Scheduled activity reorganizes circadian phase of Syrian hamsters under full and skeleton photoperiods.

Circadian rhythms can be shifted or entrained by light and by arousing nonphotic stimuli. Interactions between photic and nonphotic stimuli were examined by subjecting hamsters to a daily 3 h bout of induced running under full (FPP) or skeleton (two daily light pulses; SPP) photoperiods. Activity scheduled in mid-day of a FPP induced large phase delays (260 +/- 63 min) in hamsters that ran more than 4000 rev/3 h. Split rhythms were not evident in constant dark (DD) tests. Activity scheduled in mid-subjective day of a SPP induced 180 degrees inversions of circadian phase, apparently achieved by oscillator splitting in some cases. Activity scheduled late-day and early-night induced a mix of phase delays, advances and no responses. Activity scheduled at two phase ranges late in the night had no effect, but scheduled 1 h later (beginning the last hour of darkness) induced large phase delays (238 +/- 30 min). There was no evidence of oscillator splitting during DD tests, but free-running period was significantly longer in groups that showed large phase delays. Induced running schedules have powerful effects on the phase of photic entrainment and can alter intrinsic pacemaker properties, including internal oscillator coupling and period.

Phase shifts to refeeding in the Syrian hamster mediated by running activity.

Circadian rhythms in hamsters can be entrained by restricted daily feeding schedules. Phase control may be exerted by feeding per se, or by wheel running in anticipation of food access. Phase modulation by feeding was examined here by depriving hamsters of food for 9-24 h and refeeding at 1 of 7 different zeitgeber times on the first day of constant dim light. Significant group mean phase-advance shifts were observed only following 24 h and 17 h deprivations ending in the mid-subjective day, 7 h before the usual time of lights off (mean shifts 28 min and 66 min, respectively). The largest phase shifts were associated with wheel running during the first 6 h of refeeding. When running wheels were locked during this time in an additional group, no phase shifts were observed. A trend for small phase delays was evident for 14 h deprivations ending at the beginning of the subjective night, but no significant group mean or individual shifts were observed at other refeeding times. Refeeding after food deprivation, thus, appears to have minimal effects on circadian phase in hamsters; wheel running associated with refeeding may account for occasional shifts observed.

Nonphotic phase-shifting and the motivation to run: cold exposure reexamined.

Circadian rhythms in rodents can be phase shifted by appropriately timed activity. This may be dependent on motivational context; running induced by a novel wheel is effective, whereas running induced by cold has been inferred to be ineffective. This issue was reexamined using a different cold exposure procedure. On the first day of constant dark, 6 h before usual dark onset, Syrian hamsters were exposed to cold (+/- 4 degrees C) in their home cages, or were confined to novel wheels for 3 h. Activity rhythms were significantly phase advanced by 92 +/- 10 min following cold exposure and 86 +/- 17 min following novel wheel running, compared to 13 +/- 18 min in a control condition. Most hamsters exhibited eating, drinking, and modest levels of wheel running (1367 +/- 292 counts/6 h) during and for 3 h after cold exposure. Phase shifts following cold were not affected by food and water deprivation but were significantly attenuated by locking the wheel for 6 h beginning at cold onset (24 +/- 12 min). These data indicate that cold-induced running, even at modest levels, is an effective nonphotic Zeitgeber and do not provide support for a hypothesis that motivational contexts determine the phase-shifting value of physical activity.