Most physiology and behavior of mammalian organisms follow daily oscillations. These rhythmic processes are governed by environmental cues (e.g., fluctuations in light intensity and temperature), an internal circadian timing system, and the interaction between this timekeeping system and environmental signals. In mammals, the circadian timekeeping system has a complex architecture, composed of a central pacemaker in the brain's suprachiasmatic nuclei (SCN) and subsidiary clocks in nearly every body cell. The central clock is synchronized to geophysical time mainly via photic cues perceived by the retina and transmitted by electrical signals to SCN neurons. In turn, the SCN influences circadian physiology and behavior via neuronal and humoral cues and via the synchronization of local oscillators that are operative in the cells of most organs and tissues. Thus, some of the SCN output pathways serve as input pathways for peripheral tissues. Here we discuss knowledge acquired during the past few years on the complex structure and function of the mammalian circadian timing system.

/pdf/the-mammalian-circadian-timing-system-organization-and-54vfplckht.pdf

The mammalian circadian timing system: organization and coordination of central and peripheral clocks.

The pacemaker role of the suprachiasmatic nucleus in a mammalian circadian system was tested by neural transplantation by using a mutant strain of hamster that shows a short circadian period. Small neural grafts from the suprachiasmatic region restored circadian rhythms to arrhythmic animals whose own nucleus had been ablated. The restored rhythms always exhibited the period of the donor genotype regardless of the direction of the transplant or genotype of the host. The basic period of the overt circadian rhythm therefore is determined by cells of the suprachiasmatic region.

https://science.sciencemag.org/content/sci/247/4945/975.full.pdf

Transplanted suprachiasmatic nucleus determines circadian period

The circadian system of mammals is composed of a hierarchy of oscillators that function at the cellular, tissue, and systems levels. A common molecular mechanism underlies the cell-autonomous circadian oscillator throughout the body, yet this clock system is adapted to different functional contexts. In the central suprachiasmatic nucleus (SCN) of the hypothalamus, a coupled population of neuronal circadian oscillators acts as a master pacemaker for the organism to drive rhythms in activity and rest, feeding, body temperature, and hormones. Coupling within the SCN network confers robustness to the SCN pacemaker, which in turn provides stability to the overall temporal architecture of the organism. Throughout the majority of the cells in the body, cell-autonomous circadian clocks are intimately enmeshed within metabolic pathways. Thus, an emerging view for the adaptive significance of circadian clocks is their fundamental role in orchestrating metabolism.

/pdf/central-and-peripheral-circadian-clocks-in-mammals-2qtn9mbv5i.pdf

Central and Peripheral Circadian Clocks in Mammals

Circadian rhythms of behavior are driven by oscillators in the brain that are coupled to the environmental light cycle. Circadian rhythms of gene expression occur widely in peripheral organs. It is unclear how these multiple rhythms are coupled together to form a coherent system. To study such coupling, we investigated the effects of cycles of food availability (which exert powerful entraining effects on behavior) on the rhythms of gene expression in the liver, lung, and suprachiasmatic nucleus (SCN). We used a transgenic rat model whose tissues express luciferase in vitro. Although rhythmicity in the SCN remained phase-locked to the light-dark cycle, restricted feeding rapidly entrained the liver, shifting its rhythm by 10 hours within 2 days. Our results demonstrate that feeding cycles can entrain the liver independently of the SCN and the light cycle, and they suggest the need to reexamine the mammalian circadian hierarchy. They also raise the possibility that peripheral circadian oscillators like those in the liver may be coupled to the SCN primarily through rhythmic behavior, such as feeding.

https://science.sciencemag.org/content/sci/291/5503/490.full.pdf

Entrainment of the Circadian Clock in the Liver by Feeding

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

Anticipation of 24-hr feeding schedules in rats with lesions of the suprachiasmatic nucleus.

Entrainment of circadian rhythms by feeding schedules in rats with suprachiasmatic lesions.

Forebrain expression of c-fos due to active maternal behaviour in lactating rats

Hamsters rely on chemosensory cues from females of the same species for the initiation of copulatory behavior. While these cues are detected by both the main and accessory olfactory systems it is the central nuclei in the accessory system that regulate mating behavior. The results of the present study indicate that exposure to vaginal secretions from a female Syrian hamster (FHVS) stimulates c-fos production in the medial nucleus of the amygdala (Me), the bed nucleus of the stria terminalis (BNST) and the medial preoptic area (MPOA). Exposure to vaginal secretions from Djungarian hamsters do not stimulate neurons in these areas. Thus the present results suggest that FHVS stimulates mating behavior by activating neurons within the vomeronasal pathway.

Jennifer M. Swann

Papers

Anticipation of 24-hr feeding schedules in rats with lesions of the suprachiasmatic nucleus.

Entrainment of circadian rhythms by feeding schedules in rats with suprachiasmatic lesions.

Forebrain expression of c-fos due to active maternal behaviour in lactating rats

Pheromones induce c-fos in limbic areas regulating male hamster mating behavior.

Androgen and estrogen concentrating neurons in chemosensory pathways of the male Syrian hamster brain