Expression of Period genes: Rhythmic and nonrhythmic compartments of the suprachiasmatic nucleus pacemaker

The mammalian circadian clock lying in the suprachiasmatic nucleus (SCN) co ntrols daily rhythms and synchronizes the organism to Its environment. In a ll organisms studied, circadian timekeeping is cell-autonomous, and! rhythm icity is thought to be generated by a feedback loop involving clock protein s that inhibit transcription of their own genes. In the present study, we e xamined how these, cellular properties are organized within the SCN tissue to produce rhythmicity and photic entrainment. The results show that the SC N has two compartments regulating Period genes Per1, Per2, and Per3 mRNA ex pression differentially. One compartment shows endogenous rhythmicity in Pe r1, Per2, and Per3 mRNA expression. The other compartment does not have rhy thmic mRNA expression but has gated light-induced Per1 and Per2 and high le vels of endogenous nonrhythmic Per3 mRNA expression. These results reveal t he occurrence of differential regulation of clock genes in two distinct SCN regions and suggest a potential mechanism for producing functional differe nces in distinct SCN subregions.