Adrenergic regulation of type II 5'-deiodinase circadian rhythm in rat harderian gland

This paper reports on the regulation of the nyctohemeral profile of type II thyroxine 5'-deiodinase (T₄5'D) activity in the rat harderian gland. Harderian gland T₄5'D activity exhibits a nighttime increase with maximal values late in the dark period (0200-0400 h) and basal values during the daytime. The nocturnal rise of the deiodinating activity was prevented by either exposure of animals to light at night, injecting the animals with both α- and β-adrenergic receptor blockers, or bilateral superior cervical ganglionectomy (SCGx). However, adrenalectomy did not affect the enzyme activity in the harderian gland. In brown adipose tissue (BAT), where thyroid hormone metabolism is extremely dependent on α₁-adrenergic stimulation by blood-circulating catecholamines, adrenalectomy significantly decreased T₄5'D activity. Deiodinating activities in brain frontal cortex (BFC) and pituitary gland were unaffected by adrenalectomy. Unlike in the harderian gland, SCGx did not modify the T₄5'D activity in either BAT, BFC, or the pituitary gland. The results suggest that elevated plasma catecholamines are not required for harderian gland T₄5'D activation and that the nyctohemeral profile of the enzyme activity in the harderian gland is dependent on the noradrenergic input from the superior cervical ganglia.