The rhythms in pineal N-acetyltransferase (NAT) activity and melatonin levels were studied in the diurnally active Mexican ground squirrel and Eastern chipmunk. In the ground squirrel, both NAT activity and melatonin levels exhibited a marked nocturnal rise; these increases were not prevented by the exposure of these animals to a light irradiance of 200 μW/cm2 throughout the night. In the Eastern chipmunk, darkness at night was also associated with a marked rise in both the activity of the acetylating enzyme as well as the levels of melatonin. Again, these rhythms were not suppressed by the exposure of these animals to a light irradiance of 200 μW/cm2 for either 1 night or for 7 nights; exposure of chipmunks to light for 7 consecutive days did, however, reduce the rise in melatonin normally associated with darkness. The non-suppressibility of pineal NAT and melatonin by a 200 μW/cm2 light irradiance may relate either to the activity pattern of these animals, i.e., diurnal, or to their previous lighting history.
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