Nonphotic Phase-Shifting and the Motivation to Run: Cold Exposure Reexamined

Abstract

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°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.