Catecholaminergic control of LH secretion: E2 modulation of tyrosine hydroxylase activity in female catfish Heteropneustes fossilis

Tyrosine hydroxylase (TH) which catalyses the rate – limiting step in catecholamine (CA) synthesis shows significant annual variations with activity and kinetics increasing with the progress of gonad recrudescence up to spawning and decreasing thereafter. Estradiol-17 β (E₂) exerts biphasic effects on in vivo and in vitro enzyme activity and kinetics: low dosages/concentrations stimulated, and high dosages/concentrations inhibited them. Preincubations of hypothalamic enzyme preparations with low (10^?9 M) or high (10^?3 M) E₂ for 15 min at 30 °C, followed by cAMP (1.0 mM) for 10 min at 30 °C produced differential effects: an additive effect in the low concentration group and an inhibitory response in the high concentration group. The stimulatory or inhibitory effects on TH activity could be related to changes in apparent K_m and V_max of the enzyme for substrate and cofactor. The results suggest that TH activity and kinetics are influenced by the circulating titer of E₂ and the steroid interacts with the cAMP signaling pathway in the acute regulation of TH.