Nonphotic entrainment in a diurnal mammal, the European ground squirrel (Spermophilus citellus)

Entrainment by nonphotic, activity-inducing stimuli has been investigated i n detail in nocturnal rodents, but little is known about nonphotic entrainm ent 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 degrees out of phase in nocturnal and diurnal mammals. (2) If nonphotic phase shifts depend on the phase of the pacemaker, the two PRCs s hould be in phase. We used the diurnal European ground squirrel (Spermophil us citellus) in a nonphotic entrainment experiment to distinguish between t he two possibilities. Ten European ground squirrels were kept under dim red light (<1 lux) and 20 +/- 1 degrees 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 fre e run, and 2 possibly entrained but displayed arrhythmicity during the expe riment. In a second experiment, a photic pulse was used in a similar protoc ol. Five out of 9 squirrels entrained, 1 did not entrain, and 3 yielded amb iguous results. During stable entrainment, the phase-advancing nonphotic pu lses coincided with the end of the subjective day, while phase-advancing li ght pulses coincided with the start of the subjective day: mean psi(nonphot ic) = 11.4 h; mean psi(photic) = 0.9 h (psi defined as the difference betwe en the onset of activity and the start of the pulse). The data for nonphoti c entrainment correspond well with those from similar experiments with noct urnal Syrian hamsters where psi(nonphotic) varied from 8.09 to 11.34 h. Thi s indicates that the circadian phase response to a nonphotic activity-induc ing stimulus depends on the phase of the pacemaker rather than on the phase of the activity cycle.