Circadian Rhythms and the Gastrointestinal System

Abstract

Circadian organization comprises both hierarchical and distributed pacemakers, as well as peripheral oscillators that are coordinated by the hypothalamic suprachiasmatic nuclei (SCN). Among the peripheral tissues that are coordinated by the SCN is the gastrointestinal system via the sympathetic nervous system. At each level of gastrointestinal function, circadian clocks regulate the homeostatic functions of consuming, breaking down, digesting and absorbing nutrients. This system contains a semi-independent circadian clock capable of producing endogenously generated rhythms in gene expression, motility, and absorption both in vivo and in vitro, and capable of entraining directly to timed meals. Within the gastrointestinal lumen, a vibrant ecosystem of microorganisms, known as the gastrointestinal microbiome is regulated by the gastrointestinal clock through the presentation of nutrients, hormones, and changes in body temperature (TB), among other signals. Some of these bacteria directly respond to the host signals, while others, possessing endogenous circadian clocks themselves, may entrain to the host’s Zeitgebers, and still others may respond to signals generated by other members of the gut microbiome itself. Disruption of the circadian system may be critical in exacerbating or even causing pathological processes and diseases, including deleterious effects of aging, cancer, and inflammatory bowel disease, among others.