CHANGES IN SERUM GTH LEVELS DURING THE INDUCED OVULATION OF THE LARGE FIN LONG BARBEL CATFISH, MYSTUS MACROPTERUS AND LEIOCASSIS LONGIROSTRIS

Mystus macropterus(Bleeker)and Leiocassis longirostris Gunther were collected from the Jialing River, a tributary of the Yangtze River, during their spawning season. The homologous radioimmunoassay(RIA)for African catfish, Clarias garicpinus, gonadotropin was used to determine the serum GTH levels of these two fishes during the induced ovulation by injecting luteinizing hormone-releasing hormone analogue(LHRH-A)plus domperidone(DOM), D2 receptor antagonist of dopamine(DA)for M. macropterus and by injecting LHRH-A plus silver carp pituitary suspension(SCPS)for L. longirostris. Like most teleosts, the gonadotropin release of these two fishes was under the dual regulation Of gonadotropin-releasing-hormone(GnRH)and DA from the hypothalamus. In M. macropterus, LHRH-A alone stimulated GTH secretion, and DOM potentiated this effect of LHRH-A. Whereas DOM alone had no effect on basal GTH secretion of the fish. A preovulatory GTH surge was found prior to the ovulation of these two fishes. Ovulation cannot be successfully induced unless the preovulatory serum GTH peak could surpass a threshold. The induced serum GTH peak for males were much lower than that for females, although male's serum GTH level also elevated rapidly before spermiation. Similar cases were found in some other teleosts, both in natural and induced spawning condition.