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

Immediate-early genes (IEGs) are transiently expressed within the rodent ci rcadian system in response to nocturnal light. The two most studied light-i nduced IEGs within this system are Fos and Jun-B. Molecular expression of t hese two genes within the hypothalamic suprachiasmatic nucleus (SCN) correl ates with light-induced behavioral phase shifts. Previous studies of the ro le of Fos and Jun-B in circadian clock resetting have used light stimuli th at induce strong phase shifts. However, the relationship of Fos and Jun-B e xpression in the SCN and light-induced phase shifts in an entrainment conte xt is undocumented in rats. In this study, male rats for which the free run ning period was determined were entrained to a 0.5 h:23.5 h LD cycle. On th e fifteenth day of stable entrainment, the entraining light pulse was reduc ed to 10 min. Animals were killed 50 min later and brains were processed fo r IEG immunocytochemistry. Strong Fos induction was observed in the SCN and the intergeniculate leaflet (IGL). Strong Jun-B immunoreactivity was obser ved only in the SCN whereas Jun-B labeling in the IGL was weak. Significant correlations were obtained between the magnitude of light-induced IEGs in the SCN and the magnitude of the daily phase shift required for stable entr ainment to the 0.5 h:23.5 h LD cycle. Further, a significant correlation wa s observed between the number of Fos and Jun-B immunoreactive cells in the SCN and IGL. These data suggest that the magnitude of Fos and Jun-B inducti on within the SCN is related to the magnitude of the daily phase shift requ ired for stable entrainment. (C) 1999 Elsevier Science B.V. All rights rese rved.