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• W2912756966 abstract "Seasonal influences become increasingly important at high latitudes, where variations in external conditions are most pronounced. Mammals living in such drastic conditions have developed different forms of torpor to reduce metabolic costs in harsh periods when energy is rare. Deep hibernators like the European hamster (Cricetus cricetus) regularly decrease their body temperature (Tb) for several days to temperatures approaching ambient temperature (Ta) and thereby save a maximum of energy. In contrast some small mammals like the Djungarian hamster (Phodopus sungorus) undergo shallower bouts of daily torpor. Induced by short photoperiod they spontaneously use their circadian resting time for only a few hours of precisely timed hypothermia during which their body temperature decreases to minimum 15°C. Invariably low Tbs go along with a depression of CNS activity. Nevertheless it is crucial that functional integrity is maintained, which is why systems of vital importance during hypothermia are supposed to remain active at those low Tbs. One of the important keys during torpor is believed to be the circadian system. It measures photoperiod and thereby determines the onset of the torpor season and moreover synchronizes internal processes on a circadian basis. In this thesis we could for the first time demonstrate that the clock´s molecular machinery is still active during daily torpor in Djungarian hamsters. Alterations in phase and amplitude of clock gene expression rhythms however, point to temperature sensitivity. Decrease in protein expression during hypothermia, hence a decreased feedback might be responsible for transcriptional alterations during and after a torpor bout. To more precisely investigate phase changes seen in gene expression, we set up a long term microdialysis experiment to continuously measure melatonin, a well defined clock output, in vivo directly in the pineal gland. This method could for the first time be adapted to the very small Djungarian hamster and provides a good tool to study a circadian signal like melatonin in a seasonally heterothermic animal. During deep hibernation in European hamsters that decreased their Tb for several days ~8°C in our experimental conditions, we could not observe any rhythmic clock gene expression and thereby show that the clock stops oscillating at those low temperatures. We conclude from our data, that the circadian clock seems to be temperature compensated in a wide temperature range but once a certain low temperature is reached, oscillation is no longer possible. The histaminergic system as well has been shown to play an important role in hibernation. In hibernating ground squirrels (Spermophilus lateralis) it likely serves to purposefully suppress other neurotransmitter systems via the inhibitory autoregulatory histamine 3 (H3) receptor. In our study we also show up-regulation of this receptor during daily torpor in Djungarian hamsters in various nuclei including arcuate nucleus, dorso medial hypothalamus, suprachiasmatic nucleus and dorsal lateral geniculate, hence important relay stations for energy balance, thermoregulation, timing and vision. Those findings point to a role for the histaminergic system in daily torpor comparable to that in deep hibernation, namely actively inhibiting other neurotransmitter systems. We investigated whether reactivation of the other systems could be caused by orexins. Orexins are a group of neuropeptides that has been shown to increase wakefulness and seems moreover to be related to non-shivering thermogenesis in brown adipose tissue, the main mechanism to regain normal Tb after hypothermia. Contrary to our expectation we found orexin down-regulated during torpor as well as during arousal from torpor. This contradicts the role of orexin during arousal from hypothermia and suggests distinct mechanisms for arousal related to wakefulness and arousal from hypothermia." @default.
• W2912756966 created "2019-02-21" @default.
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• W2912756966 date "2007-01-01" @default.
• W2912756966 modified "2023-09-24" @default.
• W2912756966 title "Torpor and timing" @default.
• W2912756966 hasPublicationYear "2007" @default.
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