Phase Response Relationships between Light Pulses and the Melatonin Rhythm in Rats

Abstract

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.

Enhanced circadian photoresponsiveness after prolonged dark adaptation in seven species of diurnal and nocturnal rodents

“…In many studies, animals were kept in darkness for the duration of the study and were repeatedly exposed to light pulses at intervals of 10 to 46 days without intermediary re-entrainment to a light-dark cycle [6,[18][19][20][21]25,26,33,34,40,41], so that the actual state of dark adaptation of the animals at the time of each pulse could not be ascertained. In other studies, light-pulse tests were consistently preceded by a standardizing light-dark cycle but the interval in darkness prior to the pulse ranged from 5 to 21 days across studies [4,5,9,10,22,30,39,42,43]. Only three studies in Syrian hamsters [7,35,36] and two studies in mice [27,28] have systematically investigated the effect of duration of dark exposure on circadian photic responsiveness.…”

Timing - Understanding Central and Peripheral Clocks

“…In both animal models and humans, changes in light exposure have been shown to re-entrain, or phase shift, SCN activity in a dose dependent manner. Phase-response curves of locomotor activity in rodents (20), melatonin in rats (21) and humans (22), and core body temperature in humans (23) Figure 1. The non-visual pathway of light from the environment to the suprachiasmatic nucleus (SCN) and how it regulates the timing of the transcriptional/translational feedback loop (19).…”