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

Abstract

In the female catfish Heteropneustes fossilis, administration of thyroxine (T 4 ) , 1 µg/g body weight, i.p., in both gonadal resting and preparatory phases for 7, 14 and 21 days caused hyperthyroidism, as evidenced from a duration-dependent significant increase in serum triiodothyronine (T 3 ), and of tyrosine hydroxylase (TH) activity in telencephalon, hypothalamus-pituitary and medulla oblongata (Newman-Keuls' test; P,0·05). Hypothyroidism induced by adding 0·03% thiourea to aquarium water holding the catfish for 7, 14 and 21 days decreased serum T 3 levels in a duration-dependent manner (Newman-Keuls' test; P,0·05) and inhibited TH activity in the brain regions. T 4 replacement in 21-day thiourea-treated fish restored and even elevated significantly serum T 3 levels as well as brain TH activity in a duration-dependent manner. In general, the changes in enzyme activity were higher in the forebrain regions than medulla oblongata and in the resting phase than preparatory phase. Kinetic studies by LineweaverBurk plots showed that the stimulatory effect following T 4 administration and T 4 replacement on TH activity was due to increased affinity of the enzyme for its cofactor (6,7-dimethyl-2-amino-4-hydroxy-5,6,7,8-tetrahydropteridine), as evident from a significant decrease in apparent Michaelis-Menten constant (K m ) and an increase in apparent velocity maximum (V max ). The TH inhibition due to the thiourea treatment can be related to decreased affinity of the enzyme for its cofactor, as evident from a significant increase in apparent K m value and a significant decrease in V max . These data clearly show that circulating levels of T 4 /T 3 modulate brain TH activity by altering the kinetic properties of the enzyme, which, in turn, influence catecholaminergic activity and dependent functions.

Effect of thyroid hormone depletion on monoamines and expression patterns of catfish GnRH in the air-breathing catfish, Clarias gariepinus

“…Levels of DA remain unaltered in thyroid hormone depleted fish compared to control fish. Thyroid depletion affects several of the catecholamines, namely, L-dopa, NE and E. This is in accordance with earlier report by Chaube and Joy (2003) where an increase in the activity of tyrosine hydroxylase, a key enzyme in the catecholaminergic pathway was demonstrated following thyroxine treatment.…”

Thyroid Hormone-Otx2 Signaling Is Required for Embryonic Ventral Midbrain Neural Stem Cells Differentiated into Dopamine Neurons

“…However, much is still unknown regarding the effects of the thyroid hormone in midbrain. Some investigations indicate that the thyroid hormone influences TH activity and SNc DA neuron number [13,14]. Our results provide comprehensive evidence that the thyroid hormone is critical for DA neuron differentiation from embryonic VM NSCs.…”

“…T3 was reported to affect the morphogenesis of DA neurons from the fetal mouse mesencephalon [12]. Neonatal hypothyroidism markedly decreases the concentration of DA and the activity of its rate-limiting enzyme, tyrosine hydroxylase (TH) [13]. In a genetic thyroid-stimulating hormone receptor mutant mouse model, a reduction in the number of midbrain DA neurons has been observed [14].…”

Thyroid hormone modulation of ovarian recrudescence of air-breathing catfish Clarias gariepinus

“…The dosage of thiourea and thyroxine (T 4 ) treatment was in accordance with earlier reports and also after confirming with our pilot studies. Thyroid hormone depletion was induced by adding 0.03% of thiourea (Chaube and Joy 2003) to well-aerated aquarium tanks holding the juvenile, immature and mature catfish for 21 days. T 4 'overdose' was induced by adding 100 ppm of T 4 (T 4 sodium tablets dissolved in 3 ml of 0.1 M NaOH) to well aerated aquarium tanks holding mature catfish in 20 l of fresh water for 21 days (Plate et al 2002).…”