Phase response relationships between light pulses and the melatonin rhythm in rats

There is some controversy whether phase response curves constructed from studies conducted after acute release into constant darkness (Type II) or after prolonged constant darkness are comparable. This study investigated the effects of brief low-intensity light pulses on the onset of 6-sulphatoxymelatonin excretion in rats 48 to 60 h after lights-off and after 14 days of continuous darkness. In the former condition, maximum phase delays occurred between 4 and 6 h after expected lights-off, but no phase advances were observed within 2 days of the presentation of the stimulus. When the times of the pulses were plotted in relation to the individual onsets, peak light-induced phase delays occurred 0 to 2 h after melatonin onset. After 14 days in continuous darkness, the peak phase delays also occurred 0 to 2 h after melatonin onset and were slightly but significantly smaller. No significant phase advances were observed. In a separate small series of experiments, the temperature rhythm of rats was shown to be delayed by a comparable degree to that of melatonin by light pulses 2 and 4 h after expected lights-off under the Type II conditions. It is concluded that phase response curves conducted under Type I and Type II conditions are comparable.