Phase response relationships between light pulses and the melatonin rhythmin rats

There is some controversy whether phase response curves constructed from st udies conducted after acute release into constant darkness (Type II) or aft er prolonged constant darkness are comparable. This study investigated the effects of brief low-intensity light pulses on the onset of 6-sulphatoxymel atonin excretion in rats 48 to 60 h after lights-off and after 14 days of c ontinuous darkness. In the former condition, maximum phase delays occurred between 4 and 6 h after expected lights-off, but no phase advances were obs erved within 2 days of the presentation of the stimulus. When the times of the pulses were plotted in relation to the individual onsets, peak fight-in duced phase delays occurred 0 to 2 h after melatonin onset. After 14 days i n continuous darkness, the peak phase delays also occurred 0 to 2 h after m elatonin onset and were slightly but significantly smaller. No significant phase advances were observed. In a separate small series of experiments, th e 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 u nder the Type II conditions. It is concluded that phase response curves con ducted under Type I and Type II conditions are comparable.