Pineal N-acetyltransferase activity in hamsters maintained in shortened light cycles

Pineal serotonin N-acetyltransferase activity (NAT) was compared among Syrian hamsters maintained in light: dark (LD) cycles of 14:10 or 2:22 or in an interrupted (LD 1/4:5 3/4 × 4) photoperiodic cycle. In hamsters maintained in a LD 14:10 photoperiod, the zenith of pineal NAT activity occurs at 04:00 h, approximately 8 hours after the onset of the dark phase. When the onset of the dark phase was advanced by 8 hours (in hamsters maintained in LD 2:22 cycles), the pineal NAT activity was advanced by an equivalent length of time. Hamster pineal NAT activity was sup-pressed throughout a 24-hour period when the animals were maintained in darkness interrupted with brief periods of light at 6 h intervals (LD 1/4:5 3/4 × 4). The results suggest that melatonin syn-thesis is determined according to when light occurs during the photoperiodic cycle rather than by the total amount of darkness or the dark: light ratio to which animals are exposed in a 24-hour period.