Circadian rhythms in rat pineal melatonin synthesis under constant darkness conditions

To investigate whether rat pineal cell is capable of generating a circadian oscillation in melatonin release，the cultured pineal cell in culture medium was collected at 6∶00, 8∶00, 10∶00, 12∶00 and 15∶00 during the day, and at 18∶00, 20∶00, 22∶00, 1∶00 and 4∶00 at night respectively. The melatonin level in culture medium was measured by a direct radioimmunoassay. The complex cosine functions (CCF) statistical analysis indicates that the rat pineal cell melatonin synthesis shows an intrinsic rhythm at 3, 6, 9 day culture under constant dark conditions. However, the intrinsic rhythm disappears at 12 day culture. This result suggests that the rat individual pineal cells can function as a circadian oscillator in vitro.