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Nonphotic Entrainment in a Diurnal Mammal, the European Ground Squirrel (Spermophilus citellus)

Zoological Laboratory, University of Groningen, P.O. Box 14, 9750 AA Haren, the Netherlands

N. Mrosovsky

Departments of Zoology and Psychology, University of Toronto, Toronto, Ontario, M5S 3G5, Canada

Serge Daan

Zoological Laboratory, University of Groningen, P.O. Box 14, 9750 AA Haren, the Netherlands

Entrainment by nonphotic, activity-inducing stimuli has been investigated in detail in nocturnal rodents, but little is known about nonphotic entrainment in diurnal animals. Comparative studies would offer the opportunity to distinguish between two possibilities. (1) If nonphotic phase shifts depend on the phase of the activity cycle, the phase response curve (PRC) should be about 180° out of phase in nocturnal and diurnal mammals. (2) If nonphotic phase shifts depend on the phase of the pacemaker, the two PRCs should be in phase. We used the diurnalEuropean ground squirrel (Spermophilus citellus) in a nonphotic entrainment experiment to distinguish between the two possibilities. Ten European ground squirrels were kept under dim red light (<1 lux) and 20 ± 1° C. During the entrainment phase of the experiment, the animals were confined every 23.5 h (T) to a running wheel for 3 h. The circadian rhythms of 6 squirrels entrained, 2 continued to free run, and 2 possibly entrained but displayed arrhythmicity during the experiment. In a second experiment, a photic pulse was used in a similar protocol. Five out of 9 squirrels entrained, 1 did not entrain, and 3 yielded ambiguous results. During stable entrainment, the phase-advancing nonphotic pulses coincided with the end of the subjective day, while phase-advancing light pulses coincided with the start of the subjective day: mean [.psi]nonphotic = 11.4 h; mean [.psi]photic =0.9h([.psi] defined as the difference between the onset of activity and the start of the pulse). The data for nonphotic entrainment correspond well with those from similar experiments with nocturnal Syrian hamsters where [.psi]nonphotic varied from 8.09 to 11.34 h. This indicates that the circadian phase response to a nonphotic activity-inducing stimulus depends on the phase of the pacemaker rather than on the phase of the activity cycle.

Key Words: circadian rhythm • nonphotic entrainment • photic entrainment • diurnal mammal • phase angle • onset of activity • scheduled wheel running • ground squirrel

Journal of Biological Rhythms, Vol. 14, No. 5, 409-420 (1999)
DOI: 10.1177/074873099129000812


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