Neural and Endocrine Control of Circadian Rhythms in the Vertebrates

The nervous and endocrine systems are therefore not simply the output side of a circadian system, the interesting properties of which reside elsewhere, but rather contain the circadian system—its input sensors, its oscillators, and much of its output machinery.

Abstract

Although circadian oscillations may be generated at or below the cellular level, their behavioral expression in multicellular animals depends on neural and endocrine processes. This is true in the nontrivial sense that is implied by the fact that neural and endocrine structures have been identified that themselves oscillate, in a few cases independently of the rest of the organism. The nervous and endocrine systems are therefore not simply the output side of a circadian system, the interesting properties of which reside elsewhere, but rather contain the circadian system—its input sensors, its oscillators, and much of its output machinery. It would indeed be surprising if that were not the case. On the other hand, it is no longer necessary to accept that statement as a reasonable assumption. We have good evidence that it is true and can proceed to make it explicit by working out its details (Menaker, Taka-hashi, and Eskin, 1978). More importantly, we can begin to investigate the material basis of circadian organization and to search for general principles.