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Phase Shifting the Hamster Circadian Clock by 15-Minute Dark Pulses

Department of Psychology, 5742 Little Hall, University of Maine, Orono, ME 04469-5742, USA; e-mail: alanr{at}maine.edu

Suzanne M. Dwyer

Department of Psychology, University of Maine, Orono, ME, 04469-5742, USA; Dalhousie University, Halifax, NS, Canada

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 mirrorimage mechanism, but more recent observations suggest that dark pulseinduced 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 phaseshifting 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 phaseshifting effect of restraint at subjective dusk.

Key Words: circadian rhythm • phase shifting • phase response curve • dark pulse • hamster

Journal of Biological Rhythms, Vol. 17, No. 3, 238-247 (2002)
DOI: 10.1177/07430402017003007


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