Different populations of cells in the suprachiasmatic nuclei express c-fos in association with light-induced phase delays and advances of the free-running activity rhythm in hamsters.

Circadian rhythmicity is controlled by a light-entrainable pacemaker located in the suprachiasmatic nuclei (SCN) of the mammalian hypothalamus. Brief light exposure during the subjective night causes phase shifts of the free-running activity rhythm and expression of c-fos-related proteins (Fos) among a population of cells in the hamster SCN. Light exposure (30 lux for 15 min) during the early subjective night (CT13) causes phase delays (-60 +/- 12 min), while exposure at mid-subjective night (CT18) causes phase advances (114 +/- 48 min) of the free-running activity rhythm. Light exposure at mid-subjective day (CT6) does not cause phase alterations of the rhythm. Similarly, only light exposure at CT13 or CT18 induces Fos expression in the SCN. The distribution of Fos-immunoreactive cells in the SCN is more widespread in animals stimulated with light at CT18. In addition, a group of cells located dorsal and anterior to the SCN express Fos only after stimulation at CT18. The data are consistent with the hypothesis that Fos expression represents an event in the signal transduction pathway leading to light-induced alterations in circadian pacemaker function. Furthermore, the data raise the possibility that different populations of cells in the suprachiasmatic hypothalamus may participate in light-induced phase advances and delays of the circadian pacemaker.