Indian Journal of Experimental Biology Vol. 48, June 2010, pp. 549-553 Inhibition of hCG-induced spawning by α-methylparatyrosine, a tyrosine hydroxylase inhibitor, in the catfish Heteropneustes fossilis (Bloch) R Chaube † & K P Joy * Department of Zoology, Banaras Hindu University, Varanasi 221005, India Received 13 August, 2009; revised 15 March 2010 In this study, the effect of pharmacological inhibition of catecholaminergic activity on hCG-induced spawning was evaluated and correlated with tyrosine hydroxylase (TH) activity, the rate-limiting enzyme in catecholamine biosynthesis. Gravid female H. fossilis collected in both prespawning and spawning phases were given α-methylparatyrosine (α-MPT: 250μg/g body weight, ip, an irreversible inhibitor of TH) and human chorionic gonadotropin (hCG: 100 IU/fish, ip) alone or in combination. The fish were sampled at different intervals for measuring hypothalamic and ovarian TH activity and checking spawning response. The administration of hCG resulted in ovulation and spawning in both phases with a higher response in the spawning phase. The administration of α-MPT did not induce any response, like the control fish. In the hCG + α-MPT groups, the spawning response of hCG was significantly inhibited and delayed by the inhibitor. The spawning response of hCG was accompanied by a significant increase in both hypothalamic and ovarian TH activity at 6 and 12 h of the injection. However, at 24hr the activity decreased except in the spawning phase. The α-MPT treatment inhibited TH activity significantly in a duration–dependent manner. In the hCG + α-MPT groups, enzyme activity was inhibited at all duration. The results indicate the involvement of catecholamines during the hCG-induced spawning and the specific functional nature of the involvement needs further investigation. Keywords: Human Chorionic Gonadotropin, α-Methylparatyrosine, Ovulation, Spawning, Tyrosine hydroxylase A majority of teleosts do not breed in confined waters and breeding has to be induced artificially. Administration of exogenous gonadotropins like hCG or induction of an endogenous luteinizing hormone (LH) surge by the application of gonadotropin-releasing hormone (GnRH) or its analogue with or without a dopamine receptor (D 2 ) antagonist (for e.g. ovaprim) are the most commonly used technique to induce breeding in fish for experimental or commercial purposes. While the mechanism underlying gonadotropin-triggered meiotic maturation is well documented at the physiological and molecular level 1 , the regulatory mechanisms controlling ovulation are not clearly understood. Physiological studies have shown that central catecholamines regulate gonadotropin (GTH) secretion in teleosts 2-4 . Dopamine (DA) inhibits, and noradrenaline (NA) stimulates, GTH secretion by modulating GnRH activity 5,3 . Hypothalamic catecholaminergic system shows seasonal and differential activity during the annual reproductive cycle of the catfish, DA predominates during resting and early recrudescent phases, and NA is predominant during late recrudescent and spawning phases 6 . The GTH inhibiting action of the non-steroidal inhibitor methallibure (ICI 33, 829: N-methylthiocarbamoyl-N’- [1-methylallyl) thiocabamoyl] hydrazine) is attributed to its potent dopamine- β-hydroxylase inhibiting activity, depleting NA and accumulating DA 7 . At the ovarian level, the involvement of sympathetic nervous system in the control of its functions has been demonstrated in mammals. NA is identified as the predominant ovarian catecholmine neurotransmitter and is implicated in ovarian development, steroidogenesis and folliculogenesis, and in ovulation and corpus luteum function 8-15 . However, studies reporting the involvement of catecholamines directly on ovarian functions are —————— * Correspondent author Telephone : 091-0542-6540735 Fax: 091-0542-2368174 E-mail: kpjoy@bhu.ac.in; kpjoybhu@gmail.com † Present address: Zoology Section Mahila Mahavidyalaya Banaras Hindu University Varanasi 221005, India E-mail: chauberadha@rediffmail.com