Type II Thyroxine 5′‐Deiodinase Activity in the Rat Brown Adipose Tissue, Pineal Gland, Harderian Gland, and Cerebral Cortex: Effect of Acute Cold Exposure and Lack of Relationship to Pineal Melatonin Synthesis

The effect of acute cold exposure for 6 hours on nocturnal type II thyroxine 5′‐ deiodinase (5′‐D) activity was studied in brown adipose tissue (BAT), Harderian gland, cerebral cortex, and pineal gland of the rat. Moreover, the effect of iopanoic acid (IOP), a potent inhibitor of 5′‐D activity, on both pineal N‐acetyltransferase (NAT) activity and melatonin content in rats maintained in a cold environment was also examined. Results show that acute cold exposure significantly increases 5′‐D activity in BAT but not in either the pineal gland, Harderian gland, or cerebral cortex. In all tissues, the injection of IOP reduced dramatically 5′‐D activity, while exposure of the animals to light at night reduced 5′‐D activity in pineal gland but not in either the Harderian gland or BAT while light exposure at night increased cerebrocorticol 5′‐D activity. Cold exposure did not change either pineal NAT activity or the melatonin content of the gland. Finally, when pineal 5′‐D activity was inhibited by IOP treatment, neither nocturnal pineal NAT activity nor melatonin content was affected.