Rhythmic expression of Fos-related proteins within the rat suprachiasmatic nucleus during constant retinal illumination.
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Within the retinorecipient or ventrolateral subfield of suprachiasmatic nuclei (SCN) in rodents, expression of the protein product of the c-fos proto-oncogene, Fos, is regulated by light. In the present study, the expression of Fos and Fos-related proteins within the SCN was examined immunocytochemically for evidence of rhythmic variation in rats sacrificed at different circadian times during exposure to constant retinal illumination (LL). In all animals, nuclear Fos immunoreactivity was mainly confined to an area of the SCN that was coextensive with neuropeptide Y-immunopositive fibers distinguishing the ventrolateral subfield of the nucleus. Moreover, Fos-immunostaining within the ventrolateral SCN of rats exposed to LL fluctuated over the course of the circadian cycle, such that the density of immunopositive cells within this subfield was 2 times greater during the subjective night than during the subjective day. Since Fos expression within the SCN oscillates in the absence of photoperiodic time cues and since the peak of this oscillation coincides with the circadian times when light modulates the periodicity of the SCN pacemaker, these data provide further evidence that expression of the c-fos gene may be a molecular signal in the circadian timekeeping mechanism in the SCN and its regulation by photic stimuli.