Splitting of the Circadian Rhythm of Activity in Hamsters

Abstract

There is now unequivocal evidence, obtained from a variety of different experimental approaches, that the circadian system of multicellular organisms is composed of a population of circadian oscillators (Pittendrigh 1974, Aschoff and Wever 1976, Moore-Ede et al. 1976, Menaker et al. 1978, Block and Page 1978, Jacklet 1981). Indeed, it appears that even a single measurable circadian rhythm may in fact be regulated by more than one circadian oscillator. Strong evidence for this proposition is the observation that the circadian rhythm of locomotor activity can dissociate into two distinct components. An important feature of this dissociation is that for at least a period of time the two components, or bouts of activity, can free-run with clearly distinct periods which result in a series of changing phase relationships between the two components. Usually these components become recoupled some 12-h out of phase with each other and thereafter assume an identical free-running period. This “splitting” phenomenon is difficult to explain in terms of a single oscillator-regulating activity, but instead indicates that at least two mutually coupled circadian pacemakers underlie the circadian rhythm of activity (Pittendrigh 1974, Pittendrigh and Daan 1976, Daan and Berde 1978).