Effects of Light and Temperature Steps on Circadian Rhythms of Neurospora and Gonyaulax

Pulse experiments resulting in phase response curves, entrainment experiments, “holding” experiments, skeleton photoperiod experiments, and step experiments are performed using light, temperature and chemicals to answer a variety of questions.

Abstract

Many organisms have physiological activities that fluctuate on a daily basis; frequently these fluctuations will continue to occur on an approximately 24-hour basis even in the laboratory under constant conditions. Such rhythms are termed circadian rhythms. Light, temperature and certain chemicals have been shown to affect many circadian rhythms. These parameters are usually applied as a pulse (a step-up followed by a step-down) to a free-running rhythm under constant conditions. The pulse causes shifts from normal levels, but eventually the cycle returns into its exact normal rhythm with the only permanent change being a phase-shift relative to the control. Thus, such experiments and others have led to the development of the limit cycle model for circadian rhythms [1–4]. In an attempt to answer a variety of questions, there have been several types of experiments performed using light, temperature and chemicals. These experiments have names that are jargon to circadian rhythm researchers: pulse experiments resulting in phase response curves, entrainment experiments, "holding" experiments, skeleton photoperiod experiments, and step experiments [5]. It is important to realize, however, that all of these experiments are variations upon each other, i.e. they all involve a step-up and/or a step-down of the perturbation in question; the different experiments depend upon when these step-ups and step-downs are applied relative to each other, and when they are applied within a cycle.