Absence of fos-like-immunoreactivity within the suprachiasmatic nucleus following a transition into darkness

The effects of darkness on the circadian system have not been analyzed with respect to c-fos expression. Male Syrian hamsters (Mesocricetus auratus) were entrained to a light-dark (LD) cycle for 7 days before the transition into darkness (L/D) was advanced six hours (CT6). Induction of phase shifts in wheel-running and fos-like-immunoreactivity (FLI) within the suprachiasmatic nucleus (SCN) and intergeniculate leaflet (IGL) by the unanticipated L/D transition were examined in Experiments I and II. Photic-induction of FLI within the SCN was confirmed in Experiment III. The results showed that the unanticipated L/D transitions induced phase advances in wheel-running, but did not induce FLI within the SCN or the IGL. The lack of FLI within the entrainment pathway following L/D transitions supports other research indicating c-fos expression within the SCN is specific to photic stimulation.