Perturbations of Cellular Circadian Rhythms by Light and Temperature

Abstract

The main natural “Zeitgeber”-periodicities for circadian rhythms are the cycles of light and temperature in the course of a solar day. They entrain the endogenous circadian oscillation of organisms to exactly 24 h, or perturb (phase shift) the oscillation in cases of transmeridian flights or shift work. The entrained state itself is a dynamic state of repetitive changes of phase, amplitude and period of the endogenous oscillation caused by the Zeitgeber periodicity. Two classes of effects of the Zeitgeber can be distinguished especially in the rectangular form of light and temperature cycles in the laboratory: a) differential (phasic) effects of the different directions (up and down) of change, and b) proportional (tonic) effects of longer exposures to higher or lower intensities. Differential and proportional effects, can to some extent, be tested separately either 1) by applying steps or pulses of light, dark or different temperatures and by analysing the resulting phase shifts (Δ ρ) of the circadian oscillator, or 2) by exposing the oscillator to different constant light intensities or temperatures and by analysing the resulting changes of τ and amplitude (A). A third approach is to abolish (“hold”) the circadian oscillation by long exposures to light or high temperature and then initiate the oscillation by a single transfer to darkness or lower temperature and then to analyse the subsequent phasing of the rhythm (1,2).