The suprachiasmatic nucleus is essential for circadian body temperature rhythms in hibernating ground squirrels

Body temperature (T-b) was recorded at 10 min intervals over 2.5 years in female golden-mantled ground squirrels that sustained complete ablation of the suprachiasmatic nucleus (SCNx). Animals housed at an ambient temperature (Ta)of 6.5 degreesC were housed in a 12 hr light/dark cycle for 19 months followed by 11 months in constant light. The circadian rhythm of Tb was permanently eliminated in euthermic and torpid SCNx squirrels, but not in those with partial destruction of the SCN or in neurologically intact control animals. Among control animals, some low-amplitude T-b rhythms during torpor were driven by small (<0.1<degrees>C) diurnal changes in T-a. During torpor bouts in which T-b rhythms were unaffected by T-a, T-b rhythm period ranged from 23.7 to 28.5 hr. Both SCNx and control squirrels were more likely to enter torpor at night and to arouse during the day in the presence of the light/dark cycle, whereas entry into and arousal from torpor occurred at random clock times in both SCNx and control animals housed in constant light. Absence of circadian rhythms 2.5 years after SCN ablation indicates that extra-SCN pacemakers are unable to mediate circadian organization in euthermic or torpid ground squirrels. The presence of diurnal rhythms of entry into and arousal from torpor in SCNx animals held under a light/dark cycle, and their absence in constant light, suggest that light can reach the retina of hibernating ground squirrels maintained in the laboratory and affect hibernation via an SCN-independent mechanism.