Dissociation of the rat motor activity rhythm under T cycles shorter than 24 hours

Since the suprachiasmatic nuclei (SCN) were identified as the principal mammalian circadian clock, many studies describing their morphology and physiology have been carried out. Today, the multioscillatory nature of the SCN, which explains the dissociation of the circadian rhythms under some experimental conditions, is widely accepted. Here, we study the simultaneous presence of two circadian rhythms in the motor activity of the rat when exposed to symmetric light-dark (LD) cycles shorter than 24 h (T21, T21.5, T22, T22.5, T23, and T23.5). One rhythmic component was entrained by the external LD cycle whereas the other ran free with a period longer than 24 h. The results show that two circadian rhythms were present only when T was shorter than T23, whereas at T23.5 only one entrained component was manifested. The manifestation of the two circadian components depends quantitatively on the period of the external cycle--i.e., the strength of the entrained rhythm increases when the external T is closer to 24 h--whereas that of the nonentrained rhythm decreases. The dissociation of the motor activity rhythm and the gradual appearance of the two components are explained by considering the entrainment of a multioscillatory system as not taking place as a whole but rather in a partial manner, in such a way that some oscillators may entrain but not others. The effect of the entrained oscillators is added to the masking effect of the LD cycles.