In mammals, the suprachiasmatic nucleus (SCN) is the site of the pacemaker that is responsible for circadian rhythms in behavior and physiology1, and it is currently believed that all rhythms of SCN cells are endogenously driven and independent of extra-SCN tissues. The eye has also been shown to contain an independent circadian oscillator2, but the role of the ocular clock and whether it influences the SCN are unknown. Here we found that a rhythm of phosphorylation of mitogen-activated protein (MAP) kinase in an anatomically distinct subdivision of the SCN was completely abolished by bilateral enucleation in hamsters and mice, indicating that the eye is necessary for a circadian rhythm within the SCN.
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We thank G. Boer for VP antiserum and L. Coolen for advice and discussions.
Rhythmic pERK persists in the shell but not the core of the transplanted SCN. (a) A SCN-lesioned, wild-type hamster that received a tau homozygote donor graft, was killed during subjective day (CT6) (see Supplementary Methods). Histology of the graft (g) in this animal showing the presence of overlapping clusters of pERK and VP cells (circled areas). (b) A SCN-lesioned, tau heterozygote hamster that received a wild type graft, was sacrificed during subjective night (CT18). Histology of the graft in this animal showing the lack of pERK immunoreactivity in the CaBP region of the donor SCN (circled areas). Scale bar = 500 μm.
Retinal input to pERK cells in the core region of the SCN. (a) Hamsters injected intraocularly with the anterograde tracer cholera toxin beta subunit (CTβ), were killed several days later during the subjective night (CT16). Examination of sections double-labeled for CTβ and pERK by confocal microscopy showed that every pERK-positive cell received at least one close apposition, (arrowheads) from a CTβ-labeled bouton (green), suggesting that retinal afferents synapse onto many, if not all, pERK cells in the core region. Two serial optical sections (each 1 μm thick) are shown. Arrowheads point to close contacts between CTβ-labeled retinal afferents and pERK-positive cells. Scale bar = 10 μm. (b) Example of a section through the core region of the SCN, double-labeled for CaBP and Nissl, in an enucleated hamster sacrificed during subjective night. A defined region (white rectangular box) immediately dorsal to the CaBP subnucleus, corresponding to the location of pERK cells in control animals, was used to determine whether enucleation resulted in cell loss. Scale bar = 100 μm. (c) Bilateral enucleation had no effect on the density of Nissl-stained cells in the core region (box in b) where pERK cells are normally found during subjective night.