Diurnal Variation in Rat Liver Thyroid Hormone Receptor (TR)-α Messenger Ribonucleic Acid (mRNA) Is Dependent on the Biological Clock in the Suprachiasmatic Nucleus, whereas Diurnal Variation of TRβ1 mRNA Is Modified by Food Intake

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Abstract
Previous studies have shown a diurnal variation of certain isoforms of thyroid hormone receptors (TR) in rat liver. The genesis of these diurnal changes is still unknown. To clarify whether the biological clock, located in the hypothalamic suprachiasmatic nucleus (SCN), is involved, we made selective SCN lesions. Rats with an SCN lesion lost their circadian rhythm of plasma corticosterone and TSH when compared with intact animals. TRα1 and TRα2 mRNA expression of control rats was higher in the light period than in the dark period; changes that were abolished in the rats with SCN lesions. In contrast, liver TRβ1 mRNA of intact rats showed a diurnal variation that failed to reach statistical significance. To evaluate whether these effects could be explained indirectly by the disappearance of rhythmic feeding behavior in rats with SCN lesions, we performed a second experiment in which otherwise intact animals were subjected to a regular feeding (RF) schedule, with one meal every 4 h. When compared with rats with free access to food, RF only affected TRβ1 mRNA expression and had no effect on the diurnal changes in TRα1 and TRα2. We conclude that liver TRβ1 expression is most clearly affected by food intake. Diurnal changes in liver TRα1 and TRα2 are controlled by the biological clock in the SCN but not via changes in the daily rhythm of food intake. The findings may have physiological relevance for diurnal variation of T3-dependent gene expression, which is supported by a diurnal variation in the expression of the 5′-deiodinase gene.