The circadian system of reptiles: a multioscillatory and multiphotoreceptive system

Many parameters exhibited by organisms show daily fluctuations that may per sist when the organisms are held in constant environmental conditions. Rhyt hms that persist in constant conditions with a period close to 24 h are cal led circadian. Although nowadays most research in this field is focused on the molecular and genetic aspects - and therefore mostly on two animal mode ls (Drosophila and mouse)- the study of alternative animal models still rep resent a useful approach to understanding how the vertebrate circadian syst em is organized, and how this fascinating time-keeping system has changed t hroughout the evolution of vertebrates. The present paper summarizes the cu rrent knowledge of the circadian organization of Reptiles. The circadian or ganization of reptiles is multioscillatory in nature. The retinas, the pine al, and the parietal eye land, possibly, the suprachiasmatic nuclei of the hypothalamus, SCN) contain circadian clocks. Of particular interest is the observation that the role these structures play in the circadian organizati on varies considerably among species and within the same species in differe nt seasons. Another remarkable feature of this class is the redundancy of c ircadian photoreceptors: retinas of the lateral eyes, pineal, parietal eye, and the brain all contain photoreceptors. (C) 2001 Elsevier Science Inc. A ll rights reserved.