The circadian output signals from the suprachiasmatic nuclei

Abstract

The suprachiasmatic nuclei (SCN) of the anterior hypothalamus comprise a self-sustained biological clock generating an endogenous ∼24-h circadian rhythm, driving many overt daily rhythms in the body. An important remaining question is how the SCN neurons communicate with their efferent targets to control the daily oscillations in behavior and physiology. In this chapter, we summarize several signaling factors that may serve as such SCN output factors. Whereas vasopressin may be involved in the regulation of circadian hormone rhythms, SCN-derived prokineticin 2 (PK2), TGF-α, and cardiotrophin-like cytokine (CLC) may serve as output factors for other circadian rhythms, including locomotor activity, body temperature, and energy metabolism. The circadian rhythm in firing activity of SCN neurons is also likely to be a critical output signaling mechanism. The likely involvement of these output factors in the generation of the circadian rhythm in SCN neuronal firing activity is also discussed.