Indian Journal of Experimental Biology Vol. 52, May 2014, pp. 527-537 Aromatase activity in brain and ovary: Seasonal variations correlated with circannual gonadal cycle in the catfish, Heteropneustes fossilis Neerja Aggarwal*, Shashi Vadan Goswami, Preeti Khandelwal & Neeta Sehgal Department of Zoology, University of Delhi, Delhi 110 007, India Received 16 April 2013; revised 02 September 2013 Seasonal variations in the aromatase activity in H. fossilis estimated by a microassay were correlated with the sex steroids, vitellogenin in and ovarian weight during circannual reproductive cycle. In the female catfish, aromatase activity was detectable in the hypothalamus throughout the year whereas in ovary only during active vitellogenesis. In the catfish, hypothalamic aromatase levels increased two times during annual gonadal cycle, once in a fully gravid fish and then in a reproductively quiescent fish. On the other hand, increase in the ovarian aromatase activity was observed only during vitellogenesis, which showed a direct correlation with plasma levels of sex steroids. Further, plasma levels of testosterone and estradiol suggested a precursor-product relationship. At the completion of vitellogenesis, ovarian aromatase activity declined sharply resulting in elevation of plasma testosterone levels, which in turn could be utilized as substrate by the hypothalamic aromatase whose activity was the highest in the postvitellogenic catfish. At least two isoforms of gene, cyp19a and cyp19b, coding for aromatase in ovary and brain respectively were expressed in the catfish. Aromatase activity was more concentrated in those areas of catfish brain, which have been implicated in the control of reproduction. Keywords: Aromatase, Catfish, cyp19 gene, Heteropneustes fossilis, Hypothalamus Cytochrome P450 aromatase is predominantly expressed in brain and ovary but its presence in other tissues such as liver, kidney, adipose tissue and skin of fish has also been reported 1 . It is the key enzyme for the synthesis of estrogens from androgens. It is encoded by the CYP19 gene, a member of the cytochrome P450 gene superfamily, and catalyzes the conversion of C19 androgens to C18 estrogens. Estrogens produced by aromatization of androgens in the ovarian follicles, is essential for the biosynthesis of precursors of egg-yolk proteins by hepatocytes. It is also well characterized that estrogens play a pivotal role for sex reversal in hermaphrodites and temperature sensitive fishes 2-4 . In the brain, the synthesis of estrogen by the aromatization of androgen is related to neuroendocrine functions, sexual behavior, and differentiation during the development of the central nervous system 5,6 . Recent data also indicate that estrogens produced locally in the brain by aromatization of androgens is possibly important for neurogenesis and brain repair 7-9 . Most of the teleosts have exceptionally high aromatase activity (per unit protein) in the CNS, which is approximately ten times higher than in the ovary. In comparison with other vertebrates, the activity of this enzyme is hundred times higher in fish ovary than in the mammalian ovary 10-12 . Further, various biochemical and micro-anatomical studies have reported maximum aromatase activity in areas of brain that are associated with reproductive functions 13-23 . A perusal of literature suggests that considering high constitutive enzyme activity in brain, levels in forebrain vary seasonally, displaying a distinct peak when gonadal weight is maximum and circulating steroids are high 15,21,23-25 . Concomitantly, ovarian aromatase activity is high during vitellogenic phase that can be correlated with estrogens production and ovarian growth 11, 26-32 . Aromatase is a duplicated gene in most teleost fish 33-37 . This gene duplication gave rise to two different genes, namely cyp19a and cyp19b, which are expressed in ovary and brain respectively. The cyp19a gene is referred to as the ‘gonadal aromatase’ or ‘ovarian aromatase’, whereas the cyp19b gene as ‘neural aromatase’ or ‘brain aromatase’. The expression of duplicated cyp19 genes in two different tissues highlights the evolutionary significance of maintaining two active genes on duplicated chromosomes in fish for specific functions in the ovary and the brain 37-42 . _____________ *Correspondent author Telephone: 91 9958880622 Fax: +91 011 27666254 E-mail: neerja.agg@gmail.com