Circadian control in the timing of torpor bouts during hibernation in
European hamsters (Cricetus cricetus L.) kept under standardised photoperiod
and constant temperature
Thomas Waßmer & Franziska Wollnik
Department of Biology, University of Konstanz, P.O. Box 5560, D-78434
Konstanz, Germany
Body temperature (T-b) Of seven European hamsters maintained at constant
ambient temperature (T-a = 8 degrees C) and constant photoperiod (LD 8:16)
was recorded throughout the hibernating season using intraperitoneal temperature-sensitive
HF transmitters. The animals spent about 30% of the hibernation season
in hypothermia and 70% in inter-bout normothermy. Three types of hypothermia,
namely deep hibernation bouts (DHBs), short hibernation bouts (SHBs), and
short and shallow hibernation bouts (SSHBs), were distinguished by differences
in bout duration and minimal body temperature (T-m). A gradual development
of SSHBs from the diel minimum of T-b during normothermy could be seen
in individual hamsters, suggesting a stepwise decrease of the homeostatic
setpoint of T-b regulation during the early hibernation season. Entry into
hibernation followed a 24-h rhythm occurring at preferred times of the
day in all three types of hypothermia. DHBs and SHBs were initiated approximately
4 h before SSHBs, indicating a general difference in the physiological
initiation of SSHBs on the one hand and DHBs and SHBs on the other. Arousals
from SHBs and SSHBs also followed a 24-h rhythm? whereas spontaneous arousals
from DHBs were widely scattered across day and night. Statistical analyses
of bout length and the interval between arousals revealed evidence for
a free-running circadian rhythm underlying the timing of arousals. The
results clearly demonstrate that entries into hypothermia are linked to
the light/dark-cycle. However, the role of the circadian system in the
timing of arousals from DHBs remains unclear.
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