Fish Physiology and Biochemistry 20: 231–241, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 231 Effects of the aromatase inhibitor Fadrozole on plasma sex steroid secretion and oocyte maturation in female coho salmon (Oncorhynchus kisutch) during vitellogenesis L. O. B. Afonso 1,2,∗ , G. K. Iwama 2 , J. Smith 1 and E. M. Donaldson 1 1 West VancouverLaboratory, Fisheries and Oceans Canada, 4160 Marine Drive, West Vancouver, B.C. V7V 1N6, Canada; 2 Department of Animal Science and the Canadian Bacterial Diseases Network, 248-2357 Main Mall, Vancouver B.C. V6T 1Z4, Canada; ( ∗ Present address: Departamento de Zootecnia, Faculdade de Agronomia - UFRGS, Av. Bento Gonçalves, 7712 Caixa Postal: 776 CEP:90001-970 - Porto Alegre, RS - Brazil; Phone: 55-51-316-6034; Fax: 55-51-316-6048; E-mail: bafonso@pro.via-rs.com.br) Accepted: September 24, 1998 Key words: aromatase, atresia, brain, oocytes, 17β -estradiol, 17α-20β -dihydroxy-4-pregnen-3-one, salmon, testosterone Abstract 17β -estradiol, 17α-20β -dihydroxy-4-pregnen-3-one (17α-20β -P), and testosterone levels were measured in plasma samples obtained from vitellogenic coho salmon (Oncorhynchus kisutch) before and 32 days after injection of the aromatase inhibitor Fadrozole (AI). Plasma 17β -estradiol levels decreased significantly 6 h after injection in all AI treated fish. The higher the dose the longer the maintenance of low plasma 17β -estradiol levels. Inversely, plasma 17α-20β -P increased significantly 6 h after injection in all AI treated fish, and the higher the dose the longer the maintenance of high plasma 17α-20β -P levels. At 48 h after injection plasma testosterone levels were significantly higher in the AI treated groups. The oocyte maturation index showed that multiple injections with AI retarded oocyte development. Besides, oocyte diameter and GSI were lower in the same group, which presented high incidence of atresia of vitellogenic oocytes. The ovarian follicles and brain of the fish which received multiple injections secreted less 17β -estradiol, in vitro. These findings suggest that aromatase inhibitors such as Fadrozole may have a potential as a tool to regulate sexual development in salmon. Introduction Vitellogenesis in teleosts is promoted by a two step mechanism in which, gonadotropin increases ovar- ian secretion of 17β -estradiol (E 2 ), which in turn stimulates the hepatic synthesis and secretion of vitel- logenin (Wallace 1985; Tyler 1991). The oocyte matu- ration phase, which follows vitellogenesis is also con- trolled by gonadotropin, which stimulates the produc- tion of a maturation inducing substance by the follicles cells 17α-20β -dihydroxy-4-pregnen-3-one (17α-20β - P) (Nagahama 1987; Kagawa 1994), which in turns mediates the activation of the maturation promoting factor (MPF) in the oocytes (Nagahama and Yamashita 1989). Besides, the transition from the vitellogenic stage to the final maturation stage in salmonid fish relies on a switch in steroidogenic pathways from E 2 to 17α-20β - P. Sexual maturation in salmon is also accompanied by morphological, and physiological changes, which cause changes in pigment distribution in the skin, re- sulting in a darkening of the skin, changes in jaw morphology, decreased efficiency of the immune sys- tem, and cessation of growth, and also changes in proximate composition and in the organoleptic char- acteristics of the muscle. In terms of aquaculture these changes associated with maturation are not ben- eficial, since they result in decreased value in the market. Therefore the prevention of sexual matura- tion in species of fish such as salmonids that undergo