Nocturnal increase in pineal melatonin production in two lemming species, Dicrostonyx hudsonius and D. groenlandicus.

Pineal N-acetyltransferase (NAT) activity and melatonin levels and thyroxine 5'-deiodinase (5'D) activity in brown adipose tissue (BAT) were compared during the day and night in two species of lemmings, Dicrostonyx hudsonius and D. groenlandicus. In both species, NAT and melatonin exhibited significant night-time increases in the pineal gland. Thyroxine 5'D activity in BAT did not show a statistically significant day-night difference in either species of lemming, suggesting the night-time rise in melatonin does not induce a similar increase in BAT 5'D.