Photic entrainment and induction of immediate-early genes within the rat circadian system

Immediate-early genes (IEG) are transiently expressed within the rodent circadian system in response to nocturnal light. Immediate-early genes are part of the mechanism by which light induces phase shifts of circadian rhythmicity. The two most studied light-induced IEG within the rodent circadian system are Fos and Jun-B. Molecular expression of these two genes within the suprachiasmatic nucleus (SCN) correlates both in magnitude and in the temporal contingency with light-induced behavioral phase shifts. The determination of the role of Fos and Jun-B in circadian clock resetting was previously attempted using light stimuli that induces strong phase shifts. However, the relationship between light-induced IEG in an entrainment context is undocumented in rats. The present study examined the relation between photic induction of Fos and Jun-B in an entrainment context. Male rats, for which the free running period was determined, were entrained for 14 days on a 0.5h:23.5h LD cycle with light onset at dawn. On the fifteenth day, the entraining light pulse was reduced to 10 minutes and brains were processed for IEG immunocytochemistry. Strong Fos induction was observed in the three brain regions investigated: the SCN, intergeniculate leaflet (IGL) and anterior paraventricular thalamus (PVT). Strong Jun-B immunoreactivity was observed only in the SCN. Jun-B labeling was weak in the IGL, and absent in the PVT. Significant correlations were obtained between light-induced IEG in the SCN and the magnitude of the daily phase shift required for entrainment. Further, Fos and Jun-B immunoreactivities correlated in the SCN and in the IGL. These data suggest that the magnitude of Fos and Jun-B induction within the SCN is related to the magnitude of the daily phase shift required for entrainment