In vitro effects of catecholamines and catecholestrogens on brain tyrosine hydroxylase activity and kinetics in the female catfish Heteropneustes fossilis

Effects of catecholamines and catecholestrogens on tyrosine hydroxylase (TH) activity and kinetics were investigated in the telencephalon and hypothalamus of female Heteropneustes fossilis in gonad quiescent (resting) and recrudescent (preparatory) phases. Dopamine, noradrenaline and adrenaline and the catecholestrogen, 2-hydroxyestradiol-17beta inhibited TH activity in a concentration-dependent manner in both resting and preparatory phases, with a higher effect in the resting phase. Two- methoxyestradiol-17beta did not alter TH activity in any season. The catecholamines inhibited TH in a competitive manner increasing apparent K-m values significantly without altering the apparent V-max . Two-hydroxyestradiol-17beta inhibited significantly the enzyme in a noncompetitive manner and decreased apparent V-max without altering apparent K-m values. The apparent K-i is higher for dopamine than noradrenaline or adrenaline. The apparent K-i for 2-hydroxyestradiol-17beta is not significantly different from that of noradrenaline. The present results suggest an interaction between oestradiol-17beta (E-2 ) and catecholamine metabolism at the level of tyrosine hydroxylation and E-2 effects on catecholamines may be mediated through its 2-hydroxylation.