Photic induction and circadian expression of Fos-like protein. Immunohistochemical study in the retina and suprachiasmatic nuclei of hamster.

Fos-immunohistochemistry was performed in the retina and at four rostro-caudal levels of the suprachiasmatic nuclei (SCN) in hamsters. Animals were sacrificed at four circadian times (CT) relative to activity onset (CT12): CT07, 11, 14, 19 either in permanent darkness (DD) or 1 h after light stimulation. Quantification of immunoreactive nuclei showed (i) endogenous CT related changes exclusively within the rostral SCN with maximum immunoreactivity at CT07, (ii) CT related responses to light in retinal displaced amacrines, ganglion cells and caudal SCN (maximum at CT19), (iii) expression differences in four subsets of SCN cells according to CT. The rostral subset could be implicated in the endogenous clock mechanism since it exhibited a fluctuation of Fos immunoreactivity in DD and expression of Fos protein at CTs 06 and 18 when light provokes transients in the free-running period. In the caudal SCN, a ventro-laterally localized set responded to light at CTs 13 and 18, a dorsal crescent of cells responded only at CT18 and a group located laterally between these two responded at CT18. These cellular subsets may have different functions in the light-entrainment mechanism since light stimuli at CT13 induced phase-delays and, at CT18, phase-advances in the onset of the free-running locomotor activity rhythm.