fish treated with two levels of estrogen (2 and 10 mg  kg 1 ) and untreated control fish, fragments of go- nads were incubated with tritiated 17-hydroxyprogester- one and the metabolites identified. The ability to extract radioactivity decreased with incubation time and was lower in gonads containing a larger proportion of ovarian tissue. The difference in steroidogenic capacity between control and estrogen-treated groups was generally quan- titative rather than qualitative and paralleled the ob- served histological changes. The same metabolites were identified in all three groups, but estrogen treatment caused a marked inhibition of 5 -reduction, 3 -reduc- tion, side-chain cleavage, and 11 -hydroxylation. The main androgens identified were 11 -hydroxy-4-andro- stene-3,17-dione and 3 -hydroxy-5 -androstane-3,17- dione, and the synthesis of both steroids was inhibited by estrogen treatment. Of the more polar pregnanes, 5 - pregnane-3 ,17,20 -triol and 5 -pregnane-3 ,17,20 - triol were detected in significant amounts, but only the latter appeared to be associated with development of the testis (in the untreated fish). A feature of sea bream go- nadal steroidogenesis less common in other teleosts was the presence of 6 - and 6 -hydroxylation. © 2001 Academic Press Key Words: steroidogenesis; estrogen; Sparus aurata; teleost. INTRODUCTION In vertebrates, gonadotrophins regulate gametogen- esis by acting directly on germ cells and indirectly by stimulating steroid hormone secretion (Nagahama, 1994). In fish, early studies of steroid biosynthesis focused mainly on salmonids, where E 2 , 1 T, and 11KT or 17,20P predominate in the reproductive cycle. Fe- males often show rising levels of E 2 and T during vitellogenesis and sharp increases of 17,20P during ovulation, while males secrete androgens such as T and 11KT during early spermatogenesis, and proges- togens such as 17,20P seem to play a role in final gamete maturation. The measurement of only these steroids in many species, however, can be misleading because blood levels are frequently low and there are a variety of alternative steroid biosynthetic pathways (Scott and Canario, 1987; Kime, 1993). In vitro incuba- tions with radiolabeled precursors can identify the steroidogenic capacity of the gonad to produce ste- roids with possible functions on gamete maturation, sex differentiation, and sex inversion (Kime and Groves, 1986; Schoonen et al., 1988; Kime et al., 1991b; Guiguen et al., 1995; Ponthier et al., 1998). However, a common pattern correlating sexual status and steroid output has been difficult to establish, especially in hermaphrodite species. For example, in the hermaph- rodites, Pagellus acarne, P. erythrinus, Serranus cabrilla, and Diplodus sargus, 11T was suggested to be the 1 See Table 1 for steroid names and abbreviations. General and Comparative Endocrinology 124, 82–96 (2001) doi:10.1006/gcen.2001.7689, available online at http://www.idealibrary.com on 0016-6480/01 $35.00 Copyright © 2001 by Academic Press All rights of reproduction in any form reserved. 82