Circadian regulation of Fos B is different from c-Fos in the rat suprachiasmatic nucleus.

We measured immunoreactive Fos B protein levels in the ventrolateral subdivision of the rat suprachiasmatic nucleus (SCN) as a function of light and time of day. Immunohistochemistry revealed high levels of Fos B that were uniformly expressed throughout the 12 h:12 h light-dark cycle. Levels remained high in constant darkness and were modestly increased (about 2-fold) after a 2 h light pulse administered during the subjective night, but not after a light pulse during the subjective day. Fos B and c-Fos immunoreactivities could be colocalized within individual SCN cell nuclei using a double-label immunofluorescence method. Thus, despite their structural similarities, these two members of the fos gene family exhibit different patterns of expression in the rat SCN. These and previous data suggest that Fos/Jun DNA-binding complexes in the SCN are composed of constant, as well as variable, protein components; in at least some SCN cells, light-induced changes in the composition of these constituent proteins may lead to altered transcription of target genes.