Circadian rhythm in c-fos-like immunoreactivity in the rat brain.

The circadian variation in the expression of Fos protein(s) in the rat brain was studied immunohistochemically. The number of Fos-immunoreactive nuclei was statistically increased in the hippocampus and caudate putamen after the onset of darkness. Unlike the cells of the hippocampus and putamen the cells in the suprachiasmatic nucleus displayed a circadian variation with the lowest values during the nighttime and the highest in the morning. As the circadian rhythm of Fos expression in the normal rat brain may correlate with that reported for ACTH and corticosteroids, the results suggest that the fos gene is involved in mediating physiological activation of specific neuron populations of intact rats.