The effect of thyroid hormones on rat pineal indoleamine metabolismin vitro

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Abstract
The effect of TSH, triiodothyronine (T3) and thyroxine (T4) on basic and noradrenaline (NA)-induced indoleamine metabolism was investigated in rat pineal culture based on the observation that, under the experimental system used, melatonin is formed from its amino acid precursor tryptophan. When added to the culture medium, alone or in the presence of NA, TSH in dosages of 5 and 20 mU had no significant effect on pineal tryptophan metabolism. T4 at 2.5μg increased the melatonin concentration and, in the presence of NA 10−4 M, also induced an increase in N-acetylserotonin (NAS). T3, already at a dosage as low as 0.25μg enhanced the melatonin concentration, while 2.5μg produced significant increases in concentrations of all pineal indoleamines measured. The latter dosage of Ts also enhanced the NA-stimulated NAS. On adding larger quantities of T4 or T3 to the medium none of the changes encountered with the lower dosages could be seen. The results obtained may suggest a direct positive feedback between the thyroid and pineal glands.