SCN: ringmaster of the circadian circus or conductor of the circadian orchestra?

Abstract

The mammalian circadian system is composed of multiple circadian oscillators in both the brain and the periphery. Unravelling the organization of this system is a major challenge that the field is only beginning to take on. Clearly the suprachiasmatic nucleus of the hypothalamus (SCN) plays a key role and sits at or near the top of the organizational hierarchy, the details of which are largely unknown. The SCN has often been characterized as a 'master oscillator' that controls other oscillators downstream in the hierarchy, but there is little information about the nature of that control or how rigid or flexible it may be. Indeed, characterization of the SCN as 'master' may be exaggerated since other central circadian pacemakers are known to exist and the extent of feedback onto the SCN from other oscillators remains unexplored. We have tried to make some of the issues concerning the role of the SCN within the entire system more explicit using the somewhat fanciful metaphor referred to in the title.