Fish Physiology and Biochemistry 28: 29–32, 2003. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 29 Aromatase expression in zebrafish and channel catfish brains: changes in transcript abundance associated with the reproductive cycle and exposure to endocrine disrupting chemicals Y. Kazeto, R. Goto-Kazeto, A.R. Place and J.M. Trant ∗ Center of Marine Biotechnology, University of Maryland Biotechnology Institute, 701 East Pratt Street, Bal- timore, MD 21202, USA ∗ Author for correspondence (Phone: 1-410-234-8820; Fax: 1-410-234-8896; E-mail: trant@umbi.umd.edu) Key words: CYP19, endocrine disrupting chemical, gene expression, pituitary, reproductive endocrinology Abstract Cytochrome P450-aromatase (CYP19) is the critical steroidogenic enzyme controlling estrogen biosynthesis. In teleosts, two genes for CYP19 are found with CYP19A1 expressed mainly in the ovary and CYP19A2 expressed in both brain and pituitary. The promoters for these two homologous genes are considerably different. Exposure to estrogenic endocrine disrupting chemicals (EDC) such as ethinylestradiol and nonylphenol resulted in strong up-regulation of the CYP19A2 gene whereas the expression of CYP19A1 was largely unaffected. Moreover, CYP19A2 transcript abundance was shown to vary during the spawning cycle of both the catfish and the zebrafish which implies an association of the CYP19A2 gene in brain-pituitary endocrinology. Together these findings suggest that EDCs could affect the reproductive physiology of fish through disrupting the gene expression of CYP19A2 in the brain. Introduction Estrogens, especially estradiol-17β (E2), are key sex steroid hormones involved in various physiological events including early development, onset of puberty, and adult reproductive cycles. Cytochrome P450 aro- matase (CYP19) is the critical steroidogenic enzyme responsible for aromatization of androgens to estro- gens. Unlike other vertebrates, teleosts (including the channel catfish and zebrafish) possess two distinct CYP19 genes located on different chromosomes (Tchoudakova and Callard 1998; Chiang et al. 2001; Trant et al. 2001). CYP19A1 is predominantly ex- pressed in the ovary while the CYP19A2 transcript is mainly detected in neural tissue such as brain. The teleost pituitary also expresses large amounts of CYP19A2 (Gelinas et al. 1998) compared to the mam- malian pituitary which is aromatase-negative. These features make teleosts a unique and advantageous model for studying CYP19A2 expression and regu- lation. Unlike mammals and birds where aromatase expression is important for neural development (Beyer et al. 1994), brain sexual differentiation, and sex be- havior (Bakker et al. 2002; Balthazart et al. 1997; McEwen 1994), the function of CYP19A2 in fish is unclear. Recent molecular characterization of CYP19s in zebrafish revealed that CYP19 genes are differen- tially expressed during early development in the em- bryo/juvenile and that sex steroids differentially reg- ulate the transcription of the two genes (Trant et al. 2001). Furthermore, a number of potential promoter elements, including those for endocrine disrupting chemicals (EDC) have been identified in the 5 ′ - flanking regions of zebrafish CYP19A1 and CYP19A2 (Kazeto et al. 2001), which implies that some classes of EDCs could disrupt the expression of these genes. In the present study, we measured the changes in CYP19A2 transcript abundance in both brain and pitu- itary of zebrafish (Danio rerio) and channel catfish