Forced dissociation of food- and light- entrainable circadian rhythms of rats in a skeleton photoperiod.

To investigate the control over drinking and feeding behavior by the light-entrainable circadian pacemaker, rats were maintained in a 12 h:12 h skeleton photoperiod for 36 days with both food and water restricted to the subjective day. During the restriction period most of the food and water intake was concentrated during the first 4 h of the subjective day. The subsequent release into ad lib conditions under the skeleton photoperiod or continuous dark showed that this increased ingestion during the subjective day persisted for up to 6-10 days. This may indicate the entrainment of a food-entrainable oscillator, which has hitherto been investigated solely in anticipatory activity. The daily activity rhythm returned to its original phase position. Thus the phase of the overt daily rhythm is eventually only determined by the phase of the light-entrainable oscillator (LEO) but can temporarily be influenced by the restriction of food and water access. This restriction apparently stimulated a food-entrainable oscillator in antiphase with the light-entrainable oscillator but failed to phase shift or entrain the light-entrainable oscillator.