Selective sleep deprivation after daily torpor in the Djungarian hamster

Sleep, daily torpor and hibernation are no longer considered homologous processes. Animals emerging from these states spend most of their time in sleep. After termination of the torpor-associated hypothermia, there is an initial high electroenecephalogram (EEG) slow-wave activity (SWA; 0.75-4.0 Hz) and a subsequent monotonic decline. Both of these features are similar to the effects elicited by prolonged waking. It was previously shown that when hamsters are not allowed to sleep immediately after emerging from torpor, an additional SWA increase above the level reached after sleep deprivation (SD) alone occurs during the delayed recovery. A similar manipulation in hibernating ground squirrels abolished the subsequent SWA increase, shedding doubt on the similarity of the regulatory aspects following torpor and hibernation. To further investigate the extent to which SWA is homeostatically regulated after torpor, Djungarian hamsters were subjected to 1.5 h partial non-rapid eye movement (NREM) sleep deprivation (NSD) that either immediately followed the emergence from torpor (T + NSD) or 4-h SD (SD + NSD). The NSD was attained by disturbing the animals when they exhibited NREM sleep with high amplitude slow-waves. To investigate whether regional aspects of sleep homeostasis are similar after torpor and SD, the EEG was recorded from a parietal and frontal derivation after 4-h SD. An increase in SWA in NREM sleep occurred after all conditions in both EEG derivations. There was no significant difference in SWA during the initial 1.5-h recovery when torpor, T + NSD and SD + NSD were compared. During recovery from torpor and SD, SWA was higher in the frontal than in the parietal derivation. Our results provide further evidence that torpor and SD have similar effects on sleep. The SWA increase did not disappear after the NSD; therefore, SWA is homeostatically regulated after daily torpor. The frontal predominance of slow waves encountered both after torpor and SD indicates that waking and torpor induce similar regional changes in EEG SWA.