Embryonic development of circadian oscillations in the mouse hypothalamus.

Circadian rhythms in mammals are regulated by the hypothalamic suprachiasmatic nucleus (SCN). The generation of circadian oscillations is a cell-autonomous property, and coupling among cells is essential for the SCN to function as a pacemaker. The development of SCN anatomy and cytology has been extensively studied, but the point in development when the SCN first has the capacity to generate circadian oscillations has not been established. We therefore examined the development of circadian oscillations using per2::luc mice in which bioluminescence tracks the expression of the circadian clock protein, PER2. In vitro, hypothalamic explants first expressed consistent oscillations when isolated between 15 and 16 days postfertilization (e15). Oscillations were more robust at later ages. Explants from other brain areas did not express oscillations, indicating that the development of oscillations is not a general property of embryonic tissue. SCN explants obtained on e14 did not initially express oscillations but developed them in vitro over 4 to 6 d. Although coupling among cells is required for the long-term expression of tissue-level oscillations, explants from mice lacking the coupling peptide vasoactive intestinal peptide still developed oscillations. In the mouse, the capacity to generate molecular oscillations on e15 coincides with the completion of neurogenesis and SCN-specific transcription factor expression. Thus, within a day of its genesis at an age approximately equivalent to the end of the first trimester in humans, the SCN develops the capacity to express circadian oscillations and autonomously develops mechanisms sufficient to couple and synchronize its cells.

Resilience of circadian pacemaker development in hamsters.

Disruptions of circadian rhythms have been linked to a wide range of pathologies from sleep disorders to cancer. The extent to which disruptions of circadian rhythms during development contribute to later conditions is not known. The present study tested the hypothesis that functional properties of the central circadian pacemaker, the suprachiasmatic nucleus (SCN), are affected by abnormal entrainment during development. The SCN is specialized for the generation of robust rhythms, for direct and indirect output to physiological and behavioral systems, and for entrainment to light/dark cycles via direct retinal input. It consists of thousands of neurons and glia with distinct phenotypes and has subdivisions delineated by both anatomical and functional criteria. In rodents, SCN rhythms develop within days after SCN cells are produced and before many other aspects of differentiation, such as synaptogenesis, are complete. We demonstrated that around the time of birth, the hamster SCN in vivo can undergo repeated phase shifts by a dopamine D(1) receptor agonist (SKF-38393). For 2 days before and 2 days after birth, one group of hamsters received regular exposure to the drug at the same time of day, while another group was exposed at varying times to induce repeated phase shifts. Free-running and entrained activity rhythms were compared between the groups at different ages after weaning. Repeated phase shifts during SCN development had a significant effect on free-running period measured immediately after weaning. This effect was eliminated by subsequent entrainment to a light/dark cycle, indicating that the effect was not permanent. These and other results suggest that SCN development required for functional properties such as free-running period is resilient to perturbation.