Abstract
The circadian clock (pacemaker, oscillator) is considered endogenous, as opposed to driven, because rhythmic oscillations persist with near 24-hour periodicity under constant conditions. It is, however, connected with the environment because the rhythm is usually entrained or synchronized by the 24-hour light-dark cycle, the major environmental zeitgeber or time cue. The clock is also connected to downstream outputs; namely, the biochemical and behavioral fluctuations that are generally observed as rhythmic phenomena.
In metazoan organisms, several pacemaker locations are known (see King and Takahashi; Lamas et al.; both this volume). The eye (retina), the avian pineal, and the suprachiasmatic nucleus (SCN) all contain independent self-sustaining oscillators (Takahashi et al. 1989; Underwood et al. 1990; Klein et al. 1991; Tosini and Menaker 1996). The current prevailing view is that these tissue pacemakers are a collection of single-cell oscillators and that the tissue-pacemaker properties are similar to those of an individual cell. Original...