Temperature effects on circadian clocks

Periodic temperature changes represent one of the most effective entraining (Zeitgeber) signals for circadian clocks in many organisms. Different constant temperatures affect the circadian amplitude and ultimately the expression of circadian clocks, while the circadian period length (tau) remains approximately constant (temperature compensation). Experimental results and theoretical models are presented that may serve to explain these effects. After introducing the physico-chemical basis of temperature on enzyme-catalyzed and physiological reactions, and after describing mechanisms for temperature adaptation of physiological reactions to different thermal environments, general effects of temperature on chemical and biological oscillators are described. Kinetic models for circadian clocks and temperature compensation are presented and compared with experimental results. Special attention is given to the question how constant but different temperature levels affect clock amplitude, period length and phase. Influences of single and periodic temperature variations (steps or pulses) on circadian clocks are presented together with models which may explain the resulting phase response curves and entrainment patterns. Because temperature compensation is only one aspect of a general homeostatic mechanism that keeps the circadian period rather constant, the influence of other environmental variables and their relationship to temperature are discussed.