Daily rhythm of spontaneous immediate-early gene expression in the rat suprachiasmatic nucleus

Nocturnal light induces the expression of various immediate-early genes (IE Gs) in the suprachiasmatic nucleus (SCN), the primary pacemaker of the circ adian system of mammals, and causes phase shifts of behavioral rhythms. In the hamster SCN, some IEGs show both sensitivity to light induction at nigh t and a daily peak of spontaneous expression near dawn in different regions of the nucleus. To investigate whether both patterns of IEG expression are observed in the rat SCN, the authors studied the expression of NGFI-A, jun B, c-fos, and fosB near the time of subjective dawn in rats entrained to a light-dark 12:12 cycle and then maintained in constant total darkness for s imilar to 48 h. They found that there were two independent rhythms of expre ssion for junB and c-fos mRNAs in the SCN: (1) a rhythm of photic sensitivi ty expressed throughout the night and (2) a spontaneous rhythm of expressio n triggered around dawn and persisting for at least 2 h into the day. By co ntrast,fosB and NGFI-A transcripts were expressed only after light exposure at night and did not exhibit significant levels of spontaneous expression in the absence of photic input. These observations demonstrate that the cir cadian clock gates expression of two independent rhythms related to IEC exp ression in the rat SCN. The rhythm of sensitivity to nocturnal light exposu re is expressed more strongly in the ventral SCN and may be related to phot ic entrainment. The second rhythm is triggered spontaneously in darkness ar ound subjective dawn and is expressed in more dorsal parts of the SCN.