Circadian Entrainment to Temperature, But Not Light, in the Isolated Suprachiasmatic Nucleus

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Abstract
The suprachiasmatic nucleus (SCN) is the master pacemaker that drives circadian rhythms in mammalian physiology and behavior. The abilities to synchronize to daily cycles in the environment and to keep accurate time over a range of physiologic temperatures are two fundamental properties of circadian pacemakers. Recordings from a bioluminescent reporter ( Per1-luc ) of Period1 gene activity in rats showed that the cultured SCN entrained to daily, 1.5°C cycles of temperature, but did not synchronize to daily light cycles. Temperature entrainment developed by 1 day after birth. Light cycles failed to affect the isolated SCN of rats aged 2 to 339 days. Entrainment to a 3-h shift in the warm-cool cycle was possible in <3 days with 3°C cycles. Importantly, Per1-luc expression in vitro was similar to that seen in vivo where peak expression occurs approximately 1 h prior to the daily increase in temperature. In addition, the firing rate of individual mouse SCN neurons continued to express near 24-h rhythms from 24–37°C. At lower temperatures, the percentage of rhythmic cells was reduced, but periodicity was temperature compensated. The results indicate that normal rhythms in brain temperature may serve to stabilize rhythmicity of the circadian system in vivo and that temperature compensation of this period is determined at the level of individual SCN cells.