Development and temporal timing of torpor in Siberian hamsters (Phodopus sungorus)

The nocturnal Siberian hamster undergoes daily torpor, a form of reversible hypothermia where body temperatures can decrease to as low as 15°C. Torpor is expressed during the day light hours which coincides with the daily rest phase. Previous work emphasized that the onset of torpor is synchronized by the environmental light-dark cycle, which is controlled by a circadian clock. I tested the alternative hypothesis that the timing of normal food ingestion, not the light-dark cycle per se, constrains torpor to the day light hours. Food availability was restricted to times that differ from normal nocturnal feeding time in juvenile and adult hamsters that were commonly housed in a fixed 8:16 light-dark cycle. Hamsters fed in the second half of the dark phase and early light phase entered torpor during darkness, irrespective of the photoperiod. Hamsters fed near dark onset entered torpor during the day light hours. Increased locomotor activity preceded food availability during periods of food restriction but disappeared shortly after ad libitum feeding was reinstated.; Two other experiments explored the ontogeny of torpor in juvenile hamsters. Weanlings as young as 16 and 28 days old underwent torpor bouts in response to 2-deoxy-D-glucose (2DG) and reduced food intake, respectively. Latency for onset of spontaneous torpor was not advanced compared to adults of this species and occurred at 13 weeks of age. Juvenile Siberian hamsters, especially those born late in the breeding season, may be well-served by developing the ability to undergo torpor before the onset of winter. Another experiment addressed whether the area postrema, which monitors metabolic fuel availability, controls the onset of torpor. Hamsters that had manifested torpor spontaneously or had entered torpor in response to 2DG treatment were subjected to area postrema ablations (APx). Hamsters continued to display torpor postoperatively; most features of torpor were unaffected by AN. By contrast, decreases in food intake manifested by hamsters treated with 2DG were counteracted by AP ablation. In Siberian hamsters the AP appears to mediate effects of 2-DG on food intake but not torpor.