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 drives circadian rhythmicity but also directs the circadian organization of photoperiodic (seasonal) timekeeping. Recent evidence using electrophysiological, molecular, and genetic tools now strongly supports this conclusion. Important questions remain regarding the SCN's precise role(s) in the brain's photoperiodic circuits, especially among different species, and the cellular and molecular mechanisms for its photoperiodic "memory." New data suggesting 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.