Expression of Fos in the circadian system following nonphotic stimulation

Syrian hamsters, Mesocricetus auratus, were confined to novel running wheel s for a 3-h period, starting at approximately circadian time (CT) 4.5 (i.e. , approaching the middle of their subjective day). It can be reliably predi cted from the amount of running in this situation whether or not there will be a subsequent phase-shift. Expression of the immediate early genes c-fos and fosB was examined by immunocytochemistry in the suprachiasmatic nucleu s (SCN), the intergeniculate leaflet (IGL) of the thalamus, and the medial pretectal area of hamsters that ran vigorously in the novel wheel and would have phase-shifted. c-Fos was increased, compared to levels in a control g roup left in their home cages, in the IGL, and the pretectum (PT), but decr eased in the SCN. No significant changes in FosB were detected in any regio n examined. An additional experiment argued against the possibility that th e changes in c-Fos could be attributed to a rapid advance of the pacemaker to a different phase in the circadian cycle. Counts of c-fos-positive cells in the IGL were similar in animals given pulses of running starting at CT 4.5 and starting at CT 12.5-16 (i.e., in the subjective night when they wou ld have been active anyway). Altogether the results support the view that a ctivation of the IGL is important in nonphotic clock resetting, and raise t he possibility that the PT may also be involved in nonphotic resetting. How ever, the results also indicate that novelty-induced running does not alter c-Fos induction in a phase-specific manner in the IGL. The inhibition of c -Fos in the SCN by nonphotic phase-shifting events contrasts with the well- known inducing effects of light pulses. These different effects might under lie some of the interactions between nonphotic and photic zeitgebers when b oth act together on the circadian system. (C) 1998 Elsevier Science Inc.