Encoding le quattro stagioni within the mammalian brain: Photoperiodic orchestration through the suprachiasmatic nucleus

Within the suprachiasmatic nucleus (SCN) is a pacemaker that not only drive s circadian rhythmicity but also directs the circadian organization of phot operiodic (seasonal) timekeeping. Recent evidence using electrophysiologica l, molecular, and genetic tools now strongly supports this conclusion. Impo rtant questions remain regarding the SCN's precise role(s) in the brain's p hotoperiodic circuits, especially among different species, and the cellular and molecular mechanisms for its photoperiodic "memory." New data suggesti ng that SCN "clock" genes may also function as "calendar" genes are a first step toward understanding how a photoperiodic clock is built from cycling molecules.