The suprachiasmatic nucleus and the organization of a circadian system

Abstract

Circadian rhythms are essential features of organisms adaptation to their environment. The phenomenology and properties of circadian rhythms have been studied extensively over the last 30 years 12 and a large body of data attests to the importance of circadian rhythms in the maintenance of the temporal regulation of behavioral and internal homeostatic events. In mammals, the generation of circadian rhythms and establishment of their temporal sequence in proper phase relation to the light—dark (LD) cycle are special functions of the central nervous system (CNS). It is commonplace in neuroscience that issues of localization of function must be addressed before problems of mechanism are amenable to analysis. So it is with the problem of analysis of the neural mechanisms of circadian rhythm generation and regulation. Over the last 10 years we have obtained a large amount of new data largely directed to questions of localization of function. As this information has evolved, it indicates that the functions of circadian rhythm generation and regulation are sufficiently discrete and the CNS regions participating in them so localized that it is appropriate to speak of a circadian system. That is, we can designate areas and pathways of the CNS whose major function appears to be the generation and regulation of circadian rhythms. It is my intent here to review briefly the data that have led to this conclusion and attempt a preliminary definition of the circadian system of the mammalian brain.