Mass and in situ molar activity of tyrosine hydroxylase in the median eminence: Effect of thyroidectomy and thyroid hormone replacement

The effects of thyroidectomy and thyroid hormone replacement on the mass and in situ molar activity of tyrosine hydroxylase (TH) in the median eminence (ME) and superior cervical ganglia (SCG) of male rats were investigated. The tissue specificity of these effects were evaluated by comparing the ME with the superior cervical ganglion (SCG). All animls were thyro-paralhyroidectomized (Tx) or sham Tx. Tx rats were treated daily for 3 weeks with 0.15 M Nad (solvent vehicle) or L - thyroxine (T4). Two doses of T4, 10 and 100 μg/day/kg BW, were used. Sham Tx rats were treated with 0.15 M NaCI. All animals were studied on the day following the last treatment. The mass of TH was determined using an immunoblot assay, and the in situ activity ofTH was calculated from the rate of intracellular accumulation of L-dihydroxyphenylalanine (DOPA) after administration of an inhibitor of DOPA decarboxylase activity. In the ME, thyro parathyroidectomy resulted in a 40% increase in the mass and a 100% increase in the in situ molar activity of TH over that of sham Tx rats. Compared to Tx animals given 0.15 M NaCL, Tx rats treated with a low dose of T4 (10 μg/day/kg BW) had a reduced quantity of TH in the ME, but the molar activity of the enzyme was increased. Treatment of Tx rats with a high dose of T4 (100 μg/day/kg BW) restored TH mass but not the in situ activity of TH in the ME to the level seen in shamTx rats. The quantity and in situ molar activity of TH in the SCG were unaffected by thyroidectomy or T4 treatment. The mean concentration of dopamine in hypophysial portal plasma of Tx rats was three times that or sham-operated animals. It is concluded that increased hypothalamic secretion of dopamine observed in Tx rats is associated with an increase in the mass and in situ molar activity of TH in the ME. These data are supportive of a modulatory role or thyroid hormone(s) on the secretory activity of the tuberoinfundibular dopaminergic neurons.