Phase shifting the hamster circadian clock by 15-minute dark pulses.
The mammalian circadian pacemaker can be phase shifted by exposure to a period of darkness interrupting otherwise continuous light. Circadian phase shifting by dark pulses was interpreted originally as reflecting a photic mirror-image mechanism, but more recent observations suggest that dark pulse-induced phase shifting may be mediated by a nonphotic, behavioral state-dependent mechanism. The authors recently presented evidence indicating that the dark-pulse phase response curve (PRC) is in fact a complex function, reflecting both photic mirror image and nonphotic mechanisms at different phases of the circadian cycle. Previous studies of dark pulse-induced phase shifting have universally employed relatively long (2 to 6 h) pulses, which complicates PRC analysis due to the extended segment of the underlying PRC spanned by such a long pulse. The present study was therefore designed to examine the phase-shifting effects of brief 15-min dark pulses presented at both mid-subjective day and subjective dusk, and to explore the possible activity dependence of these effects by using physical restraint to prevent evoked locomotor activity. The results indicate that 15-min dark pulses are effective phase-shifting stimuli at both midday and dusk. Furthermore, as with longer dark pulses, phase shifting by 15-min dark pulses is completely blocked by physical restraint during subjective day but combines in a simple additive manner with the independent phase-shifting effect of restraint at subjective dusk.
Rosenwasser
Alan M
AM
Department of Psychology, University of Maine, Orono 04469-5742, USA. alanr@maine.edu
Dwyer
Suzanne M
SM
D000818:Animals; D001683:Biological Clocks / Q000502:physiology*; D002940:Circadian Rhythm / Q000502:physiology*; D006224:Cricetinae / Q000502:physiology*; D003624:Darkness; D008297:Male; D008647:Mesocricetus; D010775:Photic Stimulation; D012149:Restraint, Physical; D013997:Time Factors