Changes in hypothalamic monoamine oxidase activity in relation to season, ovariectomy, and 17β-estradiol administration in intact and ovariectomized catfish, Clarias batrachus (L.)

Abstract

Cyclic changes in hypothalamic monoamine oxidase (MAO) activity were noted during the annual ovarian cycle of Clarias batrachus, with a high level in the preparatory phase and a low level in the spawning phase. Administration of 17β-estradiol (E 2) daily for 3 days induced both season- and dose-dependent responses in enzyme activity. In the preparatory phase, 0.05, 0.1, and 0.5 μg/g doses of E 2 enhanced enzyme activity in a dose-dependent manner, with significant changes being observed in the latter two dose groups. Administration of 1.0 and 10.0 μg/g doses decreased enzyme activity, with a significant difference elicited by the latter dose. In the spawning phase, enzyme activity was significantly elevated by 0.05 and 0.1 μg/g doses. MAO activity decreased insignificantly in the 0.5 and 1.0 μg/g groups and significantly in the 10.0 μg/g E 2 group. Enzyme activity decreased significantly 2, 3, and 4 weeks after ovariectomy (prespawning phase), in a time-dependent manner, and increased after 5 weeks (not significantly different from the sham control value) and 6 weeks. Administration of E 2 to 3-week ovariectomized fish elicited dose-dependent effects on MAO activity. E 2 injections at 0.05 and 0.1 μg/g doses not only restored the ovariectomy-induced decrease, but also elevated enzyme activity significantly compared with the sham + vehicle control. The higher doses (0.5 and 1.0 μg/g) were ineffective in restoring enzyme activity, which decreased significantly in the 1.0 μg/g group compared with ovariectomy and sham control values. The results suggest that the biphasic responses of MAO may be due to differential sensitivity of the enzyme to seasonally fluctuating levels of E 2 and that E 2 participates in the negative feedback control of gonadotropin secretion by modifying hypothalamic monoamine oxidase activity.