Temporal organisation of hibernation in wild-type and tau mutant Syrian hamsters

The temporal pattern of hibernation was studied in three genotypes of Syrian hamsters with different circadian periodicity to assess a potential circadian control of alternating torpor and euthermy. We recorded the pattern of hibernation by measuring activity in continuous dim light and constant environmental temperature (6 +/- 1 degrees C). In spite of differences in the endogenous circadian period of three genotypes (tau +/+: approximately equals 24 h, tau +/-: approximately equals 22 h, and tau -/-: approximately equals 20 h) torpor bout duration was statistically indistinguishable (tau +/+: 86.9+/-5.3 h; tau +/-: 94.2+/-3.3 h; tau -/-: 88.8+/-6.2 h). The time between two consecutive arousals from torpor showed unimodal distributions not significantly different between genotypes. The first entry into torpor occurred within the active phase of the circadian cycle in all genotypes whereas the first arousal from torpor appeared to be timed randomly with respect to the prior circadian cycle. The amplitude of the activity rhythm was lower after hibernation compared with the amplitude before hibernation. The results suggest that in the Syrian hamster the circadian system does not control periodicity of torpor and arousal onsets in prolonged hibernation at 6 degrees C.