Non-photic circadian entrainment in mammals: A brief review and proposal for study during development

Abstract

A notable and phylogenetically conserved characteristic of organisms is the demonstration of endogenous 24-h (circadian) rhythms across a wide range of behavioral, physiological and molecular processes. The extensive efforts to understand how such rhythms are generated and sustained have focused on their coupling to the daily light – dark cycle, that is, on their entrainment by photic cues. Although understandable given that this is the most predictable environmental information, it is increasingly clear that recurrent, non-photic events such as the periodic availability of food also regulate circadian systems and via mechanisms different to photic cues. One of the challenges facing circadian rhythms research is to understand how organisms respond to the wealth of diurnally fluctuating information around them and integrate this to generate a coherent and adaptive system of circadian function. Here we briefly review what is known about the influence of non-photic stimuli on the circadian system. We then consider the value but inherent difficulties of a developmental approach. After considering the advantages offered by the neonatal rabbit, we close by emphasizing the importance of developing models more closely tied to the animal's natural, evolved lifestyle if sense is to be made of the diversity of information rapidly accumulating on circadian processes.

Keywords:

• Zeitgebers
• pre-visual
• suprachiasmatic nuclei
• clock genes
• newborn rabbit
• nursing

Acknowledgements

This work was supported by PAPIIT grant IN200203-3 to R.H and I.C. We thank Carolina Rojas and Diana Tejadilla for excellent technical assistance, and Steven Reppert, University of Massachusetts Medical School, for generously providing us with the Per1, Per2, Cry1 and Cry2 plasmids.