Abstract
The effect of 24-hr exposure to cold (5 degrees C) was studied in male Syrian hamsters adapted to short days (LD 8:16). Both pineal N-acetyltransferase (NAT) activity and pineal and serum concentrations of melatonin showed a clear, diel rhythm with a moderate but significant increase late in the dark period. The nighttime peak levels of NAT activity and pineal and serum melatonin were significantly higher in the animals that had been exposed to cold than in those remaining in warm (22 degrees C) conditions. The activity of type-II 5'-deiodinase (5'D) in the brown adipose tissue (BAT) was not different after 24 hr of cold exposure between cold- and warm-exposed hamsters, and the enzyme did not show any diel rhythmicity. It has been speculated that some effects of cold exposure may be simulated by melatonin treatment; the present data further support this notion. The apparent lack of response in BAT 5'D activity remains enigmatic and needs further investigation.
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