Effects of administration of some monoamine-synthesis blockers and precursors on ovariectomy-induced rise in plasma gonadotropin II in the catfish Heteropneustes fossilis

Abstract

In the present study, effects of three daily ip injections of some monoamine (MA)-synthesis blockers and precursors on plasma gonadotropin (GTH- II) levels were investigated in 3-week ovariectomized catfish, and the effects were correlated with changes in hypothalamic MA contents. Administration of α-methylparatyrosine (α-MPT; 250 μg/g BW, a tyrosine hydroxylase inhibitor) significantly decreased the ovariectomy-induced rise in GTH-II compared to that of the sham control group. Injection of L- dihydroxyphenylalanine (10 μg/g BW) in the α-MPT-treated fish elevated the GTH-II level significantly over that of the sham control group but not to the level of the ovariectomized fish. Administration of noradrenaline (NA; 5 μg/g BW) in combination with α-MPT counteracted the effect of the latter and maintained the plasma GTH-II level at that of the ovariectomized fish. Plasma GTH-II level was decreased significantly in the diethyldithiocarbamate (10 μg/g BW, a dopamine-β-hydroxylase inhibitor)-injected fish compared to that of the sham and ovariectomized control groups. Administration of para- chlorophenylalanine (p-CPA; 100 μg/g BW, a tryptophan hydroxylase inhibitor) decreased the GTH-II level significantly compared to that of the ovariectomized group. Supplementation of 5-hydroxytryptophan (20 μg/g BW) with p-CPA nullified the latter's effect and maintained the GTH-II level at that of the ovariectomized fish. The administration of both p-CPA and α-MPT significantly reduced plasma GTH-II to the lowest mark compared to that of the sham, ovariectomized, and all other treatment groups. These results clearly show that the ovariectomy-induced rise in GTH-II was mediated through simultaneous activation of hypothalamic serotonergic and NA-ergic and suppression of dopaminergic mechanisms.