Effects of glucocorticoids on plasma levels of thyroid hormones (T4 and T3) and testicular activity in catfish, Clarias gariepinus during different phases of annual breeding cycle.

Effects of short-term administration of corticosterone and cortisol on plasma levels of thyroid hormones, gonado-somatic index and testicular histology have been reported in catfish, Clarias gariepinus during different phases of its breeding cycle. Corticosterone administration had no significant effect on plasma levels of T4, T3 and T3/T4 ratio, irrespective of doses and phases of breeding cycle. However, 5 µg dose of cortisol significantly increased plasma levels of T3 and the T3/T4 ratio during quiescent and regressive phases, while it significantly decreased plasma levels of T4 during progressive phase. During breeding phase, 2 µg and 5 µg doses of cortisol significantly decreased plasma levels of T4 and T3, respectively, while 5 µg dose of cortisol alone reduced T3/T4 ratio. Irrespective of phases of annual breeding cycle and doses, short-term administration of corticosterone and cortisol had no significant effect either on GSI or testicular histology. These findings suggest that corticosterone is ineffective in stimulating plasma levels of thyroid hormones, while cortisol, depending on dose and phase/season, may differentially increase, decrease or have no effect on plasma levels of thyroid hormones in C. gariepinus.

In vitro effects of 5-hydroxytryptophan, indoleamines and leptin on arylalkylamine N-acetyltransferase (AA-NAT) activity in pineal organ of the fish, Clarias gariepinus (Burchell, 1822) during different phases of the breeding cycle.

Arylalkylamine N-acetyltransferase (AA-NAT) is the rate-limiting enzyme of melatonin biosynthetic pathway. In vitro effects of 5-hydroxytryptophan (5-HTP) and indoleamines (serotonin, N-acetylserotonin and melatonin) were studied on AA-NAT activity in the pineal organ of the fish, C. gariepinus during different phases of its annual breeding cycle. Further, in vitro effects of leptin on AA-NAT activity in the pineal organ were studied in fed and fasted fishes during summer and winter seasons. Treatments with 5-HTP and indoleamines invariably stimulated pineal AA-NAT activity in a dose-dependent manner during all the phases. However, leptin increased AA-NAT activity in a dose-dependent manner only in the pineal organ of the fed fishes, but not of the fasted fishes irrespective of the seasons.