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, neithe r the nature of these coupling signal or signals from the SCN, nor the ir target or targets in the brain, are well understood. Fiber efferent s from the SCN reach nearby hypothalamic regions, suggesting a couplin g role for neural efferent pathways. The SCN produces diffusible signa ls that reach nearby hypothalamic sites and the cerebrospinal fluid, s uggesting a role for a diffusible efferent pathway. We consider the po ssibility of redundant coupling signals of the SCN, and review evidenc e suggesting that diffusible elements may be sufficient to sustain loc omotor rhythmicity in adult animals and to restore locomotor rhythmici ty in lesioned hamsters bearing SCN grafts. We also provide data for t he occurrence of signals that synchronize oscillators, regardless of i nitial 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.