Lability of Circadian Pacemaker Amplitude in Chick Pineal Cells: A Temperature-Dependent Process

Temperature is a major regulator of circadian rhythms. The authors report here three lines of evidence that temperature modulates the amplitude of the circadian pacemaker that drives rhythmic melatonin production in chick pineal cells. (1) The melatonin rhythm persists longer in constant conditions at 40°C than at 37°C. (2) The phase response curve to low-intensity (0.15 μW/cm²) light pulses of 6-h duration has a higher amplitude at 37°C than at 40°C; a nonphotic stimulus, anisomycin, also causes larger shifts at 37°C than at 40°C. These results suggest a general increase in sensitivity to phase-shifting stimuli as temperature decreases. (3) The light intensity necessary for a critical pulse that causes arrhythmicity is lower at 37°C than at 40°C. All three of these effects of temperature can be explained in a unified manner by a limit cycle model in which temperature increases circadian pacemaker amplitude. The use of critical pulse experiments provides a novel method for estimating relative circadian pacemaker amplitude under different conditions.