PHOTIC REGULATION OF C-FOS EXPRESSION IN NEURAL COMPONENTS GOVERNING THE ENTRAINMENT OF CIRCADIAN-RHYTHMS

The rapid and transient induction of the proto-oncogene c-fos in mature neurons within the brain occurs in response to a variety of extracellular stimuli. To determine whether lighting conditions influence c-fos gene expression in the primary neural structures mediating the photoentrainment and generation of mammalian circadian rhythms, the expression of the c-fos protein (Fos) and related proteins in the retina and suprachiasmatic nuclei (SCN) of the anterior hypothalmus was examined immunohistochemically in rats exposed to a light-dark cycle of 12 h of light and 12 h of darkness (LD 12:12), constant light (LL), or constant dark (DD). The retina exhibited clear light-dark differences in the expression of Fos protein(s), such that immunopositive nuclei were readily evident during exposure to light (i.e., during the day of diurnal lighting or in LL), but were absent during exposure to darkness. In the SCN, the distribution of Fos immunoreactivity within specific subfields was differentially affected by photic conditions. Following exposure to light, a dense population of Fos-immunopositive cells was found in close association with the immunohistochemically distinct cell and fiber populations distinguishing the ventrolateral subfield of the SCN. In dark-exposed animals, Fos-immunoreactive profiles were distributed throughout the SCN in areas coextensive with the immunohistochemical localization of peptidergic neural elements in both the ventrolateral and dorsomedial subfields. As a consequence of this light-dark difference in the distribution of Fos immunoreactivity, the density of labeled cells was increased within the ventrolateral SCN, but was decreased within the dorsomedial subfield, as a result of exposure to light versus darkness. In the absence of photic time cues, temporal variation in the pattern of Fos immunostaining in the SCN, or within specific subfields of the nucleus, was evident only within dorsomedial SCN during exposure to LL, such that the density of immunopositive cells was greater during the subjective day than during the subjective night. These data demonstrate that light stimulation causes an increase in the expression of Fos protein(s) in the retina and within the ventrolateral, but not the dorsomedial, subfield of the SCN. The inductive effect of light on Fos expression within the retina and the ventrolateral or retinorecipient subfield of the SCN suggests that Fos protein(s) may play a role in the transduction of light signals by the primary neural components governing the generation and photoentrainment of circadian rhythms in mammals. © 1990 Academic Press, Inc. All rights reserved.