Circadian neural rhythms in mammals.

A number of different experimental findings indicate that endogenous circadian neural rhythms are generated within the SCN region of mammals. Rhythmic neural signals from the SCN directly, or indirectly, appear to regulate many, if not all, biochemical, physiological, and behavioral circadian rhythms. The circadian system in mammals is referred to as being "multioscillatory" in nature, although this term is often used to mean different things. The SCN itself may contain many circadian neural oscillators and the coordinated output of these multioscillators may regulate circadian rhythms. The term multioscillatory is also used to indicate the presence of anatomically distinct oscillators. However, the location and properties of circadian pacemakers that may lie outside of the SCN region remain unknown. Indeed, there is no conclusive evidence that circadian rhythms can be generated over a prolonged period of time in the absence of the SCN in mammals maintained under constant environmental conditions. Whereas the SCN appears capable of generating circadian neural signals indefinitely in the absence of any 24-hr fluctuations in the external environment, other components of the circadian system may function as damped oscillators (i.e. persist for only a few cycles in the absence of a periodic input) and/or only emit diurnal rhythms in the presence of rhythmic input from either the external (e.g. light-dark cycle) and/or internal (e.g. SCN) environment. On a conceptual as well as a physiological level, our understanding of the organization of the mammalian circadian system remains limited at the present time. Unfortunately, the limitations on our understanding of the circadian system are not always fully appreciated, and quite often a hypothesis based on limited experimental data is treated as fact. The study of the circadian system is still in the early stages of development, and further progress in elucidating the physiological mechanisms underlying the generation and expression of circadian rhythms may require new ways of looking at old problems as well as new experimental approaches. One final comment should be made concerning the importance of an increased understanding of circadian rhythms for human health. Because the study of the physiology of the circadian system is still in its early stages of development, little is known about the importance of the normal functioning of the circadian system for the health and well-being of the organism. Recent studies in humans suggest that disorders within the circadian system itself may be involved in the etiology of at least some forms of mental illness.