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Temperature Sensitivity of the Suprachiasmatic Nucleus of Ground Squirrels and Rats in vitro

Department of Biological Sciences, Stanford University, Stanford, CA 94305-5020

H. Craig Heller

Department of Biological Sciences, Stanford University, Stanford, CA 94305-5020

Temperature compensation of circadian rhythms in neuronal firing rate was investigated in the suprachiasmatic nucleus (SCN) of ground squirrels and rats in vitro. A reduction in SCN temperature from 37 to 25°C reduced peak firing rates by > 70% in rats but only by = 21% in squirrels; trough firing rates were marginally altered in both species. In the rat SCN at 25°C, the peak in neuronal activity decreased progressively on successive days and circadian rhythms no longer were present by Day 3. There was a 37% reduction in the number of single units detected and an increase in the temporal variability of peak firing rates among individual rat SCN neurons at low temperature. By contrast, single units were readily detected and circadian rhythms were robust in squirrels at 37 and 25°C; a Q₁₀ of 0.927 was associated with a shortening of tau by 2 h and a 5-h phase change after only 48 h at low temperature. These results suggest that temperature can have a substantial impact on circadian organization in a mammalian pacemaker considered to be temperature compensated.

Key Words: circadian rhythm • temperature compensation • Q10 • brain slice • single unit • hibernation

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