Daily Torpor Alters Multiple Gene Expression in the Suprachiasmatic Nucleus and Pineal Gland of the Djungarian Hamster (Phodopus sungorus)

May 12, 2006Chronobiology international

Daily torpor changes gene activity in the brain's internal clock and light-sensing glands of the Djungarian hamster

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Abstract

Daily torpor in Djungarian hamsters affects the phase and amplitude of clock gene expression in the suprachiasmatic nucleus and pineal gland.

Clock gene expression showed changes during torpor, with all investigated mRNAs increased except AVP mRNA.
Bmal1 expression rhythm was advanced on the day of torpor, indicating a shift in circadian timing.
On the day following torpor, Bmal1 and AVP rhythms exhibited severely depressed amplitude.
In the pineal gland, Aanat expression was significantly advanced during hypothermia and remained altered one day later.
These findings suggest that torpor may influence circadian rhythms through changes in gene expression and protein feedback mechanisms.

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Circadian rhythms are still expressed in animals that display daily torpor, implying a temperature compensation of the pacemaker. Nevertheless, it remains unclear how the clock works in hypothermic states and whether torpor itself, as a temperature pulse, affects the circadian system. To reveal changes in the clockwork during torpor, we compared clock gene and neuropeptide expression by in situ hybridization in the suprachiasmatic nucleus (SCN) and pineal gland of normothermic and torpid Djungarian hamsters (Phodopus sungorus). Animals from light-dark (LD) 8ratio16 were sacrificed at 8 time points throughout 24 h. To investigate the effect of a previous torpor episode on the clock, we sacrificed a group of normothermic hamsters 1 day after torpor. In normothermic animals, Per1 peaked at zeitgeber time (ZT)4; whereas, Bmal1 reached maximal expression between ZT16 and ZT19. AVP mRNA in the SCN showed highest levels at ZT7. On the day of torpor, the levels of all mRNAs investigated, except for AVP mRNA, were increased during the torpor bout. Moreover, the Bmal1 rhythm was advanced. On the day after the hypothermia, Bmal1 and AVP rhythms showed severely depressed amplitude. Those distinct amplitude changes of Bmal1 and AVP on the day after a torpor episode expression suggests that torpor affects the circadian system, probably by altered translational processes that might lead to a modified protein feedback on gene expression. In the pineal gland, an important clock output, Aanat expression, peaked between ZT16 and ZT22 in normothermic animals. Aanat levels were significantly advanced on the day of hypothermia, an effect which was still visible 1 day afterward. In summary, this study showed that daily torpor affects the phase and amplitude of rhythmic clock gene and clock-controlled gene expression in the SCN. Furthermore, the rhythmic gene expression in a peripheral oscillator, the pineal gland, is also affected.

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Daily Torpor Alters Multiple Gene Expression in the Suprachiasmatic Nucleus and Pineal Gland of the Djungarian Hamster (Phodopus sungorus)

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