Circadian modulation of c-Fos expression occurs only in the SCN and not inother visual projection areas in the rat

Brief photic stimuli at different circadian times induce differential expre ssion of c-Fos in the suprachiasmatic nuclei (SCN). Whether circadian modul ation of light-induced c-Fos expression occurs in other visual projection a reas is not known. We addressed this question by estimating the immunohisto chemical expression of c-Fos induced by 60 min light pulses at three differ ent circadian times. The areas studied were the SCN, the ventral lateral ge niculate nucleus, the intergeniculate leaflet, the ventral tegmental area, the superior colliculus and a non-visual control, the paraventricular thala mic nucleus (PVT). Light pulses induced an increase in the number of c-Fos immunoreactive cells in the SCN as a function of the circadian time. Remain ing visual structures showed a light-induced increase in c-Fos expression b ut this was not dependent on the circadian time. The non-visual control are a (PVT) did not respond to light pulses. Since no circadian modulation was found in the intergeniculate leaflet, which receives collateral projections from the same retinal ganglion cells that project to the SCN, nor in other primary visual projection areas, the present findings suggest that the cir cadian modulation of light-induced c-Fos expression in the SCN depends main ly on the functional properties of its intrinsic neurons.