Gonadotropin-Releasing Hormone Stimulates Thyroid Activity in a Freshwater Murrel, Channa gachua (Ham.), and Carps, Catla catla (Ham.) and Cirrhinus mrigala (Ham.) P. Roy, 1 M. Datta, S. Dasgupta, and S. Bhattacharya 2 Department of Zoology, School of Life Sciences, Visva Bharati University, Santiniketan-731 235, West Bengal, India Accepted November 17, 1999 Injections of mammalian GnRH (mGnRH), salmon GnRH (sGnRH), and a homologous murrel, Channa punctatus, GnRH (cGnRH) to a murrel, Channa gachua, and a carp, Catla catla, at a dose of 1 g/250 g body wt significantly increased plasma thyroxine (T 4 ) levels above control. Piscine GnRHs (sGnRH and cGnRH) had significantly greater stimulatory effects compared with mGnRH. To observe whether this stimulatory effect by GnRHs is direct or indirect, thyroid follicles were isolated from hypobranchial muscles of freshwater murrel, C. gachua and incubated (1  10 6 follicles/well) in vitro at 30 °C for 2 h without hormone and for 3 h with hormones. Addition of these three GnRHs separately at a concentra- tion of 1 g/well stimulated T 4 secretion; sGnRH and cGnRH caused greater secretion of T 4 into the medium compared with mGnRH. Specificity of GnRH action in vitro was assessed by using anti-GnRH antibody which significantly ( P F 0.01) inhibited GnRH-augmented T 4 secretion. To gain further insight, 125 I uptake by thyroid follicles and formation of [ 125 I]T 4 from this radioiodine pool was monitored in the presence or absence of sGnRH. sGnRH greatly augmented 125 I uptake by the follicles which resulted a fourfold increase in [ 125 I]T 4 formation out of this pool of 125 I compared with the control. The results indicate GnRH stimulation of thy- roid hormone formation and release in these teleosts suggesting a possible different mode of regulation of thyroid hormone secretion in teleosts.  2000 Academic Press Although the influence of thyroid hormones on teleost reproduction has been convincingly demon- strated (Ball, 1960; Ichikawa et al., 1974; Chakraborti et al., 1986; Maitra and Bhattacharya 1989; Bhattacharya et al., 1996), the effects of reproductive hormones on thyroid activity have received limited attention (Matty et al., 1958; Sage, 1973; Bandopadhyay et al., 1991). There is some information for amphibia; the highest concentrations of thyroxine occur just prior to breed- ing period in two urodeles (Suzuki and Suzuki, 1981). In Rana ridibunda, an increased plasma thyroxine (T 4 ) and testosterone coincides with the breeding season together with maximal 5'-monodeiodinating activity (Vandrope et al., 1987). Elevated T 4 and testosterone levels in the breeding season may be linked to the increased level of gonadotropin-releasing hormone (GnRH) as an intravenous injection of mammalian GnRH significantly increased plasma T 4 and testoster- one in the ranid, R. ridibunda (Jacobs et al., 1988a), and neotenic axolotl, Ambystoma mexicanum (Jacobs and Kuhn, 1988). This suggests regulation of hypophyseal 1 Present address: Department of Physiology, University of Turku, Turku, Finland. 2 To whom correspondence should be addressed at present ad- dress: Indian Institute of Chemical Biology, 4, Raja S. C. Mullick Road, Calcutta-700 032, West Bengal, India. Fax: +91 33 473 5197. E-mail: samir_bh@yahoo.com. General and Comparative Endocrinology 117, 456–463 (2000) doi:10.1006/gcen.1999.7432, available online at http://www.idealibrary.com on 456 0016-6480/00 $35.00 Copyright  2000 by Academic Press All rights of reproduction in any form reserved.