Effects of ovariectomy and oestradiol-17beta replacement on brain tyrosine hydroxylase in the catfish Heteropneustes fossilis: changes in in vivo activity and kinetic parameters

Abstract

In Heteropneustes fossilis, ovariectomy inhibited in vivo brain (hypothalamus-pituitary, telencephalon and medulla oblongata) tyrosine hydroxylase (TH) activity with significant effects in weeks 2, 3, 4 and 5 of the gonadal resting phase and in weeks 3, 4 and 5 of the prespawning phase (P<0·05, Tukey's test). Oestradiol-17 (OE 2 ) replacement in 3-week ovariectomised fish produced biphasic responses in both seasons; the low dosages of 0·05 and 0·5 µg/g body weight (BW) elevated TH activity, whereas the high dosages of 1·0 and 2·0 µg/g BW decreased it. The magnitude of the inhibition was higher in the resting phase than in the prespawning phase. The inhibitory effect of ovariectomy may be produced by elevating the apparent K m values (decreased affinity) of the enzyme for both -tyrosine (substrate) and dimethyltetrahydropteridine (cofactor) and consequently decreasing the V max . Significant changes (P<0·05) in both these parameters were noticed but showed minor differences with regard to the length of ovariectomy, season or brain regions. The biphasic effects of OE 2 replacement on TH activity seemed to be produced by differential effects on apparent K m and V max . The stimulatory effect of the low dosages of OE 2 coincides with a decrease in the apparent K m values (increased affinity) for both substrate and cofactor and an increase in the V max of the enzyme. The inhibitory effect of the high dosages of OE 2 correlated with an increase in the apparent K m values (decreased affinity) for both substrate and cofactor, and a decrease in the V max compared with the lower dosage groups. The results strongly suggested that OE 2 can modulate brain catecholaminergic activity at the level of tyrosine hydroxylation which, in turn, may alter gonadotrophin secretion. OE 2 may elicit biphasic effects by differentially altering the enzyme affinity towards the substrate and cofactor.

Thyroid hormone modulation of brain in vivo tyrosine hydroxylase activity and kinetics in the female catfish Heteropneustes fossilis

“…Further, cDNA cloning and sequencing of TH have been demonstrated in the rainbow trout (Linard et al 1998) and eel (Boularand et al 1998). In our earlier studies, we have demonstrated seasonal, diurnal, regional and sexual differences in, and effects of environmental factors (photoperiod and temperature) as well as oestrogens on brain TH activity and kinetics in the catfish (Chaube & Joy 2002, 2003. To the best of our knowledge, there are no studies relating to the role of thyroid hormones in the regulation of TH activity in non-mammals.…”

“…The increase in GSI suggests that hyperthyroidism caused a stimulation of ovarian activity. Since thyroid hormones modulate gonadal steroidogenesis (Cyr & Eales 1988, Timmermans et al 1997, the resulting strong E 2 feedback might have lessened the otherwise full-blown effect of T 4 /T 3 in the vitellogenic phase (Chaube & Joy 2002).…”

“…The telencephalon (excluding olfactory tract and bulb), and hypothalamus along with pituitary and medulla oblongata were separated as described earlier (Chaube & Joy 2002). Tissues were homogenised in 1 ml 30 mM sucrose containing 10 mM Tris-HCl buffer (pH 7·3) in a PotterElvehjem homogeniser with a loose-fitting Teflon pestle.…”

Estradiol-17β modulates dose-dependently hypothalamic tyrosine hydroxylase activity inhibited by α-methylparatyrosine in the catfish Heteropneustes fossilis

“…Tyrosine hydroxylase (TH) which catalyses the hydroxylation of L-tyrosine to L-DOPA is considered the rate-limiting enzyme in CA biosynthesis [24]. It has been shown in the catfish that TH activity varies in relation to season, and is influenced by changes in environmental photoperiod, temperature or circulating titre of E 2 [25][26][27].…”

“…Ovariectomized and E 2 -replaced catfish model, as described earlier [18,20,26,31,32] was used in the present study. The results indicate that E 2 indeed reverses the effect of the drug at low concentrations.…”