Efferent Signals of the Suprachiasmatic Nucleus

It is well established that the mammalian suprachiasmatic nucleus (SCN) is a biological pacemaker that entrains the activity of organisms to their environment and controls circadian rhythmicity. However, neither the nature of these coupling signal or signals from the SCN, nor their target or targets in the brain, are well understood. Fiber efferents from the SCN reach nearby hypothalamic regions, suggesting a coupling role for neural efferent pathways. The SCN produces diffusible signals that reach nearby hypothalamic sites and the cerebrospinal fluid, suggesting a role for a diffusible efferent pathway. We consider the possibility of redundant coupling signals of the SCN, and review evidence suggesting that diffusible elements may be sufficient to sustain locomotor rhythmicity in adult animals and to restore locomotor rhythmicity in lesioned hamsters bearing SCN grafts. We also provide data for the occurrence of signals that synchronize oscillators, regardless of initial phase. The distinct role of neural and diffusible SCN coupling signals, and the role of SCN-driven rhythmic systems (pineal melatonin rhythms, body temperature), remain to be explored.